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A new class of N–H excited-state intramolecular proton transfer (ESIPT) molecules bearing localized zwitterionic tautomers† Anton J. Stasyuk,‡ Yi-Ting Chen,‡ Chi-Lin Chen, Pei-Jhen Wu and Pi-Tai Chou* A series of new amino (NH)-type intramolecular hydrogen-bonding (H-bonding) compounds have been strategically designed and synthesized. These molecules comprise a 2-(imidazo[1,2-a]pyridin-2-yl)aniline moiety, in which one of the amino hydrogens was replaced with substituents of different electronic properties. This, together with the versatile capability for modifying the parent moiety, makes feasible comprehensive spectroscopy and dynamics studies of excited-state intramolecular proton transfer (ESIPT) as a

Received 28th July 2016, Accepted 11th August 2016

function of N–H acidity. Different from other (NH)-type ESIPT systems where the ESIPT rate and exergonicity

DOI: 10.1039/c6cp05236c

relationship among ESIPT dynamics, thermodynamics and H-bond strength. This discrepancy may be

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tautomer form, which is different from the p-delocalized tautomer form in other (NH)-type ESIPT systems.

increase with an increase in the N–H acidity and hence the H-bonding strength, the results reveal an irregular rationalized by the localized zwitterionic nature of 2-(imidazo[1,2-a]pyridin-2-yl)aniline in the proton-transfer

1. Introduction Over the past few decades, considerable attention has been paid to organic molecules undergoing excited-state intramolecular proton transfer (ESIPT). ESIPT is essentially a photo-tautomerization process that occurs in the electronically excited state of a molecule (Scheme 1a), in which the proton-transfer tautomer emission reveals an anomalously large spectral red-shift with respect to absorption. This unique photophysical property leads to numerous practical optoelectronic applications, such as bioimaging,1–3 fluorescent solar energy concentrators,4,5 and UV-photostabilizers,6–8 as well as chemosensors,9–11 organic light-emitting devices,12,13 laser dyes,14–16 and molecular switches.17–19 It has been well accepted that the driving force of ESIPT reaction comes from significant changes in the acidity and basicity of the involved groups.20,21 Proton transfer processes typically take place through the formation of a six-membered ring with a strong intramolecular hydrogen bond (H-bond) between the proton donor and acceptor groups. Five-membered22–24 and seven-membered25,26 units have also been described but they are much less common. For the vast majority of the previously described ESIPT-capable systems, a phenolic OH group acts as a proton donor, whereas a heterocyclic nitrogen atom of pyridine type or a carbonyl group serves as an acceptor of the proton.

Department of Chemistry, National Taiwan University, Taipei, 10617 Taiwan, Republic of China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp05236c ‡ These two authors contributed equally.

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Scheme 1 (a) General scheme of ESIPT and (b) advantage of 2-(imidazo[1,2a]pyridin-2-yl)aniline over 2-(imidazo[1,2-a]pyridin-2-yl)phenol in terms of impact on the acidity of a proton involved in ESIPT.

In comparison, ESIPT systems with other types of proton donors are rare. Several reports have dealt with ESIPT systems for which the amino NH group and heterocyclic nitrogen atom serve as the proton donor and acceptor, respectively.27–34 Also, there are a few cases of intramolecular proton transfer reactions through (pyrrole)N–H  N(pyridine) hydrogen bonds.35–37 In addition, the theoretical possibility of strong CH-acids acting as proton donors under photoexcitation has recently been reported.38 The aforementioned superiority of OH-type ESIPT can apparently be explained by the fact that its ground state acidity is much stronger than that of the primary amino or pyrrolic proton. This distinction can be clearly demonstrated by comparison of the acid dissociation constant measured in DMSO for

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aniline (pKa B 30.639,40) and phenol (pKa B 18.041). Assuming a similar magnitude of changes of pKa* for both N–H and O–H protons in the excited state one would expect that OH-type ESIPT is more facile than that of NH-type intramolecular H-bonding systems. Despite the significantly lower acidity of primary amine protons, compared with the hydroxyl proton, however, the NH-type H-bonding system offers an advantage in that one of the protons in the amino group can be replaced with either an electrondonating or an electron withdrawing substituent, providing the ability to efficiently influence the acidity of the proton involved in ESIPT. This additional degree of freedom thus allows ‘‘on and off’’ switching of additional deactivation channels affecting the observed optical properties of the considered NH-type intramolecular H-bonded systems (Scheme 1b). Among various OH-type ESIPT systems, 2-hydroxyphenyl substituted imidazo[1,2-a]pyridines42 have attracted consider˜ a and co-workers were the able interest in recent years. Acun first to report photoinduced intramolecular proton transfer in imidazopyridines.43 Over the next few years after this discovery the same authors proposed and experimentally proved a hypothesis about the ‘‘on–off’’ switching of the ESIPT phenomenon based on interactions between the molecule of interest and the environment (polar, protogenic solvent).44 Furthermore, great attention has been paid to the 20 -hydroxy derivative of 2-phenylimidazo[1,2-a]pyridines as an organic solid-state luminescent material. Araki and co-workers45–49 have found that 2-(20 -hydroxyphenyl)imidazo[1,2-a]pyridine is capable to form different polymorphs with substantially different optical properties (fluorescence wavelengths and quantum efficiency). This was a demonstration of a novel design concept for tunable organic luminescent solids.45 Considering that the OH-type of ESIPT has been intensively studied for 2-phenyl substituted imidazo[1,2-a]pyridine, we have chosen imidazo[1,2-a]pyridine as a main core to probe the NH-type of H-bonding systems. Herein, we report the synthesis and comprehensive photophysical studies of a new series of N(R)H-type intramolecular H-bonded system. This system bears an imidazo[1,2a]pyridine chromophore as the proton acceptor and a secondary amide as the proton donor in which the R-substituent is able to be systematically derivatized to increase the electronwithdrawing property so that the N(R)–H proton acidity and hence the H-bonding strength can be fine-tuned. Interestingly, the results of this study reveal different correlations from other (NH)-type ESIPT systems; the latter showed that the ESIPT rate and exergonicity increase with an increase in the N–H acidity and hence the H-bonding strength. This discrepancy may be rationalized by the localized zwitterionic nature of 2-(imidazo[1,2-a]pyridin-2-yl)aniline in the tautomer form, which is different from the p-delocalized tautomer form in other NH-type ESIPT systems.27–34 Details of the results and discussion are elaborated as follows.

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pyridin-2-yl)aniline (H-NHPIP) and its derivatives is apparently the reduction of 2-(2-nitrophenyl)-imidazo[1,2-a]pyridine (NO2-PIP), which in turn can be (were) obtained via a tandem Ortoleva– King–Chichibabin reaction.50,51 The advantage of the Chichibabin reaction, in addition to its simplicity, is that imidazo[1,2-a]pyridines obtained following this approach maintain an open C3 position. The presence of a relatively bulky substituent at position C3 may lead to noticeable steric hindrance and as a result of the out of planarity of the phenyl ring. The fluorescence compounds studied in this work were synthesized starting from 1-(2nitrophenyl)ethanone and 2-aminopyridine via an Ortoleva–King reaction,52 followed by ring closure and subsequent derivatization of 2(imidazo[1,2-a]pyridine-2yl)aniline (details are given in the ESI†). Recently, Gryko and co-workers53 reported that the main and well-proven reduction methods of nitroarenes, such as reduction using H2 and Pd on charcoal52 or reaction with Fe powders in concentrated hydrochloric acid failed in the case of NO2-PIP. Instead of the desired reduction of the nitro group the authors noticed a dominant side reaction – reduction of the pyridine ring. Despite the fact that Chauhan and co-workers54 reported that the reduction of NO2-PIP with Fe powder/acetic acid provided H-NHPIP in good yield, in this work, we have used the procedure developed by Kundu and co-workers55 and evaluated by Gryko – reduction of nitro-imidazo[1,2-a]pyridine with SnCl2 in ethanol – as optimal, which leads to the desired H-NHPIP in good yield. Synthetic routes for the studied compounds are depicted in Scheme 2. Having H-NHPIP as a key substrate we have then functionalized the amino group. In order to tune the acidity of this group, one of the protons has been replaced with various substituents different in electron withdrawing power. Initial prototype H-NHPIP has been transformed into several sulfonamide (CH3C6H4SulfNHPIP, C6F5Sulf-NHPIP) and acetamide (CH3CO-NHPIP, CF3CONHPIP) derivatives. It should be noted that the reaction yield for the designated sulfonamides and acetamides are generally high. Only in the case of 2,3,4,5,6-pentafluorobenzene-1-sulfonyl chloride is the reaction yield relatively low (63%), which leads to by-products. Detailed synthetic procedures and characterization of these compounds are described in the experimental section of the ESI.†

2. Results and discussion 2.1

Synthesis

A comprehensive analysis of the published literature reveals that the most straightforward synthetic strategy to 2-(imidazo[1,2-a]-

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Scheme 2 Synthetic routes for 2-(imidazo[1,2-a]pyridin-2-yl)aniline and its derivatives.

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2.2

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NMR spectroscopy

Electron withdrawing substituents have a strong impact on the electron density at the amine nitrogen. Lowering the electron density has been directly reflected in the values of the chemical shift of the denoted amide protons. Introduction of a 4-methylbenzensulfonyl substituent immediately leads to pronounced downfield shift of the sulfonamide proton signal in 1H NMR to 12.65 ppm compared to 6.55 ppm for parental H-NHPIP. Trifluoroacetyl substituent (CF3CO-NHPIP) causes an even greater shift to 14.62 ppm. Fig. 1 demonstrates scaled stacked 1 H NMR spectra for all studied compounds. This figure clearly shows the amide proton chemical shift dependence on the electron withdrawing ability of the considered substituents. Thus, minimal downfield shift has been caused by the acetyl substituent (d = 12.49), while the largest shift was observed for the trifluoroacetyl substituent. At the same time, for the studied sulfonamides, CH3C6H4Sulf-NHPIP, the proton chemical shift was fairly large (d = 12.68). The exception was C6F5Sulf-NHPIP. In contrast to other studied compounds, in this case we have been unable to detect the amide proton signal in DMSO-d6 solution. We hypothesized that such behavior may be caused by the relatively high acidity of indicated protons, and as a consequence by a strong intermolecular (solute–solvent) interaction. Based on this assumption we have carried out a series of 1 H NMR measurements for C6F5Sulf-NHPIP in various solvents with different polarities. Fig. S1 (ESI†) demonstrates the scaled stacked 1H NMR spectra of C6F5Sulf-NHPIP in various solvents. It is clear that decreasing polarity leads to an increase of the

Fig. 1 Scaled stacked 1H NMR spectra of H-NHPIP and its derivatives in DMSO-d6.

intensity of the amide proton signal. Only in benzene solution, for which a strong solute–solvent interaction seems unlikely, C6F5Sulf-NHPIP amide protons demonstrate rather a sharp signal, characteristic for other sulfonamides. The proton signal for sulfonamides is also broader than that for acetamides. The observed dependence of the proton signal width and intensity in 1 H NMR spectra on the electronic properties of the substituent at the semi-quantitative level can be described based on the acidity of the amide proton and thus the strength of the intramolecular N–H  N hydrogen bond. A widely used class of balanced reactions designated as ‘‘homodesmotic’’ has been used for the evaluation of the considered intramolecular hydrogen bond (IHB) strength. Scheme S1 (ESI†) demonstrates the formal chemical equation which has been used for IHB strength estimation. Orbital hybridization and the number of CH bonds in both parts of the equation are the same. All calculations have been performed without any solvation model, i.e. for free molecules. According to Scheme S1 (ESI†) the energy of hydrogen bonds can be estimated as a difference between the energy of the ‘‘reaction products’’ (E3 + E4) and the energy of the ‘‘reactants’’ (E1 + E2). Geometries and energies for each fragment have been calculated independently. As can be seen from Table 1, ab initio calculations performed for the studied compounds do not allow for revealing a direct relationship between the length of hydrogen bonds and chemical shift values. This discrepancy can apparently be explained by the limitation of the model used for the calculations, namely the performed calculations do not take into account any solvation model for the studied compounds, while the solvent and intermolecular solute–solvent interaction play an important role in this case. However, despite the overall roughness of the used model, the chemical shift values are in good agreement with the N–H bond length of the amide moiety, which in turn is an indicator of the acidity of the considered bond. In spite of the semi-quantitative nature of the revealed relationships and the relative coarseness of the assessment, we have focused our attention on the H-NHPIP compound. The hydrogen bond energy in this case estimated by homodesmotic reaction demonstrates a value that is at least half as low as any other compound studied in this work. The structure of the H-NHPIP compound together with the estimated value of hydrogen bond strength allowed us to suggest that this compound can exist in at least two conformers – normal (H-NHPIP) and rotated (H-NHPIP(r)) one (Scheme 3).

Table 1 Intramolecular hydrogen bond strengths (in kcal mol1) estimated using homodesmotic reaction. Experimentally measured chemical shift values (1H NMR, in ppm) for amide protons in DMSO-d6

Homodesmotic reaction R

Electronic energy (e0), kcal mol1

Sum of electronic and zero-point energies (e0 + ezpe), kcal mol1

N–H bond length,a Å

N  H hydrogen bond length,a Å

Chemical shift, ppm

H CH3CO CH3C6H4Sulf C6F5Sulf CF3CO

2.581 3.591 3.627 4.413 4.470

2.306 3.350 3.467 4.430 4.388

1.014 1.021 1.028 1.036 1.032

1.951 1.823 1.846 1.777 1.769

6.55 12.49 12.68 14.25b 14.62

a

Values of bond lengths estimated at the PBE0/6-311++G(d,p) level of theory.

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b

Chemical shift value obtained in C6D6.

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experiment by pre-selecting the Ha nuclei resonance (a large negative signal at 8.29 ppm). Integration of the signals in selective NOE spectra allows one to get the approximate ratio of the conformers in the mixture. This value is about a few percent. However, access to the quantitative or semi-quantitative level seems to be impossible due to the fact that the intensity of the signal in NOE spectroscopy is not only a function of the relative concentration but also depends on the spatial distance of the atoms undergoing cross relaxation. 2.3

Scheme 3 Spatial proximities of selected protons and NOESY spectrum (2D) in conformers of 2-(imidazo[1,2-a]pyridin-2-yl)aniline (H-NHPIP).

The small value of the hydrogen bond energy suggests a notable contribution of the H-NHPIP(r) conformer to the composition of the conformational mixture. Ab initio calculations of the Gibbs reaction free energy and the resulting concentrations of the conformers (eqn (S1)–(S4), ESI†) in the mixture demonstrate that for 2-(imidazo[1,2-a]pyridin-2-yl)aniline in polar solvent the amount of the H-NHPIP(r) conformer may reach several percent (see Table S1, ESI†). Based on this we have assumed that the presence of both conformers in polar solution can be explicitly demonstrated on the basis of twodimensional NMR spectroscopy. A careful examination of the geometrical structures of the H-NHPIP and H-NHPIP(r) conformers (Scheme 3) reveals the closest proximities of the Ha proton to the Hb and Hc protons in conformer H-NHPIP, while for the H-NHPIP(r) conformer, the Ha proton is in closest proximity to the Hb and Hd protons. This allows us to conclude that in NOESY experiment three signals corresponding to the above-mentioned interactions between protons – ‘‘Ha–Hb’’, ‘‘Ha–Hc’’ and ‘‘Ha–Hd’’ should be observed. At the same time, it should be mentioned that the signal corresponding to the ‘‘Ha–Hd’’ interaction is significantly weaker than the other two signals due to the fact that the concentration of the conformer H-NHPIP(r) is low. In full accordance with our expectations the NOESY experiment revealed three signals with coordinates [8.29, 8.57] – corresponding to ‘‘Ha–Hb’’ cross relaxation, [8.29, 7.56] – corresponding to ‘‘Ha–Hc’’ cross relaxation and [8.29, 6.56] – corresponding to ‘‘Ha–Hd’’ cross relaxation. Fig. S2 (ESI†) also demonstrates a one-dimensional

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Photophysical properties

The electronic absorption spectra and steady-state emission spectra for the titled compounds in toluene at room temperature are shown in Fig. 2, and pertinent data are presented in Table 2. Similar results were observed in cyclohexane and dichloromethane. However, the sparse solubility in cyclohexane and fast decomposition in dichloromethane under high laser power prohibit further fluorescence up-conversion measurements in these two solvents. In toluene the studied compounds all have similar electronic absorption features, consisting of an intense band peaked in the UV region (from 300 to 380 nm) with a molar absorptivity (e) ranging from 10 290 to 12 400 M1 cm1, which is ascribed to the typical p–p* transitions. This assignment is also supported by the frontier orbital analyses shown in Table S2 (ESI†), in which the S0 - S1 excitation involves mainly N(R)H-phenyl (HOMO) - imidazo[1,2-a]pyridine (LUMO) transition, manifesting its partial charge transfer character. Therefore, increasing the acidity of the N(R)–H proton by increasing the electron withdrawing strength of R should accompany the reduction of the HOMO energy and cause the blue-shift of absorption features, which is experimentally revealed in Fig. 2a. The computed lowest-lying electronic

Fig. 2 (a) Absorption and (b) normalized emission spectra of H-NHPIP (black), CH3CO-NHPIP (red), CH3C6H4Sulf-NHPIP (blue), C6F5Sulf-NHPIP (green) and CF3CO-NHPIP (magenta) in toluene at room temperature. Inset: The enlargement of H-NHPIP and CH3CO-NHPIP emission in the region of 560–800 nm.

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Paper The photophysical properties of titled compounds in toluene at room temperature

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Steady-state measurement In toluene

labs/nm (e/M1 cm1)

lema/nm

H-NHPIP

350 (11 500)

N: 390

CH3CO-NHPIP

336 (10 290)

CH3C6H4Sulf-NHPIP

333 (11 250)

C6F5Sulf-NHPIP CF3CO-NHPIP

332 (10 340) 331 (12 400)

T: 700 N: 390 T: 650 N: 380 T: 580 T: 545 N: 380 T: 510

Time-resolved measurement Ff/% 0.8 5.8 13.1 21.9 45.9

DEcal* e/kcal mol1

tobsb/ps (pre-exp. factor)

kpt/s1

kpt/s1

DEexp*/kcal mol1

N: 1.58(0.64), 19.67(0.27), 1784(0.09)c,d T: 1.58(0.42), 19.67(0.58) N: 53(0.74), 334(0.26)c T: 53(0.47), 334(0.53)c N: 16.3(0.99), 1950(0.01)c,d T: 16.3(0.38), 1950(0.62)c,d T: 3390d N: 106(0.98), 3900(0.02)c,d T: 106(0.46), 3900(0.54)c,d

4.4  1011

1.9  1011

0.5

4.3

1.4  1010

4.9  109

0.62

6.7

6.1  1010

6.1  108

2.72

14.4

— 9.3  109

— 1.4  108

— 2.47

14.8 9.5

a N = normal emission; T = tautomer (ESIPT) emission. b Lifetime was measured by using an up-conversion system with femtosecond excitation pulses (lex = 360 nm). c Lifetime measured using a TCSPC system with femtosecond excitation pulses (lex = 360 nm). d Lifetime measured using a TCSPC system with a pulsed hydrogen-filled lamp as the excitation source (lex = 360 nm). e Computed corresponding energy differences (DE* = E*(T)  E*(N)) between the normal form and tautomer form species in the lowest excited state (S1) for the titled compounds.

transition (S0 - S1) in terms of wavelength also agrees with this tendency (see Table S2, ESI†). Except for C6F5Sulf-NHPIP that exhibits solely a long wavelength emission band, all other studied compounds show dual emission behaviour (Fig. 2b) in which the short wavelength band reveals a mirror image of the absorption spectrum with a small Stokes shift ascribed to the normal emission (N*, where the asterisk denotes the excited state). The other emission band with anomalously large Stokes shifted feature (cf. absorption) is denoted as the proton-transfer tautomer emission (T*), resulting from ESIPT. An example of the studied compound H-NHPIP is given. The emission spectrum of H-NHPIP possessing an unsubstituted NH2 group has two maxima bands at 390 and 700 nm, indicative of the normal and tautomer form features, respectively. Note that this demonstrates the first case where a donor–acceptor moiety bridged by a single bond exhibits ESIPT from a weak proton donor (NH2) without an additional substituent.27,28,32,33 However, in comparison, the excitation spectrum of H-NHPIP monitored at the tautomer emission (e.g. 700 nm) is red shifted by B15 nm with respect to that monitored at the normal emission (e.g. 400 nm) (see Fig. S3, ESI†). Excluding any artefact caused by traces of impurity, the unmatched excitation spectra should be ascribed to the relatively weak intramolecular H-bond that leads to the existence of two conformers: normal H-bonded (H-NHPIP) and rotated (H-NHPIP(r)) isomers (Scheme 3) confirmed by both the NOESY spectrum (vide supra) and the computational approach (see Fig. S2 and Table S2, ESI†). The latter indicates that the normal H-bonded (H-NHPIP) form is more stable than the rotated, non-H-bonded form (H-NHPIP(r)) by only B2.3 kcal mol1. The H-bonded H-NHPIP undergoes ESIPT, whereas ESIPT is prohibited in its non-H-bonded form due to its rather long fluorescence lifetime (1.78 ns, vide infra). The red shift of 15 nm in the excitation spectrum for the H-bonded H-NHPIP (cf. H-NHPIP(r)) can be rationalized by the intramolecular H-bond that stabilizes the conformation and elongates the p-conjugation. Moreover, the excitation spectrum monitored at tautomer emission is similar to the absorption

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spectrum, supporting the H-bonded H-NHPIP to be the dominant species in the ground state. Upon increasing the acidity of the N(R)–H proton by the electron-withdrawing R group (cf. R = H for H-NHPIP), it is reasonable to expect the intramolecular H-bonded conformer to be the predominant species in the ground state. Accordingly, the proton acidity can be identified by the chemical shift from the 1H NMR spectrum (vide supra). As a result, the downfield shift of the N–H proton for the studied compounds is in the order of H-NHPIP (d B 6.55 ppm) o CH3CO-NHPIP (d B 12.49 ppm) o CH3C6H4SulfNHPIP (d B 12.68 ppm) o C6F5Sulf-NHPIP (d B 14.25 ppm) o CF3CO-NHPIP (d B 14.62 ppm). Correspondingly, the tautomer emission peak wavelength is strongly dependent on the electronic properties of the N(R) group, being blue shifted as R increases the electron-withdrawing ability (i.e., the higher acidity and hence the stronger H-bond strength). This can be rationalized by the ESIPT reaction product, i.e., a proton transfer tautomer for which the lowest lying excited state inherits a HOMO–LUMO charge transfer character from the imino (HOMO) to the imidazopyridine (LUMO) moiety. Thus, adding an electron withdrawing substituent R at the amino site results in the lowering of HOMO energy, hence the increase of the energy gap of the tautomer emission. Therefore, in good correlation with the increased N–H proton acidity, the trend of the blue shift of the proton-transfer tautomer emission in terms of peak wavelength is in the order of H-NHPIP (700 nm) 4 CH3CONHPIP (650 nm) 4 CH3C6H4Sulf-NHPIP (580 nm) 4 C6F5SulfNHPIP (545 nm) 4 CF3CO-NHPIP (510 nm) (see Fig. 2). However, very different from recent studies on other NH-type ESIPT systems, which reveal the trend of increasing ESIPT rate upon increasing the N–H acidity,28 the current system reveals certain controversial results. For example, CF3CO-NHPIP, which possesses the highest proton acidity hence the strongest H-bond among all the studied 2-(imidazo[1,2-a]pyridine-2yl)aniline derivatives, exhibits dual emission, whereas C6F5Sulf-NHPIP with lower acidity (cf. CF3CO-NHPIP) reveals exclusively tautomer emission (see Fig. 2), implying an ultrafast ESIPT process. We then made attempts to gain further insight into the ESIPT dynamics of the titled compounds using the femtosecond

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fluorescence up-conversion technique in combination with the pico-nanosecond time correlated single photon counting (TCSPC) method (see the experimental section for details). Fig. 3 shows the fluorescence up-converted signal obtained for prototypical H-NHPIP, CH3CO-NHPIP and CF3CO-NHPIP in toluene. All pertinent fitted kinetic data are listed in Table 2. As shown in Fig. 3a, for H-NHPIP in toluene, the emission monitored at the normal emission of e.g. 410 nm clearly reveals multiple decay components, consisting of short (1.6 ps) and relatively long (19.6 ps) decays, accompanied by a very long population decay rate constant that is fitted to be 1.78 ns by TCSPC. First of all, in accordance with NOESY, the excitation spectrum and the computational approach (vide supra), the long 1.78 ns decay component with a small pre-exponential factor (see Fig. 3a) can reasonably be assigned to the long population decay from the rotated non-H-bonded conformer (H-NHPIP(r)), for which ESIPT is prohibited due to the lack of intramolecular H-bonds. On the other hand, the tautomer emission monitored at e.g. 650 nm consists of a 1.6 ps rise time component and a decay time of 19.6 ps (Fig. 3 and Table 2). The shorter decay component (1.6 ps) of normal emission and the rise component (1.6 ps) of the tautomer emission are well-matched with the opposite sign in terms of the pre-exponential factor. Another result worth mentioning is the identical population decay times for both normal and tautomer emissions (B19.6 ps). The combination of these observations leads us to conclude the precursor (normal)successor (tautomer) type of ESIPT at the early stage, followed

Fig. 3 Fluorescence up-conversion decay curves obtained for (a) H-NHPIP, (b) CH3CO-NHPIP and (c) CF3CO-NHPIP in toluene. The data points in blue and red shown are monitored at normal and tautomer emissions, respectively. Solid lines depict the best biexponential fits. The fitting parameters are summarized in Table 2. lex = 360 nm. Inset: The enlargement of the early dynamic profile.

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by a fast pre-equilibrium between normal and tautomer emissions in the excited state prior to the decay of each individual emission. Supported by the identical population decay time between normal and tautomer emissions, similar excited-state equilibrium between normal and tautomer excited states is also established for CH3CONHPIP (Fig. 3b), CH3C6H4Sulf-NHPIP (Fig. S4, ESI†) and CF3CONHPIP (Fig. 3c) in toluene. We then analyzed the corresponding reaction kinetics by a coupling reaction model depicted in Scheme 4b with a kinetic expression for the concentrations of both the excited-state normal form ([N*]t) and tautomer form ([T*]t) shown in eqn (1).   kpt kpt ½N t ¼ ½N 0 et=t1 þ et=t2 kpt þ kpt kpt þ kpt ½T t ¼

kpt ½N 0 1

ðt2 Þ ðt1 Þ

ðt1 Þ1 ¼

1

  et=t1  et=t2

(1)

kN þ kT Keq ; ðt2 Þ1 ¼ kpt þ kpt 1 þ Keq

where kN* and kT* are the fluorescence decay rate constants of N* and T*, respectively. kpt denotes the forward proton transfer rate and kpt represents the reverse proton transfer rate. t2 and t1 are the observed decay time constants of the fast and slow decay components, respectively. As a result, the equilibrium constant Keq = kpt/kpt can be obtained from the ratio of the preexponential factor (at t = 0) in eqn (1), corresponding to a free energy DE* from N* to T* (DE* = T*  N*). Because 1/t2 is equivalent to kpt + kpt, the forward and backward proton transfer rate constants can be further deduced and summarized in Table 2. As a result, a plot of the forward ESIPT rate constant (kpt) and DE* as a function of N–H acidity (in terms of N–H proton NMR scale) is depicted in Fig. 4. Previous results on another N–H ESIPT system containing 2-(benzo[d]thiazol-2-yl)aniline (BTA, see Scheme 4)28 as the core moiety revealed a trend in that enhancing the N–H acidity and hence the increase of intramolecular H-bond

Scheme 4 (a) ESIPT of the BTA system through a planar-like semi-pconjugated transition state. (b) ESIPT of the N(R)H-PIP system via a plausible non-planar transition state.

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Fig. 4 A plot of the corresponding free energy from N* to T* (DE*, solid blue circle) and forward ESIPT rate constant (kpt solid red circle) in toluene as a function of N–H acidity (in terms of N–H proton NMR scale) for H-NHPIP, CH3CO-NHPIP, CH3C6H4Sulf-NHPIP and CF3CO-NHPIP.

strength increases both the ESIPT rate and exergonicity. In comparison, Fig. 4 shows very different results. In fact, the results even show the opposite trend with respect to N–H ESIPT systems bearing a BTA moiety.28 For example, CF3CO-NHPIP possesses the highest proton acidity and hence the strongest Hbond among the studied compounds, yet it exhibits the slowest forward ESIPT rate (kpt) of 9.3  109 s1 (see Table 2). Moreover, H-NHPIP has the weakest acidity in this series, yet its forward ESIPT rate (4.4  1011 s1) is the highest among the titled compounds (excluding C6F5Sulf-NHPIP that exhibits ultrafast ESIPT of o150 fs of the system response time). This discrepancy, in our viewpoint, should be due to the difference in the canonical structure between the BTA28 and current 2-(imidazo[1,2-a]pyridin-2-yl)aniline (PIP) ESIPT systems, in which the full p-conjugated, neutral structure can be drawn for the BTA systems, whereas for the PIP system only the ‘‘zwitterionic’’ structure can be depicted, localized at proton donor and acceptor sites with anionic and cationic character, respectively (see Scheme 4b). Accordingly, it is reasonable to expect the structure of the transition state to be a zwitterion-like configuration rather than a planar type of semi-p-conjugated structure proposed for the BTA system (see Scheme 4a). To gain a deeper understanding we have performed DFT/ TDDFT calculations for H-NHPIP and CH3CO-NHPIP in an attempt to reveal the possible structure for the transition state (TS*) along ESIPT. Accordingly, the structure and energetics of normal, transition state and tautomer species are depicted in Fig. S5 and S6 (ESI†), respectively. Note that the potential energy diagram shown in Fig. S6 (ESI†) is only qualitative. As shown in Fig. S5 (ESI†), the structures of TS* are closer to the tautomer (T*) species for both H-NHPIP and CH3CO-NHPIP. In other words, the computed structure confirms a zwitterionic-like configuration for TS*. However, the calculated ESIPT barrier for H-NHPIP (0.31 eV) is twice as much as that for the CH3CO-NHPIP (0.14 eV), the result of which contradicts the experimental results. Knowing that ESIPT is not solely a proton migration but involves multipledimension processes incorporating various vibrational/rotational motions, we believe that our current computational approach oversimplifies the reaction pathway. As for the zwitterion, the non-planar structure for TS*, the –R substituent at the N(R)–H site may play a key role in terms of displacement and/or orientation to optimize the geometry and hence minimize the energy of the

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transition state. For the case of H-NHPIP the –R (–H) group is subject to a very small spatial reorientation along ESIPT, whereas other groups such as R = –CF3CQO in CF3CO-NHPIP are expected to have a relatively large reorientation from the reactant (normal species in the S1 state) to the zwitterion-like transition state, such that the steric and resonance (CQO) effects are to be optimized, resulting in an appreciable barrier. On the other hand, we also made attempts to probe the reaction dynamics by the temperature dependent studies. Our current femtosecond fluorescence up-conversion setup was not suitable for this approach due to its utilization of the rotating sample cell. Alternatively, our TCSPC setup incorporating a short excitation pulse (B100 fs) and a multichannel plate offers a B 20 ps temporal resolution, which is fast enough to perform the temperature-dependent study on CF3CO-NHPIP due to its relatively slow ESIPT dynamics. The temperature dependent relaxation dynamics for CF3CONHPIP was then studied by varying the temperature from 303 to 263 K in toluene. The temperature dependent forward protontransfer rate constant kpt(T) can thus be deduced by eqn (1). As a result, Fig. 5 shows a plot for ln kpt(T) as a function of 1/T according to the Arrhenius eqn (2). ln kpt ðTÞ ¼ ln A 

Ea RT

(2)

where Ea and A denote the reaction activation energy and frequency factor, respectively. The results reveal a straight line and consequently, Ea and A for the ESIPT of CF3CO-NHPIP are calculated to be 2.43 kcal mol1 and 5.21  1011 s1, respectively. The appreciable energy barrier and the small frequency factor support the viewpoint of the relatively large amplitude motion that may be, in part, associated with the –CF3CO substituent. Nevertheless, without the comprehensive kinetic data on other compounds as well as the advanced computational approaches it will be too primitive to describe any further subtlety of the ESIPT mechanism in this system. Despite the failure of resolving the transition-state structure for the studied NH-type systems, the thermodynamics of ESIPT, i.e., DE*, can be estimated by the current time dependent-DFT (TD-DFT) approach (TD-B3LYP/6-311++G(d,p)), see the ESI,† the values of which are listed in Table 2. In comparison to the experimental data (see also Table 2) the computed DE* apparently much overestimated the reaction exergonicity. Interestingly,

Fig. 5 Arrhenius plot of the rate constant (kpt) versus reciprocal temperature of CF3CO-NHPIP in toluene from 303 to 263 K.

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however, the trend of DE* is consistent with the experimental data, being decreased in the negative number (more exergonic) in the order of H-NHPIP o CH3CO-NHPIP o CH3C6H4Sulf-NHPIP o CF3CO-NHPIP o C6F5Sulf-NHPIP. Except for CF3CO-NHPIP the results show an increase of ESIPT exergonicity upon increasing the electron withdrawing power of the –R groups and hence the increase of the N(R)–H acidity. Nevertheless, as for CF3CO-NHPIP which possesses the strongest electron withdrawing –R (CF3CO) group, its large ESIPT barrier and less reaction exergonicity is pending resolution.

3. Conclusions In summary, a new series of N–H H-bonded molecules derived from the core chromophore 2-(imidazo[1,2-a]pyridin-2-yl)aniline (PIP) have been synthesized to investigate the associated N(R)– H  N proton transfer properties in the excited state. Similar to other N–H type ESIPT systems, in a qualitative manner, the results show the correlation of the increase of the N(R)–H acidity with increasing electron withdrawing power of the –R groups and hence the increase of the hydrogen bonding strength. Also, the PIP systems show the similarity of increasing the electron withdrawing –R resulting in a hypsochromic shift of the protontransfer tautomer emission such that the tautomer emission peak wavelength can be largely tuned from 700 nm (in H-NHPIP) to 510 nm (in CF3CO-NHPIP). Despite the above similarities, however, the dissimilarity lies in that the resulting ESIPT kinetics does not reveal an increase of the ESIPT rate with an increase in the N–H acidity, as concluded in the BTA system.28 Even oppositely, CF3CO-NHPIP with the strongest electron withdrawing group reveals the slowest ESIPT rate and a large barrier. We tentatively rationalize the result by the uniqueness of the PIP system for which the proton transfer tautomer and the structure of the transition state can only be described by a zwitterionic character localized at proton donors and acceptors. Therefore, instead of the planar type semi-p-conjugated structure proposed for the BTA system, the –R configuration at the N(R)–H site in the PIP system may play a key role in terms of displacement and/or orientation to optimize the energy of the transition state. In other words, it seems like that independent of the N(R)–H acidity the structure of the transition state resembles that of the product, i.e., the tautomer species. Under these circumstances the correlations among H-bonds, ESIPT kinetics and thermodynamics may no longer hold. The results thus bring a new insight regarding the difference per se in ESIPT dynamics between fully conjugated, neutral type ESIPT and localized zwitterionic type ESIPT, which extends the horizon of N–H type ESIPT in view of fundamental and future applications.

Acknowledgements P.-T. Chou thanks the Ministry of Science and Technology, Taiwan, for the financial support. A. J. S. gratefully acknowledges The Interdisciplinary Center for Mathematical and Molecular Modelling of the University of Warsaw (ICM) for computational facilities.

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