A Theoretical Survey of the UV-Visible Spectra of Axially and Peripherally Substituted Boron Subphthalocyanines

The UV-visible spectra of a series of subphthalocyanines (SubPcs) characterized by three different axial substituents (A(n)) in combination with H, F, NO2, SO2H and SO2CH3 peripheral substituents (R-i) were predicted and analyzed by means of time-dependent DFT calculations, including chloroform as a solvent. In this analysis, we paid particular attention to the Q band, which remained almost unchanged regardless of the nature of the axial substituent. For the same axial substituent, changes in the Q band were also rather small when hydrogens at the periphery were replaced by R-1 = SO2H and R-1 = R-2 = SO2H. However, the shifting of the Q band was almost 10 times larger when R-1 = NO2 and R-1 = R-2 = NO2 due to the participation of this substituent in the pi SubPc cloud. In most cases, the characteristics of the spectra can be explained considering only the transitions involving the HOMO-1, HOMO, LUMO and LUMO + 1 orbitals, where the Q band can be decomposed into two main contributions, leading to charge separation. Only for SubPc(A(3),F,F,H) would one of the two contributions from the deepest orbital involved not lead to charge transfer. For this latter case, the HOMO-2 orbital must also be taken into account. In summary, the results obtained with the analysis of the MO indicate that the studied SubPcs are appropriate for photochemical devices.