Optimizing Three-Dimensional, π-Orbital Accessibility for Molecular Diffusion-based Photon Upconversion

Le résumé fourni par la source

Achieving high-efficiency, solution-state triplet–triplet annihilation-based photon upconversion (TTA-UC) at solar equivalent intensities is fundamentally dictated by π-orbital overlap that modulates triplet dynamics during collision. However, it has been difficult to identify the optimal intermolecular geometry of planar chromophores that maximize TTA-UC performance, as this requires non-planar, three-dimensional side group engineering. Here, we systematically introduced linear alkyl chains onto sp 3 carbons of the 5,10-dihydroindeno[2,1-a]indene (DHI) skeleton to control steric hindrance in the out-of-π-plane direction and to optimize π-orbital accessibility for efficient TTA-UC. The length-dependent, solution-state study revealed that methyl groups can maximize the TTA-UC quantum yield, whereas longer n-butyl groups are required to sufficiently suppress triplet quenching. These experimental results are supported by molecular dynamics simulations showing that the alkyl chains limit parallel intermolecular geometries at proximity. Furthermore, by using a heavy-metal-free organic sensitizer with a higher absorption coefficient, the DHI system achieves an external upconversion quantum yield of over 9% under solar-equivalent 445 nm laser excitation (1.4 mW cm −2 ). These findings provide a mechanistic blueprint for designing steric environments that modulate singlet and triplet dynamics, advancing the development of low-power, sunlight or indoor-light-driven upconversion applications.

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