Communication: Tailoring the optical gap in light-harvesting molecules

Area of Science:

• Computational chemistry
• Materials science
• Photophysics

Background:

• Designing light-harvesting molecules necessitates control over optical gaps.
• Traditional functionals in time-dependent density functional theory (TD-DFT) struggle to accurately describe charge-transfer (CT) excitations.
• Accurate prediction of CT excitations is crucial for guiding molecular design.

Purpose of the Study:

• To introduce and validate a nonempirical range-separated hybrid (NRSH) approach for predicting charge-transfer excitations.
• To demonstrate the reliability of the NRSH method for molecules of practical complexity.
• To explore the tunability of optical absorption energies in donor-acceptor-donor molecules.

Main Methods:

• Implementation and application of a nonempirical range-separated hybrid functional within TD-DFT.
• Systematic variation of molecular structure, specifically the number of thiophene rings in donor-acceptor-donor systems.
• Comparison of theoretical predictions with experimental absorption energy measurements.

Main Results:

• The NRSH approach accurately predicts charge-transfer excitations, showing good agreement between calculated and experimental absorption energies.
• Theoretical predictions indicate that the lowest optical absorption energy can be tuned by altering the number of thiophene units.
• A saturation point for tuning the optical absorption energy is observed around five thiophene rings.

Conclusions:

• The developed NRSH method provides a reliable theoretical tool for designing light-harvesting molecules by accurately predicting charge-transfer excitations.
• The study demonstrates a viable strategy for tuning the optical absorption properties of donor-acceptor-donor molecules.
• This work facilitates the rational design of materials with tailored optoelectronic properties.