Guest and solvent modulated photo-driven charge separation and triplet generation in a perylene bisimide cyclophane

Cofacial positioning of two perylene bisimide (PBI) chromophores at a distance of 6.5 angstrom in a cyclophane structure prohibits the otherwise common excimer formation and directs photoexcited singlet state relaxation towards intramolecular symmetry-breaking charge separation (tau(CS) = 161 +/- 4 ps) in polar CH2Cl2, which is thermodynamically favored with a Gibbs free energy of Delta G(CS) = -0.32 eV. The charges then recombine slowly in tau(CR) = 8.90 +/- 0.06 ns to form the PBI triplet excited state, which can be used subsequently to generate singlet oxygen in 27% quantum yield. This sequence of events is eliminated by dissolving the PBI cyclophane in non-polar toluene, where only excited singlet state decay occurs. In contrast, complexation of electron-rich aromatic hydrocarbons by the host PBI cyclophane followed by photoexcitation of PBI results in ultrafast electron transfer (<10 ps) from the guest to the PBI in CH2Cl2. The rate constants for charge separation and recombination increase as the guest molecules become easier to oxidize, demonstrating that charge separation occurs close to the peak of the Marcus curve and the recombination lies far into the Marcus inverted region.