PHOSPHORESCENCE FROM PHENAZINE IN ALKANE SOLVENTS IN THE GLASS-TRANSITION RANGE - SPIN-LATTICE, ENVIRONMENT, AND ORIENTATION RELAXATION OF MOLECULES IN THE METASTABLE TRIPLET-STATE

The phosphorescence from phenazine in two similar solvents was investigated in the glass transition range with the aim to find a correlation between spin-lattice relaxation (SLR) of molecules in the metastable triplet state (T1) and their orientational relaxation (OR). The solvents were 3-methylpentane (3MeP) and a 1:1 mixture of 3-methylpentane and isopentane (3MeP/IP). Upon laser flash excitation, apart from the decay of the total population of T1, three relaxation processes of molecules in T1 were detected: (1) the decay of the selective population of the strongly phosphorescent triplet substate T1x due to SLR, (2) a time-dependent red shift of the phosphorescence spectrum resulting from molecular environment relaxation (MER), (3) the decay of the phosphorescence polarization due to OR. All three relaxation processes are in 3MeP slower than in the less viscous solvent 3MeP/IP, but a simple relation between OR and SLR has not been found. The temperature dependence of the SLR rate constant can be represented by the sum of two terms: a T7 or Arrhenius term dominant at low temperatures (SLR1) and an Arrhenius term dominant at high temperatures (SLR(h)). SLR1 is assigned to SLR processes characteristic for a hard glass, and SLR(h) is assigned to SLR resulting from rotational diffusion. In terms of the usual distinction of alpha- and beta-relaxation processes in supercooled liquids, SLR(h) and MER are beta-processes with roughly the same activation energy, and the slow complete OR is an alpha-process with roughly the same activation energy as that of the viscosity. A fast partial OR accompanying SLR(h) and MER is predicted.