Building an emission library of donor–acceptor–donor type linker-based luminescent metal–organic frameworks

Luminescent metal–organic frameworks (LMOFs) have been extensively studied for their potential applications in lighting, sensing and biomedicine-related areas due to their high porosity, unlimited structure and composition tunability. However, methodical development in systematically tuning the emission properties of fluorescent organic linker-based LMOFs to facilitate the rational design and synthesis of target-specific materials has remained challenging. Herein we attempt to build an emission library by customized synthesis of LMOFs with targeted absorption and emission properties using donor–acceptor–donor type organic linkers. By tuning the acceptor groups (i.e. 2,1,3-benzothiadiazole and its derivatives), donor groups (including modification of original donors and use of donors with different metal–linker connections) and bridging units between acceptor and donor groups, an emission library is developed for LMOFs with their emissions covering the entire visible light range as well as the near-infrared region. This work may offer insight into well controlled design of organic linkers for the synthesis of LMOFs with specified functionality.


Introduction
Organic linker-based luminescent metal-organic frameworks (LMOFs) 1,2 have shown great potential for applications in various areas, such as solid-state-lighting, 3-7 sensing, [8][9][10][11][12] and bioimaging 13,14 due to their highly tunable structures and compositions by varying organic linkers and metal nodes and via crystal engineering under various conditions. To tune the emission properties of LMOFs in a broad energy region (including the visible light and near-infrared range) and to study the structural effect on their emission behaviors, it is essential to build design principles based on the structureproperty relationship, which will facilitate the customized synthesis and target-specic applications of LMOFs. However, tuning the emission behaviors of organic linker based LMOFs in a fully controllable manner remains a challenging task.
In our previous work, we achieved full-color emissive LMOFs using 2,1,3-benzothiadiazole and its derivative-based dicarboxylic acids and tetratopic carboxylic acids as organic linkers. 15,16 In these donor-acceptor-donor (D-A-D) based linkers, the emission tunability relies on the changeable electron-withdrawing capacity of acceptor groups. While these acceptors have been used in different D-A-D linkers to prepare LMOFs for various applications, 10,17-23 a systematic study to tune the emission properties of D-A-D linker based LMOFs and to develop a related design principle is still lacking.
For D-A-D type molecular linkers, three common strategies can be utilized to tune the emission behavior of LMOFs: (1) tuning the electron density of the acceptor groups; (2) tuning the electron density of the donor groups 15,16 and (3) tuning the bridging units between donor and acceptor groups 24,25 (Scheme 1). There are usually two approaches to changing the electron density of the donor groups: (2.1) modication of the original donor, such as addition of functional groups (i.e. NH 2 , OH, and CF 3 ) with strong electron donating or electron withdrawing capacity; (2.2) use of totally different donor groups, for which metals may link to different binding sites, e.g. to oxygen when using carboxylic acid-based donor groups or nitrogen when using pyridine or azole-based donor groups (Scheme 1). Note that changing the electron density of acceptor groups usually has little or no effect on the topology of the resultant MOFs. 15,16 However, changing the electron density of donor groups, which coordinate to the metal-containing clusters or ions to eventually form MOFs, oen leads to structure variations.
Based on these strategies, we carry out a systematic study in the present work to tune the emission properties of D-A-D type linker based LMOFs by tailoring the acceptor groups, donor groups and the bridging units, where 2,1,3-benzothiadiazole and its derivative are utilized as the acceptor groups. Among the 23 organic linkers used in this study, 13 are newly synthesized or used to prepare MOFs for the rst time, and 8 new MOF structures are obtained. For MOFs with the same structure as UiO-68, we name them UiO-68-L, in which L is the abbreviation of the organic linker; and for MOFs with new structures, we name them HIAM-N (HIAM ¼ Hoffmann Institute of Advanced Materials), where N is the number for each new MOF. This relatively large database allows us to build an emission library with a broad range of emission energies, including both visible and near infrared (NIR) regions.
By tuning the acceptor groups, the emission energies of the resultant UiO-68-type LMOFs cover the entire visible light range Scheme 1 The schematic diagram illustrating the structure and multiple strategies to tune the emission properties of D-A-D type organic linkers using 2,1,3-benzothiadiazole and its derivative as acceptors. (417, 445, 470, 520, 530, 545, 585 to 637 nm). These results further demonstrate that highly tunable emissions are achievable for D-A-D type organic linker based LMOFs by simply changing 2,1,3-benzothiadiazole and its derivative based acceptor groups.

Modifying original donor groups
As mentioned earlier, two strategies can be used to tune the donor groups: (2.1) Modifying original donor groups, where the linker-metal bond model remains intact; (2.2) Employing different types of donor groups, in which new linker-metal bonds may form, leading to different structures.
To conrm that the modication of original donor groups indeed has the capacity to tune the emission of LMOFs, four linkers, [1,2,5]thiadiazole-4,7-diyl)bis(3-aminobenzoic acid) (BTAB), were designed and synthesized with single-site modication on the donor groups ( Fig. 2a), where the order of the electron donating capacity is NH 2 > OH > OCH 3 > CF 3 . As expected, four UiO-68 type MOFs, UiO-68-BTTB, UiO-68-BTMB, UiO-68-BTHB and UiO-68-BTAB, were obtained, conrmed by the PXRD analysis (Fig. 2b). The solid-state emission of these LMOFs also covers the whole visible light spectrum from blue to red with the peak maximum at 438 nm, 520 nm, 575 nm and 650 nm for UiO-68-BTTB, UiO-68-BTMB, UiO-68-BTHB and UiO-68-BTAB, respectively (Fig. 2c). The corresponding PLQYs are 40.8%, 30.6%, 1.0% and 0.1%. It should be noted that a remarkable blue-shi, compared with UiO-68-BTMB, was observed when adding a group (such as CF 3 ) with strong electron withdrawing capacity and a signicant bathochromic shi was realized when using strong electron donating groups (i.e. OH and NH 2 ). 26 These results indicate that single-site modication of original donor groups is indeed a useful strategy to tune the emission properties of the LMOFs without changing the crystal structure.

Tuning donor groups with different types of M-L bonds
Compared with carboxylic acid-based LMOFs, the utilization of donor groups with various metal-linker bonds will not only increase the structural diversity, but also introduce new or unprecedented properties to the resultant LMOFs. Herein, we prepared two series of organic linkers using pyridine and pyrazolate as the donor groups and investigated their effects on the emission behavior of the resultant LMOFs.
When pyridine was chosen as the donor group, three linkers, , were designed and synthesized (Fig. 3a). The emission wavelengths of PBT, PNT and PNS are 452 nm, 567 nm and 611 nm, respectively. Compared to their carboxylate counter parts BTMB (500 nm), NTMB (566 nm) and NSMB (610 nm) with the same acceptor groups, a signicant blue- shi was observed for PBT ( Fig. S2 †), for which the acceptor group has a relative weak electron withdrawing capacity, like benzo[c] [1,2,5]thiadiazole. However, for acceptor groups with strong electron withdrawing capacity, almost no energy shi was observed between NTMB and PNT, and NSMB and PNS.
The pillar-layered structure of HIAM-3001 has been reported to have a pcu topology and belongs to an orthorhombic crystal system with a Pbca space group. 19 As shown in Fig. 3b, two equivalent Zn(II) are bridged by four carboxylate groups from four BDC ligands to form a binuclear "paddle-wheel" Zn 2 (COO) 4 . These 6-c second building units (SBUs) are connected by the BDC ligands to give (4, 4) layers which are further extended by PBT ligands to form the 3D network with a pcu   Fig. 3c and S3, † the SBUs are changed from the 6-c "paddlewheel" (Zn 2 (m 1,3 -COO) 4  The PXRD patterns of the as-synthesized LMOFs exhibit excellent agreement with the simulated ones, indicating the high purity of the obtained bulk samples (Fig. 3d). The peak maxima of solid-state emission of HIAM-3001, HIAM-3002 and HIAM-3003 are 499 nm, 592 nm and 678 nm, respectively (Fig. 3e), with PLQYs of 2.8%, 0.6% and 1.0% under 365 nm excitation. The lower PLQY could be attributed to the fact that the p-p stacking is much stronger in pillar-layered structures, which will cause severe non-radiative decay. The gradual red-shi was also observed in the UV-vis absorption spectra from HIAM-3001 to HIAM-3003 (Fig. 3f). Compared with UiO-68 type MOFs, HIAM-3001, 3002 and 3003 show high stability in aqueous solutions aer treatment at pH ¼ 2 to 12 for one day, conrmed by the nearly identical PXRD patterns ( Fig. S4 and S5 †). HIAM-300X (X ¼ 1-3) also exhibits high resistance to heat and stability up to 350 C for HIAM-3001 and 400 C for HIAM-3002 and HIAM-3003, respectively (Fig. S6 †). The above results conrmed our hypothesis that changing the donor group to form a different metal-linker bond model will not only give rise to tunable emission behavior, but also contribute to structural diversity of the resultant LMOFs.
The synthesis conditions for pyrazolate-based HIAM-300X (X ¼ 4 for DPBT, X ¼ 5 for DPBS and X ¼ 6 for DDPBT) are similar to those used to synthesize HIAM-3001 but without addition of 1,4-dicarboxybenzene. Single-crystal X-ray diffraction analysis reveals that HIAM-3004 crystallizes in the tetragonal crystal system with an I4 1 space group (Fig. 4b and S8 †). The Zn(II) cation is fully coordinated in a tetrahedral geometry with four nitrogen atoms from four DPBT ligands. Each DPBT ligand is fully coordinated with four Zn(II) cations, in which each pyrazolate group of the ligand connects two adjacent Zn(II) cations. The alternative connection of Zn atoms and pyrazolate groups results in an innite 4 1 helical chain along the c axis (Fig. S9 †). These screw chains are further extended by the DPBT ligand to give the 3D framework with 3D channels. Strong p-p interactions are found between the benzo[c] [1,2,5]thiadiazole rings in HIAM-3004 (centroid-to-centroid distance is 3.5822(3)Å). An identical crystal structure was formed when DPBS was employed as the luminescent linker. It should be noted that a similar MOF to HIAM-3004 was reported for photocatalytic aerobic oxidation when we prepared our manuscript, which further demonstrates the promising applications of these MOFs. 34 A totally different crystal structure was obtained when the acceptor group was changed from benzo[c] [1,2,5]thiadiazole to 5,6-dimethylbenzo[c] [1,2,5]thiadiazole for pyrazolate-based linkers. For HIAM-3006, a similar connection model to HIAM-3004 was observed where the Zn(II) cation is fully coordinated in a tetrahedral geometry with four nitrogen atoms from four DDPBT ligands. Each DDPBT ligand is fully coordinated with four Zn(II) cations, in which each pyrazolate group of the ligand connects two adjacent Zn(II) cations to give a 1D Zn chain along the a-axis. The Zn chains are further extended by the DDPBT ligand to yield a 3D framework with 1D channels along the aaxis ( Fig. 4c and S10 †).
By comparing the emission properties of UiO-68-BTMB (520 nm), HIAM-3001 (499 nm) and HIAM-3004 (563 nm), it is clear that introducing a pyridine group as the donor will induce a blue-shi, while a red-shi is observed when using pyrazolate as the donor. More importantly, with the tunable donor groups, abundant structural diversity can be realized, which might lead to new properties and applications.

Tuning bridging units
It has been reported previously that different bridging groups show signicant effects on the light absorption and emission behavior of D-A-D type compounds. For example, an emission shi from 610 nm, 622 nm to 647 nm was observed upon adding an ethynyl or a vinyl group between the donor and acceptor. 24,25 Incorporating thiophene groups into uorophores could also increase the light absorption and emission. [35][36][37] In our previous work, we found that when the acceptor groups change from benzo[c] [1,2,5]thiadiazole to naphtho [2,3-c] [1,2,5] thiadiazole, the emission shis from 520 nm to 585 nm. 15 According to these results, we believe that organic linkers with a signicant red-shi might be synthesized if these two features are combined in one structure. On the other hand, benzo[c] [1,2,5]thiadiazole and naphtho [2,3-c] [1,2,5]thiadiazole based carboxylic compounds have been utilized to prepare LMOFs with tunable emissions, and thus the biggest challenge is how to synthesize carboxylic compounds with a bathochromic shi in their emissions, which may provide the opportunity to obtain organic-linker-based NIR LMOFs.
To prove our hypothesis, six organic linkers were utilized in this section (Fig. 5a). Two of them have been reported, 4,4 0 -(benzo[c] [1,2,5]thiadiazole-4,7-diyl)dibenzoic acid (BTBA) 10 [1,2,5]thiadiazole-4,7-diyl)bis(thiophene-5,2-diyl))dibenzoic acid (BTTD). BTBA, BTTBA and BTEBA were chosen to conrm the effect of bridging groups of ethynyl and vinyl. NTEBA was designed to achieve NIR emission. BTTB and BTTD were prepared to investigate the effect of thiophene and the substitution sites on the emission behaviors. The molecular orbitals of BTBA, BTTBA, BTEBA and NTEBA were calculated using density functional theory (DFT). As depicted in Fig. 5b, a signicant increase in the highest occupied molecular orbital (HOMO) energies was observed along with a decrease in the lowest unoccupied molecular orbital (LUMO) energies when an ethynyl or a vinyl group was added. As a result, the HOMO-LUMO energy gap decreased from 3.399 eV to 2.792 eV and 2.602 eV for BTBA, BTTBA and BTEBA, respectively, indicating that the bridging groups indeed can be used to effectively tune the electronic structures. More importantly, a further decrease was obtained from 2.602 eV to 1.987 eV, when the acceptor group was changed from benzo[c] [1,2,5]thiadiazole to naphtho [2,3-c] [1,2,5]thiadiazole.
The UV-vis absorption and steady-state emission spectra of the four organic linkers in DMF solution were then measured and plotted in Fig. 5c. As expected, the emission energy varies from blue (BTBA) to NIR (NTEBA), which is consistent with the red-shis of the corresponding absorption spectra. The maximum emission peaks appear at 480, 500, 565 and 690 nm for BTBA, BTTBA, BTEBA and NTEBA, respectively. The corresponding colors of the samples under daylight and 365 nm excitation are shown in Fig. 5d. A closer look at the molecular structures reveals an interesting structure-emission correlation. (i) Compared with BTBA, when a vinyl group is added between the donor and acceptor groups, an 85 nm bathochromic shi was observed for BTEBA. The remarkable effect of vinyl groups on the emission behavior could also be proven by comparison of the emission wavelength of NTEBA and the compound without vinyl groups, 4,4 0 -(naphtho [2,3-c] [1,2,5]thiadiazole-4,9diyl)dibenzoic acid (NTB), in which a bathochromic-shi of 100 nm was recorded aer the addition of vinyl groups to NTB to form NTEBA. (ii) When acceptors were changed from benzo[c] [1,2,5]thiadiazole to naphtho [2,3-c] [1,2,5]thiadiazole, a 125 nm red-shi was realized. These results demonstrate that the combination of these two strategies together indeed offers a powerful approach to tune the emission properties and to achieve dicarboxylic acid-based NIR emissive organic linkers.
The single crystals of BTBA and BTTBA based Zr-MOFs (1 and Zr-L7) were synthesized according to literature procedures (Fig. S13 †). 10,22 A typical synthesis of BTEBA and NTEBA based Zr-MOFs, HIAM-400X (HIAM ¼ Hoffmann Institute of Advanced Materials; 40 ¼ zirconium; X ¼ 5 for BETBA and X ¼ 6 for NTEBA) is as follows: a 5 mL vial containing 54.0 mg L-proline and 63.0 mL was placed in a preheated oven at 100 C for 30 minutes; aer cooling down to room temperature, 3 mL DMF, ZrCl 4 (22.0 mg, 0.094 mmol), and an organic linker (0.094 mmol) were added to the vial, which was placed in a preheated oven at 120 C for 48 hours. Orange (HIAM-4005) and dark red (HIAM-4006) octagon-shaped single crystals were obtained (Fig. 5e). Single-crystal X-ray diffraction analysis indicates that HIAM-4005 and HIAM-4006 adopt the typical cubic structure and crystallize in the space group Fd 3m. The structure consists of two sets of independent and mutually interpenetrating UiO-type frameworks ( Fig. 5f and g). Therefore, the structure and connectivity of the SBUs in HIAM-4006 are the same as in UiO-type MOFs. The two new LMOFs show nearly identical powder X-ray diffraction (PXRD) patterns to the simulated one ( Fig. 5h), indicating that these LMOFs made of linkers with different acceptor groups belong to the same isoreticular series.
The solid-state photoluminescence and UV-vis absorption spectra were measured. As shown in Fig. 5i, the emission maxima at 522 nm, 560 nm, 607 nm and 747 nm were recorded for 1, Zr-L7, HIAM-4005 and HIAM-4006 with the corresponding PLQYs of 7.8%, 5.6%, 8.8% and 0.5% under 365 nm excitation, respectively. The NIR emissive LMOF (HIAM-4006) was obtained by the combination of an acceptor with strong electron withdrawing capacity and an optimized bridging unit. A gradual red-shi was also observed in the absorption spectra from 1 to HIAM-4006, which is consistent with their corresponding emission energies (Fig. 5j). The lowest energy absorption edge is close to 750 nm, which may be suitable for applications in photocatalytic related areas, such as photocatalytic hydrogen generation and carbon dioxide reduction.
In addition to ethynyl and vinyl bridging units, thiophene was also utilized to link the donor and acceptor groups. Compared with BTBA (480 nm), BTTBA (500 nm) and BTETA (565 nm), the maximum emission peaks of BTTB and BTTD are 553 nm and 638 nm in DMF solution under 365 nm excitation, respectively (Fig. S14 †). This result indicates that (i) thiophene is a useful bridging unit to achieve the bathochromic-shi; and (ii) the substitution position has a signicant effect on the emission properties of the resultant compounds. However, it is very difficult to obtain the crystal structure of the corresponding LMOFs, which might be attributed to the distorted molecular structures.
Based on the aforementioned results, it is clear that the emission properties of D-A-D type organic linker-based LMOFs can be systematically tuned by changing the acceptor groups, modication of the original donor groups, using different donor groups and choosing various bridging units. Therefore, an emission library can thus be built by applying different strategies described in this work (Fig. 6).

Conclusion
In conclusion, four strategies have been used to methodically tune the emission behaviors of D-A-D type organic linkerbased LMOFs, from which a large emission library can be built in a systematic manner. By precisely controlling the acceptor groups, donor groups and the bridging units, emissions of the resultant LMOFs covering the entire visible light spectrum as well as the NIR region can be achieved with abundant structural diversity. A large number of organic linkers can be designed and synthesized with varying emission behaviors from deep blue to the NIR range by gradually decreasing the electron density of acceptor groups or increasing the electron density of donor groups. This work may not only serve as a toolbox to facilitate the development of design principles for the rational design of organic linkers and customized synthesis of target-specic LMOFs but also provide a useful platform to explore NIR emissive LMOFs for biosensing and bioimaging applications.

Data availability
All crystallographic data have been deposited in the CSD. No other data is present.