Electron Paramagnetic Resonance Studies of the Lowest Triplet States of Polyphenyl Molecules in Rigid Glasses

Electron paramagnetic resonance (EPR) on the lowest excited triplet (T 1 ) 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 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 [C 6 H 5 −(1,2-C 6 H 4 )−(1,4-C 6 H 4 )−C 6 H 5 and C 6 H 5 −(1,3-C 6 H 4 )−(1,4-C 6 H 4 )−C 6 H 5, respectively] are relatively close to that of p -terphenyl, whereas that of o, m -quaterphenyl [C 6 H 5 −(1,2-C 6 H 4 )−(1,3-C 6 H 4 )−C 6 H 5 ] 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 T 1 states ( τ p 's) were measured from the decay curves of their EPR B min signals. For p -polyphenyls, the τ 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.