Formation and Reaction Kinetics of Biradicals and Triplet States in a Series of Carboxylated 1,4,5,8-Naphthalene Diimides.

The singlet-state deactivation products in a series of alkyl carboxylate substituted 1,4,5,8-naphthalene diimide () compounds were characterized using fluorescence and phosphorescence spectroscopies, as well as nanosecond laser flash photolysis. The reactive intermediates were quantified as a function of the number of methylenes in the alkyl linker. Rate constants for photoinduced electron transfer (PET) to the singlet excited state of the varied from 1.2 × 10 to 4.9 × 10 s. The yield of the long-lived amino ketyl radical ranged from 15% to 60% in compounds having 4 to 1 intervening methylenes between the chromophore and the covalently attached carboxylic acid. A predominantly triplet state was observed upon direct excitation of the compound having the slowest PET. This T state of the was reductively quenched by DABCO electron donor. The amino ketyl radical was unreactive toward electron donors but was found to react with the hydrogen atom donor β-mercaptoethanol. The compounds comprise a novel class of long-wavelength and strongly absorbing UV-activated chromophores that generate carbon-centered biradicals via direct 355 nm excitation in the absence of a cosensitizer.