Nonlinear optical properties of novel low-bandgap polythiophenes

A new approach to polythiophene thin films with variable low-energy bandgap is reported. Stille reaction [Angew. Chem. Int. Ed. Engl. 25, 508 (1986)] between a toolbox of different bis(trimethyltin)thiophenes and dihalothieno[3,4-b]pyrazines produces polythiophenes with interesting nonlinear optical properties. By variation of the substituent attached to the pyrazine ring, the electronic and optical properties of the polymer can be strongly influenced, for example, in lowering the bandgap (< 1 eV) or in increasing the oscillator strength. The improved oscillator strength is accompanied by a longer conjugation length and leads to higher nonlinearities. Third-order susceptibilities, chi((3)), of 10(-8) esu were achieved with figures of merit, chi((3))/alpha (where alpha is the linear absorption coefficient), of as much as 2 x 10(-13) esu cm. Frequency-tunable degenerate four-wave mixing experiments revealed a strong wavelength dispersion of the figure of merit above the bandgap, in contrast to scaling laws empirically found by other groups. (C) 1998 Optical Society of America.