Introduction

One key essence of reticular chemistry is modular designability, where molecular building blocks can be linked into crystalline frameworks in rational and predictable manners1,2. Such modular designability requires that molecular building blocks with suitable linkages can be combined almost arbitrarily to give crystalline frameworks that form a chemical space. For metal-organic frameworks (MOFs), such combinatorial chemical spaces construction has been observed for a rather large set of linkers and metal building units, which provides an enormous library of MOFs with vast structural diversities. In stark contrast to the great structural diversities of crystalline frameworks, only a few of them can be vitrified through melt-quenching or perturbative methods3,4,5,6,7,8,9,10,11,12. Thus, expansion of reticular chemistry into the regime of glassy materials requires establishing molecular network-forming glasses chemical spaces with modular designability and systematic understanding of their glass-forming abilities.

Glasses can be defined as nonequilibrium and non-crystalline condensed matters that can undergo glass transition, which is a continuous solid-to-liquid transition characterized by heat capacity jump and rheological behaviors due to the emergence of collective configurational motions13,14. However, achieving such configurational degree-of-freedom in covalently linked networks are challenging due to the constraint posed by strong and rigid covalent linkages between molecular building blocks. The conventional melt-quenching method essentially uses high temperature to break these covalent linkages and achieve liquification, which often causes the thermally labile organic linker to decompose and thus has seen very limited success in producing network-forming glasses from crystalline frameworks incorporating organic building blocks3.

Although the linkages between building blocks are strong and directional, they can also be reversible and dynamic. Harnessing the intrinsic reversibility and dynamics of these bonds represent a viable method to achieve covalently linked networks with suitable connectivity and thus preserve configurational degree of freedom necessary for a glass transition. Introducing monodentate modulators represent a generalizable approach to facilitate linkage reversibility through modulator-linker exchange15,16,17. The competition between linkers and modulators can result in missing-linker defects that reduces network connectivity, which also alleviates the constraint on configurational motion posed by the covalent linkage18,19,20. Increasing linker flexibility is also an intuitive way to promote network motion, which may add to the degree-of-freedom provided by network modulator to give collective configurational motion necessary for glass transition.

Alkoxide linked networks are promising candidates for constructing chemical spaces of molecular network-forming glasses, where the charged alkoxide linkage is adequate to give high thermal stability and its non-chelating feature can enable facile ligand exchange. Although several examples of monoliths and glasses are known to be made of metal-oxo clusters and linkers with hydroxyl functional groups21,22, the systematic understandings necessary for modular tunability and rational design has yet to be achieved. Specifically, a potentially generalizable method to produce glassy alkoxide networks involves evaporating alcohol solvents from a solution of the node and linker. As the alcohol solvents also play the role of network modulator, its removal would cause increase in network connectivity and viscosity. Comprehensive understanding of such continuous evolution from solutions to viscous liquids and glasses is vital for achieving processability and rational synthesis for these glasses, which requires elucidating the relationship between the composition of the node-linker-modulator tertiary mixtures and their glass transition behavior and viscosity. To achieve modular design, it is also important to expand the types of metal nodes that can be incorporated into the alkoxide network-forming glasses and demonstrate their effect on framework properties.

In this report, we show that node-linker-modulator represents a generic formula to construct glasses and liquids based on alkoxide linkages, where nodes and linkers with suitable flexibility can form covalently linked networks and the modulator would affect configurational motion through reduction in network connectivity and modulating noncovalent interactions between building blocks. The dynamic exchange between alcohol-based linker and monodentate alcohol modulators enables a generic evaporative synthetic method to produce these liquids and glasses from solutions of volatile alcohol solvents. We demonstrate such chemical space construction with titanium- and zirconium- oxo cluster nodes, multi-dentate alcohol linkers with various flexibility and monodentate alcohol modulators with various molar ratio (Supplementary Table 1). Systematic compositional, rheological calorimetric, spectroscopic and total scattering studies are carried out to elucidate their structure and properties. The node-linker-modulator strategy is then applied to networking forming glasses with organic composition to produce glassy borate ester networks, which echoes the synthesis of first covalent organic framework using the boronic ester linkage23. The modular tunability achieved in the node-linker-modulator chemical spaces enables the incorporation of fluorescent molecular motifs into network-forming glasses, which is then incorporated in a proof-of-concept electroluminescent device. We believe this research would expand the library of molecular network-forming glasses and represent an important step to expand reticular chemistry into glassy materials.

Result and discussion

Although reticular design of crystalline frameworks and glasses all targeted networks linked with strong bonds, their design strategy is different: crystalline frameworks employ rigid building blocks with well-defined connectivity that can direct the formation of ordered structures; glassy networks would prefer flexible building blocks and versatile connectivity that can increase configurational entropy. Thus, we chose a series of titanium alkoxide networks with flexible linkers as the model system to study the effect of modulator in the node-linker-modulator chemical space, which incorporates in situ formed titanium-oxo cluster as the metal nodes, 2,2’,2”-((ethane-1,1,1-triyltris(benzene-4,1-diyl))tris(oxy))tris(ethan-1-ol) (TB) as the linker and 3,4,5-trimethoxybenzyl alcohol (TMBA) as modulator (Fig. 1a, Supplementary Figs. 1–6): (1) titanium-oxo clusters produced by heating titanium isopropoxide (TTIP) in alcohol solvents have high solubility, multiple coordination sites for alcohol linkers and are known to undergo facile coordinative ligand exchange24; (2) TB linker is highly flexible and has alcohol functional groups that can form dynamic titanium-alkoxide bonds with titanium-oxo clusters and its tri-dentate rigid core would facilitate network formation and preservation of pores (3) TMBA modulator has the alcohol functional group to coordinatively exchange with the TB linker and can dissolve TB and titanium-oxo cluster without phase separation. TMBA also has high boiling point and would not evaporate at room temperature, thus allowing the construction of node-linker-modulator tertiary systems with well-defined compositions.

Fig. 1: The glass transition and rheological measurements for the Ti-TB0.67TMBAx series.
Fig. 1: The glass transition and rheological measurements for the Ti-TB0.67TMBAx series.
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a The chemical structure of Ti16O16(OEt)32 cluster, TB and TMBA that constitute the Ti-TB0.67TMBAx glasses. Titanium, oxygen and carbon atoms are represented as blue, red and black spheres, respectively. b Structure model of Ti-TB0.67M1.33 network reflecting its connectivity (M denotes modulator), the bridging TBs are highlighted with orange color. c Pair-distribution function of the Ti-TB0.67TMBA1.33 compared with the simulated pattern using the structure model of Ti-TB0.67M1.33. d The exponential viscosity increases of Ti-TB0.67TMBAx series. e Storage modulus and loss modulus of Ti-TB0.67TMBAx (x = 4, 3, 2) showing the change from viscous liquid to elastic solids between Ti-TB0.67TMBA3 and Ti-TB0.67TMBA2. f Angell Plot of Ti-TB0.67TMBAx (x = 4, 3, 2). g Tangent delta from dynamic mechanical analysis measured at 1 Hz showing softening near Tg for Ti-TB0.67TMBAx (x = 6, 4, 2). h DSC curve of the Ti-TB0.67TMBAx series showing the increase of Tg and broadening of glass transition with decreased amount of TMBA. i Heat capacity jump associated with glass transition for the Ti-TB0.67TMBAx series. For d and i, the samples are Ti-TB0.67TMBAx (x = 18, 8, 6, 4, 3, 2) in the order of increased OHTB/(OHTB + OHTMBA) value, which is also listed in Supplementary Table 1.

The Ti-TB-TMBA series can be made by heating an ethanol solution of TTIP, TB and TMBA to reflux for overnight followed by evaporating ethanol solvent at elevated temperature (Methods). Under the synthetic condition, TTIP is known to form titanium-oxo clusters through partial hydrolysis, which often have a distribution of size and composition at a given condition25. The molar ratio between Ti and TB is initially chosen to be 3:2, giving a titanium-to-alkoxide ratio of 1:2, same as a prominent Ti16O16(OEt)32 cluster, and the amount of TMBA can be varied systematically to give a series of Ti-TB0.67TMBAx (x = 18–0.22) with x denoting the molar ratio of TMBA compared to titanium. The Ti-TB0.67TMBAx materials have the appearance of liquid (x = 18, 8, 6, 4), viscoelastic material (x = 3, 2, 1.33), and glassy solid (x = 0.86, 0.5, 0.22) with different amount of modulator (Supplementary Fig. 1), which all show non-crystalline feature in X-ray diffraction (XRD, Supplementary Fig. 2). The molar ratio of TB and TMBA can be experimentally verified through digested NMR (Supplementary Figs. 3, 4, Supplementary Table 2), and the titanium contents are determined by inductively coupled plasma mass spectrometry (ICP-MS, Supplementary Text 1). The Ti-TB-TMBA samples are also characterized by infrared spectroscopy (IR, Supplementary Fig. 5), which shows the preservation of TB linker. Notably, with low content of TMBA (x smaller than 4), residue ethanol is found in the products by digested NMR even after prolonged heating at 85 °C (Supplementary Fig. 3), which indicates these ethanol molecules, along with TMBA and TB, are deprotonated and exist as ethoxide ligand coordinated to the titanium-oxo cluster. Consequently, the Ti-TB0.67TMBAx series all have thermal stability above 120 °C as evidenced by TGA (Supplementary Fig. 6).

The statistically average connectivity of Ti-TB0.67TMBAx series can be analyzed based on the stoichiometry of the titanium node and the linkers (Supplementary Text 2). Notably, various titanium oxo clusters can be produced by partial hydrolysis of simple titanium alkoxides depending on the reaction conditions and the alkoxide ligands25. Under the synthetic conditions of Ti-TB-TMBA where mixed alcohols are used, there would be a complex mixture of titanium oxo clusters in the final product, which would have a distribution in size and structure. The Ti16O16(OEt)32 cluster is used as the representative node structure for this analysis, considering its synthetic conditions (i.e. temperature, solvent) are similar to that of Ti16O16(OEt)32. In addition, most known titanium oxo clusters produced in relevant conditions have similar structure features and stoichiometry as the Ti16O16(OEt)32 cluster. However, we note that the following analysis based on Ti16 cluster is only meant to convey a semi-quantitative picture of how network connectivity changes with the amount of modulator. While the analysis shows certain consistency between structure speculations and physical characterizations, due to the inherent complexity of the materials and uncertainty associated with the detailed structure of inorganic node, molecularly precise chemical structures or rigorously quantitative analysis on network connectivity are not feasible.

For simplicity, here after we denote the Ti16O16(OEt)32 cluster as Ti16 cluster as an abbreviation. For the Ti16 cluster, there are two alkoxide ligands per titanium atom. With excess amount of TMBA (x large than 4), on average the TB has less than one hydroxyl group coordinated to the Ti16 cluster, giving a TMBA solution of Ti16 clusters and non-coordinating TB, consistent with their liquid like appearance (Supplementary Text 2). Decreasing the amount of TMBA would have TB coordinate to Ti16 cluster, initially giving TB and TMBA coordinated Ti16 clusters that form supramolecular assembly (Ti-TB0.67TMBA3). Further reducing TMBA to x lower than 2 would allow substantial amount of TB to bridge Ti16 clusters, which would give oligomer, linear coordination polymer and eventually network structure, corresponding to the change of physical form from viscous liquid to viscoelastic solid and brittle monoliths.

To give a chemically intuitive picture of Ti-TB0.67Mx series (M denotes modulator), Polymatic program is used to model their structures in a supercell. In a preliminary analysis, 5 Ti16 clusters and 50 TB linkers are put in a cubic cell 3.4 nm in length (Fig. 1b), and the presence of modulator can be accounted for by randomly capping a given number of coordinative sites while allowing the remaining sites to link with TB. The structure modeling for Ti-TB0.67M1.33 give a network oligomer with 4 TB bridging 5 Ti16 clusters and the rest of TB molecules coordinating with one hydroxyl group (Fig. 1b, Supplementary Text 3, Supplementary Fig. 7). As the modulator content of Ti-TB0.67M1.33 is in the middle of this series, this structure model is a corroborating evidence that the connectivity of the Ti-TB0.67M series can span from coordination oligomer to coordination network. Comparison between pair-distribution function (PDF) simulated from the structure model and the experimental pattern shows qualitative consistency (Fig. 1c), where the short-range features can be allocated to atom pairs in the Ti16 cluster and organic linker (Supplementary Fig. 8). Notably, due to inherent disordering in the structure, the PDF mostly reflects short-range structure corresponding to C-C, C-O, Ti-O bonds, phenyl rings and Ti-O-Ti motifs of the titanium-oxo cluster. Conversely, information on the intermediate size regime, such as the overall structure of the titanium oxo cluster extending several Ti-O-Ti motifs and the connection mode of TB linker (bridging/dangling), are hidden in the broad features of the pattern and cannot be directly obtained from the PDF analysis. Similarly, long-range structure regime reflecting the network connectivity is also unavailable from total scattering analysis. Nevertheless, based on well-known chemical knowledge on titanium alkoxide chemistry, the combination of above-mentioned stoichiometric analysis, structure modeling and short-range ordering obtained from PDF provides a plausible picture for the tunable network connectivity of Ti-TB0.67TMBAx, which show consistency with physical property characterizations and compositional analysis in the sections below.

Rheology measurement is employed to quantitatively study the change of physical form of Ti-TB0.67TMBAx from liquid to viscoelastic solid with decreasing TMBA (x from 18 to 2), which can be viewed as TMBA solution of TB and titanium-oxo cluster with increasing concentration. the viscosity of Ti-TB0.67TMBAx series increase exponentially with decreased amount of TMBA, indicating the percolation of Ti-TB coordination polymer with increasing molecular weight at higher concentration (Fig. 1d). In comparison, the viscosity of regular polymer solution would increase in a polynomial manner with increased polymer concentration26,27. The network formation can be more clearly shown by its transition from viscous liquid to elastic solid as evidenced by the relative value of storage and loss modulus of Ti-TB0.67TMBAx with x changing from 4 to 3 and 2. For Ti-TB0.67TMBA2 (Fig. 1e), its storage modulus is larger than loss modulus, indicating it behaves as elastic solid. The frequency dependent rheology measurement at room temperature finds that the Ti-TB0.67TMBA3 and Ti-TB0.67TMBA2 samples show shear thinning (Supplementary Fig. 9), which is often allocated to polymer chain entanglement and orientation change, thus indicating the presence of polymeric coordinative networks. Although the rheology data does not provide the full temperature range to quantitatively extract fragility, from the slope of the Angell plot (Fig. 1f) it can be seen that higher linker/modulator ratio tend to give less fragile liquid behavior. Considering that many network-forming glasses such as SiO2 and low-density liquid phase of ZIF-4 show strong liquid behavior28, the decrease of Angell pot slope for Ti-TB0.67TMBAx with smaller x value may be viewed as a corroborating evidence supporting the increase in network connectivity with lower modulator content. For Ti-TB0.67TMBAx networks with x lower than 2, their elasticity become rather low and are thus not suitable for rheological measurement. From a practical perspective, these observation in rheological measurements convey information on the initial stage of continuous transformation from solution to glass triggered by modulator evaporation in the synthesis of glassy alkoxides with volatile modulators, where the viscosity change strongly affects the processability and shaping of the final product.

The glass transition behavior of the Ti-TB-TMBA series is studied by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) to track their evolution from solution to supramolecular assembly, coordination polymer and glassy network with decreased TMBA (Fig. 1g–i, Supplementary Figs. 10–13). With increased network ratio, the glass transition temperature of Ti-TB0.67TMBAx gradually increase from the Tg of TMBA (i.e. −42 °C, Supplementary Fig. 12) to −16 °C for Ti-TB0.67TMBA2 accompanied by monotonic decrease of the heat capacity jump associated with glass transition (Fig. 1h, i). The trend in heat capacity jump is an indication that the configurational degree-of-freedom of the corresponding super-cooled liquid becomes larger when the amount of TMBA is increased. DMA shows material softening at Tg, which indicates the configurational motion associated with glass transition involves the entirety of the tertiary system and not solely originate from the motion of TMBA (Fig. 1g). For Ti-TB0.67TMBAx (x < 2), these samples also do not possess adequate mechanical strength for direct DMA measurement but are also not suitable for powder measurement kit used for samples with higher TMBA content. The glass transitions of Ti-TB0.67TMBAx (x < 2) start to show significant broadening in the DSC curve that prevents reliable extraction of delta Cp values (Supplementary Fig. 13). Due to the broadening, although the Tg values can still be nominally extracted from the DSC curve using inflection point method, these values are used only for comparison within the Ti-TB-modulator series to show general trends. Such broadenings in DSC curve are typically observed for asymmetric mixtures composed of materials with distinctly different Tg and indicate the presence of dynamic decoupling and heterogeneity driven by strong concentration fluctuation29. In the Ti-TB0.67TMBAx (x < 2), such dynamic heterogeneity would arise from the presence of coordination networks with different connectivity and conformations. Notably, for the Ti-TB0.67TMBAx series, the viscosity increases to solidification and the broadening of DSC curve both occur at around x = 2, which is the point where the connectivity analysis starts to give substantial bridging of TB linker and form coordination polymer (Supplementary Text 2), thus providing a rather consistent picture of the chemical space.

As standard DSC up scan curve give broad features, the glass transition of Ti-TB0.67TMBAx (x < 2) is further elucidated by measuring the enthalpy relaxation, which is a Cp overshoot in the glass transition region during heating that reflects the recovery of the enthalpy lost during prior annealing13. Theoretically, the enhancement of this overshoot is the direct consequence of the nonexponentiality and nonlinearity of the glass relaxation process30, and the extent of the overshoot increases with extending the prior annealing time around Tg. For enthalpy relaxation measurements, the Ti-TB0.67TMBA0.5 and Ti-TB0.67TMBA0.22 are annealed for an hour at different temperatures near their Tg. The subsequent DSC up scan (exo up) show enthalpy relaxation as a depression with convex shape, which correspond to an overshoot compared to the broad feature in standard DSC up scan and confirm presence of glass transition (Supplementary Figs. 14, 15). Notably, for different annealing temperatures, the overshoot peaks corresponding to enthalpy relaxations appear at slightly different temperatures, which is also a manifestation of dynamic heterogeneity. For Ti-TB0.67TMBA0.5, annealing at 30 °C for an hour prior to up scan give enthalpy relaxation peak at around 45 °C. For Ti-TB0.67TMBA0.22, annealing at 45 °C for an hour prior to up scan give enthalpy relaxation peak at around 70 °C. These enthalpy relaxation measurements provide important evidence supporting the presence of glass transitions for Ti-TB0.67TMBAx (x < 2) and the increase in Tg with decreased TMBA content, consistent with increase in coordinative network connectivity.

The presence of coordinative network for Ti-TB-TMBA series with low TMBA content can also be quantitatively shown by comprehensive composition analysis (Supplementary Text 1). For Ti-TB0.67TMBA0.22, its empirical formula is determined by a combination of elemental analysis, digested NMR and ICP analysis to be Ti16O16TB10.6TMBA3.5(OEt)7.4, where TB to modulator (ethoxide and TMBA) ratio reaches 1:1. Consequently, from the perspective of network connectivity, Ti-TB0.67TMBA0.22 correspond to Ti-TB0.67M0.67 (M includes TMBA and ethoxide) and would have higher connectivity than Ti-TB0.67M1.33, which is shown to have a oligomeric coordination network (Supplementary Text 3). Thus Ti-TB0.67TMBA0.22 would have a network structure. Without TMBA, Ti-TB0.67 has white precipitate and cannot be made into transparent monolithic network. Decreasing the Ti to TB ratio to 1.2 give Ti-TB0.82 as a glassy network without TMBA, which shows high thermal stability in TGA, non-crystalline feature in XRD and enthalpy relaxation in DSC (Supplementary Figs. 16–19). Thus, we have shown that by tuning the amount of modulator, the node-linker-modulator tertiary system can give a series of glasses and liquids with different connectivity and glass transition behavior, which could be a generalizable formula for designing molecular network-forming glasses.

The modular designability of the node-linker-modulator chemical space for network-forming glass can be established by changing the molecular building blocks and studying the properties of the resulting materials to unravel general design rules and handles for tunability (Fig. 2a). Glasses incorporating titanium-oxo cluster node and different modulators and linkers are obtained in similar synthetic condition as the Ti-TB-TMBA series, which shows similarity in the short and medium range in PDF consistent with Ti-O bonds and Ti…Ti correlations of the titanium-oxo clusters (Fig. 2b, Supplementary Fig. 8). Specifically, both 3.1 and 3.8 Å atom pairs are characteristic length between two titanium atoms in Ti-O-Ti motifs of Ti16 cluster. As is stated above, the titanium oxo clusters formed by titanium alkoxide partial hydrolysis could have slight differences in the intermediate size regime beyond 4 Å25, and the flexible TB linkers would also give almost random features in this regime, thus resulting in the lack of clear features in pair distribution functions beyond 4 Å.

Fig. 2: Study of titanium alkoxide glasses incorporating TB, GDPA, PEG5 linkers and using BAMH and TMBA as modulators.
Fig. 2: Study of titanium alkoxide glasses incorporating TB, GDPA, PEG5 linkers and using BAMH and TMBA as modulators.
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a The chemical structure diagram of TB, GDPA, PEG5, BAMH and TMBA that constitute the Ti-TB0.67BAMHx, Ti-GDPA1BAMHx, Ti-PEG51TMBAx, Ti-PEG50.75TMBAx and Ti-PEG50.5TMBAx series. Titanium, oxygen and carbon atoms are represented as blue, red and black spheres, respectively. b The PDF patterns for Ti-TB0.67BAMH0.22, Ti-GDPA1 and Ti-PEG51TMBA0.22 as the representative samples for each series with highest network connectivity. The peak assignments are shown in Supplementary Fig. 8. c The trend in glass transition temperatures for Ti-TB0.67TMBAx, Ti-TB0.67BAMHx and Ti-GDPA1BAMHx showing consistent increase of Tg with reduced modulators. d DSC up scan curves for the Ti-PEG50.75TMBAx series with scan rate of 10 °C/min. e The trend in glass transition temperatures for Ti-PEG51TMBAx, Ti-PEG50.75TMBAx and Ti-PEG50.5TMBAx series showing the effect of the interplay between noncovalent interactions and coordinative network formation on Tg.

To shed light on the effect of modulator on the node-linker-modulator chemical space, a solid-state modulator methyl 4-(hydroxymethyl)benzoate (BAMH, Supplementary Fig. 20) is chosen to produce Ti-TB0.67BAMHx series (x = 4.7, 3, 2, 1.33, 0.86, 0.22), which are also characterized by digested NMR to give the relative ratio between TB, BAMH and residue ethoxide ligands corroborated by IR spectra (Supplementary Figs. 21–23, Supplementary Table 3). The samples in this series show non-crystalline features in XRD (Supplementary Fig. 24) and similar thermal stability as the Ti-TB0.67TMBAx series (Supplementary Fig. 25). Changing from the liquid modulator TMBA to a solid-state modulator would affect the energy landscape of the Ti-TB-modulator chemical space and unravel the factors affecting the glass transition. While BAMH and TB and commonly known titanium-oxo clusters are all solids at room temperature, the Ti-TB0.67BAMH4.7 and Ti-TB0.67BAMH3 have the appearance of liquids at room temperature. Ti-TB0.67BAMH4.7 shows glass transition temperature of −46 °C (Supplementary Fig. 26), which is close to 2/3 of the BAMH melting point of 50 °C. Considering the empirical rule that material Tg would be close to 2/3 of the melting point, such agreement indicates that the Tg of node-linker-modulator system with large excess of modulator is primarily determined by the noncovalent interactions of the modulators, which is consistent with the physical picture that Ti-TB0.67Mx (x > 4) can be viewed as solutions with modulator as solvent and the linker and node as solute. With lower amount of BAMH, the Tg of Ti-TB0.67BAMHx increases and start to converge with that of Ti-TB0.67TMBAx with x reaches 2, indicating the glass transition in this region is dominated by the property of the coordination network (Fig. 2c, Supplementary Fig. 26). This is also the x value where polymeric networks start to form according to the analysis above (Supplementary Text 2). Ti-TB0.67BAMH0.22 show similar enthalpy relaxation as Ti-TB0.67TMBA0.22 (Supplementary Fig. 27), and its composition analysis also indicates the presence of coordinative network (Supplementary Text 1). The consistent increase in Tg for Ti-TB0.67TMBAx and Ti-TB0.67BAMHx for x smaller than 2 indicates the presence of modulator promotes configurational motion, which echoes the evaporation induced vitrification process observed for metal alkoxides and phenolate glasses. Notably, the alkoxide ligand exchange on Ti16 cluster can take place at rather low temperature, far below the boiling point of ethanol (Supplementary Fig. 28). From a molecular perspective, the modulator affects the material energy landscape through reduction in network connectivity, maintaining free volume and modulating the noncovalent interactions between linkers and nodes.

To demonstrate the modular design of linker in the node-linker-modulator chemical space, we synthesized Ti-GDPA1BAMHx (x = 3, 1.3, 0.5, 0) series using a di-topic, diethylene glycol functionalized diphenylanthracene linker (denoted as GDPA), which also has a fluorescent aromatic core and flexible terminal motifs similar to TB. Ti-GDPA1BAMHx series are synthesized with a similar method as Ti-TB0.67BAMHx, with the ratio between titanium and linker hydroxyl groups kept at 1:2 and an ethanol/tetrahydrofuran solution mixture. The Ti-GDPA1BAMHx series are also characterized by digested NMR and IR spectra (Supplementary Figs. 29–31, Supplementary Table 4). The samples in this series show non-crystalline features in XRD (Supplementary Fig. 32) and no weight loss in TGA until 120 °C (Supplementary Fig. 33). Ti-GDPA1BAMH3 can also be viewed as a BAMH solution of titanium-oxo cluster and linker, and its Tg of −27 °C coincide with Ti-TB0.67BAMH3, which is a manifestation of the dominant role of BAMH. With increased linker-to-modulator ratio, the Tg of Ti-GDPA1BAMHx series increases (Fig. 2c, Supplementary Figs. 34, 35). Without BAMH modulator, the Ti-GDPA1 glassy network also shows broad DSC curve. With annealing temperature of 65 °C, Ti-GDPA1 show enthalpy relaxation peak above 80 °C, which is a manifestation of glass transition in this temperature regime (Supplementary Fig. 36).

To further demonstrate the modular designability of tertiary node-linker-modulator series and shed light on the how the interplay between covalent linkages and noncovalent interactions affect their properties, a more flexible di-topic linker, pentaethylene glycol (PEG5), is employed to synthesis a series of Ti-PEG5-TMBA glasses. PEG5 is a liquid at ambient temperature and has a Tg lower than −75 °C (Supplementary Fig. 37). The binary mixtures of PEG5 with TMBA have increasing Tg with higher TMBA content, which is a manifestation of noncovalent interactions between TMBA and PEG molecules (Fig. 2e, Supplementary Fig. 38). With the addition of TTIP, in situ formed titanium node can connect PEG5 into networks, where reducing TMBA modulator would cause increased network connectivity and Tg. Consequently, in Ti-PEG5-TMBA series, the decrease of TMBA modulator would have a negative effect on Tg from the perspective of noncovalent interactions and a positive effect on Tg from the perspective of increased network connectivity, and the overall effect on Tg would depend on the interplay between covalent linkage and noncovalent interactions. To study such interplay, we synthesized three series of Ti-PEG5-TMBA glasses with different titanium content: Ti-PEG51TMBAx, Ti-PEG50.75TMBAx, and Ti-PEG50.5TMBAx series, which are all characterized by digested NMR and IR spectra to confirm the ratio between linker and modulators (Supplementary Figs. 39–45, Supplementary Table 5). All samples show non-crystalline features in XRD (Supplementary Figs. 46–49) and TGA of representative samples show thermal stability up to 120 °C (Supplementary Figs. 50, 51). The PDF of a representative sample, Ti-PEG51TMBA0.22 show consistency with the titanium oxo cluster linked network (Fig. 2b, Supplementary Fig. 8). Among the three series, the Ti-PEG51TMBAx series has lowest titanium content and thus lowest connectivity with same linker-to-modulator ratio. For this series, the sample viscosity also increases exponentially with reduced amount of TMBA similar to the trend observed for Ti-TB0.67TMBAx series, showing the gradual formation of coordination network (Supplementary Fig. 52). However, the Tg of this series decrease with lower TMBA modulator content, which aligns with the TMBA-PEG5 binary system and indicates the inter-molecular interaction play relatively dominant role (Fig. 2e, Supplementary Fig. 53). For Ti-PEG50.75TMBAx series, the sample Tg first decrease then increase with reduced TMBA, indicating the increase in coordinative connectivity start to prevail against noncovalent interactions in the energy landscape at low modulator regime where coordinative cross-linking start to occur (Fig. 2d). For Ti-PEG50.5TMBAx, the Tg increase more profoundly with decreased modulator from x = 1.5, indicating the presence of higher network connectivity dominate the energy landscape (Supplementary Fig. 54). Thus, the trend of Tg of the Ti-PEG5-TMBA series has comprehensively show the interplay between inter-molecular interactions and coordinative network formation for affecting the glass transition, which is a manifestation of collective configurational motions of the network. Ti-PEG51 is chosen to be the representative modulator-free sample for full compositional analysis (Supplementary Text 1). For Ti-PEG0.75, its high connectivity lead to rather broad DSC curve, and the glass transition feature can be analyzed by enthalpy relaxation (Supplementary Fig. 55).

To expand the chemical space of alkoxide glassy networks and achieve modular designability in terms of the metal node, we synthesized a series of alkoxide glasses based on TB and GDPA linker using zirconium butoxide precursor (Fig. 3). While known zirconium alkoxide clusters with well-defined crystal structure often incorporates mixed carboxylate and alkoxide ligand, the Zr3O(OR)5(OMc)5 (OMc denotes methacrylate) type of cluster is known to form with comparably small amount of acid from zirconium butoxide precursor and can undergo facile exchange with monodentate alcohol, which is a rather plausible structure model for the metal node in these glasses. Similar to the titanium alkoxide glass series, the exact structure of the zirconium cluster in this synthesis cannot be rigorously determined. The cluster presence would have a finite distribution, and the Zr3O(OR)5(OMc)5 is only used as an over-simplified model for zirconium node to give an plausible structure that is also chemically intuitive31. The PDF pattern of Zr-GDPA1BAMH2 and Zr-TB0.67TMBA2 show Zr-O bond distance of 2.1 Å and Zr…Zr correlation of 3.5 Å consistent with the simulated PDF of the Zr3O(OR)5(OMc)5 cluster, supporting the presence of zirconium oxo clusters (Fig. 3a, b, Supplementary Fig. 56). The Zr-TB0.67TMBAx series, Zr-GDPA1 and Zr-GDPA1BAMH2 are characterized by digested NMR, IR, TGA and XRD (Supplementary Figs. 57–64, Supplementary Tables 6, 7). The Tg of Zr-TB0.67TMBAx series increase from −43 °C to −30 °C with x decrease from 8 to 2. However, it has higher tendency towards crystallization or phase separation into unknown white precipitate especially at lower TMBA content range, which prevents synthesizing samples with x < 2 and the study of glassy Zr-TB network with higher connectivity. The Zr-TB0.67TMBA2 glass, which has the highest connectivity in the series, have an empirical formula of Zr3O(TB)2.2(TMBA)6.9, consistent with a network structure (Supplementary Text 1). With GDPA linker and BAMH modulator, Zr-GDPA1BAMH2 can also be obtained as monolithic glass with well-defined Tg of 20 °C. Without non-volatile modulator, Zr-GDPA1 can be synthesized in similar way as Ti-GDPA1, which also show enthalpy relaxation in DSC heating curve at around 60 °C when annealed in the cooling run, indicating the presence of glass transition type transformation (Supplementary Fig. 65). The empirical formula for the Zr-GDPA1 glass is determined to be Zr3O(GDPA)2.65(OBu)1.5(OH)3.6, which would have most OH group of GDPA deprotonated and link to zirconium (Supplementary Text 1).

Fig. 3: The structure model, total scattering and glass transitions of Zr-TB0.67TMBAx, Zr-GDPA1 and Zr-GDPA1BAMH2.
Fig. 3: The structure model, total scattering and glass transitions of Zr-TB0.67TMBAx, Zr-GDPA1 and Zr-GDPA1BAMH2.
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a The core structure of Zr3O(OR)10 cluster as the proposed inorganic node for structure modeling of the zirconium alkoxide-based glasses and liquids. Zirconium, oxygen and carbon atoms are represented as blue, red and gray spheres, respectively. b PDF patterns for Zr-TB0.67TMBA2 and Zr-GDPA1 showing consistency with the simulated PDF pattern of the Zr3O(OR)10 cluster. c DSC curves of the Zr-TB0.67TMBAx (x = 8, 4.7, 3, 2) series with scan rate of 10 °C/min. d DSC curves of the Zr-GDPA1 and Zr-GDPA1BAMH2 with scan rate of 10 °C/min.

In the materials described above, we have demonstrated the chemical space construction for metal-organic network-forming glasses by varying the linker, metal and modulator. The node-linker-modulator formula to construct network-forming glass can be further generalized to covalent organic networks. We demonstrate such possibility by reacting GDPA with trimethyl borate to construct borate ester networks with dynamic B-O bonds as linkage (Fig. 4)32,33. Both B-GDPA1 and B-GDPA1POE0.65 (POE denotes 2-phenoxyethanol) are characterized by digested NMR, XRD, IR and TGA analysis (Supplementary Figs. 66–70, Supplementary Table 8). B-GDPA1 and B-GDPA1POE0.65 network show well-defined Tg at 15 and −2 °C, respectively. The lower Tg of B-GDPA1POE0.65 can be rationalized in similar ways as the node-linker-modulator metal-organic network-forming glasses reported above, where the POE modulator, with Tg near −60 °C, promotes configurational motion by reducing network connectivity and altering the energy landscape of noncovalent interactions. Notably, the B-GDPA1 network without non-volatile modulator can be made into uniform transparent plates (Fig. 4a) and show a much more pronounced glass transition in DSC curve compare to Ti-GDPA1 and Zr-GDPA1 (Fig. 4b). PDF analysis mainly show features of the GDPA linker, which is also consistent with a structure model that links GDPA to boron atom with the hydroxyl group (Fig. 4c, Supplementary Figs. 72–73). Combined NMR, elemental analysis and ICP give an empirical formula of B(GDPA)(EtO)0.53(OH)0.47 based on structure model of B(OR)332,33,34. The B NMR of DMSO solution of B(OMe)3 and GDPA confirms the presence of boronic esters in the B-GDPA series (Supplementary Fig. 74)35.

Fig. 4: The synthesis, glass transitions and total scattering of B-GDPA1 and B-GDPA1POE0.65.
Fig. 4: The synthesis, glass transitions and total scattering of B-GDPA1 and B-GDPA1POE0.65.
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a Chemical diagram of the B(OMe)3 and POE modulator and the digital photograph of monolithic B-GDPA1. b DSC curves of B-GDPA1 and B-GDPA1POE0.65 with scan rate of 10 °C/min. c PDF of B-GDPA1 and B-GDPA1POE0.65 showing features consistent with simulated pattern using a molecular B-GDPA3 motif (Supplementary Fig. 73).

The modular designability of the node-linker-strut chemical space provide unique opportunity to develop fluorescent molecular network-forming glasses that can be used for optoelectronic devices. As a proof-of-concept demonstration, the fluorescent GDPA linker is used to synthesis molecular network-forming glasses with Ti, Zr and B based precursors and ethanol solvent, which is then evaporated under elevated temperature. The Ti-, Zr-, B-GDPA1 show blue fluorescence with 350 nm excitation with quantum yield of 3.5%, 43% and 66%, respectively (Fig. 5a, Supplementary Fig. 75, Supplementary Table 9). The B-GDPA1 glass with high brightness is used to fabricate a proof-of-concept electroluminescence device (Fig. 5b). In this demonstration, a simple light emitting capacitor is constructed with carbon nanotube network as source contact, gold as current collector, Si as backgate and SiO2 as gate oxide36. The B-GDPA1 is dropcasted on the top CNT network as the emitter layer (Fig. 5d). With alternating gate voltage at 400 kHz, oppositely charged carriers are injected to B-GDPA1 to give electroluminescence starting from 25 V (Fig. 5c, e, f). It is worth noting that such device is a simple demonstration of the potential of B-GDPA1 in constructing electroluminescence device, and it only takes relatively routine engineering optimization to lower the operating voltage and increase brightness for these device37,38. We believe the incorporation of fluorescent linker and screening of the suitable metal node for high quantum yield successfully demonstrate the power of modular and reticular design for molecular network-forming glasses.

Fig. 5: Fluorescent glassy networks based on GDPA linker for electroluminescence devices.
Fig. 5: Fluorescent glassy networks based on GDPA linker for electroluminescence devices.
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a Photoluminescence quantum yield (PLQY) of Ti-, B- and Zr-GDPA1 networks. b Fluorescence spectra of Ti-, B- and Zr-GDPA1 networks (insert: fluorescence image under UV illumination for B-GDPA1). c Electroluminescence spectrum of B-GDPA1. d The structure of AC driven electroluminescence device using B-GDPA1 as the emitter layer. e Optical image of the electroluminescent device, with 20-micron gap between the finger electrodes. f Electroluminescence of the device incorporating B-GDPA1. For c and f, the device was operated at 400 kHz and 80 V peak-to-peak AC gate voltage. Scale bars in (e) and (f) are both 100 micrometers.

In summary, we have reported a general strategy to construct alkoxide-based molecular network-forming glass with node-linker-modulator formula, which can achieve modular designability and substantially expand the material scope for glassy materials. A series of alkoxide-based glasses can be constructed in a combinatorial manner similar to vector direct product of node, linker and modulators, thus enabling modular designability and structural diversity. The modular design has allowed for incorporation of functional molecular building blocks, which we demonstrate by designing glasses with high fluorescent quantum yield that can be used for electroluminescent devices. We believe the current work represent an important step to expand the concept of reticular material design into glasses.

Methods

Synthesis

All the samples were synthesized based on the stoichiometry in Supplementary Table 1. Ti-TB0.67TMBA18 is obtained by dissolving TB (109.5 mg, 0.25 mmol) and TMBA (1337.8 mg, 6.75 mmol) in 3 mL ethanol followed by adding TTIP (106.5 mg, 0.375 mmol) to this solution. The reaction was carried out at 80 °C for 24 h. The mixture was poured into a petri dish at 80 °C for 1 h to give the product as a liquid. Ti-TB0.67TMBAx (x = 8, 6), are synthesized using the same method with different amount of TMBA. Ti-TB0.67TMBAx (x < 6) are viscous liquids and solids, thus are synthesized with a smaller scale to ensure uniformity during evaporation (Supplementary Table S1). For these samples, the evaporation is carried out at 85 °C for 4 h to ensure the complete removal of unbounded EtOH. Ti-TB0.67BAMH, Ti-PEG5-TMBA series are synthesized with similar procedures. For Ti-GDPA1BAMHx, a mixed solvent of tetrahydrofuran/ethanol (1:1) was used to improve solubility of GDPA. For Zr based liquids and glasses are synthesized with the same procedure using zirconium butoxide butanol solution (80 wt%) as the zirconium source, and the Zr-GDPA1BAMHx is also synthesized with a mixed solvent of tetrahydrofuran/ethanol (1:1). B-GDPA1POEx is synthesized with B(OMe)3 as boron precursor and tetrahydrofuran/ethanol (9:1) as solvent.

AC electroluminescence device

SiO2/p++Si substrate with 100 nm oxide layer was washed by piranha solution, followed by O2 plasma treatment for 3 min. Poly-l-lysine solution was dropped on the clean substrate and then washed with Ultrapure water. Afterwards, semiconducting carbon nanotubes solution was used to soak the substrate for 15 min followed by washing with ultrapure water. The substrates were annealed in a mixed gas (5% H2 in Ar) atmosphere at 250 °C for 1 h. After deposition of the source electrodes (Au) using Ebeam (Electron Beam Evaporation), the metal electrodes were patterned to form the fork finger electrodes using ML3 (laser direct writing technique). Finally, carbon nanotubes outside the device area were etched with O2 plasma using UV lithography. The B-GDPA solution was stirred for 24 h, dropped on the CNT top layer of the device and then heated at 80 °C for 1 h. The square wave AC signal to drive the device is generated from the ATG-2021H power signal generator of Aigtek Electronic Technology Co., Ltd.

X-ray diffraction and total scattering

Powder X-ray diffraction data were collected on a Rigaku MiniFlex instrument using CuKα (λ  =  1.5418 Å) radiation, which are specified in the Supplementary Figs. The X-ray total scattering measurements were performed at room temperature at BL13SSW of the Shanghai Synchrotron Radiation Facility with wavelength of 0.247 Å. The raw data is processed with Dioptas software and pdfgetX3 program.

Differential scanning calorimetry and Thermogravimetric analysis

The glass temperature transition of samples was measured using a TA Instruments DSC 250 differential scanning calorimeter with RCS 90 cooler. The samples of ~5 mg were loaded in Tzero Aluminium hermetic pans. The DSC curves are measured with ramp rate of 10 °C/min unless otherwise noted. The glass transition temperatures and delta Cp are determined in TA Instruments Trios software with the midpoint of the transition determined by inflection point method and the onset and end of the transition determined by tangent method. For enthalpy relaxation measurements, the samples were first annealed at a given temperature before subsequent heating run. Thermogravimetric analysis (TGA) was conducted using a TA Instrument TGA 55 instrument under an Nitrogen atmosphere with a heating rate of 10 °C/min in the temperature range from 30 °C to 300 °C.

Dynamic mechanical analysis

DMA was carried out by a PerkinElmer DMA8000 dynamic mechanical analysis machine with powder kits with 0.02 mm strain at 1 Hz. The Tan delta was defined as the ratio of the loss modulus E” to the energy storage modulus E′.

Rheology measurements

The rheology measurements were performed on a TA Instrument ARES-G2 with a 25 mm aluminum parallel plate, the low viscosity sample was measured by slow-sweep mode; otherwise samples with high modulus was measured by small amplitude oscillatory shear mode. all the samples were tested at 25 °C expect for varied temperature measurements. For slow sweep mode: shear rate 1–100 1/s; points per decad: 5; off equilibration time 10.0 s; averaging time 30.0 s. In small amplitude oscillatory shear mode, active mode in axial force adjustment was used for compression, the samples were applied to 0.01% strain rate, and oscillated at a logarithmic sweep angular frequency of 100 to 0.1 rad/s with 6 points per decade.

Photoluminescence and infrared spectroscopy

PL spectra were obtained with a Meta Test Corporation equipped with mLaser_C03. Infrared spectroscopy was measured with a Perkin Elmer FT-IR Spectrometer Frontier instrument.

Photoluminescence quantum yield (PLQY)

HORIBA Fluorolog-3 is used to measure PLQY. The sample is evenly dispersed on a quartz glass substrate while taking a blank quartz glass sheet of the same size as the reference. The prepared sample is placed in the integrating sphere, and the monochromatic light of 350 nm is introduced into the sample in the integrating sphere through the optical fiber. By keeping the excitation wavelength unchanged at 350 nm, the number of photons absorbed by the sample can be determined by integrating the fluorescence spectrum of the sample and the blank reference at 330–370 nm, and the number of photons emitted by the sample can be determined by integrating the fluorescence spectrum between 370 and 600nm with the same method.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.