External heavy atom effect, exciton-phonon coupling, and triplet energy transfer in a novel crystalline complex between naphthalene and 1,4-diiodotetrafluorobenzene

The crystalline complex naphthalene: 2(1,4-diiodotetrafluorobenzene), the formation of which is being reported here for the first time, was investigated by a combination of low temperature Raman spectroscopy and electronic spectroscopy. The Raman investigation reveals that this crystalline complex is ordered but stabilized, primarily, by the van der Waals interaction. The phonon bands of the two components in the lattice are found to be largely decoupled. The liquid helium temperature electronic spectroscopy suggests the absence of any low-lying charge-transfer band. The lowest triplet state in the complex is that of naphthalene which shows a strong manifestation of the external heavy-atom effect due to the iodine atoms. Because of this heavy-atom effect, the S₀ → T₁ absorption of naphthalene, in this complex, can be seen with a 50% absorption in a crystal of 0.5 mm thickness. This triplet-exciton transition is found to have a very narrow exciton bandwidth, and it shows a strong phonon sideband in contrast to what is observed for the triplet transition of naphthalene in neat and isotopically mixed crystals. Yet, the fluorescence of naphthalene in the complex does not show such an enhancement of the phonon-sideband intensity. A theoretical description is presented, whereby both the strong heavy-atom effect and the selective increase of the exciton-phonon coupling for the triplet transition of naphthalene in the complex are explained by mixing of this transition with the triplet transition of 1,4-diiodotetrafluorobenzene.