Specific features of the electron structure of ZnTPP aggregated forms: data of optical measurements and quantum-chemical calculations

Zinc tetraphenylporphyrin (ZnTPP) films and ZnTPP-based composites are promising materials of organic photonics. Porphyrins are inclined towards self-assembly and the formation of molecular ensembles or differently structured aggregates. The energies of the formation of aggregates and ordered structures and modifications of their electron structures compared to that of a free ZnTPP molecule have not been adequately explored. In the study, the first comprehensive investigation of the structure, absorption and luminescence spectra, and photoluminescence kinetics of structurally perfect ZnTPP thin films produced under quasi-equilibrium conditions in vacuum is conducted. It is shown that changes in the absorption spectra and the red shift of the luminescence spectra of films by 0.15 eV from the spectra observed for ZnTPP solutions in toluene can be interpreted as a result of the formation of an ordered thin-film structure through the dimerization of porphyrin planar molecules under nearly equilibrium conditions. The optimal geometric structure, the energy of the ground state, and the electronic spectrum of excitations of the dimerized (ZnTPP)$_2$ state are calculated in the context of density functional theory and time-dependent density functional theory. The energy gain on the formation of a dimer of symmetry $C_i$ compared to two separate molecules is 0.23 eV per dimer; the HOMO–LUMO gap for the dimer is decreased by 70 meV. The radiative emission time in the ordered solid phase is an order of magnitude shorter than that in the solution in toluene and corresponds to 277 ps, which is typical of $J$ aggregates of porphyrins.