Optical nonlinearity of a sigma-pi photoconductive copolymer, poly(disilanylene-2, 2 ' -bipyridine-5, 5 ' -diyl) ruthenium complex, has been investigated in chloroform solution of various concentrations. The nonlinear optical absorption resulted in optical limiting, optical bistability, and laser pulse compression for nanosecond pulses at the wavelength of 532 nm. Theoretical analyses showed that the optical absorptive nonlinearity was dependent on the photogenerated excitons and exciton-exciton couplings.