Benchmarking triplet-triplet annihilation photon upconversion schemes.

Area of Science:

• Photochemistry and Photophysics
• Computational Chemistry
• Materials Science

Background:

• Photon upconversion using triplet-triplet annihilation in molecular systems offers a route to harness sub-bandgap photons for photovoltaic applications.
• Computational chemistry can accelerate the design of novel molecular systems for efficient photon upconversion, reducing the need for extensive experimental synthesis.

Purpose of the Study:

• To benchmark various time-dependent density functional theory (TD-DFT) methods for their accuracy in describing photophysical properties of sensitizer/annihilator pairs.
• To provide guidelines for selecting appropriate computational methods for future design of molecular systems for photon upconversion.

Main Methods:

• Benchmarking of time-dependent density functional methods, including meta-GGA, hybrid, and range-separated hybrid functionals.
• Evaluation of the performance of functionals like B3LYP, PBE0, M06-L, and M11-L in describing singlet absorptions and triplet energetics.
• Assessment of the impact of the Tamm-Dancoff approximation (TDA) on TD-DFT accuracy for these systems.

Main Results:

• Hybrid functionals (B3LYP, PBE0) with 20-25% exact exchange accurately describe singlet absorptions.
• Meta-GGA functionals (M06-L, M11-L) without exact exchange provide the best description of triplet energetics.
• The Tamm-Dancoff approximation generally does not improve the accuracy of singlet or triplet energy calculations for these molecular pairs.

Conclusions:

• The choice of functional is critical and depends on whether singlet or triplet states are of primary interest for photon upconversion systems.
• Specific hybrid functionals are recommended for singlet state calculations, while specific meta-GGA functionals are preferred for triplet state calculations.
• TDA is not essential for accurate TD-DFT calculations of these sensitizer/annihilator pairs.