Correlation among Hydrogen Bond, Excited-State Intramolecular

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C: Energy Conversion and Storage; Energy and Charge Transport

Correlation among Hydrogen Bond, Excited-State Intramolecular ProtonTransfer Kinetics and Thermodynamics for –OH Type Proton Donor Molecules Zong-Ying Liu, Jiun-Wei Hu, Chi-Lin Chen, Yi-An Chen, Kew-Yu Chen, and Pi-Tai Chou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07433 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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The Journal of Physical Chemistry

Correlation among Hydrogen Bond, Excited-State Intramolecular Proton-Transfer Kinetics and Thermodynamics for –OH Type Proton Donor Molecules Zong-Ying Liu, Jiun-Wei Hu, Chi-Lin Chen, Yi-An Chen, Kew-Yu Chen,*, Pi-Tai Chou,*,

Department of Chemistry, National Taiwan University, Taipei, 10617 Taiwan, R.O.C.

Department of Chemical Engineering, Feng Chia University, Taichung, 40724 Taiwan, R.O.C.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (P.-T. Chou) *Email: [email protected] (K.-Y. Chen)

ABSTRACT: A series of new molecules bearing alkyl-substitutes on the parent molecule 1hydroxy-11H-benzo[b]fluoren-11-one (HBF) have been designed and synthesized, which possess an intramolecular hydrogen bond (H-bond) between -OH proton donor and carbonyl proton acceptor. All studied molecules present an equilibrium type of excited-state intramolecular proton transfer (ESIPT) at 298 K. The alkyl- substitutions at various positions

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subtly alter the intramolecular H-bond strength, which then fine-tune the excited-state equilibrium and hence thermodynamics between normal and tautomer species. These, in combination with finite rates of ESIPT resolved by femtosecond fluorescence up-conversion techniques, lead to the establishment of an empirical relationship among H-bond strengths, ESIPT kinetics and thermodynamics for HBF series of molecules, in which the stronger H-bond is, the faster and more exergonic ESIPT is. The intensity ratio for normal versus proton-transfer tautomer emission can be systematically tuned, demonstrating the harness of –OH type ESIPT reaction via facile alkyl- substituent perturbation.

INTRODUCTION

Excited-state intramolecular proton transfer (ESIPT) has drawn many researchers’ attention due to its fundamental importance.1-7 ESIPT is a phenomenon of transferring a proton from proton-donor to proton-acceptor through the existing hydrogen bond (H-bond). Owing to the dramatic changes in electronic configuration, the proton-transfer tautomer exhibits an unusually large Stokes shifted emission, which makes ESIPT molecules promising in various applications, such as organic light-emitting diode,8-14 lasing materials,14-16 UV photostabilizer,17-19 chemosensors20-22 and photochemical reaction.23-29 One of recent interests is the white-light generation accessed by a single type ESIPT molecule.9, 12 This, in theory, requires equilibrium between excited normal and proton-transfer tautomer states, such that both normal and tautomer emissions can be harvested, achieving panchromatic emission. However, an experimental challenge lies in that most of the ESIPT molecules possess strong O-H type H-bond, for which ESIPT is nearly barrierless and highly exergonic, giving solely a proton-transfer tautomer emission.30-33 Therefore, systematically harnessing O-H type ESIPT in both thermodynamics and


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dynamics is challenging. In this regard, it should be noted that a number of ESIPT coupled charge transfer (CT) molecules did show environmentally tuning thermodynamics and dynamics for ESIPT. Because of large changes of dipole moment amid CT coupled ESIPT, solvent polarity relaxation plays a key role, which channels into the proton transfer dynamics as well as alters the thermodynamics.34-40 In this case the solvent polarity serves as an external, not an internal tuning factor. From the fundamental viewpoint, one of the intrinsic factors to tune ESIPT parameters lies in the H-bonding strength in terms of distance and/or angle. In this regard, recent studies of N-H proton-donor type ESIPT have gained much insight into structure-ESIPT relationship.41-47 Different from the O-H type proton donor for which further chemical modification on the –OH group is not possible, NR-H ESIPT systems provide an opportunity for varying -R such that the acidity of N-H proton can be fine-tuned to harness the H-bonding strength. As a result, recent advance has shown remarkable correlation among hydrogen-bonding strength, ESIPT kinetics and thermodynamics, in which the stronger H-bond is, the faster and more exergonic the N-H ESIPT is.46 This empirical rule, on the one hand, establishes the relationship between kinetics and thermodynamics, which is generally irrelevant in chemical reactions involving bond rearrangement. On the other hand, from the application point of view, it can systematically harness the relative energy between normal and tautomer excited states and hence the exploitation of ratiometric emission intensity for versatile applications.48-52 Knowing that majority of ESIPT molecules possess –OH type of H-bonds, one important issue is thus to probe if the -OH ESIPT pattern follows a similar H-bonding strength-kineticsthermodynamics relationship established in the NR-H type proton-donor ESIPT. This is a challenging task because of the aforementioned unresolvable ultrafast –OH ESIPT dynamics in


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common and lack of freedom of modification at the –OH site. We propose that this obstacle may be circumvented by exploiting a few–OH type ESIPT molecules that exhibit equilibrium, kinetically resolvable ESIPT due to their relatively weak H-bonds.12, 53-57 Therefore, adding a small perturbation such as facile alkyl substitution in these molecules may fine-tune the H-bond strength and equilibrium and hence thermodynamics, while the ESIPT kinetics are still resolvable. As a result, pertinent ESIPT parameters may be accessed experimentally to probe the correlation among hydrogen bond, ESIPT kinetics and thermodynamics for theses –OH type proton donor molecules. In this study, standing on the –OH proton donor molecule 1-hydroxy-11H-benzo[b]fluoren-11one (HBF), which was known to undergo equilibrium type of ESIPT with resolvable reaction dynamics,12 we then introduced a small perturbation by the insertion of a modest electrondonating t-butyl substituent at various positions of the HBF moiety. All structures of HBF derivatives were identified by x-ray crystallography (see supporting information, SI), while the corresponding H-bonding strength was qualitatively accessed via 1H-NMR measurement, together with computational approach. Concomitantly, the steady-state and time-resolved spectroscopies were carried out to provide all necessary kinetic and thermodynamic parameters and to discuss the H-bonding strength-kinetics-thermodynamics relationship established in the NR-H ESIPT systems. Results and corresponding discussion are elaborated below. EXPERIMENTAL METHOD Steady-State and Time-Resolved Fluorescence Spectroscopy. Steady-state absorption and emission measurements were performed by U3310 spectrophotometer (Hitachi) and FLS980 fluorescence spectrometer (Edinburgh), respectively.


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The subnanosecond to nanosecond time-resolved measurements were carried out by a timecorrelated single photon counting (TCSPC) system (OB-900L lifetime spectrometer, Edinburgh) with the excitation light source from second harmonic generation (SHG) of 400 nm of pulseselected tsunami femtosecond laser pulses at 800 nm (Spectra-Physics). The temporal resolution, after removing the instrument broadening partially, is about 20 ps. The ultrafast time-resolved spectroscopic studies were recorded by a FOG100 femtosecond up-conversion system (CDP) pumped by the same femtosecond pulse laser. The femtosecond time-resolved data were fitted to the sum of exponential functions convoluted with the instrument response function (IRF), which is fitted to 150 fs determined by Raman scattering signal. Computational Methodology. All the theoretical calculations were carried out by the Gaussian 09 program. The ground-state geometry optimization for HBF and its derivatives in cyclohexane were performed using density functional theory DFT with M06-2X hybrid function in combination with a polarizable continuum model (PCM). The first excited-state structures and oscillator strength were carried out by time-dependent DFT using the same hybrid function. The 6-31+G(d,p) basis sets were employed for all atoms. RESULTS AND DISCUSSION Scheme 1. The synthetic routes and the structures of tert-butyl substituted HBF derivatives.


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Scheme 1 depicts the chemical structures and the synthetic routes of tert-butyl substituted HBF derivatives. Alkylation of HBF with tert-butyl alcohol and sulfuric acid, detected by 1H NMR in CDCl3, gave a mixture of 2-substituted and 4-substituted regioisomers 2TB-HBF and 4TB-HBF in 4:1 ratio, which were separated by HPLC. Further alkylation of either 2TB-HBF or 4TB-HBF afforded a 2,4-disubstituted product (2,4-DTB-HBF). Alternatively, alkylation of HBF with tert-butyl chloride and aluminum trichloride (AlCl3) changed the regioselectivity to only 8-positions, giving 8TB-HBF in 90% yield. This high regioselectivity is rationalized by the fact that chelation of the carbonyl and phenolic oxygens to the electron-deficient AlCl3 decreases electron density of the phenolic ring, resulting in alkylation at the electron-rich naphthalene ring preferentially. Further alkylation of 8TB-HBF with tert-butyl alcohol and sulfuric acid gave a 2,8-disubstituted product (2,8-DTB-HBF). Molecular structures of all obtained compounds were


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confirmed by single-crystal X-ray diffraction analyses (see Figures S12-16). Detailed synthetic procedures and product characterizations are provided in the experimental section of the supplementary information.

Figure 1. Normalized steady-state absorption spectra (dotted-solid lines, normalized at S1 band) and emission spectra (solid lines) for titled compounds in cyclohexane at 298 K. λex = 400 nm. Figure 1 shows the steady-state absorption and emission spectra of titled compounds measured in cyclohexane at 298 K. Pertinent data are summarized in Table 1. The reference compound HBF shows the lowest lying absorption band at 430 nm with a molar absorptivity (ε) 1300 M1

cm-1, which, supported by the TD-DFT calculated frontier orbitals (see Figure 2 and Table S1),

is attributed to a partial charge transfer band from phenol to the indenone moiety.12 The charge-


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transfer character increases acidity and basicity of proton donor (–OH) and acceptor (–C=O), respectively, rendering the driving force for ESIPT. These new alkylated HBF derivatives exhibit similar but slightly red-shifted absorption spectra relatively to that of HBF. Computational results indicate that electron density of HOMO is distributed at C2, C4 and C8 positions (see Figure 2). Therefore, adding electron-donating t-butyl to these positions should raise HOMO energy with negligible perturbation onto LUMO, rationalizing the absorption spectral red shift for these derivatives.

Figure 2. Computed HOMO (orange area) and LUMO (blue area) for normal form of 4TB-HBF involved in the first singlet excitation. The numbering in black and red is for carbon and oxygen atoms, respectively. Upon excitation (e.g. 400 nm, see Figure 1) all HBF derivatives in this study exhibit dual emission that can be well resolved with band maxima at ~450 nm and ~640 nm. Slight difference in peak wavelength among them is attributable to the small perturbation of t-butyl substituent in various numbers and at different positions (vide supra). For all t-butyl derivatives, monitoring at different emission bands gives the identical excitation spectra that are also similar to the absorption profiles (see Figure S17), indicating that these two emission bands originate from the same ground state. Rationally, the short-wavelength emission band is ascribed to normal


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emission (N*) while the other with anomalously large Stokes shift is assigned to the protontransfer tautomer emission (T*).

Absorption S0→S1

8TB-HBF

HBF

λexp/nm (ε/M-1cm1 )

λcalc/nm (f)a

437 (2000)

358 (0.1141)

432 (1300)

2,8-DTBHBF

447 (2600)

2TB-HBF

441 (2200)

4TB-HBF

2,4-DTBHBF

Emission S1→S0

435 (1800)

450 (4000)

352 (0.1033)

365 (0.1236)

360 (0.1164)

354 (0.1463)

363 (0.1634)

λexp/nma

λcalc/nm (f)a

Q.Y.a

τexp/ps (pre-exp. factor)b

N:482

N:452 (0.0924)

T:626

T:525 (0.3503)

600 nm: 15 (-0.31), 1012 (0.69)c

N:449

N:447 (0.0715)

480 nm: 15.5 (0.66), 530 (0.34)c

T:622

T:516 (0.3749)

N:500

N:463 (0.1065)

T:639

T:553 (0.3363)

N:491

N:459 (0.09)

480 nm: 20 (0.4), 1012 (0.6)c 0.032

0.015

600 nm: 15(-0.4), 530 (0.6)c 480 nm: 1.9 (0.57), 485 (0.43)c 0.014 600 nm: 1.9 (-0.28), 485 (0.72)c

0.01 T:637

T:545 (0.3402)

N:483

N:458 (0.0834)

T:630

T:526 (0.3947)

N:499

N:470 (0.1077)

T:646

T: 546 (0.3819)

480 nm: 1.8 (0.77), 350 (0.23)c 600 nm: 1.8 (-0.3), 350 (0.7)c 480 nm: 1.4 (0.84), 370 (0.16)c

0.011 600 nm: 1.4 (-0.26), 370 (0.74)c 480 nm: 0.4 (0.9), 325 (0.1)c 0.011

600 nm: 0.4 (-0.12), 325(0.88)c

a

N denotes as normal, T denotes as tautomer, f means oscillator strength, Q.Y. represents quantum yield. b Emission lifetime (τexp) recorded by femtosecond fluorescence up-conversion. c Population decay time constant determined from subnanosecond Time Correlated Single Photon Counting (TCSPC) was applied for the fitting.

Table 1. Photophysical properties of titled compounds in cyclohexane at 298 K.

Despite the sameness in the absorption and emission peak wavelengths, the intensity ratio for normal versus tautomer emission in each derivative shows remarkable difference. Using HBF as a reference for the N*/T* emission intensity ratio, adding a tert-butyl group on C8, 8TB-HBF


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increases the N*/T* ratio, while the t-butyl addition on C2 and C4 or on both, forming 2TBHBF, 4TB-HBF and 2,4-DTB-HBF, respectively, decreases the N*/T* ratio. Interestingly, 2,8DTB-HBF, for which the tert-butyl group is added at both C2 and C8, gives rise to a similar N*/T* ratio to that of HBF. These results imply subtle changes of ESIPT kinetics and/or thermodynamics fine-tuned by t-butyl substituents, as supported by the following time-resolved measurements.

Figure 3. Fluorescence up conversion decay curve recorded for 4TB-HBF in cyclohexane. The data points (circle and triangle) were recorded by monitoring at the corresponding normal and tautomer emission. Solid lines represent the best exponential fitting. Inset: the enlargement of kinetic traces from 0 to 30 ps. To gain further insight into the dynamics of ESIPT, time-resolved fluorescence spectroscopy in femtosecond to nanosecond ranges was carried out by fluorescence up-conversion and timecorrelated single photon counting (TCSPC) techniques, respectively (see supporting information for detail). Table 1 summarizes pertinent time-resolved data. Figure 3 shows a typical fluorescence up-conversion signal for 4TB-HBF in cyclohexane monitored at different emission


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regions. Upon monitoring at the normal emission band (e.g., 480 nm) the corresponding relaxation dynamics of 4TB-HBF was well fitted by biexponential decay, consisting of a short component fitted to be 1.4 ± 0.1 ps and a long component that revealed slight decay within the 400 ps acquisition window. The long decay component was further resolved to be 370 ps using TCSPC. On the other hand, the tautomer emission monitored at e.g. 600 nm comprises a 1.4 ± 0.2 ps rise time component and a long population decay component of 370 ps. Within experimental error, the fast decay component of normal emission matches the rise time of tautomer emission very well. This, in combination with the identical population decay time for both emission bands, can be well described by a precursor (normal)-successor (tautomer) type of ESIPT under a fast excited-state pre-equilibrium (see Scheme 2). Similarly to HBF12 and 4TBHBF, the rest HBF derivatives all undergo a pre-equilibrium type of ESIPT, i.e. a biexponential decay for the normal emission, in which the fast decay corresponds to the rise of the tautomer, while both normal and tautomer emission exhibit the identical population decay (see Table 1 and Figure S18). Scheme 2 also gives the relaxation kinetics model for both time resolved [N*]t and [T*]t in terms of fluorescence intensity, which can be expressed as equations 1 and 2 (for detailed derivation, see SI). [ ∗ ] [ ] = ⋅ − X ⋅ e + X − ⋅ e (1) − ∗

⋅ [ ∗ ] [ ] = ⋅ e − e (2) − ∗

where ≃

∗ + ∗ ⋅ 1 + !"

!"

; = + (3)


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Table 2. Observed 1H NMR chemical shift and O1-O2 distances in crystal, calculated O1H-O2 distances in ground state and the first excited-state, experimental energy differences between normal and tautomer in excited states (∆E*) and rate constant (kpt) of forward ESIPT for the titled compounds. 1

H NMR

O1-O2 distances

O1H-O2 distances

O1H-O2 distances

(S0) (Å)

(S1) (Å)

∆E* (kcal/mol)

%&' (s-1)

1.968

0.25

1.98×1010

2.081

1.963

-0.40

4.27×1010

2.824

1.993

1.878

-0.16

2.99×1011

9.5

2.842

1.991

1.872

-0.73

4.31×1011

4TB-HBF

9.7

2.783

1.970

1.856

-0.99

6.02×1011

2,4-DTBHBF

10.5

2.713

1.900

1.769

-1.28

1.79×1012

(hydroxyl H) (ppm)

(X-ray) (Å)

8TB-HBF

8.6

2.919

2.078

HBF

8.6

2.879

2,8-DTBHBF

9.5

2TB-HBF

As a result, the pre-exponential factors of the fast and slow decay components of [ ∗ ] , denoted as ( and ( , respectively, can be expressed as ( ≃

(4) +

( ≃

(5) +

where ( and ( can be obtained from the biexponential fitting of [ ∗ ] using extrapolation to ) = 0 experimentally. Accordingly, the equilibrium constant

!" + ⁄ -

can be obtained by

the ratio of (4) versus (5). The energy differences Δ/ ∗ between two species, defined as Δ/ ∗ = ∗ ∗ /0123!4 − /524306 , can thus be deduced from 7/ ∗ = −89:

!" ,

the values of which are listed

in Table 2. Also, the observed rate of fast decay component is equivalent to + (see eq.


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1), which, together with

!" +=

⁄ -, can deduce individual forward ( ) and backward

( ) proton-transfer rate constant. As a result, and values are listed in Table 2. Careful analyses of Table 1 and Table 2 indicate that increases in the order of 8TB-HBF (51 ps)-1 < HBF (23 ps)-1 < 2,8-DTB-HBF (3.3 ps)-1 < 2TB-HBF (2.3 ps)-1 < 4TB-HBF (1.7 ps)-1 < 2,4-DTB-HBF (0.56 ps)-1. Scheme 2. The proposed ESIPT model for HBF derivatives.

In another approach, we calculated the H-bond distance by accessing the O1H…O2 distance (see Figure 2 for the numbering of oxygen atom) in both ground (S0) and the first excited (S1) states using DFT and TD-DFT approaches, respectively. The resulting data is listed in Table 2. In comparison to HBF, the t-butyl substituted derivatives at C2, C4 or both C2/C4 show decrease of O1H…O2 distance, implying an increase of H-bonding strength in both S0 and S1 states. As for the ground state, the results correlate well with the 1H-NMR measurement (in CDCl3, see Table 2), in which the O-H protons for C2, C4 and C2/C4 substituted HBFs are all further downfield shifted than that of HBF. Also, analyzed by the X-ray crystallography (see Table 2), the O1…O2 distances for C2, C4 and C2/C4 substituted HBFs are all shorter than that


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of HBF. If one neglects the perturbation of lattice energy, the results of calculation and experimental measurements are mutually consistent for the ground state. Accordingly, the O-H chemical shift of 2,4-DTB-HBF (10.5 ppm) exhibits the most downfield, and the O-O distance (2.713 Ǻ, measured by the X-ray crystallography) is the shortest among the titled compounds. On the other hand, calculation also shows that 8TB-HBF with t-butyl substitution at C8 gives the longest O1H…O2 distance hence the weakest H-bond in both S0 and S1 states (see Table 2). As for the ground state, the result is also confirmed by the least downfield of the –OH 1H-NMR peak (8.6 ppm) and the longest O-O distance (2.919 Ǻ, measured by the X-ray crystallography) for 8TB-HBF among the titled HBF derivatives. One may expect that adding t-butyl electron-donating group at C2, C4 or both C2/C4 positions should decrease the acidity of the –OH proton, resulting in the decrease of the proton donating ability and hence weakening the O1H…O2 H-bonding strength. The apparent opposite result can be rationalized by the canonically drawn resonant/inductive effect by t-butyl electron-donating group at C2, C4 (or both C2/C4), which increases the basicity of the carbonyl oxygen (O2, see Figure 2). The net results are the increase of the H-bond strength. It is the balance between the acidity (-O1H) and basicity (-O2) that makes subtle tuning the H-bond and thus the ESIPT equilibrium in the excited state.


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Figure 4. (a)The plot of log of ESIPT versus H-bond O1H-O2 distances calculated in S0 and S1 states. (b) The plot of energy difference between T* and N*, ∆E* versus H-bond O1H-O2 distances calculated in S0 and S1 states. (c) The plot of log versus ∆E*. For clarity, the studied molecules are numbered as 1 (2,4-DTB-HBF), 2 (4TB-HBF), 3 (2TB-HBF), 4 (2,8DTB-HBF), 5 (HBF), 6 (8TB-HBF). With all necessary data provided, we then make attempts to explore the correlation among Hbonding strength, ESIPT kinetics and thermodynamics. Figure 4a shows the logarithm plot of ESIPT rate as a function of the O1H…O2 H-bonding distance that is calculated in both S0 and S1 states. Note that the logarithm plot of is to simulate the barrier of the ESIPT. Due to the thermal expansion and contraction of the sample disk, it is unfortunately not feasible for us to carry out the femtosecond fluorescence up-conversion in a wide temperature range in order to deduce the reaction barrier. Nevertheless, within the experimental uncertainty for the rate of , the tendency of increasing log upon decreasing the O1H…O2 H-bonding distance is obvious. In other words, the stronger the hydrogen bond is, the greater the rate constant of proton-transfer


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would be. In a qualitative manner, the result implies that stronger H-bond requires less adjustment of the O1H…O2 geometry regarding either distance or angle amid ESIPT and hence the smaller reaction barrier for ESIPT. Figure 4b shows the plot of experimentally deduced ∆E* versus the O1H…O2 H-bonding distance calculated at either S0 or S1 state. The plot reveals a trend in that ∆E* becomes more exergonic (decrease in energy) as the H-bond distance decreases, i.e. as the H-bonding strength increases. Chemically, this correlation is explainable by the effect of π electron delocalization. In the same class (moiety) of ESIPT molecules, the stronger intramolecular H-bond induces more π electron delocalization, stabilizing the tautomer T* state and hence the higher exergonicity of ∆E*.46 Combining Figures 4a and 4b leads to the establishment of the relationship between ∆E* and logarithm of ESIPT rate depicted in Figure 4c, which clearly reveals the correlation that the more exergonic the reaction is, the faster rate and smaller barrier the ESIPT shows. As a result, Figures 4a-c clearly shows the hallmark regarding the correlation among intramolecular H-bonding, ESIPT kinetics and thermodynamics for the –OH proton donor molecules.


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Figure 5. Normalized solid state (solid powder) emission spectra of titled compounds at 298 K. λex=400 nm. Finally, in the solid state, it is worth of note that all studied HBF derivatives show a similar trend of t-butyl substituent dependent ratiometric emission (see Figure 5 and inset for emission color) to that in solution. The result demonstrates that a facile chemical modification of HBF is able to fine-tune the excited-state equilibrium between N* and T* states, making possible the harness of dual emission for the ubiquitous O-H proton donor ESIPT systems in lighting applications. CONCLUSION In summary, we emphasize here that most –OH type ESIPT molecules studied so far, due to their intrinsic strong intramolecular H-bond, are subject to barrierless and ultrafast ESIPT. This, together with the lack of substitution at the –OH site (cf. the NR-H group), makes systematic


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studies of correlation among intramolecular H-bonding, ESIPT kinetics and thermodynamics difficult. In this study, based on subtle t-butyl substitution to the parent molecule HBF that exhibits equilibrium type ESIPT, fine-tuning the H-bond strength becomes possible, which leads us to be able to probe the trend of ESIPT rates and reactant-product energetics as a function of H-bond distance. The correlations show several remarks for the studied HBF derivatives: In the same ESIPT core moiety, the stronger H-bonding strength gives faster rate of ESIPT, which points to smaller reaction barrier. Also the stronger H-bonding strength renders more exergonic ESIPT. The combination of these indicates that the more exergonic the ESIPT is, the faster rate, i.e., the smaller barrier, the reaction shows. It is thus of great interest to know if similar correlation can be applied to other –OH proton donor molecules such that an empirical relationship can be set up for the ubiquitous –OH proton donor ESIPT systems. ASSOCIATED CONTENT Supporting Information. Additional NMR, crystal structure of compounds, computational results, and supplementary photophysics are provided. This

material is available free of charge via the Internet at

http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (P.-T. Chou) *Email: [email protected] (K.-Y. Chen) Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT We appreciate the ministry of science and technology, Taiwan, for generous supports. REFERENCES (1) Mcmorrow, D.; Kasha, M. Intramolecular Excited-State Proton-Transfer in 3Hydroxyflavone - Hydrogen-Bonding Solvent Perturbations. J. Phys. Chem. 1984, 88, 2235-2243. (2) Seo, J.; Kim, S.; Park, S. Y. Strong Solvatochromic Fluorescence from the Intramolecular Charge-Transfer State Created by Excited-State Intramolecular Proton Transfer. J. Am. Chem. Soc. 2004, 126, 11154-11155. (3) Lochbrunner, S.; Wurzer, A. J.; Riedle, E. Microscopic Mechanism of Ultrafast ExcitedState

Intramolecular

Proton

Transfer:

A

30-fs

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of

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Correlation among Hydrogen Bond, Excited-State Intramolecular

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