Effect of substituent on the behaviour of the excited singlet and triplet states in carbonyl derivatives of anthracene of the type 9-X·CO·A

Abstract

Kinetic and spectroscopic studies relevant to the excited singlet and triplet states have been carried out on eighteen carbonyl compounds of anthracene, principally of the type 9-X·CO·A (where A is anthracene), aiming to reveal distinctly how a substituent X controls the excited-state behaviour. The compounds examined are classified into six groups based on the nature of X. Group I compounds, where X is an alkyl group, are non-fluorescent in any solvent at room temperature, but they start to fluoresce strongly near 77 K with fluorescence lifetimes of ca. 10 ns. When X is capable of conjugation with the carbonyl group, the derived compounds (group II) totally lack fluorescence even at 77 K. The carbonyl groups of anthracene-9-carbaldehyde and 2-methylaceanthren-1-one (group III) differ from the others in that they form hydrogen bonds in protic solvents. Only group V compounds, with an electron-donating substituent such as an amino, methoxy or hydroxy group, are modestly fluorescent at room temperature. Intersystem crossing from S₁ to T_nπ*, whose location relative to S₁ is affected by X, appears to play an important role in the appearance of fluorescence. Group VI compounds show that the introduction of another substituent at the 10-position also affects the fluorescence properties. In contrast to the wide variation seen in the fluorescence properties, the lowest triplet states are found to be similar to each other and only slightly different from those of anthracene. The shape of the phosphorescence and triplet–triplet absorption spectra and the lifetimes and energies of the lowest triplet states are also similar to those of anthracene. This similarity suggests that the lowest triplet states of the examined compounds are not of nπ* but of ππ* character, originating from the anthracene group.