INFLUENCE OF SOLVENT ON ULTRAFAST KINETICS OF ELECTRON PHOTOEJECTION
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Picosecond spectroscopy revealed that sodium salt of tetraphenylethylene dianion (T2-,2Na+) is bleached by a 530-nm pulse whether dissolved in dioxane or tetrahydrofuran (THF). It was established that in either solvent T2-,2Na+, and not T2-,Na+, is the photolyzed species. The original absorbance returns within about 10 psec when dioxane solution is photolyzed, but the bleached state persists for at least a nanosecond when the reaction occurs in THF. The different structure of the aggregate, tighter in dioxane and looser in THF, is reflected in the different behavior of the excited state. The absorbance of T- radical anion (max = 660 nm) is not observed in either system within the time scale of our experiments. Although electron ejection follows the photolysis of T2-,2Na+ in THF, either only a small fraction of excited aggregates undergoes autoionization (making the detection of T- hardly possible) or the resulting T- retains the geometry of T2- for at least 60 psec (the long time of our observations) and the distorted radical anion apparently does not absorb in the expected region. To differentiate between these alternatives, sodium salt of tetracene dianion was flash photolyzed in dioxane and in THF. Bleaching and the appearance of tetracene radical anion was observed in both solvents. It is probable, therefore, that the substantially different shapes of T2- and of T- prevent the observation of T- formation in dioxane. 1975, American Chemical Society. All rights reserved.
