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

Abstract 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.