Excited-State Vibrations, Lifetimes, And Nonradiative Dynamics Of Jet-Cooled 1-Ethylcytosine

The S-1 excited-state lifetime of jet-cooled 1-ethylcytosine (1ECyt) is similar to 1 ns, one of the longest lifetimes for cytosine derivatives to date. Here, we analyze its S-0 -> S-1 vibronic spectrum using two-color resonant two-photon ionization and UV/UV holeburning spectroscopy. Compared to cytosine and 1-methylcytosine, the S-0 -> S-1 spectrum of 1ECyt shows a progression in the out-of-plane "butterfly" mode nu 1 ', identified by spin-component scaled-second-order coupled-cluster method ab initio calculations. We also report time-resolved S-1 state nonradiative dynamics at similar to 20 ps resolution by the pump/delayed ionization technique. The S-1 lifetime increases with the number of nu 1 ' quanta from tau = 930 ps at v1 '=0 to 1030 ps at v1 '=2, decreasing to 14 ps at 710 cm(-1) vibrational energy. We measured the rate constants for S-1 S-0 internal conversion and S-1 T-1 intersystem crossing (ISC): At the v ' = 0 level, k(IC) is 8 x 10(8) s(-1) or three times smaller than 1-methylcytosine. The ISC rate constant from v ' = 0 to the T-1((3)pi pi*) state is k(ISC) = 2.4 x 10(8) s(-1), 10 times smaller than the ISC rate constants of cytosine, but similar to that of 1-methylcytosine. Based on the calculated S-1((1)pi pi*) state radiative lifetime tau(rad) = 12 ns, the fluorescence quantum yield of 1ECyt is phi(fl) similar to 7% and the intersystem crossing yield is phi(ISC) similar to 20%. We measured the adiabatic ionization energy of 1-ethylcytosine via excitation of the S-1 state as 8.353 +/- 0.008 eV, which is 0.38 eV lower than that of amino-keto cytosine. Measurement of the ionization energy of the long-lived T-1(pi pi*) state formed via ISC reveals that it lies 3.2-3.4 eV above the S-0 ground state. Published under license by AIP Publishing.