Reactivities of carboxyalkyl-substituted 1,4,5,8-naphthalene diimides in aqueous solution

A series of water-soluble 1,4,5,8-naphthalene diimide derivatives has been prepared and their redox and photophysical properties characterized. From laser flash photolysis studies, the triplet excited state of N,N'-bis[2-(N-pyridinium)ethyl]-1,4,5,8-naphthalene diimide (NDI-pyr) was found to undergo oxidative quenching with the electron donors DABCO, tyrosine, and tryptophan as expected from thermodynamics. Interestingly, the reactivities of naphthalene diimides (NDI) possessing alpha- and beta-carboxylic acid substituents (R = -CH2COO-, -C(CH3)2COO-, and -CH2CH2COO-) were strikingly different. In these compounds, the transient produced upon 355 nm excitation did not react with the electron donors. Instead, this transient reacted rapidly (k > 10(8)-10(9) M-1 s-1) with known electron acceptors, benzyl viologen and ferricyanide. The transient spectrum of the carboxyalkyl-substituted naphthalimides observed immediately after the laser pulse was nearly identical to the one-electron-reduced form of 1,4,5,8-naphthalene diimide (produced independently using the bis-pyridinium-substituted naphthaldiimide). From our studies, we conclude that the transient produced upon nanosecond laser flash photolysis of NDI-(CH2)nCOO- is the species produced upon intramolecular electron transfer from the carboxylate moiety to the singlet excited state of NDI. In separate experiments, we verified that the singlet excited state of NDI-pyr does, indeed, react intermolecularly with acetate, alanine, and glycine. The process is further substantiated using thermodynamic driving force calculations. The results offer new prospects of the efficient photochemical production of reactive carbon-centered radicals.