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

Abstract

Intersystem crossing rates of S 1 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 S 1 ( 1 ππ * ) 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 ∼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 corr ISC = 1%-5%. These are combined with previously measured vibronic state-specific decay rates, giving ISC rates k ISC = 0.4-1.5 · 10 9 s −1 , the corresponding S 1 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 · 10 9 s −1 to the T 1 ( 3 ππ * ) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of 1 n O π * character into the S 1 ( 1 ππ * ) 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 T 1 ( 1 ππ * 3 ππ * ) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S 1 T 1 ISC is moderately fast (k ISC = 3 · 10 9 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. C 2015 AIP Publishing LLC. [http://dx

The excited-state structure, vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-methylcytosine

“…16 More recently, the intersystem crossing (ISC) quantum yield and ISC rates of Cyt have been measured as a function of the S 1 state vibrational energy. 17 A long-lived excited state with a lifetime of several hundred ns has also been observed for 1MCyt following excitation near the 0 0 0 band and ionization at 157 nm. 9 The interest in the photophysics of Cyt has motivated many theoretical studies, whose main aims were to explain the ultrashort excited-state lifetimes and the formation of the long-lived triplet state.…”

“…Therefore, the most likely mechanism for ISC from ( 1 ππ * ) Min at low excitation energies involves population of the vibrationally excited levels of T 1 , similar to what was recently proposed for Cyt. 17 We have also calculated the triplet energies along the singlet decay paths (dotted lines in Figures 13(a) and 13(b)). The S 1 /T 1 energy gap remains small along both paths, ≤3000 cm −1…”

“…The relative singlet and triplet ion signals in the ns pump/ionization transients depend on the lifetime of the S 1 level and on the ratio of the ionization cross sections σ + ion (S 1 )/σ + ion (T 1 ), both must be taken into account when modeling the ns excitation/ionization transients, as previously discussed for cytosine. 17 We discuss the fitting of the ISC rates in combination with the ps excitation/ionization transients in Section III E. . The black curve is measured at the prompt ionization peak, corresponding to ionization of the S 1 state.…”

“…38 More recently, S 1 → T 1 ISC has been suggested as the mechanism for formation of the triplet after excitation near the origin. 17 In this context, our joint experimental and theoretical study combining R2PI and UV/UV holeburning spectroscopy of 1MCyt with high level ab initio calculations addresses several goals: (1) To interpret and explain the vibronic structure of the S 0 → S 1 R2PI spectrum, which depends on the change between the S 0 -and S 1 -state structures and the excited-state vibrations that are activated by the optical excitation. In order to check the possible involvement of the methyl group rotation, we have also measured the isotopomer…”

Probing the excited state relaxation dynamics of pyrimidine nucleosides in chloroform solution

“…Estimated triplet state yields for cytosine are at most a few percent, either in the gasphase 39,81 or aqueous solution. 17 Our measurements suggest that for the nucleoside in chloroform, triplet state formation is also negligible.…”