Structural characteristics that make chlorophylls green: interplay of hydrocarbon skeleton and substituents

Abstract

Understanding the effects of substituents on natural photosynthetic pigments is essential for gaining a deep understanding of why such pigments were selected over the course of evolution for use in photosynthetic systems. This knowledge should provide for a more thoughtful design of artificial light-harvesting systems. The hydrocarbon skeleton of all chlorophylls is phorbine, which contains an annulated five-membered (isocyclic) ring in addition to the reduced pyrrole ring characteristic of chlorins. A phorbine and a 13 1 -oxophorbine (which bears an oxo group in the isocyclic ring) were synthesized as benchmark molecules for fundamental spectral and photophysical studies. The phorbine and 13 1 -oxophorbine macrocycles lack peripheral substituents other than a geminal dimethyl group in the reduced ring to stabilize the chlorin chromophore. The spectral properties and electronic structure of the zinc or free base 13 1 -oxophorbine closely resemble those of the corresponding analogues of chlorophyll a. Accordingly, the fundamental electronic properties of chlorophylls are primarily a consequence of the 13 1 -oxophorbine base macrocycle.

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

“…The Φ T value for BC (0.62) is greater than that for (NC) 2 BC , reflecting the effect of the cyano groups to draw electron density from the macrocycle and thereby diminish spin‐orbit coupling, which underlies the intersystem‐crossing process. The Φ T values obtained here for the dicyanobacteriochlorins are consistent with the results of prior studies on synthetic bacteriochlorins, which give average yields of ∼0.5 for free base bacteriochlorins, ∼0.7 for zinc chelates,∼0.8 for indium chelates and ∼1 for palladium chelates . Essentially quantitative intersystem crossing is also found for palladium‐substituted bacteriochlorophyll and derivatives thereof .…”

“…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 .…”

De NovoSynthesis of Gem-Dialkyl Chlorophyll Analogues for Probing and Emulating Our Green World

“…289 Acidic bromination in conjunction with α-arylation enabled preparation of the core molecular skeleton of chlorophylls. Thus, reaction of 8,9-dibromodipyrromethane-1-carboxaldehyde ( 120a-Br 8,9 , see Scheme 60) with Western half 118 afforded the 13-bromochlorin ZnC-Br 13 .…”

“…289 (A conceptual framework for understanding the origin of the intrinsic spectral properties of tetrapyrrole macrocycles is provided by Gouterman’s four-orbital model, 346 which is outside the scope of the present review.) Zinc chelates were employed for comparison with the zinc chelate of chlorophyll a (zinc pheophytin a , abbreviated Zn-Pheo a ) given the greater chemical stability of zinc versus magnesium chelates.…”

“…Indeed, access to the 2, 255,296,298,299 3, 265,290,298,299 5, 254,256,269 7, 277,283,2908,277 10, 254,256,269 12, 255,284,299 13, 265,286,289,299 15, 263,270,283,286,299 17, 257,270,299 18, 257 and 20 263,283 positions has been achieved, as has installation of the isocyclic ring 267,289 (and 6-membered imide analogues 286,300 ) characteristic of native chlorophylls. The following sections describe novel chlorin architectures obtained chiefly by application of aforementioned methodology for (1) precursor synthesis, (2) macrocycle formation, and (3) bromination of the chlorin macrocycle under neutral or acidic conditions.…”