Michael Evan Chandross

Designing π-conjugated polymers with light emission in the infrared

There is currently a great need for solid state lasers that emit in the infrared. Whether or not conjugated polymers that emit in the IR can be synthesized is an interesting theoretical challenge. We show that emission in the IR can be achieved in designer polymers in which the effective Coulomb correlation is smaller than that in existing systems. We also show that the structural requirement for having small effective Coulomb correlations is that there exist transverse π-conjugation over a few bonds in addition to longitudinal conjugation with large conjugation lengths.

Prediction of infrared light emission from pi-conjugated polymers: a diagrammatic exciton basis valence bond theory

There is currently a great need for solid-state lasers that emit in the infrared, as this is the operating wavelength regime for applications in telecommunications. Existing π-conjugated polymers all emit in the visible or ultraviolet, and whether or not π-conjugated polymers that emit in the infrared can be designed is an interesting challenge. On the one hand, the excited-state ordering in trans-polyacetylene, the π-conjugated polymer with a relatively small optical gap, is not conducive to light emission because of electron-electron interaction effects. On the other hand, excited-state ordering opposite to that in trans-polyacetylene is usually obtained by chemical modification that increases the effective bond alternation, which in turn increases the optical gap. We develop a theory of electron correlation effects in a model π-conjugated polymer that is obtained by replacing the hydrogen atoms of trans-polyacetylene with transverse conjugated groups and show that the effective on-site correlation in this system is smaller than the bare correlation in the unsubstituted system. An optical gap in the infrared as well as excited-state ordering conducive to light emission is thereby predicted with similar structural features.

Theory of bi-excitons in /spl pi/-conjugated polymers

Summary form only given. Theoretical models of /spl pi/-conjugated polymers that emphasize the Coulomb electron-electron correlations predict that the lowest optidal excitation is to an exciton. We have extended our theoretical studies to an energy region that is much higher, to probe specifically the states that are reached by two-electron excitations from the ground state. Theoretical evidence for the biexciton, a bound state of two excitons that occurs below the two-electron continuum, is found. The lowest biexciton state is of even parity, can be reached by optical excitatioll from the IB/sub u/ exciton, and in principle also by two-photon excitation from the ground state. Interpretation of a recent picosecond photoinduced absorption experiment on polyparavinylene (PPV) in terms of the biexciton will be given. Very similar experimental results have been obtained also in pump-probe experiments with other conjugated polymers (polydiacety-enes, PTV, etc.) in the past. These results will be discussed within the context of our theory. Our earlier work on optical nonlinearity is /spl pi/-conjugated polymers already predicted stable biexcitons, and the present results therefore also confirm the validity of the mechanism of third order optical nonlinearity proposed by us.