| Research Abstract |
In this project, we have first tried to improve the sensitivity of the time-resolved infrared system, and then we have measured transient infrared spectra of molecules described in the following, and have discussed molecular and electronic structures of the excited species. As hydrocarbon molecules with symmetric structures, we have studied on trans-stilbene and diphenylacetylene in polar and non-polar solvents. As"push-pull"molecules with electron donating (ex. amino or pyrrolyl) and accepting (ex. cyano) groups linked by conjugated pi-electron systems (benzene rings), we have studied on (dimethylamino)benzonitrile (DMABN), cyanobenzquinuclidine (CBQ), and (cyanophenyl)pyrrole (CPP). On molecules of this type, characteristic structures of charge-transfer (CT) excited states in polar environments, with the amino or pyrrolyl groups twisted perpendicularly to the benzene ring plane, have been discussed. However, experimental evidence for the structure is scant and this problem is still a matter of controversy among researchers. In the present study, we have measured picosecond infrared spectra of CT and locally excited (LE) states of DMABN and the isotope-substituted species, and have discussed on the CT molecular structure based on the reliable vibrational assignments, as well as from the comparison with CBQ and CPP as molecules with related structures. We have succeeded in establishing vibrational assignments of infrared bands of the CT state of DMABN, and have revealed that the electronic structure of the CT state contains both quinoidal and benzenoidal contributions. However, in the present stage, we do not have the final conclusion (planar or twisted) about the molecular structure, unfortunately.
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