Control of Photophysical Properties and Photoreactions of Aromatic Imides by Use of Intermolecular Hydrogen Bonding

The effect of addition of alcohols on the photophysical properties and the photoreactions of N-methylphthalimide (1), N-methyl-1,8-, -2,3-, and -1,2-naphthalimide (2−4), and N-methyl-9,10-phenanthrenedicarboximide (5) has been investigated. The UV and IR spectra of aromatic imides 1−5 showed the presence of the intermolecular hydrogen bonding between the carbonyl group of the aromatic imides and the alcohols in less polar solvents. The equilibrium constants K for the hydrogen bonding were determined by the UV spectra. The fluorescence intensities of 2 and 1 were found to be remarkably and moderately enhanced by the addition of alcohols, respectively, though those of 3−5 were little enhanced by the addition of alcohols. On the other hand, photochemical cyclobutane formation of 2 with styrene (6) was found to be enhanced by the addition of 2,2,2-trifluoroethanol in benzene. Enhancement of the fluorescence quantum yields Φf and the photoreaction of 2 by the hydrogen bond formation was explained by the decrease of the efficiency of the intersystem crossing from 1(ππ*) to 3(nπ*), whose energy was increased by the hydrogen bonding.