Synthesis and excited-state photodynamics of perylene-porphyrin dyads. 1: Parallel energy and charge transfer via a diphenylethyne linker

The photophysical properties of a perylene-porphyrin dyad have been examined with the aim of using this construct for molecular photonics applications. The dyad consists of a perylene-bis(imide) dye (PDI) connected to a zinc porphyrin (Zn) via a diphenylethyne linker (pep). In both polar and nonpolar solvents, the photoexcited perylene unit (PDI*) decays very rapidly (lifetimes of 2.5 (toluene) and 2.4 ps (acetonitrile)) by energy transfer to the porphyrin, forming PDI-pep-Zn* in high yield (80%, toluene; 70% acetonitrile), and hole transfer to the porphyrin, forming PDI^--pep-Zn⁺ in lesser yield (20%, toluene; 30% acetonitrile). In both toluene and acetonitrile, the Zn* excited state subsequently decays with a lifetime of 0.4 ns primarily (80%) by electron transfer to the perylene (forming PDI^--pep-Zn⁺). In the nonpolar solvent (toluene), the PDI^--pep-Zn⁺ charge-transfer product has a lifetime of > 10 ns and decays by charge recombination primarily to the ground state but also by thermal repopulation of the Zn* excited state. The occurrence of the latter process provides a direct experimental measure of the energy of the charge-separated state. In the polar solvent (acetonitrile), the PDI^--pep-Zn⁺ charge-separated state decays much more rapidly (<0.5 ns) and exclusively to the ground state. In general, the complementary perylene and porphyrin absorption properties together with very fast and efficient PDI*-pep-Zn → PDI-pep-Zn* energy transfer suggest that perylenes have significant potential as accessory pigments in porphyrin-based arrays for light-harvesting and energy-transport applications. Furthermore, the finding of fast energy transfer initiated in PDI*, charge-transfer reactions that can be elicited either in PDI* or Zn*, and a charge-separated state (PDI^--pep-Zn⁺) that can be long- or short-lived depending on solvent polarity, indicates the versatility of the perylene-porphyrin motif for a variety of applications in molecular photonics.