Bridge-Length- and Solvent-Dependent Charge Separation and Recombination Processes in Donor-Bridge-Acceptor Molecules

Area of Science:

• Photochemistry
• Molecular Electronics
• Electron Transport

Background:

• Donor-bridge-acceptor (D-B-A) molecules are crucial for understanding electron transport.
• Intramolecular charge separation (CS) and recombination (CR) are key phenomena in D-B-A systems.
• Pyrene (Pyr) and triarylamine (TAA) moieties offer tunable electronic properties for CS studies.

Purpose of the Study:

• To investigate photoinduced CS and CR in D-B-A molecules with varying phenylene bridge lengths.
• To understand the influence of solvent polarity and electronic coupling on CS behavior.
• To determine parameters governing CS efficiency and charge-separated state lifetime.

Main Methods:

• Femtosecond transient absorption (fs-TA) spectroscopy for real-time CS monitoring.
• Steady-state spectroscopy and chemical calculations to analyze solvent effects and electronic coupling.
• Analysis of distance-dependent decay constants and Marcus electron transfer theory.

Main Results:

• CS behavior exhibits a sharp switch with increasing solvent polarity and bridge length.
• Sufficient electronic coupling enables CS even in low-polarity solvents for short distances.
• Distance-dependent decay constant of CS rate is determined as ~0.53 Å⁻¹, indicating superexchange tunneling.
• Electronic coupling (VDA) is the dominant factor for CS rate, while bridge rotation affects charge-separated state lifetime.

Conclusions:

• Solvent polarity and bridge length critically modulate CS efficiency in D-B-A dyads.
• Superexchange tunneling governs CS rates, with electronic coupling being the key parameter.
• Strategies for high CS efficiency and long-lived charge-separated states in TAA-B-Pyr derivatives are elucidated.