Enol-keto tautomerism of aromatic photochromic Schiff base N,N'-bis(salicylidene)-p-phenylenediamine: ground state equilibrium and excited state deactivation studied by solvatochromic measurements on ultrafast time scale

A photochromic symmetric Schiff base, N,N'-bis(salicylidene)-p-phenylenediamine, is proposed as a probe for the study of solvent dependent enol-keto tautomerism in the ground and excited states. The ground state equilibrium between the enol-keto tautomers is found to depend mainly not on polarity but on the proton donating ability of the solvent. Upon selective excitation of each of these tautomers, the same excited state of a keto tautomer is created: in enol, after the ultrafast excited state intramolecular proton transfer (ESIPT), reaction, and in keto tautomer, directly. Then some part (<30%) of excited molecules are transferred to the photochromic form in its ground state. The evidence of another ultrafast deactivation channel in the excited enol tautomer competing with ESIPT has been found. The solvent does not influence the ESIPT dynamics nor the efficiency of the creation of the photochrome.     

Ultrafast branching of reaction pathways in 2-(2'-hydroxyphenyl)benzothiazole in polar acetonitrile solution

In a combined study on the photophysics of 2-(2'-hydroxyphenyl)-benzothiazole (HBT) in polar acetonitrile utilizing ultrafast infrared spectroscopy and quantum chemical calculations, we show that a branching of reaction pathways occurs on femtosecond time scales. Apart from the excited-state intramolecular hydrogen transfer (ESIHT) converting electronically excited enol tautomer into the keto tautomer, known to be the dominating mechanism of HBT in nonpolar solvents such as cyclohexane and tetrachloroethene, in acetonitrile solution twisting also occurs around the central C-C bond connecting the hydroxyphenyl and benzothiazole units in both electronically excited enol and keto tautomers. The solvent-induced intramolecular twisting enables efficient internal conversion pathways to both enol and keto tautomers in the electronic ground state. Whereas relaxation to the most stable enol tautomer with twisting angle Θ = 0° implies full ground state recovery, a small fraction of HBT molecules persists as the keto twisting conformer with the twisting angle Θ = 180° for delay times extending beyond 120 ps.     

Coupled electron and proton transfer processes in 4-dimethylamino-2-hydroxy-benzaldehyde

TDDFT calculations, picosecond transient absorption, and time-resolved fluorescence studies of 4-dimethylamino-2-hydroxy-benzaldehyde (DMAHBA) have been carried out to study the electron and proton transfer processes in polar (acetonitrile) and nonpolar (n-hexane) solvents. In n-hexane, the transient absorption (TA) as well as the fluorescence originate from the ππ* state of the keto form (with the carbonyl group in the benzaldehyde ring), which is produced by an intramolecular proton transfer from the initially excited ππ* state of the enol form (OH group in the ring). The decay rate of TA and fluorescence are essentially identical in n-hexane. In acetonitrile, on the other hand, the TA exhibits features that can be assigned to the highly polar twisted intramolecular charge transfer (TICT) states of enol forms, as evidenced by the similarity of the absorption to the TICT-state absorption spectra of the closely related 4-dimethylaminobenzaldehyde (DMABA). As expected, the decay rate of the TICT-state of DMAHBA is different from the fluorescence lifetime of the ππ* state of the keto form. The occurrence of the proton and electron transfers in acetonitrile is in good agreement with the predictions of the TDDFT calculations. The very short-lived (～1 ps) fluorescence from the ππ* state of the enol form has been observed at about 380 nm in n-hexane and at about 400 nm in acetonitrile.     

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.     

Photochrome that was not: 2-hydroxynaphtylidene-(8-aminoquinoline)

We report the results of quantum-chemical calculations, which show that the keto form of 2-hydroxynaphtylidene-(8-aminoquinoline) (HNAQ) is slightly more stable than the enol form both in the ground and first excited ππ* electronic states. The barrier for proton transfer between the enol and the ketone in the ground state is ca. 3300 cm(-1) (HF), and 770 cm(-1) (B3LYP), indicating a very fast (ps scale) exchange of protons between the two tautomeric forms. This barrier decreases slightly in the first excited ππ* electronic state (2500 cm(-1) - CIS), making proton exchange even faster. We show that the ππ* state of the ketone tautomer is prone to radiationless transition to a state with nearly perpendicular orientation of the two ring systems, similarly to other Schiff bases that are photochromes (for instance salicydeneaniline). This state arises when an electron from the highest occupied molecular orbital (HOMO) of the ketone ring system is transferred to a LUMO localized on the CHNH group of the bridge connecting the two ring systems of the molecule. The energy minimum of this "perpendicular" state lies only ca. 0.09 eV from the ground state potential-energy surface, thus it is prone to extremely rapid radiationless decay. Further relaxation on this surface leads to a metastable conformation that lies ca. 4440 cm(-1) above the planar, hydrogen-bonded, ketone conformation. Unfortunately, photochromism of this metastable conformation does not occur, since its absorption spectrum overlaps the spectrum of the stable species (with the predicted absorption around 438 nm vs calculated 440.6 nm in the stable ketone).     

Photo-deactivation pathways of a double H-bonded photochromic Schiff base investigated by combined theoretical calculations and experimental time-resolved studies

The photophysics of N,N'-bis(salicylidene)-p-phenylenediamine (BSP) is analyzed both theoretically and experimentally. The alternative intramolecular proton-transfer reactions lead to three different tautomers. We performed DFT and TDDFT calculations to analyze the topography of the reactions connecting the three tautomers. Deactivation paths through a Conical Intersection (CI) region are also analyzed to explain the low fluorescence quantum yield of the phototautomers. The complex molecular structure of BSP provides a large number of deactivation paths, almost all of them energetically available following the initial photoexcitation. Femtosecond (fs) time-resolved emission studies in solution and flash photolysis experiments (nano to millisecond regime) were performed to get detailed information on the time domain of the full photocycle. The picture that emerges by combining theoretical and experimental results shows a very fast (less than 100 fs) photoinduced single proton transfer process leading to a phototautomer where a single proton has moved. This species may deactivate through a low-energy CI leading in about 20 ps to a rotameric form in the ground state that has a lifetime of several tens of microseconds in solution. This process competes with another deactivation path taking place prior to the proton-transfer reaction which involves a low-energy CI leading to a rotamer of the enol structure. In the flash photolysis studies, the rotamer of the enol structure was directly identified by the positive transient absorption band in the 250-260 nm and its lifetime in n-hexane (10 ms) is almost 3 orders of magnitude longer than the lifetime of the photochrome (around 40 μs). Our findings do not exclude a double proton transfer reaction in the excited enol form to give a tautomer in less than 100 fs during the first (impulsive) phase of the reaction which reverts back to the photoproducts of the simple proton transfer in 1-3 ps.     

Excited state proton transfer reaction of two new intramolecularly hydrogen bonded Schiff bases at room temperature and 77K.

Two new orthohydroxy Schiff bases, 7-phenylsalicylidene benzylamine (PSBA) and 7-ethylsalicylideneaniline (ESA) have been synthesized. The excited state intramolecular proton transfer (ESIPT) and the structure of PSBA and ESA in its crystalline form and in the solvents n-hexane, n-heptane and 1,4-dioxane have been investigated by means of absorption, emission and nanosecond spectroscopy at room temperature and 77K. One ground state species has been detected both in neutral and basic solutions of both PSBA and ESA: the cis-enol form with an intramolecular hydrogen bond. The ESIPT and formation of keto tautomer are evidenced by a large Stokes shifted emission (approximately 12000 cm(-1)) at room temperature only in the case of ESA. On the other hand the keto tautomer is the predominant species at 77K in a solid matrix and as a solid sample at room temperature both in the case of ESA and PSBA. In the case of both ESA and PSBA the more intense, higher energy emission is due to the species which has not undergone ESIPT and attributed mainly due to cis-enol form. The trans-enol form is also observed by changing the excitation wavelength. Both the compounds are found to undergo a structural change to a zwitterionic and intermolecular hydrogen bonded form in the presence of a strong base like triethylamine. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rates of proton transfer reaction in the case of PSBA. Our theoretical calculation at the AM1 level of approximation shows that the ground singlet state has a rather large activation barrier both in the case of PSBA and ESA. The barrier height is much lower on the corresponding excited singlet surface only in the case of ESA. The process is predicted to be endothermic in the ground state and exotherrmic in the excited singlet state.     

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.     

Femtosecond studies of charge-transfer mediated proton transfer in 2-butylamino-6-methyl-4-nitropyridine N-oxide

We have unraveled the effects of an amino substituent in the ortho position on the excited-state dynamics of 4-nitropyridine N-oxide by studying the picosecond fluorescence kinetics and femtosecond transient absorption of a newly synthesized compound, 2-butylamino-6-methyl-4-nitropyridine N-oxide, and by quantum chemical calculations. Similar to the parent compound, the S(1) state of the target molecule has significant charge-transfer character and shows a large (approximately 8000 cm(-1)) static Stokes shift in acetonitrile. Analysis of the experimental and the theoretical results leads, however, to a new scenario in which this intramolecular charge transfer triggers in polar, aprotic solvents an ultrafast (around 100 fs) intramolecular proton transfer between the amino and the N-O group. The electronically excited N-OH tautomer is subsequently subject to solvent relaxation and decays with a lifetime of approximately 150 ps to the ground state.     

Femtosecond dynamics of piroxicam structures in solutions

We report on studies of femtosecond dynamics of a nonsteroidal anti-inflammatory drug, piroxicam (1), in water at three different pHs and for comparison in two aprotic solvents. An ultrafast excited-state proton-transfer (ESIPT) process takes place in neutral and cationic enol-type structures. Femtosecond emission and transient absorption experiments show that this reaction is barrierless, and the proton-transferred keto tautomer is formed in less than 100 fs in both organic solvents and acidic water. In neutral and alkaline water, the ESIPT is not present because of the prevalence of the anion structures at the ground state. For the excited anions (pH = 7, 11) and formed keto cations (pH approximately 3), an intramolecular charge-transfer process takes place in times shorter than 0.3 ps. The formed structures have a fluorescence lifetime of approximately 2-5 ps, depending on pH. In contrast, the internal twisting motion in organic solvents is slower (approximately 0.5-1 ps) and gives rotamers with lifetimes of tens of picoseconds. These results clearly show strong interactions of 1 with water, significantly distinct from those present in organic aprotic solvents. We believe that the results are important for a better understanding on short time interactions of drugs with their environment.     

The ground state tautomer in the excited state relaxation process of 3-hydroxyflavone at room temperature

Very recently, the ground state reverse proton transfer in 3-hydroxyflavone (3-HF) has been reported to take place within 30 ps by picosecond transient absorption spectroscopy by Aartsma and co-workers. Here we present evidence of intervention of a long-lived ground state tautomer involved in the excited state relaxation process of 3-HF by transient absorption and two-step laser excitation fluorescence spectroscopy.     

Tautomerism in 7-hydroxyquinoline: a combined experimental and theoretical study in water

Tautomerism in the ground and excited states of 7-hydroxyquinoline (7HQ) was studied in different solvents using steady-state and lifetime spectroscopic measurements, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. Equilibrium between the enol and the keto/zwitterion tautomers exists in 7HQ, which is solvent-dependent. Of the solvents used in this study, only in water does the absorbance spectrum of 7HQ show absorption from both the enol and zwitterion tautomers. In addition, in aqueous media, fluorescence is observed from the zwitterion tautomer only, which is attributed to self-quenching of the enol fluorescence by energy transfer to the ground-state zwitterion tautomer and energetically favorable excited-state proton transfer. Solvation of the hydrogen bonding sites of 7HQ was studied in binary mixtures of 1,4-dioxane and water, and three water molecules were estimated to connect the polar sites and induce intermolecular proton transfer. The results are confirmed by DFT calculations showing that three water molecules are the minimum number required to form a stable solvent wire. Mapping the water density around the polar sites using MD simulations shows well-defined hydrogen bonds around the amino and hydroxyl groups of the enol tautomer and slightly less well-defined hydrogen bonds for the zwitterion tautomer. The presence of three-member water wires connecting the polar centers in 7HQ is evident in the MD simulations. The results point to the unique spectral signatures of 7HQ in water, which make this molecule a potential probe to detect the presence of water in nanocavities of macromolecules.     

基于异噁唑酮类份菁染料的激发态分子内质子转移的研究

运用量子化学理论计算方法研究了3-甲基-4-(1H-吲哚-3-次甲基)-异噁唑-5-酮(A)及其衍生物份菁染料的激发态分子内质子转移性质.研究表明:在基态3种染料AH(R=H),AO(R=—O(H3))和AP(R=—O(H2Ph))只存在酮式构型,在激发态AH与AP存在酮式和烯醇式2种构型,而AO存在酮式、烯醇式和仲胺式3种构型.红外光谱表明化合物从基态跃迁到激发态存在分子内的氢键增强作用,势能曲线显示激发态的质子转移为放热反应且能垒较低,通过分析电子光谱得到具有较大斯托克位移的激发态分子内质子转移的荧光发射峰,前线分子轨道理论计算进一步说明了其质子转移的发生过程.     

The effect of hydrogen bonding on the excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: a TDDFT molecular dynamics study

The dynamics of the excited-state proton transfer (ESPT) in a cluster of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and hydrogen-bonded water molecules was investigated by means of quantum chemical simulations. Two different enol ground-state structures of HBT interacting with the water cluster were chosen as initial structures for the excited-state dynamics: (i) an intramolecular hydrogen-bonded structure of HBT and (ii) a cluster where the intramolecular hydrogen bond in HBT is broken by intermolecular interactions with water molecules. On-the-fly dynamics simulations using time-dependent density functional theory show that after photoexcitation to the S(1) state the ESPT pathway leading to the keto form strongly depends on the initial ground state structure of the HBT-water cluster. In the intramolecular hydrogen-bonded structures direct excited-state proton transfer is observed within 18 fs, which is a factor two faster than proton transfer in HBT computed for the gas phase. Intermolecular bonded HBT complexes show a complex pattern of excited-state proton transfer involving several distinct mechanisms. In the main process the tautomerization proceeds via a triple proton transfer through the water network with an average proton transfer time of approximately 120 fs. Due to the lack of the stabilizing hydrogen bond, intermolecular hydrogen-bonded structures have a significant degree of interring twisting already in the ground state. During the excited state dynamics, the twist tends to quickly increase indicating that internal conversion to the electronic ground state should take place at the sub-picosecond scale.     

生色团连接的苯骈三氮唑衍生物的激发态分子内质子转移

用从头算和密度泛函理论研究了对硝基二苯乙烯作为生色团连接的2-(2-羟基-苯基)-苯骈三氮唑的衍生物2-羟基-5-[对硝基-二苯乙烯基-氧亚甲基]-苯基-(2H-苯骈三氮唑)(C1)和4′-硝基-3,4-二[2-羟基-(2H-苯骈三氮唑)-苄氧基]-二苯乙烯(C2)发生激发态分子内质子转移(ESIPT)的可能性.系统研究了C1和C2发生ESIPT的互变异构体的基态与激发态的性质变化,包括相关的键长、键角等结构参数,Mulliken电荷和偶极矩,前线轨道以及势能曲线.计算结果表明,对于C1来讲,酮式(keto)的基态(K)不存在稳定结构,因此发生基态分子内质子转移(GSIPT)可能性很小.酮式的激发态(K*)的氢键强度要远强于烯醇式(enol)的激发态(E*)的氢键强度.分子在光致激发后,质子供体所带负电荷减小而质子受体所带负电荷增加.在K*,HOMO→LUMO的电子跃迁导致电子密度从"酚环"向质子化杂环转移.E*→K*跃迁只需要克服较小的能垒(约41 kJ.mol-1).计算结果表明C1发生ESIPT的可能性很大.C2由于具有高能量,其具有基态的单质子转移特征的异构体EK(同时含烯醇E与酮K结构)、具有基态的双质子转移特征的异构体2K(含有双酮结构),以及具有双酮结构特征的激发态2K*均无法获得它们的稳定结构,因此,基态分子内单或双质子转移和激发态分子内双重质子转移发生的可能性极小.然而,由于双烯醇式的激发态(2E*)和EK的激发态(EK*)存在稳定结构,且2E*→EK*跃迁具有低能垒,因此C2有可能发生激发态分子内单重质子转移.本文进一步计算了两个分子的紫外-可见吸收光谱与荧光发射光谱,获得了具有较大斯托克位移的ESIPT的荧光发射峰.     

Excited state properties of 7-hydroxy-4-methylcoumarin in the gas phase and in solution. A theoretical study

TDDFT/B3LYP and RI-CC2 calculations with different basis sets have been performed for vertical and adiabatic excitations and emission properties of the lowest singlet states for the neutral (enol and keto), protonated and deprotonated forms of 7-hydroxy-4-methylcoumarin (7H4MC) in the gas phase and in solution. The effect of 7H4MC-solvent (water) interactions on the lowest excited and fluorescence states were computed using the Polarizable Continuum Method (PCM), 7H4MC-water clusters and a combination of both approaches. The calculations revealed that in aqueous solution the pi pi* energy is the lowest one for excitation and fluorescence transitions of all forms of 7H4MC studied. The calculated excitation and fluorescence energies in aqueous solution are in good agreement with experiment. It was found that, depending on the polarity of the medium, the solvent shifts vary, leading to a change in the character of the lowest excitation and fluorescence transition. The dipole-moment and electron-density changes of the excited states relative to the ground state correlate with the solvation effect on the singlet excited states and on transition energies, respectively. The calculations show that, in contrast to the ground state, the keto form has a lower energy in the pi pi* state as compared to enol, demonstrating from this point of view the energetic possibility of proton transfer from the enol to the keto form in the excited state.     

Electronic and quantum dynamical insight into the ultrafast proton transfer of 1-hydroxy-2-acetonaphthone

The ultrafast proton-transfer dynamics of 1-hydroxy-2-acetonaphthone has been theoretically analyzed in the ground and first singlet excited electronic states by density functional theory calculations and quantum dynamics. The potential energies obtained in the ground electronic state reveal that the proton-transfer process does not lead to a stable keto tautomer unless the transfer of the hydrogen from the enol form is accompanied by an internal rotation of the newly formed O-H bond. Calculations in the first singlet excited electronic state point to a very low barrier for the formation of the keto tautomer. The analysis of the calculated frequencies of the two tautomers in the excited state unveils a coupling of the skeletal motions (low frequency modes) with the proton-transfer process, as it has been stated from time-resolved experiments. The electronic energies obtained by the time-dependent density functional theory formalism have been fitted to a monodimensional potential energy surface in order to perform an exact quantum dynamics study of the process. Our results show that the proton-transfer process is completed within 25.5 fs, in remarkable good agreement with experiments.     

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.     

Excited‐State Prototropic Equilibrium Dynamics of 6‐Hydroxyquinoline Encapsulated in Microporous Catalytic Faujasite Zeolites

The excited‐state proton transfer and geminate recombination of 6‐hydroxyquinoline (6HQ) encaged in catalytic Na⁺‐exchanged faujasite zeolites X (NaX) and Y (NaY) have been explored by measuring steady‐state and picosecond time‐resolved spectra. The pathways and rate constants of proton transfer of excited 6HQ are determined by the microscopic environment of zeolitic hosts surrounding the guest molecules. The excited‐state proton transfer of a 6HQ molecule encapsulated in a zeolitic nanocavity is initiated by deprotonation of the enolic group to form an anionic intermediate and completed by subsequent protonation of the imino group to form a zwitterionic tautomer. Geminate recombination occurs to compete with proton transfer at each tautomerization step of excited‐state 6HQ because of the confined environment of dehydrated zeolitic supercages. Consequently, excited‐state equilibria among three prototropic species of 6HQ are established in microporous catalytic faujasite zeolites. Kinetic differences in NaX and NaY are attributed to dissimilarities in acidity/basicity.     

Spectral and acid-base features of 3,7-dihydroxy-2,8-diphenyl-4H,6H-pyrano[3,2-g]chromene-4,6-dione (diflavonol)--a potential probe for monitoring the properties of liquid phases

Diflavonol is a molecule that can exist in neutral or anionic form and in several tautomeric forms in ground and excited states. Absorption and emission spectroscopy combined with theoretical calculations have shown that only one tautomer of neutral diflavonol exists in the ground state, but two exist in the excited state. In the latter case, one is the tautomer originating from the ground state tautomer, which exists in strongly protic solvents, the other is the phototautomer occurring in weakly protic or aprotic solvents as a result of the intramolecular transfer of one proton. The OH groups present in diflavonol and involved in weak intramolecular hydrogen bonds exhibit a proton-donating ability reflected by the experimental values of acidity constants or theoretical enthalpies and free energies of proton detachment. The electronically excited molecule is a relatively strong acid when it loses one proton. With increasing basicity of the medium, monoanionic and dianionic forms occur which exhibit spectral characteristics and an emission ability different from those of neutral diflavonol. These interesting features of diflavonol open up possibilities for the analytical use of the compound and its application as a spectral probe sensitive to the properties of liquid phases.