Intersystem crossing rates of S1 state keto-amino cytosine at low excess energy.

The amino-keto tautomer of supersonic jet-cooled cytosine undergoes intersystem crossing (ISC) from the v = 0 and low-lying vibronic levels of its S₁(¹ππ*) state. We investigate these ISC rates experimentally and theoretically as a function of S₁ state vibrational excess energy E_exc. The S₁ vibronic levels are pumped with a ~5 ns UV laser, the S₁ 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 S₁ and T₁ 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 S₁ ⇝S₀ internal conversion (IC) rates are 30-100 times larger. Theoretical ISC rates are computed using SCS-CC2 methods, which predict rapid ISC from the S₁; v = 0 state with k_ISC = 3. 10⁹ s^-1 to the T₁(3ππ*) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of ¹n_Oπ* character into the S₁(¹ππ*) 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 S₁ state, S₁ ⇝S₀ internal conversion dominates the nonradiative decay with k_IC ≥ 2 . 10¹⁰ s^-1, (2) the calculated S₁⇝T₁ (¹π#960;*³ππ*) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S₁ ⇝ T₁ ISC is moderately fast (k_ISC = 3. 10⁹ s^-1), and not ultrafast, as claimed by other calculations, and (4) at E_exc ~ 550 cm^-1 the IC rate increases by ~50 times, probably by accessing the lowest conical intersection (the C5-twist CI) and thereby effectively switching off the ISC decay channels. [ABSTRACT FROM AUTHOR]