The fluorescence of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in nonpolar organic liquids for excitation energies below and above the photoionization threshold

Abstract

The fluorescence from TMPD has been studied in the solvents n-hexane, methylcyclohexane, cyclohexane, isooctane, and tetramethylsilane as a function of excitation wavelength from λex=300 nm to λex≃170 nm. In no solvent is there observed any apparent decline in the fluorescence yield Φf for excitation energies to ≈0.7-0.9 eV above the photoionization threshold. At higher energies Φf declines slightly. However, in the presence of low concentrations of either carbon tetrachloride, chloroform, ethylbromide, or n-perfluoroheptane there is observed an abrupt increase in the quenching efficiency of the TMPD fluorescence almost precisely at an excitation energy equal to the threshold energy for photoionization εt. The quenchable species Se generated at εt, is found to have the following properties: (a) the probability of its quenching follows the usual Stern-Volmer form characteristic of the quenching of a spatially coherent excited state and not of a geminate ion pair; (b) the quantum yield ηe for the production of Se in isooctane exhibits two maxima at εex=5.64eV (ηe=0.41) and εex=7.1 eV (ηe=0.66) and appears generally to be relatively insensitive to the nature of the solvent; (c) the quenching constant of Se exhibits a strong variability with the nature of the quencher and of the solvent which is qualitatively similar to the variability of the reaction rate constant of a "quasifree" electron with these quenchers in these solvents; (d) the quenching constant of Se continuously changes as εex increases above εt but the quenching probability continues to retain the Stern-Volmer form. All of these properties are shown to be consistent with the identification of Se as a diffuse but coherent excited state of TMPD and as the precursor (rather than a geminate ion pair) of the "escaped" electron observed in photoionization experiments. Copyright © 1977 American Institute of Physics.