Effects of water on the phosphorescence spectra of aromatic carbonyl compounds. Part 3.—Dual- or multiple-component phosphorescence of 1-indanone at 77 K in some protic solvents containing water

Abstract

Phosphorescence spectra of 1-indanone (IND) at 77 K in protic solvents such as EPA (diethyl ester–isopentane–ethanol, 5 ∶ 5 ∶ 2 volume ratio) ME (methanol–ethanol, 1 ∶ 1 volume ratio) and ethanol (each containing water) have been found to exhibit dual phosphorescence with short (2.5–2.7 ms) and long (18.4–30.0 ms) lifetimes. Since the phosphorescence spectrum with the short lifetime is identical to that found in an aprotic solvent such as methylcyclohexane (without the addition of H₂O), the short-lived emitting species is attributed to free triplet IND of nπ* character, while the long-lived emitting species is attributed to a complex (triplet IND–H₂O with a fixed number of H₂O molecules) of mixed nπ*–ππ* character. The phosphorescence decay curve of IND in a strong hydrogen-bonding solvent such as 2,2,2-trifluoroethanol (TFE)(containing H₂O) can also be reproduced by a biexponential function, but the corresponding lifetimes are found to change with the sweeping time of the oscilloscope. It is supposed that only the shortest and longest lifetimes are true values, while the other lifetimes are fortuitous values owing to the existence of various types of emitting species. Changing the H₂O content, moreover, it is found that the shortest lifetime is independent of the H₂O content, while the longest lifetime increases linearly with increasing H₂O content; the maximum lifetime (603 ms) is reached at an H₂O content of ca. 7 vol. %. Since no clear emission peaks reflecting the progression of the carbonyl vibration can be seen, the emitting species with the shortest lifetime (2.9 ms) is ascribed to triplet IND (hydrogen-bonded with TFE) of mixed nπ*–ππ* character, while the long-lived emitting species are attributed to several types of complexes (triplet IND–H₂O or triplet IND–H₂O–TFE with various numbers of H₂O molecules) of mixed nπ*–ππ* or ππ* character depending on the number of H₂O molecules contained in the complex. No long-lived emission originating from triplet IND hydrogen-bonded with the alcohol molecule is observed.