Yasushi Numata

Electronic spectra of jet-cooled 1-phenylpyrrole: Large-amplitude torsional motion and twisted intramolecular charge-transfer phenomenon

The fluorescence excitation spectrum and the single vibronic level dispersed fluorescence spectra in the region of the S0⇆S1 transition were measured for jet-cooled 1-phenylpyrrole. The 0–0 band was observed at 35 493 cm−1. Long and low-frequency progressions with somewhat irregular intensity distributions appeared on both spectra, and were assigned to torsional motion. The torsional energy levels in the S0 and S1 states were obtained up to 25 and 16 quanta, respectively. The torsional potentials in both states could be determined from the sufficient number of energy levels observed. In the S0 state the most stable conformation was determined to be a twisted form with a dihedral angle of 38.7°, where the planar barrier height was calculated to be 457 cm−1, and the perpendicular to be 748 cm−1. On the other hand, it was discovered that 1-phenylpyrrole in the S1 state also had a twisted form with a somewhat smaller dihedral angle of 19.8°, and that the barrier to planarity was 105 cm−1 and to perpendiculari...

S1-State Internal Conversion of Isolated Azulene Derivatives

The S0-S1 hole-burning spectra of azulene and its derivatives, 1-methyl, 2-methyl, 4-methyl, 1-cyano, and 2-cyanoazulenes, were measured under the isolated condition in order to gain an insight into the internal-conversion mechanism. The width of every 0-0 band was dependent on its transition energy and independent of the density of the S0-state vibrational levels isoenergetic to its zero level of the S1 state. On the contrary, the vibronic-band broadening of each molecule progressed in proportion to the vibrational excess energy of the S1 state. In the low-energy region, widths gradually increased, which is attributed to the normal internal conversion. A drastic increase was observed in the medium-energy region in azulene and three methyl derivatives but not in the two cyano ones. This is considered to be the onset of the relaxation process due to the conical intersection suggested by Bearpark et al. [J. Am. Chem. Soc. 1996, 118, 169]. Anomalous width behavior was found for two vibronic bands whose widths were still narrow even above the onset. One was 0 + 2659 cm(-1) band of azulene, that had been already reported by Ruth et al. [Phys. Chem. Chem. Phys. 1999, 1, 5121], and we could reproduce it by the hole-burning method. Another was 0 + 2878 cm(-1) band of 2-methylazulene. This is the vibronic selectivity in competition between the relaxation process and the normal internal conversion. The amplitude vectors of these modes were similar, including the in-plane bending of the CH bond and the stretching of the transannular bond.

Spectroscopy and Photophysics of Low-Lying Excited Singlet States of α,ω-Dithienylbutadiene and α,ω-Dithienylethylene Vapors

Fluorescence and excitation spectra and fluorescence lifetimes have been measured for alpha,omega-dithienylbutadiene (DTB) and alpha,omega-dithienylethylene (DTE) in a jet-cooled He expansion and in the static vapor phase. It is shown that the emission of DTB in the static vapor phase consists of the S1(2(1)A(g)) and S2(1(1)B(u)) fluorescence, while the emission in a jet consists of solely the S1 fluorescence. The fluorescence quantum yields of DTB and DTE vapors decrease significantly with increasing excitation energy. The conformer-specific fluorescence was measured for DTB and DTE in a jet. The false S1 (2(1)A(g)) origin of the most stable conformer of DTB was observed at 25470.5 cm(-1) in a jet, which is located approximately 2800 cm(-1) below the S2 (1(1)B(u)) origin. The S1 origins of the two conformers of DTE were observed at 28648 and 28966 cm(-1) in a jet, and the S1 state of the most stable conformer is assigned as 1(1)B(u).

Quantitative Determination of Glycine, Alanine, Aspartic Acid, Glutamic Acid, Phenylalanine, and Tryptophan by Raman Spectroscopy

ABSTRACT The development of a new quantitative method for amino acids using Raman spectroscopy is reported. Raman spectra of glycine, alanine, aspartic acid, glutamic acid, phenylalanine, and tryptophan were measured. The band ratio between the Raman intensity of the amino acid and that of acetonitrile as an external standard was calculated to remove the influence of factors such as laser power intensity and instrumental effects. The calibration curves were obtained by plotting the band ratios against the concentrations of the amino acids. The curves were linear with coefficient correlations of over 0.99 for all amino acids. The Raman spectra of known concentration samples were measured to confirm the reproducibility of this method. The relative errors were small, indicating that the concentrations of amino acids can be determined using Raman spectroscopy. The limits of detection and quantitation were determined as thrice and 10 times the standard deviation of the background signal to be 0.007 and 0.02 mol L−1, respectively. Raman spectra of aspartic acid at 0.02 mol L−1 were measured several times and the uncertainty was 7%.

Photophysics of chlorofluorobenzenes: enhancement of sensitized phosphorescence due to cluster formation

Abstract The photodissociation of o- , m- , and p -chlorofluorobenzene and their clusters in a supersonic jet were studied by sensitized phosphorescence excitation spectroscopy and resonance-enhanced multiphoton ionization time of flight (REMPI TOF) mass spectroscopy. While bands of both monomer and the cluster were observed together in the fluorescence excitation spectrum, we found that the only the cluster bands appeared in the sensitized phosphorescence excitation spectrum. This shows that the sensitized phosphorescence yield in the cluster is much larger than that of the monomer. It also suggests that the CCl bond cleaves rapidly in the triplet state of the monomer. However, we reveal that the fast vibrational relaxation due to cluster formation which is competitive with the dissociation of CCl bond takes place in the triplet cluster.

Intersystem crossing in jet-cooled naphthalene clusters as studied by sensitized phosphorescence excitation spectroscopy

Abstract The fluorescence excitation and sensitized phosphorescence excitation spectra of jet-cooled naphthalene clusters were measured. The intensity of the cluster bands in the sensitized phosphorescence excitation spectrum depends on both the cluster size and the excess vibrational energy. The intensity of the trimer is 1.3 times larger than that of the monomer in the 8 1 0 region. However, the intensity of the tetramer is weaker than that of the monomer in the same excitation region. The trimer band intensity in the sensitized phosphorescence excitation spectrum decreases with an increase in the excitation energy compared with that in the fluorescence excitation spectrum. These results indicate that intersystem crossing of the clusters competes with faster relaxation at higher excitation energies. It is considered that the fast process is associated with the relaxation of the clusters to an excimer state.