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 (eg) 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.