Figure 1 The conceptual scheme of A-type and molecular A-type nanogrids (AGs) structures as a model of the nanolinkage.
synthesizing AGs, yielding 9–30%. Additionally, we observed that the yield of AGs obtained with R /S -BINDFOH was significantly higher than that obtained with Mix-BINDFOH. Meanwhile, theoretical calculations and experimental results demonstrate that a smaller dihedral angle of the axial chiral group improves the yield of AGs. Finally, the nanogrid effect of AGs is confirmed by OFET memory devices.
Before synthesizing AGs, their structures were optimized and calculated density functional theory (DFT) at the B3LYP/6-31+G(d) level of theory. The structure of AGs (AG1 as an example) has six stable stereoisomers that can be divided into two pairs ofC2 -symmetric rac -isomers (aRSS -AG1, and aSRR -AG1; aRRR -AG1 and aSSS -AG1) and two asymmetric meso -configurations (aRSR -AG1 andaSRS -AG1) (Figure S1). Moreover, using the energy ofaRSS -AG1 and aSRR -AG1 (0 kcal/mol) as reference, the total energies of the other four stereoisomers are 5.968 kcal/mol (aRSR -AG1), 5.968 kcal/mol (aSRS -AG1), 14.411 kcal/mol (aRRR -AG1), and 14.411 kcal/mol (aSSS -AG1), respectively (Figure S2). These results suggest that achieving stereoselective synthesis of AGs could be challenging because the total energies of each stereoisomer are relatively similar. Meanwhile, to investigate the effect of the chain length of B2 synthons on the properties of AGs, we calculate the strain energies and dihedral angle (on axial chirality) of AG1 (11.533 kcal/mol and 83o), AG4 (32.854 kcal/mol and 91o) and AG5 (40.728 kcal/mol and 107o) (Figure S3, Table S1). These results shown that as the chain length of B2 synthons increases, the strain energies and dihedral angle of AGs also increases gradually.
Results and Discussion
To optimize the reaction conditions, we employed 1,1′-binaphthyl difluorenols (Mix-BINDFOH, A2) and thiophene as model reactions under different Brønsted or Lewis acid mediators, solvent, time and acids concentration conditions (Table S2). BF3·Et2O exhibited the best performance, providing a yield of 9% for AG1. Meanwhile, during such process, byproducts such as UG1, dimer gridization product DG1, trimer gridization product TG1, and oligomeric gridization product NG1 (N>3) (N represents the number of duplicate units) were obtained in 9%, 51%, 12% and 16%, respectively.
Next, to further investigate the effect of the chirality of binaphthalene on the yields of nanogrids, we examined (R )-BINDFOH and (S )-BINDFOH under the optimized conditions. Surprisingly, the yields of AG2 (16%) and AG3 (20%) significantly increased compared to Mix-BINDFOH when R /S -BINDFOH was used as the substrate. Moreover, the yield of DG2 (38%), DG3 (43%), and ungridization UGs (trace) decreased accordingly (Figures 2a and 2b). The results suggest that both Mix-BINDFOH and R /S -BINDFOH support a gridization pathway rather than a linear polymerization pathway, with their primary product being DGs, probably due to the smaller strain energies of DGs than AGs. Notably, the proportion of AGs produced byR /S -BINDFOH was significantly higher than that of Mix-BINDFOH, which may be due to more competitive reactions. Thirdly, to study the effect of the dihedral angle of axial chirality on the gridization reaction, we explored different sizes of B2 synthons. We tested this protocol using bithiophene (BTh) and terthiophene (TTh), which have longer molecular chains than thiophene but the same reactive site. As a result, the expect AGs (AG4 and AG5) were not observed, due to the larger dihedral angle of AG4 and AG5 compared to AG1. Instead, oligomeric nanogrids DGs (DG4, 30%; DG5, 22%), TGs (TG4, 23%; TG5, 30%) and NGs (NG4, 37%; NG5, 40%) were obtained (Figure 2c). These results indicated that i) as the dihedral angle of the axial chirality increases, the reaction does not support the formation of AGs and ii) as the chain length of B2 synthons increases, the reaction tends to generate oligomeric nanogrids with large molecular weight. Finally, to verify the above results, we further examined biphenyl difluorenols (DPDFOH, mixed stereoisomers) under the optimized conditions. Similarly, we found that when thiophene was used as B2 synthon, AG6 (dihedral angle, 79o, Table S1) could be obtained in 28% yield and oli-gomeric nanogrids (DG6, 16%; TG6, 5%) were found. However, with bithiophene and terthiophene, AG7 (dihedral angle, 109o, Table S1) and AG8 (dihedral angle, 162o, Table S1) were not