Triplet-Triplet Electronic Absorption Spectroscopy Using a Fourier Transform Infrared Spectrometer

Abstract

The nature and location of the triplet states of organic molecules has been of considerable interest for 40 years. Because of the metastable character of the lowest triplet (T1) levels, it is possible to obtain a considerable population in T1 by optical pumping. Transitions from this triplet to higher lying triplets may be induced by simultaneous excitation with a suitable probe source. Lewis and Lipkin were the first to observe triplet-triplet (T-T) absorption in an organic molecule. Since that time many investigators have successfully used the method to investigate excited triplet states. Two basic techniques for T-T absorption have been developed. In the first, introduced by McClure, steady-state illumination of the sample is used. The second, developed by Porter and co-workers, uses pulsed pump and probe radiation sources. By and large, in the great majority of reports on T-T absorption published to date, using either CW or pulsed techniques, the probe radiation has been confined to the visible and ultraviolet regions. These high probe energies ensure that only those excited triplets which lie at energies far removed from T1 will be investigated. It is, however, generally expected that those triplets lying in closer proximity to T1 will play a significant role in the mechanisms of nonradiative decay, photodegradation, etc., yet no systematic studies of T-T absorption in the infrared range have appeared. For a number of different classes of organic molecules (e.g., aromatic hydrocarbons and dicarbonyl compounds) there are theoretical predictions that one or more T-T transitions should lie in this range. To investigate these possibilities we have embarked on a program of T-T absorption in the infrared range using a Fourier transform (FT) IR spectrometer. In this note we report preliminary results on phenanthrene in a poly(methylmethacrylate) matrix that show that the application of an FT-IR spectrometer to T-T absorption measurements is feasible.