Two-Step Charge Separation Passing Through the Partial Charge-Transfer State in a Molecular Dyad

Area of Science:

• Molecular photophysics and charge transfer dynamics.

Background:

• Charge separation (CS) is typically observed in weakly coupled donor-acceptor systems.
• Understanding the intermediate states in CS is vital for optimizing molecular devices.

Purpose of the Study:

• To investigate a novel two-step charge separation process in a synthesized diketopyrrolopyrrole-pyrrolopyrrole (DPP-PP) dyad.
• To elucidate the role of partial charge-transfer (CT) states in efficient charge separation.

Main Methods:

• Utilized transient absorption, fluorescence upconversion, and transient impulsive stimulated Raman spectroscopy.
• Analyzed excited-state absorption spectra and fluorescence quenching.
• Investigated vibrational frequency shifts to quantify CT character.

Main Results:

• Observed a two-step CS process initiated by photoexcitation, proceeding through a partial CT state before reaching the final CS state.
• Determined relaxation times to the partial CT state influenced by solvent and structural fluctuations (1-20 ps in toluene, 700 fs in THF).
• Quantified CT character of the bright and partial CT states (0.1 e and 0.5 e, respectively) using vibrational frequency analysis.

Conclusions:

• The DPP-PP dyad exhibits efficient charge separation via a distinct partial CT state, facilitated by strong electronic communication within the pyrrolo[3,2-b]pyrrole core.
• This mechanism offers potential applications in organic photovoltaics and light-emitting diodes.
• The study provides quantitative insights into CT dynamics and the influence of molecular structure on charge separation efficiency.