Experimental and Calculated Spectra of π‑Stacked Mild Charge-Transfer Complexes: Jet-Cooled Perylene·(Tetrachloroethene)_<i>n</i>, <i>n</i> = 1,2

The S₀ ↔ S₁ spectra of the mild charge-transfer (CT) complexes perylene·tetrachloroethene (P·4ClE) and perylene·(tetrachloroethene)₂ (P·(4ClE)₂) are investigated by two-color resonant two-photon ionization (2C-R2PI) and dispersed fluorescence spectroscopy in supersonic jets. The S₀ → S₁ vibrationless transitions of P·4ClE and P·(4ClE)₂ are shifted by δν = −451 and −858 cm^–1 relative to perylene, translating to excited-state dissociation energy increases of 5.4 and 10.3 kJ/mol, respectively. The red shift is ∼30% larger than that of perylene·<i>trans</i>-1,2-dichloroethene; therefore, the increase in chlorination increases the excited-state stabilization and CT character of the interaction, but the electronic excitation remains largely confined to the perylene moiety. The 2C-R2PI and fluorescence spectra of P·4ClE exhibit strong progressions in the perylene intramolecular twist (1a_u) vibration (42 cm^–1 in S₀ and 55 cm^–1 in S₁), signaling that perylene deforms along its twist coordinate upon electronic excitation. The intermolecular stretching (T_z) and internal rotation (R_c) vibrations are weak; therefore, the P·4ClE intermolecular potential energy surface (IPES) changes little during the S₀ ↔ S₁ transition. The minimum-energy structures and inter- and intramolecular vibrational frequencies of P·4ClE and P·(4ClE)₂ are calculated with the dispersion-corrected density functional theory (DFT) methods B97-D3, ωB97X-D, M06, and M06-2X and the spin-consistent-scaled (SCS) variant of the approximate second-order coupled-cluster method, SCS-CC2. All methods predict the global minima to be π-stacked centered coplanar structures with the long axis of tetrachloroethene rotated by τ ≈ 60° relative to the perylene long axis. The calculated binding energies are in the range of −<i>D</i>₀ = 28–35 kJ/mol. A second minimum is predicted with τ ≈ 25°, with ∼1 kJ/mol smaller binding energy. Although both monomers are achiral, both the P·4ClE and P·(4ClE)₂ complexes are chiral. The best agreement for adiabatic excitation energies and vibrational frequencies is observed for the ωB97X-D and M06-2X DFT methods

Watson–Crick and Sugar-Edge Base Pairing of Cytosine in the Gas Phase: UV and Infrared Spectra of Cytosine·2-Pyridone

While keto-amino cytosine is the dominant species in aqueous solution, spectroscopic studies in molecular beams and in noble gas matrices show that other cytosine tautomers prevail in apolar environments. Each of these offers two or three H-bonding sites (Watson–Crick, wobble, sugar-edge). The mass- and isomer-specific <i>S</i>₁ ← <i>S</i>₀ vibronic spectra of cytosine·2-pyridone (Cyt·2PY) and 1-methylcytosine·2PY are measured using UV laser resonant two-photon ionization (R2PI), UV/UV depletion, and IR depletion spectroscopy. The UV spectra of the Watson–Crick and sugar-edge isomers of Cyt·2PY are separated using UV/UV spectral hole-burning. Five different isomers of Cyt·2PY are observed in a supersonic beam. We show that the Watson–Crick and sugar-edge dimers of keto-amino cytosine with 2PY are the most abundant in the beam, although keto-amino-cytosine is only the third most abundant tautomer in the gas phase. We identify the different isomers by combining three different diagnostic tools: (1) methylation of the cytosine N1–H group prevents formation of both the sugar-edge and wobble isomers and gives the Watson–Crick isomer exclusively. (2) The calculated ground state binding and dissociation energies, relative gas-phase abundances, excitation and the ionization energies are in agreement with the assignment of the dominant Cyt·2PY isomers to the Watson–Crick and sugar-edge complexes of keto-amino cytosine. (3) The comparison of calculated ground state vibrational frequencies to the experimental IR spectra in the carbonyl stretch and NH/OH/CH stretch ranges strengthen this identification

NH₃ as a Strong H‑Bond Donor in Singly- and Doubly-Bridged Ammonia Solvent Clusters: 2‑Pyridone·(NH₃)_<i>n</i>, <i>n</i> = 1–3

Mass- and isomer-selected infrared spectra of 2-pyridone·(NH₃)_<i>n</i> clusters with <i>n</i> = 1–3 were measured in the NH and CH stretch fundamental region (2400–3700 cm^–1) using infrared (IR) laser depletion spectroscopy combined with resonant two-photon ionization UV laser detection. The IR depletion spectra reveal three different H-bonding topologies of these clusters: The <i>n</i> = 1 and 2 clusters form ammonia bridges stretching from the N–H to the CO group of the <i>cis</i>-amide function of 2-pyridone (2PY), giving rise to intense and strongly red-shifted (2PY)­NH and ammonia NH stretch bands. For <i>n</i> = 3, two isomers (3X and 3Y) are observed in the IR spectra: The spectrum of 3X is compatible with an ammonia-bridge structure like <i>n</i> = 2, with the third NH₃ accepting an H-bond from C⁶–H of 2PY. The IR spectrum of 3Y exhibits a broad IR band in the 2500–3000 cm^–1 range and is characteristic of a bifurcated double-bridged structure in which the first NH3 accepts an H-bond from the (2PY)­NH and donates two H-bonds to the other two ammonias, both of which donate to the CO group of 2PY. This double-donor/double-bridge H-bonding pattern increases the acceptor strength of the first ammonia and dramatically lowers the (2PY)­NH stretching frequency to ∼2700 cm^–1. For all clusters the ammonia 2ν₄ HNH bend overtones in the 3180–3320 cm^–1 region gain intensity by anharmonic coupling (Fermi resonance) to the hydrogen-bonded ammonia NH stretches, which are red-shifted into the 3250–3350 cm^–1 region. The experimental results are supported by optimized structures, vibrational frequencies, and IR intensities calculated using density-functional theory with the B3LYP and PW91 functionals, as well as with the more recent functionals B97-D and M06-2X, which are designed to include long-range dispersive interactions

Structure and Intermolecular Vibrations of Perylene·<i>trans</i>-1,2-Dichloroethene, a Weak Charge-Transfer Complex

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene <i>trans</i>-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S₀ ↔ S₁ spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (<i>T</i>_vib ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” R_c intermolecular vibration with very low frequencies of 11 (S₀) and 13 cm^–1 (S₁). The intermolecular T_<i>z</i> stretch and the R_a and R_b bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a_u “twisting” vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from <i>D</i>_2<i>h</i> to <i>D</i>₂. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked “centered” structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of <i>D</i>_e = −20.8 to −26.1 kJ/mol. The 0₀⁰ band of the S₀ → S₁ transition is red-shifted by δν = −301 cm^–1 relative to that of perylene, implying that the <i>D</i>_e increases by 3.6 kJ/mol or ∼15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated R_c, T_z, R_a, and R_b vibrations by comparing to the observed/calculated frequencies and S₀ ↔ S₁ Franck–Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results

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

Excitonic Splitting and Vibronic Coupling Analysis of the <i>m</i>‑Cyanophenol Dimer

The <i>S</i>₁/<i>S</i>₂ splitting of the <i>m</i>-cyanophenol dimer, (mCP)₂ and the delocalization of its excitonically coupled <i>S</i>₁/<i>S</i>₂ states are investigated by mass-selective two-color resonant two-photon ionization and dispersed fluorescence spectroscopy, complemented by a theoretical vibronic coupling analysis based on correlated <i>ab initio</i> calculations at the approximate coupled cluster CC2 and SCS-CC2 levels. The calculations predict three close-lying ground-state minima of (mCP)₂: The lowest is slightly <i>Z</i>-shaped (<i>C</i>_<i>i</i>-symmetric); the second-lowest is <5 cm^–1 higher and planar (<i>C</i>_2<i>h</i>). The vibrational ground state is probably delocalized over both minima. The S₀ → S₁ transition of (mCP)₂ is electric-dipole allowed (A_<i>g</i> → A_<i>u</i>), while the S₀ → S₂ transition is forbidden (A_<i>g</i> → A_<i>g</i>). Breaking the inversion symmetry by ¹²C/¹³C- or H/D-substitution renders the S₀ → S₂ transition partially allowed; the excitonic contribution to the S₁/S₂ splitting is Δ_<i>exc</i> = 7.3 cm^–1. Additional isotope-dependent contributions arise from the changes of the <i>m</i>-cyanophenol zero-point vibrational energy upon electronic excitation, which are Δ_<i>iso</i>(¹²C/¹³C) = 3.3 cm^–1 and Δ_<i>iso</i>(H/D) = 6.8 cm^–1. Only partial localization of the exciton occurs in the ¹²C/¹³C and H/D substituted heterodimers. The SCS-CC2 calculated excitonic splitting is Δ_<i>el</i> = 179 cm^–1; when multiplying this with the vibronic quenching factor Γ_{<i>vibron</i>}^<i>exp</i> = 0.043, we obtain an exciton splitting Δ_{<i>vibron</i>}^<i>exp</i> = 7.7 cm^–1, which agrees very well with the experimental Δ_<i>exc</i> = 7.3 cm^–1. The semiclassical exciton hopping times range from 3.2 ps in (mCP)₂ to 5.7 ps in the heterodimer (mCP-<i>h</i>)·(mCP-<i>d</i>). A multimode vibronic coupling analysis is performed encompassing all the vibronic levels of the coupled <i>S</i>₁/<i>S</i>₂ states from the <i>v</i> = 0 level to 600 cm^–1 above. Both linear and quadratic vibronic coupling schemes were investigated to simulate the S₀ → S₁/S₂ vibronic spectra; those calculated with the latter scheme agree better with experiment

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

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