Water-Mediated Electronic Modulation in Boron–Nitrogen Multi-resonance Thermally Activated Delayed Fluorescence Emitters

Using the prototypical “multiple resonance” (MR) emitters CzBN and BCzBN (also known as DtBuCzB), we uncover a previously unrecognized yet crucial complexation process between B/N MR cores and water molecules that profoundly alters their ground- and excited-state photophysics and photochemistry. This discovery originated from an unexpected new blue-shifted band of BCzBN (360 nm absorption/375 nm emission), which appeared, alongside the parent 467/480 nm bands, when trace water was present in organic solvents, such as tetrahydrofuran (THF). The same phenomenon was subsequently observed for CzBN. Water titration experiments with CzBN in THF reveal a 1(CzBN)/3(H2O) stoichiometry complex with a binding constant of 10.65 ± 0.66 M–3. Quantum-chemical calculations further support that a linear relay water trimer engages the boron center through an O(H2O) → B(CzBN) Lewis acid–base interaction and forms H(H2O)···N(CzBN) hydrogen bonds, thereby perturbing the MR-core planarity. This interaction raises the LUMO energy while preserving the alternating HOMO/LUMO distribution, allowing both the parent CzBN (475 nm) and water complex (370 nm) emissions to exhibit thermally activated delayed fluorescence (TADF). Further fluorescence titration and time-resolved emission studies reaffirm a ground-state equilibrium between CzBN, the 3H2O–CzBN complex, and second-shell water-solvated 3H2O–CzBN. Upon excitation, expulsion of water from the boron center occurs in solvated 3H2O–CzBN, creating a branching pathway that competes with TADF and yields the characteristic 475 nm CzBN emission. Water complexation is also observed in other BCzBN derivatives with enhanced boron Lewis acidity but is absent in those with diminished acidity, indicating that the static interaction of O(H2O) → B(MR) with water is indispensable for forming water–B/N MR complexes. These results uncover water as a previously overlooked yet ubiquitous perturbing agent in B/N-type MR systems, opening new opportunities for understanding and exploiting their behavior under aqueous influence.