Article pubs.acs.org/JPCB
Describing Excited State Intramolecular Proton Transfer in Dual Emissive Systems: A Density Functional Theory Based Analysis Liam Wilbraham,†,‡ Marika Savarese,§,∥ Nadia Rega,§,∥ Carlo Adamo,†,⊥ and Ilaria Ciofini*,† †
Institut de Recherche de Chimie Paris IRCP, CNRSChimie ParisTech, PSL Research University, 11 Rue Pierre et Marie Curie, F-75005 Paris, France ‡ Department of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom § Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso Universitario di M.S. Angelo, via Cintia, 80126 Napoli, Italy ∥ Center for Advanced Biomaterials for Health Care@CRIB, Istituto Italiano di Tecnologia, Largo Barsanti e Matteucci n, 53 80125 Napoli, Italy ⊥ Institut Universitaire de France, 103 Boulevard Saint Michel, F-75005 Paris, France ABSTRACT: The excited state intramolecular proton transfer (ESIPT) reaction taking place within 2-(2-hydroxyphenyl)benzoxazole (HBT) and two recently experimentally characterized napthalimide derivativesknown as N-1 and N-4has been investigated in order to identify and test a possible protocol for the description and complete mechanistic and electronic characterization of the reaction at the excited state. This protocol is based on density functional theory, time-dependent density functional theory, and a recently proposed electron density based index (DCT). This method is able to identify all stable species involved in the reaction, discriminate between possible reaction pathways over potential energy surfaces (PES), which are intrinsically very flat and difficult to characterize, and quantitatively measure the excited state charge transfer character throughout the reaction. The photophysical properties of the molecules (i.e., absorption and emission wavelength) are also quantitatively determined via the implicit inclusion of solvent effects in the case of toluene and, the more polar, tetrahydrofuran. The accuracy obtained with this protocol then opens up the possibility of the ab initio design of molecules exhibiting ESIPT for tailored applications such as highly selective molecular sensors.
1. INTRODUCTION Excited state intramolecular proton transfer (ESIPT) has drawn a tremendous amount of research interest and is well documented.1−3 Generally, the ESIPT process requires, at the ground state, an intramolecular hydrogen bond between the proton donor (here, an oxygen atom; Figure 1) and proton
the re-distribution of electronic density across the molecule upon excitation. This results in fast proton transfer from the proton donor to the proton acceptor, i.e., a tautomeric transformation at the excited state converting the excited state enol form (E*) to the excited state keto form (K*). This excited state (K*) can then undergo radiative emission to the ground state keto form (K), before returning to the E state via reverse proton transfer. This mechanism is shown schematically in Figure 1. Overall, if the excited state enol form is stable enough, this process can actually give rise to a dual-emission effect corresponding to the presence of two emission bands in the electronic spectraone corresponding to enol emission and another corresponding to keto emission. This dualemission phenomenon has been exploited in the synthesis of various novel chemosensors for use in various target applictions4−9
Figure 1. Schematic diagram showing energy levels and structure of enol and keto forms involved in a generic ESIPT reaction.
acceptor (here, a nitrogen atom; Figure 1) groups of the molecule which must be in close proximity. In the ground state, molecules capable of ESIPT usually exist exclusively in the most energetically favorable enol (E) form. Upon the absorption of a photon, the acidity and basicity of the proton donor and acceptor groups, respectively, are increased as a consequence of © XXXX American Chemical Society
Special Issue: Photoinduced Proton Transfer in Chemistry and Biology Symposium Received: July 24, 2014 Revised: September 9, 2014
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previously at the TD-DFT level,24−27 but similar approaches have not yet been applied to derivatives of this molecule. This work is structured as follows: after a description of the computational protocol (section 2), the results obtained in terms of stable intermediates and transition energies for the HBT molecule in relation to experimental data are discussed in section 3.1. The potential energy (PES) and charge transfer index surfaces for HBT are also discussed in section 3.1, before results concerning the napthalimide derivatives N-1 and N-4 are explored in section 3.2. Finally, some general conclusions are drawn.
Among ESIPT dyes showing dual emission, probably the 2(2-hydroxyphenyl)benzothiazole (HBT) and 2-(2hydroxyphenyl)benzoxazole (HBO)10 are the most experimentally studied and well known prototypes. Nonetheless, both dyes actually display weak absorption in the visible range, so intensive effort has been devoted to the functionalization of such skeletons in order to enhance their absorption behavior,10−18 although rarely with success.19 Recently, Ma and co-workers20 reported a series of HBT derivatives (the N-X, X = 1−4, family) functionalized with napthalimide groups (NI) showing enhancedi.e. redshiftedabsorption and evidence of the dual emission attributed to ESIPT reactions. To ensure the electronic coupling between the HBT core and the NI substituent, different π conjugated bridges were used. In the present work we will investigate the ground and excited state properties of two such compounds (namely, the N-1 and N-4 molecules), which actually represent the systems with the shortest and the longest bridges of the N-X family (Figure 2), and compare their photophysical behavior to that of the HBT molecule.
2. COMPUTATIONAL PROTOCOL DFT and TD-DFT28−30 were used to explore the ground and excited state properties of each molecule, and all calculations, unless otherwise stated, were carried out at the PBE031/631+G(d)32 and CAM-B3LYP33/6-31+G(d) levels of theory for the ground state. TD-CAM-B3LYP/6-31+G(d) and TD-PBE0/ 6-31+G(d) levels were used for excited state calculations. The CAM-B3LYP functional is introduced because it contains a correction for long-range behavior and performs well in the description of excited states (ESs) of molecules with significant through-space charge transfer.33 In analogy with experiments, both toluene and tetrahydrofuran (THF) were considered as solvents and included using an implicit conductor-like polarizable continuum model (CPCM).34 The ground and excited state structures and energetic minima of the enol and keto forms of each molecule were determined. A two-dimensional (2D) PES of HBT was characterized via a series of partial geometric optimizations, constraining the O−H distance between 1.0 and 1.9 Å in increments of 0.1 Å and the O−N distance between 2.54 and 2.66 Å in increments of 0.02 Å, to form a grid on which the PES is mapped. All calculations were conducted for a planar structure. All calculations were performed using the Gaussian 09 suite of programs,35 with the ground and excited state densities computed using the Gaussian cubegen utility. At least three excited states are kept in all of the calculations for each molecule, with the first being the one of interest. The density grids calculated using the cubegen Gaussian utility are required for the calculation of the density based index. The DCT density based index23 allows the quantification of the spatial extent of charge transfer within a molecule upon vertical excitation, and it has recently been applied for the characterization of excited state processes, including ESPT reactions.21,22,36,37 In order to summarize the construction of the index, we must define ρGS(r) and ρEX(r), the electron densities of the molecule at the ground and excited states, respectively. The change in the density of a local point within the molecule upon excitation is then
Figure 2. Molecular structures of molecules investigated in this work.
Indeed, because the various stable intermediate species involved in ESIPT reaction correspond to different energetic minima, it is possible to use an approach based on density functional theory (DFT) to identify and characterize them.21,22 In this work, in order to capture both the geometric and electronic aspects of the ESIPT reaction as well as the photophysical properties of each of the molecules investigated, an approach utilizing DFT, time-dependent density functional theory (TD-DFT), and a recently proposed density based charge transfer index (DCT)23 will be employed. These tools will be used to explore the potential energy surface and through-space charge transfer character at the excited state. The ESIPT reaction in HBT and similar molecules has been studied
Δρ(r ) = ρEX (r ) − ρGS (r )
(1)
Regions in space that experience an increase in electron density can be represented by the function ρ+(r), and those that experience a decrease in density are described by ρ−(r)these correspond to positive and negative values of Δρ(r), respecively: ⎧ if Δρ(r ) > 0 ⎪ Δρ(r ) ρ+ (r ) = ⎨ ⎪ if Δρ(r ) < 0 ⎩0 B
(2)
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⎧ Δρ(r ) if Δρ(r ) < 0 ρ− (r ) = ⎨ if Δρ(r ) > 0 ⎩0
conclusion that the PT reaction must be a product of the absorption of the photon and the corresponding excitation of HBT. From the analysis of the trans-forms of HBT, it can be seen that the trans-keto configuration is less favorable at the ground state but that it becomes closer in energy with the keto form at the excited state, with a difference between keto and trans-keto of less than 7 and 4 kcal/mol in toluene and THF, respectively, highlighting the trans-keto form of HBT as a potential emitting species. This is in good agreement with the long-lived transketo form reported by Barbatti and co-workers.26 Therefore, even if the trans-keto configuration is less favorable when compared to the enol and keto forms of HBT at the ground state, this confomer is too identified as a possible emitting species that can contribute to experimental spectra. Table 2 shows the calculated and experimental20 photophysical properties of all previously identified species of HBT in
⎪ ⎪
(3)
If we define the barycenters of the regions associated with these functions as R+ and R−, respectively, the charge transfer index is simply defined as the distance between them: DCT = |R+ − R −|
(4)
This index is used in this work to quantify the spatial extent of charge transfer excitations in HBT as a result of the ESIPT mechanism. These DCT values will next be compared to that computed in the cases of N-1 and N-4.
3. RESULTS AND DISCUSSION In order to validate our computational protocol, first the photophysical properties of HBT will be discussed, since for this molecule extensive experimental and theoretical works are available.26,27 Next, the properties of the N-1 and N-4 derivatives will be analyzed (section 3.2). 3.1. HBT Ground and Excited State Properties. Structural optimizations at the ground and excited states were conducted for HBT in both toluene and THF. As well as the enol and keto tautomers, trans-enol and trans-keto rotational isomers (Figure 2) were also considered as it has been shown previously using femtosecond pump−probe experiments that these forms exist for times of around 100 ps.26 It is noted that all conformers considered correspond to a planar structure. Indeed, although the existence of a stable twisted conformation for the excited keto form was previously identified by Barbatti and collaborators,26 at the level of theory here reported we were unable to locate such a conformation. The relative stabilities of the different forms computed at ground and excited state are reported in Table 1.
Table 2. Calculated Photophysical Properties of Various Forms of the HBT Moleculea λABS
molecular species
relative GS energyb
relative ES energyc
toluene
enol keto trans-enol trans-keto enol keto trans-enol trans-keto
0.00 8.38 13.03 15.75 0.00 7.25 11.45 11.79
0.00 −7.32 13.92 −0.69 0.00 −7.62 11.86 −3.90
THF
molecule species
toluene
enol keto trans-enol trans-keto enol keto trans-enol trans-keto
THF
330 406 315 412 327 401 314 408
λEM
calc
expt
(0.580) (0.444) (0.710) (0.447) (0.570) (0.435) (0.689) (0.434)
337 337 335 335
calc 373 470 377 475 384 471 391 477
(0.655) (0.354) (0.766) (0.358) (0.856) (0.463) (0.932) (0.461)
expt 514 514 363 363
a
For each solvent, the calculated absorption and emission energies (nm) are given for each species along with corresponding experimental values.20 Computed oscillator strengths (a.u.) for each transition are given in parentheses.
Table 1. Relative Energies (kcal/mol) Computed for the Various Forms of the HBT Molecule at Ground and Excited Statesa solvent
solvent
both toluene and THF. The predicted absorption and emission wavelengths agree well with the experimental data. In particular, errors ranging from 0.08 to 0.24 eV are observed, in agreement with the level of accuracy expected at the TD-DFT level. Furthermore, the large Stokes’ shift observed in experiment in toluene (of 1.2 eV) is accurately reproduced with calculated values of 1.1 eV. This large Stokes’ shift results from the tautomerisation from the enol to the keto form. By comparison of the experimental and calculated absorption data, the experimental absorption peak corresponds to that calculated for the enol form of HBT in both toluene and THF. This suggests that absorption takes place exclusively by the enol form of HBT and agrees well with the calculated relative stabilities reported in Table 1 which show that the proton transfer does not occur at the ground state and thus the keto form will not absorb. The similarity between the behavior of the HBT molecule in the two solvents ends there, however, as the maximum of the emission band observed in experiment corresponds to that of the keto form in toluene but to that of the enol form in THF. This suggests that the ESIPT reaction is inhibited by the THF solvent. By examining the calculated oscillator strengths and the experimental emission spectra,20 it is possible to further discriminate between the emitting species. In THF, the combined oscillator strength of the enol (0.856) and transenol (0.932) is approximately 2.5 times that of the combined values for the keto (0.354) and trans-keto (0.358) in toluene.
a
Ground state and excited state energies were computed at the PBE0/ 6-31+G(d)/CPCM level and TD-PBE0/6-31+G(d)/CPCM level of theory, respectively. bRelative to the GS enol form (E). cRelative to the ES enol form (E*).
Ignoring the trans-forms of the HBT molecule for the moment, from Table 1 it can be seen that, in both toluene and THF, the minimum energy conformation of HBT at the ground state is represented by the enol tautomer, with the keto form being higher in energy. Conversely, at the excited state, the keto tautomer is the energetically preferred conformation. Together, these allow spontaneous proton transfer at the ground state and reverse proton transfer at the excited state to be ruled out and define the one-way loop of the ESIPT reaction as depicted in Figure 1. All together these findings afford the C
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combined with the reduced contribution on the oxygen relative to the HOMO, is expected for a spontaneous ESIPT reaction in HBT. To better understand the ESIPT mechanism, the partial structural optimization of HBT in each solvent, at various OH and ON distances, was conducted in order to construct 2D potential energy surfacesplotted as contour maps in Figure 4. Note in this case we will not consider possible cis−trans isomerization but consider only the normal confomer. Two potential reaction pathways are identified: a direct pathway which corresponds to a direct proton transfer (i.e., a transfer occurring at almost fixed ON distance) and an indirect, minimal energy pathway which corresponds to a skeletal contraction before the proton transfers. After the proton transfer via the minimal energy path, the reaction pathway takes the route of steepest energy descent before reaching the minimum at the known excited state keto structure. From the computed data, it is clear that the skeletal contraction is the energetically preferred mechanism and it is interesting to note that the maximum energy value in this path (the transition state) corresponds to the minimum ON distance. Comparing the PES of each solvent, it is clear that the reaction is less favored in THF as previously suggested via the comparison of the calculated and experimental emission data in Table 2. This is shown quantitatively in Table 3 which reports
This is in agreement with the relative intensities in the experimental spectrum20 and provides further evidence that the keto species is the dominant emitter in toluene while the enol species remains prevalent in THF. Notably, the trans-isomer of the HBT molecule has predicted emission wavelengths close to those of the cis-isomers so that, based on the sole photophysical data, it is not possible to discriminate the rotational isomer present. Therefore, based on the analysis of the data collected in Tables 1 and 2, it would rather seem that, in the case of the more polar THF solvent, irradiation may induce significant isomerization more than ESIPT. From an electronic point of view, all of these transitions correspond to HOMO−LUMO excitations. The orbitals involved in the case of the HBT enol and keto tautomers in THF are reported in Figure 3. Clearly the transitions are of
Table 3. Estimated Reaction Barriers (kcal/mol) for Both the Direct Proton Transfer and the Minimal Energy (Skeletal Contraction) Mechanisms reaction pathway
toluene
THF
direct minimal
4.2 1.9
4.9 3.0
the estimated reaction barrier for ESIPT in toluene and THF. From this data, it appears the reaction barrier is higher in THF with respect to toluene and that the reaction barrier corresponding to the skeletal contraction (named as minimal) is, as expected, always lower than the direct proton transfer in both solvents. Indeed this finding simply conveys that, during a PT, the donor and acceptor atoms get closer first and then the proton is transferred. While the PES gives an insight into the mechanical aspects of ESIPT, it does not allow the direct monitoring of the nature of
Figure 3. Calculated orbital of HBT in toluene: (A) enol HOMO; (B) keto HOMO; (C) enol LUMO; (D) keto LUMO.
π−π* type, and the solvent seems to have no significant effect on the orbital character. Furthermore, the large orbital contribution computed on the nitrogen for the LUMO,
Figure 4. Two-dimensional excited state potential energy surface (PES) computed in (A) toluene and (B) THF. Energies are in kilocalories per mole relative to the keto form, and OH and ON distances are in angstroms. Black arrows indicate potential reaction pathways. The energetic minima of the excited state enol (E*) and keto (K*) forms are indicated. D
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Figure 5. Two-dimensional DCT surfaces computed in (A) toluene and (B) THF. Distances are in angstroms. Arrows indicate potential reaction pathways.
the electronic rearrangement taking place in HBT during the reaction. In order to capture the electronic changes during ESIPT, the previously defined DCT index was used to construct a “DCT surface” for each solvent, as shown in Figure 5. For both toluene and THF, it is predicted that, as the proton is transferred, the DCT index increases to a maximum value before decaying to a final value at the keto form of the molecule. The increase in the charge separation to a maximum is expected as it corresponds to the reaching of a maximum CT distance. After the transfer of the proton takes place, and the molecule retracts, the charge transfer character decreases. It is interesting to note that the DCT value does not decay significantly after proton transfer. This is due to the intramolecular nature of the reaction, in which the proton is held in place after the reaction has completed. In another of our studies,21 conducted using the same index, the DCT value was seen to decrease much further as this reaction was intermolecular in nature so the proton was able to “escape” once it has transferred to the acceptor molecule, resulting in a more dramatic decrease in the charge separation. In general the quantitative agreement in photophysical properties obtained for the HBT molecule allowed us to validate the level of theory used for the study of the N-1 and N4 derivatives. 3.2. N-1 and N-4 Derivatives. As for HBT, ground and excited state optimizations were conducted for the N-1 molecule using the PBE0 functional. These calculations, as well as those for N-4, were also performed using the CAMB3LYP functional in order to verify the quality of the results since a significant charge transfer character is expected. The results obtained are summarized in Table 4. N-1, as for HBT, is more stable in the enol form at the ground state while the keto form is lower in energy at excited state for both toluene and THF when the PBE0 functional is used. This, again, rules out spontaneous proton transfer reaction at the ground state for the N-1 molecule. Nonetheless, the relative difference in total energy between the N-1 enol and keto forms at ground and especially at the excited state is smaller when compared to those calculated for HBT. This suggests both that the N-1 molecule has a flatter PES than HBT and that the red shift of the emission energy between the enol and keto forms of N-1 is expected to be lower. Contrary to this, using the CAM-B3LYP functional, the ESIPT reaction is predicted to be slightly unfavored, though the relative total energies of the enol and keto forms are very close (within 0.5 kcal/mol). Analogous with HBT, the trans-enol and trans-keto
Table 4. Relative Energies of Various Forms of the N-1 and N-4 Molecules (kcal/mol) at Ground and Excited Statesa molecule/functional N-1/PBE0
solvent toluene THF
N-1/CAM-B3LYP
toluene
THF
N-4/CAM-B3LYP
toluene THF
molecule species
relative GS energyb
relative ES energyc
enol keto enol keto enol keto trans-enol trans-keto enol keto trans-enol trans-keto enol keto enol keto
0.00 6.71 0.00 5.50 0.00 7.39 9.15 13.82 0.00 6.09 8.87 11.13 0.00 7.99 0.00 6.70
0.00 −0.63 0.00 −1.77 0.00 0.28 9.94 4.91 0.00 −0.61 9.61 3.04 0.00 4.45 0.00 5.28
a
Ground and excited state energies for N-1 were calculated both using PBE0 and CAM-B3LYP functionals although only the latter values are reported in the table. N-4 calculations utilise the CAM-B3LYP functional only. bRelative to the Enol form (E). cRelative to the ES enol form (E*).
forms of N-1 are 8.8 and 11.1 kcal/mol higher in energy than the enol form at the ground state, respectively, while at the excited state that the total energy of the trans-keto form is within 4 kcal/mol of the cis-keto form which is qualitatively similar to results obtained for the HBT molecule. In contrast to N-1 and HBT, the enol form of the N-4 molecule (in both solvents) has lower total energy at both the ground and excited stateevidence that ESIPT does not occur spontaneously for this molecule. This, together with the results for the N-1 molecule, is early evidence that the experimental strategy of the addition of napthalimide derivatives to HBT to shift its absorption and emission wavelengths may not deliver the desired dual-emission phenomenon. Calculated photophysical properties of the N-1 and N-4 molecules are given in Table 5. At first glance and focusing on the enol and keto forms of N-1 to begin with, there is good agreement between the experimental and calculated absorption and emission energies using both functionals. Examining the mean average error (MAE) at the ground and excited states between predicted and experimental data, it is found that the E
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Table 5. Calculated Photophysical Properties of Various Forms of N-1 and N-4 Moleculesa λABS molecule/functional N-1/PBE0
molecule species
calc
expt
calc
toluene
enol keto enol keto enol keto trans-enol trans-keto enol keto trans-enol trans-keto enol keto enol keto
439 (1.010) 501 (1.043) 442 (0.976) 505 (1.005) 375 (1.209) 425 (1.098) 371 (1.139) 440 (1.109) 375 (1.186) 421 (1.141) 372 (1.1212) 434 (1.1158) 389 (1.302) 419 (0.432) 392 (1.288) 412 (0.362)
405
490 (1.104) 551 (1.131) 514 (1.232) 581 (1.265) 447 (1.291) 490 (1.323) 442 (1.225) 502 (1.291) 470 (1.406) 520 (1.542) 465 (1.3469) 529 (1.5318) 466 (1.457) 482 (0.467) 493 (1.590) 485 (0.553)
THF N-1/CAM-B3LYP
toluene
THF
N-4/CAM-B3LYP
λEM
solvent
toluene THF
384 405 405 384 384 433 429
expt 545 592 545 545 592 592 491 553
a
For each solvent, the calculated absorption and emission energies (nm) are given for each species along with corresponding experimental values.20 Computed oscillator strengths (a.u.) for each transition are given in parentheses.
PBE0 functional yields an MAE of 0.33 and 0.03 eV for the absorption and emission energies, respectively. This is unexpected as we would expect the discrepancy between calculated and experimental data to be larger for emission values. It can be concluded, then, that the greater accuracy observed for the calculated emission energies in N-1 is the result of some kind of error cancelation, prompting the introduction of the CAM-B3LYP functional in an attempt to address this inconsistency. Using the CAM-B3LYP functional, the MAE for calculated absorption and emission energies is 0.16 and 0.27 eV, respectively. This level of accuracy is more reasonable, particularly for absorption energies, which are expected to be typically accurate to within 0.2 eV of experimental values at the TD level. Following this, we can conclude that the CAMB3LYP functional is more suitable concerning the larger N-1 and N-4 molecules with significant charge transfer character (see below), so calculations involving the largest molecule, N-4, were conducted using this functional only. Analyzing the experimental emission spectra,20 we can note that, for both molecules in toluene, only a single distinct band is observed. This is in excellent agreement with the calculated emission energies which predicted a red shift of approximately 0.24 eV for N-1 and 0.09 eV for N-4not sufficient to observe distinct dual emission in either molecule. It is noted that the proton transfer reaction was earlier deemed unfavorable for the N-4 molecule (Table 4), which is also in keeping with the single emission band observed. Actually, this result is also reflected in the calculated molecular orbitals shown in Figure 6. It can be seen that the napthalimide moieties in both N-1 and N-4 act to withdraw electrons from the HBT moiety upon excitation from HOMO to LUMO. Comparing the enol and keto transitions, they are observed as almost identical so it is reasonable to assume that the transition energies, as a result, are similar. In fact, it was observed that the orbital shape was unaffected by the solvent used, which suggests that the HOMO−LUMO transition energies for the enol and keto forms in THF, as for toluene, are similar. From Table 5, the calculated emission energy for the trans-enol form compares well with the second, smaller
emission band observed in THF, at approximately 475 nm. This, the orbital shapes and the total energies given in Table 4, gives strong evidence that the dual emission observed in THF for N-1 is not the product of the ESIPT reaction but the result of the formation of a rotational confomer, namely, the transenol form. Finally, the absence of any contribution to the LUMO from orbitals centered on atoms involved in the ESIPT for N-4 complements the earlier suggestion that the ESIPT reaction is not favored for this molecule. The calculated DCT values for the enol and keto forms of N-1 and N-4 (all around 4.0 Å) show that, compared with HBT, the through-space charge transfer character is much larger after the addition of the napthalimide moiety. From this, it is expected that electronic transitions within N-1 and, by extension, N-4 are dominated by charge transfer and are hardly influenced by the ESIPT reaction itself. This is, again, reflected in the orbital shapes for these molecules (Figure 6) which show negligible contribution to the LUMO from orbitals centered on atoms involved in the ESIPT and thus suggests that it is unlikely that dual emission would be observed as a result of ESIPT for N-1 and N-4.
4. CONCLUSION In summary, it is clear that the PBE0/6-31+G(d)/CPCM level of theory performs well in the prediction of ground and excited state properties of HBT and that the minimal energy intermediate structures involved in ESIPT can be successfully identified. Given the qualitative and quantitative agreement with experimental data, the protocol of using the PBE0/6-31G +(d)/CPCM level of theory to conduct a scan over OH and ON distances is indeed able to identify, and discriminate between, potential reaction pathways in different solvents across a very flat PES. While the PESs can describe the mechanical aspects of the ESIPT reaction very well, they say nothing of the change in electronic arrangement of the molecule as a result of proton transfer, soin order to fully describe ESIPTthis was incorporated via a density based index (DCT). It has been shown that the charge separation quality of the ESIPT reaction can be characterized with the DCT index using a F
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napthalimide moieties, or equally strong acceptor units, to the base HBT molecule in fact reduces the red shift between enol and keto tautomers and obscures any true dual emission as a result. In fact, in the more extreme case of the N-4 molecule, the ESIPT reaction is not favored at all. To conclude, it has been shown that computationally affordable computational protocols such as those here described can be of valuable help in the ab initio screening and design of new molecules that exhibit the ESIPT reaction for use in novel applications in molecular probes.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ilaria.ciofi[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.W. acknowledges the University of Strathclyde for an Erasmus grant. I.C. acknowledges support of IDRIS (Orsay) within the framework of the project “DARI i2014087160”.
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REFERENCES
(1) LeGourriérec, D.; Kharlanov, V. A.; Brown, R. G.; Rettig, W. Excited-State Intramolecular Proton Transfer (ESIPT) in 2-(2′Hydroxyphenyl)-oxazole and -thiazole. J. Photochem. Photobiol., A 2000, 130, 101−111. (2) Ameer-Beg, S.; Ormson, S. M.; Brown, R. G.; Matousek, P.; Towrie, M.; Nibbering, E. T.; Foggi, P.; Neuwahl, F. V. Ultrafast Measurements of Excited State Intramolecular Proton Transfer (ESIPT) in Room Temperature Solutions of 3-Hydroxyflavone and Derivatives. J. Phys. Chem. A 2001, 105, 3709−3718. (3) Lim, S.-J.; Seo, J.; Park, S. Y. Photochromic Switching of ExcitedState Intramolecular Proton-Transfer (ESIPT) Fluorescence: A Unique Route to High-Contrast Memory Switching and Nondestructive Readout. J. Am. Chem. Soc. 2006, 128, 14542−14547. (4) Liu, B.; Wang, H.; Wang, T.; Bao, Y.; Du, F.; Tian, J.; Li, Q.; Bai, R. A New Ratiometric ESIPT Sensor for Detection of Palladium Species in Aqueous Solution. Chem. Commun. (Cambridge, U. K.) 2012, 48, 2867−2869. (5) JináKang, H. A Highly Selective Fluorescent ESIPT Probe for the Dual Specificity Phosphatase MKP-6. Chem. Commun. (Cambridge, U. K.) 2009, 5895−5897. (6) Udhayakumari, D.; Saravanamoorthy, S.; Ashok, M.; Velmathi, S. Simple Imine Linked Colorimetric and Fluorescent Receptor for Sensing Zn2+ Ions in Aqueous Medium Based on Inhibition of ESIPT Mechanism. Tetrahedron Lett. 2011, 52, 4631−4635. (7) Wu, Y.; Peng, X.; Fan, J.; Gao, S.; Tian, M.; Zhao, J.; Sun, S. Fluorescence Sensing of Anions Based on Inhibition of Excited-State Intramolecular Proton Transfer. J. Org. Chem. 2007, 72, 62−70. (8) Chen, W.-H.; Xing, Y.; Pang, Y. A Highly Selective Pyrophosphate Sensor Based on ESIPT Turn-on in Water. Org. Lett. 2011, 13, 1362−1365. (9) Ulrich, G.; Nastasi, F.; Retailleau, P.; Puntoriero, F.; Ziessel, R.; Campagna, S. Luminescent Excited-State Intramolecular ProtonTransfer (ESIPT) Dyes Based on 4-Alkyne-Functionalized [2, 2′Bipyridine]-3, 3′-Diol Dyes. Chem.Eur. J. 2008, 14, 4381−4392. (10) Ikegami, M.; Arai, T. Photoinduced Intramolecular Hydrogen Atom Transfer in 2-(2-hydroxyphenyl) Benzoxazole and 2-(2hydroxyphenyl) Benzothiazole Studied by Laser Flash Photolysis. J. Chem. Soc., Perkin Trans. 2 2002, 1296−1301. (11) Lukeman, M.; Wan, P. A New Type of Excited-State Intramolecular Proton Transfer: Proton Transfer from Phenol OH to a Carbon Atom of an Aromatic Ring Observed for 2-Phenylphenol. J. Am. Chem. Soc. 2002, 124, 9458−9464.
Figure 6. Calculated orbital of N-1 and N-4 in toluene: (A) N-1 enol HOMO; (B) N-1 keto HOMO; (C) N-1 enol LUMO; (D) N-1 keto LUMO (E) enol HOMO N-4; (F) N-4 keto HOMO; (G) N-4 enol LUMO; (H) N-4 keto LUMO.
similar “scan” protocol in the construction of the excited state PESs. It was found that increased charge separation is achieved in solutions of THF compared with toluene, despite the reduced tendency for HBT to enter the keto form. The transition state was also identified as the point at which the reaction path reaches maximum DCT. The calculated values of DCT for N-1 and N-4 show that the electronic transitions within this molecule are dominated by charge transfer and have little influence from the ESIPT reaction. From a computational point of view for N-1 and N-4, where the long-range interelectronic behavior must be accounted for, the CAM-B3LYP functional is better suited for application in TD-DFT calculations while the PBE0 functional remains sufficient for ground state DFT calculations. From a more application-oriented point of view, it has also been shown that the dual emission observed by Ma and coworkers for the N-1 napthalimide derivative of HBT is likely due to rotational isomerism of the molecule, and not the ESIPT mechanism itself. In fact, the molecule exhibits a very small red shift between the enol and keto forms and these arguments are supported by the calculations for the N-4 molecule. These findings together support that the strategy of the addition of G
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(12) Lukeman, M.; Wan, P. Excited-State Intramolecular Proton Transfer in o-Hydroxybiaryls: A New Route to Dihydroaromatic Compounds. J. Am. Chem. Soc. 2003, 125, 1164−1165. (13) Lukeman, M.; Wan, P. Excited State Intramolecular Proton Transfer (ESIPT) in 2-Phenylphenol: An Example of Proton Transfer to a Carbon of an Aromatic Ring. Chem. Commun. (Cambridge, U. K.) 2001, 1004−1005. (14) Wang, Y.-H.; Wan, P. Excited State Intramolecular Proton Transfer (ESIPT) in Dihydroxyphenyl Anthracenes. Photochem. Photobiol. Sci. 2011, 10, 1934−1944. (15) Ulrich, G.; Ziessel, R.; Harriman, A. The Chemistry of Fluorescent Bodipy Dyes: Versatility Unsurpassed. Angew. Chem., Int. Ed. 2008, 47, 1184−1201. (16) Ziessel, R.; Harriman, A. Artificial Light-Harvesting Antennae: Electronic Energy Transfer by Way of Molecular Funnels. Chem. Commun. (Cambridge, U. K.) 2011, 47, 611−631. (17) Seo, J.; Kim, S.; Park, S. Y. Strong Solvatochromic Fluorescence from the Intramolecular Charge-Transfer State Created by ExcitedState Intramolecular Proton Transfer. J. Am. Chem. Soc. 2004, 126, 11154−11155. (18) Baiz, C. R.; Ledford, S. J.; Kubarych, K. J.; Dunietz, B. D. Beyond 7-Azaindole: Conjugation Effects on Intermolecular Double Hydrogen-Atom Transfer Reactions. J. Phys. Chem. A 2009, 113, 4862−4867. (19) Yang, P.; Zhao, J.; Wu, W.; Yu, X.; Liu, Y. Accessing the LongLived Triplet Excited States in Bodipy-Conjugated 2-(2-Hydroxyphenyl) Benzothiazole/Benzoxazoles and Applications as Organic Triplet Photosensitizers for Photooxidations. J. Org. Chem. 2012, 77, 6166−6178. (20) Ma, J.; Zhao, J.; Yang, P.; Huang, D.; Zhang, C.; Li, Q. New Excited State Intramolecular Proton Transfer (ESIPT) Dyes Based on Naphthalimide and Observation of Long-Lived Triplet Excited States. Chem. Commun. (Cambridge, U. K.) 2012, 48, 9720−9722. (21) Savarese, M.; Netti, P. A.; Adamo, C.; Rega, N.; Ciofini, I. Exploring the Metric of Excited State Proton Transfer Reactions. J. Phys. Chem. B 2013, 117, 16165−16173. (22) Savarese, M.; Netti, P. A.; Rega, N.; Adamo, C.; Ciofini, I. Intermolecular Proton Shuttling in Excited State Proton Transfer Reactions: Insights from Theory. Phys. Chem. Chem. Phys. 2014, 16, 8661−8666. (23) Le Bahers, T.; Adamo, C.; Ciofini, I. A Qualitative Index of Spatial Extent in Charge-Transfer Excitations. J. Chem. Theory Comput. 2011, 7, 2498−2506. (24) Mohammed, O. F.; Luber, S.; Batista, V. S.; Nibbering, E. T. Ultrafast Branching of Reaction Pathways in 2-(2′-Hydroxyphenyl) benzothiazole in Polar Acetonitrile Solution. J. Phys. Chem. A 2011, 115, 7550−7558. (25) Luber, S.; Adamczyk, K.; Nibbering, E. T.; Batista, V. S. Photoinduced Proton Coupled Electron Transfer in 2-(2′-Hydroxyphenyl)-Benzothiazole. J. Phys. Chem. A 2013, 117, 5269−5279. (26) Barbatti, M.; Aquino, A.; Lischka, H.; Schriever, C.; Lochbrunner, S.; Riedle, E. Ultrafast Internal Conversion Pathway and Mechanism in 2-(2′-hydroxyphenyl)benzothiazole: A Case Study for Excited-State Intramolecular Proton Transfer Systems. Phys. Chem. Chem. Phys. 2009, 11, 1406−1415. (27) Tsai, H.-H. G.; Sun, H.-L. S.; Tan, C.-J. TD-DFT Study of the Excited-State Potential Energy Surfaces of 2-(2′-Hydroxyphenyl) benzimidazole and Its Amino Derivatives. J. Phys. Chem. A 2010, 114, 4065−4079. (28) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. Molecular Excitation Energies to High-Lying Bound States from TimeDependent Density-Functional Response Theory: Characterization and Correction of the Time-Dependent Local Density Approximation Ionization Threshold. J. Chem. Phys. 1998, 108, 4439−4449. (29) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. An Efficient Implementation of Time-Dependent Density-Functional Theory for the Calculation of Excitation Energies of Large Molecules. J. Chem. Phys. 1998, 109, 8218−8224.
(30) Furche, F.; Ahlrichs, R. Adiabatic Time-Dependent Density Functional Methods for Excited State Properties. J. Chem. Phys. 2002, 117, 7433−7447. (31) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110, 6158−6170. (32) Francl, M.; Pietro, W.; Hehre, W.; Binkley, H.; Gordon, M.; Defrees, D.; Pople, J. Self-Consistent Molecular-Orbital Methods. 23. A Polarization-Type Basis Set for 2nd-Row Elements. J. Chem. Phys. 1982, 77, 3654−3665. (33) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange− Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51−57. (34) Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995−2001. (35) Frisch, M.; et al. Gaussian 09, Revision A.02; Gaussian: Wallingford, CT, USA, 2009; p 200. (36) Savarese, M.; Raucci, U.; Adamo, C.; Netti, P. A.; Ciofini, I.; Rega, N. Modeling of Charge Transfer Processes to Understand Photophysical Signatures: The Case of Rhodamine 110. Chem. Phys. Lett. 2014, 610, 148−152. (37) Savarese, M.; Raucci, U.; Adamo, C.; Netti, P. A.; Ciofini, I.; Rega, N., Non-Radiative Decay paths in Rhodamines: New Theoretical Insight Phys. Chem. Chem. Phys. 2014, DOI: 10.1039/C4CP02622E.
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Describing Excited State Intramolecular Proton Transfer in Dual Emissive Systems: A Density Functional Theory Based Analysis Liam Wilbraham,†,‡ Marika Savarese,§,∥ Nadia Rega,§,∥ Carlo Adamo,†,⊥ and Ilaria Ciofini*,† †
Institut de Recherche de Chimie Paris IRCP, CNRSChimie ParisTech, PSL Research University, 11 Rue Pierre et Marie Curie, F-75005 Paris, France ‡ Department of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom § Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso Universitario di M.S. Angelo, via Cintia, 80126 Napoli, Italy ∥ Center for Advanced Biomaterials for Health Care@CRIB, Istituto Italiano di Tecnologia, Largo Barsanti e Matteucci n, 53 80125 Napoli, Italy ⊥ Institut Universitaire de France, 103 Boulevard Saint Michel, F-75005 Paris, France ABSTRACT: The excited state intramolecular proton transfer (ESIPT) reaction taking place within 2-(2-hydroxyphenyl)benzoxazole (HBT) and two recently experimentally characterized napthalimide derivativesknown as N-1 and N-4has been investigated in order to identify and test a possible protocol for the description and complete mechanistic and electronic characterization of the reaction at the excited state. This protocol is based on density functional theory, time-dependent density functional theory, and a recently proposed electron density based index (DCT). This method is able to identify all stable species involved in the reaction, discriminate between possible reaction pathways over potential energy surfaces (PES), which are intrinsically very flat and difficult to characterize, and quantitatively measure the excited state charge transfer character throughout the reaction. The photophysical properties of the molecules (i.e., absorption and emission wavelength) are also quantitatively determined via the implicit inclusion of solvent effects in the case of toluene and, the more polar, tetrahydrofuran. The accuracy obtained with this protocol then opens up the possibility of the ab initio design of molecules exhibiting ESIPT for tailored applications such as highly selective molecular sensors.
1. INTRODUCTION Excited state intramolecular proton transfer (ESIPT) has drawn a tremendous amount of research interest and is well documented.1−3 Generally, the ESIPT process requires, at the ground state, an intramolecular hydrogen bond between the proton donor (here, an oxygen atom; Figure 1) and proton
the re-distribution of electronic density across the molecule upon excitation. This results in fast proton transfer from the proton donor to the proton acceptor, i.e., a tautomeric transformation at the excited state converting the excited state enol form (E*) to the excited state keto form (K*). This excited state (K*) can then undergo radiative emission to the ground state keto form (K), before returning to the E state via reverse proton transfer. This mechanism is shown schematically in Figure 1. Overall, if the excited state enol form is stable enough, this process can actually give rise to a dual-emission effect corresponding to the presence of two emission bands in the electronic spectraone corresponding to enol emission and another corresponding to keto emission. This dualemission phenomenon has been exploited in the synthesis of various novel chemosensors for use in various target applictions4−9
Figure 1. Schematic diagram showing energy levels and structure of enol and keto forms involved in a generic ESIPT reaction.
acceptor (here, a nitrogen atom; Figure 1) groups of the molecule which must be in close proximity. In the ground state, molecules capable of ESIPT usually exist exclusively in the most energetically favorable enol (E) form. Upon the absorption of a photon, the acidity and basicity of the proton donor and acceptor groups, respectively, are increased as a consequence of © XXXX American Chemical Society
Special Issue: Photoinduced Proton Transfer in Chemistry and Biology Symposium Received: July 24, 2014 Revised: September 9, 2014
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previously at the TD-DFT level,24−27 but similar approaches have not yet been applied to derivatives of this molecule. This work is structured as follows: after a description of the computational protocol (section 2), the results obtained in terms of stable intermediates and transition energies for the HBT molecule in relation to experimental data are discussed in section 3.1. The potential energy (PES) and charge transfer index surfaces for HBT are also discussed in section 3.1, before results concerning the napthalimide derivatives N-1 and N-4 are explored in section 3.2. Finally, some general conclusions are drawn.
Among ESIPT dyes showing dual emission, probably the 2(2-hydroxyphenyl)benzothiazole (HBT) and 2-(2hydroxyphenyl)benzoxazole (HBO)10 are the most experimentally studied and well known prototypes. Nonetheless, both dyes actually display weak absorption in the visible range, so intensive effort has been devoted to the functionalization of such skeletons in order to enhance their absorption behavior,10−18 although rarely with success.19 Recently, Ma and co-workers20 reported a series of HBT derivatives (the N-X, X = 1−4, family) functionalized with napthalimide groups (NI) showing enhancedi.e. redshiftedabsorption and evidence of the dual emission attributed to ESIPT reactions. To ensure the electronic coupling between the HBT core and the NI substituent, different π conjugated bridges were used. In the present work we will investigate the ground and excited state properties of two such compounds (namely, the N-1 and N-4 molecules), which actually represent the systems with the shortest and the longest bridges of the N-X family (Figure 2), and compare their photophysical behavior to that of the HBT molecule.
2. COMPUTATIONAL PROTOCOL DFT and TD-DFT28−30 were used to explore the ground and excited state properties of each molecule, and all calculations, unless otherwise stated, were carried out at the PBE031/631+G(d)32 and CAM-B3LYP33/6-31+G(d) levels of theory for the ground state. TD-CAM-B3LYP/6-31+G(d) and TD-PBE0/ 6-31+G(d) levels were used for excited state calculations. The CAM-B3LYP functional is introduced because it contains a correction for long-range behavior and performs well in the description of excited states (ESs) of molecules with significant through-space charge transfer.33 In analogy with experiments, both toluene and tetrahydrofuran (THF) were considered as solvents and included using an implicit conductor-like polarizable continuum model (CPCM).34 The ground and excited state structures and energetic minima of the enol and keto forms of each molecule were determined. A two-dimensional (2D) PES of HBT was characterized via a series of partial geometric optimizations, constraining the O−H distance between 1.0 and 1.9 Å in increments of 0.1 Å and the O−N distance between 2.54 and 2.66 Å in increments of 0.02 Å, to form a grid on which the PES is mapped. All calculations were conducted for a planar structure. All calculations were performed using the Gaussian 09 suite of programs,35 with the ground and excited state densities computed using the Gaussian cubegen utility. At least three excited states are kept in all of the calculations for each molecule, with the first being the one of interest. The density grids calculated using the cubegen Gaussian utility are required for the calculation of the density based index. The DCT density based index23 allows the quantification of the spatial extent of charge transfer within a molecule upon vertical excitation, and it has recently been applied for the characterization of excited state processes, including ESPT reactions.21,22,36,37 In order to summarize the construction of the index, we must define ρGS(r) and ρEX(r), the electron densities of the molecule at the ground and excited states, respectively. The change in the density of a local point within the molecule upon excitation is then
Figure 2. Molecular structures of molecules investigated in this work.
Indeed, because the various stable intermediate species involved in ESIPT reaction correspond to different energetic minima, it is possible to use an approach based on density functional theory (DFT) to identify and characterize them.21,22 In this work, in order to capture both the geometric and electronic aspects of the ESIPT reaction as well as the photophysical properties of each of the molecules investigated, an approach utilizing DFT, time-dependent density functional theory (TD-DFT), and a recently proposed density based charge transfer index (DCT)23 will be employed. These tools will be used to explore the potential energy surface and through-space charge transfer character at the excited state. The ESIPT reaction in HBT and similar molecules has been studied
Δρ(r ) = ρEX (r ) − ρGS (r )
(1)
Regions in space that experience an increase in electron density can be represented by the function ρ+(r), and those that experience a decrease in density are described by ρ−(r)these correspond to positive and negative values of Δρ(r), respecively: ⎧ if Δρ(r ) > 0 ⎪ Δρ(r ) ρ+ (r ) = ⎨ ⎪ if Δρ(r ) < 0 ⎩0 B
(2)
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⎧ Δρ(r ) if Δρ(r ) < 0 ρ− (r ) = ⎨ if Δρ(r ) > 0 ⎩0
conclusion that the PT reaction must be a product of the absorption of the photon and the corresponding excitation of HBT. From the analysis of the trans-forms of HBT, it can be seen that the trans-keto configuration is less favorable at the ground state but that it becomes closer in energy with the keto form at the excited state, with a difference between keto and trans-keto of less than 7 and 4 kcal/mol in toluene and THF, respectively, highlighting the trans-keto form of HBT as a potential emitting species. This is in good agreement with the long-lived transketo form reported by Barbatti and co-workers.26 Therefore, even if the trans-keto configuration is less favorable when compared to the enol and keto forms of HBT at the ground state, this confomer is too identified as a possible emitting species that can contribute to experimental spectra. Table 2 shows the calculated and experimental20 photophysical properties of all previously identified species of HBT in
⎪ ⎪
(3)
If we define the barycenters of the regions associated with these functions as R+ and R−, respectively, the charge transfer index is simply defined as the distance between them: DCT = |R+ − R −|
(4)
This index is used in this work to quantify the spatial extent of charge transfer excitations in HBT as a result of the ESIPT mechanism. These DCT values will next be compared to that computed in the cases of N-1 and N-4.
3. RESULTS AND DISCUSSION In order to validate our computational protocol, first the photophysical properties of HBT will be discussed, since for this molecule extensive experimental and theoretical works are available.26,27 Next, the properties of the N-1 and N-4 derivatives will be analyzed (section 3.2). 3.1. HBT Ground and Excited State Properties. Structural optimizations at the ground and excited states were conducted for HBT in both toluene and THF. As well as the enol and keto tautomers, trans-enol and trans-keto rotational isomers (Figure 2) were also considered as it has been shown previously using femtosecond pump−probe experiments that these forms exist for times of around 100 ps.26 It is noted that all conformers considered correspond to a planar structure. Indeed, although the existence of a stable twisted conformation for the excited keto form was previously identified by Barbatti and collaborators,26 at the level of theory here reported we were unable to locate such a conformation. The relative stabilities of the different forms computed at ground and excited state are reported in Table 1.
Table 2. Calculated Photophysical Properties of Various Forms of the HBT Moleculea λABS
molecular species
relative GS energyb
relative ES energyc
toluene
enol keto trans-enol trans-keto enol keto trans-enol trans-keto
0.00 8.38 13.03 15.75 0.00 7.25 11.45 11.79
0.00 −7.32 13.92 −0.69 0.00 −7.62 11.86 −3.90
THF
molecule species
toluene
enol keto trans-enol trans-keto enol keto trans-enol trans-keto
THF
330 406 315 412 327 401 314 408
λEM
calc
expt
(0.580) (0.444) (0.710) (0.447) (0.570) (0.435) (0.689) (0.434)
337 337 335 335
calc 373 470 377 475 384 471 391 477
(0.655) (0.354) (0.766) (0.358) (0.856) (0.463) (0.932) (0.461)
expt 514 514 363 363
a
For each solvent, the calculated absorption and emission energies (nm) are given for each species along with corresponding experimental values.20 Computed oscillator strengths (a.u.) for each transition are given in parentheses.
Table 1. Relative Energies (kcal/mol) Computed for the Various Forms of the HBT Molecule at Ground and Excited Statesa solvent
solvent
both toluene and THF. The predicted absorption and emission wavelengths agree well with the experimental data. In particular, errors ranging from 0.08 to 0.24 eV are observed, in agreement with the level of accuracy expected at the TD-DFT level. Furthermore, the large Stokes’ shift observed in experiment in toluene (of 1.2 eV) is accurately reproduced with calculated values of 1.1 eV. This large Stokes’ shift results from the tautomerisation from the enol to the keto form. By comparison of the experimental and calculated absorption data, the experimental absorption peak corresponds to that calculated for the enol form of HBT in both toluene and THF. This suggests that absorption takes place exclusively by the enol form of HBT and agrees well with the calculated relative stabilities reported in Table 1 which show that the proton transfer does not occur at the ground state and thus the keto form will not absorb. The similarity between the behavior of the HBT molecule in the two solvents ends there, however, as the maximum of the emission band observed in experiment corresponds to that of the keto form in toluene but to that of the enol form in THF. This suggests that the ESIPT reaction is inhibited by the THF solvent. By examining the calculated oscillator strengths and the experimental emission spectra,20 it is possible to further discriminate between the emitting species. In THF, the combined oscillator strength of the enol (0.856) and transenol (0.932) is approximately 2.5 times that of the combined values for the keto (0.354) and trans-keto (0.358) in toluene.
a
Ground state and excited state energies were computed at the PBE0/ 6-31+G(d)/CPCM level and TD-PBE0/6-31+G(d)/CPCM level of theory, respectively. bRelative to the GS enol form (E). cRelative to the ES enol form (E*).
Ignoring the trans-forms of the HBT molecule for the moment, from Table 1 it can be seen that, in both toluene and THF, the minimum energy conformation of HBT at the ground state is represented by the enol tautomer, with the keto form being higher in energy. Conversely, at the excited state, the keto tautomer is the energetically preferred conformation. Together, these allow spontaneous proton transfer at the ground state and reverse proton transfer at the excited state to be ruled out and define the one-way loop of the ESIPT reaction as depicted in Figure 1. All together these findings afford the C
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combined with the reduced contribution on the oxygen relative to the HOMO, is expected for a spontaneous ESIPT reaction in HBT. To better understand the ESIPT mechanism, the partial structural optimization of HBT in each solvent, at various OH and ON distances, was conducted in order to construct 2D potential energy surfacesplotted as contour maps in Figure 4. Note in this case we will not consider possible cis−trans isomerization but consider only the normal confomer. Two potential reaction pathways are identified: a direct pathway which corresponds to a direct proton transfer (i.e., a transfer occurring at almost fixed ON distance) and an indirect, minimal energy pathway which corresponds to a skeletal contraction before the proton transfers. After the proton transfer via the minimal energy path, the reaction pathway takes the route of steepest energy descent before reaching the minimum at the known excited state keto structure. From the computed data, it is clear that the skeletal contraction is the energetically preferred mechanism and it is interesting to note that the maximum energy value in this path (the transition state) corresponds to the minimum ON distance. Comparing the PES of each solvent, it is clear that the reaction is less favored in THF as previously suggested via the comparison of the calculated and experimental emission data in Table 2. This is shown quantitatively in Table 3 which reports
This is in agreement with the relative intensities in the experimental spectrum20 and provides further evidence that the keto species is the dominant emitter in toluene while the enol species remains prevalent in THF. Notably, the trans-isomer of the HBT molecule has predicted emission wavelengths close to those of the cis-isomers so that, based on the sole photophysical data, it is not possible to discriminate the rotational isomer present. Therefore, based on the analysis of the data collected in Tables 1 and 2, it would rather seem that, in the case of the more polar THF solvent, irradiation may induce significant isomerization more than ESIPT. From an electronic point of view, all of these transitions correspond to HOMO−LUMO excitations. The orbitals involved in the case of the HBT enol and keto tautomers in THF are reported in Figure 3. Clearly the transitions are of
Table 3. Estimated Reaction Barriers (kcal/mol) for Both the Direct Proton Transfer and the Minimal Energy (Skeletal Contraction) Mechanisms reaction pathway
toluene
THF
direct minimal
4.2 1.9
4.9 3.0
the estimated reaction barrier for ESIPT in toluene and THF. From this data, it appears the reaction barrier is higher in THF with respect to toluene and that the reaction barrier corresponding to the skeletal contraction (named as minimal) is, as expected, always lower than the direct proton transfer in both solvents. Indeed this finding simply conveys that, during a PT, the donor and acceptor atoms get closer first and then the proton is transferred. While the PES gives an insight into the mechanical aspects of ESIPT, it does not allow the direct monitoring of the nature of
Figure 3. Calculated orbital of HBT in toluene: (A) enol HOMO; (B) keto HOMO; (C) enol LUMO; (D) keto LUMO.
π−π* type, and the solvent seems to have no significant effect on the orbital character. Furthermore, the large orbital contribution computed on the nitrogen for the LUMO,
Figure 4. Two-dimensional excited state potential energy surface (PES) computed in (A) toluene and (B) THF. Energies are in kilocalories per mole relative to the keto form, and OH and ON distances are in angstroms. Black arrows indicate potential reaction pathways. The energetic minima of the excited state enol (E*) and keto (K*) forms are indicated. D
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Figure 5. Two-dimensional DCT surfaces computed in (A) toluene and (B) THF. Distances are in angstroms. Arrows indicate potential reaction pathways.
the electronic rearrangement taking place in HBT during the reaction. In order to capture the electronic changes during ESIPT, the previously defined DCT index was used to construct a “DCT surface” for each solvent, as shown in Figure 5. For both toluene and THF, it is predicted that, as the proton is transferred, the DCT index increases to a maximum value before decaying to a final value at the keto form of the molecule. The increase in the charge separation to a maximum is expected as it corresponds to the reaching of a maximum CT distance. After the transfer of the proton takes place, and the molecule retracts, the charge transfer character decreases. It is interesting to note that the DCT value does not decay significantly after proton transfer. This is due to the intramolecular nature of the reaction, in which the proton is held in place after the reaction has completed. In another of our studies,21 conducted using the same index, the DCT value was seen to decrease much further as this reaction was intermolecular in nature so the proton was able to “escape” once it has transferred to the acceptor molecule, resulting in a more dramatic decrease in the charge separation. In general the quantitative agreement in photophysical properties obtained for the HBT molecule allowed us to validate the level of theory used for the study of the N-1 and N4 derivatives. 3.2. N-1 and N-4 Derivatives. As for HBT, ground and excited state optimizations were conducted for the N-1 molecule using the PBE0 functional. These calculations, as well as those for N-4, were also performed using the CAMB3LYP functional in order to verify the quality of the results since a significant charge transfer character is expected. The results obtained are summarized in Table 4. N-1, as for HBT, is more stable in the enol form at the ground state while the keto form is lower in energy at excited state for both toluene and THF when the PBE0 functional is used. This, again, rules out spontaneous proton transfer reaction at the ground state for the N-1 molecule. Nonetheless, the relative difference in total energy between the N-1 enol and keto forms at ground and especially at the excited state is smaller when compared to those calculated for HBT. This suggests both that the N-1 molecule has a flatter PES than HBT and that the red shift of the emission energy between the enol and keto forms of N-1 is expected to be lower. Contrary to this, using the CAM-B3LYP functional, the ESIPT reaction is predicted to be slightly unfavored, though the relative total energies of the enol and keto forms are very close (within 0.5 kcal/mol). Analogous with HBT, the trans-enol and trans-keto
Table 4. Relative Energies of Various Forms of the N-1 and N-4 Molecules (kcal/mol) at Ground and Excited Statesa molecule/functional N-1/PBE0
solvent toluene THF
N-1/CAM-B3LYP
toluene
THF
N-4/CAM-B3LYP
toluene THF
molecule species
relative GS energyb
relative ES energyc
enol keto enol keto enol keto trans-enol trans-keto enol keto trans-enol trans-keto enol keto enol keto
0.00 6.71 0.00 5.50 0.00 7.39 9.15 13.82 0.00 6.09 8.87 11.13 0.00 7.99 0.00 6.70
0.00 −0.63 0.00 −1.77 0.00 0.28 9.94 4.91 0.00 −0.61 9.61 3.04 0.00 4.45 0.00 5.28
a
Ground and excited state energies for N-1 were calculated both using PBE0 and CAM-B3LYP functionals although only the latter values are reported in the table. N-4 calculations utilise the CAM-B3LYP functional only. bRelative to the Enol form (E). cRelative to the ES enol form (E*).
forms of N-1 are 8.8 and 11.1 kcal/mol higher in energy than the enol form at the ground state, respectively, while at the excited state that the total energy of the trans-keto form is within 4 kcal/mol of the cis-keto form which is qualitatively similar to results obtained for the HBT molecule. In contrast to N-1 and HBT, the enol form of the N-4 molecule (in both solvents) has lower total energy at both the ground and excited stateevidence that ESIPT does not occur spontaneously for this molecule. This, together with the results for the N-1 molecule, is early evidence that the experimental strategy of the addition of napthalimide derivatives to HBT to shift its absorption and emission wavelengths may not deliver the desired dual-emission phenomenon. Calculated photophysical properties of the N-1 and N-4 molecules are given in Table 5. At first glance and focusing on the enol and keto forms of N-1 to begin with, there is good agreement between the experimental and calculated absorption and emission energies using both functionals. Examining the mean average error (MAE) at the ground and excited states between predicted and experimental data, it is found that the E
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Table 5. Calculated Photophysical Properties of Various Forms of N-1 and N-4 Moleculesa λABS molecule/functional N-1/PBE0
molecule species
calc
expt
calc
toluene
enol keto enol keto enol keto trans-enol trans-keto enol keto trans-enol trans-keto enol keto enol keto
439 (1.010) 501 (1.043) 442 (0.976) 505 (1.005) 375 (1.209) 425 (1.098) 371 (1.139) 440 (1.109) 375 (1.186) 421 (1.141) 372 (1.1212) 434 (1.1158) 389 (1.302) 419 (0.432) 392 (1.288) 412 (0.362)
405
490 (1.104) 551 (1.131) 514 (1.232) 581 (1.265) 447 (1.291) 490 (1.323) 442 (1.225) 502 (1.291) 470 (1.406) 520 (1.542) 465 (1.3469) 529 (1.5318) 466 (1.457) 482 (0.467) 493 (1.590) 485 (0.553)
THF N-1/CAM-B3LYP
toluene
THF
N-4/CAM-B3LYP
λEM
solvent
toluene THF
384 405 405 384 384 433 429
expt 545 592 545 545 592 592 491 553
a
For each solvent, the calculated absorption and emission energies (nm) are given for each species along with corresponding experimental values.20 Computed oscillator strengths (a.u.) for each transition are given in parentheses.
PBE0 functional yields an MAE of 0.33 and 0.03 eV for the absorption and emission energies, respectively. This is unexpected as we would expect the discrepancy between calculated and experimental data to be larger for emission values. It can be concluded, then, that the greater accuracy observed for the calculated emission energies in N-1 is the result of some kind of error cancelation, prompting the introduction of the CAM-B3LYP functional in an attempt to address this inconsistency. Using the CAM-B3LYP functional, the MAE for calculated absorption and emission energies is 0.16 and 0.27 eV, respectively. This level of accuracy is more reasonable, particularly for absorption energies, which are expected to be typically accurate to within 0.2 eV of experimental values at the TD level. Following this, we can conclude that the CAMB3LYP functional is more suitable concerning the larger N-1 and N-4 molecules with significant charge transfer character (see below), so calculations involving the largest molecule, N-4, were conducted using this functional only. Analyzing the experimental emission spectra,20 we can note that, for both molecules in toluene, only a single distinct band is observed. This is in excellent agreement with the calculated emission energies which predicted a red shift of approximately 0.24 eV for N-1 and 0.09 eV for N-4not sufficient to observe distinct dual emission in either molecule. It is noted that the proton transfer reaction was earlier deemed unfavorable for the N-4 molecule (Table 4), which is also in keeping with the single emission band observed. Actually, this result is also reflected in the calculated molecular orbitals shown in Figure 6. It can be seen that the napthalimide moieties in both N-1 and N-4 act to withdraw electrons from the HBT moiety upon excitation from HOMO to LUMO. Comparing the enol and keto transitions, they are observed as almost identical so it is reasonable to assume that the transition energies, as a result, are similar. In fact, it was observed that the orbital shape was unaffected by the solvent used, which suggests that the HOMO−LUMO transition energies for the enol and keto forms in THF, as for toluene, are similar. From Table 5, the calculated emission energy for the trans-enol form compares well with the second, smaller
emission band observed in THF, at approximately 475 nm. This, the orbital shapes and the total energies given in Table 4, gives strong evidence that the dual emission observed in THF for N-1 is not the product of the ESIPT reaction but the result of the formation of a rotational confomer, namely, the transenol form. Finally, the absence of any contribution to the LUMO from orbitals centered on atoms involved in the ESIPT for N-4 complements the earlier suggestion that the ESIPT reaction is not favored for this molecule. The calculated DCT values for the enol and keto forms of N-1 and N-4 (all around 4.0 Å) show that, compared with HBT, the through-space charge transfer character is much larger after the addition of the napthalimide moiety. From this, it is expected that electronic transitions within N-1 and, by extension, N-4 are dominated by charge transfer and are hardly influenced by the ESIPT reaction itself. This is, again, reflected in the orbital shapes for these molecules (Figure 6) which show negligible contribution to the LUMO from orbitals centered on atoms involved in the ESIPT and thus suggests that it is unlikely that dual emission would be observed as a result of ESIPT for N-1 and N-4.
4. CONCLUSION In summary, it is clear that the PBE0/6-31+G(d)/CPCM level of theory performs well in the prediction of ground and excited state properties of HBT and that the minimal energy intermediate structures involved in ESIPT can be successfully identified. Given the qualitative and quantitative agreement with experimental data, the protocol of using the PBE0/6-31G +(d)/CPCM level of theory to conduct a scan over OH and ON distances is indeed able to identify, and discriminate between, potential reaction pathways in different solvents across a very flat PES. While the PESs can describe the mechanical aspects of the ESIPT reaction very well, they say nothing of the change in electronic arrangement of the molecule as a result of proton transfer, soin order to fully describe ESIPTthis was incorporated via a density based index (DCT). It has been shown that the charge separation quality of the ESIPT reaction can be characterized with the DCT index using a F
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napthalimide moieties, or equally strong acceptor units, to the base HBT molecule in fact reduces the red shift between enol and keto tautomers and obscures any true dual emission as a result. In fact, in the more extreme case of the N-4 molecule, the ESIPT reaction is not favored at all. To conclude, it has been shown that computationally affordable computational protocols such as those here described can be of valuable help in the ab initio screening and design of new molecules that exhibit the ESIPT reaction for use in novel applications in molecular probes.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ilaria.ciofi[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.W. acknowledges the University of Strathclyde for an Erasmus grant. I.C. acknowledges support of IDRIS (Orsay) within the framework of the project “DARI i2014087160”.
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REFERENCES
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Figure 6. Calculated orbital of N-1 and N-4 in toluene: (A) N-1 enol HOMO; (B) N-1 keto HOMO; (C) N-1 enol LUMO; (D) N-1 keto LUMO (E) enol HOMO N-4; (F) N-4 keto HOMO; (G) N-4 enol LUMO; (H) N-4 keto LUMO.
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H
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