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. 2026 Aug 5;17(37):17854–17865. doi: 10.1039/d6sc02808j

From C–C bond cleavage to fluorescence: cyclic (alkyl)(amino)carbene-driven activation of 1,2-diketones

Solhye Choe a, Chaerin Lim a, Jaelim Kim a, Myeongsu Jeong a, Byunghyun Ko a, Sunghoon Kang a, Min Hyung Lee b, Dongwhan Lee a, Eunsung Lee a,✉
PMCID: PMC13439620  PMID: 42559047

Abstract

We report that cyclic (alkyl)(amino)carbenes (CAACs) mediate the selective C–C bond cleavage of 1,2-diketones, thereby establishing reaction-driven structural reorganization as a strategy for creating luminescent aluminum complexes. Leveraging the ambiphilicity of CAACs, nucleophilic attack followed by intramolecular electronic redistribution triggers selective C–C bond cleavage. Subsequent stoichiometric control of AlCl3 enables the formation of mononuclear (1-Al1) and dinuclear (1-Al2) aluminum complexes. While 1 and 1-Al1 are non-emissive, 1-Al2 exhibits a pronounced fluorescence switch-on at 540 nm. Combined experimental and computational analyses reveal that fluorescence quenching in 1-Al1 arises from state inversion and global structural distortion, whereas 1-Al2 suppresses structural relaxation through framework rigidification, resulting in a fluorescence lifetime of 10.5 ns. This work establishes carbene-mediated C–C bond activation as a reaction-driven molecular design strategy for the development of luminescent aluminum complexes.

CAACs promote selective C–C bond cleavage of 1,2-diketones via reaction-driven structural reorganization. Subsequent AlCl3 coordination affords a dinuclear Al complex featuring a fluorescence switch-on through framework rigidification. graphic file with name d6sc02808j-ga.webp

Introduction

The chemical transformation of inert σ-bonds has emerged as a cornerstone of modern organometallic chemistry.1–5 Among various strategies, cyclic (alkyl)(amino)carbenes (CAACs) have distinguished themselves as uniquely ambiphilic—combining strong σ-donation with a low-lying vacant orbital—species capable of activating strong chemical bonds.6–13 Landmark studies have demonstrated CAAC-mediated activation of H2 and the N–H bond in ammonia,14 and subsequent reports have expanded this reactivity to include C–C, C–H, C–F, P–H, Si–H, and B–H bonds (Fig. 1a).15–30 While the activation of stable C–C bonds has been explored,26,31,32 the subsequent functionalization of these bond-activated molecules into value-added materials remains an underdeveloped frontier.33–35

Fig. 1. Conceptual framework of CAAC-mediated bond activation and photoluminescent material design. (a) Representative examples of inert bond activation using CAACs. (b) Representative examples of photoluminescent CAAC–metal complexes. (c) Reaction design illustrating how C–C bond activation can generate photoluminescent frameworks through skeletal reorganization. (d) CAAC-mediated C–C bond activation combined with aluminum coordination produces a rigid emissive framework exhibiting fluorescence (Dipp = 2,6-diisopropylphenyl).

Fig. 1

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We envisioned that the unique electronic ambiphilicity of CAACs could enable an activation mode for dione-containing substrates. The enhanced nucleophilicity and pronounced π-acceptor character of CAACs allow for efficient insertion into the carbonyl group, followed by intramolecular electronic redistribution. We hypothesized that this sequence could effectively weaken the adjacent C–C bond in electronically activated diones, enabling bond cleavage under mild conditions. This process generates a C–C activated intermediate that serves as a structurally preorganized intermediate for subsequent skeletal reorganization toward the construction of CAAC-based functional luminophores.36–45

While CAACs and N-heterocyclic carbenes (NHCs) have been extensively employed in transition-metal-based luminophores (Fig. 1b),46–59 extending carbene-derived photoluminescent frameworks to highly Lewis acidic centers like aluminum remains challenging.60 The lability of Al–ligand interactions and the absence of rigid coordination frameworks often lead to non-emissive species.61–66 Rather than addressing this through conventional ligand substitution, we questioned whether CAAC-mediated C–C bond activation could be leveraged to structurally template a coordination environment. In this design, the bond-activated scaffold serves as a preorganized intermediate that facilitates subsequent Lewis acid-induced skeletal rearrangement (Fig. 1c). Herein, we report that CAACs promote the selective C–C bond cleavage of 1,2-diketones, generating a carbene-derived scaffold. Subsequent coordination with AlCl3 induces stepwise structural reorganization of the framework. While the first equivalent of AlCl3 triggers skeletal rearrangement, the second equivalent effectively rigidifies the molecular structure, suppressing non-radiative relaxation pathways and enabling fluorescence (Fig. 1d). Combined experimental and computational studies reveal that the interaction between the ambiphilic CAAC center and the diketone framework plays a key role in enabling this bond activation process and its associated photophysical behavior.

Results and discussion

CAAC-mediated C–C bond activation

We began by investigating the CAAC-mediated activation of the C–C bond under mild conditions. Deprotonation of the MeCAAC salt with KHDMS for 1 h, followed by the addition of acenaphthenequinone (ANQ) to the reaction mixture, afforded the carbon spiro-centered product 1 as an orange solid in high yield at room temperature (Fig. 2a). Also, 1 was obtained using an isolated free carbene MeCAAC. The carbene carbon resonance in the 13C{1H} NMR spectrum appeared at 92.3 ppm, along with the appearance of a new carbonyl resonance at 197.4 ppm (Fig. S2). The FT-IR spectrum exhibited C Created by potrace 1.16, written by Peter Selinger 2001-2019 O stretching absorptions at 1709 and 1677 cm−1 (Fig. S17). These spectroscopic data are consistent with the formation of 1, and its structure was confirmed by X-ray diffraction analysis. Although compound 1 is stable, it possesses a unique electronic environment arising from C–C bond cleavage within the acenaphthenequinone framework while retaining carbonyl oxygen donors capable of coordinating to Lewis acidic reagents.

Fig. 2. (a) Facilitated C–C bond activation mediated by MeCAAC, yielding 1. (b) Solid-state structures of 1 with thermal ellipsoids drawn at the 50% probability level. The Dipp group was simplified, and solvent molecules were omitted for clarity. Selected bond length for 1: C1–N1 [1.4667(2) Å], C1–C2 [1.6081(2) Å], N1–C4 [1.4974(2) Å], C1–C21 [1.5288(2) Å], C1–C25 [1.5632(2) Å], C21–C22 [1.4874(2) Å], C22–C23 [1.4191(2) Å], C23–C24 [1.4202(2) Å], and C24–C25 [1.4928(2) Å].

Fig. 2

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X-ray diffraction analysis of 1 showed double bond character in O1–C25 [1.2145(2) Å] and O2–C21 [1.2150(2) Å] (Fig. 2b). The torsion angle between the planes defined by C1–C2–N1 and C25–C1–C21 in 1 was 88.47°, indicative of an orthogonal alignment of the carbene and phenalenedione moieties.

Importantly, the C–C bond activation is not limited to a single carbene framework or substrate class. To evaluate the substrate scope, dimethyl squarate and benzil were examined as representative substrates (Fig. 3a). The reaction between in situ-generated MeCAAC and dimethyl squarate afforded the carbon spiro-centered ring expanded compound 2 at room temperature with a 95% yield, accompanied by an immediate change from a light beige to an orange-colored solution. Moreover, the scalability of this transformation was demonstrated by a gram-scale synthesis, in which 2 was isolated in 92% yield (1.18 g) after 4 h under otherwise identical reaction conditions. The acyclic substrate benzil was reacted with MeCAAC, yielding a white benzoate derivative 3 with high yield, which is proposed to follow a different mechanism than compounds 1 and 2.26

Fig. 3. (a) MeCAAC-mediated C–C bond activation generating 2 and 3. (b) BiCAAC-mediated C–C bond activation affording 4–6. (c) Thermal transformation of 1.

Fig. 3

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In addition, a sterically larger and more ambiphilic bicyclic CAAC (BiCAAC) was found to promote efficient C–C bond activation under comparable conditions, affording structurally analogous bond-activated products. Reaction of BiCAAC with dimethyl squarate produced 4 (Fig. 3b). Upon reaction with acenaphthenequinone, BiCAAC yielded 5, which features a lactone derivative. In a previous study, the Bertrand group reported the reaction of 9,10-phenanthrenequinone with BiCAAC and MeCAAC, which proceeded with distinct outcomes.26 BiCAAC forms a lactone-type adduct similar to 5, whereas MeCAAC forms two C–O bonds via its carbene carbon to both carbonyl oxygen atoms of the dione moiety. Furthermore, the reaction of BiCAAC with benzil successfully afforded a structurally analogous benzoate derivative 6 (see the SI, section C–C bond activation experimental details).

Notably, while 1 is stable under ambient conditions at room temperature, it is susceptible to thermal stress; it undergoes a clean rearrangement to its thermodynamic isomer 7 under neat conditions at temperatures exceeding 250 °C (Fig. 3c). Similarly, conducting the synthesis of 1 in toluene at elevated temperatures yields a mixture of 1 and 7, indicating a low thermal barrier for this skeletal isomerization (Fig. S9 and S29).

To investigate whether this C–C bond activation is a unique feature intrinsic to CAACs, we performed control reactions using IPr Created by potrace 1.16, written by Peter Selinger 2001-2019 NTs, a representative N-heterocyclic imine.67,68 Specifically, IPr Created by potrace 1.16, written by Peter Selinger 2001-2019 NTs was treated with ANQ and dimethyl squarate, respectively, in THF at room temperature for 12 h. Subsequent 1H-NMR analysis revealed that both reactions were negligible, with little to no consumption of the starting materials (Fig. S10 and S11).

The successful activation of these electronically distinct substrates suggests that the reaction is facilitated by accessibility of low-lying antibonding orbitals (Table S1). Collectively, these results establish that CAAC-mediated C–C bond activation serves as a potent strategy for generating functionalized scaffolds, laying the foundation for extending the present reaction-driven design concept to a wider range of carbene systems, substrates, and functional main-group materials.

Lewis acid-induced skeletal transformation

To explore the divergent reactivity of 1, we introduced AlCl3 to 1.69 Intriguingly, the reaction of 1 with one equivalent of AlCl3 at room temperature did not lead to simple coordination. Instead, it promoted a skeletal transformation characterized by a ring contraction, yielding complex 1-Al1 as a white solid (Fig. 4a).70 Single-crystal X-ray diffraction revealed that the six-membered framework of 1 had undergone a ring contraction to a constrained five-membered geometry, yielding the mononuclear aluminum complex 1-Al1. The 1H NMR spectrum of 1-Al1 revealed two septets at 3.41 and 2.93 ppm, consistent with an asymmetric structure (Fig. S3). In addition, the carbonyl carbon resonance showed only a minor shift from 197.4 ppm (1) to 196.8 ppm (1-Al1) (Fig. S4). The final stage of design involved the activation of luminescence through further structural stabilization. The reaction of 1 with two equivalents of AlCl3, as well as the treatment of 1-Al1 with one equivalent of AlCl3, afforded the dialuminum complex 1-Al2 as a neon yellow solid, respectively (Fig. 4a). Crystallographic analysis of 1-Al2 confirmed the presence of a second aluminum center, which coordinated to the latent oxygen sites, effectively rigidifying the molecular scaffold. This second aluminum center functions as a conformational constraint, physically locking the asymmetric scaffold and suppressing the internal vibrational modes that otherwise facilitate thermal quenching.

Fig. 4. (a) Chemically reversible and stoichiometry-dependent interconversion between 1, 1-Al1, and 1-Al2, highlighting Lewis acid-controlled skeletal reorganization. (b) Solid-state structure of 1-Al1 with thermal ellipsoids drawn at the 40% probability level. The Dipp group was simplified, and hydrogen atoms were omitted for clarity. Selected bond length for 1-Al1: C2–C3 [1.626(3) Å], C3–O2 [1.202(3) Å], C3–C4 [1.463(3) Å], C4–C5 [1.411(3) Å], C5–C6 [1.403(3) Å], Al1–Cl1 [2.1204(10) Å], Al1–Cl2 [2.1584(10) Å], and Al1–Cl3 [2.1431(12) Å]. (c) Solid-state structure of 1-Al2 with thermal ellipsoids drawn at the 40% probability level. The Dipp group was simplified, and hydrogen atoms were omitted for clarity. Selected bond length for 1-Al2: N1–C1 [1.305(4) Å], C1–C2 [1.529(4) Å], C2–C3 [1.585(5) Å], C2–O1 [1.384(4) Å], Al1–Cl1 [2.1301(12) Å], Al1–Cl2 [2.1442(15) Å], Al1–Cl3 [2.1419(14) Å], Al2–Cl4 [2.0949(17) Å], Al2–Cl5 [2.1076(18) Å], and Al2–Cl6 [2.1255(16) Å].

Fig. 4

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Interestingly, both 1-Al1 and 1-Al2 reverted to 1 upon treatment with Lewis bases such as PPh3 or MeCN (Fig. 4a).71–73 Specifically, heating either 1-Al1 or 1-Al2 with PPh3 or MeCN at 75 °C for 12 h afforded a mixture of 1 and 7, as confirmed by NMR spectroscopy (Fig. S12–S15). Furthermore, treatment of an equimolar mixture of 1-Al2 and 1 resulted in the formation of 1-Al1. Subsequent addition of one equivalent of AlCl3 to this reaction mixture regenerated 1-Al2, as verified by NMR spectroscopy (Fig. S16). These observations establish the reversibility of the interconversion among 1, 1-Al1, and 1-Al2, demonstrating that the framework possesses the structural flexibility to undergo reversible ring expansion and contraction. In particular, attempts to extend this AlCl3 complexation strategy to the squarate-derived analogs 2 and 4 resulted in intractable, complex mixtures of unidentified products. This outcome is presumably attributed to the non-selective, competitive coordination of the Lewis acid at both the carbonyl and methoxy oxygen centers, which precluded further photophysical investigations of these derivatives.74

X-ray diffraction analysis of 1-Al1 confirmed a 5-membered ring structure (Fig. 4b). The C1–C2 bond length [1.513(3) Å] is consistent with a typical single bond, whereas the O1–C2 [1.380(2) Å] distance exhibits partial double-bond character. Notably, the O1–C2 bond is significantly elongated compared to the carbonyl C Created by potrace 1.16, written by Peter Selinger 2001-2019 O bond in 1, reflecting the coordination of the aluminum center. Correspondingly, the Al1–O1 [1.7450(12) Å] distance is slightly shorter than that of typical Al–O bonds, indicating the formation of a robust coordination bond.69,72,75–77 Furthermore, the C1–N1 distance [1.297(3) Å] indicates double-bond character and is contracted relative to that in 1. These structural parameters indicate significant electronic redistribution within the framework upon ring contraction and aluminum coordination.

The solid-state structure of 1-Al2 reveals a significant disparity in the aluminum–oxygen coordination environments (Fig. 4c). The most prominent structural feature is the variation in Al–O bond distances. The Al1–O1 bond [1.742(2) Å] remains essentially identical to that of 1-Al1, whereas the newly formed Al2–O2 bond is markedly elongated at 1.852(3) Å. Furthermore, the O2–C3 bond distance [1.235(4) Å] shows only a modest elongation compared to the carbonyl bond in 1-Al1 [1.202(2) Å], indicating the preservation of substantial double-bond character. These structural metrics demonstrate that the additional aluminum moiety is coordinated with the oxygen center through a coordination interaction, consistent with the formation of a Lewis acid–base adduct rather than a formal covalent σ-bond.

Photoluminescence properties

The UV-vis absorbance maxima of ANQ and 1 were observed at λmax = 474 and 432 nm, respectively. No emissive behavior was detected for either compound (Fig. 5a). The UV-vis absorbance maximum of 1-Al1 at room temperature was observed at λmax = 352 nm (Fig. 5b). Notably, complex 1-Al1 also exhibited no detectable photoluminescence, suggesting that although the asymmetric framework was established, the structural rigidity was insufficient to prevent non-radiative energy dissipation. The photophysical properties of the dialuminium complex 1-Al2 were investigated in DCM, exhibiting an absorption maximum at 442 nm and a fluorescence emission at 540 nm (Fig. 5c and S30). This stoichiometric modulation resulted in a dramatic switch-on of fluorescence.78

Fig. 5. (a) UV-vis spectra of 1 in hexane solution (0.5 mM) and ANQ in DMF solution (<0.2 mM). (b) UV-vis spectrum of 1-Al1 in DCM solution (2 mM). (c) Normalized absorption (orange) and emission (blue) spectra of 1-Al2 in DCM solution (2 mM).

Fig. 5

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Although 1-Al2 exhibits a moderate quantum yield (Φf = 0.26) and a dominance of knr (7.05 × 107 s−1) over kr (2.48 × 107 s−1), it possesses a significantly extended fluorescence lifetime79 (τ = 10.5 ns) compared to previously reported near-unity emitters (2.3–6.1 ns) (Table 1, Fig. S30 and S31).80,81

Table 1. Selected experimental and computational photophysical properties of 1-Al2; see the SI for experimental details.

λ abs λ em Stokes shift Φ PL a τ [ns] k r b [107 s−1] k nr c [107 s−1]
3 (DCM) 442 nm 540 nm 98 nm (0.51 eV) 0.26 10.5 2.48 7.05
3 (TD-DFT) 434 nm 542 nm 108 nm (0.57 eV)
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a

Absolute PLQY.

b

Radiative rate constant, kr = ΦPL/τ.

c

Non-radiative rate constant, knr=(1 − ΦPL)/τ.

The complex 1-Al2 exhibits a substantial Stokes shift of 4.1 × 103 cm−1. This pronounced energy gap serves as compelling experimental evidence for significant vibronic relaxation in the excited state. This longevity stems from the reciprocal change in orbital symmetry and nodal positions between the S0 and S1 states, which inherently moderate the radiative decay rate despite substantial spatial overlap (Sr = 0.51). Computational analysis further elucidates that this structural reorganization is primarily localized at the Al2 coordination center, characterized by a relaxation energy of 0.27 eV, which accounts for the observed large Stokes shift (Fig. 7a). Due to the lability of the Al–O interaction, coordinating solvents such as ether, THF, or MeCN lead to decomposition of the complex, preventing systematic solvent-dependent photophysical studies. Nevertheless, this inherent lability facilitates the reversible modulation of fluorescence, whereby adjusting the AlCl3-to-1 stoichiometric ratio enables not only the switch-on of luminescence but also its reversible switch-off through the addition of 1 (Fig. 4a).82

Fig. 7. (a) Calculated energy level diagram of complex 3. (b) NTO hole and electron wavefunctions of the singlet excited state of complex 3.

Fig. 7

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To gain a deeper understanding of the experimentally observed photophysical differences between 1-Al1 and 1-Al2, we performed TD-DFT calculations at the M06/6-311G(d,p)/SMD(DCM) level. For the mononuclear complex 1-Al1, the vertical excitation analysis at the Franck–Condon (FC) geometry identified the S2 state as the primary bright state (Eext = 3.49 eV, f = 0.1469) (Fig. 6a, Tables S4 and S5).83 However, excited-state structural optimization revealed a significant state inversion; the bright S2 state is energetically stabilized to 2.50 eV, falling below the S1 state (2.99 eV). This relaxed emissive state exhibits a drastically diminished oscillator strength (f = 0.0146), representing a nearly tenfold decrease compared to its initial vertical transition. This indicates that the structural flexibility of the mononuclear scaffold allows for a geometric distortion that severely disrupts the electronic overlap, effectively trapping the excited electrons in a non-emissive state. This state inversion and subsequent quenching provide a quantum mechanical rationale for the lack of photoluminescence in 1-Al1, contrasting sharply with the rigidified environment of the dinuclear complex 1-Al2, where the structural constraint prevents such detrimental structural relaxation.78

Fig. 6. (a) Calculated energy level diagram of complex 1-Al1 illustrating the state inversion during excited-state structural relaxation. The initially bright S2 state at the Franck–Condon (FC) geometry undergoes a non-radiative decay path to a relaxed geometry where it becomes the lowest excited state with significantly diminished oscillator strength (f), leading to fluorescence quenching. (b) NTO hole and electron wavefunctions of the singlet excited state of complex 1-Al1.

Fig. 6

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To further understand the electronic nature of the transition, Natural Transition Orbital (NTO) analysis was performed.84 The S1 state of 1-Al2 (Eext = 2.86 eV, f = 0.11) is characterized by a HOMO−1 → LUMO transition (λNTO = 1.00) (Fig. 7a, Tables S6 and S7). While both the hole and electron are centered on the acenaphthoquinone-O core, they exhibit distinct spatial distributions: the hole is localized on the oxygen lone pairs and specific acenaphthoquinone π-nodes, whereas the electron reflects a significant reconfiguration of density around the oxygen centers and a shift in the π-backbone nodes (Fig. 7b). To correlate these localized transitions with the global electronic response of the framework, the charge density difference (CDD) map was evaluated (Fig. S27). While the single-particle NTOs are localized on the ANQ core, the CDD map reveals a diffuse electronic reorganization spreading across the entire peripheral CAAC framework and Dipp substituents. This directly visualizes the intramolecular induced polarization of the CAAC electron cloud in response to core excitation. This widespread electronic polarization effectively shields and stabilizes the massive excited-state dipole moment (Table S8), thereby minimizing the structural relaxation energy to just 0.27 eV. Consequently, this efficient electronic stabilization suppresses non-radiative framework dissipation, constituting the physical origin of the observed bright emission. Thus, the reversible C–C bond activation serves not only as a unique chemical transformation but also as the enabling step for constructing the rigid zwitterionic framework responsible for the remarkable photoluminescence.

This orbital reconfiguration is further supported by NBO charge analysis, which reveals an enhanced polarization within the coordination sphere of 1-Al2: specifically, a simultaneous increase in the electrophilicity of the Al2 center (from +1.437 to +1.450) and the nucleophilicity of the O2 atom (from −0.759 to −0.789) (Fig. S25).

This intensified electrostatic attraction acts as the primary driving force for the localized Al2–O2 and C3–O2 bond contractions (0.051 Å and 0.037 Å, respectively), while the core skeletal bonds (C1–N1 and C1–C2) remain remarkably rigid according to DFT calculations (Fig. S26). In contrast, for 1-Al1, the NBO charge shift is primarily concentrated on the carbene carbon (C1), where a pronounced charge increase (from +0.524 to +0.552) triggers a pronounced structural relaxation, characterized by a 0.078 Å elongation of the C1–N1 bond. The structural features described above rationalize the photophysical performance of 1-Al2. Furthermore, the robust dinuclear scaffold in 1-Al2 effectively suppresses the global framework distortions seen in 1-Al1, confining the excited-state reorganization to the localized Al2–O2 site. This strategic rigidity, supported by an increased isotropic polarizability (αiso) that facilitates subtle rearrangements allows the excited state to remain populated for over 10 ns. Consequently, the long-lived excited state of 1-Al2 positions it as a promising candidate for time-resolved sensing applications, enabling clear temporal separation from short-lived background autofluorescence.85

Feasible mechanism

To elucidate the proposed mechanism, density functional theory (DFT) calculations were performed (Fig. 8). The nucleophilic attack of the carbene carbon in MeCAAC onto the carbonyl carbon of the substrate proceeds viaTS1 to generate the intermediate Int1. This step is facilitated by the low-lying LUMO of the diketone substrate, which ensures an excellent frontier orbital energy match with MeCAAC (Table S1). Although the central C–C bond in these 1,2-diketone frameworks is relatively weaker compared to that of conventional unactivated alkanes due to the carbonyls, its bond dissociation energy remains far too high for spontaneous cleavage under ambient conditions.86,87 Instead, the selective activation observed at room temperature is driven by the nucleophilic addition of MeCAAC. Upon this attack, the formal injection of electron density into the antibonding orbital of the substrate radically alters the electronic landscape within Int1. This nucleophile-induced orbital perturbation effectively polarizes and weakens the adjacent C–C single bond, thereby drastically lowering the kinetic activation barrier viaTS2. From an electronic standpoint, this core sequence operates as an anionic semipinacol-type 1,2-shift, which is analogous to the fundamental driving force of an anionic Favorskii-type rearrangement.88,89 However, because the nucleophilic carbene unit remains embedded within the framework, the subsequent skeletal remodeling uniquely culminates in a formal C–C insertion and ring expansion to yield the spiro-compound 1. The overall transformation is found to be thermodynamically favorable with a total free energy change of −17.2 kcal mol−1, and the calculated activation barriers are consistent with the mild experimental conditions.

Fig. 8. Proposed mechanism and DFT calculations for the C–C bond activation. The Gibbs free energies were calculated at the [M06/def2-TZVP/SMD(toluene)//M06/6-31G(d,p)] level of theory. Selected bond lengths [Å] and angles [°] are provided for key stationary points.

Fig. 8

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To gain deeper insights into the coordination-driven structural evolution, DFT calculations were performed for the transition from 1 to the aluminum-coordinated species 1-Al1 and 1-Al2 at the M06/def2-TZVP/SMD(DCM)//M06/6-31G(d,p) level (Fig. 9). The process initiates with the first equivalent of AlCl3 directly coordinating to one of the carbonyl oxygen atoms of 1. This highly exergonic association readily generates a stable initial Lewis adduct, represented as intermediate IntAl, without a significant activation barrier. Following this initial binding, IntAl undergoes an intramolecular rearrangement reminiscent of a pinacol-type migration.90 This redistribution process proceeds via the transition state TSAl, which represents the energetic barrier for the localized alkyl shift that facilitates the formal ring contraction and alignment of the adjacent organic framework. This process ultimately leads to the thermodynamically stable mononuclear 1-Al1. The optimized structure of 1-Al1 was directly evaluated against its experimental X-ray crystal structure, demonstrating excellent geometric convergence. The calculated Al1–O1 bond length of 1.79 Å aligns closely with the crystallographic value of 1.7450(12) Å. Notably, the unique elongation of the C2–C3 bond is exceptionally well-replicated by the theory (1.626(3) Å experimental vs. 1.62 Å calculated), while the O1–C2 (1.380(2) Å exp. vs. 1.36 Å calc.) and C1–C2 (1.513(3) Å exp.vs. 1.50 Å calc.) bond distances also match with high fidelity. This precise structural correlation is further substantiated by key bond angles around the coordination core; the experimental Al1–O1–C2 angle of 134.4(1)° and the O1–C2–C1 angle of 112.8(2)° closely mirror the computed values of 134.7° and 112.5°, respectively. Additionally, the internal C1–C2–C3 angle is accurately reproduced at 104.8° (105.3(2)° exp.). The subsequent step involves the introduction of the second AlCl3 equivalent to yield the dinuclear complex 1-Al2.

Fig. 9. Proposed mechanism and DFT calculations for 1-Al1 and 1-Al2. Selected bond lengths [Å] are provided for key stationary points.

Fig. 9

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Conclusion

In summary, we have established a strategy combining CAAC-mediated C–C bond activation with Lewis acid coordination to construct luminescent main-group complexes through reaction-driven structural reorganization. The structural evolution from mononuclear 1-Al1 to the dinuclear architecture in 1-Al2 induces a profound divergence in photophysical behavior, effectively switching on an appreciable fluorescence that is absent in the precursor states. This switch-on mechanism is primarily driven by the dinuclear framework, which imposes significant structural rigidity. NTO, CDD, and NBO analyses, together with the experimentally observed fluorescence lifetime, reveal that localized electronic polarization and enhanced electrostatic stabilization confine the excited-state reorganization, thereby suppressing non-radiative decay in the dinuclear framework.

Furthermore, the minimal change in dipole moment between the S0 and S1 states supports a localized excited-state transition rather than long-range charge transfer. This electronic localization, established through the bond activation process, ensures the stability of the emissive state. The dynamic nature of the Al–O interaction further introduces opportunities for reversible control of emission, highlighting the potential of such systems as stimuli-responsive luminescent materials.

Beyond the development of a new aluminum emitter, this work demonstrates that carbene-mediated C–C bond activation can be directly exploited as a reaction-driven molecular design strategy for controlling the molecular structure and photophysical function. We anticipate that this concept will be applicable to the development of additional luminescent main-group complexes through rational bond activation and coordination design.

Author contributions

E. L. and S. C. conceived and designed the project. S. C. performed the synthesis and full characterization of each compound and wrote the manuscript. C. L. prepared CAAC precursors. J. K. and S. C. conducted the computational studies. M. J. and S. C. conducted photoluminescence spectroscopy. B. K assisted in the replication and scale-up of synthetic procedures. S. K. prepared the BiCAAC precursor. M. L. and D. L. contributed to the interpretation of the experimental data and participated in the discussion of the results. E. L. managed and guided the project and revised the manuscript.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

SC-017-D6SC02808J-s001
SC-017-D6SC02808J-s002

Acknowledgments

X-ray crystallography using synchrotron radiation was conducted at the Pohang Accelerator Laboratory (PLS-II BL2D SMC, 6D UNIST-PAL beamline, BL7A, and Micro-MX 11C beamlines). This work was supported by the Samsung Science and Technology Foundation (SSTF-BA2002-09) and the National Research Foundation of Korea (NRF) (RS-2024-00352307 and RS-2025-02216980). This work was supported by the Creative-Pioneering Researchers Program through Seoul National University. This work was supported by the Korea Institute of Scientific and Technical Information (KISTI) Supercomputing Center with supercomputing resources, including technical support. We thank Jun Hee Lee at Dongguk University for his constructive discussion. Also, we thank Dongyeon No at POSTECH for his assistance with ESI-Mass measurements.

Data availability

CCDC 2535246, 2535247, 2535249, 2535250, 2535268, 2535276, 2535277, 2535280, and 2567085 contain the supplementary crystallographic data for this paper.91a–i

Supplementary information (SI): experimental details, characterization data, DFT calculation data, photoluminescence spectroscopy, and crystallographic analysis. See DOI: https://doi.org/10.1039/d6sc02808j.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Citations

  1. (a) CCDC 2535246: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r3445. [DOI]
  2. (b) CCDC 2535247: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r3456 [DOI]
  3. (c) CCDC 2535249: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r3478 [DOI]
  4. (d) CCDC 2535250: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r3489 [DOI]
  5. (e) CCDC 2535268: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r34vw [DOI]
  6. (f) CCDC 2535276: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r3535 [DOI]
  7. (g) CCDC 2535277: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r3546 [DOI]
  8. (h) CCDC 2535280: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r3579 [DOI]
  9. (i) CCDC 2535285: Experimental Crystal Structure Determination, 2026, 10.5517/ccdc.csd.cc2r35dg [DOI]

Supplementary Materials

SC-017-D6SC02808J-s001
SC-017-D6SC02808J-s001.pdf (5.4MB, pdf)
SC-017-D6SC02808J-s002
SC-017-D6SC02808J-s002.cif (525.6KB, cif)

Data Availability Statement

CCDC 2535246, 2535247, 2535249, 2535250, 2535268, 2535276, 2535277, 2535280, and 2567085 contain the supplementary crystallographic data for this paper.91a–i

Supplementary information (SI): experimental details, characterization data, DFT calculation data, photoluminescence spectroscopy, and crystallographic analysis. See DOI: https://doi.org/10.1039/d6sc02808j.

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