Unraveling the Mechanism of ACQ-to-AIE Transformation of 3,5-Diacetyl-1,4-dihydro-2,6-lutidine: Dark-State Suppression via Hydrogen-Bond-Mediated Molecular Packing.

Aggregation-induced emission (AIE) materials have attracted broad interest due to their efficient solid-state luminescence, yet a direct and predictable conversion of aggregation-caused quenching (ACQ) molecules into AIE systems remains a significant challenge. Recent experimental studies suggest that water (H2O) can modulate molecular packing to induce ACQ-to-AIE transitions, but the underlying molecular mechanism remains elusive. Here, we employ the density functional theory (DFT), time-dependent DFT (TD-DFT), ONIOM (QM/MM), and molecular dynamics simulations to systematically unravel the H2O-mediated ACQ-to-AIE mechanism in 3,5-diacetyl-1,4-dihydro-2,6-lutidine (DDL). Our results reveal that the nonradiative decay of DDL is associated with the couplings between the ππ* and nπ* states. In solution, the weak fluorescence of DDL originates from efficient nonradiative decay associated with the strong couplings between the ππ* and nπ* states, where acetyl-group-related vibrational modes make an important contribution to the nonradiative process. In pure DDL aggregates, the nπ* state remains energetically competitive, thereby facilitating dark-state-mediated nonradiative relaxation and giving rise to ACQ. Upon incorporation of H2O into the crystal lattice, π-π stacking is disrupted, the slip angles increase, and molecular vibrations are restricted. More importantly, the ππ*/nπ* energy gap at the Franck-Condon region is enlarged, which weakens the vibronic interaction between the bright ππ* state and the dark nπ* state. As a result, access to the dark nπ* state is suppressed, and radiative decay from the ππ*-min becomes more competitive. Notably, H2O molecules act not only as hydrogen-bond acceptors and donors, but also form a stable intermolecular hydrogen-bonding network that restricts the structural relaxation of the acetyl group. The mechanism is consistent with the experimentally observed enhancement of fluorescence quantum yield from 0.95% to 25.6%. This work elucidates the molecular principles underlying the H2O-mediated ACQ-to-AIE transitions and offers a conceptually significant framework for designing solid-state luminescent materials without chemical modification.