Systematic characterization of excited states in conjugated polymers

We present the first complete systematic characterization of the excited states in conjugated polymers. Our results are relevant for the understanding of the photophysics of these materials. We perform full configuration interaction calculations in an exciton basis within which a long chain polymer is considered as coupled molecular units. Complete pictorial descriptions of all excited states are obtained. In linear chain polymers such as the polyacetylenes and polydiacetylenes the 1B_u is an exciton, and the fundamental two-photon states can be broadly classified into triplet-triplet (TT), charge-transfer (CT) and singlet-singlet (SS) excitations. In the above CT refers to charge-transfer from one unit to another, and TT and SS are two electron-two hole excitations. In TT the spin angular momenta of two different triplet excitations combine to give an overall singlet, while the individual excitations are singlets in SS. The 2A_g is classified as TT. The mA_g, an even parity state that plays a strong role in nonlinear optics, is a correlated CT state. The SS states occur higher in energy and for moderate exciton binding split into the biexciton and two-exciton continuum. The calculations can be easily extended to the polyphenylenes, for which the characterization of excited states continues to be possible. These theoretical results are useful in explaining a variety of third order nonlinear optical spectroscopic measurements as well as picosecond photoinduced absorption.