Effects of substituents on the third order nonlinear optical properties of poly[(3-alkyl thiophene-2,5-diyl)-(benzylidenequinomethene-2,5-diyl)] derivatives

Five poly[(3-alkylthiophene) benzylidenequinomethene] derivatives: poly[(3-butylthiophene-2,5-diyl)- (p-nitrobenzylidenequinomethene-2,5-diyl)] (PBTNBQ), poly[(3-hexylthiophene-2,5-diyl)- (p-nitrobenzylidenequinomethene-2,5-diyl)] (PHTNBQ), poly [(3-hexylthiophene-2,5-diyl)-(p-N,N-dimethylamino)- benzylidenequinomethene-2,5-diyl] (PBTDMABQ), poly[(3-hextylthiophene-2,5-diyl)-(p-N,N-dimethylamino) benzylidenequinomethene-2,5-diyl] (PHTDMABQ) and poly[(3-octylthiophene-2,5-diyl) [(p-N,N-dimethylamino) benzylidenequinomethene-2,5-diyl](POTDMABQ) were synthesized. According to the function of band gap and energy of the incident photons, the band gaps of these polymers are calculated as 1.82, 1.85, 1.71, 1.78 and 1.67 eV, respectively. The homogenous films of these five polymers are prepared and their third-order nonlinear optical properties are studied with the backward degenerate four-wave mixing (DFWM) at 532 nm. By using the relative measurement technique, the thirdorder nonlinear optical susceptibilities of PBTNBQ, PHTNBQ, PBTDMABQ, PHTDMABQ and POTDMABQ films are measured as big as 1.74×10^-8, 1.82×10^-8, 5.62×10^-9, 8.64×10^-9 and 1.22×10^-8 esu respectively. It is found that the substitutents have a strong effects on nonlinear optical properties of poly[(3-alkylthiophene) benzylidenequinomethene] derivatives, the effects of substituents on the third order nonlinear properties of poly-[(3-alkylthiophene) benzylidenequinomethene] derivatives are illuminated with the delocalization degree of conjugated π electronics and the polarity of the polymers.