PHOTOSTATIONARY FLUORESCENCE EMISSION AND TIME RESOLVED SPECTROSCOPY OF SYMMETRICALLY DISUBSTITUTED ANTHRACENES ON THE MESO AND SIDE RINGS - THE UNUSUAL BEHAVIOR OF THE 1,4 DERIVATIVE

Steady state and time resolved fluorescence emission properties of symmetrical dialkoxyanthracenes (especially substituted on the side rings) 1-X, Y were studied in methylcylohexane. At room temperature, the fluorescence spectra of 1-X, Y show bands in the region of 380-550 nm and quantum yields (phi(F)) in the range of 0.2-1. The fluorescence emission decays were found to be single exponential. The determination of the intersystem crossing quantum yields (phi(isc)) for the weakly fluorescent compounds (1-1,5, 1-1,8 and 1-2,3) demonstrates that internal conversion is negligible compared with fluorescence emission and intersystem crossing, as previously observed for other anthracene derivatives. The fluorescence emission efficiency of compounds 1-X, Y is controlled by the relative mutual positions of the second triplet T2 (whose energy varies significantly with substitution) and the first excited singlet S1 states, respectively. An unusual solvatochromism was found for compound 1-1,4 which has a very weak permanent dipole moment in the ground state. This behavior was assigned to strong changes in the electronic densities between the excited singlet state and the ground state.