First Observation of Monomer and Dimer Radical Cation upon Photoreduction of Cyanopyridinium Derivatives in Solution by Electron Spin Resonance and Absorption Spectroscopy

Abstract

The monomer radical and the dimer radical cation of cyanopyridinium derivatives have been studied by absorption and electron spin resonance (ESR) spectroscopies at room temperature in acetonitrile upon steady photolysis. The stable cyanopyridinyl radical of 1,3-bis(4-cyanopyridiniumyl)propane (acceptor) formed by one-electron transfer from tetraphenylborate anion (donor) interacted intramolecularly with a parent cation, forming a dimer radical cation. A charge resonance band was observed at 1360 nm and assigned to the formation of intramolecular dimer radical cations. The ESR spectra revealed that one unpaired electron in the intramolecular dimer radical cation was completely shared between two chromophores of photoreduced 1,3-bis(4-cyanopyridiniumyl)propane (1). On the other hand, the intermolecular dimer radical cation of 1-(4-cyanopyrdiniumyl)hexadecane (2) as a monochromophoric compound was not formed because no charge resonance band was observed in the near-infrared region. Clearly resolved ESR spectra of both the dimer radical cation and the monomer radical were observed, for the first time, upon photoreduction of these cyanopyridinium derivatives. The effect of the chromophore size on the charge resonance band in the near-infrared region was also demonstrated by homo- and hetero-type bichromophoric compounds containing 4-cyano- and 4-nitrostyryl-pyridinium.