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Proton Transfer Dynamics of 4'-N,N-dimethylamino-3-hydroxyflavone Observed in Hydrogen-bonding Solvents and Aqueous Micelles Deborin Ghosh, Shaikh Batuta, Sreeparna Das, Naznin Ara Begum, and Debabrata Mandal J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 01 Apr 2015 Downloaded from http://pubs.acs.org on April 2, 2015
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Proton Transfer Dynamics of 4-N,N-Dimethylamino-3-Hydroxyflavone Observed in Hydrogen-Bonding Solvents and Aqueous Micelles
Deborin Ghosha, Shaikh Batutab, Sreeparna Dasb, Naznin Ara Begumb, Debabrata Mandala* a
Department of Chemistry, University College of Science & Technology, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India E-Mail:
[email protected] b
Bio-Organic Chemistry Lab, Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, India,
Abstract
Photophysical studies on the 4-N,N-Dimethylamino-3-hydroxyflavone fluorophore were performed in hydrogen-bonding solvents. Both in hydrogen-bonding acids and bases, clear evidence of excited state intramolecular proton transfer (ESIPT) emerged from steady-state and time-resolved spectroscopies. The same was also observed for the fluorophores residing in the hydrophilic shell region of aqueous micelles, where they come into close contact with water molecules at the micelle-water interface. Slow ~100 ps ESIPT time-constants were determined in these systems that correlated well with solvation dynamics. The slow ESIPT time-constants are attributed to activated barrier crossing from the solvent-relaxed enol form to tautomer form in the excited state energy surface of the flavone. In contrast to the barrier-less ESIPT occurring in early (< 1 ps) time-scales, this activated proton-transfer event necessarily requires extensive reorganization of flavone···solvent intermolecular hydrogen bonds, a process heavily modulated by the relatively slower dynamics of solvent relaxation around the excited fluorophore.
Keywords:
ESIPT, solvent relaxation, slow dynamics
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1.
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Introduction: The 3-hydroxyflavone (3HF) molecule is widely regarded as a prototype for observing
excited state intramolecular proton transfer (ESIPT). Following S0S1(ππ*) excitation, 3HF and its derivatives undergo ESIPT from the initially excited enol-like E* species to the keto-like tautomer T*,often producing dual fluorescence
1-9
. For 3HF in aprotic solvents, ESIPT is
essentially barrier-less, occurring in femtosecond time-scales and leading to drastic suppression of the E* fluorescence
1-3
. However, in protic environments, specific solvation and
intermolecular H-bonding may impose a large activation barrier across the ESIPT pathway4-9, allowing E* fluorescence to appear. On the other hand, the 4-N,N-dialkylamino substituted 3-hydroxyflavones exhibit prominent E* fluorescence even in aprotic solvents10-19. Moreover, the E* fluorescence is highly sensitive to solvent polarity: it becomes more intense and red-shifted in more polar solvents 10-25, in contrast to the T* fluorescence which remains almost un-shifted. Introduction of the strong electron-donating N,N-dialkylamino group severely perturbs the -electron distribution of the molecule, conferring a prominent charge-transfer character to the initially excited E* species. Subsequent ESIPT from 3-hydroxy to 4-carbonyl causes a reverse displacement of charge, thus diminishing the polar character of the T* species. In fact, dipole moments of the T* species and ground-state enol are found to be nearly the same, while that of the excited-state enol E* is ~10 D higher
19,26-28
. This stark difference in dipole moments necessarily introduces a solvent
reorganization barrier between the E* and T* species in polar solvents, making the overall ESIPT an activated process 19-22. The nature of solvent also leaves a deep impact on the ESIPT dynamics of 4-N,Ndialkylamino-3-hydroxyflavones. The shortest ESIPT time-constants ( -1,
where Li(,t=t’) is the TRES for the ith species (enol or tautomer) at the time-delay t=t', hi(t') is its relative amplitude, νi,max(t') its peak position, γi(t') its asymmetric parameter, and Δi(t') its bandwidth parameter, all at time-delay t=t'. All these parameters were assumed to vary with the time-delay. The fitting curves in Fig. 4 show that the double-lognormal fit works well with the spectra in both methanol and formamide. Next, for a given spectrum, the enol and tautomer components were re-constructed using the parameters associated with Lenol(,t=t’) and Ltauto(,t=t’), respectively. These components are also appended to Fig. 4. A careful inspection of
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the spectra reveal that the tautomer emission remains almost un-shifted, while the enol emission undergoes a very small time-dependent Stokes shift (TDSS) which rapidly dies out in~250 ps. The difference in enol and tautomer behavior is obviously due to the large difference of dipole moment between E* and T*: the more polar E* species is stabilized by solvation while the less polar T* species is immune to it. From the peak emission wavenumber at different time-delays max(t), the solvent relaxation function for DMA3HF enol in methanol and formamide are calculated as: C(t) = [max(t) - (max(t=)]/ [max(t=0) - (max(t=)]
(4)
The C(t) vs time-delay profiles are plotted in Fig. 5a, and fitted to a single-exponential decay with decay time-constants of 7010 ps. Since the solvation time-constants obtained thus are of the same order as our instrumental time-resolution, and due to the rather large relative error in these obtained values, it is difficult to distinguish between the solvation dynamics for methanol and formamide under our experimental conditions. Meanwhile, the TDSS, given by max(t=0) - (max(t=), is found to be ~100 cm-1 in both solvents. At first sight, this miniscule TDSS looks quite puzzling. However, this may be only a fraction of the total TDSS, since the max(t=0) term in Eq.(4) has been calculated on the basis of our picosecond measurements, while solvent relaxation in polar solvents becomes operative right from sub-picosecond time-delays42. Fee and Maroncelli46 had developed a method to estimate the actual max(t=0) in a polar solvent, i.e., the emission peak prior to any solvent relaxation, with the help of the absorption and fluorescence spectra of the fluorophore recorded in a non-polar solvent: flmid,polar(t=0) = abmid,polar– [abmid,nonpolar- flmid,nonpolar]
(5)
where the subscripts “abmid” and “flmid” refer to the midpoint of the absorption and fluorescence spectra, respectively. Using the flmid,polar(t=0) so calculated, fluorescencemax(t=0) can be further obtained46. This method has been successfully implemented in several works on ultrafast solvation dynamics47-50. In the present case, the absorption and fluorescence spectra of DMA3HF in pure alkanes are reported, where the E* emission peak occurs at 440 nm11. Thus, using the spectral data of DMA3HF, its actual fluorescence max(t=0) in the H-bonding solvents methanol and formamide were found to be ~21000 cm-1. By comparison, the max(t=) in these solvents appear in the range of 18800 – 18950 cm-1. Hence, the total TDSS is >2000 cm-1 in both
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solvents. This implies that ~95% of the total TDSS has escaped detection in our work, because it got completed at a much shorter time-scale than our instrumental time-resolution. Out of the entire solvent relaxation event, we could capture only the very minor, ~100 cm-1 tail, associated with the 7010 ps time-constant. Intensity ratio of tautomer peak to enol peak in the same time-window are also plotted in Fig. 5a: the ratio is found to increase initially but quickly saturates just beyond 100 ps. This behavior clearly indicates that ESIPT dynamics of DMA3HF in the H-bonding solvents faithfully reflects the dynamics of solvent relaxation in these solvents. As mentioned above, slow 10-100 ps ESIPT time-constants are not entirely unknown for DMA3HF in polar solvents like acetonitrile, DMF, ethyl acetate and secondary alcohols 23,25
19, 22,
. Chou et al. have proposed an elegant mechanism to explain the solvent dependence of
ESIPT time-constants, in terms of a potential energy surface expressed as a function of two independent coordinates: one for solvent relaxtion and the other for proton transfer19,
51, 52
.A
scheme based on their mechanism is depicted in Fig. 6, where the overall dynamics has been projected separately along solvation (Fig. 6a) and proton transfer coordinates (Fig. 6b). In Fig. 6a, ultra-fast photo-excitation of the ground-state enol E launches it into Ei*, the initial chargetransferred enol on the excited-state surface. This step is too rapid for the solvent molecules to respond, so that they remain frozen in the same configuration adapted to ground-state DMA3HF, which naturally favors E-state over Ei*-state. Here the scheme makes a critical assumption: that although the dipole moment of Ei* is extremely larger than that of E, those of T* and E are not very different19. This implies that the solvent configuration immediately after the excitation event continues to favour T* over Ei*-state, depicted as solvent configuration "Ci" in Fig. 6b. Under this configuration, there is hardly any barrier for proton-transfer, so that Ei* is free to undergo ultrafast barrierless ESIPT into T* along the proton-transfer coordinate. However, at the same time, the solvent configuration begins to adjust to the newly formed Ei*-state, causing it to relax into the Esolv* state following the solvation coordinate (Fig. 6a). The solvent configuration corresponding to Esolv* is described as "C#" in Fig. 6b. Evidently, a nascent barrier has begun to form along the proton-transfer channel. Under this configuration, the enol and tautomer have nearly the same energy, and reversible ESIPT occurs via activated barrier crossing. The solvent relaxation may proceed even further to generate configuration "CE*" of Fig. 6b, where Esolv* is energetically more favorable than T*.
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Now, due to the much weaker dipole moment of T* compared to E*, the former undergoes negligible solvent relaxation. Hence, in all situations, the Esolv*T* barrier height is determined primarily by the energy-level of the Esolv* basin, which is deeper in more polar solvents
51,52
.In solvents which behave as H-bond acids or bases (H-X or Y, as in Fig. 1b), this
activated barrier crossing must be strongly coupled with breaking and realignment of DMA3HFsolvent intermolecular H-bonds. Hence its dynamics should necessarily reflect that solvent relaxation/reorganization around the fluorophore. According to literature values42, solvation dynamics in all the four chosen solvents spans time-scales ranging from ~10 fs to 100 ps, with small but non-negligible contribution from time-constants of several tens of picoseconds 42
. Under the present conditions of study, it is these “slower” solvation components that play a
decisive role in controlling the activated ESIPT rates. Finally, there might be a sizeable number of DMA3HF molecules which do not participate in any intermolecular H-bonding interactions with the solvent. For these, the ESIPT is decoupled from reorganization dynamics of DMA3HFsolvent intermolecular H-bonds. Hence, ESIPT for these molecules will proceed in a much faster time-scale of
