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Ultraviolet photoelectron spectroscopy (UPS) has been applied to the investigation of the electronic structure of oligothiophenes with 4-8 thiophene rings. In a series of a-linked oligomers (an with n being the number of rings), a systematic evolution of the 1T band is observed. Several peaks which correspond to the 1T band are observed in the region of 0.7-3 e V below the Fermi level (E F ), and the bandwidth becomes broader with increasing n. The nonbonding 1Tband is observed at 3.5 eV below EF and its energy is almost independent of the number of thiophene units. UPS spectra of a 7 and a 8 are fairly similar to the spectra of poly thiophene, showing that these oligomers are good model compounds of the polymer. The ionization threshold energy of a 7 and poly thiophene was observed to be 5.3 eV. The effect of irregularity on the 1T-electron system was also studied by using oligomers which contain a (3 iinkage or a vinylene group at the middle of the molecule. The UPS spectra showed that the (3 linkages significantly affect the electronic structure of poly thiophene, while the vinylene group does not. In order to analyze the UPS spectra and to investigate the electronic structures of oligomers, the orbital energies and the geometries of these oligomers are calculated by the semiempirical MNDO-SCF-MO (modified neglect of diatomic overlap self-consistent-field molecular orbital) method. Theoretically simulated spectra of these oligothiophenes derived from the obtained orbital energies by Gaussian broadening are compared with the observed ones. The agreement between the observed and calculated spectra is very good, particularly in the 1T region. It is shown from the optimized geometry that (I) an's have planar structure and 1T electrons are delocalized, (2) the oligomer with (3 linkages has non planar structure leading to limited delocalization of 1T electrons, and (3) the oligomers with a vinylene group are almost planar and the disturbance by the vinylene group on the delocalization is small..) Present address:

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2-(1,3-Dimethylimidazolidinio)diselenocarboxylate ( 16) was obtained in 48% yield as thermally stable, dark green crystals by reaction of 2-methylene-1,3-dimethylimidazolidine (7) with Se 2 Cl 2 in the presence of Et 3 N. The reaction of 16 with elemental sulfur gave 2-(1,3-dimethylimidazolidinio)dithiocarboxylate (20) in 95% yield by exhaustive selenium-sulfur exchange, whereas the use of a limited amount of sulfur allowed the preparation of 2-(1,3-dimethylimidazolidinio)selenothiocarboxylate ( 19) as dark red crystals. X-ray crystallographic analyses revealed that the planes of the carbenium ion and CX 2 -(X ) S, Se) parts of 16 and 20 are nearly perpendicular to each other. The observed geometries (including bond angles and lengths) of 16 and 20 were reproduced satisfactorily by ab initio calculations (B3LYP/6-31G* level). Discussion is also made on the structures of 16, 19, and 20 on the basis of spectroscopic data (NMR, IR, and UV/vis). Methylation of 16 with MeI furnished the carbenium iodide (26a), which possesses a diselenoester moiety, as thermally labile, green crystals. The reaction of 16 with two molar amounts of DMAD provided the 1:2 adduct (30a) in 67% yield. The reaction of 16 with N- [(p-tolylsulfonyl)imino]phenyliodinane gave the inner salt (35) in 68% yield with loss of selenium atom from the initial adduct (33).

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