Photochemical reactions of aromatic aldehydes and ketones : higher triplet state reactions and radiationless transitions

Abstract

Photoreduction and photoaddition reactions were investigated on those compounds, 9-CH_3CO-A, 9-CHO-A, 1-CH_3CO-A, 2-CH_3CO-A, 3-CH_3CO-P and 3-CHO-P, which are the carbonyl derivatives of anthracene (-A) or pyrene (-P). These compounds bad the ππ state as their lowest triplet states. 9-CHO-A, 1-CHO-A, 3-CH_3CO-P and 3-CHO-P underwent the above photoreactions. The other compounds were not photoreactive at all. Quantum yields of the photoreactions on the photoreactive compounds were measured, and it was found that the value for 3-CHO-P was larger than that for 9-CHO-A. The relative photoreduction rates were also obtained in several solvents. Most compounds were non-fluorescent is non-polar solvents but showed fluorescence in various degrees in polar media. In spite of the supposition that those phenomena reflected the natures of the excited states of the compounds, they had no relation to the photoreactivity. The typical examples were 1-CH_3CO-A and 1-CHO-A, both of which showed a similar solvent effect on fluorescence spectra but had a different photoreactivity; the former was non-reactive. From these results and the temperature dependence of the lowest triplet state yield, the relative positions of the several excited states of these compounds were estimated. It was supposed that the magnitudes of the decay rate of the higher excited triplet state T (nπ) determined the photoreactivity. The lifetimes of the lowest triplet state T1(ππ) were also measured on the above compounds. It was found that the compounds having the shorter lifetime of the lowest triplet state were photoreactive. Particularly the lifetime of photoreactive 9-CHO-A was much shorter than that of non-reactive 9-CH_3CO-A, which had a similar lifetime to anthracene. With the aid of the theories of Robinson and El-Sayed and the first order perturbation, it will be shown that this lifetime is correlated to the molecular structure which seems to have a close connection with the radiationless transition process and the photoreactivity of T (nπ).