Assemblies of gold nanorods capped with poly-diarylethene (DE) were chemically prepared in a solution. Plasmon assisted optical responses of the poly(DE) have been studied. Fluorescence from the open-form of DE was mostly quenched near 550nm due to interband transition of gold, while fluorescence near 700nm was slightly enhanced by the longitudinal plasmon resonance. Cycloreversion reaction of the gold nanorodpoly(DE) has been also studied, and it has been found that the reaction rate is enhanced by the plasmon excitation and increases nearly two-fold in the assembly. The magnitude of the enhancement is qualitatively consistent with the spatially and orientationally averaged value of the simulated optical field enhancements in the vicinity of the nanorod.