Luminescence of oxotetrahydrobenzo[c]phenanthridine in polar and nonpolar solvents:: Inversion of the energy levels of the Sππ* and Tnπ* states

The spectral-luminescent properties and electronic-level structure of a derivative of oxotetrahydrobenzo[c]phenanthridine-3,3-dimethyl-1,2,3,4-tetrahydrobenzo[c]phenanthridine-1-one (DTP)-in polar and nonpolar solutions are studied by the luminescence and picosecond spectroscopy techniques at 293 and 77 K. It is shown that DTP exhibits pipi* fluorescence only in polar proton-containing methanol, ethanol, and n-propanol and that this fluorescence is associated with a chromophore + alcohol complex. Solutions of DTP in n-propanol and the nonpolar mixture methylcyclohexane, + toluene (6:1) at 77 K display pipi* phosphorescence with a quantum yield Phi(Ph) = 0.1, a lifetime tau(Ph) = 1.9 s, and a frequency of the 0-0 transition equal to 21550 cm(-1). The fact that DTP fluorescence is absent in aprotic solvents and exists in alcohols is explained by the inversion of the energy levels of the S-pipi* and T-npi* states, as a result of which the energy level of the fluorescing S-pipi* state in alcohol becomes the lowest. The picosecond spectra and kinetics of the induced S-n(pipi*) <-- S-1(pipi*) and T-k(pi*) <-- T-1(pipi*) absorption made it possible to estimate the lifetime of the S-1(pipi*) state of DTP in n-propanol at 293 K (20 +/- 10 ps) and the position of the triplet-triplet absorption peak (495 nm). (C) 2003 MAIK "Nauka/Interperiodica".