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 (similar to3.5 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 greater than or equal to 1 ns in both toluene and benzonitrile. The pathway for generating PDI- MgPO+ 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.