Hiroyuki Saigusa

Internal heavy atom effect on the triplet spin sublevels of the lowest triplet state of naphthalene. I. Radiative and nonradiative decays of the spin sublevels of 1‐halonaphthalenes

The mechanism of the internal heavy atom effect is discussed from the viewpoints of the dynamic and spectral properties of the triplet spin sublevels of 1‐halonaphthalenes. The heavy atom effect on the radiative rate constant for phosphorescence emission in the O–O band is most prominent in the Tx emission, somewhat less in the Ty emission, and is small in the Tz emission. (The x and y axes correspond to the long and short axes of the naphthalene molecule, and the z axis is perpendicular to the molecular plane.) The heavy atom effect on the nonradiative decay rate constant is most prominent in the decay of the Tx and Ty sublevels and is small in the decay of the Tz sublevel. These experimental results are interpreted in terms of a mechanism in which spin‐orbit coupling involving the halogen atomic orbital plays an essential role. The heavy atom enhancement in the radiative and nonradiative decay of the Tx sublevel is mainly governed by the spin–orbit coupling between the (ππ*) triplet state and (σπ*) or (...

Microhydration of the guanine-guanine and guanine-cytosine base pairs.

Monohydration structures of the guanine-guanine and guanine-cytosine base pairs have been elucidated by IR-UV double resonance spectroscopy combined with ab initio calculations. The systems studied consist of the homodimer of 9-methylguanine and the heterodimer of 9-methylguanine and 1-methylcytosine in which the methyl group is introduced to mimic the presence of the sugar-phosphate backbone and to block specific tautomerization. The monohydrate of the homodimer is identified as that of the most stable symmetric structure formed by the keto tautomers of guanine, which demonstrates that the base pair structure is not influenced by the hydration. It is also shown that at least two structural isomers, one of which retains the Watson-Crick GC pair structure, contribute the monohydrated cluster of the heterodimer. Although stacked base pairs are suggested to be significantly stabilized by the addition of water, the result shows no clear indication for the presence of stacked monohydrates in either homodimer or heterodimer case.

Twisted S1 excited state geometries in 4-dimethylaminobenzonitrile and dimethylaniline: New -d6 origin bands

The S1←S0 electronic spectra of 4-dimethylaminobenzonitrile-h6 and -d6 (DMABN) and dimethylaniline-h6 and -d6 have been reexamined, and new electronic origins have been observed for the -d6 species, approximately 65 cm−1 lower in energy than previously reported. The spectra of DMABN-h3d3 and several other isotopomers of DMABN are reported for the first time. A prominent low-frequency progression is assigned to dimethylamino torsion, and the S1 states are found to be twisted by about 26° with a small 190 cm−1 barrier to planarity. Other bands are tentatively assigned to inversion and methyl torsional motions.

Picosecond photodissociation study of the excimer formation in van der Waals dimers of aromatic molecules

Abstract The excimer formation that takes place following electronic excitation of van der Waals dimers of fluorene and dibenzofuran has been investigated in real-time using a picosecond pump-probe photodissociation technique. For both dimers, excimer formation is found to occur on a ≈40 ps time scale. The result indicates that the broadening in the excitation spectrum of the dibenzofuran dimer is due to inhomogeneous sources.

Ab initio studies on the photophysics of uric acid and its monohydrates: role of the water molecule.

The photophysical behavior of three lowest-energy tautomers of uric acid and seven most stable isomers of uric acid monohydrate is comprehensively studied by ab initio calculations. Ground-state energies are calculated with the CCSD(T) method, while excitation and ionization energies as well as excited-state potential energy profiles of photoinduced processes are calculated with the CC2 method. For the (1)ππ* state, it is found that the excitation energy of the monohydrate cluster is significantly lower than that of isolated uric acid when the water molecule is hydrogen-bonded at a specific carbonyl group. The calculated excited-state potential energy profiles suggest that some monohydrate isomers can undergo a migration of the water molecule from one site to another site in the (1)ππ* state with a small energy barrier. It is also found for both uric acid and its monohydrate that nonradiative decay via the NH bond dissociation in the (1)πσ* state is likely to occur at higher excitation energies. On the basis of the computational results, possible mechanisms for the absence of specific isomers of uric acid monohydrate from the resonant two-photon ionization spectrum are discussed.

Multiple Hydrogen-Bonding Interactions of Uric Acid/9-Methyluric Acid with Melamine Identified by Infrared Spectroscopy

Hydrogen-bonded complexes of uric acid/9-methyluric acid (UA/9MUA) with melamine (MEL) are prepared by the combined technique of laser desorption and supersonic-jet expansion, and their stable structures are investigated by infrared spectroscopy and theoretical calculations. It is shown that the 1:1 complex formed between UA and MEL is of nonplanar type, in which the two chromophore planes are significantly folded and thus allow for triple hydrogen-bonding interactions. An anomalously broad IR band is observed in the low-frequency range 2500-2800 cm(-1), which is taken as evidence for the formation of a strong hydrogen bond between one of the NH sites of UA and MEL. In the case of 9MUA, in which hydrogen bonding to the N9H site of UA is blocked by the methyl group, two planar pairs formed of 9MUA and MEL are found to coexist. The nature of the multiple hydrogen-bonding interactions in these complexes is discussed based on the natural bond orbital analysis and compared with those of the guanine-cytosine base pair. The results are expected to provide important information on the structural characterization of urinary stones developed in infants after ingesting MEL-contaminated formula.

Specific deuterium isotope effects on the intramolecular charge-transfer dynamics of 4-(dimethylamino)benzonitrile

The photoinduced charge-transfer (CT) dynamics of 4-(dimethylamino)benzonitrile (DMABN) are studied for methyl-deuterated isotopomers DMABN-d6 (with two CD3 groups) and DMABN-h3d3 (with one CH3 and one CD3 group) by steady-state and time-resolved fluorescence spectroscopy. The asymmetrically deuterated species DMABN-h3d3 is found to exhibit less CT fluorescence in sufficiently polar solvents than the symmetric isotopomers of DMABN-h6 and DMABN-d6. The results of fluorescence decay measurements are also reported, which suggest that the rate of backward electron transfer is increased upon asymmetric substitution of the dimethylamino group.

Effective Strategy for Conformer-Selective Detection of Short-Lived Excited State Species: Application to the IR Spectroscopy of the N1H Keto Tautomer of Guanine

The ultrafast deactivation processes in the excited state of biomolecules, such as the most stable tautomers of guanine, forbid any state-of-the-art gas phase spectroscopic studies on these species with nanosecond lasers. This drawback can be overcome by grafting a chromophore having a long-lived excited state to the molecule of interest, allowing thus a mass-selective detection by nanosecond R2PI and therefore double resonance IR/UV conformer-selective spectroscopic studies. The principle is presently demonstrated on the keto form of a modified 9-methylguanine, for which the IR/UV double resonance spectrum in the C═O stretch region, reported for the first time, provides evidence for extensive vibrational couplings within the guanine moiety. Such a successful strategy opens up a route to mass-selective IR/UV spectroscopic investigations on molecules exhibiting natural chromophores having ultrashort-lived excited states, such as DNA bases, their complexes as well as peptides containing short-lived aromatic residues.