Planarizing cytosine: The S 1 state structure, vibrations, and nonradiative dynamics of jet-cooled 5,6-trimethylenecytosine

We measure theS0→S1spectrum and time-resolvedS1state nonradiative dynamics of the “clamped”cytosine derivative 5,6-trimethylenecytosine (TMCyt) in a supersonic jet, using two-color resonanttwo-photon ionization (R2PI), UV/UV holeburning, and ns time-resolved pump/delayed ionization.The experiments are complemented with spin-component scaled second-order approximate cou-pled cluster (SCS-CC2), time-dependent density functional theory, and multi-state second-orderperturbation-theory (MS-CASPT2)ab initiocalculations. While the R2PI spectrum of cytosine breaksoff∼500 cm1above its 000band, that of TMCyt extends up to +4400 cm1higher, with over a hun-dred resolved vibronic bands. Thus, clamping the cytosine C5–C6bond allows us to explore theS1state vibrations andS0→S1geometry changes in detail. The TMCytS1state out-of-plane vibra-tionsν′1,ν′3, andν′5lie below 420 cm1, and the in-planeν′11,ν′12, andν′23vibrational fundamentalsappear at 450, 470, and 944 cm1.S0→S1vibronic simulations based on SCS-CC2 calculationsagree well with experiment if the calculatedν′1,ν′3, andν′5frequencies are reduced by a factorof 2–3. MS-CASPT2 calculations predict that the ethylene-typeS1S0conical intersection (CI)increases from +366 cm1in cytosine to>6000 cm1in TMCyt, explaining the long lifetime andextendedS0→S1spectrum. The lowest-energyS1S0CI of TMCyt is the “amino out-of-plane”(OPX) intersection, calculated at +4190 cm1. The experimentalS1S0internal conversion rateconstant at theS1(v′=0) level iskIC=0.98–2.2·108s1, which is∼10 times smaller than in1-methylcytosine and cytosine. TheS1(v′=0) level relaxes into theT1(3ππ∗) state by intersystemcrossing withkISC=0.41–1.6·108s1. TheT1state energy is measured to lie 24 580±560 cm1above theS0state. TheS1(v′=0) lifetime isτ=2.9 ns, resulting in an estimated fluorescencequantum yield ofΦfl=24%. Intense two-color R2PI spectra of the TMCyt amino-enol tautomersappear above 36 000 cm1. A sharpS1ionization threshold is observed for amino-keto TMCyt,yielding an adiabatic ionization energy of 8.114±0.002 eV

The elusive S2 state, the S1/S2 splitting, and the excimer states of the benzene dimer

We observe the weak S 0 → S 2 transitions of the T-shaped benzene dimers (Bz)2 and (Bz-d 6)2 about 250 cm−1 and 220 cm−1 above their respective S 0 → S 1 electronic origins using two-color resonant two-photon ionization spectroscopy. Spin-component scaled (SCS) second-order approximate coupled-cluster (CC2) calculations predict that for the tipped T-shaped geometry, the S 0 → S 2 electronic oscillator strength f el (S 2) is ∼10 times smaller than f el (S 1) and the S 2 state lies ∼240 cm−1 above S 1, in excellent agreement with experiment. The S 0 → S 1 (ππ ∗) transition is mainly localized on the “stem” benzene, with a minor stem → cap charge-transfer contribution; the S 0 → S 2 transition is mainly localized on the “cap” benzene. The orbitals, electronic oscillator strengths f el (S 1) and f el (S 2), and transition frequencies depend strongly on the tipping angle ω between the two Bz moieties. The SCS-CC2 calculated S 1 and S 2 excitation energies at different T-shaped, stacked-parallel and parallel-displaced stationary points of the (Bz)2 ground-state surface allow to construct approximate S 1 and S 2 potential energy surfaces and reveal their relation to the “excimer” states at the stacked-parallel geometry. The f el (S 1) and f el (S 2) transition dipole moments at the C 2v -symmetric T-shape, parallel-displaced and stacked-parallel geometries are either zero or ∼10 times smaller than at the tipped T-shaped geometry. This unusual property of the S 0 → S 1 and S 0 → S 2 transition-dipole moment surfaces of (Bz)2 restricts its observation by electronic spectroscopy to the tipped and tilted T-shaped geometries; the other ground-state geometries are impossible or extremely difficult to observe. The S 0 → S 1/S 2 spectra of (Bz)2 are compared to those of imidazole ⋅ (Bz)2, which has a rigid triangular structure with a tilted (Bz)2 subunit. The S 0 → S 1/ S 2 transitions of imidazole-(benzene)2 lie at similar energies as those of (Bz)2, confirming our assignment of the (Bz)2 S 0 → S 2 transition

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

We have investigated the S0 → S1 UV vibronic spectrum and time-resolved S1 state dynamics of jet-cooled amino-keto 1-methylcytosine (1MCyt) using two-color resonant two-photon ionization, UV/UV holeburning and depletion spectroscopies, as well as nanosecond and picosecond timeresolved pump/delayed ionization measurements. The experimental study is complemented with spin-component-scaled second-order coupled-cluster and multistate complete active space second order perturbation ab initio calculations. Above the weak electronic origin of 1MCyt at 31 852 cm−1 about 20 intense vibronic bands are observed. These are interpreted as methyl group torsional transitions coupled to out-of-plane ring vibrations, in agreement with the methyl group rotation and out-of-plane distortions upon 1ππ∗ excitation predicted by the calculations. The methyl torsion and ν′1 (butterfly) vibrations are strongly coupled, in the S1 state. The S0 → S1 vibronic spectrum breaks off at a vibrational excess energy Eexc ∼ 500 cm−1, indicating that a barrier in front of the ethylene-type S1 S0 conical intersection is exceeded, which is calculated to lie at Eexc = 366 cm−1. The S1 S0 internal conversion rate constant increases from kIC = 2 · 109 s−1 near the S1(v = 0) level to 1 · 1011 s−1 at Eexc = 516 cm−1. The 1ππ∗ state of 1MCyt also relaxes into the lower-lying triplet T1 (3ππ∗) state by intersystem crossing (ISC); the calculated spin-orbit coupling (SOC) value is 2.4 cm−1. The ISC rate constant is 10–100 times lower than kIC; it increases from kISC = 2 · 108 s−1 near S1(v = 0) to kISC = 2 · 109 s−1 at Eexc = 516 cm−1. The T1 state energy is determined from the onset of the time-delayed photoionization efficiency curve as 25 600 ± 500 cm−1. The T2 (3nπ∗) state lies >1500 cm−1 above S1(v = 0), so S1 T2 ISC cannot occur, despite the large SOC parameter of 10.6 cm−1. An upper limit to the adiabatic ionization energy of 1MCyt is determined as 8.41 ± 0.02 eV. Compared to cytosine, methyl substitution at N1 lowers the adiabatic ionization energy by ≥0.32 eV and leads to a much higher density of vibronic bands in the S0 → S1 spectrum. The effect of methylation on the radiationless decay to S0 and ISC to T1 is small, as shown by the similar break-off of the spectrum and the similar computed mechanismsThis research has been supported by the Schweiz. Nationalfonds (Grant Nos. 121993 and 132540), the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) from Catalonia (Spain) (Grant No. 2014SGR1202), the Ministerio de Economía y Competividad (MINECO) from Spain (Grant No. CTQ2015-69363-P), and the National Natural Science Foundation of China (Grant No. 21303007

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

The <i>S</i>₀ → <i>S</i>₁ vibronic spectrum and <i>S</i>₁ state nonradiative relaxation of jet-cooled keto-amino 5-fluorocytosine (5FCyt) are investigated by two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm^–1 resolution. The 0₀⁰ rotational band contour is polarized in-plane, implying that the electronic transition is ¹<i>ππ</i>*. The electronic transition dipole moment orientation and the changes of rotational constants agree closely with the SCS-CC2 calculated values for the ¹<i>ππ</i>* (<i>S</i>₁) transition of 5FCyt. The spectral region from 0 to 300 cm^–1 is dominated by overtone and combination bands of the out-of-plane ν₁^′ (boat), ν₂^′ (butterfly), and ν₃^′ (HN–C₆H twist) vibrations, implying that the pyrimidinone frame is distorted out-of-plane by the ¹<i>ππ</i>* excitation, in agreement with SCS-CC2 calculations. The number of vibronic bands rises strongly around +350 cm^–1; this is attributed to the ¹<i>ππ</i>* state barrier to planarity that corresponds to the central maximum of the double-minimum out-of-plane vibrational potentials along the ν₁^′, ν₂^′, and ν₃^′ coordinates, which gives rise to a high density of vibronic excitations. At +1200 cm^–1, rapid nonradiative relaxation (<i>k</i>_nr ≥ 10¹² s^–1) sets in, which we interpret as the height of the ¹<i>ππ</i>* state barrier in front of the lowest <i>S</i>₁/<i>S</i>₀ conical intersection. This barrier in 5FCyt is 3 times higher than that in cytosine. The lifetimes of the ν′ = 0, 2ν₁^′, 2ν₂^′, 2ν₁^′ + 2ν₂^′, 4ν₂^′, and 2ν₁^′ + 4ν₂^′ levels are determined from Lorentzian widths fitted to the rotational band contours and are τ ≥ 75 ps for ν′ = 0, decreasing to τ ≥ 55 ps at the 2ν₁^′ + 4ν₂^′ level at +234 cm^–1. These gas-phase lifetimes are twice those of <i>S</i>₁ state cytosine and 10–100 times those of the other canonical nucleobases in the gas phase. On the other hand, the 5FCyt gas-phase lifetime is close to the 73 ps lifetime in room-temperature solvents. This lack of dependence on temperature and on the surrounding medium implies that the 5FCyt nonradiative relaxation from its <i>S</i>₁ (¹<i>ππ</i>*) state is essentially controlled by the same ∼1200 cm^–1 barrier and conical intersection both in the gas phase and in solution

Excited-State Structure and Dynamics of Keto–Amino Cytosine: The ¹ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast

We have measured the mass- and tautomer-specific S₀ → S₁ vibronic spectra and S₁ state lifetimes of the keto–amino tautomer of cytosine cooled in supersonic jets, using two-color resonant two-photon ionization (R2PI) spectroscopy at 0.05 cm^–1 resolution. The rotational contours of the 0₀⁰ band and nine vibronic bands up to +437 cm^–1 are polarized in the pyrimidinone plane, proving that the electronic excitation is to a ¹ππ* state. All vibronic excitations up to +437 cm^–1 are overtone and combination bands of the low-frequency out-of-plane ν₁^′ (butterfly), ν₂^′ (boat), and ν₃^′ (H–N–C⁶–H twist) vibrations. UV vibronic spectrum simulations based on approximate second-order coupled-cluster (CC2) calculations of the ground and ¹ππ* states are in good agreement with the experimental R2PI spectrum, but only if the calculated ν₁^′ and ν₂^′ frequencies are reduced by a factor of 4 and anharmonicity is included. Together with the high intensity of the ν₁^′ and ν₂^′ overtone vibronic excitations, this implies that the ¹ππ* potential energy surface is much softer and much more anharmonic in the out-of-plane directions than predicted by the CC2 method. The ¹ππ* state lifetime is determined from the Lorentzian line broadening necessary to reproduce the rotational band contours: at the 0₀⁰ band it is τ = 44 ps, remains at τ = 35–45 ps up to +205 cm^–1, and decreases to 25–30 ps up to +437 cm^–1. These lifetimes are 20–40 times longer than the 0.5–1.5 ps lifetimes previously measured with femtosecond pump–probe techniques at higher vibrational energies (1500–3800 cm^–1). Thus, the nonradiative relaxation rate of keto–amino cytosine close to the ¹ππ* state minimum is <i>k</i>_nr ∼ 2.5 × 10¹⁰ s^–1, much smaller than at higher energies. An additional nonradiative decay channel opens at +500 cm^–1 excess energy. Since high overtone bands of ν₁^′ and ν₂^′ are observed in the R2PI spectrum but only a single weak 2ν₃^′ band, we propose that ν₃^′ is a promoting mode for nonradiative decay, consistent with the observation that the ν₃^′ normal-mode eigenvector points toward the “C⁶-puckered” conical intersection geometry

STRUCTURAL STUDIES OF PYRROLE-BENZENE COMPLEXES BY CHIRPED-PULSE ROTATIONAL SPECTROSCOPY

Author Institution: Department of Chemistry, University of Virginia, McCormick Rd., Charlottesville, VA 22904; Departement fur Chemie und Biochemie, Universitat Bern, Freiestrasse 3, 3012 Bern, SwitzerlandNon-covalent intermolecular interactions are important in structural biology. The N-H $\cdots$ $\pi$ hydrogen bond between amino acid side chains is an important structural determinant and highly affects the secondary structure of proteins. The pyrrole-benzene complex can be viewed as a model system for studying these fundamental interactions. Previous IR and UV spectroscopic studies of the pyrrole-benzene complex by Dauster \textit{et al.} nderline{\textbf{10}}, 2827 (2008)} and Pfaffen \textit{et al.} nderline{\textbf{13}}, 14110 (2011)} support a T-shaped structure with an N-H $\cdots$ $\pi$ hydrogen bond to the benzene ring. In order to obtain accurate structural information we have investigated the broadband rotational spectrum of the supersonic-jet cooled complexes of pyrrole with benzene and benzene-\textit{d}$_{1}$ in the 2-18 GHz frequency range. In addition to the hetero dimer we have also observed the two cyclic mixed trimers (pyrrole)$_{2}$-benzene and pyrrole-(benzene)$_{2}$

Gas-Phase Cytosine and Cytosine‑N₁‑Derivatives Have 0.1–1 ns Lifetimes Near the S₁ State Minimum

Ultraviolet radiative damage to DNA is inefficient because of the ultrafast S₁ ⇝ S₀ internal conversion of its nucleobases. Using picosecond pump–ionization delay measurements, we find that the S₁(¹<i>ππ</i>*) state vibrationless lifetime of gas-phase keto-amino cytosine (Cyt) is τ = 730 ps or ∼700 times longer than that measured by femtosecond pump–probe ionization at higher vibrational excess energy, <i>E</i>_exc. N₁-Alkylation increases the S₁ lifetime up to τ = 1030 ps for N₁-ethyl-Cyt but decreases it to 100 ps for N₁-isopropyl-Cyt. Increasing the vibrational energy to <i>E</i>_exc = 300–550 cm^–1 decreases the lifetimes to 20–30 ps. The nonradiative dynamics of S₁ cytosine is not solely a property of the amino-pyrimidinone chromophore but is strongly influenced by the N₁-substituent. Correlated excited-state calculations predict that the gap between the S₂(¹<i>n</i>_Oπ*) and S₁(¹<i>ππ</i>*) states decreases along the series of N₁-derivatives, thereby influencing the S₁ state lifetime