Excited-State Structure, Vibrations, and Nonradiative Relaxation of Jet-Cooled 5-Fluorocytosine

Locating Cytosine Conical Intersections by Laser Experiments and Ab Initio Calculations

The decay mechanism of S₀ → S₁ excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at N1, C5, and C6, rapid internal conversion sets in at 250-1200 cm^-1 above the 0₀⁰ bands. The break-off in the spectra correlates with the calculated barriers toward the "C5-C6 twist" conical intersection, which unambiguously establishes the decay mechanism at low S₁ state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5, and C6 following (¹ππ*) excitation. The R2PI spectra of the clamped derivatives 5,6-trimethyleneCyt (TMCyt) and 1-methyl-TMCyt (1M-TMCyt), which decay along an N3 out-of-plane coordinate, extend up to +3500 and +4500 cm^-1.

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

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, k_IC 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 k_ISC = 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 τ_rad = 12 ns, the fluorescence quantum yield of 1ECyt is Φ_fl ∼ 7% and the intersystem crossing yield is Φ_ISC ∼ 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.

Planarizing cytosine: The S1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine

We measure the S₀ → S₁ spectrum and time-resolved S₁ state nonradiative dynamics of the "clamped" cytosine derivative 5,6-trimethylenecytosine (TMCyt) in a supersonic jet, using two-color resonant two-photon ionization (R2PI), UV/UV holeburning, and ns time-resolved pump/delayed ionization. The experiments are complemented with spin-component scaled second-order approximate coupled cluster (SCS-CC2), time-dependent density functional theory, and multi-state second-order perturbation-theory (MS-CASPT2) ab initio calculations. While the R2PI spectrum of cytosine breaks off ∼500 cm^-1 above its 0₀⁰ band, that of TMCyt extends up to +4400 cm^-1 higher, with over a hundred resolved vibronic bands. Thus, clamping the cytosine C⁵-C⁶ bond allows us to explore the S₁ state vibrations and S₀ → S₁ geometry changes in detail. The TMCyt S₁ state out-of-plane vibrations ν₁^', ν₃^', and ν₅^' lie below 420 cm^-1, and the in-plane ν₁₁^', ν₁₂^', and ν₂₃^' vibrational fundamentals appear at 450, 470, and 944 cm^-1. S₀  →  S₁ vibronic simulations based on SCS-CC2 calculations agree well with experiment if the calculated ν₁^', ν₃^', and ν₅^' frequencies are reduced by a factor of 2-3. MS-CASPT2 calculations predict that the ethylene-type S₁ ⇝ S₀ conical intersection (CI) increases from +366 cm^-1 in cytosine to >6000 cm^-1 in TMCyt, explaining the long lifetime and extended S₀ → S₁ spectrum. The lowest-energy S₁ ⇝ S₀ CI of TMCyt is the "amino out-of-plane" (OP_X) intersection, calculated at +4190 cm^-1. The experimental S₁ ⇝ S₀ internal conversion rate constant at the S₁(v^'=0) level is k_IC=0.98-2.2⋅10⁸ s^-1, which is ∼10 times smaller than in 1-methylcytosine and cytosine. The S₁(v^'=0) level relaxes into the T₁(³ππ*) state by intersystem crossing with k_ISC=0.41-1.6⋅10⁸ s^-1. The T₁ state energy is measured to lie 24 580±560 cm^-1 above the S₀ state. The S₁(v^'=0) lifetime is τ=2.9 ns, resulting in an estimated fluorescence quantum yield of Φ_fl=24%. Intense two-color R2PI spectra of the TMCyt amino-enol tautomers appear above 36 000 cm^-1. A sharp S₁ ionization threshold is observed for amino-keto TMCyt, yielding an adiabatic ionization energy of 8.114±0.002 eV.

How nature covers its bases

The response of nucleobases to UV radiation depends on structure in subtle ways, as revealed by gas-phase experiments.

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((1)ππ(∗)) state. We investigate these ISC rates experimentally and theoretically as a function of S1 state vibrational excess energy Eexc. 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 QISC (corr)=1%-5%. These are combined with previously measured vibronic state-specific decay rates, giving ISC rates kISC = 0.4-1.5 ⋅ 10(9) 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 kISC = 3 ⋅ 10(9) s(-1) to the T1((3)ππ(∗)) triplet state. The surprisingly high rate of this El Sayed-forbidden transition is caused by a substantial admixture of (1)nOπ(∗) character into the S1((1)ππ(∗)) wave function at its non-planar minimum geometry. The combination of experiment and theory implies that (1) below Eexc = 550 cm(-1) in the S1 state, S1⇝S0 internal conversion dominates the nonradiative decay with kIC ≥ 2 ⋅ 10(10) s(-1), (2) the calculated S1⇝T1 ((1)ππ(∗)⇝(3)ππ(∗)) ISC rate is in good agreement with experiment, (3) being El-Sayed forbidden, the S1⇝T1 ISC is moderately fast (kISC = 3 ⋅ 10(9) s(-1)), and not ultrafast, as claimed by other calculations, and (4) at Eexc ∼ 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.