Photochemistry of Intramolecular Charge Transfer Excited States in Donor-Acceptor-Substituted Diamines

Article abstract of DOI:10.1021/j100018a019

The photochemistry and photophysics of 1,2-diamines C5H10NCHPhCHPhNHC6H4R (1, R=CN, and 2, R=4-pyridyl) have been examined.These compounds display a strong absorption band in the near-UV which is due to intramolecular charge transfer from the secondary amine group to the 4-cyanophenyl or (4-pyridyl)phenyl acceptor unit.Photoexcitation into this absorption band leads to moderately intense fluorescence from the ¹LE state of the charge transfer chromophore and to homolytic bond fragmentation across the 1,2 C-C bond with moderate quantum efficiency.Detailed photochemical and photophysical studies reveal that the bond fragmentation reaction ensues from a second intramolecular charge transfer excited state (denoted ¹CT) which is based on electron transfer from the tertiary piperidine nitrogen to the 4-cyanophenyl or (4-pyridyl)phenyl unit.Photochemical product analysis reveals that erythro -> threo (or threo -> erythro) isomerization occurs under both argon-degassed and air-saturated conditions.This observation indicates that recombination of the radicals formed by bond fragmentation occurs, both within the geminate pair and between free radicals that have escaped the solvent cage.Analysis of fluorescence, transient absorption, and steady-state photochemical kinetics data indicates that (1) internal conversion from the ¹LE state to the ¹CT state by intramolecular electron transfer occurs with k>/=10⁹ s^-1 in all of the diamines; (2) bond fragmentation within the ¹CT state occurs with k>/=10⁸ s^-1 in each of the diamines; (3) bond fragmentation may be faster in erythro-1 than in threo-1.