Near-Unity Triplet Generation Promoted via Spiro-Conjugation.

Spiro-linked bichromophoric molecular systems are superior for triplet excited state generation. First, the two chromophores are tethered into a close proximity not accessible via alternative linkage, promoting spiro-conjugation mediated charge-transfer (CT). Second, the spiro-orthogonality inhibit FRET-type non-radiative deactivation and high rigidity largely suppresses vibronic energy dissipation. An intersystem crossing (ISC) rate constant of 1.0×10 11 s -1 was previously registered with a spiro-bis-benzophenone scaffold. We envisage that triplet generation efficiency could be further enhanced by stabilizing the spiro-CT state and rationally designed a family of spiro-compounds (SCTs), which consist of electron-rich diphenyl ether as the spiro-CT donor and electron-deficient dinaphthyl ketone as the spiro-CT acceptor. Through fine-tuning of the energy level between the CT and high energy triplet states, near-unity triplet generation quantum yield was achieved and the underlying ISC mechanism is revealed by using ultrafast spectroscopy and quantum chemical calculations. Finally, potential triplet sensitizing application was demonstrated in SCTs. Our findings suggest that spiro-bichromophoric molecular system with an enhanced spiro-charge transfer warrants efficient triplet generation and represents a powerful design strategy of heavy-atom-free triplet sensitizers with predictable ISC property.