A Theoretical Study of Brominated Porphycenes: Electronic Spectra and Intersystem Spin-Orbit Coupling

In this paper, a time-dependent density functional theoretical study (TDDFT) has been carried out for brominated 2,7,12,17-tetra-n-propylporphycenes. Their potential therapeutic use in photodynamic therapy (PDT), a noninvasive medical treatment of cancer diseases, is due to the strong absorbance in the red part of the visible spectrum and the presence of heavy atoms (bromine). The prediction of electronic spectra for photosensitizer molecules can be a valuable tool in the design of drugs for application in PDT. Singlet and triplet vertical excitation energies have been calculated by means of the nonempirical hybrid functional PBE0 in conjunction with a split valence basis set (SVP), on previously optimized, at the density functional level of theory, ground state geometries. In particular, the quantum-chemical simulation of their absorption electronic spectra, both in vacuo and in solvent environments (dichloromethane and bromobenzene), has evidenced the red shift maxima wavelengths for the Q bands (or lower energy bands) with an increasing number of bromine atoms, in agreement with experimental results. The mean absolute deviation for the Q-electronic bands is about 0.3 eV. Calculated vertical triplet energies are between 1.04 (for tetra-brominated derivative) and 1.20 eV (for dibrominated derivative). The influence of bromine atoms on intersystem spin crossing has been investigated by applying a computational code which calculates spin-orbit matrix elements between singlet and triplet excited state wave functions weighted by the TDDFT transition coefficients.