Cyclotriphosphazene appended porphyrins and fulleropyrrolidine complexes as supramolecular multiple photosynthetic reaction centers: steady and excited states photophysical investigation

Abstract

New multiple photosynthetic reaction centers were constructed from cyclophosphazene decorated multiporphyrin chromophores and a fulleropyrrolidine having a pyridine ligand (FPY). The excited state electron transfer in the self-assembled donor–acceptor assembly was investigated by using steady state absorption and emission, time-resolved emission spectroscopy and nanosecond laser flash photolysis. The effect of metal (Zn²⁺) coordination to porphyrin units in the multiporphyrin arrays on cyclophosphazine scaffold (P3N3Zn) was studied by comparing with metal free porphyrin assembly on a cyclophosphazene scaffold (P3N3). In P3N3Zn, the decrease of absorption and fluorescence intensity and the lowering of the amplitude of longer fluorescence lifetime with increase of FPY concentration reflect the formation of a ground state complex with an association constant of ∼14 910 M⁻¹. When compared to the metal-free complex P3N3, the metal-coordinated derivative P3N3Zn exhibited shortening of the singlet and triplet state lifetimes and lowering of the singlet and triplet quantum yields. The cause of the decrease of the triplet quantum yields by insertion of zinc metal is discussed along with the possible non-planarity of the porphyrin ring. From the fluorescence lifetime measurements for the P3N3Zn–FPY mixture, it is proposed that self-assembly of the donor–acceptor complex leads to charge separated species with a rate constant of 7.1 × 10⁹ s⁻¹. The decrease of triplet state intensity and lifetime of the P3N3Zn in the P3N3Zn–FPY complex from the nanosecond transient absorption studies support the occurrence of intermolecular electron transfer in the triplet state.