Kimio Akiyama

Twist conformational effects on the excited triplet states of aromatic ketones studied by multifrequency TREPR and pulsed EPR spectroscopy

Employing X- and Q-band time-resolved EPR and pulsed EPR techniques, we have investigated effects of twist conformation on the electronic structures and magnetic properties of the lowest excited triplet states of alkyl phenyl ketones. The conformational variation due to steric hindrance of a bulky substituent on the α-position drastically leads to a decrease of the zerofield splitting constant of |D| and an increase of the energy gap between 3nπ* and 3ππ* states. Hydrogen bonding with H2O destabilized significantly the 3nπ* state and switched the T1 state to the 3ππ* state. t-Butyl phenyl ketone gave unusually broad EPR spectra in rigid glassy matrices. The multifrequency and pulsed EPR measurements clarified that the inhomogeneously broadening spectra were ascribable to the distribution of the dihedral angle about a twist conformation. Asymmetric distribution of the ZFS parameters was interpreted in terms of the perturbation of spin—orbit interactions to the spin sublevels.

Positive exchange interaction in the radical ion pair of benzophenone anion and 1,4-diazabicyclo[2,2,2]octane cation radicals studied by FT-EPR spectroscopy

Abstract Electron spin polarization generated from the photoreduction of benzophenone (BP) and its derivatives in the presence of 1,4-diazabicyclo[2,2,2]octane (DABCO) was studied in various solvents. The DABCO cation radical obtained showed a CIDEP spectrum with A/E (absorption/emission) polarization by RPM, while other neutral radicals gaee an E/A pattern. Using triplet quenchers, it was confirmed that these RPM signals were generated through the triplet reaction process. The present results revealed that the radical ion pair including the BP anion and DABCO +. cation radicals has a positive J , while the neutral radical pairs generated under the same condition have a negative J . The sign of J is independent of the polarity of the organic solvents used.

Electronic structure of the excited triplet state of nonphosphorescent tropolone studied by time-resolved EPR

Time-resolved EPR measurements and semi-empirical MO calculations have been carried out to clarify the electronic structure of the T1 state of nonphosphorescent tropolone. The observed zero-field splitting parameters indicate that the T1 state has a pure ππ* character and the energy order among the spin sublevels was TY (in-plane)>TX (in-plane)>0>TZ (out-of-plane), which is the same as tropone for a parent molecule. However, the principal X-axis slightly turns out of the carbonyl bond direction so that the intersystem crossing from the S1 state to the TY sublevel becomes more active. The spin densities obtained from hyperfine splittings of protons and MO calculation of AM1 method show an asymmetric distribution of unpaired π-electrons over two oxygens as well as the seven-membered ring. This unbalanced distribution makes the deviation of the X-axis from the carbonyl bond clear. We found two sites that can exchange each other by intramolecular proton transfer in a durene single crystal.

TIME-RESOLVED AND FOURIER TRANSFORM EPR STUDIES OF TYPE I PHOTOCHEMICAL REACTION. DIRECT DETERMINATION OF KINETIC PARAMETERS OF C-C BOND CLEAVAGE

Continuous wave time-resolved EPR (CW-TREPR) and pulsed Fourier transform EPR (FT-EPR) experiments with a resolution of ≈10 ns were carried out to investigate the Type I photocleavage. A two-pulse echo detection method was used to eliminate the dead-time problem in the FID measurements. An analysis of the buildup kinetics of the FT-EPR signal intensity due to t-butyl radicals provided the rates of the α-cleavage of t-butylphenyl ketone at various temperatures. We obtained two kinetic parameters at low and high temperatures which correspond to the non-radiative decay rate of the excited triplet state of the ketone and the rate of the Type I photocleavage, respectively.

Coherent and incoherent hopping of triplet exciton in quinoxaline-annelated and naphthalene-annelated dimers

Abstract Both a quinoxaline-annelated dimer and a naphthalene-annelated dimer showed excimer-type fluorescence spectra. Excimer-type phosphorescence was observed for the quinoxaline dimer, while the phosphorescence of the naphthalene dimer was not much different from that of the monomer type. In contrast, the EPR spectra clearly showed delocoalization of the unpaired spins in the two chromophores for both molecules. The present results can be interpreted by the models of coherent supermolecule delocalization of the unpaired spins and incoherent hopping of triplet exciton for the quinoxaline and naphthalene dimers, respectively.

Effects of Lewis acid on the sign of exchange interaction of photogenerated radical ion pairs

FTEPR study shows that the sign of the exchange interaction of the nradical ion pair including the 4,4′-dimethoxybenzophenone radical nanion and 1,4-diazabicyclo[2,2,2]octane (DABCO) radical cation changes nfrom positive to negative by addition of BF3.