Triplet–triplet
annihilation photon upconversion (TTA-UC)
is a process able to repackage two low-frequency photons into light
of higher energy. This transformation is typically orchestrated by
the electronic degrees of freedom within organic compounds possessing
suitable singlet and triplet energies and electronic couplings. In
this work, we propose a computational protocol for the assessment
of electronic couplings crucial to TTA-UC in molecular materials and
apply it to the study of crystal rubrene. Our methodology integrates
sophisticated yet computationally affordable approaches to quantify
couplings in singlet and triplet energy transfer, the binding of triplet
pairs, and the fusion to the singlet exciton. Of particular significance
is the role played by charge-transfer states along the b-axis of rubrene crystal, acting as both partial quenchers of singlet
energy transfer and mediators of triplet fusion. Our calculations
identify the π-stacking direction as holding notable triplet
energy transfer couplings, consistent with the experimentally observed
anisotropic exciton diffusion. Finally, we have characterized the
impact of thermally induced structural distortions, revealing their
key role in the viability of triplet fusion and singlet fission. We
posit that our approaches are transferable to a broad spectrum of
organic molecular materials, offering a feasible means to quantify
electronic couplings.