Using pulse radiolysis and triplet energy transfer has enabled us to measure the triplet energies in a broad range of different $\pi$-conjugated polymers. In all cases we find that the $1{}^3{B}_u$ is of order 0.6 to 1 eV below the $1{}^1{B}_u$, indicative of localized triplet states with strong electron-electron correlation. We also observe that the $1{}^1{A}_g-1{}^3{B}_u$ gap decreases linearly as the $1{}^1{A}_g-1{}^1{B}_u$ gap decreases even though polymers with very different structure have been studied. This surprising result suggests that polymers with singlet gap $<1.3\mathrm{}\mathrm{eV}$ will have a triplet ground state.

Quantum yields for triplet formation have been determined for seven common π-conjugated polymers in benzene solution using time-resolved photoacoustic calorimetry (PAC) in conjunction with fluorescence quantum yields, singlet and triplet energies. The polymers studied include three poly(thiophenes), poly(2-methoxy,5-(2^′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), a cyano derivative of MEH-PPV, a ladder type poly(p-phenylene) (MeLPPP), and a poly(fluorene). Yields of singlet oxygen formation have also been determined for these polymers in benzene by time-resolved phosphorimetry, and are in reasonable agreement with triplet yields obtained by PAC. Polythiophenes show the highest intersystem crossing yields, which are suggested to result from extensive spin-orbit coupling. Where singlet oxygen yields are less than triplet yields, it is suggested that interaction of molecular oxygen with the ground state of the polymers may be involved.