The photophysics of selectively metallated arrays of quinoxaline-fused tetraarylporphyrins

A comprehensive study of the photophysical interactions occurring in tris-porphyrin and tetrakis-porphyrin arrays 1 and 2 has been undertaken. The arrays consist of porphyrins with quinoxaline units fused at the β,β′-pyrrolic faces of the macrocycle. The linkage geometry is such that these arrays resemble the arrangement of chromophores that constitute natural photosynthetic reaction centres (PRCs). Selective metallation of the terminal chromophores of the arrays with Zn(II) and Au(III) allows directional electron and energy transfer processes to occur. The results show that excitation at any chromophore of the arrays leads to efficient charge transfer across the length of the arrays, distances of 35 and 50 Å for 1 and 2, respectively. Charge recombination is several orders of magnitude slower in both cases. The excellent performance of these arrays is attributed in part to the use of quinoxalinoporphyrins. Linked appropriately, π-system-fused porphyrins can exhibit strong electronic communication in the excited state, whilst being effectively insulated in the ground state. As such, arrays of these porphyrins might find use as components of photovoltaic devices.