TD-DFT assessment of the excited state intramolecular proton transfer in hydroxyphenylbenzimidazole (HBI) dyes.

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A TD-DFT Assessment of the Excited State Intramolecular Proton Transfer in HydroxyphenylBenzImidazole (HBI) Dyes Ymène Houari, Siwar Chibani, Denis Jacquemin, and Adèle D. Laurent J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp505036d • Publication Date (Web): 26 Nov 2014 Downloaded from http://pubs.acs.org on November 30, 2014

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The Journal of Physical Chemistry

A TD-DFT Assessment of the Excited State Intramolecular Proton Transfer in HydroxyphenylBenzImidazole (HBI) Dyes Ymène HOUARI,† Siwar CHIBANI,† Denis JACQUEMIN,†,‡ and Adèle D. LAURENT∗,† Laboratoire CEISAM - UMR CNR 6230, Université de Nantes, 2 Rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France, and Institut Universitaire de France, 103, bd Saint-Michel, F-75005 Paris Cedex 05, France. E-mail: [email protected]

∗

To whom correspondence should be addressed CEISAM, Nantes ‡ IUF, Paris †

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Abstract Dyes undergoing excited state intramolecular proton transfer (ESIPT) received an increasing attention during the last decades. If their unusual large Stokes shifts and sometimes dual-fluorescence signatures have paved the way towards new applications, the rapidity of ESIPT often prevents its investigation with sole experimental approaches, and theoretical simulations are often welcome, if not necessary, to obtain a full rationalization of the observations. In the present paper, we evaluate both the absorption and fluorescence spectra of, respectively, the enol and keto form of a series of hydroxyphenylbenzimidazole (HBI) using a robust protocol based on Time-Dependent Density Functional Theory (TD-DFT). Optical spectra were obtained accounting for both vibronic and environmental effects. The aim of this work is therefore not to evaluate the radiationless pathway going through the twisted ESIPT structures, though excited-state reaction paths between enol and keto form have been rationalized. First we have compared three dyes differing by the strength of the donor groups and we have quantified the impact of the flexible butyl chain substituting the imidazole side. In accordance with experiments, we show that the presence of a dialkylamino auxochrome allows to tune the excited-state potential energy surface leading to a weaker tendency to ESIPT. This trend is rationalized in terms of both structural and electronic effects. Next larger hydroxyphenyl-phenanthroimidazole (HPI) were considered to assess the impact of a stronger π-delocalisation. 0-0 energies and vibrationally resolved spectra of the corresponding fluoroborate derivatives were studied as well. The dialkylamino auxochrome significantly decreases the 0-0 energies due to the presence of an important charge transfer character while the addition of a BODIPY moiety induce a change of the emission signature now localized on the BODIPY side rather than on the NBO core.


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Graphical abstract



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Introduction Recently, proton transfer (PT) and more specifically excited state intramolecular proton transfer (ESIPT) processes have attracted countless works in both chemistry and biochemistry fields. 1–31 Indeed, a large panel of applications are based on ESIPT and this includes UV photostabilizers, 32,33 fluorescent chemosensors, 34–37 and photoswitches. 11,38,39 ESIPT is a photoinduced intramolecular PT typically characterized by very short lifetimes (k ∼ 1013 s−1 ) often relatively low fluorescence quantum yields, 40 but large Stokes shifts (as large as 10 000 cm−1 ) which enables to obtain emission at large wavelengths even with a UV stimulus. This particular PT process has already been observed in many molecules, e.g., 2,5-bis(20 -benzoxazolyl)-hydroquinone, 41 methyl salicylate, 42–46 salicylic acid, 47,48 ohydroxyacetophenone, 49,50 2-(20 -pyridyl)pyrrole, 51 3-hydroxy flavones 44,52–60 and 2-(20 -hydroxyphenyl)-benzothiazole. 7,61–65 In most ESIPT dyes, the solvatochromic effects as well as the presence of auxochromes play a critical role in tuning the macroscopic response. A full experimental probing of ESIPT generally implies extensive - and expensive - experimental studies of the PT dynamics with steady state spectroscopy, time-resolved luminescence spectroscopy and/or ultrafast IR spectroscopy. 66–75 In this contribution, we investigate a series of hydroxyphenylbenzimidazole (HBI) and hydroxyphenyl-phenanthroimidazole (HPI) ESIPT dyes that have been very recently synthetized and characterized by Ziessel and coworkers (see Scheme 1). 76 In these molecules, like in many others ESIPT dyes, the phenol acts as a proton donor (acid) and the nitrogen atom as a proton acceptor (base). In the ground state (GS), the enol form (E) of HBI/HPI dyes is the only stable isomer, but in the excited-state (ES), the keto tautomer (K*) becomes accessible and even dominant in these particular compounds (see Scheme 2). Upon absorption of light, there is a significant reorganization of the electronic cloud, and the relative acid and basic strengths of the atoms implied in the intramolecular hydrogen bond differ significantly in the two electronic states. Indeed, the ESIPT process occurs from the excited enol form (E*) that tautomerizes into the corresponding keto form (K*) from which 4 ACS Paragon Plus Environment

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emission occurs. Fluorescence efficiency of such ESIPT dyes is known to be decreased by the presence of a radiativeless desactivation pathway that is arising from the twisting in the ESIPT dyes (sometimes incorrectly denoted twisted intramolecular charge transfer, TICT) through a conical intersection, i.e the twisted keto form (K∗twist ). We wish here to clarify the difference between the twisting in ESIPT and TICT; the latter is related to the stabilization of a charge transfer state through a twist (occurring around a single bond) between acceptor and donor moeities inducing a red shift in the fluorescence spectrum. The twisting in the ESIPT dyes leading to the global minimum in the excited state (K∗twist ) takes place around a double-like bond and this induces a competition and a possible decrease of the K∗ fluorescence quantum yield due to fast radiation’s deexcitation from K∗twist . Through radiative (or radiativeless) desactivation, K∗ (K∗twist ) returns to its unstable ground state keto form which, in turns, restores the original enol form (E) by spontaneous PT. An obvious way of completely preventing ESIPT (and to impede twisting) is to complex the dye with an excess of boron trifluoride leading to fluoroborate derivatives (see Scheme 2) that can only present “direct” emission. Ziessel’s group has measured large Stokes shifts (∆SS ) between 6400 and 10300 cm−1 for the HBI/HPI shown in Scheme 1 and logically interpreted these ∆SS as signatures of the E∗ to K∗ tautomerization characteristic of an ESIPT process. Additionally, they observed shoulder(s) in the fluorescence spectra, and these extra bands, especially marked in 3b, could be related to a combination of overlapping E∗ and K∗ fluorescences.



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R R

R2 N

N

R1

N

N

N

H O

N H O

1a R1 = H R2 = Me

1b R1 = H R2 = nBu

2a R1 = OMe R2 = Me

2b R1 = OMe R2 = nBu

3a R1 = NEt2 R2 = Me

3b R1 = NEt2 R2 = nBu

4

R = Me

H O 5

R=

tBu

Scheme 1: The enol form of the HBI (1, 2 and 3) and HPI (4 and 5) compounds investigated in this work. Note that for the three first, the actually synthesized molecules are denoted b, whereas model molecules used during test calculations are labelled a.

N

N

N H O E

N

N H

O K

N F

B O F

Scheme 2: Representation of enol (E) and keto (K) forms in equilibrium as well as of the corresponding fluoroborate derivative (right) for the investigated dyes. Only the HBI core is represented for the sake of compactness.

In that framework, we note that interpreting complex band shapes is one of the problem for which theoretical chemistry can be helpful to assist experimental evidences. If a variety of highly refined post-Hartree-Fock methods including coupled cluster (CC), configuration interaction, and multiconfigurational self-consistent field theories have been employed to reach an in depth understanding of the ESIPT in model compact compounds, e.g., 2(1H -pyrazole-5-yl)pyridines (2-PPs), 77 cytosine 78 and (2,20 -bipyridyl)-3,30 -diol(BP(OH2 ), 79 only Density Functional Theory (DFT), and more precisely its Time Dependent counterpart (TD-DFT), offers the opportunity to study “real-life” systems in condensed phase by providing an adequate balance between accuracy and computational effort. It is therefore not surprising that (TD-)DFT has been intensively used to characterize ESIPT in 6 ACS Paragon Plus Environment

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o-hydroxybenzaldehyde, 80?

–82

salicylic acid, 80,81 7-hydroxy-1-indanone, 80,81 salicylideneani-

line, 83 2-(20 -hydroxyphenyl)benzoxazole, 84 2-(20 -hydroxyphenyl)benzoxthiazole, 85 1-hydroxy11H-benzofluoren-11-one, 86 N-(3-(benzo[d]thiazol-2-yl)-4-(tert-butyldiphenylsilyloxy)phenyl)benzamide 87 and hydroxyphenylbenzoxazole, 88 to cite a few examples. Some studies also compared the relative merits of wavefunction and density based approaches for modeling ESIPT. For instance, in their seminal work Sobolewski and co-workers 89 first calculated GS potential-energy profiles at the second-oder Møller-Plesset level 90 whereas they optimized ES structures with both Complete Active Space-Self Consistent Field 91 and Configuration Interaction Single 92 methods. Next, they performed single point calculations along the reaction path using both Complete Active Space Perturbation Theory to second order 93 (CAS-PT2) and TD-DFT (with the B3P86 exchange-correlation functional). They concluded that TDDFT successfully reproduced the energetic properties. Later, Aquino et al. 80,81 applied both TD-DFT (B3LYP) and resolution-of-the identity second order coupled cluster 94 (RI-CC2) approaches, and found that TD-DFT reasonably reproduced experimental data. In the seminal work of Barbatti on 2-(20 -hydroxyphenyl)benzoxthiazole internal conversion pathway has been extensively studied by comparing TD-DFT(B3LYP), RI-CC2, CASSCF and MRCI+Q methods to evaluate excited state pathway. They clearly show that single reference TD-DFT and RI-CC2 give acceptable results until the stage of the twisting in ESIPT characterized by a biradical K∗twist form for which a multireference method is necessary. 85 We also underline that several ESIPT works have accounted for medium effects, 77,86–88,95–98 typically thanks to the use of the Polarizable Continuum Model, 99 though we found one investigation relying on the Reference Interaction Site Model. 82 However, these environmental corrections are, in the vast majority of cases, only included as corrections to the vertical transition energies. Recently, we have shown that the presence of a blend of E∗ and K∗ emission band in HBO derivatives can be explained when both vibronic and refined solvent models are accounted simultaneously throughout the calculations, 88 an approach that we are following here. Few theoretical studies dealt with the simulation of the ES potential energy surfaces



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(PES) of HBI dyes. Tsai and co-workers have investigated the first singlet excited state of a specific series of small HBI derivatives using a PCM-TD-DFT protocol. 100 The main objective was to understand the interplay between ESIPT and the twisting in ESIPT processes after photoexcitation. Chipem’s group has been interested in rationalizing the effect of nitrogen substitution into both benzimidazole 101 and phenolic moeities of HBI, 102 as well as understanding temperature effects measured in various solvents 103 typically using CIS method to optimize the ES geometry and B3LYP to perform single point computations. Depending on the nitrogen position they found out that the twisting in ESIPT processes might compete within the ESIPT phenomenon of such dyes. In the early 2000s two joint experimental and theoretical papers using a semi-emperical procedure (CNDO/S-CI//AM1) also pointed out that twisting in the ESIPT process to explain a decrease of the fluorescence quantum yield. 104,105 Crystal structure of two HBI polymorphs were recently obtained and time-resolved spectroscopy indicated that a twisting in the ESIPT process was occurring in one of the polymorphs. Those data have been validated by the finding of a non planar structure (dihedral angle of 300 ) employing PBE0 method. 106 For the specific real-life dyes shown in Scheme 1 (1b-3b, 4 and 5), the present work is therefore the first within the computational chemistry field to evaluate vibrationally resolved absorption and emission spectra of ESIPT dyes and of their corresponding fluoroborate complexes. The aim being to reproduce the position and the shape of original experimental spectra, 76 we will not evaluate the fluorescence quantum yield meaning the simulation of the biradical character K∗twist form. This paper is organized as follows. First, we detail the selected methods used for obtaining reaction paths, vibronic structure and the optical spectra. Second, we discuss our results, starting with the model structures of Scheme 1 (1a, 2a and 3a) that allow to ensure the quality of the chosen methodology. Next, we explore the actually synthesized HBI dyes to provide direct comparisons with experiments. Eventually, fluoroborate and more extended molecules are discussed before the concluding remarks.


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Methods All calculations used Gaussian09 107 and relied on the M06-2X hybrid exchange-correlation functional, 108,109 a choice justified by numerous previous benchmarks demonstrating the accuracy of this approach for predicting both vertical and adiabatic transition energies as well as for reproducing band shapes of absorption and fluorescence spectra. 110–113 Moreover, this functional has already successfully reproduced ESIPT 88 signatures as well as the vibronic spectra of fluoroborate derivatives having a NBO core. 114 Here, we have applied a previously proposed strategy 111 which consists in determining the geometrical and vibrational parameters with a rather compact atomic basis set, 6-31G(d), and computing the transition energies with a much larger basis set, namely 6-311+G(2d,p), so that all energies shown in the main text are corrected for basis set effects. Within this approach, both 0-0 energies (that can be compared to the experimental absorption–fluorescence crossing point) and band shapes (see below) 111,115 are computed, allowing physically-meaningful theory–experiment comparisons. For each molecule, both the ground and first excited states have been fully optimized using DFT and TD-DFT analytical gradients, respectively. Of course, both the enol and keto forms of each dye have been considered. To numerically achieve accurate values, we have tightened self consistent field (10−10 a.u.) and geometry optimization (10−5 a.u.) convergence thresholds as well as a used a (75,302) pruned DFT integration grid (so called fine grid). All other Gaussian09 defaults 107 for thresholds and algorithms have been applied. Vibrationally resolved spectra of fluoroborate complexes, that present a specific shape, have been obtained using the FCclasses program, 116–118 applying the Frank-Condon (FC) approximation as strongly dipole-allowed ES are considered. The reported spectra have been simulated using a convoluting Gaussian function presenting a half width at half maximum that has been adjusted to allow accurate comparisons with experiments (typical value: 0.08 eV). A maximal number of 25 overtones for each mode and 20 combination bands on each pair of modes were included in the calculation. The maximum number of integrals to be computed for each class was, at most, set to 1012 to reach FC factors higher than 0.9 for all 9 ACS Paragon Plus Environment


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presented vibronic spectra. Potential energy surfaces have been explored using relaxed potential energy scan going from E to K form, constraining one internal coordinate (the O-H bond length) and minimizing all other coordinates at every scan point. As TD-DFT is not suitable to describe the biradicalic character of K∗twist , we did not explore potential energy beyond the almost planar K∗ . Actually as in the Barbatti paper, the K∗twist TD-DFT optimization leads to the most stable form in the excited state by 5.8 kcal/mol and induces a dihedral angle of 67o between the two moeities that actually is due to the limitation of single determinant method. Next starting with the data of the scan, transition states have been located in both GS and ES. All E, K, E∗ and K∗ stationary points have been characterized as true minimum by confirming that all eigenvalues of the Hessian are positive whereas the transition states of both states (TS and TS*) do present one imaginary frequency that mainly implies the displacement of the relevant transferred proton. To confirm the reaction paths in the ground and the excited states, the intrinsic reaction coordinate 119–123 approach was applied but it did not lead surprising informations (see the Supporting Information for IRC graphs for 1b, 2b and 3b) Environmental effects (here dichloromethane as in the experiment) have been accounted for using the well-known Polarizable Continuum Model (PCM model). 99 While geometry optimizations, Hessian and Gibbs energies have been determined within the popular linearresponse (LR) PCM approach, all excited-state and transition energies have been corrected using the corrected LR scheme (cLR). 124,125 The cLR scheme allows to correct the cavity polarization by accounting for the change of electron density upon electronic transition through the calculation of the actual electronic density of the excited-state. Therefore, it allows not only to compare structures having different variations of dipole moment amplitudes between the two states but also to accurately estimate emission wavelengths (and hence ∆SS ). Of course, while we applied the equilibrium PCM limit for both geometry optimization and vibrational TD-DFT calculations (slow phenomena), absorption and fluorescence wavelengths


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are computed within the non-equilibrium limit of cLR-PCM (fast phenomena). To ensure the validity of the selected methodology, additional tests have been performed. First, for molecule 1a, we have compared TD-DFT/DFT results to those obtained with the highly correlated equation-of-motion coupled-cluster single and double (EOM-CCSD) approach. 126,127 EOM-CCSD allows to describe a variety of electronic excitations and especially those containing a double-excitation character that are inaccurately described by standard TD-DFT. Second, to observe the dynamic of the system, we have employed classical molecular dynamic simulations of a single HBI molecule in dichloromethane solution using the NAMD program. 128 Parameters of the solvent and the solute have been generated with ParamChem 129 using the CGen force field. 130 Starting with the M06-2X/6-31G(d) optimized molecule in solution, the structure is placed at the center of a cubic simulation box of dimension (50x50x50). After minimization, the solvent is relaxed whereas the HBI is frozen for 100 ps with a timestep of 1.0 fs in the NPT ensemble. Finally, all the system is relaxed at 298 K in the NPT ensemble for 2 ns using a timestep of 2.0 fs.

Results and discussion Model structures The method described above has been firstly applied to model structures of HBI dyes (1a, 2a and 3a, see Scheme 1). In these model molecules, we have replaced the flexible n-butyl chain at R2 position by a methyl group. Our aim is to test the methodology on smaller systems and to reach first insights regarding the ESIPT of these HBI dyes that are different from one another by the nature of the R1 group, presenting increasing donor strength (R1 = H, OMe and NEt2 for 1a, 2a and 3a, respectively). In addition, we have performed vertical EOM-CC calculations for molecule 1a, considering the (TD-)DFT optimal structures to verify the accuracy of TD-DFT energies. These calculations have been performed in gas phase with the 6-31G(d) atomic basis set to ensure a straightforward comparison between 11 ACS Paragon Plus Environment


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the two theoretical approaches. First, EOM-CC results revealed no significant contributions from doubly excited-states. Second, in the GS, both DFT and CCSD predict a K tautomer much less stable than its E counterpart, with energy difference reaching 0.53 and 0.58 eV for DFT and CCSD, respectively. At the ES, the relative energies (taking the GS E isomer as reference) of E*, TS* and K∗ are 4.15 (4.31), 4.16 (4.31) and 3.78 (3.77) eV at the TD-DFT (EOM-CCSD) level of theory. In other words, though TD-DFT slightly overstabilizes E∗ and TS*, both schemes predict a very small ESIPT barrier [0.01 (0.00) eV] and a more stable K∗ form in the ES, confirming that M06-2X is suited for our purposes of simulating absorption and emission spectra. This analysis shows that we can be confident in the TD-DFT values in the following. Energies of the E (E*) and K (K*) isomers of the three model systems were investigated and the data are listed in Table 1 while the energetic profiles together with transition energies are available in Figure 1. As expected, in the GS, the enol form is clearly the most stable for all three molecules. Actually, the K isomer lies ca. 0.38 eV above E in the GS on the energy (E) scale, and the related TS is very close to K in energy. It is crystal clear that no keto isomer can be thermally formed at room temperature. 131 There is for sure important changes at the ES, as the K∗ isomer is expected to become the most stable tautomer for all cases, which is fully consistent with ESIPT. For instance, the relative free energy (∆G) of 1a is for 0.00 and 0.36 eV for E and K, respectively but attains 3.83 and 3.46 eV for E∗ and K*, and similarly for the other molecules. On the energy PES of the ES, the energy barriers going from E∗ to K∗ are 0.01, 0.07 and 0.19 eV for 1a, 2a and 3a. Interestingly, on the G scale that includes zero-point vibrational and entropic effects, E∗ does not constitute a true minimum anymore for both 1a and 2a (R1 =H and OMe) as the corresponding TS is below E*. In other words, in these two structures, the ESIPT process is barrierless and should occur extremely rapidly after photon absorption. On the contrary, with the strong amino donor group of 3a, both E∗ and K∗ are true minima, of nearly equivalent energies, separated by a small positive barrier on the G scale (0.07 eV). Consequently, we can already suggest


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the emission spectra of the two first molecules are only related to K∗ while normal emission from the E∗ is possible in the later dye. As we already observed for HBO dyes, 88 we can suppose that a mixture of E∗ and K∗ signatures might appear in the fluorescence spectra of 3a, though we cannot estimate the relative ratios of these emissions. In short, the energetic profiles indicate that the most (less) marked ESIPT profile is reached for 1a (3a). Table 1: Relative free energies (∆G in eV), total energies (∆E in eV) and (absolute) dihedral angle between the phenol ring and the benzimidazole part (|d| in degree) for the three first dyes (see Scheme 1). All results at cLR-PCMTD-M06-2X/6-311+G(2d,p)//LR-PCM-M06-2X/6-31G(d) level of theory considering dichloromethane as solvent. The most stable enol GS isomer is used as reference to determine the relative energies. Dye 1a

State GS

ES

2a

GS

ES

3a

GS

ES

Form enol TS keto enol TS keto enol TS keto enol TS keto enol TS keto enol TS keto

∆G 0.00 0.32 0.36 3.83 3.77 3.46 0.00 0.32 0.35 3.84 3.82 3.56 0.00 0.29 0.34 3.62 3.69 3.58

∆E 0.00 0.38 0.39 4.12 4.13 3.69 0.00 0.36 0.37 4.13 4.20 4.07 0.00 0.36 0.37 3.87 4.06 3.83

|d| Dye 24.3 1b 6.7 8.37 10.9 8.41 25.5 23.0 2b 6.32 7.75 12.4 7.58 26.9 21.4 3b 5.68 7.26 13.8 9.47 28.3

State GS

ES

GS

ES

GS

ES

Form enol TS keto enol TS keto enol TS keto enol TS keto enol TS keto enol TS keto

∆G 0.00 0.27 0.30 3.85 3.74 3.46 0.00 0.23 0.22 3.79 3.74 3.52 0.00 0.30 0.34 3.66 3.68 3.63

∆E 0.00 0.30 0.35 4.11 4.04 3.63 0.00 0.28 0.29 4.10 4.11 3.71 0.00 0.35 0.36 3.86 4.04 3.83

|d| 32.5 6.52 23.1 16.7 5.16 32.6 31.2 5.8 6.96 18.4 4.71 33.8 20.8 6.46 7.26 12.8 7.4 25.3

We have also investigated the variations of the geometries between the GS and ES as well as between both tautomers. It turns out that the most significant parameter (beyond the distances of the atoms implied in the intramolecular hydrogen bond that, of course, changes between E and K), is the dihedral angle between the phenol and benzimidazole rings (d in Table 1) that is significantly modified when going from GS to ES and for the E and K 13 ACS Paragon Plus Environment


tautomers. First, we note that there is a significant twist (ca. 20o to 25o ) in the GS E isomer and that this twist is reduced to ca. 10o to 13o in E*. Interestingly, the twist angle increases again to larger values in K*, so that the most stable isomers of both states (E and K*) depart more importantly from planarity than the least stable tautomers (K and E*).

1

We also note that the dihedral angle in E∗ follows the ∆G of the ESIPT process: the more planar the structure after photon absorption, the stronger the ESIPT. Indeed, the |d| in E∗ rank: 10.9o (1a) < 12.4o (2a) < 13.8o (3a), whereas the corresponding thermodynamic excited-state ∆G are -0.37 eV > -0.28 eV > -0.04 eV. For 3a, relaxed scan of the dihedral angle as a function of the O-H bond length are available in the Supporting Information (SI). 6

6 S1

5

5

4

4

S1

5

S1

1.0

1.5 x(O-H,

2.0 Å)

2.5

391 nm

335 nm

347 nm

E (eV)

401 nm

333 nm

304 nm S0

1 0

0 0.5

3 2

S0

1

0

330 nm

2

S0

1

3

281 nm

E (eV)

411 nm

2

345 nm

3

331 nm

4

280 nm

E (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5

1.0

1.5 x(O-H,

2.0

2.5

Å)

0.5

1.0

1.5 x(O-H,

2.0

2.5

Å)

Figure 1: The PES curves computed of 1a (left), 2a (center) and 3a (right) in the ∆E scale. The vertical absorption (blue) and emission (red) transition energies are also given in nm. The enol and keto isomers correspond to small and large O-H distances, respectively.

A qualitative analysis of the above-mentioned trends can also be reached by examining the electronic density differences represented in Figure 2. Upon the photoexcitation, the phenol group of HBI becomes more acidic (decrease of electron density in blue) and the nitrogen atom becomes more basic (increase of electron density in red), which is consistent with the expected ESIPT picture (see Introduction). More importantly, significant differences between the three model molecules appear due to the nature of the R1 group. Indeed, while variations of the density on the nitrogen side is almost constant, it is evident from Figure 2 that upon addition of electron donors at R1 position, the depletion of density on the oxygen Note that this slightly twisted form is not the K∗twist that is poorly optimized using TD-DFT method (67 diedral angle) due to its biradicalic character. 1

o


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atom of the phenol decreases, indicating a smaller enhancement of the acidity from the GS to the ES in 3a than in 1a. Indeed, the blue domain around that oxygen atoms clearly follows 1a > 2a > 3a. In other words, the different behavior are guided by the change of the acidity of the alcohol rather than by variations of the basicity of the nitrogen atom.

Figure 2: Electronic density differences for 1a (left), 2a (center) and 3a (right) using a contour threshold of 0.0004 a.u. The blue (red) zones indicate decrease (increase) of electron density upon absorption.

Eventually, a verification of the possible presence of TICT was performed for 1a as TICT might be a possible alternative path for deexcitation compared to ESIPT as seen in the Introduction. In the TICT model, the twist angle between the two subunits of the K∗ form reaches 90◦ and a biradical character appeared. It is well known that standard hybrid functionals such as B3LYP or PBE0 that contain a small amount of exact exchange tends to overfavored twisted forms. 132–134 Using the M06-2X method, no minima suggesting a TICT process could be found for 1a. However further studies would be welcome to elucidate this non-radiative pathway as TD-DFT methods are generally not the most adequate and results obtained with multireference methods would be prefer to describe such a competitive process.

Synthesized HBI and HPI dyes In this Section, we focus on the actual HBI dyes (with a n-butyl chain), namely 1b, 2b and 3b and we also comment on the larger HPI dyes, 4 and 5. As the n-butyl chain is quite flexible, we have started to investigate 1b, 2b and 3b with classical molecular dynamic (MD, see Methods) considering the enol isomer. The average |d| angles of these three dyes reach ca. 22◦ − 24◦ in the dynamics: they are significantly 15 ACS Paragon Plus Environment


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smaller than the static values listed in Table 1 for the two former compounds. However, the MD simulation confirms that the hydrogen bond between the phenol and the lone pair of the nitrogen is conserved during the whole simulation, as measured by the N. . .H distance that is 1.96 ± 0.1 ˚ A, which is much smaller than the sum of van der Waals radii (2.65 ˚ A) indicating a strong hydrogen bond. 135 If one measures the impact of the n-butyl chain by comparing the dihedral angles of the a and b series of the E form, one clearly notes a strong increase of the dihedral angles for 1b and 2b, of ca. 8◦ for the most stable isomers (see Table 1), the variations being significantly smaller in 3b. Indeed, in the latter structure, the strong electron donating character of the dialkylamino group contributes to the planarity of the compound, irrespective of the use of a methyl or n-butyl substituent on the imidazole side. Like in the model compounds, the most stable tautomer is more twisted than both the least stable isomers and the TS, and this holds for both electronic states. Let us now turns towards the relative energies of the different isomers that are listed Table 1 to allow direct comparisons with the model systems discussed in the previous Section. For the GS, we note that the keto tautomers remain much less stable than their enol counterparts and present similar energies to the TS connecting the two structures: they are not formed experimentally. With respect to the model systems, we note however that the keto form is slightly less unfavorable in 1b and 2b than in 1a and 2a. As expected, the keto form becomes favored on the ES both in terms of total and free energies. The situation is very similar to the one discussed in the previous Section, that is, the ESIPT is barrierless for 1b and 2b but presents a small barrier in 3b if one considers the free energy PES. For 3b, the enol and keto tautomers present almost the same energies (∆G=-0.03 eV), the forward and reverse barriers being as trifling as +0.02 eV and +0.05 eV, respectively. These are smaller than in the model compound, but the same conclusion is reached: dual emission is only possible in 3b. In the SI, the density difference plots for 1b–3b are given, and they are completely similar to the one shown in Figure 2, as the n-butyl chain does obviously not participate in the π-conjugated path. Experimentally, the emission band detected presents a clear ESIPT


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signature, 76 for all three dyes, which is consistent with a favored K∗ tautomer. In 3b (and to a significantly smaller extend in 2b) one nevertheless notices not perfectly resolved emission shoulder at smaller wavelengths, which might indicate a partial emission from E*, which is again consistent with the trends provided by our ab initio simulations. To gain more insights, let us now compare experimental and theoretical spectroscopic signatures. Maximum absorption (λabs ) and emission (λemi ) wavelengths as well as the Stokes shifts (∆SS ) are listed in Table 2 considering both enol and keto isomers (see the SI for data for the model compounds). We are mainly interested in the λabs of E, λemi of K∗ and the corresponding ∆SS , as the above analysis indicates that these forms are favored at the GS and ES, respectively. From a methodological point of view, we note that the equilibrium/nonequilibrium limits provide very similar data (maximal discrepancy 4 nm for the absorption of 1b), and we only discuss the latter in the following. Quantitatively, we observe that all computed wavelengths are underestimated compared to experiment, i.e., the selected procedure overshoots the transition energies. This is a well-known tendency of M06-2X, 111,136 and as a discussion of the pros and cons of specific exchange-correlation functional(s) is beyond our scope here, we focus on the chemical effects (auxochromic variations) that are nicely reproduced. Indeed, going from 1b to 2b and next to 3b induces successive λabs bathochromic shifts of +1 nm and +24 nm experimentally, whereas TD-DFT yields +2 nm and +26 nm, respectively. For λemi , the 1b → 3b shift attains -36 nm (-41 nm) experimentally (theoretically). The intermediate position of λemi of 2b is also restored though in that case, the absolute shift compared to 1b is underrated by theory (-22 nm versus -11 nm). The computed Stokes shifts are also slightly too large, though again, the strong decrease of ∆SS when donor groups are added are nicely reproduced by the selected approach, which gives confidence in the TD-DFT analysis. Clearly, the Stokes shifts computed for K∗ are in much better agreement with experiment than those of E*, further confirming that the main emission band originates from ESIPT for the present set of compounds. In the experimental fluorescence spectra of 3b there is a not well-resolved shoulder at ca. 400 nm in, that is



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at ca. -45 nm compared to the main band. 76 Given the fact that theory foresees an enol emission displaced from -41 nm compared to the keto band, it is quite safe to attribute the shoulder in the experimental spectra to a signature of E*. Table 2: Maximum absorption and emission wavelengths (in nm) computed for the three dyes. All values obtained with the cLR-PCM model in dichloromethane considering both equilibrium (eq) and non-equilibrium (neq) limits. On the rightmost columns, the Stokes shifts (in cm−1 ) are given as well, considering the enol absorption as reference. The experimental data are from Ref. 76.

1b 2b 3b

Form enol keto enol keto enol keto

Absorption eq neq Exp. 278 277 321 405 401 280 279 322 335 333 307 305 346 336 335

Emission eq neq Exp. 337 337 424 429 480 338 338 414 418 458 347 347 386 388 444

Stokes shift neq Exp. 6427 12791 10300 6526 11919 9200 3968 7013 6400

In short, the addition of donor groups on the phenol side induces an increase of the E∗ → K∗ ESIPT barrier and a decrease of the Stokes shift related to modifications of both the absorption and emission wavelengths. These trends are consistent with experimental measurements. Two more extended ESIPT dyes (4 and 5) proposed by Ziessel’s group have been investigated using the same methologogy. Both are also characterized by an intramolecular hydrogen bond between the phenol group and one nitrogen atom of the phenanthroimidazole and by an additional phenyl group or a phenyl alcyne phenyl (PhAlPh) group bonded to the nitrogen atom of the phenanthroimidazole not participating in ESIPT. In the GS, the K forms could not be located hinting that the K tautomer is even less stable than in HBI dyes. Similarly to HBI, there is a twist between the phenol and the phenanthroimidazole (see |d| in Table 3) of 16.5◦ and 19.8◦ for 4 and 5, respectively. It is worth noting that the phenyl group of 4 and the PhAlPh of 5 are almost perpendicular (79◦ and 75◦ , respectively) to the core of the molecule. Upon photon absorption those two geometrical parameters sig18 ACS Paragon Plus Environment

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nificantly change. Actually the dihedral angle involved in the hydrogen bond core become nearly coplanar in 4 with a decrease of 16◦ of the dihedral angle, while the phenyl is perpendicular to the rest of the molecule in the E∗ form. On the contrary hydrogen bond core of 5 is only slightly modified (15.3◦ ) and the PhAlPh group twisted by ca. 66◦ after photon absorption. Energetically the E∗ in HPI are relatively more stable than in HBI by 0.2 eV, as the hydrogen bond is less twisted. In addition on both the E and G scale 5 is slightly more stable than 4 by 0.08 eV and 0.03 eV, respectively. On such large molecules the ES TS were not looked for the computational cost is too demanding; however, K∗ tautomers for both HPI were optimized and their ∆G values are found smaller than E∗ forms by 0.01 and 0.04 eV suggesting a possible ESIPT process. Indeed, on a qualitative picture, a decrease of the electron density on the hydroxyl group of the phenol moiety upon absorption is observed on Figure 3 showing the significant increase of acidity as for HBI dyes. This Figure confirms also that the phenyl group of 4 does not participate in the electronic transition while the PhAlPh only slightly contribute to the optical phenomena. As for HBI λabs of E, λemi of K∗ and the corresponding ∆SS have been determined and are listed in Table 4. Concerning the absorption and emission spectra of HPI we observe that eq and neq values are rather similar (3 nm difference). As expected HPI being much more conjugated than HBI due to by the presence of a phenanthroimidazole, the absorption spectra of the E form is red shifted. Indeed going from 1b to 4 and to 5 experimentally induces a bathochromic shift in λabs of +42 nm and +41 nm that is well reproduced by theory with estimates of +30 nm and +29 nm, respectively. Interestingly the experimental emission spectra is blue (red) shifted by 8 nm (9 nm) when going from 1b to 4 (5) and this trend is qualitatively restored by theory (-22 nm and +2 nm).



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Table 3: Relative free energies (∆G in eV), total energies (∆E in eV) and (absolute) dihedral angle between the phenol ring and the benzimidazole part (|d| in degree) for the two large ESIPT dyes 4 and 5 (see Scheme 1). All results at cLRPCM-TD-M06-2X/6-311+G(2d,p)//LR-PCM-M06-2X/6-31G(d) level of theory considering dichloromethane as solvent. The most stable enol GS isomer is used as reference to determine the relative energies. 4

5

State GS ES GS ES

Form enol enol keto enol enol keto

∆G 0.00 3.61 3.60 0.00 3.58 3.54

∆E 0.00 3.84 3.82 0.00 3.76 3.75

|d| 16.5 0.49 19.3 19.8 15.3 24.9

Figure 3: Electronic density differences for 4 (left) and 5 (right) using a contour threshold of 0.0004 a.u. The blue (red) zones indicate decrease (increase) of electron density upon absorption.


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Table 4: Maximum absorption and emission wavelengths (in nm) computed for 4 and 5. All values obtained with the cLR-PCM model in dichloromethane considering both equilibrium (eq) and non-equilibrium (neq) limits. On the rightmost columns, the Stokes shifts (in cm−1 ) are given as well, considering the enol absorption as reference. The experimental data are taken from Ref. 76.

4 5

Form enol keto enol keto

Absorption eq neq Exp. 307 307 363 – – 306 306 362 – –

Emission eq neq Exp. 353 353 405 407 472 374 375 428 431 489

Stokes shift neq Exp. 4245 8003 6400 6013 9477 7200

Fluoroborate derivatives Let us now turn towards the fluoroborate complexes synthetized as well by Ziessel and coworkers (see Scheme 3). 76 The latter compounds are based on the NBO atomic sequence on which we already performed significant efforts to simulate both the 0-0 energies corresponding to the crossing point between absorption and emission bands (AFCP energies: Absorption/Fluorescence Crossing Point) and the band shapes. 114 In this Section, we focus on the simulation of the optical signature of these borate complexes, more precisely, on the calculation of the AFCP energies (Table 5) and vibronically refined absorption and emission spectra.

Scheme 3: Representation of borate complexes investigated herein.



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Going from 6a to 6b, that is replacing an hydrogen atom by a methoxy group induces an increase of the 0-0 energies by +0.07 eV (see Table 5). This modest variation is consistent with the similar topologies of the density difference plots (see Figures in the SI). However we do not reach the same trend for 6b than experiment as a -0.16 eV shift is observed. Using a recently developed approach, 137,138 we have quantified the charge transfer (CT) parameters for these dyes. For 6a (6b), the amount of CT is 0.50 e (0.49 e), and the CT distance is 0.98 ˚ A (0.98 ˚ A). However, changing the donor group from OMe (6b) to NEt2 (6c) provokes an important decrease of the 0-0 energy (-0.27 eV). In opposition to 6a and 6b, 6c presents a significant charge transfer character with a dCT (charge transfer distance) of 2.16 ˚ A (see Table 5) related to the strong donor character of NEt2 . This illustrates that OMe is not a strong donor group. In Figure 4, we have simulated absorption and emission vibronic spectra of 6b (see SI for 6c). For the band shapes of 6b, the agreement with experiment is very satisfactory (see experimental spectra in Ref. 76). We do distinguish two important GS and ES modes located at 1662 cm−1 and 1589 cm−1 , respectively that contribute to the specific band shape. These two modes correspond to stretching in the aromatic rings, while they do not involve BF2 (see movies in the SI).


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Figure 4: Convoluted and stick spectra for 6b.

Table 5: Theoretical best estimates of the experimental 0-0 energies obtained with the PCM-TDDFT-M06-2X approach, the CT distance (dCT in ˚ A) and the transferred charge (q CT in e) for the dyes shown in Scheme 3. Dye 6a 6b 6c 7 8 9

AFPC energies Theory Exp. 76 3.78 3.70 3.85 3.54 3.58 3.24 3.70 3.31 3.64 3.35 2.80 2.46

dCT

q CT

0.98 0.98 2.16 0.64 0.85 0.82

0.50 0.49 0.54 0.48 0.49 0.36

For the three latter molecules, that present almost the same skeleton, the 0-0 energies decrease in the order: 7 > 8 > 9. Interestingly, adding a boron-dipyrromethene (BODIPY) moiety (9) does induce a strong decrease of the 0-0 energy. The density difference plots for the first and the third excited states of 9, that present larger oscillator strengths, are shown in Figure 5. For the first ES, the density is purely located on the BODIPY core, and the NBO core do not play any role in the optical transition. This is due to the orthogonality of the phenyl rings. This is consistent with the experiment, as the irradiation in the NBO 23 ACS Paragon Plus Environment


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core at 360 nm leads to the unique emission of BODIPY moiety at much longer wavelength, indicating that the lowest state is localized on the BODIPY. For the third ES of 9, one notices a charge transfer character from the phenyl to the BODIPY. The dCT increases from 0.82 ˚ A (9 first ES) to 4.03 ˚ A (9 third ES), this state being therefore more solvatochromic.

Figure 5: Electronic density difference plots for the first (left) and third excited states (right) of 9.

Conclusions A complete computational study has been carried out on both hydroxyphenylbenzimidazole (HBI) and hydroxyphenyl-phenanthroimidazole (HPI) derivatives as well on their fluoroborate compounds. The former molecules undergo an ESIPT process upon photon absorption. By comparing a highly correlated method to TD-DFT/DFT, we observe a rather small variation in the GS and ES energetics. Both approaches predict the enol form as the most stable tautomer in the GS while in the ES TD-DFT underestimates the relative energies of E∗ and TS* by ca. 0.15 eV. However, E∗ →K∗ energy barriers computed at the TD-DFT level of theory are similar to their EOM-CCSD counterparts which ensure the validity of


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interpretations based on TD-DFT results. We wish to underline that the biradical K∗twist is poorly described by TD-DFT method and should be treated with multireference methods. Starting from model systems (methyl replaced a n−butyl), we were able to reproduce the experimental ESIPT trend. Actually the E∗ → K∗ is barrierless for 1a and 2a meaning that the fluorescence can only originate from the K∗ isomer while a barrier of 0.07 eV is computed for 3a and this is enough to observe two different emissions band induced by E∗ and K*. A qualitative analysis confirms that the hydroxyl group of 3a, influenced by a strong donor group at R1 position is less acidic than in 1a and 2a. These quantitative and qualitative pictures are similar in the ”real“ ESIPT dyes (series b) hinting that the n−butyl chain is not modifying the ESIPT process. Computed absorption and emission spectra of HBI and HPI were in good qualitative agreement with experiment as theory was able to reproduce the measured red or blue shifts according to the nature of the substituants. The comparisons of 0-0 energies of the fluoroborates derivates indicate that strong donor groups on such molecules induce a large charge transfer modifying the AFCP energies. The addition of a BODIPY moiety induces a strong modification of the emission spectra as the lowest excited state becomes localized on the BODIPY moiety. A perspective to this work would be to evaluate internal conversion pathway, that is explore the entire potential energy surface of the excited state up to the biradical K∗twist .

Acknowledgement The authors acknowledge J. Graton for sharing his knowledge on hydrogen bonds. Y.H. thanks the European Research Council (ERC, Marches - 278845) and the Région des Pays de la Loire for her PhD grant. S.C. thanks the European Research Council (ERC, Marches - 278845) for her PhD grant. D.J. acknowledges the European Research Council (ERC) and the Région des Pays de la Loire for financial support in the framework of a Starting Grant (Marches - 278845) and a recrutement sur poste stratégique, respectively. This research used resources of 1) the GENCI-CINES/IDRIS (Grants c2013085117), 2) CCIPL (Centre de 25 ACS Paragon Plus Environment


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Calcul Intensif des Pays de Loire) and 3) a local Troy cluster.

Supporting Information Available Relationship between dihedral angle and OH bond length for one model molecule, imaginary frequencies of the TS for 1a–3a. Absorption and emission wavelengths for 1a–3a. PES, density plots and GS scans IRC for 1b–3b. Density plots for 6–8 as well as the vibronic spectra of 6c. Movies showing the stretching modes involved in the shape of 6b spectra. This material is available free of charge via the Internet at http://pubs.acs.org/.

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1 2 3 4 5 6 Paragon Plus Environmen ACS 7 8

Excited-State Intramolecular Proton Transfer of the Natural Product Quercetin.

Excited-state intramolecular proton transfer: photoswitching in salicylidene methylamine derivatives.

colorimetric cyanide-selective sensor based on excited-state intramolecular charge transfer-excited-state intramolecular proton transfer switching.

TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: key role of the excited-state hydrogen-bond strengthening.

TDDFT investigation of the excited state proton transfer reaction of fisetin chromophore.

TDDFT study of the polarity controlled ion-pair separation in an excited-state proton transfer reaction.

TDDFT study of twisted intramolecular charge transfer and intermolecular double proton transfer in the excited state of 4'-dimethylaminoflavonol in ethanol solvent.

Time-dependent density functional theory study on the excited-state intramolecular proton transfer in salicylaldehyde.

Correlation between excited-state intramolecular proton-transfer and singlet-oxygen quenching activities in 1-(acylamino)anthraquinones.

Describing excited state intramolecular proton transfer in dual emissive systems: a density functional theory based analysis.

Excited state intramolecular proton transfer in π-expanded phenazine-derived phenols.

Excited state intramolecular proton transfer dynamics of 1-hydroxy-2-acetonaphthone.

Photoinduced excited state intramolecular proton transfer and spectral behaviors of Aloesaponarin 1.

Ratiometric fluorescent chemosensor for fluoride ion based on inhibition of excited state intramolecular proton transfer.

Excited-state intramolecular proton transfer to carbon atoms: nonadiabatic surface-hopping dynamics simulations.

Linear energy relationships in ground state proton transfer and excited state proton-coupled electron transfer.

Steric hindrance inhibits excited-state relaxation and lowers the extent of intramolecular charge transfer in two-photon absorbing dyes.

A novel non-fluorescent excited state intramolecular proton transfer phenomenon induced by intramolecular hydrogen bonds: an experimental and theoretical investigation.

Study of fluorescence probe transfer mechanism based on a new type of excited-state intramolecular proton transfer.

Exploring the metric of excited state proton transfer reactions.

Excited-State Proton Transfer of Weak Photoacids Adsorbed on Biomaterials: Proton Transfer on Starch.

Atypical energetic and kinetic course of excited-state intramolecular proton transfer (ESIPT) in room-temperature protic ionic liquids.

Intersystem Crossing Pathway in Quinoline-Pyrazole Isomerism: A Time-Dependent Density Functional Theory Study on Excited-State Intramolecular Proton Transfer.

Fluorescent amino acid undergoing excited state intramolecular proton transfer for site-specific probing and imaging of peptide interactions.