Electron paramagnetic resonance studies of the lowest triplet states of polyphenyl molecules in rigid glasses

Electron paramagnetic resonance (EPR) on the lowest excited triplet (TI) states of polyphenyl [diphenyl(polyphenylene)] molecules were studied in rigid organic glasses at 77 K, We observed the very interesting differences among their three groups of o-, m-, and p-polyphenyls [diphenyl(poly(1,2-, 1,3-, and 1,4-phenylene)), respectively]. For p-polyphenyls, the /D/ value decreases with the increase of the number of the composed benzene rings, whereas it is scarcely changed at about 0.109 cm(-1) for m-polyphenyls and at about 0.086 cm(-1) for o-polyphenyls except for o-quaterphenyl. Because there are several conformers for o- and cm m-polyphenyls composed of more than three benzene rings, the /E/ values obtained are distinguishable in some cases of m-polyphenyls with planar conformations but could not generally be separable for o-polyphenyls with nonplanar conformations which have changeable twist angles around the C-C bond connecting the adjacent benzene rings. For the quaterphenyls having two different groups, the: EPR spectra of o,p- and m,p- quaterphenyls [C6H5-(1,2-C6H4)-(1,4-C6H4)-C6H4 and C6H5-(1,3-C6H4)-(1,4-C6H4)-C6H5, respectively] are relatively close to that of p-terphenyl, whereas that of o,m-quaterphenyl [C6H5 -(1,2-C6H4)-(1,3-C6H4)-C6H5] appears approximately to be a superposition of those of o- and m-terphenyls, These relations can be elucidated from the viewpoints of the geometrical and electronic structures. The lifetimes of their TI states (tau (p)'s) were measured from the decay curves of their EPR B-min signals. For p-polyphenyls, the tau (p) decreases with the increase of the number of the composed benzene rings, whereas it is scarcely changed at about 5.0 s for m-polyphenyls and at about 2.2 s for o-polyphenyls except for o-terphenyl. These trends are generally similar to those of the /D/ values.