Effects of water on the phosphorescence spectra of aromatic carbonyl compounds. Part 1.—Dual phosphorescence of benzophenone at 77 K in 2,2 2-trifluoroethanol–water

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Abstract
From measurements of the phosphorescence spectrum and its decay at 77 K, the lowest triplet state of benzophenone (BP) at low concentrations (ca. 1 × 10–4 mol dm–3) has been assigned to be of nπ* character even in a strong hydrogen-bonding solvent such as 2,2,2-trifluoroethanol (TFE). At high sample concentrations (ca. 1 × 10–3 mol dm–3), another nπ*-type phosphorescence due to aggregated BP is observed. Addition of water (H2O or D2O, 1–4 vol.%) to BP–TFE solutions causes dual phosphorescence with lifetimes of 3.6 and 15.9 ms, but only the long-lived phosphorescence is observed in TFE–water ( 5 vol.%): The nπ*-type short-lived phosphorescences at low and high sample concentrations are attributed to free triplet BP (or BP hydrogen-bonded with TFE) and aggregated triplet BP, respectively, while the long-lived phosphorescence is attributed to a complex of triplet BP–water–TFE (1 : 1 : 1) or triplet BP–water (1 : 1) of mixed nπ*–ππ* character.