Retro-synthetic analysis (Corey and Cheng, 2009) revealed that there are several strategies to assemble the four-layer 3D molecular framework. These strategies are mainly based on utilizing dual Suzuki–Miyaura cross-couplings (Miyaura and Suzuki, 1995) as the key steps, as represented by the case of target 8a, in which two fragments (diboronic ester 1aa and bromide 1a) would be joined (Figure 1). In our previous synthesis, boronic esters proved to be more effective than boronic acids in multilayer synthesis via Suzuki–Miyaura couplings. Therefore, they were selected for the present assembly. We made many efforts to synthesizing diboronic ester 1aa for this purpose, but we failed. Similarly, low chemical yields were encountered during the synthesis of diphenyl(1-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)naphthalen-2-yl)phosphine oxide 8a. For this reason, they are excluded from our RSA design. Two key precursors, 1,8-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2- yl)phenyl)naphthalene 6a and (1-bromonaphthalen-2-yl)diphenylphosphine oxide 7a, can be conveniently obtained, making us choose it as the major route (the top part of Figure 1) for this work. The precursor 6a was readily derived from the carbon–boron coupling of naphthalene-1,8-diylbis (4,1-phenylene) bis(trifluoromethanesulfonate) 5a, which originated from the dual Suzuki–Miyaura cross-couplings of 1,8-dibromonaphthalene 1a with (4-methoxyphenyl)boronic acid 2, both of which are commercially available.

The assembly was represented by the synthesis of targets 6a and 6b, in which different conditions are explored for two steps to achieve higher efficiencies (Scheme 2). The synthesis of the building block 6a was started from Suzuki–Miyaura coupling of 1,8-dibromonaphthalene 1a with (4-methoxyphenyl)boronic acid 2 by employing Pd(OAc)₂ as a catalyst and K₂CO₃ as a base in the DMF/H₂O co-solvent at 100°C, leading to the formation of 1,8-bis(4-methoxyphenyl)naphthalene 3a in an 85% yield. The precursor 3a was transformed into 4,4'-(naphthalene-1,8-diyl)diphenol 4a via demethylation in the presence of BBr₃ in DCM by changing the temperature from −10°C to room temperature to afford an 88% yield. The precursor 4a was allowed to react with excess trifluoromethanesulfonic anhydride (Tf₂O) by using pyridine and 4-dimethylaminopyridine (DMAP) as bases to yield naphthalene-1,8-diylbis (4,1-phenylene) bis(trifluoromethanesulfonate) 5a in a 96% yield. The reaction between 5a and bis(pinacolato)diboron (B₂Pin₂) in in situ catalytic systems involve using KOAc as a base additive and (1,1-bis(diphenylphosphino)ferrocene) dichloropalladium (II) as a catalyst in 1,4-dioxane at 80°C to afford 1,8-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)naphthalene 6a as double-layer reactants (Scheme 2A). The synthesis of the building block of 5,6-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,2-dihydroacenaphthylene 6b was also started from Suzuki–Miyaura coupling by treating 5,6-dibromo-1,2-dihydroacenaphthylene 1b with (4-methoxyphenyl)boronic acid 2 by using Pd(PPh₃)₄ as the catalyst and Na₂CO₃ as the base in DMF/H₂O as a mixed solvent at 100°C, to yield 5,6-bis(4-methoxyphenyl)-1,2-dihydroacenaphthylene 3b in a 57% yield. The precursor 3b was converted into 4,4'-(1,2-dihydroacenaphthylene-5,6-diyl)diphenol 4b via demethylation in the presence of BBr₃ in DCM by gradually changing temperature from −78°C to room temperature to afford an 86% yield. The two steps shown in Scheme 2B were performed under the same conditions as the aforementioned synthetic route to yield (1,2-dihydroacenaphthylene-5,6-diyl)bis (4,1-phenylene) bis(trifluoromethanesulfonate) 5b and 5,6-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1,2-dihydroacenaphthylene 6b chemical yields of 92% and 87%, respectively.

The synthesis of another key precursor is represented by the generation of (1-bromonaphthalen-2-yl)diarylphosphine oxides 7 by the following literature procedures (Wu et al., 2021a). It was started with the protection of 1-bromo-2-naphthol with Tf₂O to yield 1-bromonaphthalen-2-yl trifluoromethanesulfonate in the presence of pyridine. The second step was conducted through the C-P coupling with diaryl phosphine oxide by taking advantage of Pd₂ (dba)₃ and 1,3-bis(diphenylphosphino)propane (DPPP) as the catalytic combination (Ji et al., 2020). In addition, substrates 7b–7o were synthesized starting with the nucleophilic substitution of diethyl phosphite with arylmagnesium bromide to yield bisaryl phosphine oxides, followed by subjecting to the catalytic coupling with 1-bromonaphthalen-2-yl trifluoromethanesulfonate.

The final step was to assemble the four-layer targets by treating 1,8-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)naphthalene 6a with (1-bromonaphthalen-2-yl)diarylphosphine oxides 7a in the presence of a Pd(PPh₃)₄ catalyst as the key step, delivering various four-layered folding phosphine oxides 8a in good yields (Scheme 1). At this step, it is necessary to optimize the conditions since Suzuki–Miyaura coupling between the double-layer diboronic ester 6a and bromide 7a did not result in ideal yields under the aforementioned catalytic systems. Different catalysts, solvents, and bases were screened, and the results are shown in Table 1 (entries 1–9). In the beginning, the reaction of 6a and 7a in a 1:2.5 mol ratio was carried out in the presence of 10 mol% Pd(PPh₃)₄ and 3.0 equiv of K₂CO₃ in THF/H₂O (5:1, v/v) at 90 °C for 48 h, and the desired [1 + 4+1] multilayered 3D product 8a was obtained in 86% yield through dual Suzuki–Miyaura couplings (entry 1). Other Pd catalysts, including Pd₂ (dba)₃, PdCl₂, and Pd(OAc)₂, were then examined together with K₂CO₃ in this transformation, but all yielded unsatisfactory results (entries 2–4). Similarly, experimentation with various solvent systems, such as toluene/H₂O, DME/H₂O, and 1,4-dioxane/H₂O did not produce satisfactory results either (entries 5–7). We next attempted to optimize conditions by exploiting K₃PO₄ and Cs₂CO₃ as bases and found that both attempts did not show poor chemical yields of 37% and 40%, respectively (entries 8 and 9).

Since there are two polar -POPh₂ groups existing in the products, the purification of resulting crude products at this step can be readily obtained through the group-assisted purification (GAP) (Kaur et al., 2010; Kaur et al., 2011; An et al., 2015) chemistry/technology, eliminating the need for chromatography and recrystallization. The pure product 8a and its derivatives 8b—8p were conveniently obtained by simply washing the crude products with 95% EtOH, making this synthesis much greener and environmentally friendly.

Having established the optimal reaction conditions, we next investigated the scope of the double Suzuki–Miyaura cross-coupling reaction by using a variety of preformed 2-diarylphosphinyl-1-naphthyl bromide 7. As shown in Scheme 3, the influence of substituents in the aryl moiety of 7 was first evaluated. The reactions of 2-diarylphosphinyl-1-naphthyl bromide 7 with either electron-rich groups (Me 7b, OMe 7c, Ph 7d, SMe 7e, and NMe2 7f) or electron-poor groups (F 7g, Cl 7h, and OCF₃ 7i) at the para position of the aryl moiety of 7 could tolerate this reaction system, leading to the corresponding products 8b–8i in 54%–91% yield. Similarly, the meta-substituent of the aryl unit of 7 (Me 7j, OMe 7k, and F 7l) still showed a high reactivity profile, providing access to the corresponding multilayered 3D products 7j–7l in 62%–93% yield. It is noteworthy that the ortho-methyl substituted analog 7m was a suitable surrogate for this coupling reaction, which could work smoothly to deliver the product 8m in 52% yield. To our delight, both 3,5-dimethyl-substituted arylphosphine oxide 7n and 1-naphthyl-substituted phosphine oxide 7o were adopted to demonstrate the compatibility of this protocol and furnished the target products 8n–8o in 88% and 64% yields, respectively. Furthermore, the dihydroacenaphthylene-derived double-layer diboronic ester 6b was then allowed to react with 2-diphenylphosphinyl-1-naphthyl bromide 7a under standard conditions. As anticipated, the reactions proceeded smoothly, enabling the Pd-catalyzed coupling to yield the corresponding product 8p in 87% yield. The structures of all products were fully characterized by carbon and proton NMR spectroscopic and HRMS analyses.

Furthermore, the resulting multilayer framework has been unambiguously assigned by X-ray structural analysis of one of the products, 8a (Figure 2). This structure clearly presents two groups of nearly parallel units: four phenyl rings in the middle and two naphthyl rings at one end. These two planar units plus a single naphthyl ring anchor consist of a [1 + 4+2] multilayer framework, which added one more layer in the middle, as compared with our previous [1 + 3+1] type of multi-layer counterparts in the middle columns of their structures. It should be noted that our preliminary results on assembling a maximum of five-layer counterparts show promising results, leading to a [2 + 5+2] multilayer framework.

Among the products listed in Figure 3 and Figure 4, several representatives, 8a, 8c, 8g, 8i, and 8n, were examined for their behaviors on UV–vis absorption, photoluminescence (PL), and aggregation-induced emission (AIE). As shown in Figure 3A, UV-vis absorption spectra were recorded for these compounds with the same concentration in THF (Figure 3A). The highest absorptions of four samples (8a, 8g, 8i, and 8n) displayed wide absorption between 280 nm and 360 nm, except for molecule 8c. The wide absorption of 8c was observed to be between 260 and 210 nm; the highest position appeared at 280 nm, and the second highest position was around 340 nm.

The photoluminescence spectra of these compounds upon excitation exhibit bands at slightly different curves with regard to emission strengths and wavelengths (Figure 3B). Excitation wavelengths at 344 nm for 8a, 8c, 8i, and 8g and at 352 nm for 8n were utilized for measuring their photoluminescence. As shown in the spectrum, 8c displays its maximum emission at 400 nm. As compared with 8c, 8a displays its maximum emission at 430 nm shifted downfield and 8a, 8g, and 8i at 344 nm shifted upfield, respectively. The PL performance seems complicated, being attributed to solvent–target interactions and electronic and conformational steric effects of aromatic rings attached to the phosphorus center. Compared with 8a, which has no functional group on its two phenyl rings of the P=O center, the presence of both the electron-donating (OMe in 8c) and electron-withdrawing groups (CF₃O and F in 8g and 8i, respectively) and the steric effect (two methyl groups in 8n) all resulted in upfield emission.

Fluorescence spectroscopic analysis was conducted using 8n as a representative for aggregation-induced emission (AIE). As shown in Figure X, the water fractions (f_w) were increased from 0% to 60%, resulting in a steady emission enhancement from 352 nm to 555 nm. This emission change is attributed to the intermolecular packing of the molecular matrix, indicating the existence of aggregation-induced emission (AIE) by this four-layer compound. Although an obvious GAP exists between emission in f _w = 0% (in pure THF) and the other four fw slots, the emission at the later four curves (10%–60%) does not display obvious differences.

As usual, the intermolecular aggregation largely suppresses the rotational motions of aromatic rings so that the exciton energy cannot be depleted by the radiation-less decay, thus making the present AIE observation possible. The intermolecular packing process would have an impact on the intramolecularly layered framework, but in a diluted environment, the movements between molecules would diminish as the poor solvent (water) became more prevalent. The intramolecular stacking would become more regular and predominantly controlled by suppressing the whole framework while water was added to the solvent mixture as soon as the acceptable saturation value of f _w = 60% was reached. The partial AIE activities could also exist because of this compressed packing model’s contribution to more efficient through-space interactions. This result is in accordance with our earlier research on multilayer molecules (Wu et al., 2021a; Wu et al., 2021b; Wang et al., 2022).

An interesting shape of emission appeared in pure and transparent THF, which indicates some degrees of molecular aggregation exist in this system. This observation would benefit organic synthesis during condition modifications by taking advantage of aggregates. It should be noted that THF is among the most common solvents in organic synthesis, particularly in asymmetric synthesis and catalysis. Our laboratory has recently proven that chiral aggregates can enhance asymmetric control and can even switch the stereo configuration of resulting chiral products (Rouh et al., 2022; Tang et al., 2023). Chiral aggregates were directly confirmed by AIE, AIP (aggregation-induced polarization) (Tang et al., 2022c; Tang et al., 2022d), and dynamic light scattering (DLS) experiments in THF-water and THF-ethanol co-solvents. Both stoichiometric and catalytic asymmetric reactions have been carried out successfully, defined as aggregation-induced asymmetric synthesis (AIAS) (Tang et al., 2023) and aggregation-induced asymmetric catalysis (AIAC) (Jin et al., 2022b).