The X-ray diffraction of crystalline 1,2-dihydro-3H-benz[e]inden-3-one (DHBI) reveals that the molecular geometry is fully planar in the electronic ground state. Glassy solutions of naphthaldehyde, 2-acetonaphthone and methyl 2-naphthoate in the mixtures methylcyclohexane/iso-pentane (MIP) and methanol/ethanol (ME) are phosphorescent. DHBI in ME shows phosphorescence, but in MIP it is non-phosphorescent. The phosphorescence spectra of these compounds and of naphthalene have a strong resemblance. This is in accordance with a molecular distortion in the lowest triplet state, which decouples the π electron systems of the carbonyl group and the naphthyl group. The absence of phosphorescence of DHBI in MIP, indicates a geometry of the triplet state, having a non-planar naphthalene ring, when the molecule is in the non-hydrogen bonded form.