Spectral evidence for the existence of green “incipient” dimers in single crystals of anthracene

Abstract

The fluorescence spectra and lifetimes of ultra pure anthracene crystals subjected to photodimerization and photo-oxidation have been investigated. Whereas surface photo-oxidation does not change the spectral characteristics of the anthracene even at 4 K, photodimerization significantly modifies both the spectrum and the fluorescence lifetimes of the crystals. Thus a broad unstructured a polarized emission with a maximum at ∼475 nm is observed in photodimerized samples (it is also present as a weak feature in the spectrum of the so-called pure parent crystal) together with the characteristic anthracene progression at 406, 421 and 444 nm that contains broad, vibronic maxima and is essentially unpolarized. These emissions are attributed, respectively, to traps which are interpreted as anthracene “incipient” dimer (“sandwich” dimer) and single, displaced anthracene molecules in close proximity to the stable photodimer. The long and short components of the time resolved emission observed are consistent with such an assignment. For the singlet exciton the two trap depths may be assigned to ∼4000 cm^–1 and ∼300 cm^–1 respectively with a distribution for the latter extending to around ±50 cm^–1. This shallow trap, consisting of displaced anthracene molecules, is similar to the one previously identified in deformed crystals of anthracene.