Sulfur-Based Intramolecular Hydrogen-Bond: Excited-State Hydrogen-Bond On/Off Switch with Dual Room-Temperature Phosphorescence

We report O–H----S hydrogen-bond (H-bond) formation and its excited-state intramolecular H-bond on/off reaction unveiled by room-temperature phosphorescence (RTP). In this seminal work, this phenomenon is demonstrated with 7-hydroxy-2,2-dimethyl-2,3-dihydro-1H-indene-1-thione (DM-7HIT), which possesses a strong polar (hydroxy)-dispersive (thione) type H-bond. Upon excitation, DM-7HIT exhibits anomalous dual RTP with maxima at 550 and 685 nm. This study found that the lowest lying excited state (S₁) of DM-7HIT is a sulfur nonbonding (n) to π* transition, which undergoes O–H bond flipping from S₁(nπ*) to the non-H-bonded S′₁(nπ*) state, followed by intersystem crossing and internal conversion to populate the T′₁(nπ*) state. Fast H-bond on/off switching then takes place between T′₁(nπ*) and T₁(nπ*), forming a pre-equilibrium that affords both the T′₁(nπ*, 685 nm) and T₁(nπ*, 550 nm) RTP. The generality of the sulfur H-bond on/off switching mechanism, dubbed a molecule wiper, was rigorously evaluated with a variety of other H-bonded thiones, and these results open a new chapter in the chemistry of hydrogen bonds.