Electronic Absorption and Fluorescence Spectra of Anthracene, [D10]-Anthracene, and Acridine in n-Alkane Matrices at 4°K

Abstract

A systematic examination has been made of the electronic absorption and fluorescence spectra of anthracene and [D10]-anthracene in n-hexane, n-heptane, and n-octane matrices at 4°K. Vibrational analyses have been made and the correspondence between most of the modes of the two molecules has been established. A number of errors in previously published anthracene data are pointed out. The existence, in the fluorescence spectrum of anthracene in n-heptane at 4°K, of a line which lies 391 cm−1 above the electronic origin has been shown to be part of the environment multiplet structure characteristic of these matrix spectra. It is clear that the line has no connection with the intense 390-cm−1 vibrational frequency since its position with respect to the main origin is the same in the spectrum of [D10]-anthracene, for which the vibrational frequency is 376 cm−1. A possible hypothesis that fluorescence was occurring from vibrationally excited molecules is therefore ruled out. The distinction between (a) “environment multiplets” for molecules in different crystal environments and (b) spectral components deriving from local lattice vibrations is stressed. The environment multiplet structure of the undeuterated and deuterated molecules are found to be virtually identical. The 4°K spectrum of acridine in n-heptane shows some weak sharp lines on broad background of absorption. The line shapes of the 77°K anthracene and 4°K acridine spectra are discussed in terms of the relative half-widths and intensities of components representing no-phonon and phonon transitions, respectively.