Intramolecular proton transfer from the highest singlet states in 3-hydroxyflavone

It is found that the excitation spectra of the dual fluorescence of 3-hydroxyflavone are different for different recording wavelengths and that the intensity ratio of the emission of the normal and tautomeric (with intramolecular proton transfer) forms upon selective UV excitation in the regions of the S ₁, S ₂, and S ₃ singlet absorption bands strongly depends on the excitation wavelength. The results obtained directly point to the existence of an additional channel of population of the excited state of the tautomeric form and are explained by the intramolecular proton transfer through the S ₂ and S ₃ excited singlet states of fluorophore molecules. The constants of this transfer are estimated using analytical relations for the steady-state fluorescence excitation.     

Investigation of reactions from the highest excited states of molecules by fluorescent spectroscopy methods

The effect of the highest excited states on the yield of photoproducts that are usually formed upon excitation of the first singlet electronic state of polyatomic molecules is discussed. It is shown that the excitation of molecular objects through the highest singlet states can, in some cases, increase the yield of reaction products. This allows one to estimate the probabilities of reactions from the corresponding states. The consideration concerns a wide range of primary photoreactions, including the electronic density redistribution (the intramolecular electron transfer) in the excited state, the protolytic reactions, the intramolecular proton transfer (the phototautomerization), the hydrogen bond formation, and the formation of excimers and exciplexes. The relations obtained are used to analyze the experimental fluorescence spectra of 3-hydroxyflavone solutions, excited by electromagnetic radiation with different wavelengths in the region of the S ₁, S ₂, and S ₃ absorption bands. The analysis fulfilled shows that the highest singlet states play an important role in the formation of tautomers in 3-hydroxyflavone due to the intramolecular proton transfer.     

Investigation of excited-state proton transfer in 3-hydroxyflavone molecules

The dual fluorescence spectra of 3-hydroxyflavone molecules excited by electromagnetic radiation in the region of the S ₁ and S ₂ absorption bands in the temperature region of 20–80°C are studied using the dynamic quenching of the excited state. An analysis of the fluorescence parameters shows that heating the solution from room temperature to 60°C increases the proton transfer rate by a factor of 1.24 in the case of standard excitation into the main absorption band and even stronger (by a factor of 6.9) in the case of excitation into the second absorption band. The presence of a quencher reduces the yield of the two emission bands and noticeably increases the proton transfer rate, by a factor of 1.16 at room temperature and by a factor of 1.25 at 80°C. Upon excitation into the second singlet band, the transfer rate increases even more (especially at higher temperatures), by a factors of 1.24 and 3.5 for the same temperatures. The temperature dependences of the transfer rate constant allowed us to estimate the activation energies of the proton transfer reaction under different physical conditions and reach conclusions about the mechanism by which this reaction proceeds. It is found that the proton transfer activation energy decreases from 500 to 360 cm⁻¹ when measured in temperature ranges of 20–40 and 20–60°C. The introduction of a quencher with a concentration of 5 × 10⁻³ M increases the activation barrier to 534 and 471 cm⁻¹ in the same temperature ranges.     

Studying reactions that proceed from highest excited states of molecules using fluorescence quenching

The steady-state monochromatic excitation of a luminophore that has fluorescing products is considered. The effect of dynamic quenching of highest excited states on the fluorescence of singlet states under its excitation via singlet S ₁ and S _n (n ≥ 2) states is discussed. It is shown that the use of the method of fluorescence dynamic quenching by foreign impurities opens new possibilities for studying photoreactions that proceed via S _n singlet states. A large number of primary photoprocesses are considered which include the electron density redistribution (the internal electron transfer) in the excited state, protolytic reactions, intramolecular proton transfer (phototautomerization), hydrogen bonding, and formation of excimers and exciplexes. It is shown that, upon dynamic quenching, the bimolecular quenching constant of an excited level depends on the amount of thermal energy released in the luminophore before the occurrence of the light emission event. Based on the experimental measurements of the fluorescence spectra at different quencher contents, the calculation of the Stern-Volmer constant for reaction products is considered in detail. It is shown that this constant can be most reliably determined from the dependence of the fluorescence intensity ratio of the initial reagents and the quencher product rather than from the dependence of the fluorescence intensity of the products on the concentration of the quencher. The relations determined are used in analysis of the experimental fluorescence spectra of solutions of 3-hydroxyflavone excited by radiation with different wavelengths lying in the range of the S ₁ and S ₂ absorption bands. The temperature behavior of the Stern-Volmer constant for different fluorescence bands of 3-hydroxyflavone is considered. It is shown that, if these constants for the normal and tautomeric forms are correctly determined, their temperature dependences are similar.     

Structural dynamics of 4‐formaldehyde imidazole and imidazole in light absorbing S2(ππ*) state

The short‐time structural dynamics of 4‐formaldehyde imidazole and imidazole in light absorbing S₂(ππ*) state were studied by using resonance Raman spectroscopy and quantum mechanical calculations. The vibrational spectra and ultraviolet absorption spectra of 4‐formaldehyde imidazole were assigned. The resonance Raman spectra of imidazole and 4‐formaldehyde imidazole were obtained in methanol and acetonitrile with excitation wavelengths in resonance with the first intense absorption band to probe the short‐time structural dynamics. complete active space self‐consistent field calculations were carried out to determine the minimal singlet excitation energies and structures of S₁(nπ*), S₂(ππ*), and conical intersection point S₁(nπ*)/S₂(ππ*). The results show that the A‐band structural dynamics of imidazole is predominantly along the N₁H/C₄H/C₅H/C₂H in‐plane bending reaction coordinate, which suggests that excited state proton or hydrogen transfer reaction takes place somewhere nearby the Franck–Condon region. The significant difference in the short‐time structural dynamics between 4‐formaldehyde imidazole and imidazole is observed, and the underlying mechanism is interpreted in term of excited state charge redistribution. Copyright © 2015 John Wiley & Sons, Ltd.     

Direct observation of proton transfer from the excited <Emphasis Type="Italic">S</Emphasis><Subscript>2</Subscript> state in the 3-hydroxyflavone molecule

The spectra of dual fluorescence of 3-hydroxyflavone molecules excited by 44-ps pulses in the region of the S ₁ and S ₂ absorption bands are measured with a picosecond resolution. The dynamics of the spectra directly demonstrates the time development of the proton transfer from the carboxyl to the carbonyl group of the molecule. Upon excitation into the main absorption band, the transfer process occurs for about 210 ps. The excitation into the region of the S ₂ band results in a faster (～170 ps) process, and the relative contribution made to the total spectrum by the long-wavelength band, which belongs to the proton-transfer state, is higher in this case for all the time ranges of luminescence recording. The data obtained directly point to an additional channel of proton transfer via the S ₂ state. The probability of this process is estimated to be 0.84 × 10¹² s^?1.     

ESIPT reaction starting from S2 and S3 singlet states in 3-hydroxyflavone

Spectra fluorescence at excitation of emission within spectral intervals of S₃, S₂ and S₁ bands of absorption, and excitation at registration of fluorescence at maxima of blue and green bands of emission have been studied in 3-hydroxyflavone solution. A possibility of ESIPT reaction starting from S₃ and S₂ states of parent 3HF molecule was shown for the first time follows from observations of differences in excitation spectra of normal and tautomer forms of 3HF, and from different ratios of these forms intensities upon excitation in different ranges of UV spectra of absorption.     

Excitation transfer into bound and continuum states investigated by optical and electron spectroscopy

We present energy spectra of electrons formed in the reaction of He(2³ S, 2¹ S) with NO₂ which have significantly improved counting statistics and resolution compared to earlier work. Further, we show spectra of the fluorescence light emitted in these reactions. The data are recorded in the same molecular beam apparatus as the electron spectra. For the metastable singlet state He(2¹ S) the spectra have not been measured before. We find that in addition to ionization excitation transfer takes place into Rydberg states of NO₂**. Subsequently, the highly excited NO₂** molecules dissociate into NO and atomic O* Rydberg atoms.     

The ESIPT mechanism of dibenzimidazolo diimine sensor: a detailed TDDFT study

Spectroscopic investigations on excited state proton transfer of a new dibenzimidazolo diimine sensor (DDS) were reported by Goswami et al. recently. In our present work, based on the time‐dependent density functional theory (TDDFT), the excited‐state intramolecular proton transfer (ESIPT) mechanism of DDS is studied theoretically. Our calculated results reproduced absorption and fluorescence emission spectra of the previous experiment, which verifies that the TDDFT method we adopted is reasonable and effective. The calculated dominating bond lengths and bond angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared vibrational spectra. Further, hydrogen bonding strengthening manifests the tendency of ESIPT process. The calculated frontier molecular orbitals further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O–H coordinate, the potential energy barrier of about 5.02 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 0.195 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Determination of the stern-volmer constant in quantum systems with dual fluorescence

The mathematical relations describing the properties of the steady-state spontaneous emission of quantum systems with dual fluorescence under conditions of dynamic quenching of excited states by foreign impurities are analyzed. The direct dependence of the intensity and yield of the photoproduct fluorescence on the quencher concentration is not simple and cannot serve as a convenient base for determining the Stern-Volmer constant. It is shown that, in the case of a kinetic character of product formation, the fluorescence intensity ratio of the initial dye and its photoproduct linearly increases with the quencher concentration. The relation obtained can be used to determine the constant of bimolecular quenching of the excited states of reaction products. This conclusion is based on the analysis of the experimental fluorescence spectra of 3-hydroxyflavone, obtained upon excitation in the region of the S ₁ absorption band under conditions of dynamic quenching by potassium iodide. This analysis can be applied to a wide range of luminophores with photoreactions accompanied by dual fluorescence (charge transfer, proton transfer, phosphorescence, complexation, etc.).     

Dynamic quenching of the multiband fluorescence of 3-hydroxyflavone

The properties of emission, absorption, and dual fluorescence excitation of 3-hydroxyflavone in acetonitrile are studied under the conditions of dynamic quenching by potassium iodide with concentrations up to 4 × 10^?2 M. The normal and tautomeric forms undergo quenching, which is more efficient for the tautomeric form. An interesting circumstance is that the absorption in the S ₀ → S ₁ and S ₀ → S ₂ singlet bands of the solution increases with increasing quencher concentration in the whole region of concentrations used, the steepest rise being recorded in the concentration region from 0 to 5 × 10^?3 M. The intensities and quantum yields of the two fluorescence bands show rather complicated nonlinear dependences on the quencher concentration. The long-wavelength fluorescence band, which belongs to the tautomeric form of 3-hydroxyflavone, is quenched considerably stronger. The experimental results reveal the kinetic character of the excited-state proton transfer in molecules of 3-hydroxyflavone in acetonitrile.     

Intra- and intermolecular proton transfer in methyl-2-hydroxynicotinate

Spectral characteristics of methyl 2-hydroxynicotinate (MEHNA) have been studied using absorption, fluorescence excitation and fluorescence spectroscopy, as well as, using single photon counting nanosecond spectrofluorimeter. MEHNA is present as enol in less polar solvents and keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to phototautomer, formed by excited state intramolecular proton transfer (ESIPT), whereas fluorescence is only observed from keto form in polar solvents. In aqueous and polar solvents monocation (MC) is formed by protonating the exo carbonyl oxygen atom in the ground state (S₀) and in the first excited singlet state (S₁), MC is obtained by protonating carbonyl oxygen atom of the ester. It is formed by ESIPT from exo carbonyl proton to carbonyl oxygen atom of the ester. Dication is formed by protonating both the oxygen atoms. Two kinds of monoanions formed by deprotonating phenolic proton or >N-H proton of keto suggest the presence of enol and keto in aqueous solution. In cyclohexane MC is formed by protonating carbonyl oxygen in both S₀ and S₁ states. The electronic structure calculations were performed on each species using semi-empirical quantum mechanical AM1 method and density functional theory B3LYP with 6-31G^** basis set using Gaussian 98 program, along with potential energy mapping, to characterize the particular species.     

黄芩素激发态分子内质子转移耦合电荷转移反应的实验和理论研究

通过稳态光谱实验和量子化学计算相结合,研究了黄芩素激发态质子转移耦合电荷转移的反应. 实验和计算中S₁态吸收峰的缺失表明S₁态是暗态. S₁暗态导致在实验中观察不到黄芩素在乙醇溶液中的荧光峰,且固体的荧光峰很弱. 黄芩素分子的前线分子轨道和电荷差异密度表明S₁态是电荷转移态,然而S₂态是局域激发态. 计算的黄芩素分子的势能曲线在激发态只有一个稳定点,这表明了黄芩素激发态分子内质子转移的过程是一个无     

Theoretical investigation on the excited‐state intramolecular proton transfer mechanism of 2‐(2′‐benzofuryl)‐3‐hydroxychromone

Spectroscopic studies on excited‐state proton transfer of a new chromophore 2‐(2′‐benzofuryl)‐3‐hydroxychromone (BFHC) have been reported recently. In the present work, based on the time‐dependent density functional theory (TD‐DFT), the excited‐state intramolecular proton transfer (ESIPT) of BFHC is investigated theoretically. The calculated primary bond lengths and angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared (IR) vibrational spectra as well as the calculated hydrogen bonding energies. Further, hydrogen bonding strengthening manifests the tendency of excited state proton transfer. Our calculated results reproduced absorbance and fluorescence emission spectra of experiment, which verifies that the TD‐DFT theory we used is reasonable and effective. The calculated Frontier Molecular Orbitals (MOs) further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O―H coordinate, the potential energy barrier of about 14.5 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 5.4 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

The influence of temperature and dynamic quenching on the properties of 3-hydroxyflavone excited states

The influence of temperature and dynamic quenching on the properties of excited states of the normal and tautomeric 3-hydoxyflavone forms was studied. The stationary two-band fluorescence spectra of this luminophore in acetonitrile were recorded and analyzed. The spectra were observed under excitation by electromagnetic radiation in the region of the S ₁ absorption band over the temperature range 20–80°C. TEMPO was used as a quencher of the excited state. Heating caused temperature quenching of luminescence, and the tautomer formed via the excited state of the normal form of the luminophore was quenched more strongly both in pure solvent and in the presence of the quencher. An analysis of two-band fluorescence parameters led us to conclude that solution heating over the temperature range studied increased the rate of proton transfer by 1.25 times. The introduction of the quencher also accelerated proton transfer by 1.16–1.25 times as the temperature increased from room temperature to 80°C.     

Vapor-phase fluorescence spectra from the second excited singlet state of pyrene and its derivatives

Vapor-phase fluorescence spectra have been measured for pyrene and its simple derivatives, i.e., pyrene-d₁₀, 1-methylpyrene, and 4-methylpyrene. Each of these derivatives shows a weak fluorescence emission which is similar to the fluorescence from the second excited singlet state (S₂) of pyrene and is to be assigned to the S₂-fluorescence. The methyl substitution causes frequency shifts (Δν) in both absorption and fluorescence, and the Δν values for absorption and fluorescence transitions that are associated with the same excited state, i.e., the first excited singlet state (S₁) or the second excited singlet state (S₂), are approximately equal to each other. When the excitation energy increases, the S₂-fluorescence shifts gradually to the red in almost the same way as the S₁-fluorescence. The S₂-fluorescence spectrum has a sort of mirror-image relation to the S₂-absorption. A comparison of S₂-emissions of pyrene and pyrene-d₁₀ suggests that the ratio between the quantum yields of S₂- and S₁-fluorescence may be related to a ratio $\text{?}{}_2\text{?}{}_1$, where ?₁ and ?₂ are the densities of vibrational states in S₁ and S₂ at the energy of excitation.     

Theory of Direct Photoisomerization of Stilbene

A microscopic model for describing the direct trans-cis photoisomerization of stilbene is developed and an interpretation of the experimental results found from time-resolved fluorescence and absorption spectroscopy of stilbene in solution is given. According to this model the photoisomerization of trans-stilbene is proposed to involve the radiationless transition from the trans singlet state S₁(trans), prepared by direct optical excitation, into the perp singlet state S₁ (perp) and the radiationless decay of this state into the quasi-isoenergetic trans triplet state T₄ (trans).     

Intersystem crossing in oligothiophenes studied by fs time-resolved spectroscopy

High triplet quantum yields of more than 90% for bithiophene and terthiophene have to be connected with very fast and effective formation of triplets after excitation. We studied these processes with fs pump–probe spectroscopy. The time behaviour of transient optical spectra within the singlet and triplet manifold was examined for bi- and terthiophene (2T and 3T) in solution. For 2T we used two-photon absorption for excitation. We found transient spectra of the excited singlet state, the triplet state and that of radical cations. The kinetics of the excited-state absorption was described by a bi-exponential function. Additionally we observed formation and recombination of radical cations. The recombination is connected with triplet formation. Both processes could be described by a time constant of 62 ps±9 ps. For 3T we found a dependence of the processes on excitation energy using one-photon absorption. The triplet quantum yield increased with higher excitation energy. The kinetics becomes bi-exponential with increasing amplitude of the short time constant of 2 ps at increasing excitation energy. The main reasons for the effective intersystem crossing (ISC) in both oligothiophenes are – besides the high spin-orbit coupling factor introduced by the sulphur atom – the almost isoenergetic positions of the S ₁ and T ₂ states, detected by PD-PES [1]. At higher photon excitation energy for 3T above the band gap an additional channel for ISC was detected. We believe that during the geometric change from the non-relaxed non-planar to the relaxed planar excited state S ₁, ultrafast intersystem crossing takes place. Received: 6 December 1999 / Published online: 2 August 2000     

Transient absorption spectra of tetraazaporphine and its reduced derivatives upon picosecond excitation

Transient absorption spectra of tetraazaporphine and substituted derivatives of tetraazachlorin and tetraazabacteriochlorin were obtained upon picosecond excitation with various probe pulse delay times in order to resolve the open question about the reasons for fluorescence quenching in tetraazaporphine hydrogenated derivatives. The quantum yield of triplet state T ₁ formation has been estimated. It has been shown that in all investigated cases radiationless de-excitation of the fluorescent level S ₁ occurs by two channels: S ₁ ⤳ T ₁ and S ₁ ⤳ S ₀, the latter being predominant. As the S ₁ level becomes lower, the transition rate for this channel increases. For tetraazaporphine and its derivatives, a quasi-photochemical mechanism is proposed that accounts for the anomalous efficiency of the S ₁ ⤳ S ₀ channel in the dissipation of the electronic excitation energy.