The fluorescence property and molecular structure of a specifically substituted cellulose derivative, 6-O-alpha-(1-naphthylmethyl)-2,3-di-O-pentylcellulose (NPeC), were investigate d. Steady-state and time-resolved measurements of fluorescence spectra were carried out for a dilute solution of NPeC in tetrahydrofuran at different temperatures. It was found that the cellulose derivative forms a naphthyl excimer intramolecularly via a dynamic process requiring quite a small activation energy (E(a) = 2.8 kcal mol(-1)). The intramolecular excimer formation in NPeC is interpreted as a consequence of the short-range approach of two adjacent naphthyl chromophores after photoexcitation of one of them; possibly, the substituents are originally allowed to be fairly close to each other, owing to a special arrangement of the carbohydrate backbone. In combination with the conformational analysis performed on a model disaccharide as a dimer unit of NPeC, it is suggested that the backbone chain of this polymer prefers to assume a "twisted" structure.