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

## TL;DR

This paper shows how water molecules interact with boron-nitrogen compounds, changing their light-emitting properties and revealing new ways to control their behavior.

## Contribution

The study identifies a new water-mediated complexation process in boron-nitrogen MR emitters that alters their photophysics and enables TADF.

## Key findings

- Water forms a 1:3 complex with CzBN in THF, with a binding constant of 10.65 ± 0.66 M–3.
- Water complexation shifts emission to 370 nm and enables TADF by perturbing MR-core planarity.
- The interaction involves a linear water trimer and is essential for forming water–B/N MR complexes.

## Abstract

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.