Theoretical study of the electronic excitations of free-base porphyrin–Ar₂ van der Waals complexes

The intermolecular interaction of free-base porphine (FBP)–Ar₂ and free-base tetraazaporphyrin (FBPz)–Ar₂ van der Waals (vdW) complexes was calculated in the ground state and vertical excitations that correspond to the Q- and B-bands using the many-body wavefunction theory of the symmetry-adapted cluster-configuration interaction (SAC-CI) method and time-dependent density functional theory (TDDFT). For the 1¹B_3u state of FBP–Ar₂ a blueshift (high-energy shift) of excitation energy was calculated using the SAC-CI method; such a blueshift was not obtained by TDDFT calculations. This calculated blueshift corresponds to the experimentally observed blueshift in the Q_x-band of FBP for FBP–Ar_n complexes. For FBPz–Ar₂, blueshifts of the Q-band were not obtained using SAC-CI and TDDFT. These behaviors of the energy shift of the Q-bands could not be explained by the point dipole–point dipole interaction model. Large redshifts (low-energy shift) were obtained for the B-band states (2¹B_3u and 2¹B_2u) of FBP and FBPz. The energy shift showed the inverse sixth-power dependence on the intermolecular distance. The point dipole–point dipole interaction model can describe the redshift of the B-band. For the excited states that exhibit large redshifts, the TDDFT can qualitatively describe the vdW interaction in the excited states by supermolecular calculations. The solvatochromic shifts for FBP and FBPz in an Ar matrix were examined by the linear-response polarizable continuum model and TDDFT. The magnitude of calculated solvatochromic redshifts is proportional to the square of the transition dipole moment.