Distinct Excited-State Dynamics of Near-Orthogonal Perylenimide Dimer: Conformational Planarization versus Symmetry Breaking Charge Transfer

Efficient photoinduced charge separation in artificial multichromophoric architectures relies on two critical factors, (i) electronic coupling and (ii) solvation. While the coherent exciton interactions delocalize the excitation energy among molecules, the solvation-dependent dynamical disorder tends to localize it. Local environments such as solvent polarity/dielectric environments exhibit profound effect on mediating the excited-state relaxation dynamics via specific electronic/geometric changes in chromophore multimers. Herein, a comprehensive account of the solvent governed distinct exciton coupling and symmetry breaking charge separation in a near-orthogonal perylenimide dimer (PP) is presented employing steady-state, femtosecond transient absorption measurements and quantum chemical calculations. Steady-state absorption measurements of the PP dimer reveal apparent electronic coupling between the two monomeric units, wherein the fluorescence measurements reveal a strong fluorescence character in nonpolar solvent, but a significantly quenched state is observed in polar solvent. Ultrafast transient absorption measurements reveal that the fluorescence quenching in polar solvent arises from a photoinduced symmetry-breaking charge transfer (SBCT) process and a subsequent population of the charge-separated radical ion-pair state. Contrastingly, in nonpolar solvent, the charge transfer is endothermic and energetically not feasible. Manifestly, the dimer in nonpolar solvent undergoes a conformational planarization within 20 ps accompanied by excitation delocalization over the two identical monomers in the lowest excited singlet state as evident from the dominant stimulated emission (around 580 nm) and the excited-state absorption (around 740 nm) in the femtosecond transient absorption spectra. Observed solvent-mediated selective control on the excited-state relaxation pathways in the near-orthogonal PP dimer can help shed light on the mechanisms of energy/charge transfer in molecular systems and guide the design of novel high-performance photovoltaic materials.