Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

The fluorescence characteristics of 4-substituted chloro and fluorobenzenes I and II were studied in isooctane and acetonitrile solutions. It was found that all compounds exhibit a weak (substituent and solvent dependent) fluorescence in the range 305-370 nm with quantum yields 1.2 . 10^-2-2.3 . 10^-1. The relation between the substituent nature and fluorescence band position may be quantified by log ν_f^~ = ρσ_p + log ν_f^~0, the magnitude of the shift paralleling the donor strength of the substituent. Fluorescence quantum yields are increased approximately by a factor 2 on going from isooctane to acetonitrile and solvent-induced shifts are proportional to the static dipole moment change Δμ which occurs upon excitation. Radiative decay rate varies only slightly around a mean value of 5 . 10⁷ s^-1 and shows no substantial difference between chloro (I) and fluoro (II) derivatives. The non-radiative decay rate (of the order ~ 10⁹ s^-1) was found to be about 5 times higher in the case of chloro compounds due to the more efficient S₁ - T_n intersystem crossing.