Synthesis and properties of weakly coupled dendrimeric multiporphyrin light-harvesting arrays and hole-storage reservoirs

A convergent synthesis employing porphyrin building blocks has afforded dendrimeric multiporphyrin arrays containing n Zn-porphyrins (n = 4, 8, or 20) and one free base- (Fb-) porphyrin joined via diarylethyne linkers. Size exclusion chromatography was used extensively for purification. The arrays have sufficient solubility in toluene or other solvents for routine handling. With increasing size, the intense near-UV Soret (S₀→S₂) absorption band broadens, splits, and red shifts due to inter-porphyrin exciton coupling. In contrast, the weaker visible bands (S₀→S₁ remain essentially unchanged in position or width in proceeding from the monomer all the way to the 21-mer; however, the molecular extinction coefficients of the visible bands scale with the number of porphyrins. Similarly, the one-electron oxidation potentials, of the porphyrins are virtually unchanged as the arrays get larger. These results are indicative of relatively weak (but significant) electronic coupling between ground states and between the photophysically relevant lowest-excited-singlet states of the diarylethyne-linked porphyrins; thus, the characteristic properties of the individual ufiits are retained as the architectures increase in complexity. Efficient excited-singlet-state energy transfer occurs among the Zn-porphyrins and ultimately to the sole Fb-porphyrin in each of the arrays, with the overall arrival time of energy at the trapping site increasing modestly with the number of Zn-porphyrins= 1 (45 ps), 2 (90 ps), 8 (105 ps), and 20 (220 ps). The overall energy-transfer efficiencies are 98%, 96%, 96%, and 92% in the same series. The ground-state hole-storage properties of the 21-mer (Zn₂₀Fb) were examined. Bulk electrolysis indicates that 21 (or more) electrons can be removed from this array (e.g., one hole resides on each porphyrin) to yield a stable "super-charged" π-cation radical. Taken together, these results indicate that the convergent building-block synthesis approach affords dendrimeric multiporphyrin arrays with favorable properties for light-harvesting and hole storage.