Synthesis and excited-state photodynamics of perylene-bis(imide)-oxochlorin dyads. A charge-separation motif

Three perylene-oxochlorin dyads have been prepared and characterized with the goal of identifying charge-injection or molecular-switching motifs for use in molecular photonics. Each dyad consists of a perylene-bis(imide) dye (PDI) joined at the 10-position of a magnesium, zinc, or free base (Fb) oxochlorin via a diphenylethyne linker. Each dyad has been studied in both polar and nonpolar media using static and time-resolved optical spectroscopy and electrochemical techniques. Dyad PDI-MgO is an excellent charge-separation unit in which the excited perylene (∼ 3.5 ps lifetime) or the excited oxochlorin (lifetimes of 0.5 ns in benzonitrile and 1.0 ns in toluene) give rise to state PDI^- MgO⁺ in high overall yield (>90%); the charge-separated state has a lifetime of ≥ 1 ns in both toluene and benzonitrile. The pathway for generating PDI^- MgO⁺ from the excited perylene (PDI*) involves both hole transfer and energy transfer to the oxochlorin followed by electron transfer from the resulting MgO* to PDI. Similar decay of PDI* by energy transfer and hole transfer is found for dyads PDI-ZnO and PDI-FbO. However, electron-transfer quenching of the excited oxochlorin in these two dyads either does not occur or occurs to a much lesser degree than for PDI-MgO in both polar and nonpolar solvents. For PDI-FbO the decay of the charge-separated state occurs significantly by charge recombination to give the excited oxochlorin, making this a good light-harvesting system even though the early stages of the dynamics include charge separation/recombination. The observed differences in the extent of the possible excited-state processes (energy, hole, and electron transfer) among the dyads and in polar versus nonpolar media are consistent with the estimated energy ordering of the excited- and charge-separated states. This study has provided a new class of arrays containing perylene accessory pigments and oxochlorin chromophores that can be utilized for applications in light harvesting and molecular optoelectronics.