Photoinduced Proton-transfer Processes Research Articles

Conformational effects on the photoinduced proton-transfer processes in 1-methyl-2-(3′-hydroxy-2′-pyridyl)benzimidazole

The ground- and excited-state behaviour of 1-methyl-2-(3′-hydroxy-2′-pyridyl)benzimidazole (MeHPyBI) in various solvents has been studied by UV–Vis absorption spectroscopy and by time-resolved and steady-state fluorescence spectroscopy. Three species were detected in the ground state in aqueous solutions of pH near to neutrality: a non-planar enol form, a planar cis-enol form and a planar keto tautomer. Electronic excitation of both the cis-enol form and the keto tautomer yields the excited keto form, the former through an ultrafast excited-state intramolecular proton-transfer (ESIPT). The non-planar enol form cannot undergo ESIPT and partly deprotonates at the hydroxy group to afford the anion. In non-aqueous solvents only the cis-enol form was detected in the ground state, yielding the keto tautomer in the excited state. The first protonation of MeHPyBI takes place at the benzimidazole N(3). Whereas the photoexcited cation shows fluorescence in acidified acetonitrile, it deprotonates at the hydroxy group in acidified ethanol, yielding the keto tautomer. In acidic aqueous solution, the excited cation undergoes a two-step tautomerization by two different routes to afford a keto cation, protonated at the pyridyl N. All the protonated and deprotonated forms of MeHPyBI are non-planar in the ground state both in water and ethanol, but they adopt a planar structure in the excited state. Comparison of the behaviour of MeHPyBI with related molecules reveals that the existence of planar forms and the neutral keto species in the ground state is favoured by the presence of the pyridyl nitrogen. The conformational equilibria in the ground state are crucial for the excited-state behaviour of these types of molecules.

Hyper-Rayleigh scattering investigation of nitrobenzyl pyridine model compounds for optical modulation of the hyperpolarisability

Hyper-Rayleigh scattering (HRS) is used to determine the molecular hyperpolarisability (β) of model compounds for the two tautomeric forms of nitrobenzyl pyridine (NBP) derivatives. For each derivative large differences (a factor of 10 to 200) are found between the two forms which indicates that the NBP system can be used to optically modulate second-harmonic generation through a photoinduced proton transfer process.

Femtosecond‐Picosecond Laser Photolysis Studies on Proton Transfer Process of Excited 1‐Pyrenol‐Triethylamine Hydrogen Bonding Complex in Solutions

AbstractThe proton transfer (PT) process in the excited hydrogen bonding (HB) complex of 1‐pyrenol and triethylamine, leading to the formation of a hydrogen‐bonded ion pair (HBIP*) of excited pyrenolate anion and triethylammonium cation, PyO−* ··· H‐N+(C2H5)3, and subsequent reaction processes of HBIP* in various solvents were investigated by picosecond and femtosecond laser photolysis, as well as by steady‐state absorption and fluorescence measurements. Time constants for the PT process taking place in the time region of a few picoseconds were almost independent of the polarity of the solvent and it was suggested that the intermolecular slow motion in the HB complex, such as the intermolecular vibration and/or the torsional motion of the two moieties, might regulate the PT process. Moreover, the ionic dissociation process following the photoinduced PT process in polar solvent was directly measured. The dynamic behaviors of the excited intermolecular HB complex and underlying mechanisms are discussed by comparing these results with those on the intramolecular photoinduced PT process and with results of investigations of electron transfer systems.