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

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 13¹-oxophorbine (which bears an oxo group in the isocyclic ring) were synthesized as benchmark molecules for fundamental spectral and photophysical studies. The phorbine and 13¹-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 13¹-oxophorbine closely resemble those of the corresponding analogues of chlorophyll a. Accordingly, the fundamental electronic properties of chlorophylls are primarily a consequence of the 13¹-oxophorbine base macrocycle.