PHOTOCHEMISTRY OF INTRAMOLECULAR CHARGE-TRANSFER EXCITED-STATES IN DONOR-ACCEPTOR-SUBSTITUTED DIAMINES

The photochemistry and photophysics of 1,2-diamines C(5)H(10)NCHPhCHPh NHC(6)H(4)R (1, R = CN, and 2, R = 4-pyridyl) have been examined. Thes e compounds display a strong absorption band in the near-UV which is d ue 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 fr om the (1)LE state of the charge transfer chromophore and to homolytic bond fragmentation across the 1,2 C-C bond with moderate quantum effi ciency. Detailed photochemical and photophysical studies reveal that t he bond fragmentation reaction ensues from a second intramolecular cha rge transfer excited state (denoted (CT)-C-1) which is based on electr on transfer from the tertiary piperidine nitrogen to the 4-cyanophenyl or (4-pyridyl)phenyl unit. Photochemical product analysis reveals tha t erythro --> three (or three --> erythro) isomerization occurs under both argon-degassed and air-saturated conditions. This observation ind icates 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 abs orption, and steady-state photochemical kinetics data indicates that ( 1) internal conversion from the (1)LE state to the (CT)-C-1 state by i ntramolecular electron transfer occurs with k greater than or equal to 10(9) s(-1) in all of the diamines; (2) bond fragmentation within the (CT)-C-1 state occurs with k greater than or equal to 10(8) s(-1) in each of the diamines: (3) bond fragmentation may be faster in erythro- 1 than in threo-1.