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This paper presents a study of singlet-triplet transition of naphthalene in naphthalene; 1, 4 dibromotetrafluorobenzene and naphthalene: 1, 4 diiodotetrafluorobenzene. The formation of these complexes is being reported here for the first time. In the naphthalene: 1, 4-diiodotetrafluorobenzene crystalline complex, the $T1\leftrightarrow S0$ transition of naphthalene can be observed in a 0.5 mm thick crystal due to the external heavy atom effect. This transition isotopic mixing of naphthalene. The nature of Exciton-phonon interaction is elucidated by examining the effect of isotopic substitution of naphthalene on the structures of the phonon sideband. The phonon spectra of absorption and emission are compared. Triplet Exciton migration is studied from the phosphorescence of the isotopic mixed crystals which contain a supertrap as the exciton trapping center. The variation in the lifetime of phosphorescence of naphthalene in the two complexes provide an opportunity to derive the mode of energy transport.