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 T_1x 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 3 MeP/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 T^η 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 α-and β-relaxation processes in supercooled liquids, SLR_h and MER are β-processes with roughly the same activation energy, and the slow complete OR is an α-process with roughly the same activation energy as that of the viscosity. A fast partial OR accompanying SLR_h and MER is predicted.