Andrzej Mordziński

Intramolecular proton transfer in excited benzoxazoles

Abstract Exicted-state proton transfer in 2-(2′-hydroxyphenyl)benzoxazole (PBX-OH) was quantitatively studied using model compounds PBX and PBX-OCH 3 . Low temprature as well as OH → OD substitution slow proton transfer. No isotope effect on tautomeric fluorescence or primary phosphorescence life times is observed. Proton transfer in excited PBX-OH occurs from the S 1 state with very low activation energy.

Intramolecular single and double proton transfer in benzoxazole derivatives

Abstract The “double” derivatives of benzoxazole, bis-2,5-(2-benzoxazolyl)hydroquinone (II) and bis-3,6-(2-benzoxazolyl)-pyrocatochol (III), have been investigated. In (II), only one proton is transferred in the S 1 state. Primary and tautomeric forms exist in a rapidly established equilibrium. In (III), two tautomers were detected. One is generated in the S 1 state by a double proton transfer without a potential barrier, while the other, generated by a single proton transfer, is already present in trace amounts in the S 0 state.

Laser-induced absorption and fluorescence studies of photochromic Schiff bases

Abstract Three photochromic Schiff bases: N-salicylideneaniline (SA), N-salicylidene-1-naphthylamine (SN), and N,N′-bis-(salicylidene)- p -phenylenediamine (BSP), were studied in acetonitrile by means of steady-state and time-resolved absorption and fluorescence spectroscopy, as well as semiempirical quantum chemical calculations. In all these molecules, the transient absorption and two-step laser-induced fluorescence spectra of long-lived transients are remarkably similar. The photochromic species is tentatively assigned to the non-hydrogen bonded form of the proton transfer reaction product with a considerable contribution of zwitterionic character.

Proton tunnelling in porphycene seeded in a supersonic jet

Abstract Porphycene, a constitutional isomer of porphyrin, was investigated in a supersonic jet expansion using laser-induced fluorescence and hole burning techniques. The lines recorded in the fluorescence excitation spectrum were split into doublets. No splitting was detected if one or both inner hydrogen atoms were replaced by deuterium. The doublet structure also disappeared upon formation of a complex between porphycene and water or alcohol. These findings indicate that the origin of the doublet structure in bare porphycene is the tunneling splitting due to the exchange of two inner protons between the nitrogen atoms. From the spectral response to deuteration, and to a change in cooling conditions, it is deduced that the tunneling splitting is higher in the ground state than in the first excited singlet state.

Picosecond kinetics of the excited-state intramolecular proton transfer reaction: Confirmation of the intrinsic potential barrier in 2,5-bis-(benzoxazolyl)-hydroquinone

Abstract The kinetics of previously reported excited-state intramolecular proton transfer (ESIPT) reactions were investigated using single-photon counting techniques of high temporal resolution: 2,5-bis-(benzoxazolyl)-hydroquinone undergoing single PT emits two fluorescence bands, the primary (450 nm) and tautomeric (603 nm). The bimodal kinetics of the equilibrated reversible reaction was confirmed as well as the intramolecular character of the primary emission. Two kinetic components were detected: one representing forward and back PT reactions (≈20 ps), and the second representing the common decay of its partners (≈960 ps . The kinetics of an ESIPT has thus been directly observed at room temperature.

The role of the triplet state in depopulation of the electronically excited proton-transferring system [2,2′-bipyridyl]-3,3′-diol

Abstract Electronic excitation of [2,2′-bipyridyl]-3,3′-diol (BP(OH) 2 ) generates the diketo-tautomeric form of the system. The long-lived transient absorption (TA) of BP(OH) 2 was assigned as the T-T absorption of the diketo-tautomer. The effect of the stimulated emission pumping on TA excludes the essential contribution of the absorption from the ground-state diketo form. The rate constants of T-T annihilation and of oxygen quenching are k TTA = 2.4 × 10 10 M −1 s −1 and k q O 2 = (2–3) × 10 9 M −1 s −1 . The contribution of the second-order process to the overall decay of the triplet BP (OH) 2 , and the origin of the delayed fluorescence of the diketo form, are discussed.

Reversible intramolecular proton-transfer reactions of electronically excited “double” benzoxazoles: A direct observation of the effect of the intrinsic barrier

Abstract Picosecond kinetics of excited-state single- or double-proton transfer along intramolecular hydrogen bonds were analyzed for three molecules: 2,5-bis-(2-benzoxazolyl)hydroquinone, its monomethoxy-derivative and 3,6-bis-(2-benzoxazolyl)pyrocatechol. Forward and back proton-transfer rate constants were determined. They are all of the order of 10 10 s −1 , i.e. considerably lower than expected for this class of reactions.

Excited state proton transfer in jet-cooled 2,5-di-(2-benzoxazolyl)phenol

Abstract The molecule 2,5-di-(2-benzoxazolyl)phenol (DBP) has been studied by laser spectroscopy in a supersonic free jet. In the jet the fluorescence with a Stokes shift of 7000 cm −1 arises exclusively from the proton-transferred species. The fluorescence excitation spectrum of DBP exhibits the broad fluorescence background underlying some weak, sharp features. The first prominent band has been observed at 27830 cm −1 , but from 250 cm −1 above the origin the spectrum becomes congested and broad. OH→OD substitution of DBP produces a blue shift of 103 cm −1 . For the deuterated sample (DBP-d 1 ) resolved line structure in the fluorescence excitation spectrum has been observed up to an excess energy of 400 cm −1 . To identify spectral features corresponding to deuterated DBP and other species double-resonance depletion spectroscopy has been applied. From analysis of the width of the vibronic bands of DBP-d 1 , we estimated an upper limit of the excited state deuteron transfer rate constant, k DT =1.7×10 12 s −1 .

Mechanism of excited-state proton transfer in “double” benzoxazoles: Bis-2,5-(2-benzoxazolyl)hydroquinone

Abstract Two molecules, bis-2,5-(2-benzoxazolyl)hydroquinone (II) and bis-3,6-(2-benzoxazolyl)pyrocatechol (III), undergo a very effective excited-state intramolecular proton transfer. The single proton transfer in II involves an energy barrier on the potential energy curve. The proton-transfer reaction is described as a fully reversible process, Δ H 0 = −0.5 kcal/mole. MoIecule III shows a barrierless, double proton transfer; the activation energy of the radiationless decay rate of the excited tautomer, Δ E , is 4.1 kcal/mole.

The electronic spectrum of benz[a]anthracene. Linear and magnetic circular dichroism and fluorescence polarization studies

Abstract The electronic transitions of benz[a]anthracene are investigated by linear dichroism spectroscopy in stretched polyethylene, fluorescence polarization spectroscopy, magnetic circular dichroism spectroscopy, and by quantum mechanical calculations, leading to the assignment of at least eight and possibly ten transitions in the near-ultraviolet region. Accurate experimental moment directions are derived for six electronic transitions. Moment directions and MCD B -terms predicted by a recently developed LCOAO procedure are in good agreement with the experimental results. Standard CNDO and INDO methods fail to predict the moment directions for weak transitions, a failure which seems to be associated with a too strong breakdown of the alternant hydrocarbon pairing symmetry in these methods.