Recombination luminescence of tetramethyl- p-phenylene diamine in glassy solutions

Abstract

Recombination luminescence (RL) induced by U.V. illumination of glassy solutions of N,N,N′,N′-tetramethyl- p-phenylenediamine (TMPD) in dipropyl ether (DPE) or in a mixture of methylcyclohexane and isopentane has been studied. Under given conditions of U.V. illumination the sum of the amounts of light emitted isothermally at 77 K, S 1, and on warming the glass, S 2, is independent of any variation in S 1 and S 2, showing that the probability of photon emission is independent of the recombination temperature (in the range 77–113 K for DPE). During RL, under any conditions of afterglow, there is no proportionality between the fraction of the total light emission produced and the fraction of TMPD + having disappeared: in some cases about 50 per cent of the total emission can be produced without any measurable decrease in TMPD + concentration. The spectrum of RL is a mixture of fluorescence, F, and phosphorescence, P, of TMPD. The ratio P/F depends strongly on the RL conditions: if the glass is warmed immediately after the end of an U.V. illumination only phosphorescence is observed, and the ratio, total number of photons emitted/total number of TMPD + formed, is equal to (0·1±0·0.4) δ, where δ is the probability of photon emission from the triplet state of TMPD. Only a small part of electron-TMPD + recombinations lead to photon emission. The quantum efficiencies both of trapped electron photobleaching and photostimulated RL increase monotonically with exciting wavelength from 1600 to 800 nm. In the presence of electron acceptors (naphthlene, biphenyl, benzophenone) the concentration of trapped electrons decreases much more than the emission of light. In contradistinction to hydrocarbon solutions, the addition of electron acceptors to DPE strongly diminishes the emission due to warming the glass, as a result of the secondary processes taking place on softening the glass. The mechanism of RL is discussed in terms of the hypothesis of electron tunnelling.