Knowledge Bank

The electronic states of the naphthalene crystal were studied for the dual purpose of providing support for the analysis of the gas phase fluorescence and testing the existing theories of the excited states of molecular crystals. We first studied the polarized emission of the three crystallographic planes of single oriented naphthalene crystals at $20\ast K$ and $77\ast K$. All the lines of the emission are predominantly polarized along the b-axis. The unpolarized spectrum is the same as was found by Obreimov and $Shabaldas1$ and shows many similarities to the gas phase fluorescence, which was recently investigated by the $authors.2$ According to theory, 3, 4, 5 two electronic states belonging to different representations of the factor group of the crystal should appear in the crystal for every electronic state of the free molecule. The fluorescence is resolved into two band systems, one beginning at $31060cm-1$ and the other at $29945cm-1$. The latter system appears with greater sharpness in the emission at $77\ast$K from the AC plane than does the 31060 system. Furthermore, it is difficult to find a reasonable vibrational analysis which sits the 29945 system into the 31060 system. The two systems are both strongly polarized along the b-axis. It can then be argued, with certain assumptions, that the transition in the free molecules is along the short axis of the molecule. According to the vapor fluorescence $analysis2$ and the vapor absorption $analysis6$ the longest wavelength transition is symmetry forbidden. The theory of symmetry forbidden transitions in molecular crystals has not been developed, however, and the above conclusions are valid only after extrapolation of the existing theory for allowed transitions. We have also reexamined the absorption of naphthalene single crystals and have studied the absorption and fluorescence of mixed crystals containing naphthalene. These experimental data provide a basis for a correlation between the electronic states of the free molecule and those of the crystal.