Intersystem crossing rates of S-1 state keto-amino cytosine at low excess energy

摘要

The amino-keto tautomer of supersonic jet-cooled cytosine undergoes intersystem crossing (ISC) from the v = 0 and low-lying vibronic levels of its S-1((1)pi pi*) state. We investigate these ISC rates experimentally and theoretically as a function of S-1 state vibrational excess energy E-exc. The S-1 vibronic levels are pumped with a similar to 5 ns UV laser, the S-1 and triplet state ion signals are separated by prompt or delayed ionization with a second UV laser pulse. After correcting the raw ISC yields for the relative S-1 and T-1 ionization cross sections, we obtain energy dependent ISC quantum yields Q(ISC)(corr) = 1%-5%. These are combined with previously measured vibronic state-specific decay rates, giving ISC rates k(ISC) = 0.4-1.5 . 109 s(-1), the corresponding S-1 (sic) S-0 internal conversion (IC) rates are 30-100 times larger. Theoretical ISC rates are computed using SCS-CC2 methods, which predict rapid ISC from the S-1; v = 0 state with k(ISC) = 3 . 109 s(-1) to the T-1((3)pi pi*) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of (1)n(O)pi* character into the S-1((1)pi pi*) wave function at its non-planar minimum geometry. The combination of experiment and theory implies that (1) below E-exc = 550 cm(-1) in the S-1 state, S-1 S-0 internal conversion dominates the nonradiative decay with k(IC) >= 2 . 10(10) s(-1), (2) the calculated S-1 (sic) T-1 ((1)pi pi*(sic)(3)pi pi*) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S-1 (sic) T-1 ISC is moderately fast (k(ISC) = 3 . 109 s(-1)), and not ultrafast, as claimed by other calculations, and (4) at E-exc similar to 550 cm(-1) the IC rate increases by similar to 50 times, probably by accessing the lowest conical intersection (the C5-twist CI) and thereby effectively switching off the ISC decay channels. (C) 2015 AIP Publishing LLC.