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 (τ$_{CS}$ = 161 +/- 4 ps) in polar CH$_2$Cl$_2$, which is thermodynamically favored with a Gibbs free energy of ΔG$_{CS}$ = -0.32 eV. The charges then recombine slowly in τ$_{CR}$ = 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 CH$_2$Cl$_2$. 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

Ultrafast Photoinduced Symmetry-Breaking Charge Separation and Electron Sharing in Perylenediimide Molecular Triangles

We report on a visible-light-absorbing chiral molecular triangle composed of three covalently linked 1,6,7,12-tetra(phenoxy)perylene-3,4:9,10-bis- (dicarboximide) (EDT) units. The rigid triangular architecture reduces the electronic coupling between the PDIs, so ultrafast symmetry-breaking charge separation is kinetically favored over intramolecular excimer formation, as revealed by femtosecond transient absorption spectroscopy. Photoexcitation of the PDI triangle dissolved in CH2Cl2 gives PDI+center dot-PDI-center dot in tau(CS) = 12.0 +/- 0.2 ps. Fast subsequent intramolecular electron/hole hopping can equilibrate the six possible energetically degenerate ion-pair states, as suggested by electron paramagnetic resonance/electron nuclear double resonance spectroscopy, which shows that one-electron reduction of the EDT triangle results in complete electron sharing among the three PDIs. Charge recombination of PDI+center dot PDI-center dot to the ground state occurs in tau(CR) = 1.12 +/- 0.01 ns with no evidence of triplet excited state formation

Solvent-Templated Folding of Perylene Bisimide Macrocycles into Coiled Double-String Ropes with Solvent-Sensitive Optical Signatures

A series of semirigid perylene bisimide (PBI) macrocycles with varied ring size containing two to nine PBI chromophores were synthesized in a one-pot reaction and their photophysical properties characterized by fluorescence, steady-state, and transient absorption spectroscopy as well as femtosecond stimulated Raman spectroscopy. These macrocycles show solvent-dependent conformational equilibria and excited-state properties. In dichloromethane, the macrocycles prevail in wide-stretched conformations and upon photoexcitation exhibit symmetry-breaking charge separation followed by charge recombination to triplet states, which photosensitize singlet oxygen formation. In contrast, in aromatic solvents folding of the macrocycles with a distinct odd–even effect regarding the number of PBI chromophore units was observed in steady-state and time-resolved absorption and fluorescence spectroscopy as well as femtosecond stimulated Raman spectroscopy. These distinctive optical properties are attributable to the folding of the even-membered macrocycles into exciton-vibrational coupled dimer pairs in aromatic solvents. Studies in a variety of aromatic solvents indicate that these solvents embed between PBI dimer pairs and accordingly template the folding of even-membered PBI macrocycles into ropelike folded conformations that give rise to solvent-specific exciton-vibrational couplings in UV–vis absorption spectra. As a consequence of the embedding of solvent molecules in the coiled double-string rope architecture, highly solvent specific intensity ratios are observed for the two lowest-energy exciton-vibrational bands, enabling assignment of the respective solvent simply based on the absorption spectra measured for the tetramer macrocycle