Computational and Spectroscopic Analysis of β-Indandione Modified Zinc Porphyrins

Porphyrins have characteristic optical properties which give them the potential to be used in a range of applications. In this study, a series of β-indandione modified zinc porphyrins, systematically changed in terms of linker length and substituent, resulted in absorption spectra that are dramatically different than that observed for the parent zinc porphyrin (ZnTXP, 5,10,15,20-tetrakis(3,5-dimethylphenyl)porphyrinato zinc(II)). These changes include strong absorptions at 420, 541, and 681 nm (110.2, 57.5, and 29.2 mM^-1 cm^-1, respectively) for the most perturbed compound. Computational studies were conducted and showed the different optical effects are due to a reorganization of molecular orbitals (MOs) away from Gouterman's four-orbital model. The substituent effects alter both unoccupied and occupied MOs. An increased length of linker group raised the energy of the HOMO-2 such that it plays a significant role in the observed transitions. The degenerate LUMO (e_g) set are split by substitution, and this splitting may be increased by use of a propylidenodinitrile group, which shows the lowest-energy transitions and the greatest spectral perturbation from the parent zinc porphyrin complex. These data are supported by resonance Raman spectroscopy studies which show distinct enhancement of phenyl modes for high-energy transitions and indandione modes for lower-energy transitions.