Phosphorescence excitation spectroscopy in supersonic jets. The lowest triplet state of pyrazine

Jet-Cooled Phosphorescence Excitation Spectrum of the T1(n,π*) ← S0 Transition of 4H-Pyran-4-one

The 4H-pyran-4-one (4PN) molecule is a cyclic conjugated enone with spectroscopically accessible singlet and triplet (n,π*)excited states. Vibronic spectra of 4PN provide a stringent test of electronic-structure calculations, through comparison of predicted vs measured vibrational frequencies in the excited state. We report here the T1(n,π*) ← S0 phosphorescence excitation spectrum of 4PN, recorded under the cooling conditions of a supersonic free-jet expansion. The jet cooling has eliminated congestion appearing in previous room-temperature measurements of the T1 ← S0 band system and has enabled us to determine precise fundamental frequencies for seven vibrational modes of the molecule in its T1(n,π*) state. We have also analyzed the rotational contour of the 000 band, obtaining experimental values for spin-spin and spin-rotation constants of the T1(n,π*) state. We used the experimental results to test predictions from two commonly used computational methods, equation-of-motion excitation energies with dynamical correlation incorporated at the level of coupled cluster singles doubles (EOM-EE-CCSD) and time-dependent density functional theory (TDDFT). We find that each method predicts harmonic frequencies within a few percent of observed fundamentals, for in-plane vibrational modes. However, for out-of-plane modes, each method has specific liabilities that result in frequency errors on the order of 20-30%. The calculations have helped to identify a perturbation from the T2(π,π*) state that leads to unexpected features observed in the T1(n,π*) ← S0 origin band rotational contour.

Cavity ringdown spectrum of the T(1)(n,pi*) <-- S(0) transition of 4-cyclopentene-1,3-dione

The cavity ringdown absorption spectrum of 4-cyclopentene-1,3-dione was recorded near 487 nm in a room-temperature gas cell. The very weak band system (epsilon approximately 0.05 dm3 mol-1 cm-1) in this region is due to the T1(n,pi*) <-- S0 electronic transition. The origin-band maximum was observed at 20540.0 +/- 0.5 cm-1. We have assigned about 40 vibronically resolved bands in a region extending to +1100 cm-1 relative to the origin. Assignments were aided by quantum-chemical calculations of the T1 <-- S0 0-0 excitation energy as well as ground-state vibrational frequencies. From the CRD spectral assignments, we determined fundamental frequencies for several vibrational modes in the T1 excited state, including the lowest-energy ring-bending and -twisting modes, nu19' (b1) and nu14' (a2), respectively. Their fundamentals in the T1 state are 160.5 and 246 cm-1, compared to 99 and 239 cm-1, respectively, in the S0 ground state. The increases in these ring frequencies upon electronic excitation signify that the nominal n --> pi* chromophore is delocalized to include the conjugated ring atoms. The extent of this delocalization is different in the T1(n,pi*) vs S1(n,pi*) excited states. This conclusion is based on observed differences in T1 vs S1 ring fundamental frequencies.

Phosphorescence excitation spectrum of the T(1)(n,pi*)<--S(0) transition of 4H-pyran-4-one

The phosphorescence excitation (PE) spectrum of 4H-pyran-4-one (4PN) vapor at 40-50 degrees C was recorded near 366 nm. The most intense vibronic feature in this region of the spectrum is the T(1)(n,pi*)<--S(0) origin band. The value of nu(0) for the 0(0)(0) transition was determined to be 27 291.5 cm(-1) by comparing the observed spectrum to a simulation in the T(1)<--S(0) origin-band region. Attached to the origin band in the PE spectrum are several Deltav=0 sequence bands involving low-frequency ring modes. From the positions of these bands, together with the known ground-state combination differences, fundamental frequencies for nu(18') (ring bending), nu(13') (ring twisting), and nu(10') (in-plane ring deformation) in the T(1)(n,pi*) excited state were determined to be 126, 269, and 288 cm(-1), respectively. These values represent drops of 15%, 32%, and 43%, compared to the respective fundamental frequencies in the S(0) state. The changes in these ring frequencies indicate that the effects of T(1)(n,pi*)<--S(0) excitation extend beyond the nominal carbonyl chromophore and involve the conjugated ring atoms as well. The delocalization may be more extensive for T(1)(n,pi*) than for S(1)(n,pi*) excitation.

Twisted intramolecular charge transfer states: Rotationally resolved fluorescence excitation spectra of 4,4′-dimethylaminobenzonitrile in a molecular beam

We report the observation at high resolution of seven vibronic bands that appear within approximately 200 cm(-1) of the electronic origin in the S(1)-S(0) fluorescence excitation spectrum of 4,4'-dimethylaminobenzonitrile (DMABN) in a molecular beam. Surprisingly, each band is found to be split into two or more components by a (coordinated) methyl group tunneling motion which significantly complicates the analysis. Despite this fact, high quality [(Observed-Calculated)< or =30 MHz] fits of each of the bands have been obtained, from which the rotational constants, inertial defects, torsion-rotation interaction constants, methyl group torsional barriers, and transition moment orientations of DMABN in both electronic states have been determined. The data show that DMABN is a slightly pyramidalized (approximately 1 degree) but otherwise (heavy-atom) planar molecule in its ground S(0) state, and that its electronically excited S(1) state has both a more pyramidalized (approximately 3 degrees) and twisted (approximately 25 degrees) dimethylamino group. Large reductions in the methyl group torsional barriers also show that the S(1)<--S(0) electronic transition is accompanied by significant charge transfer from the nitrogen atom to the pi* orbitals of the aromatic ring. Thereby established is the participation of all three vibrational coordinates in the dynamics leading to the "anomalous" emissive behavior of DMABN in the condensed phase.

Density functional study of the S0 (X1Ag) and T1 (a3Au) states of the glyoxal molecule

The density functional theory (DFT) calculations in different approximations have been performed for the geometries and vibrational states of the trans-glyoxal molecule in the ground state S0 (X1Ag) and in the lowest excited triplet state T1 (a3Au, n-pi* type). Eight typical combinations of exchange and correlation functionals have been used. Comparative Hartree-Fock (HF) calculations have also been performed. For the open shell a3Au state the standard spin-unrestricted Hartree-Fock and Kohn-Sham approaches (UHF, UKS) as well as the restricted open-shell versions (ROHF, ROKS) have been applied. The calculated frequencies have been compared, among others, with the data resulting from the most recent phosphorescence exicitation spectra of glyoxal cooled in the supersonic molecular beam, recorded with the cooperation of one of us (JH) for the spin-forbidden S0-T1 transition. The most realistic description of the vibrational frequencies, within the unscaled harmonic approximation, can be obtained using the 3-parameter Becke-93 exchange functional (B3), whereas this description practically does not depend on the correlation functional used. Our calculations support the recently reexamined experimental energy of the symmetric CH-rocking fundamental for the T1 state.