Fluorescence quenching in bichromophoric systems with nonconjugated chromophores: 5-substituted derivatives of l,3,5-triaryl-2-pyrazoline

The spectral and fluorescent properties of l,3,5-triphenyl-2-pyrazoline derivatives with acceptor substituents in benzene ring in the 5 position of the pyrazoline cycle were investigated. The corresponding pyrazoline derivative, in which phenyl radical in the 5 position is replaced by a fragment with an extended conjugated system-2.5-diphenyloxazolyl-was studied as well. The noticeable decrease in fluorescence quantum yield in comparison with the nonsubstituted molecule is characteristic of the compounds studied, which belong to the broader class of bichromophoric molecules with nonconjugated chromophores. The existence of excited states of “ mixed ” or “cross” (charge transfer) type, formed with participation of occupied molecular orbitals of one chromophoric fragment and vacant molecular orbitals of another, is typical of such a bichromophoric system. On the basis of our experimental and theoretical data, the assumption was made that the fluorescence quenching in the investigated bichromophoric molecules could be connected with the thermal population of the excited states of “mixed” type.