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.