Ab initio study of nonhomogeneous broadening of the zero-field splitting of triplet guest molecules in diluted glasses

Nonhomogeneous broadening of phosphorescence lines and microwave signals in optical detection of magnetic resonance (ODMR) has been calculated using multiconfigurational self-consistent field wave functions and the polarized continuum model. The solvent effects on the zero-field splitting (ZFS) parameters in the low-lying triplet states of aza-aromatic molecules are found to be linearly dependent on the solvent-induced shifts in the phosphorescence frequency in agreement with experimental data. The main contribution to the ZFS originates in the dipolar interaction of the two electron spins, the spin–spin coupling. The spin–orbit coupling (SOC) contribution to the ZFS parameter is much larger for the ³$nπ*$ state of pyrazine compared to the ³$ππ*$ states of quinoline. The second-order SOC contribution to the splitting of the ³$nπ*$ state in the pyrazine molecule does not show any appreciable dependence on the dielectric constant of the solvent. This raises doubts about earlier theories for explaining the inhomogeneous broadening in triplet-state spectra based exclusively on the SOC-induced mixing of the singlet and triplet states. We complete the interpretation of the ODMR spectrum of pyrazine by calculating the hyperfine coupling (HFC) tensors in the lowest triplet state using the UB3LYP hybrid functional. An appreciable solvent-induced rotation of the anisotropic HFC tensor axes has been obtained for the ³$nπ*$ state of pyrazine, in particular for ¹³$C$ and ¹⁴$N$ nuclei. This produces additional nonhomogeneous broadening not only in electron–nuclear double resonance spectra, but also in electron paramagnetic resonance signals because the anisotropic HFC perturbation results in an intensity redistribution among the magnetic transitions between the spin sublevels. A small in-plane rotation of the ZFS tensor axes upon solvation has been predicted for quinoline. Rotation of the magnetic axes induced by the interaction with isotropic solvents can provide a new mechanism for spin-lattice relaxation in the triplet state.