Femtosecond dynamics on excited-state proton/charge-transfer reaction in 4'-N,N-diethylamino-3-hydroxyflavone. The role of dipolar vectors in constructing a rational mechanism

The excitation behaviors for 4'-N,N-diethylamino-3-hydroxyflavone (Ia) have been investigated via femtosecond fluorescence upconversion approaches to gain detailed insights into the mechanism of the proton/charge-transfer coupling reaction. In polar solvents such as CH2Cl2 and CH3CN, in addition to a slow, solvent-polarity-dependent rate (a few tens of picoseconds(-1)) of excited-state intramolecular proton transfer (ESIPT) reported previously, early femtosecond relaxation dynamics clearly reveal that the proton-transfer tautomer emission consists of a rise component of a few hundred femtoseconds. The temporal spectral evolution at the time domain of zero to a few hundred femtoseconds further resolves two distinct emission bands consisting of a proton-transfer tautomer emission and a time-dependent Stokes shifted emission. The results, in combination with ab initio calculations on the dipolar vectors for normal and tautomer species, lead us to unveil the importance of the relationship of the dipolar vectors among various states, and hence the corresponding solvation energetics in the overall ESIPT reaction. We conclude a similar dipolar character between ground-state normal (N) and excited proton-transfer tautomer (T*) species, whereas due to the excited-state intramolecular charge transfer (ESICT), the normal excited state (N*) possesses a large dipolar change with respect to N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvation relaxation process. After reaching the solvent equilibration, there exists an equilibrium between N* and T* states in, for example, CH3CN. Due to the greatly different equilibrium polarization between N* and T*, both forward and reversed ESIPT dynamics are associated with a solvent-induced barrier. The latter viewpoint of the equilibrium type of ESIPT in Ia is in agreement with the previous reports based on steady-state, picosecond, and femtosecond dynamic approaches.     

Origin of the nonexponential dynamics of excited-state proton transfer in wt-green fluorescent protein

We used an inhomogeneous excited-state proton-transfer kinetics model to explain the origin of the non-exponential time-resolved emission of the A-band of wt-green fluorescence protein. The calculated fit is rather good for both H 2O and D 2O samples in a wide temperature range of 80-229 K. We attribute the inhomogeneous kinetics to the distance dependence of the excited-state proton-transfer rate between the proton donor (the hydroxyl group of the chromophore) and the oxygen of a nearby water molecule.     

Rotamerism, tautomerism, and excited-state intramolecular proton transfer in 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazoles: novel benzimidazoles undergoing excited-state intramolecular coupled proton and charge transfer

The solvent and temperature dependence of the phototautomerization of 1-methyl-2-(2'-hydroxyphenyl)benzimidazole (4) and the novel compounds 2-(4'-amino-2'-hydroxyphenyl)benzimidazole (1), 2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (2), and 1-methyl-2-(4'-N,N-diethylamino-2'-hydroxyphenyl)benzimidazole (3), together with the ground-state rotamerism and tautomerism of these new compounds, have been studied by UV-vis absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. A solvent-modulated rotameric and tautomeric equilibrium is observed in the ground state for 1, 2, and 3. In cyclohexane, these compounds mainly exist as a planar syn normal form, with the hydroxyl group hydrogen-bonded to the benzimidazole N3. In ethanol, the syn form is in equilibrium with its planar anti rotamer (for 1 and 2), with the phenyl ring rotated 180 degrees about the C2-C1' bond and with a nonplanar rotamer for compound 3. In aqueous solution, a tautomeric equilibrium is established between the anti normal form (or the nonplanar rotamer for 3) and the tautomer (with the hydroxyl proton transferred to the benzimidazole N3). The syn normal form of these compounds undergoes in all the solvents an excited-state intramolecular proton-transfer process from the hydroxyl group to the benzimidazole N3 to yield the excited tautomer. The tautomer fluorescence quantum yield of 2, 3, and 4 shows a temperature-, polarity-, and viscosity-dependent radiationless deactivation, connected with a large-amplitude conformational motion. We conclude that this excited-state conformational change experienced by the tautomer is associated with an intramolecular charge transfer from the deprotonated dialkylaminophenol or phenol (donor) to the protonated benzimidazole (acceptor), affording a nonfluorescent charge-transfer tautomer. Therefore, these compounds undergo an excited-state intramolecular coupled proton- and charge-transfer process.     

Intramolecular Proton Transfer in 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine: Evidence for Tautomer in the Ground State

The intramolecular proton transfer in a newly synthesized molecule, 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine (HPOP) is studied using UV‐visible absorption, fluorescence emission, fluorescence excitation and time‐resolved fluorescence spectroscopy. In the ground state, the molecule exists as cis‐ and trans‐enol in all the solvents. However, in dioxane, alcohols, acetonitrile, dimethylformamide and dimethylsulfoxide the keto tautomer is also observed in the ground state. Dual fluorescence is observed in HPOP where the large Stoke shifted emission is due to emission from the excited‐state intramolecular proton transfer product, whereas the other emission is the normal emission from enol form. The fluorescence (both normal and tautomer emission) of HPOP is less than those of corresponding benzoxazole and imidazopyridine derivatives. This reveals that the nonradiative decay becomes more efficient upon substitution of electronegative atom on the charge acceptor group. The pH studies substantiate the conclusion that (unlike in its imidazole analog) the third ground state species is the keto tautomer and not the monoanion. The effect of temperature on cis‐enol‐trans‐enol‐keto equilibrium and the nonradiative deactivation from the excited state are also investigated.     

Modulation of intramolecular proton transfer by electronic excitation and environment: 2-Pyridone as a case study

The kinetics and thermodynamics of lactam/lactim tautomerization in 2-pyridone have been investigated, with special attention to direct and assisted proton transfer mechanisms in the ground and first excited electronic state. Specific interactions with a single water molecule strongly enhance the reaction rate and shift the equilibrium toward the lactam form. The effect of bulk solvent is comparatively negligible, although the lactam form is further stabilized. Electron excitation strongly destabilizes the saddle point for proton transfer and, especially, the lactim form with respect to the lactam species. As a consequence, the direct reaction barrier is increased, but the reverse barrier is lowered. Nonpotential energy effects are relatively small and do not modify the aforementioned general trends. © 1994 by John Wiley & Sons, Inc.     

Excited-State Double Proton Transfer in 1-Azacarbazole Hydrogen Bonded Complexes

Excited-state double proton transfer (ESDPT) has been studied in a variety of 1-azacarbazole (1AC) hosted hydrogen-bonded complexes. In 1 AC/carboxylic acids hydrogen bonded complexes, large association constants of > 1.0 × 10⁴ M^?1 are measured in the ground state and the rate of ESDPT is » 5.0 × 10⁹ s^?1, resulting in a dominant proton-transfer tautomer emission. In several 1 AC/lactam hydrogen bonded complexes, however, spectral and dynamic results show the existence of a fast excited-state equilibrium between normal and proton-transfer tautomer states. The result can be tentatively rationalized by a non-catalytic ESDPT mechanism incorporating tautomerization energy of the guest molecule.     

Excited-state proton transfer through water bridges and structure of hydrogen-bonded complexes in 1H-pyrrolo[3,2-h]quinoline: adiabatic time-dependent density functional theory study

Proton transfer reaction is studied for 1H-pyrrolo[3,2-h]quinoline-water complexes (PQ-(H(2)O)(n), n = 0-2) in the ground and the lowest excited singlet states at the density functional theory (DFT) level. Cyclic hydrogen-bonded complexes are considered, in which water molecules form a bridge connecting the proton donor (pyrrole NH group) and acceptor (quinoline nitrogen) atoms. To understand the effect of the structure and length of water bridges on the excited-state tautomerization in PQ, the potential energy profile of the lowest excited singlet state is calculated adiabatically by the time-dependent DFT (TDDFT) method. The S(0) --> S(1) excitation of PQ is accompanied by significant intramolecular transfer of electron density from the pyrrole ring to the quinoline fragment, so that the acidity of the N-H group and the basicity of the nitrogen atom of the quinoline moiety are increased. These excited-state acid-base changes introduce a driving force for the proton transfer reaction. The adiabatic TDDFT calculations demonstrate, however, that the phototautomerization requires a large activation energy in the isolated PQ molecule due to a high energy barrier separating the normal form and the tautomer. In the 1:1 cyclic PQ-H(2)O complex, the energy barrier is dramatically reduced, so that upon excitation of this complex the tautomerization can occur rapidly in one step as concerted asynchronous movements of the two protons assisted by the water molecule. In the PQ-(H(2)O)(2) solvate two water molecules form a cyclic bridge with sterically strained and unfavorable hydrogen bonds. As a result, some extra activation energy is needed for initiating the proton dislocation along the longer hydrogen-bond network. The full tautomerization in this complex is still possible; however, the cooperative proton transfer is found to be highly asynchronous. Large relaxation and reorganization of the hydrogen-bonded water bridge in PQ-(H(2)O)(2) are required during the proton translocation from the pyrrole NH group to the quinoline nitrogen; this may block the complete tautomerization in this type of solvate.     

Ground- and excited-state double proton transfer in lumichrome/acetic acid system: theoretical and experimental approach

Experimental time-resolved spectral and photon counting kinetic results confirm formation of an isoalloxazinic excited state via excited-state double proton transfer (ESDPT) catalyzed by a carboxylic acid molecule that forms a hydrogen-bond complex with the parent alloxazine molecule. This isoalloxazinic tautomer manifests itself as a distinct long-lived emissive species formed only in such alloxazine derivatives that were not substituted at the N1 nitrogen atom, being a product of the excited-state reaction occurring from the alloxazinic excited state. Theoretical calculations support the idea that the ESDPT occurs by the concerted mechanism. The calculated activation barrier in the excited state is much lower than the same barrier in the ground state and even disappears for the HOMO-1 to LUMO excitation, which explains the fact that the reaction takes place in the excited-state only. The reaction rate estimated from the emission kinetics is ca. 1.4 x 10(8) dm3 mol(-1) s(-1) in ethanolic solutions of lumichrome with added acetic acid.     

6-hydroxyquinoline-N-oxides: a new class of "super" photoacids

N-Oxidation of hydroxyquinolines leads to a dramatic increase in their excited-state acidity. Time-resolved and steady-state emission characterization of 6-hydroxyquinoline-N-oxide and 2-methyl-6-hydroxyquinoline-N-oxide reveals a rich but less complex proton-transfer behavior than that of its parent hydroxyquinoline. The electronic effect of the oxidized heterocyclic nitrogen atom makes the excited state both less basic and more acidic than the parent and adds hydroxyquinoline N-oxides to the class of high-acidity excited-state proton donors in photochemistry and photobiology. Adiabatic photoinduced proton transfer is accompanied by the efficient nonreversible deoxygenation and 1-2 oxygen migration.     

Intramolecular proton-transfer processes starting at higher excited states: a fluorescence study on 2-butylamino-6-methyl-4-nitropyridine N-oxide in nonpolar solutions

This article describes the exceptional photophysics of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M). It is known from the literature that this compound may undergo excited-state intra- or intermolecular proton-transfer reactions. In nonpolar solvents, 2B6M exhibits an unusual fluorescence behavior: there is a substantial difference between the relative band intensities of the excitation and absorption spectra. Furthermore, in emission two bands are observed, and their relative intensities depend on the excitation wavelength, thus violating the Kasha-Vavilov rule. It is the objective of this research to interpret these results. For this purpose, steady-state fluorescence excitation and emission spectra in the liquid state were recorded and quantum yields were determined for the two types of emission. In addition, absorption spectra were measured at room temperature and under low-temperature conditions. Finally, fluorescence lifetimes of the emitting species were determined using the time-correlated single photon counting technique. The results suggest that in the liquid state only one (monomeric) ground state species dominates, which can emit via two different pathways (from the normal and the tautomeric excited state). The excitation spectra point at two different internal proton-transfer processes, one starting at the S1 state and one starting at the S2 state. On the basis of the measured lifetimes and fluorescence quantum yields, a kinetic scheme was completed that can quantitatively explain the observations.     

Inter- and intramolecular excited state proton transfer in 2-hydroxy-9H-carzole-1-carboxylic acid

Absorption, fluorescence and fluorescence excitation spectroscopy and single photon counting time dependence spectrofluorimetry have been used to study the inter- and intramolecular excited state proton transfer (ESIPT) reactions in 2-hydroxy-9H-carbazole-1-carboxylic acid (2-HCA). Except in cyclohexane and water (pH 5) dual fluorescence is observed in rest of the solvents used. Normal Stokes shifted band seems to originate from 2-HCA-1-c and tautomer emission band from the tautomer formed by ESIPT in 2-HCA-1-c followed by structural reorganization. Both these emission band systems originate from the same ground state species. AM1 and CNDO/S-CI calculations have been carried out to establish the identity of the species. Different prototropic equilibria have been determined and discussed.     

Excitation wavelength dependence of the proton-transfer reaction of the green fluorescent protein

Picosecond time-correlated single-photon counting was used to measure the proton-transfer rate of green fluorescent protein (GFP) excited by several wavelengths between 266 and 405 nm. When samples of GFP in water and D2O are excited at short wavelengths, lambda(ex) < 295 nm, the fluorescence properties are largely modified with respect to excitation at a wavelength around 400 nm, the peak of the absorption band of the S0 --> S1 transition of the ROH form of the chromophore. The shorter the excitation wavelength, the longer the decay time of the ROH emission band at 450 nm and the longer the rise time of the RO- emission band at 512 nm. The proton transfer is slower by an order of magnitude and about a factor of 3 when GFP in water and D2O are excited by 266 nm, respectively.     

Synthesis of 7-azaserotonin: its photophysical properties associated with excited state proton transfer reaction

We report the synthesis of 3-(2-aminoethyl)-5-ol-1H-pyrrolo[2,3-b]pyridine (7-azaserotonin), which may potentially serve as an agonist or antagonist of serotonin receptors. In alcohols, the solvent (e.g., ethanol) catalyzed proton-transfer reaction takes place for 7-azaserotonin in the excited state, resulting in dual emission. Conversely, excited-state deprotonation takes place in neutral aqueous solution. The unique excitation behavior makes 7-azaserotonin versatile as a potential bioprobe.     

Photobase-Driven Excited-State Intramolecular Proton Transfer (ESIPT) in a Strapped π-Electron System

We report a new design strategy for an excited-state intramolecular proton transfer (ESIPT) fluorophore that can be used in acidic media. A photobasic pyridine-centered donor-acceptor-donor-type fluorophore is combined with a basic trialkylamine “strap”. In the presence of an acid, protonation occurs predominantly at the amine moiety in the ground state. A single-crystal X-ray diffraction analysis confirmed the formation of a pre-organized intramolecular hydrogen-bonded structure between the resulting ammonium moiety and the pyridine ring. Upon excitation, the intramolecular charge-transfer transition increases the basicity of the pyridine moiety in the excited state, resulting in proton transfer from the amine to the pyridine moiety. Consequently, the fluorophore takes on a polymethine-dye character in the ESIPT state, which gives rise to significantly red-shifted emission with an increased fluorescence quantum yield.     

Excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: formation of the anion in polar solvents

Excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole (HBT) dissolved in pyridine is investigated. New absorptions and fluorescence bands are detected after addition of water or NaOH to the solution. The spectra are identical to the HBT anion absorption and fluorescence. Picosecond spectroscopy is used to determine the kinetics of electronically excited states. The excited-state lifetime of the anion is 3.5 ns. The tautomeric fluorescence of HBT after proton transfer builds up within 4 ps after excitation and decays with a time constant of 20 ps.     

Excited State Intramolecular Proton Transfer of 1-Hydroxyanthraquinone

The excited state intra-molecular proton transfer dynamics of 1-hydroxyanthraquinone in solution are investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. Two characteristic bands of excited state absorption and stimu-lated emission are observed in transient absorption spectra with the excitation by the pump wavelength of 400 nm. From the delayed stimulated emission signal, the time scale of the intra-molecular proton transfer is determined to be about 32 fs. The quantum chemistry calculations show that the molecular orbits and the order of the S₂ and S₁ states are rever-sal and a conical intersection is demonstrated to exist along the proton transfer coordinate. After proton transfer, the second excited state of tautomer populated via the conical intersection undergoes the internal conversion with ～200 fs and the following intermolecular energy relaxation with ～16 ps. The longer component 300 ps can be explained in terms of the relaxation from excited-state tautomer to its ground state. From our observations, two proton transfer pathways via a conical intersection are proposed and the dominated one preserves the molecular orbits.     

Spectroscopy and femtosecond dynamics of 7-N,N-diethylamino-3-hydroxyflavone. The correlation of dipole moments among various states to rationalize the excited-state proton transfer reaction

Comprehensive excitation behaviors of 7-N,N-diethylamino-3-hydroxyflavone (I) have been investigated via steady state, temperature-dependent emission, and fluorescence upconversion to probe the excited-state intramolecular proton transfer (PT) reaction. Upon excitation, I undergoes ultrafast (<120 fs), adiabatic type of charge transfer (CT), so that the dipolar vector in the Franck-Condon excited state is much different from that in the ground state. In polar solvents such as CH2Cl2 and CH3CN, early relaxation dynamics clearly reveals the competitive rates between solvent relaxation and PT dynamics. After reaching thermal equilibrium, a relatively slow, solvent-polarity-dependent rate (a few tens of picoseconds(-1)) of PT takes places. Firm support of the early relaxation dynamics is rendered by the spectral temporal evolution, which resolves two distinct bands ascribed to CT and PT emission. The results, in combination with ab initio calculations on the dipolar vectors for various corresponding states, led us to conclude that excited-state normal (N*) and excited proton-transfer tautomer (T*) possesses very different dipole orientation, whereas the dipole orientation of the normal ground state (N) is between that of N* and T*. PT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvent relaxation dynamics induced by CT. Unlike the well-known PT system, 4'-N,N-diethylamino-3-hydroxyflavone, in which equilibrium exists between solvent-equilibrated N(eq)* and T(eq)*, N(eq)* --> T(eq)* PT for I is a highly exergonic, irreversible process in all solvents studied. Further temperature-dependent studies deduce a solvent-polarity-perturbed energy barrier of 3.6 kcal/mol for the N(eq)* --> T(eq)* PT in CH3CN. The proposed dipole-moment-tuning PT mechanism with the associated relaxation dynamics is believed to apply to many PT molecules in polar, aprotic solvents.     

Comprehensive studies on an overall proton transfer cycle of the ortho-green fluorescent protein chromophore

Initiated by excited-state intramolecular proton transfer (ESIPT) reaction, an overall reaction cycle of 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI), an analogue of the core chromophore of the green fluorescent protein (GFP), has been investigated. In contrast to the native GFP core, 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (p-HBDI), which requires hydrogen-bonding relay to accomplish proton transfer in vivo, o-HBDI possesses a seven-membered-ring intramolecular hydrogen bond and thus provides an ideal system for mimicking an intrinsic proton-transfer reaction. Upon excitation, ESIPT takes place in o-HBDI, resulting in a ～600 nm proton-transfer tautomer emission. The o-HBDI tautomer emission, resolved by fluorescence upconversion, is comprised of an instantaneous rise to a few hundred femtosecond oscillation in the early relaxation stage. Frequency analysis derived from ultrashort pulse gives two low-frequency vibrations at 115 and 236 cm(-1), corresponding to skeletal deformation motions associated with the hydrogen bond. The results further conclude that ESIPT in o-HBDI is essentially triggered by low-frequency motions and may be barrierless along the reaction coordinate. Femtosecond UV/vis transient absorption spectra also provide supplementary evidence for the structural evolution during the reaction. In CH(3)CN, an instant rise of a 530 nm transient is resolved, which then undergoes 7.8 ps decay, accompanied by the growth of a rather long-lived 580 nm transient species. It is thus concluded that following ESIPT the cis-proton transfer isomer undergoes cis-trans-isomerization. The results of viscosity-dependent dynamics are in favor of the one-bond-flip mechanism, which is in contrast to the volume-conserving isomerization behavior for cis-stilbene and p-HBDI. Further confirmation is given by the picosecond-femtosecond transient IR absorption spectra, where several new and long-lived IR bands in the range of 1400-1500 cm(-1) are assigned to the phenyl in-plane breathing motions of the trans-proton transfer tautomer. Monitored by the nanosecond transient absorption, the 580 nm transient undergoes a ～7.7 μs decay constant, accompanied by the growth of a new ～500 nm band. The latter is assigned to a deprotonated tautomer species, which then undergoes the ground-state reverse proton recombination to the original o-HBDI in ～50 μs, achieving an overall, reversible proton transfer cycle. This assignment is unambiguously supported by pump-probe laser induced fluorescence studies. On these standpoints, a comparison of photophysical properties among o-HBDI, p-HBDI, and wild-type GFP is discussed in detail.     

Cryogenic fluorescence and absorption spectroscopy studies on monomeric and dimeric species of 2-butylamino-6-methyl-4-nitropyridine N-oxide

2-Butylamino-6-methyl-4-nitropyridine-N-oxide (2B6M) belongs to a group of compounds that can undergo not only excited-state intra-, but also intermolecular proton transfer. The latter of course requires the presence of dimeric species. Previously, we have shown that for 2B6M in aprotic non-polar solvents in the liquid state such dimers play no role. Under these conditions, only one single monomeric species exists, exhibiting anomalous fluorescence behavior, i.e. proton transfer not only starting from the lowest excited electronic singlet state, but also from higher excited states. However, we also noted that under frozen, crystalline matrix conditions more species show up in the spectra. In order to study this multi-species system in more detail, we present absorption and fluorescence experiments on 2B6M, recorded in n-octane at various temperatures between 293 and 5 K. High-resolution spectra are included, not only in fluorescence but also in absorption. We demonstrate that under cryogenic conditions three species can be discerned, two of these providing high-resolution spectra with their main 0-0 lines around 452 and 465 nm, respectively. A detailed vibrational analysis of their emission spectra is included. The third species gives broad-banded spectra, in absorption extending to about 520 nm with its long-wavelength maximum around 460 nm, in emission with a maximum around 535 nm. We tentatively assign the three species to a monomer, a H-bonded dimer and a strongly interacting (pi-pi-stacked) dimer, respectively. We conclude from the excitation spectra that (anomalous) intramolecular proton transfer at higher excited states is still operative under cryogenic conditions. Indications for excited-state intermolecular proton transfer in the stacked dimeric species were not found.     

REINVESTIGATION OF THE PHOTOPHYSICS OF 2-(2'-HYDROXY-4'-DIETHYLAMINOPHENYL)BENZOTHIAZOLE

Abstract— The photophysical properties of 2-(2'-hydroxy-4'-diethylaminophenyl) benzothiazole (HABT) have been investigated by steady-state and time-resolved spectroscopies. In n-heptane HABT exhibits both normal and tautomer emissions with ∼equal fluorescence intensity at room temperature, in contrast to a previous report in which negligible tautomer emission was observed. The normal/tautomer (400/500 nm) ratio of emission intensity increases as the temperature decreases. Two possible excited-state intramolecular proton transfer (ESIPT) mechanisms are proposed, which cannot be resolved at the present stage. One proposed mechanism incorporates state mixing between -OH and -N(C₂H₅)₂ charge transfer states, resulting in a significant energy barrier for ESIPT. An alternative mechanism is also proposed in which fast proton tunneling may take place between enol and keto forms, which are in equilibrium in the excited singlet state.