Triplet excited states of organic molecules play essential roles in photocatalysis, photodynamic therapy, photon upconversion, and organic optoelectronic applications. However, achieving efficient triplet generation in purely organic molecules remains challenging because of their intrinsically weak spin–orbit coupling. Spin–orbit charge-transfer intersystem crossing (SOCT-ISC) has emerged as a promising approach, but designing molecules that simultaneously exhibit strong charge-transfer (CT) absorption and efficient triplet formation remains a long-standing challenge.

Recently, a research team led by Prof. WU Kaifeng from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences has identified a new molecular design principle for balancing these competing requirements in organic donor–acceptor systems.

By controlling the conformational dynamics of donor-acceptor molecules, the researchers revealed that competing charge recombination pathways determine the efficiency of triplet generation.

The study was published in the Journal of the American Chemical Society.

Using a perylene diimide-based donor–acceptor molecule (PBI-4Cz) as a model system, the team demonstrated that an intermediate donor–acceptor dihedral angle provides strong electronic coupling for long-wavelength CT absorption while maintaining efficient SOCT-ISC pathways. Temperature-dependent ultrafast spectroscopy revealed that restricting molecular conformational fluctuations can suppress undesired singlet charge recombination and promote triplet formation.

Unlike conventional strategies that simply pursue a nearly orthogonal donor-acceptor geometry, this study shows that a balance among electronic coupling, spin-related interactions, and structural rigidity is essential for efficient triplet photosensitizer design.

Furthermore, the resulting molecule enabled red-light-driven triplet–triplet annihilation photon upconversion, highlighting its potential for photochemical energy conversion and related applications.

"Our study provides new insights into controlling excited-state dynamics in metal-free organic molecules and offers a general strategy for developing high-performance triplet photosensitizers for photochemistry and optoelectronic applications," WU said.