The Metastability of the Lowest Excited Singlet Level of Naphthalene

Abstract

The fluorescence lifetime of naphthalene has been determined by direct observation to be on the order of 10⁻⁶ sec. The comparatively weak absorption band on the long‐wave‐length side of the ultraviolet spectrum of naphthalene is thereby established as a separate electronic transition to a metastable level, and as the lowest singlet‐singlet transition. This weak absorption band and the fluorescence and phosphorescence bands of naphthalene in rigid glass solution at 77°K are presented. The absorption (Prikhotko) and fluorescence (Obreimov and Shabaldas) spectra of single crystals of naphthalene at 20°K are reinterpreted in terms of the above results; in particular, the lowest singlet‐singlet band in the crystal is shown to be the 31060 cm⁻¹ band. An anomalous crystal absorption band at 29945 cm⁻¹ is accounted for in terms of absorption to an excited triplet state. A π‐electron energy scheme for naphthalene for the experimentally observed singlet and triplet levels below 50000 cm⁻¹ is given.