Triplet Energies of $\mathit{\pi }$-Conjugated Polymers

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.