Excimer evolution hampers symmetry-broken charge-separated states

Abstract

Achieving long-lived symmetry-broken charge-separated states in chromophoric assemblies is quintessential for enhanced performance of artificial photosynthetic mimics. However, the occurrence of energy trap states hinders exciton and charge transport across photovoltaic devices, diminishing power conversion efficiency. Herein, we demonstrate unprecedented excimer formation in the relaxed excited-state geometry of bichromophoric systems impeding the lifetime of symmetry-broken charge-separated states. Core-annulated perylenediimide dimers (SC-SPDI₂ and SC-NPDI₂) prefer a near-orthogonal arrangement in the ground state and a π-stacked foldamer structure in the excited state. The prospect of an excimer-like state in the foldameric arrangement of SC-SPDI₂ and SC-NPDI₂ has been rationalized by fragment-based excited state analysis and temperature-dependent photoluminescence measurements. Effective electronic coupling matrix elements in the Franck-Condon geometry of SC-SPDI₂ and SC-NPDI₂ facilitate solvation-assisted ultrafast symmetry-breaking charge-separation (SB-CS) in a high dielectric environment, in contrast to unrelaxed excimer formation (Ex*) in a low dielectric environment. Subsequently, the SB-CS state dissociates into an undesired relaxed excimer state (Ex) due to configuration mixing of a Frenkel exciton (FE) and charge-separated state in the foldamer structure, downgrading the efficacy of the charge-separated state. The decay rate constant of the FE to SB-CS (k _FE→SB-CS) in polar solvents is 8-17 fold faster than that of direct Ex* formation (k _FE→Ex*) in non-polar solvent (k _FE→SB-CS≫k _FE→Ex*), characterized by femtosecond transient absorption (fsTA) spectroscopy. The present investigation establishes the impact of detrimental excimer formation on the persistence of the SB-CS state in chromophoric dimers and offers the requisite of conformational rigidity as one of the potential design principles for developing advanced molecular photovoltaics.

Fig. 1. Molecular structures of monomeric SPDI and NPDI (a) and dimeric SC-SPDI₂ and SC-NPDI₂ (b). Optimized geometries of SC-SPDI₂ (c) and SC-NPDI₂ (d) with the corresponding dihedral angles between the monomeric planes. Normalized UV-vis absorption and emission spectra (e) and fluorescence decay profile (f) of SC-SPDI₂ and SC-NPDI₂ in toluene at room temperature.

Fig. 2. Excited-state structural relaxation of SC-SPDI₂ dimer upon photoexcitation.

Fig. 3. The temperature-dependent photoluminescence spectra of (a) SC-SPDI₂ and (b) SC-NPDI₂ in toluene.

Fig. 4. (a) Femtosecond transient absorption contour maps (top) and spectra (bottom) of SC-SPDI₂ in toluene (TOL) showing the excited-state dynamics after photoexcitation at 470 nm. (b) Evolution associated difference spectra reconstructed from global analysis of the A → B → C → D model, where FE is the Frenkel exciton/singlet excited state; Ex* is the unrelaxed excimer state; Ex is the relaxed excimer state. (c) Schematic representation of excited-state dynamics of SC-SPDI₂ in toluene at room temperature.

Fig. 5. (a) Femtosecond transient absorption contour maps (top) and spectra (bottom) of SC-SPDI₂ in acetonitrile (ACN) showing the excited-state dynamics after photoexcitation at 470 nm. (b) Evolution associated difference spectra reconstructed from global analysis of the A → B → C → D model, where FE is the Frenkel exciton/singlet excited state; SB-CS is the symmetry-breaking charge-separated state; Ex is the excimer state. (c) Schematic representative excited-state dynamics of SC-SPDI₂ in acetonitrile at room temperature.

Fig. 6. The potential energy diagram summarizes the excited-state dynamics of SC-SPDI₂ and SC-NPDI₂ undergoing structural relaxation in polar solvents (ACE and ACN) and non-polar solvent (TOL), where FE is the Frenkel exciton, Ex* is the unrelaxed excimer state, Ex is the stable excimer state and SB-CS is the symmetry-breaking charge-separated state. The x-axis represents the reaction coordinate and E on the y-axis indicates the potential energy.

Fig. 7. Pictorial representation of the delocalized Frenkel exciton (left), charge resonance states (middle) of FC geometry, and the excimer state (right) of the relaxed foldamer structure of SC-SPDI₂. (a) Schematic representation of hole–electron distribution in the dimer, the rectangular box represents each fragment, dashed orange circles depict holes, and solid blue circles depict excited electrons. Hole–electron correlation plots (b) and the corresponding isosurface of the hole (c, orange) and electron (d, blue) distribution of different excited-states of SC-SPDI₂. The CT and PR values are given at the bottom to define the nature of excitations.