Lessons from Chlorophylls: Modifications of Porphyrinoids Towards Optimized Solar Energy Conversion

Practical applications of photosynthesis-inspired processes depend on a thorough understanding of the structures and physiochemical features of pigment molecules such as chlorophylls and bacteriochlorophylls. Consequently, the major structural features of these pigments have been systematically examined as to how they influence the S-1 state energy, lifetimes, quantum yields, and pigment photostability. In particular, the effects of the macrocyclic pi-electron system, central metal ion (CMI), peripheral substituents, and pigment aggregation, on these critical parameters are discussed. The results obtained confirm that the p-electron system of the chromophore has the greatest influence on the light energy conversion capacity of porphyrinoids. Its modifications lead to changes in molecular symmetry, which determine the energy levels of frontier orbitals and hence affect the S-1 state properties. In the case of bacteriochlorophylls aggregation can also strongly decrease the S-1 energy. The CMI may be considered as another influential structural feature which only moderately influences the ground-state properties of bacteriochlorophylls but strongly affects the singlet excited-state. An introduction of CMIs heavier than Mg2+ significantly improves pigments' photostabilities, however, at the expense of S-1 state lifetime. Modifications of the peripheral substituents may also influence the S-1 energy, and pigments' redox potentials, which in turn influence their photostability.