Quantum-chemical modeling of spectroscopic and photophysical properties of ethylene-bridged octaethylporphyrin dimer

In a solution, a dimer of octaethylporphyrin meso-linked by an ethylene bridge (tbis-OEP) may adopt conformational forms the lifetimes of the S ₁ state of which are considerably different (7 ns for the P′ and ∼6–9 ps for the P and U forms). In this work, we propose new structural models for these forms based on calculations of the geometry, vibrational, and electronic states of tbis-OEP and related model systems using methods of the density functional theory. The modeling of the absorption spectra of tbis-OEP in toluene shows that they can be well described assuming that the distribution of conformers with values of the dihedral angle α (which determines the positions of the macrocycles with respect to the ethylene bridge) in the range of 10°–90° is Boltzmanian. We have found that a considerable difference between the spectral properties of tbis-OEP in toluene and chloroform is related to particular features of the structure of the corresponding solvation complexes. Interactions with chloroform molecules in solution facilitate the predominant stabilization of conformers with α ∼ 90°, while, in the case of solvation complexes with toluene molecules, conformers with α ∼ 40° are more energetically favorable. Based on the optimization of the geometry of tbis-OEP in the S ₁ state and on the calculations of the matrix elements of vibronic interaction, we propose mechanisms by which a considerable shortening of the lifetime of the excited state of tbis-OEP and experimental regularities of variation of this parameter upon variation of the solution viscosity and freezing of the solution can be explained.