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) st ates of polyphenyl [diphenyl(polyphenylene)] molecules were studied in rigi d organic glasses at 77 K, We observed the very interesting differences amo ng 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 dec reases with the increase of the number of the composed benzene rings, where as it is scarcely changed at about 0.109 cm(-1) for m-polyphenyls and at ab out 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 t hree benzene rings, the /E/ values obtained are distinguishable in some cas es of m-polyphenyls with planar conformations but could not generally be se parable for o-polyphenyls with nonplanar conformations which have changeabl e twist angles around the C-C bond connecting the adjacent benzene rings. F or 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] appea rs approximately to be a superposition of those of o- and m-terphenyls, The se relations can be elucidated from the viewpoints of the geometrical and e lectronic structures. The lifetimes of their TI states (tau (p)'s) were mea sured 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 benze ne rings, whereas it is scarcely changed at about 5.0 s for m-polyphenyls a nd at about 2.2 s for o-polyphenyls except for o-terphenyl. These trends ar e generally similar to those of the /D/ values.