Porphodilactones
as Synthetic Chlorophylls: Relative
Orientation of β‑Substituents on a Pyrrolic Ring Tunes
NIR Absorption
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Abstract
Porphodilactones
represent the porphyrin analogues, in which the
peripheral bonds of two pyrrole rings are replaced by lactone moieties.
They provide an opportunity to investigate how β-substituent
orientation of porphyrinoids modulates the electronic structures and
optical properties, in a manner similar to what is observed with naturally
occurring chlorophylls. In this work, a comprehensive description
of the synthesis, characterization, and optical properties of <i>meso</i>-tetrakispentafluorophenylporphodilactone isomers is
first reported. The β-dilactone moieties are found to lie at
opposite pyrrole positions (<i>trans</i>- and <i>cis</i>-configurations are defined by the relative orientations of the carbonyl
group when one lactone moiety is fixed), in accordance with earlier
computational predictions (Gouterman, M. <i>J. Am. Chem. Soc.</i> <b>1989</b>, <i>111</i>, 3702). The relative orientation
of the β-dilactone moieties has a significant influence on the
electronic structures and photophysical properties. For example, the
Q<sub><i>y</i></sub> band of <i>trans</i>-porphodilactone
is red-shifted by 19 nm relative to that of the <i>cis</i>-isomer, and there is a 2-fold increase in the absorption intensity,
which resembles the similar trends that have been reported for natural
chlorophyll <i>f</i> and <i>d</i>. An in depth
analysis of magnetic circular dichroism spectral data and TD-DFT calculations
at the B3LYP/6-31G(d) level of theory demonstrates that the <i>trans</i>- and <i>cis</i>-orientations of the dilactone
moieties have a significant effect on the relative energies of the
frontier π-molecular orbitals. Importantly, the biological behaviors
of the isomers reveal their different photocytotoxicity in NIR region
(>650 nm). The influence of the relative orientation of the β-substituents
on the optical properties in this context provides new insights into
the electronic structures of porphyrinoids which could prove useful
during the development of near-infrared absorbing photosensitizers