Polarized fluorescence of β-carotene and related polyenes

Abstract

The spectral and polarization characteristics of trans-isomers ofβ-carotene, lycopene, retinal, and 1,6-diphenylhexatriene (DPH) in solutions have been investigated by the methods of polarized fluorescence and semiempirical calculations of AM1 and RM3. The high and constant values of the degree of polarization P ∼0.46, closely coinciding with the limiting value P0=0.5, according to the S2→S0-fluorescence and S2←S0-fluorescence excitation spectra ofβ-carotene and lycopene in n-hexane at room temperature and also high and coinciding according to the S1→S0-fluorescence and S2←S0-fluorescence excitation spectra for retinal in EPA at 77 K and DPH in a polyvinylbutyral film, point to the coincidence of the directions of absorption and emission oscillators. These oscillators are aligned along the major axis of the polyene molecule, and the longwave S2←S0-absorption band is formed only by one electronic1B u + ←1A g − -transition. The oscillators of shortwave Sa←S0-absorption bands are aligned at an angle to the oscillators of the S2←S0-transitions, which is related to the distortions in the structure of molecules (β-carotene, lycopene) or, as in the case of DPH, to the influence of transitions localized on the orbitals of phenyl rings and polarized perpendicularly to the major axis of the molecule.