*3.2. Solvatochromism*

Their PXRD patterns (Figure 6) show that the sorption of H2O and NH3 by **1d** formed new phases (**1dw** and **1dNH3**, respectively) with noticeable colour changes from red to khaki (Figure 7). Upon desorption, both **1dw** and **1dNH3** resulted in purple powder phases, which are amorphous (**1dwTG** and **1dNH3TG**). However, the crystallinity, as well as their khaki colours, were restored after reabsorption (**1dwTGw** and **1dNH3TGNH3**). Solvatochromism in MOFs has been reported to be the result of the supramolecular interactions such as hydrogen bonding and/or the coordination of the solvent molecules to the metal centres in the frameworks [27,44,47]. These interactions affect the energy associated with d-d transitions resulting in visible colour changes [27,39].

**Figure 6.** PXRD patterns for reversible sorption for ammonia, and H2O by **1d**.

**Figure 7.** Reversible sorption of H2O and NH3 in **1d** and associated colour changes.

### *3.3. Kinetics of Desorption from* **1** *and* **3**

TGA may be used to determine the activation energy (Ea) of the gues<sup>t</sup> desorption process. We used the Ozawa model-free method [48] to study the removal of guests DMF, NH3, and H2O for both systems reported here. Samples of mass 1–2 mg were heated at different heating rates (5, 10, 20, and 30 ◦C min−1) in order to determine the activation energy associated with the removal of gues<sup>t</sup> molecules from **1**, **3**, **1dw**, **3dw**, **1dNH3**, and **3dNH3** (Figure S7). Percentage mass losses along with the corresponding temperature at each heating rate were used to determine the activation energy (Ea) according to the equation:

$$
\log \beta\_{\infty} = \log(\text{A}\_{\text{α}} \text{ E}\_{\text{α}\text{α}} / \text{g}(\text{α}) \text{R}) - 2.315 - 0.457(\text{E}\_{\text{α}\text{α}} / \text{RT}\_{\text{α}}) \tag{2}
$$

where βα is the heating rate, Aα is the frequency factor, Eaα is the activation energy, Tα is the temperature at each conversion level, and g(α) refers to the kinetic model. Figure S8 presents the plots of logβα versus reciprocal absolute temperature (in the form of 1000/T <sup>K</sup>−1). Equating the slope to −0.457(Ea/RT) allows one to calculate the activation energies, which are given in Table 5.


**Table 5.** Activation energy associated with removal of gues<sup>t</sup> molecules.

The activation energies determined for desorption from **1d** are higher than the corresponding desorption from **3d**. This may be attributed to the difference in the metal centre as well as the solvent-accessible volume of the channels, *viz*. 549.0 Å3 in **1d** and 571.4 Å3 in **3d**, as the size of the cavities influences the supramolecular interactions possible between host and gues<sup>t</sup> [47,49]. Activation energies associated with the desorption of DMF and H2O are similar to one another but are higher than that of NH3. Higher activation energies are generally associated with stronger host-guest interactions. The activation energies for desorption of DMF from **1d** and **3d** are comparable to those reported for the related MOF {[Co(34pba)2]·DMF}n [47], while the average activation energies for the desorption of H2O for **1d** and **3d** are also comparable to those reported for [Co(34pba)2] isomers and chromium(III) terephthalate (MIL-101) [27,50]. There are no previous reports of desorption of ammonia from MOFs, so we compared our values to those reported for the desorption of NH3 from Brønsted acid sites in zeolite ZSM-5 derivatives [51], which were found to have activation energies between 50 and 60 kJ mol−1. Activation energies determined in this study are of the same order of magnitude, suggesting that intermolecular interactions such as hydrogen bonding with the channel walls are of approximately the same strength as those in the zeolite.
