NONLINEAR-OPTICAL PROPERTIES OF NOVEL LOW-BANDGAP POLYTHIOPHENES

A new approach to polythiophene thin films with variable low-energy ba ndgap is reported. Stille reaction [Angew. Chem. Int. Ed. Engl. 25, 50 8 (1986)] between a toolbox of different bis(trimethyltin)thiophenes a nd dihalothieno[3,4-b]pyrazines produces polythiophenes with interesti ng nonlinear optical properties. By variation of the substituent attac hed to the pyrazine ring, the electronic and optical properties of the polymer can be strongly influenced, for example, in lowering the band gap (< 1 eV) or in increasing the oscillator strength. The improved os cillator strength is accompanied by a longer conjugation length and le ads to higher nonlinearities. Third-order susceptibilities, chi((3)), of 10(-8) esu were achieved with figures of merit, chi((3))/alpha (whe re 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 t he bandgap, in contrast to scaling laws empirically found by other gro ups. (C) 1998 Optical Society of America.