Narrow-band-gap 2,5-thienylene-divinylene (ThV) units were incorporated into the poly(fluorene vinylene) backbone via a Gilch reaction as an energy trap with various feed ratios; this yielded pronounced changes in the electrochemical and optical properties of the material. The energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the polymers {poly(9,9-di-iso-octylfluorene vinylene) [poly(fluorene vinylene-co-thiophene vinylene (FV))], C1, and C2} were estimated to be -5.53 to -5.10 eV and -2.98 to -2.84 eV, respectively, by cyclic voltammetry measurements. In comparison with poly(FV), the HOMO energy levels of polymers poly(fluorene vinylene-co-thiophene vinylene (FV) (90: 10) (Cl) and poly(fluorene vinyl ene-co-thiophene vinylene (FV) (80 : 20) (C2) were significantly increased, but their LUMO energy levels were slightly decreased. The optical properties were investigated by absorption and emission spectra of the polymers. The good spectral overlap between the emission of poly(FV) and the absorption of polymers C1 and C2 revealed a sufficient energy transfer from the majority of 9,9-di-iso-octylfluorene vinylene units to the minority of ThV units. The reduction of self-absorption losses of polymers C1 and C2 due to spectral separation caused by the incorporation of ThV units could be indirectly confirmed by nonlinear optical (NLO) properties. The result of the NLO properties of the polymers showed that the third-order NLO coefficients of poly(FV), C1, and C2 were 8.1 x 10(-10), 1.35 x 10(-9), and 1.51 x 10(-9) esu, respectively. (C) 2008 Wiley Periodicals, Inc.