*3.7. UV-Vis Absorption Spectra*

In order to investigate the optical properties of the title compounds, UV-Vis absorption and optical diffuse reflectance spectra of **1**–**3** were obtained in the wavelength range of 190–800 nm. As shown in Figure S5 (Supplementary Materials), the optical band gaps of **1**–**3** are 2.60, 2.59, and 2.56 eV, respectively, which are comparable to other Ni6-substitute POMs, including [Ni6(μ3-OH)3(en)2(dien)(H2O)5(B-α-PW9O34)]·3H2O (2.42 eV), [Ni6(μ3- OH)3(dap)2(py)6- (H2O)(B-α-PW9O34)]·H2O (2.37 eV), and [Ni(en)2][Ni6(μ3-OH)3(en)3(1,3 bdc)(H2O)2(B-α- PW9O34)]·9H2O (2.53 eV) [20,29]. It was found that the band gaps of **1–3** are in the order of **3** < **2** < **1**, which conforms to the band gaps of the compounds decreasing with the increasing dimensionality or complexity of the structures, as proposed by Kanatzidis and Papavassiliou [40].

#### **4. Conclusions**

In summary, three new TMSPs containing {Ni6PW9} units were designed and synthesized from monomers to 1D POMCOC under hydrothermal conditions. **1** is a monomer with DACH molecules decorating the Ni6 cluster. In order to construct POMCOF on the basis of **1**, the rigid aromatic MIP ligand was first incorporated and the anionic monomeric POM **2** was obtained. HMIP still acts as a decorating group on the Ni6 cluster but fails to bridge adjacent {Ni6PW9} units. By analyzing the orientations and steric hindrance between adjacent {Ni6PW9} units of **2**, the aliphatic AP ligand was purposely chosen to replace HMIP on the base of **2**, which resulted in the formation of **3**, a new 1D POMCOC with novel helical chain. Owing to the good flexibility of the AP linker, **3** represents the first 1D POMCOC with a helical chain. This work is an example of our continued work of constructing POMCOFs with hexa-NiII substituted TMSP SBUs. The successfully designed syntheses from **1** to **3** provide us with a new strategy of using chainlike dicarboxylate acid as a linker to make POMCOFs, which may lead to some intriguing structures that cannot be found with rigid aromatic linkers. Further works with this strategy are in progress.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27134295/s1, Table S1: Comparisons of the bond lengths and bond angles in Ni6-subsi- tuted TMSPs; Figure S1: Asymmetric units of **1**–**3**; Figure S2: [Ni(DACH)2] 2+ complex in **2** and **3**; Figure S3: PXRD of **1**–**3**. Figure S4: IR spectra of **1**–**3**; Figure S5: UV-Vis spectra of **1**–**3**; Figure S6: TG curves of **1**–**3**. [41–43] are cited in the Supplementary Materials.

**Author Contributions:** Conceptualization, C.-A.C.; methodology, C.-A.C. and Y.L.; formal analysis, C.-A.C. and Y.L.; data curation, Y.L.; writing—original draft, preparation, C.-A.C.; writing—reviewing and editing, supervision, and funding acquisition: G.-Y.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant numbers 21831001, 21571016, 91122028 and 20725101.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are available from the authors.
