Excited-state vibrations, lifetimes, and nonradiative dynamics of jet-cooled 1-ethylcytosine

Abstract

The S1 excited-state lifetime of jet-cooled 1-ethylcytosine (1ECyt) is ∼1 ns, one of the longest lifetimes for cytosine derivatives to date. Here, we analyze its S0 → S1 vibronic spectrum using two-color resonant two-photon ionization and UV/UV holeburning spectroscopy. Compared to cytosine and 1-methylcytosine, the S0 → S1 spectrum of 1ECyt shows a progression in the out-of-plane “butterfly” mode ν1′, identified by spin-component scaled-second-order coupled-cluster method ab initio calculations. We also report time-resolved S1 state nonradiative dynamics at ∼20 ps resolution by the pump/delayed ionization technique. The S1 lifetime increases with the number of ν1′ quanta from τ = 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 S1 ⇝ S0 internal conversion and S1 ⇝ T1 intersystem crossing (ISC): At the v′ = 0 level, kIC is 8 × 108 s−1 or three times smaller than 1-methylcytosine. The ISC rate constant from v′ = 0 to the T1(3ππ*) state is kISC = 2.4 × 108 s−1, 10 times smaller than the ISC rate constants of cytosine, but similar to that of 1-methylcytosine. Based on the calculated S1(1ππ*) state radiative lifetime τrad = 12 ns, the fluorescence quantum yield of 1ECyt is Φfl ∼ 7% and the intersystem crossing yield is ΦISC ∼ 20%. We measured the adiabatic ionization energy of 1-ethylcytosine via excitation of the S1 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 T1(ππ*) state formed via ISC reveals that it lies 3.2–3.4 eV above the S0 ground state.

Locating Cytosine Conical Intersections by Laser Experiments andAb InitioCalculations

“…Nanosecond laser two-color resonant two-photon ionization (2C-R2PI) spectra of the amino-keto tautomers of (a) 6-methylCyt, (b) 1-ethylCyt, , (c) Cyt (with atom numbering), − (d) 1-methylCyt, , (e) 5-methylCyt, (f) 5-fluoroCyt, (g) 5,6-trimethyleneCyt, and (g) 1-methyl-5,6-trimethyleneCyt. Ionization wavelength: 226 nm for panels c and e, 223 nm for panel d, 220 nm for panels a and b, and 245 nm for panels f–h.…”

“…Figure shows the 2C-R2PI spectra of the eight cytosines of our set. The spectra of 6-methylCyt and 1M-TMCyt in Figure a,h are reported for the first time, and the previously published spectra ,,,− are included for completeness. The S 0 → S 1 spectra all exhibit vibronic progressions in the low-wavenumber “butterfly” and “boat” vibrational modes of the cytosine chromophore, reflecting the out-of-plane framework deformations that occur upon optical excitation of the S 1 ( 1 ππ *) state. − , The spectra of 6-methylCyt, Cyt, 1-methylCyt, and 1-ethylCyt break off at low E exc = 250–550 cm –1 , as marked by vertical red arrows in Figure .…”

“…The spectra of 6-methylCyt and 1M-TMCyt in Figure a,h are reported for the first time, and the previously published spectra ,,,− are included for completeness. The S 0 → S 1 spectra all exhibit vibronic progressions in the low-wavenumber “butterfly” and “boat” vibrational modes of the cytosine chromophore, reflecting the out-of-plane framework deformations that occur upon optical excitation of the S 1 ( 1 ππ *) state. − , The spectra of 6-methylCyt, Cyt, 1-methylCyt, and 1-ethylCyt break off at low E exc = 250–550 cm –1 , as marked by vertical red arrows in Figure . The spectra of 5-methylCyt and 5-fluoroCyt break off at substantially higher energies, E exc = 750 and 1200 cm –1 , while those of TMCyt and 1M-TMCyt extend up to almost 4000 cm –1 and >4500 cm –1 , respectively.…”

Ultrafast excited‐state dynamics of gas‐phase 5‐methylcytosine tautomers

“…The fastest decay component (τ 1 ) is present in transients measured at all λ pump 's and exhibits a robust time constant of about 0.25 ps. Because it has been shown that the excited-state deactivation rates of Cy derivatives are very sensitive to the excess energy above the origin, [9,12,[15][16][17]26] this rather independence of the τ 1 component on λ pump over a large energy range ($0.7 eV) suggests that it is not likely to be due to an excited-state population decay of 5mCy tautomers. Indeed, its time scale suggests that it is more likely to be due to an initial intramolecular vibrational-energy redistribution (IVR) occurring in excited 5mCy tautomers.…”

“…[3][4][5][6] Among all nucleobases, cytosine (Cy) and its derivatives are probably the most studied system experimentally in the gas phase. [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Ultrafast excited-state dynamics of Cy in the gas phase has been explored by several groups. [22][23][24][25][26] Our group has previously used femtosecond pump-probe photoionization spectroscopy to investigate the ultrafast excited-state dynamics of Cy and its microsolvated complexes.…”