Porphodilactones as Synthetic Chlorophylls: Relative Orientation of β-Substituents on a Pyrrolic Ring Tunes NIR Absorption

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 meso-tetrakispentafluorophenylporphodilactone isomers is first reported. The β-dilactone moieties are found to lie at opposite pyrrole positions (trans- and cis-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. J. Am. Chem. Soc. 1989, 111, 3702). The relative orientation of the β-dilactone moieties has a significant influence on the electronic structures and photophysical properties. For example, the Qy band of trans-porphodilactone is red-shifted by 19 nm relative to that of the cis-isomer, and there is a 2-fold increase in the absorption intensity, which resembles the similar trends that have been reported for natural chlorophyll f and d. 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 trans- and cis-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.

Gadolinium(III) Porpholactones as Efficient and Robust Singlet Oxygen Photosensitizers

“…Notably, Gd‐4 showed an extinction coefficient ratio of Q y (0,0) band and Soret band ( ɛ Q(0,0) / ɛ Soret ) up to 0.56. Such redshifted absorption with increasing intensity, along with the replacement of β‐oxazolone moieties and the orientation of β‐dioxazolone moieties, is consistent with our previously reported Zn, Pt, and Pd analogs, and suggests the capability for effective excitation in the deep‐red to NIR region.…”

Indachlorins: Nonplanar Indanone‐Annulated Chlorin Analogues with Panchromatic Absorption Spectra between 300 and 900 nm

“…The similarity of the conformational effects of a pyrrolic β,β′‐double bond and a lactone moiety was amply demonstrated before 25c. 26b, 32b, 37…”

“…Both the spectral resemblance of the β,β′‐lactone moiety to a double bond and its bathochromic effect in chlorin‐type chromophores were observed before 25c. 26b, 32b, 38…”

Structure Modification and Spectroscopic Properties of Artificial Porphyrinoids

“…Shen and Zhang et al. reported the synthetic porphodilactones 44 and 45 , in which two pyrrole rings of the porphyrin ligand were replaced by oxazolone moieties (Scheme ) 32. Porphodilactone isomers 40 were obtained by the reaction of porphyrin 39 or porpholatone 46 with an excess of Oxone® ( 39 : 12 eq and 46 : 4 eq), using RuCl 3 and 2,2′‐bipyridine as catalysts,33 in 45 and 53 % yield, respectively.…”

“…A value of 0.87 was obtained for cis ‐porphodilactone zinc complex 42 irradiated with a 635 nm laser beam, while free‐base cis ‐porphodilactone 45 showed no singlet oxygen production at all under the same treatment. Further photocytotoxicity studies have demonstrated the potential utility of porphodilactones 41 , 42 , and 44 as NIR PDT photosensitizers 32…”