A copolymer of cholesteryl 6-(methacryloyloxy)hexanoate and a small amount of(1-pyrenylmethyl) 6-(methacryloyloxy)hexanoate (Py-C-5-MA) was prepared by free radical copolymerization. ii copolymer of 1-eicosanylmethacrylate and a small amount of Py-C-5-MA was also prepared as a reference copolymer. A wide-angle X-ray diffraction pattern for an n-hexane solution of the cholesterol(Chol)-containing copolymer showed a peak corresponding to a spacing of 5.3 Angstrom. In n-hexane, the hydrodynamic radius (R-h) for the Chol-containing copolymer (M-w = 7.8 x 10(4)) was 8.1 nm, while that of the eicosanyl-containing copolymer (M-w = 4.9 x 10(4)) was 9.6 nm, R-h for the former being smaller than that for the latter, although M-w for the former was higher than that of the latter. H-1-NMR spectra of the Chol-containing polymer in n-hexane-d(14) indicated a strong restriction of local motions of pendant Chol groups. Fluorescence spectra of the Chol-containing copolymer in n-hexane indicated that each pyrene group was isolated from others. In n-hexane/benzene mixed solutions of the Chol-containing polymer, the ratio of the intensity of the excimer relative to the monomer emission decreased with increasing the ratio of n-hexane in the mixed solvent. Electron transfer from N,N-dimethylaniline to singlet-excited pyrene chromophores was suppressed in the Chol-containing copolymer in n-hexane. The pyrene chromophores exhibited a long triplet lifetime in n-hexane. These observations led us to conclude that Chol groups formed stacks in n-hexane, and that the pyrene chromophores were trapped in the Chol stacks, leading to the "protection" of pyrene from the bulk phase.