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Quantum modeling of opticaland magnetic properties of the triplet state

This thesis represents a quantum chemical treatment of optically detected magnetic resonance (ODMR) spectra of organic molecules excited to the triplet state ( ). The state is invariably of lower energy than the excited singlet ( ) state, derived from the same single-electron transition. The triplet state has three spin sublevels which are split even in the absence of an external magnetic field. The zero-field splitting (ZFS) is determined mostly by spin-spin coupling (SSC) and is a unique characteristic of the orbital nature of the excited state. Each spin sublevel has its own transition probability to the ground state, since it is governed by spin-orbit coupling (SOC) with specific selection rules. The difference in the magnetic sublevel radiative rates makes it possible to detect microwave transitions between triplet spin substates via changes in phosphorescence intensity, which is the origin of the ODMR technique in triplet state spectroscopy. The electric dipole activity of the spin sub-levels in the phosphorescent triplet-singlet transitions is estimated in this thesis by means of the quadratic response technique. The first ab initio attempt is performed in order to understand the origin of spin-selective solvent effects on ODMR signals. To the first order of perturbation theory SOC does not affect the energy of the triplet spin states. The second order SOC contribution to the splitting of the states is found to be almost negligible. Solvent effects on the zero-field splitting constants induced by electron spin-spin coupling in the triplet states of azaaromatic molecules are calculated using multi-configuration self-consistent field wave functions and the polarized continuum model. The correlation between the solvent-induced shift in the ZFS and phosphorescence frequency that has been observed in ODMR experiments in low temperature glasses is reproduced by our direct SSC and SOC calculations. It is found that account of SOC perturbation produces only a minor influence on the solvent-induced ZFS shift even for states. Solvent effects on hyperfine structure of triplet states are also studied in this thesis. The applicability of the MCSCF technique for ZFS and hyperfine constant calculations in organic triplet molecules is proved by similar studies of rotational fine-structure in optical spectra of diatomic molecules. Experimental research in these two branches of molecular spectroscopy are developed independently, but ab initio theory can treat both types of spectra on the same background of spin-selectivity in magnetic perturbations. Using density functional theory (DFT) it is possible to treat also large molecular systems. In this respect special attention in this thesis is paid to theoretical studies of triplet state properties of the free-base porphin molecule. Comparative studies of singlet-triplet transition moments by time-dependent MCSCF and DFT methods in three-atomic molecules show similar results and indicate that DFT is a good approach of similar quality as explicitly correlated methods for small molecules.