Singlet-Singlet Electronic Excitation Energy Transfer in a Bichromophore Cooled in a Supersonic Jet

Abstract

The fluorescence excitation spectra and polarization of the fluorescence of a bichromophore cooled in a supersonic jet are measured. The bichromophore consists of 2,5-diphenyloxazole (PPO) and 1-(5 ′ -phenyl-1 ′ ,3 ′ -oxazole-2 ′ -yl)-4-(5 ′ -phenyl-1 ′ ,3 ′ ,4 ′ -oxadiazole-2 ′ -yl)-benzene (POPDP) connected by the methylene bridge -(CH 2 )-. Based on the analysis of these components and theoretical calculations, it has been proven that the total intramolecular singlet-singlet transfer of the energy of electronic excitation from the energy donor PPO to the acceptor POPDP occurs in a time of 10 −12 sec.Introduction. In a condensed medium, the properties of bichromophore molecules have been studied rather adequately. The electronic excitation energy transfer for molecules having continuous absorption and fluorescence spectra is described by inductive-resonance and exchange-resonance mechanisms [1]. At the spaces between interacting molecules comparable with the Van der Waals radii the approximations underlying this theory are disturbed. In [2,3], the bichromophore molecule is considered as a single system with intramolecular radiationless energy transfer. Recently, works have been carried out [4,5] dealing with the study of the transfer of electronic energy in aromatic bichromophores cooled in supersonic jets.In the present work, for the first time in investigations of electronic excitation energy transfer in bichromophore molecules cooled in a supersonic jet we have used fluorescence polarization. The object of investigation was the heterocyclic bichromophore molecule consisting of 2,5-diphenyloxazole (PPO) and 1-(5 ′ -phenyl-1 ′ ,3 ′ -oxazole-2 ′ -yl)-4-(5 ′ -phenyl-1 ′ ,3 ′ ,4 ′ -oxadiazole-2 ′ -yl)-benzene (POPDP) -a molecule that has not been studied before. The PPO fragment of the bichromophore acts as the energy donor and POPDP -as its acceptor. Chromophores are connected by the methylene -CH 2 -bridge. The absorption spectrum of POPDP is well overlapped with the fluorescence spectrum of PPO in both an ethanol solution and a high-temperature vapor, which provides favorable conditions for efficient intermolecular energy transfer. These compounds are active media of lasers on solutions, and PPO -also of the vapor laser.Experimental. A spectrometric complex for measuring the excitation spectra and degree of polarization of fluorescence of jet-cooled complex molecules has been described in [6]. The vapor of the bichromophore studied was heated to a temperature of 530-570 K, and its mixture with helium at a pressure of up to 3 atm admitted to a vacuum chamber by a pulse valve with a nozzle of diameter 0.7 mm at a repetition rate of 4 Hz. The bichromophore molecules not interacting with each other were cooled to a rotational temperature of 7 K. Excitation of the molecules of interest was done at a distance of 12 mm from the nozzle by radiation of the second harmonic of a dye laser with a spectral width of the line of 1 cm −1 tunable in a region of 28,000-34,000 cm