Vladimir Kharlanov

Excited-state intramolecular proton transfer (ESIPT) in 2-(2′-hydroxyphenyl)-oxazole and -thiazole

The azoles 2-(2 0 -hydroxyphenyl)oxazole (HPO) and 2-(2 0 -hydroxyphenyl)-4-methylthiazole (HPT) have been synthesised and studied in order to compare their photophysical characteristics. Their absorption and emission properties are reported in non-polar, alcoholic and aqueous media. Ground and excited state pK data were determined by spectroscopy and a model is proposed to explain the behaviour of HPT and HPO as a function of the pH. Excitation spectra and quantum chemical calculations suggest an equilibrium of ground state conformers. The calculations also predict a small energy barrier for rotation in the first excited singlet state for the proton transferred tautomers. The resulting twisted structure of the tautomer form possesses a biradicaloid nature, and is near-degenerate in energy with the first excited triplet state. ©2000 Elsevier Science S.A. All rights reserved.

Apparent Stoichiometry of Water in Proton Hydration and Proton Dehydration Reactions in CH3CN/H2O Solutions

Gradual solvation of protons by water is observed in liquids by mixing strong mineral acids with various amounts of water in acetonitrile solutions, a process which promotes rapid dissociation of the acids in these solutions. The stoichiometry of the reaction XH(+) + n(H(2)O) = X + (H(2)O)(n)H(+) was studied for strong mineral acids (negatively charged X, X = ClO(4)¯, Cl¯, Br¯, I¯, CF(3)SO(3)¯) and for strong cationic acids (uncharged X, X = R*NH(2), H(2)O). We have found by direct quantitative analysis preference of n = 2 over n = 1 for both groups of proton transfer reactions at relatively low water concentrations in acetonitrile. At high water concentrations, we have found that larger water solvates must also be involved in the solvation of the proton while the spectral features already observed for n = 2, H(+)(H(2)O)(2), remain almost unchanged at large n values up to at least 10 M of water.

Experimental and Theoretical Study of Excited-State Structure and Relaxation Processes of Betaine-30 and of Pyridinium Model Compounds

The long-wavelength absorption band of 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (betaine-30, B30) in ethanol and 1-chlorobutane shifts to the blue by cooling in the temperature range of 294-128 K. In addition, B30 shows fluorescence in both solvents at 77 K, which is absent at room temperature. The study of the ground and excited state of B30 and its model compound 4-(pyridinium-1-yl)-phenolate PyPo by DFT/TDDFT calculations indicates that for the perpendicular conformation, the ground state possesses a charge-separated closed-shell hole pair (hp) electronic configuration, and the S(1) state corresponds to a biradicaloid electronic structure (dd) with a small dipole moment caused by an unpaired electron on each of the orthogonal fragments. Following the absorption process, there is a driving force for geometrical relaxation within the S(1) state toward an orthogonal arrangement of the phenolate and the pyridinium ring. In this final S(1) equilibrium geometry, the energy gap between the excited and the ground state is strongly reduced and causes very efficient radiationless deactivation of the S(1) state at room temperature. At 77 K, the viscous barrier hindering large-amplitude motion enables the appearance of the fluorescence from the partially or nonrelaxed excited Franck-Condon structure. By variation of the donor and acceptor strength of the two moieties, the energy gap for perpendicular systems can be tuned, allowing, in principle, the switching between the two cases hp < dd and hp > dd. This enables a new access to the efficient construction of mnemonic systems and NLO dyes.

Excited-state relaxation properties of ionic and nonionic donor–acceptor biphenyl derivatives

Abstract A cationic and a neutral donor–acceptor biphenyl-type system are compared with respect to their photophysical properties. The strong intramolecular fluorescence quenching observed only for the ionic system can be traced back to an excited state twisting relaxation. This process is suppressed either by chemical bridging or by rigidizing the environment at low temperatures. Quantum chemical calculations indicate a strongly different shape of the excited state potential energy surface for the ionic compound, namely a barrierless intramolecular twisting towards perpendicularity, and a significant narrowing of the S 1 –S 0 energy gap upon twisting relaxation in the excited state. This suggests a close-lying conical intersection in the ionic case and explains the strongly different fluorescence behaviour of the two compounds.

Excited-state intramolecular proton transfer (ESIPT) in 2-(2'-hydroxyphenyl)pyridine and some carbon-bridged derivatives

Abstract The absorption and emission properties of 2-(2′-hydroxyphenyl)pyridine (HPP), 10-hydroxy-5,6-dihydrobenzo[h]quinoline (HdihBQ) and 10-hydroxybenzo[h]quinoline (HBQ), are reported in non-polar, alcoholic and aqueous environments. Ground and excited state pK values are presented, and compared with those of related derivatives. The important role of intramolecular rotational mechanisms and structural flexibility in the behaviour of these systems is suggested by (1) the temperature dependence of the fluorescence and (2) the increase in the quantum yields of the tautomeric emission across the series HPP - HdihBQ - HBQ. The existence of conformers of HPP is proposed and their energy as a function of the torsional angle is estimated by quantum chemical calulations. It is concluded that two non-radiative channels operate in HPP tautomer, one connected to twisting and the other related to nπ* deactivation.

Optical, Redox, and DNA-Binding Properties of Phenanthridinium Chromophores: Elucidating the Role of the Phenyl Substituent for Fluorescence Enhancement of Ethidium in the Presence of DNA

The phenanthridinium chromophores 5-ethyl-6-phenylphenanthridinium (1), 5-ethyl-6-methylphenanthridinium (2), 3,8-diamino-5-ethyl-6-methylphenanthridinium (3), and 3,8-diamino-5-ethyl-6-(4-N,N-diethylaminophenyl)phenanthridinium (4) were characterized by their optical and redox properties. All dyes were applied in titration experiments with a random-sequence 17mer DNA duplex and their binding affinities were determined. The results were compared to well-known ethidium bromide (E). In general, this set of data allows the influence of substituents in positions 3, 6, and 8 on the optical properties of E to be elucidated. Especially, compound 4 was used to compare the weak electron-donating character of the phenyl substituent at position 6 of E with the more electron-donating 4-N,N-diethylaminophenyl group. Analysis of all of the measurements revealed two pairs of chromophores. The first pair, consisting of 1 and 2, lacks the amino groups in positions 3 and 8, and, as a result, these dyes exhibit clearly altered optical and electrochemical properties compared with E. In the presence of DNA, a significant fluorescence quenching was observed. Their binding affinity to DNA is reduced by nearly one order of magnitude. The electronic effect of the phenyl group in position 6 on this type of dye is rather small. The properties of the second set, 3 and 4, are similar to E due to the presence of the two strongly electron-donating amino groups in positions 3 and 8. However, in contrast to 1 and 2, the electron-donating character of the substituent in position 6 of 3 and 4 is critical. The binding, as well as the fluorescence enhancement, is clearly related to the electron-donating effect of this substituent. Accordingly, compound 4 shows the strongest binding affinity and the strongest fluorescence enhancement. Quantum chemical calculations reveal a general mechanism related to the twisted intramolecular charge transfer (TICT) model. Accordingly, an increase of the twist angle between the phenyl ring in position 6 and the phenanthridinium core opens a nonradiative channel in the excited state that depends on the electron-donating character of the phenyl group. Access to this channel is hindered upon binding to DNA.

Multiple emission of N-(1-naphthyl)-pyridinium

The trimethyl derivative of N-(1-naphthyl)-pyridinium shows temperature dependent multiple fluorescence. Quantum chemical calculations indicate that several minima on the S1-hypersurface are responsible for this behaviour. Locally excited, resonance-type charge shift and biradicaloid charge shift states could be characterised.

Photophysics of arylsubstituted tropylium ions

Abstract Phenyl tropylium ion (TRPH) bearing different donor substituents were investigated both experimentally (absorption and fluorescence) and theoretically (semiempirical and ab initio calculations for ground and excited state structures and transitions). Generally, an increased donor strength of the substituent leads to a favoring of the planar geometry in the ground state. In the S 1 -state, however, energetic minimum develops for orthogonal structures, which are characterized by strong charge shift (CSh) (electron transfer from the substituted aryl to the tropylium fragment), forbidden transition moments and near-zero singlet–triplet energy gaps. These results can serve as a basis to interpret the completely non-emissive behavior of the tropylium derivatives with strong donor substituents.

Theoretical and Experimental Studies of N,N-Dimethyl-N′-Picryl-4,4′-Stilbenediamine

N,N-dimethyl-N′-picryl-4,4′-stilbenediamine (DMPSDA) was prepared, purified and crystallised in a form of black lustrous crystals, and its absorption and fluorescence spectra were recorded in cyclohexane, acetonitrile and dimethyl sulfoxide. Non-emissive intramolecular charge transfer state (ICT) was clearly observed in this molecule in all three solvents. Theoretical calculations demonstrating a betaine electronic structure of the trinitrophenyl group in the ground state of the molecule and a charge transfer nature of the long wavelength transition S0 → S1 supported the experimental observations of the ICT formation in the molecule.

Theoretical and spectroscopic studies of the photochemistry of 3-(4-dimethylaminophenyl)-7-methoxy-cyclohepta-1,3,5-triene

Abstract In this paper the photophysics and photochemistry of the title compound has been examined by laser, flash lamp and steady state methods. In protic solvents, photoheterolysis is able to compete with a rapid 1,7-hydrogen shift reaction to give the substituted tropylium methoxide, with the lifetime of the tropylium ion strongly depending on the nature of the alcohol. Hydrogen bonds between the solvent and the methoxy group of the cycloheptatriene derivative favor bond breakage and the solvation of the methoxide ion by protic solvents increases the lifetime of the free ions. Ab initio calculations suggest that the photoheterolysis in the gas phase takes places via a consecutive photohomolysis under formation of a radical pair, which then is undergoing an intra-pair electron transfer to form the tropylium and the methoxide ions. The 1,7-hydrogen shift reaction leads exclusively to the production of 5-(4-dimethylaminophenyl)-1-methoxy-cyclohepta-1,3,5-triene. The fluorescence of the title compound exhibits a very large Stokes shift indicating considerable geometrical changes in the relaxed excited state and according to semi-empirical and ab initio calculations, the seven-membered ring is planar in the fluorescing state.