Three-Dimensional Polarization Analysis of the Phosphorescence of Halonaphthalenes in Host Crystals

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Abstract
The polarization ratios b/a, b/c′, and a0/c0 of the phosphorescence emission of 2-chloro-, 2-bromo-, 2-iodo-, and 2, 3-dibromonaphthalene in biphenyl host crystal and 1-bromonaphthalene in naphthalene host crystal are determined at 77°K. The relative transition probability of the emitting oscillators along the long axis (PL), short axis (PM) and normal to the halonaphthalene molecular plane (PN) has been determined for each vibronic band in the spectrum. This is accomplished by a three-dimensional analysis, using the observed polarization ratios from any two host crystal faces and the normalization condition: PL+PM+PN=100, applied to any vibronic emission band. The relative importance of the different spin-allowed transitions in rendering the triplet—singlet spin-forbidden transition allowed is concluded from the three-dimensional analysis. In all the compounds studied, the out-of-plane electric dipole allowed transitions contribute 20%—40% of the intensity of the phosphorescence of the halonaphthalenes by direct spin—orbit perturbation [Subspectrum I:T. Pavlopoulos and M. A. El-Sayed, J. Chem. Phys. 41, 1082 (1964)]. The in-plane electric dipole allowed transitions contribute 60%—80% of the intensity via spin—orbit-vibronic perturbation (Subspectrum II). The relative amounts of the long- and short-axis polarized emissions are very sensitive to the position of the halogen atom in the naphthalene ring. The long-axis polarized emission constitutes 52%, 21%, and 39% of the total emission of the 2-halo-, 1-bromo-, and 2, 3-dibromonaphthalenes, respectively. The short-axis polarized emission constitutes 10%—20%, 36%, and 44% of the total emission of the 2-halo-, 1-bromo-, and 2, 3-dibromonaphthalenes, respectively. These observed variations in the long- and short-axis in-plane polarized emission are readily explained by the valence-bond method if the perturbing transitions involve the (intramolecular) charge-transfer (ionic) states formed from the linear combination of the ionic structures resulting from transferring electrons from the halogen atoms to the ring or to other halogens, if present. The relative amount of the out-of-plane emission is found to be largest for the 2, 3-dibromo-derivative (∼35%) in rigid glasses but lowest (∼17%) for the same compound in biphenyl host. This result shows that the decrease of the relative amount of the out-of-plane emission of 2, 3-dibromonaphthalene in biphenyl host is a result of host—guest and not bromine—bromine interaction. The fact that the different bands involving the lattice vibrations of the host are found to have polarization similar to that of the phononless band indicates that the host—guest interaction is static in nature. Two mechanisms are proposed to explain the results. In the first mechanism, the intramolecular spin—orbit coupling scheme is changed by forcing the halogen to be slightly nonplanar with the aromatic ring due to the packing forces of the host lattice. In the second mechanism, mixing between the host and guest electronic states is proposed.