Triplet-triplet energy-transfer coupling: theory and calculation

Area of Science:

• Quantum Chemistry
• Photochemistry
• Molecular Physics

Background:

• Triplet-triplet (TT) energy transfer is crucial for various photochemical processes.
• Understanding the electronic coupling is essential for predicting TT energy transfer rates.
• Existing methods for calculating TT coupling have limitations.

Purpose of the Study:

• To develop and validate methods for calculating electronic coupling strengths in TT energy transfer.
• To investigate the factors influencing TT coupling, such as molecular geometry and orbital overlap.
• To provide insights into the timescales of TT energy transfer.

Main Methods:

• Direct calculation of off-diagonal Hamiltonian matrix elements using spin-localized unrestricted Hartree-Fock wave functions.
• Energy gap calculations derived from the configuration-interaction-singles (CIS) scheme.
• Analysis of TT coupling in face-to-face ethylene and stacked polyene systems.

Main Results:

• Both proposed methods yield similar results for TT coupling strengths.
• The exponential attenuation factor for face-to-face ethylene is 2.59 A⁻¹.
• TT coupling is sensitive to molecular stacking, with partial stacking significantly reducing and accelerating the decay of coupling.
• Coupling is primarily determined by the overlap of frontier orbitals in regions of close contact.

Conclusions:

• The developed methods accurately quantify TT electronic coupling.
• Molecular geometry, particularly the degree of stacking, critically influences TT energy transfer.
• Calculated coupling strengths suggest that TT energy transfer can occur on nanosecond to picosecond timescales.