Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 3: The Distinctive Impact of Auxochromes at the 7‐ versus 3‐Positions

Abstract

Assessing the effects of substituents on the spectra of chlorophylls is essential for gaining a deep understanding of photosynthetic processes. Chlorophyll a and b differ solely in the nature of the 7-substituent (methyl versus formyl), whereas chlorophyll a and d differ solely in the 3-substituent (vinyl versus formyl), yet have distinct long-wavelength absorption maxima: 665 (a) 646 (b) and 692 nm (d). Herein, the spectra, singlet excited-state decay characteristics, and results from DFT calculations are examined for synthetic chlorins and 13(1)-oxophorbines that contain ethynyl, acetyl, formyl and other groups at the 3-, 7- and/or 13-positions. Substituent effects on the absorption spectra are well accounted for using Gouterman's four-orbital model. Key findings are that (1) the dramatic difference in auxochromic effects of a given substituent at the 7- versus 3- or 13-positions primarily derives from relative effects on the LUMO+1 and LUMO; (2) formyl at the 7- or 8-position effectively "porphyrinizes" the chlorin and (3) the substituent effect increases in the order of vinyl < ethynyl < acetyl < formyl. Thus, the spectral properties are governed by an intricate interplay of electronic effects of substituents at particular sites on the four frontier MOs of the chlorin macrocycle.

Molecular Electronic Tuning of Photosensitizers to Enhance Photodynamic Therapy: Synthetic Dicyanobacteriochlorins as a Case Study

“…Such trends can be seen in the redox properties and the energies of the frontier molecular orbitals (Table ) . Furthermore, the strategic choice and placement of substituents allow some control over the relative oxidation and reduction potentials ( via HOMO and LUMO energies), as has been shown for synthetic chlorins, bacteriochlorins and related oxophorbines and bacteriooxophorbines that incorporate the keto‐bearing five‐membered ring of the native photosynthetic chromophores .…”

“…Additionally, the native‐like bacteriochlorins bear a 3‐acetyl group and the related keto group in the fused five‐membered ring spanning the 12‐ to 15‐positions (Chart 1). These groups are basically electron withdrawing in character in that they lower the HOMO and LUMO energies and make the chromophore harder to oxidize and easier to reduce than bacteriochlorins that do not contain such substituents , and the same is true of chlorin analogues . Thus, the photostability of WST9 and WST11 is likely provided by effects of both substituents and the electronegative palladium ion to reduce photooxidation.…”

“…As described above, the same is true of the bacteriochlorins bearing 3,13 substituents that are the focus of this work and our recent investigation of the PDT efficacy of these photosensitizers . On the basis of shifts in MO energy and redox properties obtained upon incorporation of cyano versus acetyl groups at similar positions (Table ) , one would expect the combined effects on oxidation potentials [photostability and photoactivity via Eq. (1A)] and reduction potentials [photoactivity via Eq.…”

Optical absorption and electronic spectra of chlorophylls a and b

“…For the B3LYP/6-31G* level and grid basis set, the dipole moment is obtained to be 6.49 D, 2.73 D and 4.92 D, 1.14 D, respectively, for Chl a and Chl b. Note that the dipole moment of a water molecule is about 1.85-2.6 D, 58 whereas the reported value using the cavity model for the dipole moment of Chl a is about 4.6 D. 59,60 The orientation of the electric dipole moment of Chl a and Chl b and partial charge distributions are shown in Fig. 1.…”

Photophysical Properties and Electronic Structure of Bacteriochlorin–Chalcones with Extended Near‐Infrared Absorption

“…chlorins and porphyrins) (14,15). The incorporation of auxochromes (16) such as phenyl, vinyl and acetyl groups at selected positions allows wavelength tuning of the strong NIR absorption band (17)(18)(19)(20), known as the Q y band (21). Chart 1 shows representative free-base (metal-free) bacteriochlorins B1-B11 (14,(17)(18)(19)(20) that bear such auxochromes at the 3-position, 13-position or both positions.…”