Framework formation and Crystal structure. Single crystals of EuTTA were obtained by reacting H2TTA with EuCl3∙6H2O in water and acetonitrile (4:1, v/v) at 150°C for 48 hours. The yellow crystals are not photoluminescent.58 The X-ray structure of the 3D net of EuTTA features parallel Eu-carboxyl rods (along the c axis) in a quadrangle array (Fig. 1). The neighboring Eu atoms exhibit alternating distance (3.948 and 4.517 Å): the short pair is straddled by four carboxyl groups, and the long pair by two carboxyl groups and one aqua bridge. Together the asymmetric formula of the unit cell is Eu(C12H4O4S4)1.5(H2O)0.5, with one centric TTA piece (with lower site multiplicity) contributing only as half. The bulk sample features the same crystalline phase (PXRD patterns in Fig. 2A). Elemental analysis found [C (31.65%), H (1.24%), S (24.62%], fitting the formula Eu2(C12H4O4S4)3(H2O)2 with a calculated profile: C (31.77%), H (1.18%), S (28.26%).
The spacing of the TTA molecules along the Eu-carboxyl rod is also uneven (Fig. 1C): with a close pair (interplanar gap: 3.7 Å; the pair is related by a center of symmetry, but each being non-centric) alternating with the (centric) TTA at ca. 4.6 Å. The C2H2 flank of the latter is disordered over two sets of positions, in line with the more open setting. The opening, however, is limited: it allows the atoms to wiggle more, but not N2 or CO2 guests to sorb (Figure S18).
The impact on reactivity, notably, is decisive. For when the EuTTA crystal is heated to 230 °C (to form EuTTA-230), the loose (centric) TTA loses one (S or C) atom on each wing, and forms benzodithiophene (BDT) or bis(dithiole) rings (as illustrated in Scheme 1 and Figures 1C); the other two (i.e., the close pair of) TTA, by comparison, remain unchanged. As evidence, the X-ray structure is telling (Figure 1C). The heated sample (EuTTA-230) retains the same net and the same space group [C2/c: 16.385(3), 16.484(3), 16.295(3) Å, 116.11(3)°; cell volume: 3952.0 Å3], cf. the pristine EuTTA [16.5676(15), 16.3937(14), 16.2839(14) Å; cell volume: 4045.5 Å3]. Revealingly, the sulfur atoms on the transformed linker become only partially occupied (i.e., contrasting the full occupancy of the dithiin S sites in the other two unchanged TTA pieces). The disordering of the S and C sites on the transformed linker, however, precludes a qualitative assignment of the benzodithiophene (BDT) or bis(dithiole) components. But the presence of the reported anti-benzo[1,2-b:5,4-b’]dithiophene molecule (anti-BDT)59 is supported by solution NMR data (from the EuTTA-230 sample dissolved in HF/DMSO-d6; mostly dissolved, but with a little black solid remaining), see Figure S20, while the distinct EPR signal of the EuTTA-230 solid (Figure S24) is consistent with formation of the open-shell dithiole species. Besides the anti-BDT signals, the NMR data (Figure S20) also feature additional peaks, suggesting other molecular products in the bulk sample, including possibly the syn-BDT isomer and the halfway, dithiin-benzothiophene product DTBT as drawn in Figure S20.
The ring contraction lessens the steric repulsion on the carboxyl groups, and more coplanarity is seen between the benzothiophene and carboxyl moieties, with the dihedral angle (5.16°) much smaller than that of the dithiin-based TTA precursor (48.65°). The coplanarity helps align the benzothiophene length along the Eu-carboxyl rod, and swings its wing atoms closer to the neighboring dithiin molecules (at C∙∙∙C contact of 3.49 Å). The wiggle room for the molecules is therefore compressed, apparently stopping the two remaining TTA linkers from undergoing reactions at this temperature (230°C).
At higher temperatures (e.g., 350°C for 2 hours), the remaining TTA linkers also react. Specifically, the resulted crystals (EuTTA-350) become darker (black) (Figure S17) and slower to dissolve in HF/DMSO-d6; and 1H NMR (Figure S20) of the dissolved portion (mostly dissolved, with some black solid remaining—more than the EuTTA-230 case) features only two pairs of peaks (one from the known anti-BDT, the other possibly from the syn-BDT isomer), without any dithiin C2H2 signal from the TTA molecule remaining.
Radical analysis of EuTTA-350. Most notable of the EuTTA-350 crystals is the stronger and stable paramagnetic signals. The EPR signal centers at g = 2.002 (Figure 2B), and is indicative of organic radicals. No EPR peaks for Eu(II) (e.g. g ≈ 2.0, 2.8, 3.4, 4.5 and 6.0)60 were detected, with the 4f6 Eu3+ (in the 7F0 ground state) being diamagnetic/EPR-silent. No reduction of Eu(III) to Eu(II) during thermal treatment was thus found, which is also confirmed by the X-ray photoelectron spectroscopy (XPS) signals at 1165.5 and 1135.4 eV corresponding to the Eu(III) 3d3/2 and 3d5/2 peaks which are also observed in the EuTTA precursor (Figure 2C, S21, S22).61,62 The EPR signal of EuTTA-350 remains significant after heating at 300 °C in air for 2 hours, or boiling in water for 24 hours (Figure 3A). Notably, the organic components of EuTTA-350 can be extracted into an acidic solution (e.g., into DMSO/HCl), and then precipitated (e.g., by adding water) to give an organic solid that retains strong EPR signal, further showcasing the stability of the radical species thereof (Figure S25).
The stronger EPR signal suggests further formation of 1,3-dithiole-based radicals in EuTTA-350, i.e., from the two remaining TTA linkers, in addition to the radical species initially formed (e.g., at 230°C, as in the above mentioned EuTTA-230). The strong paramagnetism complicates the direct detection of the dithiole products by NMR. To verify the formation of a bis(dithiole) molecule, it is key to demonstrate the retention of the four sulfur atoms (vs. only two S atoms in the benzodithiophene case). For this, oxidative treatment to convert the bis(dithiole) into the more tractable tetrasulfonic acid derivative (Figure S26)63 was attempted. Specifically, the soluble fraction of the EuTTA-350 crystals were first extracted into DMSO/HCl, and then precipitated out by adding water. The black precipitate was treated with 30% H2O2 to give a red-brown solution (with some red solid remaining); the solution was then evaporated, and the residue (dissolved in D2O) was found by both 1H and 13C NMR to feature the expected tetrasulfonic acid derivative (Figure S26).
Single crystal X-ray diffraction of EuTTA-350 remains strong (albeit with peaks of poor shapes), and indicates the structural integrity of the Eu-carboxylate framework. But the severe disorder on the linker portion makes it hard to pinpoint the C and S atoms on the two S-heterocycle wings (see the .res file for a plausible but non-definitive model for the flanking S/C sites). So the X-ray data of EuTTA-350 do not clarify the crosslinks among the linker molecules; but the lowered solubility of the EuTTA-350 crystals in HF/DMSO-d6 (relative to the readily soluble EuTTA) suggests some degree of crosslinking might have occurred, as is tentatively proposed in Figure S27; the scheme therein features the formula Eu4(C24H6O8S4)(C23H6O8S8)2(H2O)14, and a calculated elemental profile [C (30.64%), H (1.69%), S (23.37%)] consistent with the elemental analysis results [C (30.71%), H (1.85%), S (25.02%] for EuTTA-350.
Variable-temperature magnetic susceptibility measurement of EuTTA and EuTTA-350 were carried out at a dc field of 1000 Oe at 2‒300 K. From \({}_{M}T\) versus T plots (Fig. 2D), \({}_{M}T\) of EuTTA is 2.38 cm3 K mol-1 at room-temperature and decreased to 0.025 cm3 K mol-1 at 2 K. The paramagnetism of EuTTA in 2‒300 K is caused by coupling between nonmagnetic 7F0 ground state and closely lying 7F1 excited state of Eu(III),64 so the \({}_{M}T\) contributed by Eu(III) is 2.36 cm3 K mol-1. As for EuTTA-350 [molar mass 1209: 90% of that of EuTTA, Eu2(C12H4O4S4)3(H2O), 1343; based on TGA weight loss of 10% from EuTTA to EuTTA-350, Figure S14], the \({}_{M}T\) is 10.06 cm3 K mol-1 at room-temperature and decreased to 8.09 cm3 K mol-1. The decrease is caused by Eu(III), while the remaining \({}_{M}T\) value of 8.09 cm3 K mol-1 can be ascribed to the organic radicals in EuTTA-350. The corresponding effective magnetic moment µeff = (8\({}_{M}T\))1/2 = 8.04 B.M., which approximates that of 6 unpaired electrons: µeff = \(g\sqrt{S(S+1)}\) = 6.93 (S = 6/2 = 3, with g = 2). Because of little linker loss (e.g., < 5% from TGA data, Figure S14) from EuTTA to EuTTA-350, we assume the formula unit of EuTTA-350 to retain 3 linkers; the above numbers thus suggest each linker bearing 2 unpaired electrons as a diradical. The biradical is fitting for the bis(dithiole) product and was also consistent with the tetrasulfonic derivative (Figure S26); As for the benzodithiophene (BDT) components, radicals can arise and stabilize from thiophene crosslinks as illustrated in Figure S27.6
DFT calculation. For illustration theoretical calculation was conducted on the protonated form of the bis(dithiole) diradical (Figure 2E). The results show that spin densities locate mainly on the two sulfur-flanked C atoms (0.71 and 0.63 spin, respectively, Table S1,); the S atoms also take up almost the remaining spin densities (0.28 and 0.17 spin on each set of S atoms, Table S1). Atoms of the benzenoid core also share spin densities but these are far smaller than the spin densities on the side S‒C‒S moieties. The spin natural orbitals (SNOs) of two independent spins (Figure 2E) suggest that (1) two spins mainly localize on the S‒C‒S wings (71.72% and 88.48% orbital contribution, Table S2) and (2) the two spins are unlike to mix as reflected by the separate orbitals occupied by each spin. The spin densities and SNO results thus consistently indicate the diradicals to mostly localize on the S‒C‒S units of the dithiole rings.
Photothermal conversion and water evaporation. The radical-rich EuTTA-350 solid strongly absorbs across the broad visible/near-infrared regions (see Fig. 3B for the diffuse reflectance spectra), suggesting photothermal conversion uses. Also, the thermal conductivity of EuTTA-350 was found to be as low as 0.2 W K−1 m−1 (like that of rubber or mineral oil) at room temperature (Figure S28), indicating good thermal insulating property suited for photothermal conversion applications. As is monitored by an infrared camera, the temperature of EuTTA-350 powder rises rapidly under a xenon lamp (1 kW m-2, 420–2500 nm; to simulate 1-sun illumination): as shown in Figs. 4A and 4B, within 480 seconds, it increases by 47°C to reach 69.2°C, which is a highest jump among MOF materials, second only to the Zr-Fc solid (Figure S30).65−68 By comparison, EuTTA and EuTTA-230 reach only 41.4°C and 47.6°C, respectively, under the same conditions. In addition, the photothermal performance of EuTTA-350 is stable: in all 5 illumination cycles tested, the temperature consistently rises to 65°C within in 480 seconds (Fig. 4C); and PXRD indicates the sample remains crystalline afterwards (Figure S29).
So we use EuTTA-350 to build a solar-driven water evaporation device (Figure S35). As EuTTA-350 is light-weight and hydrophobic (with a high contact angle of 85.4°; Figure S36), its powder disperses to form a floating thin film, facilitate heat transfer to the water body for efficient interfacial evaporation. Under 1-sun (1 kW m-2) exposure in air (using 50 mg of EuTTA-350), the evaporation rate (see the plot in Fig. 4D, see also plots in Figure S33 and S34 for other amounts of EuTTA-350) can reach 1.44 kg m-2 h-1, 4 times that of pure water (0.36 kg m-2 h-1), and also better than EuTTA (0.62 kg m-2 h-1) and EuTTA-230 (1.12 kg m-2 h-1), with the temperature equilibrating at 47.4°C after 1 hour (Figure S31). The infrared thermal image showed that the energy conversion takes place at the MOF layer (Figure S31). The solar-driven water evaporation efficiency is calculated to be 97.9%,69 among the highest of all photothermal materials (Fig. 4E, Table S3).67,68,70−78