Direct Observation of Ultrafast Relaxation Dynamics of a Mixed Excimer State in Perylene Monoimide Dimer by Femtosecond Transient Absorption

A J-type dimer PMI-2, two perylene monoimides linked by butadiynylene bridger was prepared, and its excited-state dynamics was studied using ultrafast femtosecond transient absorption spectroscopy, along with steady-state spectroscopy and quantum chemical calculations. It is evidently demonstrated that the symmetry-breaking charge separation (SB-CS) process in PMI-2 is positively mediated by an excimer, which is mixed by localized Frenkel excitation (LE) and an interunit charge transfer (CT) state. Kinetic studies show that, with the polarity increasing of the solvent, the transformation of excimer from a mixture to the CT state (SB-CS) is accelerated, and the recombination time of the CT state is reduced obviously. Theoretical calculations indicate that these are due to PMI-2 obtaining more negative free energy (ΔGcs) and lower CT state energy levels in highly polar solvents. Our work suggests that the mixed excimer can be formed in a J-type dimer with suitable structure, in which the charge separation the process is sensitive to the solvent environment.