Excited-state symmetry-breaking charge separation (SB-CS) can offer an efficient pathway to solar energy capture and conversion. We synthesized a series of 1,6,7,12-tetrakis­(4-t-butylphenoxy)­perylene­(3,4:9,10)­bis­(dicarboximide) cyclophane dimers with m-xylylene, p-xylylene, and 4,4′-diyldimethane-1,1′-biphenyl spacers and studied them with steady-state and time-resolved optical spectroscopies. Photoinduced SB-CS occurs in all three cyclophanes in CH2Cl2, with the SB-CS rate decreasing as the interchromophore distance is increased. Time-resolved emission spectroscopy and kinetic modeling reveal that the charge-separated state exists in pseudoequilibrium with the excited state prior to decay. Notably, the meta-spaced cyclophane also undergoes SB-CS in toluene within ∼100 ps, despite the lack of charge stabilization by the low dielectric constant solvent. These results demonstrate that SB-CS can occur across long distances and in weakly polar environments, which offers the possibility of harnessing SB-CS for solar energy capture and conversion.

Designing molecular systems that exploit vibronic coherence to improve light harvesting efficiencies relies on understanding how interchromophoric interactions, such as van der Waals forces and dipolar coupling, influence these coherences in multichromophoric arrays. However, disentangling these interactions requires studies of molecular systems with tunable structural relationships. Here, we use a combination of two-dimensional electronic spectroscopy and femtosecond stimulated Raman spectroscopy to investigate the role of steric hindrance between chromophores in driving changes to vibronic and vibrational coherences in a series of substituted perylenediimide (PDI) cyclophane dimers. We report significant differences in the frequency power spectra from the cyclophane dimers versus the corresponding monomer reference. We attribute these differences to distortion of the PDI cores from steric interactions between the substituents. These results highlight the importance of considering structural changes when rationalizing vibronic coupling in multichromophoric systems.

Singlet fission (SF) creates two triplet excitons following absorption of a photon by two electronically interacting chromophores. Quaterrylene-3,4:13,14-bis(dicarboximide) (QDI) is a strongly absorbing chromophore that readily fulfills the energetic requirements for SF, E(S 1 ) > 2E(T 1 ), and thus should undergo rapid and efficient SF. SF was studied in thin films of the QDI derivative N,N-bis(2,6-diisopropylphenyl)-QDI (ArQDI), which undergoes SF in <300 fs to form the correlated triplet pair state, 1 (T 1 T 1 ), which dissociates with a (7.3 ± 1.2 ns) −1 rate constant. The observed triplet yield for a thin film that has been solvent-vapor annealed with CH 2 Cl 2 is 135 ± 20% instead of 200%, which is typically expected of chromophores that undergo ultrafast formation of the 1 (T 1 T 1 ) state. The lower SF yield in ArQDI results from the failure of the 1 (T 1 T 1 ) state to dissociate before returning to the ground state. In contrast to other molecules, like hexacene, which have low triplet energies, the SF rate in ArQDI is not limited by a multiphonon relaxation bottleneck, largely due to the fact that the S−T energy gap in the film is substantially smaller than that measured for monomeric ArQDI. The ability to maintain a favorable S−T energy gap in a film is a design consideration when chromophores are considered for use to enhance solar cell performance.

Covalent chromophore dimers having the required energetics can undergo intramolecular singlet fission (SF) in solution; however, in the solid state, intra- and intermolecular SF can compete. Here, the structure and excited-state dynamics of a linear terrylene-3,4:11,12-bis­(dicarboximide) (TDI) dimer, TDI-Ph-TDI, in which the two TDI molecules are linked via one of their imide nitrogen atoms to a 2,5-di-t-butylphenyl spacer at its 1,4-positions are studied in solution and in thin films to understand the interplay between these two SF mechanisms. TDI-Ph-TDI undergoes slow intramolecular SF in toluene due to weak through-bond interactions and a lack of through-space electronic coupling. The TDI-Ph-TDI dimers in the films are π-stacked, allowing for through-space interactions between neighboring TDI moieties. As a result, TDI-Ph-TDI undergoes intermolecular SF two orders of magnitude faster than intramolecular SF. Using film-processing techniques, the SF dynamics can be modified. The (T1T1) states in the unannealed and thermally annealed films dissociate to form free triplet excitons, whereas a long-lived (T1T1) state with mixed charge-transfer character is observed in a chlorobenzene solvent vapor annealed film. Although intramolecular SF in TDI-Ph-TDI cannot compete with intermolecular SF, the structure of TDI-Ph-TDI has a strong influence on the possible film morphologies and the role of the charge-transfer state in SF.

Recent advances in two-dimensional electronic spectroscopy (2DES) have enabled identification of fragile quantum coherences in condensed phase systems near the equilibrium molecular geometry. In general, traditional 2DES cannot measure such coherences associated with photophysical processes that occur at times significantly after the initially prepared state has dephased, such as the evolution of the initial excited state into a charge transfer state. We demonstrate the use of transient two-dimensional electronic spectroscopy (t-2DES) to probe coherences in an electron donor-acceptor dyad consisting of a perylenediimide (PDI) acceptor and a perylene (Per) donor. An actinic pump pulse prepares the lowest excited singlet state of PDI followed by formation of the PDI •-Per •+ ion pair, which is probed at different times following the actinic pulse using 2DES. Analysis of the observed coherences provides information about electronic, vibronic, and vibrational interactions at any time along the reaction coordinate for ion pair formation. TOC Graphic: conventional 2DES experiments have already dephased. The t-2DES experimental technique, therefore, can be employed to investigate the role of vibronic coupling on photoinduced electron transfer and other photophysical processes in chemical, biological, and materials systems through initiation and measurement of coherences at arbitrary positions along the reaction coordinate.