Effect of temperature and phase on the photoionization energy threshold of TMPD in hydrocarbon solvents

Abstract

The temperature dependence of the TMPD photoionization energy threshold, I, in a set of eight hydrocarbon solvents has been systematically studied, from room temperature down to 77 K. For this purpose ion-pair formation is detected either by measuring the photocurrents in the liquid phase or by measuring the recombination luminescence intensity in the solid phase. It is found that I(liquid) increases linearly when the temperature is decreased down to the liquid–solid transition temperature. At this temperature I undergoes an abrupt increase of 0.2–0.6 eV depending on the hydrocarbon and the nature of the phase transition. Any subsequent solid–solid phase change is accompanied by a new shift towards higher energy. In the low temperature phase of all the studied crystals, I(solid) has a constant value down to 77 K. Glass-forming liquids have a very different behavior: I varies linearly in the liquid and the straight line extrapolates to the I(glass) value at 77 K. The applicability of the two methods is discussed. From the present data it is concluded that the conduction state energy, V0, is constant in the low-temperature phase of crystals. By calculating the polarization energy due to the TMPD+ cation and from data on the temperature dependence of V0 in the liquid phase, we have estimated V0 in crystalline n-hexane (0.64 eV) and 2,2,4-trimethylpentane (0.44 eV) and in the plastic phase of cyclohexane (0.47 eV) and 2,2-dimethylbutane (0.01 eV). Finally a correlation of I with the medium density is described.