EFFECTS OF TEMPERATURE ON THE TRIPLET BEHAVIOR OF THE STYRYLPHENANTHRENE ISOMERS

The properties of the triplet state of five styrylphenanthrene (StPh) trans isomers were studied in 2-methyltetrahydrofuran (MTHF) as a function of temperature. At room temperature the T-T absorption was observed only for 4- and 9-StPh, while under these conditions 1-, 2-, and 3-StPh have too low a quantum yield of triplet formation (Phi(T) < 0.02); their T-T absorption spectra were obtained at low temperature. Phi(T) of 1- and 2-StPh increases more than tenfold on going from 293 to 77 K, and the triplet lifetime (tau(T)) increases by four orders of magnitude and approaches values of 5-40 ms at 77 K. The change in tau(T) is explained in terms of an equilibrium between trans and perpendicular (perp) conformations of the lowest triplet state in fluid solution and temperature and viscosity effects on the trans --> perp rotation. Evidence is presented for the existence of two conformeric trans triplet states of 3-StPh at 77 K. Semi-empirical calculations were performed to obtain the energy of the triplet state, the wavelengths of several T-T absorption maxima (lambda(T)), and the oscillator strength. The calculated lambda(T) values coincide with those measured in n-hexane.