Pulse radiolysis-optical absorption studies on the triplet states of p-phenylenevinylene oligomers in solution

The triplet excited states (B-3(u)) of phenyl-capped, n-octyloxy substitute d p-phenylenevinylene oligomers have been generated by pulse radiolysis in benzene at room temperature. They were characterized by their UV/vis absorp tion spectra, kinetics of formation and decay, and quenching by oxygen. The triplet states were formed either by energy transfer from the solvent exci ted singlet followed by inter-system crossing (phi(ISC) approximate to 0.06 ) or by energy transfer from the naphthalene triplet. They exhibited a stro ng absorption (E approximate to 10(5) M-1 cm(-1)) at photon energies that d epended strongly on the chain length. Comparison with poly(2-methoxy-5-(2-m ethoxy-5-(2'-ethylhexoxy)-p-phenylenevinylene) (MEH - PPV) suggests that th e conjugation in the polymer extends over approximately 15 monomer units, i .e., similar to the conjugation length in the solid state at low temperatur e. In oxygen-free solution, the triplets had lifetimes of the order of tens of microseconds. They were quenched by oxygen with bimolecular rate consta nts of ca. 10(9) M-1 s(-1). Radical-anions and radical-cations of the short er oligomers could also be observed. The radical-anions were efficiently qu enched by oxygen (k approximate to 10(10) M-1 s(-1)), while the radical-cat ions were unaffected by O-2.