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The excitonic splitting between the S1 and S2 electronic states of the doubly hydrogen-bonded dimer 2-pyridone·6-methyl-2-pyridone (2PY·6M2PY) is studied in a supersonic jet, applying two-color resonant two-photon ionization (2C-R2PI), UV-UV depletion, and dispersed fluorescence spectroscopies. In contrast to the C2h symmetric (2-pyridone)2 homodimer, in which the S1 ← S0 transition is symmetry-forbidden but the S2 ← S0 transition is allowed, the symmetry-breaking by the additional methyl group in 2PY·6M2PY leads to the appearance of both the S1 and S2 origins, which are separated by Δexp = 154 cm-1. When combined with the separation of the S1 ← S0 excitations of 6M2PY and 2PY, which is δ = 102 cm-1, one obtains an S1/S2 exciton coupling matrix element of VAB, el = 57 cm-1 in a Frenkel-Davydov exciton model. The vibronic couplings in the S1/S2 ← S0 spectrum of 2PY·6M2PY are treated by the Fulton-Gouterman single-mode model. We consider independent couplings to the intramolecular 6a′ vibration and to the intermolecular σ′ stretch, and obtain a semi-quantitative fit to the observed spectrum. The dimensionless excitonic couplings are C(6a′) = 0.15 and C(σ′) = 0.05, which places this dimer in the weak-coupling limit. However, the S1/S2 state exciton splittings Δcalc calculated by the configuration interaction singles method (CIS), time-dependent Hartree-Fock (TD-HF), and approximate second-order coupled-cluster method (CC2) are between 1100 and 1450 cm-1, or seven to nine times larger than observed. These huge errors result from the neglect of the coupling to the optically active intra- and intermolecular vibrations of the dimer, which lead to vibronic quenching of the purely electronic excitonic splitting. For 2PY·6M2PY the electronic splitting is quenched by a factor of ∼30 (i.e., the vibronic quenching factor is Γexp = 0.035), which brings the calculated splittings into close agreement with the experimentally observed value. The 2C-R2PI and fluorescence spectra of the tautomeric species 2-hydroxypyridine·6-methyl-2-pyridone (2HP·6M2PY) are also observed and assigned. [ABSTRACT FROM AUTHOR]

The S1↔S0 vibronic spectra of supersonic jet-cooled 2-pyridone [pyridin-2-one (2PY)] and its N–H deuterated isotopomer (d-2PY) have been recorded by two-color resonant two-photon ionization, laser-induced fluorescence and emission, and fluorescence depletion spectroscopies. By combining these methods, the B origin of 2PY at 000+98 cm-1 and the bands at +218 and +252 cm-1 are identified as overtones of the S1 state out-of-plane vibrations ν1′ and ν2′, as are the analogous bands of d-2PY. Anharmonic double-minimum potentials are derived for the respective out-of-plane coordinates that predict further ν1′ and ν2′ overtones and combinations, reproducing ∼80% of the vibronic bands up to 600 cm-1 above the 000 band. The fluorescence spectra excited at the electronic origins and the ν1′ and ν2′ out-of-plane overtone levels confirm these assignments. The S1 nonplanar minima and S1←S0 out-of-plane progressions are in agreement with the determination of nonplanar vibrationally averaged geometries for the 000 and 000+98 cm-1 upper states by Held et al. [J. Chem. Phys. 95, 8732 (1991)]. The fluorescence lifetimes of the S1 state vibrations show strong mode dependence: Those of the out-of-plane levels decrease rapidly above 200 cm-1 excess vibrational energy, while the in-plane vibrations ν5′, ν8′, and ν9′ have longer lifetimes, although they are above or interspersed with the “dark” out-of-plane states. This is interpreted in terms of an S1′ state reaction with a low barrier towards a conical intersection with a prefulvenic geometry. Out-of-plane vibrational states can directly surmount this barrier, whereas in-plane vibrations are much less efficient in this respect. Analysis of the fluorescence spectra allows to identify nine in-plane S0′ state fundamentals, overtones of the S0 state ν1″ and ν2″ out-of-plane vibrations, and >30 other overtones and combination bands. The B3LYP/6-311++G(d,p) calculated anharmonic wave numbers are in very good agreement with the observed fundamentals, overtones, and combinations, with a deviation Δrms=1.3%. [ABSTRACT FROM AUTHOR]

The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)[sub 2] and its [sup 13]C-, d[sub 1]-, and d[sub 2]-isotopomers were investigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C[sub 2h] symmetric (2PY)[sub 2]–h[sub 2] and (2PY)[sub 2]–d[sub 2] complexes, the 2PY moieties are equivalent and the S[sub 1]↔S[sub 0] ([sup 1]A[sub g]↔[sup 1]A[sub g]) transition is forbidden. A single H/D or [sup 12]C/[sup 13]C isotopic substitution reduces the symmetry to C[sub s], so that (2PY)[sub 2]–[sup 13]C and (2PY)[sub 2]–d[sub 1] both exhibit S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] transitions. The S[sub 1]/S[sub 2] state exciton splittings are 43.6 cm[sup -1] and 52.4 cm[sup -1], respectively. These are analyzed in terms of a Frenkel model and compared to calculated splittings based on ab initio monomer transition dipole moments. For (2PY)[sub 2]–d[sub 1], whose 2PY subunits are different, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S[sub 1]↔S[sub 0] and S[sub 2]↔S[sub 0] spectra are analyzed and assigned. Several b[sub u] intermolecular vibrations of S[sub 1] appear via vibronic coupling to the S[sub 2](B[sub u]) state. Combination of the fluorescence data from excitation of the S[sub 1] and S[sub 2] origins and vibrational excitations of (2PY)[sub 2]–h[sub 2], (2PY)[sub 2]–d[sub 1], and (2PY)[sub 2]–d[sub 2] allows the determination of the six S[sub 0] state intermolecular vibrational frequencies.© 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jet-cooled phenol·(H2O)3 and d1-phenol·(D2O)3 in the S0 and S1 electronic states. Two-color resonant two-photon ionization combined with time-of-flight mass spectrometry and dispersed fluorescence emission spectroscopy provided mass-selective vibronic spectra of both isotopomers in both electronic states. In the S0 state, eleven low-frequency intermolecular modes were observed for phenol·(H2O)3, and seven for the D isotopomer. For the S1 state, several intermolecular vibrational excitations were observed in addition to those previously reported. Ab initio calculations of the cyclic homodromic isomer of phenol·(H2O)3 were performed at the Hartree–Fock level. Calculations for the eight possible conformers differing in the position of the ‘‘free’’ O–H bonds with respect to the almost planar H-bonded ring predict that the ‘‘up–down–up–down’’ conformer is differentially most stable. The calculated structure, rotational constants, normal-mode eigenvectors, and harmonic frequencies are reported. Combination of theory and experiment allowed an analysis and interpretation of the experimental S0 state vibrational frequencies and isotope shifts. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The supersonically cooled 7-hydroxyquinoline·(H[sub 2]O)[sub 3] cluster was investigated by mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectroscopy. UV(ultraviolet)/UV-holeburning measurements prove that a single cluster isomer is formed, although different tautomers (7-keto- and 7-hydroxyquinoline), rotamers (cis and trans) and isomers (cyclic and water-wire) are possible. Ab initio calculations of structures and vibrations of different tautomers and isomers of this cluster predict that the cis-enol-7-hydroxyquinoline·(H[sub 2]O)[sub 3] "water-wire" cluster is the most stable species. The experimental S[sub 0] and S[sub 1] inter- and intramolecular vibrational frequencies are in good agreement with the calculated harmonic frequencies. S[sub 1]←S[sub 0] excitation leads to contraction of all four hydrogen bonds along the hydrogen bonded water wire, inducing intense intermolecular stretching vibrations of the O-H...O and HO-H...N(quinoline) hydrogen bonds which terminate the water-wire. There are no spectroscopic signs of proton transfer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass- and isomer-selected S[sub 1]←S[sub 0] resonant two-photon ionization and S[sub 1]→S[sub 0] fluorescence spectra were obtained for the supersonically cooled 7-hydroxyquinoline·(H[sub 2]O)[sub 2] cluster. UV/UV-holeburning measurements show that >98% of the spectrum is due to a single "water-chain" cluster isomer, although two different tautomers (7-keto- and 7-hydroxyquinoline), two different rotamers (cis- and trans-hydroxy), and two torsional conformers of the chain are possible. Ab initio calculations of structures and vibrations of five different tautomers/ rotamers/ conformers of this cluster are reported. These predict that the cis-7-hydroxyquinoline·(H[sub 2]O)[sub 2] "up/down" water-chain form is the most stable cluster. The experimentally observed S[sub 0] and S[sub 1] state vibrational frequencies agree well with those calculated for this isomer. We find no evidence for either the trans-rotamer or the keto tautomer clusters. S[sub 1]←S[sub 0] excitation leads to contraction of all three hydrogen-bonds along the hydrogen-bonded water chain, inducing intermolecular stretching vibrations, but no proton transfer. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The S1/S2 splitting of the m-cyanophenol dimer, (mCP)2 and the delocalization of its excitonically coupled S1/S2 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 ab initio 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 Z-shaped (Ci-symmetric); the second-lowest is <5 cm-1 higher and planar (C2h). The vibrational ground state is probably delocalized over both minima. The S0 → S1 transition of (mCP)2 is electric-dipole allowed (Ag → Au), while the S0 → S2 transition is forbidden (Ag → Ag). 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 Δexc = 7.3 cm-1. Additional isotope-dependent contributions arise from the changes of the m-cyanophenol zero-point vibrational energy upon electronic excitation, which are Δiso(12C/13C) = 3.3 cm-1 and Δiso(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 Δel = 179 cm-1; when multiplying this with the vibronic quenching factor Γvibronexp = 0.043, we obtain an exciton splitting Δvibronexp = 7.7 cm-1, which agrees very well with the experimental Δexc = 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- h )·(mCP- d ). A multimode vibronic coupling analysis is performed encompassing all the vibronic levels of the coupled S1/S2 states from the v = 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. [ABSTRACT FROM AUTHOR]