Synthesis and excited-state photodynamics of perylene-porphyrin dyads. 4. Ultrafast charge separation and charge recombination between tightly coupled units in polar media

New perylene-porphyrin dyads that have excellent light-harvesting and energy-utilization capabilities in nonpolar media are found to exhibit efficient, ultrafast and tunable charge-transfer activity in polar media. The dyads consist of a perylene-monoimide dye (PMI) connected to a porphyrin (Por) via an ethynylphenyl (ep) linker. The porphyrin constituent of the PMI-ep-Por arrays is either a zinc or magnesium complex (Por = Zn or Mg) or a free-base form (Por = Fb). Following excitation of the perylene in each array in acetonitrile, PMI* decays in less than or equal to0.4 ps by a combination of energy transfer to the ground-state porphyrin (forming Por*) and hole transfer (forming PMI(-)Por(+)). The excited porphyrin formed by energy transfer (or via direct excitation) then undergoes effectively quantitative electron transfer back to the perylene (tau = 1, 1, 700 ps for Por = Mg, Zn, Fb). Subsequently, charge recombination within PMI- Por(+) returns each dyad quantitatively to the ground state (tau = 2, 4, 8 ps for Por = Mg, Zn, Fb). The dynamics of the PMI Por* --> PMI- Por(+) and PMI- Por(+) --> PMI- Por(+) charge-transfer processes can be modulated by altering the type of polar solvent (acetonitrile, benzonitrile, tetrahydrofuran and 2,6-lutidine). The charge-separation times for PMI-ep-Zn are 1, 6, 9 and 22 ps in these solvents, while the charge-recombination times are 4, 24, 38 and 34 ps. The efficient, rapid and tunable nature of the charge-transfer processes in polar media makes the PMI-ep-Por dyads useful units for performing molecular-switching functions. These properties when combined with the excellent light-harvesting and energy-transfer capabilities of the same arrays in nonpolar media afford a robust perylene-porphyrin motif that can be tailored for a variety of functions in molecular optoelectronics.