Abstract
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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.