ECAMP 15

Recently there has been growing interest in TPPS4 for applications in photodynamic therapy as photosensibilizator [1]. The monomers of this porphyrin efficiently self-associate into H- and J-aggregates in aqueous media. It is not fully understood what kind of TPPS4 configurations are building blocks of molecular aggregates. For these reasons, the aim of this study is to determine tetramers and calculate their absorption and circular dichroism (CD) spectra.
First of all, quantum mechanical (QM) calculations were performed on geometry optimizations of the structures of TPPS4 zwitterionic monomers. Two different types of the zwitterionic form of the TPPS4 molecules were considered with respect to position of SO3H protonized groups: either these groups are opposite (Z1 monomer) or adjacent (Z2 monomer). QM calculation were performed using the DFT B3LYP/6-311G(d,p) basis set and polarizable continuum model (PCM) methods. The Gaussian 16 program was used. The next step was to take parameters from the General Amber force field (GAFF). It was also adjusted several parameters of monomers. Molecular mechanics (MM) geometry optimization was also performed. The second step was to construct Z1 and Z2 tetramers. The obtained tetramers were solvated with water boxes. Then molecular dynamics (MD) simulation was performed with AMBER 22 program.
Theoretical calculations of the TPPS4 tetramer’s absorption and CD spectra were performed for Z1 and Z2 tetramers. The Frenkel excitonic model was used for this task as the electrons are localized to each molecule so we were able to treat each excitation as an exciton. These calculations were performed by associating 4 optical transitions to each molecule constituting the aggregate, which are in turn characterized by their respective transition energies for the Q and B band and an optical transition dipole moment vector for each transition (μ): i, j, k and l, the first two vectors corresponding to the Q band and vectors three and four to the B band. Each vector’s coordinates were associated with the coordinates of each molecule’s central nitrogen atoms. The tetramer's spectral density was calculated and then averaged for all 5000 frames of the simulation and normalized to a maximum of 1.
It was determined two Z1 and Z2 linear tetramers. A comparison was performed of Z1 and Z2 tetramers absorption spectra with the measured spectra from the previous published article [2]. Our analysis of results demonstrates that larger peak shift of Z1 tetramer’s spectra are more similar to the experimental data, however other configurations can be possible as well.

References
[1] Q. Xiao, J. Wu, X. Pang, Y. Jiang, P. Wang, A. W. Leung, L. Gao, S. Jiang, and C. Xu, “Discovery and development of natural products and their derivatives as photosensitizers for photodynamic therapy,” Curr. Med. Chem. 25(7), 839–860 (2018).
[2] L. Baliulyte, D. Abramavicius, S. Bagdonas, A. Kalnaityte, V. Poderys, R. Rotomskis, and V. Barzda, “Comparative quantum chemical and spectral characterization of meso-tetra (4-sulfonatophenyl) porphine forms as seeds for J-and H-aggregates,” AIP Advances 13(10) (2023).