Photoinduced Charge Transfer at Discrete Molecular Interfaces in Cocrystals.

Area of Science:

• Materials Science
• Organic Chemistry
• Photophysics

Background:

• Precise construction of molecular heterostructures in organic donor-acceptor (D-A) cocrystals is crucial for understanding charge transfer (CT) dynamics and developing high-performance optoelectronic materials.
• While densely packed D-A arrays are common, discrete heterojunctions at the molecular scale remain underexplored.

Purpose of the Study:

• To demonstrate an approach for creating discrete molecular interfaces in D-A cocrystals.
• To investigate the impact of these discrete interfaces on charge transfer dynamics and optoelectronic properties.

Main Methods:

• Synthesis of a tetracationic naphthalenediimide-based macrocycle (NBox^4+) and its monomeric analogue (NPy^2+).
• Cocrystallization of NBox^4+ and NPy^2+ with electron-rich pyrene (Pyr).
• Characterization using UV-vis absorption spectroscopy and femtosecond transient absorption microscopy.

Main Results:

• The NBox·Pyr cocrystal, featuring A-D-A discrete interfaces, showed a 20 nm red shift in UV-vis absorption and a ~0.1 eV lower CT state energy compared to the NPy·Pyr cocrystal with 1D D-A stacks.
• Femtosecond transient absorption microscopy revealed a significantly shorter CT state lifetime (203 ps) in NBox·Pyr compared to NPy·Pyr (1083 ps), indicating faster charge recombination.
• Stronger electronic coupling at the discrete molecular interfaces in NBox·Pyr was observed.

Conclusions:

• Discrete molecular interfaces play a critical role in tailoring CT interactions and excited-state dynamics in solid-state materials.
• This research offers a versatile strategy for designing optoelectronic materials with molecular-level spatial control.
• The NBox^4+ macrocycle facilitates the formation of discrete interfaces, enabling enhanced charge transfer properties.