Excited-state Tautomerization Research Articles

How does tautomerization affect the excited-state dynamics of an amino acid-derivatized corrole?

In the first two decades of the XXI century, corroles have emerged as an important class of porphyrinoids for photonics and biomedical photonics. In comparison with porphyrins, corroles have lower molecular symmetry and higher electron density, which leads to uniquely complementary properties. In macrocycles of free-base corroles, for example, three protons are distributed among four pyrrole nitrogens. It results in distinct tautomers that have different thermodynamic energies. Herein, we focus on the excited-state dynamics of a corrole modified with l-phenylalanine. The tautomerization in the singlet-excited state occurs in the timescales of about 10–100 picoseconds and exhibits substantial kinetic isotope effects. It, however, does not discernably affect nanosecond deactivation of the photoexcited corrole and its basic photophysics. Nevertheless, this excited-state tautomerization dynamics can strongly affect photoinduced processes with comparable or shorter timescales, considering the 100-meV energy differences between the tautomers in the excited state. The effects on the kinetics of charge transfer and energy transfer, initiated prior to reaching the equilibrium thermalization of the excited-state tautomer population, can be indeed substantial. Such considerations are crucially important in the design of systems for artificial photosynthesis and other forms of energy conversion and charge transduction.

Probing ultrafast reaction mechanisms of photo-excited dithizone through transient absorption spectroscopy and computational CASSCF studies

Dithizone, with its derivatives and complexes, is characterized by seven classes of chromic reactions, which make it a remarkable candidate for diverse sensor applications. We now present dithizone’s chromic response to experimental photoexcitation as studied by ultrafast laser spectroscopy, density functional theory (DFT), and complete active space self-consistent field (CASSCF) calculations. After vertical excitation to the first excited state, within 240 fs the reaction proceeds towards an intersystem crossing (ISC) that is coupled with a conical intercept (CI), while the second path proceeds via a second CI. The ISC leads to excited state tautomerization within 1.6 ps to form a triplet excited state product that takes a longer time to return back to reactant ( ∼ 5 n s ), while the first and second CIs both lead to the ground state within 1.6 ps and 12 ps, respectively. Additionally, dynamics through both CIs allow for efficient repopulation of the reactant ground state. Theoretical calculations using minimum energy path analysis and CASSCF calculation (including n-electron valence state perturbation theory multireference perturbation theory correction) gave additional insight into ground and excited state conformational changes that are observed by means of experimental femtosecond laser pulse excitations of dithizone.

Following Bimolecular Excited-State Proton Transfer between Hydroxycoumarin and Imidazole Derivatives.

The ultrafast dynamics of a bimolecular excited-state proton transfer (ESPT) reaction between the photoacid 7-hydroxy-4-(trifluoromethyl)-1-coumarin (CouOH) and 1-methylimidazole (MI) base in aprotic chloroform- d1 solution were investigated using ultrafast transient infrared (TRIR) and transient absorption (TA) spectroscopies. The excited-state lifetime of the photoacid in solution is relatively short (52 ps), which at the millimolar photoacid and base concentrations used in our study precludes any diffusion-controlled bimolecular ESPT reactions. This allows the prompt ESPT reaction between hydrogen-bonded CouOH and MI molecules to be studied in isolation and the "contact" ESPT dynamics to be unambiguously determined. Our time-resolved studies reveal that ultrafast ESPT from the CouOH moiety to hydrogen-bonded MI molecules occurs within ∼1 ps, tracked by unequivocal spectroscopic signatures of CouO-* photoproducts that are formed in tandem with HMI+. Some of the ESPT photoproducts subsequently π-stack to form exciplexes on a ∼35 ps time scale, minimizing the attractive Coulombic forces between the oppositely charged aromatic molecules. For the concentrations of CouOH and MI used in our study (up to 8 mM), we saw no evidence for excited-state tautomerization of coumarin anions.

Halogen substituent effect on the water-assisted excited-state tautomerization of 2, 7-diazaindole-H2O complex in aqueous solution: A theoretical study.

We studied the ESPT process in the 2-DAI-H2O complex theoretically for the first time, and compared the kinetics of 2-DAI-H2O with those features of 7-DAI-H2O. The substituted effect on the dynamics of excited-state double proton transfer in 2-DAI-H2O and 7-DAI-H2O clusters in water were also investigated at the TD-M06-2X/6-311+G(d, p) level. In this work, 2,7-DAI-H2O is also expressed as 2-DAI-H2O and 7-DAI-H2O, in which correspond to different ESPT reactions and generate two tautomers (N2H form and N7H form). In both the 2-DAI-H2O and 7-DAI-H2O complexes, ESPT processes happened in a concertedly but asynchronously protolysis pathway. The ESPT process preferred to occur in the 7-DAI-H2O complex due to its lower barrier height. For the 3-X-2-DAI-H2O and 3-X-7-DAI-H2O (X = H, F, Cl, Br) complexes, the replacement of halogen atom did not influence the ESPT mechanism. However, the replacement of halogen atom changed the structural parameters evidently, reduced the barrier height up to 4-5 kcal/mol, and enlarged the asynchronicity of ESPT apparently. ∆(R1+R2) values in the 3X-2-DAI-H2O and 3X-7-DAI-H2O complexes have linear correlation to the ZPE-corrected ESPT barrier height linearly.

Experimental and theoretical study of the excited-state tautomerism of 6-azauracil in water surroundings

The irradiation of water solution of 6-azauracil showed that one of the photoreactions is the phototautomerization of the oxo tautomer of the compound into the hydroxy tautomer. The photoreaction was identified by a thorough analysis of experimentally obtained UV-spectra and the theoretical spectra simulation of several hydroxy tautomers of the compound. Furthermore we performed DFT level (BLYP/aug-cc-pVDZ) investigations of the mechanism of phototautomerization of the oxo tautomer of 6-azauracil in water surroundings applying PCM. The complete excited-state reaction path of the mechanism was proposed. It was found that the phototautomerization occurs along the 1πσ∗ excited-state reaction path. This reaction path was found to connect the two conical intersections S0/S1 of the H-detachment processes in the oxo and hydroxy tautomers of 6-azauracil.

Theoretical study on excited-state multiple proton-transfer process of 7-azaindole with ammonia and mixed water-ammonia: Stepwise or concerted?

A detailed study of the excited-state tautomerization of 7AI-(NH3)n (n = 1, 2), 7AI-H2O-NH3 and 7AI-NH3-H2O complex in the gas phase were investigated at the CASSCF(10, 9)/6-31G(d, p) level theoretically. The effect of dynamic electron correlation on the barrier height and reaction energy was considered by MRPT2. The excited-state double proton transfer in 7AI-NH3 takes place in a concerted but asynchronous protolysis pathway. However, the triple proton transfer in 7AI cluster along the H-bonded ammonia dimer prefers a two-step process with a distinct intermediate. Each proton transfer step was a concerted but asynchronous path. The formation of H-bonded chain consisting of mixed water-ammonia affected the excited-state proton transfer mechanism. With the stabilization effect of the second solvent molecule, the first solvent molecule with large or small proton-accepting ability made the ESTPT occur in a stepwise or concerted pattern.

Effect of different alkyl groups on excited-state tautomerization of 7AI-azaindole-H2O: A theoretical study

The effect of substituted alkyl groups at different substituted position on the first excited-state proton transfer of nR7AI-H2O (n = 2–6; R = CH3, C2H5, CF3) complexes were theoretically investigated at the TD-M06-2X/6–31 + G(d, p) level. Here n value denoted the substituted position Cn of R group. The replacement of alkyl R group had no effect on the features of HOMO and LUMO, but influenced the S0 → S1 adiabatic transition energies of the nR7AI-H2O complex. Through computation, we found that the double proton transfer took place in a concerted but asynchronous protolysis pattern regardless of substituted group R and substituted position in the nR7AI-H2O complex. The vibrational-mode specific nature of ESPT was verified. The alkyl group R changed the geometrical parameters of TS, and resulted in enlarging/narrowing the asynchronousity of ESPT. The ESPT barrier height was also affected by the substituted group and position.

Non-typical fluorescence studies of excited and ground state proton and hydrogen transfer

Fluorescence studies of tautomerization have been carried out for various systems that exhibit single and double proton or hydrogen translocation in various environments, such as liquid and solid condensed phases, ultracold supersonic jets, and finally, polymer matrices with single emitters. We focus on less explored areas of application of fluorescence for tautomerization studies, using porphycene, a porphyrin isomer, as an example. Fluorescence anisotropy techniques allow investigations of self-exchange reactions, where the reactant and product are formally identical. Excitation with polarized light makes it possible to monitor tautomerization in single molecules and to detect their three-dimensional orientation. Analysis of fluorescence from single vibronic levels of jet-isolated porphycene not only demonstrates coherent tunneling of two internal protons, but also indicates that the process is vibrational mode-specific. Next, we present bifunctional proton donor-acceptor systems, molecules that are able, depending on the environment, to undergo excited state single intramolecular or double intermolecular proton transfer. For molecules that have donor and acceptor groups located in separate moieties linked by a single bond, excited state tautomerization can be coupled to mutual twisting of the two subunits.

Hydrogen-bonded channel-dependent mechanism of long-range proton transfer in the excited-state tautomerization of 7-hydroxyquinoline: a theoretical study

The first excited-state proton transfer (ESPT) in 7-hydroxyquinoline (7HQ) mediated by water and methanol molecules in a nonpolar solution was studied by quantum mechanical calculations. The multiple proton transfer through a hydrogen-bonded channel consisting of two or three hydroxyl molecules occurs in a concerted and asynchronous pathway. When the H-bonded channel molecules are water or methanol, proton transfer takes place in a solvolytic or protolytic fashion, respectively. The barrier height of the ESPT depends on the basicity of the H-bonded channel. The predicted barrier height is 2.97 kcal/mol lower with two CH3OH than with two H2O since monomeric methanol is more basic than monomeric water in heptane and DPE. The ESPT barrier heights are 3–4 kcal/mol lower with three H-bonded molecules than with two. The rate constants and KIEs for the ESPT of 7HQ-(ROH)2 (ROH = H2O, CH3OH) were calculated and compared to the experimental values.

Theoretical study on excited-state proton transfer via hydrogen-bonded ethanol (EtOH) wire for 7AI in the gas phase

Systematic studies of the excited-state tautomerization in the 7-azaindole–(EtOH) n (n = 1, 2) complexes in the gas phase were theoretically investigated. Structures and energies for reactant, transition state and product were computed at the CASSCF levels with the 6-31G(d,p) and 6-311G(d,p) basis sets. The barrier heights and reaction energies were corrected by the second-order multireference perturbation theory (MRPT2) to consider the dynamic electron correlation. The excited-state double-proton transfer in 7AI–EtOH occurs in a concerted but asynchronous mechanism. Similarly, such paths are also found in the two transition states during the excited-state triple-proton transfer of 7AI–(EtOH)2 complex. One path is that the proton from the pyrrole ring moved first to ethanol, and the other path is that the ethanol proton moved first to pyridine ring. The CASSCF level with the MRPT2 correction clearly showed that the former path was much preferable to the latter. The preferable barrier height for the 7-azaindole–(EtOH)2 complex was 4.3 kcal/mol with the zero-point energy correction. Additionally, the effect on the substitution of the ethyl group for the methyl group in the 7AI–(MeOH) n (n = 1, 2) was discussed. The replacement of the methyl group by the ethyl group obviously increased the barrier height and the asynchronousity of proton transfer in the 7AI–(EtOH)2 complex and had little effect on the 7AI–(EtOH) complex.

Control of electronic populations in porphycene with chirped ultrashort pulses

The degree of optical excitation of porphycene in solution can be controlled by manipulating linear chirp of high intensity ultrashort laser pulses while keeping their energies and spectra constant. Transient absorption, which reflects molecular populations in the ground and the first singlet excited state, varies by over 100% between optimal positively and negatively chirped excitation pulses. The results are discussed invoking coherent vibrational wavepacket dynamics. Full Text: PDF References J. Waluk, Porphycenes: Spectroscopy, Photophysics, and Tautomerism. In CRC Handbook of Organic Photochemistry and Photobiology, M. Oelgemoeller, A. Griesbeck, F. Ghetti, Eds. (CRC Press: Boca Raton, FL, 2012), pp 809-829. CrossRef P. Fita, N. Urbańska, C. Radzewicz, J. Waluk, "Ground- and Excited-State Tautomerization Rates in Porphycenes", Chem. Eur. J. 15, 485 (2009). CrossRef P. Ciąćka, P. Fita, A. Listkowski, M. Kijak, S. Nonell, D. Kuzuhara, H. Yamada, C. Radzewicz, and J. Waluk, J. Phys. Chem. B 119, 2292 (2015). CrossRef P. Nuernberger, G. Vogt, T. Brixner and G. Gerber, "Femtosecond quantum control of molecular dynamics in the condensed phase", Phys. Chem. Chem. Phys. 9, 2470 (2007). CrossRef G. Cerullo, C.J. Bardeen, Q. Wang, C.V. Shank, "High-power femtosecond chirped pulse excitation of molecules in solution", Chem. Phys. Lett. 262 362 (1996). CrossRef C.J. Bardeen, Q. Wang, C.V. Shank, "Selective Excitation of Vibrational Wave Packet Motion Using Chirped Pulses", Phys. Rev. Lett. 75, 3410 (1995). CrossRef P. Tournois, "http://dx.doi.org/10.1016/S0030-4018(97)00153-3", Opt. Comm. 140, 245 (1997). CrossRef W. Demtroder, Laser spectroscopy: basic concepts and instrumentation, 3rd ed. (Springer Press, Berlin, Heidelberg, New York, 2003). CrossRef A. Starukhin, E. Vogel, J. Waluk, J. Phys. Chem. A 102, 9999 (1998). CrossRef P. Fita, C. Radzewicz, J. Waluk, J. Phys. Chem. A, 112, 10753 (2008). CrossRef

Stepwise vs concerted excited state tautomerization of 2-hydroxypyridine: Ammonia dimer wire mediated hydrogen/proton transfer.

The stepwise and concerted excited state intermolecular proton transfer (PT) and hydrogen transfer (HT) reactions in 2-hydroxypyridine-(NH3)2 complex in the gas phase under Cs symmetry constraint and without any symmetry constraints were performed using quantum chemical calculations. It shows that upon excitation, the hydrogen bonded in 2HP-(NH3)2 cluster facilitates the releasing of both hydrogen and proton transfer reactions along ammonia wire leading to the formation of the 2-pyridone tautomer. For the stepwise mechanism, it has been found that the proton and the hydrogen may transfer consecutively. These processes are distinguished from each other through charge translocation analysis and the coupling between the motion of the proton and the electron density distribution along ammonia wire. For the complex under Cs symmetry, the excited state HT occurs on the A″((1)πσ*) and A'((1)nσ*) states over two accessible energy barriers along reaction coordinates, and excited state PT proceeds mainly through the A'((1)ππ*) and A″((1)nπ*) potential energy surfaces. For the unconstrained complex, potential energy profiles show two (1)ππ*-(1)πσ* conical intersections along enol → keto reaction path indicating that proton and H atom are localized, respectively, on the first and second ammonia of the wire. Moreover, the concerted excited state PT is competitive to take place with the stepwise process, because it proceeds over low barriers of 0.14 eV and 0.11 eV with respect to the Franck-Condon excitation of enol tautomer, respectively, under Cs symmetry and without any symmetry constraints. These barriers can be probably overcome through tunneling effect.

Food Colors as Intrinsic Luminescent Sensors in Edible Products

Limited use of luminescence spectroscopy in food and pharmaceutical applications can be related to two main causes: a)the inherent properties of most useful fluorophores (low availability, toxicity, high price and restricted solubility), and b)incomplete photophysical characterization of edible and safe-to-ingest fluorophores.To expand the use of luminescence spectroscopy to monitor quality and safety of edible goods, photophysical properties of five generally-recognized-as-safe(GRAS) food colors that are routinely added to foods or pharmaceuticals were assessed. The sensitivity of the food dyes’ fluorescence emission intensity to the surrounding medium's rigidity and polarity were also determined.Environmental polarity moderately impacted the location of the fluorescence intensity peaks and bathochromic shifts were observed for all dyes as the polarity of the solvents increased. The Stokes shifts,(λem-λexc), were estimated to be 45-90nm depending on the medium and molecular structure of the synthetic color.All these food dyes were practically non emissive in common fluid solvents, which can explain the limited information on their photophysical properties. Excited state tautomerization and/or internal twisting, already reported in synthetic non-edible dyes in low viscosity solutions, can also constitute the predominant non- radiative relaxation pathway of the studied food colors in fluid environments. The medium's rigidity was altered by changing temperature and composition. As the viscosity and consistency index of the surrounding medium increased, the dyes’ fluorescent emission intensity also increased, which suggests the molecular rotor character of these dyes. The maximum fluorescence intensity of each dye vs. viscosity relationship was characterized by a power law model and the sensitivity of each dye to changes in viscosity was evaluated in terms of its parameters.In principle, the large Stokes shift and sensitivity to viscosity supports the potential use of these food colors as probes of microviscosity or molecular crowding in edible goods.

Excited-State Tautomerization of Gas-Phase Cytosine

In order to investigate experimentally observed phototautomerization of gas-phase cytosine, several excited-state tautomerization mechanisms were characterized at the EOM-CCSD and TDDFT levels. All pathways that took place exclusively on the S1 surface were found to have significant barriers that were much higher than the barriers involved in radiationless decay of cytosine tautomers through conical intersections back to the ground state; tautomerization in this fashion cannot compete with radiationless relaxation. However, an alternative possibility is that the conical intersections that facilitate radiationless decay could also facilitate tautomerization. Barrierless pathways indicate that it is energetically possible that bifurcation at the conical intersections can lead to a subset of the population reaching different tautomers. This could be an explanation for the observed tautomerization of keto cytosine after exposure to low-energy UV light.

Excited-State Tautomerization of 7-Azaindole in Nonpolar Solution: A Theoretical Study Based on Liquid-Phase Potential Surfaces of Mean Force.

Excited state tautomerization of a 7-azaindole (7AI) complex with one methanol molecule in heptane was studied using variational transition state theory including multidimensional tunneling (VTST/MT) with the dielectric continuum model for the solvent effect. Electronic structures and energies for reactants and transition state (TS) in solution were computed at the complete active space self-consistent field (CASSCF) level with second-order multireference perturbation theory (MRPT2) to take into consideration of dynamic electron correlation. The polarizable continuum model using the integral equation formalism (IEFPCM) and the SMD model were used for the excited-state solvent effect. Excited-state surfaces of potential of the mean force in solution were generated for the first time at the MRPT2//SMD/CASSCF(10,9)/6-31G(d,p) level. The position of TS on the reaction coordinate substantially depended on the dynamic electron correlation. The two protons in the excited-state tautomerization were transferred in a concerted but asynchronous process. Calculated HH/DD kinetic isotope effect (KIE) and the ratio of Arrhenius pre-exponential factors, A(HH)/A(DD), agreed very well with the corresponding experimental values. The shape of the adiabatic energy surfaces in the excited-state strongly depended on the position of isotopes due to the asynchronicity of the reaction path, and the tunneling effect was essential for reproducing experimental KIEs. The pyrrolic proton moved a twice longer distance by tunneling than the hydroxyl proton in the most probable tunneling path at 292 K. This study strongly suggests that the mechanism of the excited-state double proton transfer in heptane is triggered by proton transfer from the pyrrolic nitrogen of 7AI to alcohol (protolytic pathway), rather than by proton transfer from alcohol to the pyridine nitrogen of 7AI (solvolytic pathway).

Direct observation of ground-state lactam–lactim tautomerization using temperature-jump transient 2D IR spectroscopy

We provide a systematic characterization of the nanosecond ground-state lactam-lactim tautomerization of pyridone derivatives in aqueous solution under ambient conditions using temperature-jump transient 2D IR spectroscopy. Although electronic excited-state tautomerization has been widely studied, experimental work on the ground electronic state, most relevant to chemistry and biology, is lacking. Using 2D IR spectroscopy, lactam and lactim tautomers of 6-chloro-2-pyridone and 2-chloro-4-pyridone are unambiguously identified by their unique cross-peak patterns. Monitoring the correlated exponential relaxation of these signals in response to a laser temperature jump provides a direct measurement of the nanosecond tautomerization kinetics. By studying the temperature, concentration, solvent, and pH dependence, we extract a thermodynamic and kinetic characterization and conclude that the tautomerization proceeds through a two-state concerted mechanism. We find that the intramolecular proton transfer is mediated by bridging water molecules and the reaction barrier is dictated by the release of a proton from pyridone, as would be expected for an efficient Grothuss-type proton transfer mechanism.

Wagging motion of hydrogen-bonded wire in the excited-state multiple proton transfer process of 7-hydroxyquinoline·(NH3)3 cluster

In this work, the dynamics of hydrogen bonds (as well as the hydrogen-bonded wire) in excited-state tautomerization of 7-hydroxyquinoline·(NH3)3 (7HQ⋅(NH3)3) cluster has been investigated by using time-dependent density functional theory (TDDFT). It shows that upon an excitation, the hydrogen bond between –OH group in 7-hydroxyquinoline (7HQ) and NH3 moiety would extremely strengthened in S1 state, which could effectively facilitate the releasing of the proton from the phenolic group of 7HQ moiety to the hydrogen-bonded wire and the forming an Eigen-like cationic wire (NH3⋯NH4+⋯NH3) in the cluster. To fulfill the different optimal angles of NH4+ in the wire, a wagging motion of hydrogen-bonded wire would occur in excited state. Moreover, the wagging motion of the hydrogen-bonded wire would effectively promote excited-state proton transfer reaction. As the results, an excited-state multiple proton transfer (ESMPT) mechanism containing two concerted and asymmetrical processes has been proposed for the proton transfer dynamics of 7HQ⋅(NH3)3 cluster.

Fast excited state dynamics in the isolated 7-azaindole-phenol H-bonded complex

The excited state dynamics of the H-bonded 7-azaindole-phenol complex (7AI-PhOH) has been studied by combination of picosecond pump and probe experiments, LIF measurements on the nanosecond time scale and ab initio calculations. A very short S(1) excited state lifetime (30 ps) has been measured for the complex upon excitation of the 0(0)(0) transition and the lifetime remains unchanged when the ν(6) vibrational mode (0(0)(0) + 127 cm(-1)) is excited. In addition, no UV-visible fluorescence was observed by exciting the complex with nanosecond pulses. Two possible deactivation channels have been investigated by ab initio calculations: first an excited state tautomerization assisted by a concerted double proton transfer (CDPT) and second an excited state concerted proton electron transfer (CPET) that leads to the formation of a radical pair (hydrogenated 7AIH(●) radical and phenoxy PhO(●) radical). Both channels, CDPT and CPET, seem to be opened according to the ab initio calculations. However, the analysis of the ensemble of experimental and theoretical evidence indicates that the excited state tautomerization assisted by CDPT is quite unlikely to be responsible for the fast S(1) state deactivation. In contrast, the CPET mechanism is suggested to be the non-radiative process deactivating the S(1) state of the complex. In this mechanism, the lengthening of the OH distance of the PhOH molecule induces an electron transfer from PhOH to 7AI that is followed by a proton transfer in the same kinetic step. This process leads to the formation of the radical pair (7AIH(●)···PhO(●)) in the electronically excited state through a very low barrier or to the ion pair (7AIH(+)···PhO(-)) in the ground state. Moreover, it should be noted that, according to the calculations the πσ* state, which is responsible for the H loss in the free PhOH molecule, does not seem to be involved at all in the quenching process of the 7AI-PhOH complex.

Excited state tautomerization of 7-azaindole catalyzed by pyrazole

Pyrazole, a five member cyclic azole, is reported here as an efficient catalyst for excited state tautomeric conversion of 7-azaindole. In hydrocarbon solution the two compounds efficiently form a doubly hydrogen-bonded 1:1 cyclic complex whose association constant value is found comparable with 7-azaindole dimerization constant, and according to B3LYP/6-311G++∗∗ calculation the binding energies of the complex and dimer are nearly same. In the excited state (S1), the TDDFT calculation predicts tautomer of the complex to be 13.4kcal/mol more stable than normal form. Fluorescence spectra reveal that upon UV excitation the complex emits exclusively from the tautomeric form.

Excited State Tautomerization of 7-Azaindole in a 1:1 Complex with δ-Valerolactam: A Comparative Study with the Homodimer

A comparative analysis for relative stability between normal and tautomeric forms in the excited electronic states of 7-azaindole···δ-valerolactam 1:1 complex and 7-azaindole homodimer has been presented. The tautomeric configuration of the complex is estimated to be ~6 kcal/mol more stable than normal form, and the same for homodimer appears to be ~10 kcal/mol. Consistent with these estimates both the complex and homodimer undergo facile double proton transfer tautomerization upon UV excitation in hydrocarbon solutions (Chou; et al. J. Am. Chem. Soc. 1995, 117, 7259). However, we notice that such similarity in photophysical behavior of the two hydrogen-bonded systems is lost completely in a cold supersonic jet expansion. The jet-cooled homodimer emits only the tautomer fluorescence in the visible spectral region, but the complex emits exclusively from the locally excited state in ultraviolet. We have interpreted this contrast by arguing that the effective barrier for excited state double proton exchange tautomerization of the complex is larger compared to that of the homodimer, and the difference originates because of asymmetric nature of the two hydrogen bonds of the complex.