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 (NDI) 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 NDIs varied from 1.2 × 1010 to 4.9 × 1010 s-1. The yield of the long-lived amino ketyl radical ranged from 15% to 60% in compounds having 4 to 1 intervening methylenes between the NDI chromophore and the covalently attached carboxylic acid. A predominantly triplet state was observed upon direct excitation of the compound having the slowest PET. This T1 state of the NDIs 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.