Dr Jinyu Sheng

Dr Jacopo Perego

Silvia Bracco

Prof Włodzimierz A Czepa

Dr Wojciech Danowski
email: danowski@unistra.fr

Dr Simon Krause

Piero Sozzani

Artur Ciesielski

Angiolina Comotti
email: angiolina.comotti@unimib.it

Ben L Feringa
email: b.l.feringa@rug.nl.

Prof. B L Feringa

Prof S Bracco

prof. P Sozzani

prof A Comotti

Prof A Ciesielski

Construction of multi-stimuli responsive highly porous switchable frameworks by in-situ solid-state generation of spiropyran switches

Keywords: photoswitch, porous aromatic framework, porous material, smart material, multistimuli responsiveness, spiropyran, solid-state isomerization contaminant removal, and water harvesting

Stimuli-responsive molecular systems supported within permanently porous materials offer the opportunity to host dynamic functions operating multifunctional smart materials. However, the construction of highly porous frameworks featuring external-stimuli responsiveness, for example by light excitation, is still in its infancy. Here we present a general strategy to construct spiropyranfunctionalized highly porous switchable aromatic frameworks (PSFs) by modular and high-precision anchoring of molecular hooks and an innovative in-situ solid-state grafting approach. Three spiropyran-grafted frameworks bearing distinct functional groups exhibiting various stimuliresponsiveness were generated by two-step post-solid-state synthesis of a parent indole-based material. The quantitative transformation and preservation of high porosity were demonstrated by spectroscopic and gas adsorption techniques. For the first time, we provide a highly efficient strategy to construct multi-stimuli-responsive, yet structurally robust, spiropyran materials with high pore capacity which was proved essential for the reversible and quantitative isomerization in the bulk as demonstrated by solid-state NMR spectroscopy. The overall strategy allowed to construct dynamic materials which undergo reversible transformation of spiropyran to zwitterionic merocyanine, by

Introduction

Stimuli-responsive molecules [1][START_REF] Feringa | Molecular Switches[END_REF][START_REF] Zbigniew | Molecular Photoswitches[END_REF] offer great opportunities as building blocks for the construction of adaptive materials capable of undergoing reversible conformational changes under the effect of external physical or chemical stimuli. [START_REF] Theato | [END_REF][5][6][7][8][9][10] In this context, spiropyrans (SPs) [11][12][13] are among the most prolific representatives of molecular switches due to their prominent multi-stimuli responsiveness encompassing light, mechanical force, pH, temperature, polarity of media, metal ions and redox potential. [11,14,15] The isomerization from cyclized nonpolar form to zwitterionic conjugated merocyanine (MC) form induces drastic changes in both molecular structure and physicochemical properties. [12,[15][16][17] In addition, a careful choice of substituents allows for the regulation of thermal stability of the MC form, the absorption wavelength and their photostability as well as the robustness to hydrolysis. [14,17] Due to these versatile features, SPs were widely used for construction of responsive materials including polymers, [17][18][19] gels, [START_REF] Francis | Sensors Actuators[END_REF][21][START_REF] Morim | Proc. Natl. Acad. Sci[END_REF][START_REF] Li | [END_REF] thin films [24] or monolayers, [25,26] smallmolecule assemblies, [27,28] biohybrid system [29] and porous materials. [30][31][32][33][34][35][36][37][38][39] The development of porous solids capable of reversibly interconverting between two or more states in response to one or more stimuli offers opportunities to manipulate noninvasively the properties of the material enabling new functions with the advantage of broad flexibility with the choice of the applied stimulus. In this context, the incorporation of SPs into porous systems offers a general and convenient approach to develop multi-stimuli responsive porous materials for applications ranging from cargo capture, delivery and release to chemical sensing. [13,40] Pioneering studies by Klajn and coworkers on the incorporation of SPs in nanoporous materials indicated the partially-preserved multi-stimuli responsiveness of the SP-moiety with respect to that in solution. [30] Recently, major attention was focused on the incorporation of SPs as guests or pendant moieties in metal-organic frameworks (MOFs) proving a suitable strategy to achieve fast photoisomerization, [31] modulation of conductivity, [33][34][35] capability of water desalination [36] and tuning of gas adsorption. [41,42] However, the effective integration of SPs in highly porous covalent frameworks and the preservation of their isomerization behavior remains a major challenge. [43] In addition, the fabrication of SP-functionalized porous switchable frameworks with large accessible surface areas is unexplored. A typical strategy to construct highly porous materials is to exploit the efficient Yamamoto Ullmann-type coupling reaction, which unlike Suzuki-type coupling, forms highly porous aromatic frameworks (PAFs) with unparalleled surface areas. [44,45] By contrast, as a result of high reactivity of the Ni(0) complex, the number of the functional groups tolerated in these reactions are severely limited. Hence, apart from rare examples of overcrowded alkenes, [46,47] molecular switches are largely incompatible with the Yamamoto coupling reaction, consequently compromising the fabrication methods of switchable aromatic frameworks.

Here, we provide a judicious post-synthetic strategy for the construction of SP-grafted highly porous switchable frameworks (SP-PSFs). We exploit in-situ solid-state synthesis of the SPs moieties in the pores of the framework via a pre-anchored indole precursor inside a porous aromatic frameworks (PAF) followed by methylation of the indole and finally the grafting of the SP moiety by condensation of the indoline Fisher-base with a variety of salicylaldehydes. This approach bypasses the limited functional group-tolerance of Yamamoto coupling as a result of the high stability of indole moiety and allows for fabrication of materials decorated with SP moieties exhibiting BET surface area as high as 1608 m 2 g -1 and 1.21 cm 3 g -1 total pore volume. Due to the modularity and high precision of this approach, three novel SP-PSFs with distinct properties and functions were fabricated illustrating the scope and versatility of SP switches. In addition to the high porosity, all the synthesized porous materials possess multi-stimuli responsiveness comparable to that of the SPs in solution, owing to the high porosity minimizing the interactions of the responsive moieties with the aromatic framework.

These highly porous and responsive materials were tested for various applications, including acidic gas adsorption and detection, as well as removal of metal ion from solution (Figure 1).

Results and discussion

Synthesis and characterization of PAFs and NSP-PSF.

The parent indole-grafted porous aromatic framework (indole-PAF) was synthesized by a Yamamoto-Ullmann cross-coupling reaction. Since monofunctional or linear co-monomers are expected to drastically decrease the porosity of the frameworks, [46,48] here a tritopic indole-appended building block (1-Br 3 , Figure 2a) was designed. The copolymerization of tetratopic and di-or tritopic monomers by Yamamoto coupling is recognized for yielding materials with reduced surface area in comparison to the frameworks constructed exclusively from the tetratopic monomers. [46,48] With the expectation that a higher porosity would facilitate subsequent post-synthetic transformations inside the pore space while still preventing the tight packing of the spiropyrans, [17] the 4:1 proportion of porogenic tetra-p-bromo-phenylmethane (TPM-Br 4 ) and functional monomers (1-Br 3 )was selected to balance the porosity of the materials and the concentration of indole moieties in the framework. [46,48] As fabricated indole-PAF material exhibits high pore capacity as established by N 2 adsorption measurements at 77 K. A typical type IV isotherm with large hysteresis is observed for this material (Figure 2b, grey) indicating the presence of micropores and expanded capacity in the mesopore region at high gas loading. The Langmuir and Brunauer-Emmett-Teller (BET) surface areas were as large as 1913 and 1708 m 2 g -1 , respectively.

The pore-size distribution was calculated using non-local density functional theory (NL-DFT), which indicated that micropores constitute ca. 32% of pore volume, with the total pore volume of 1.26 cm 3 g - 1 , highlighting significant tendency of the material to generate mesopores. Subsequent N-methylation of indole-PAF with methyl iodide afforded indolium-PAF (Figure 2a) with reduced BET surface area and total pore volume to 1310 m 2 g -1 and 0.81 cm 3 g -1 , respectively. Such a significant drop in the available voids is expected given the presence of bulky iodide anions in the structure, as confirmed by energy-dispersive X-ray spectroscopy (EDS) (Figure S34). Interestingly, while the microporosity dropped from 0.54 cm 3 g -1 to 0.44 cm 3 g -1 (percentage decrease = 18%), the mesoporosity was more affected with a 48% decrease of mesopore volume. Subsequent condensation of iodolium-PAF with 5-nitro-salicylaldehyde in the presence of base yielded a nitro-appended spiropyran porous switchable framework (NSP-PSF). The NSP-PSF material exhibits higher porosity than the preceding indolium-PAF with BET surface area and total pore volume of 1606 m 2 g -1 and 1.08 cm 3 g -1 , respectively (Table S1, see SI section 4.1 for the discussion on the impact of changes in molecular weight of the monomeric units on the porosity of the framework), which is consistent with removal of the looselybound bulky iodides from the pores of the framework. The lower BET surface area of the NSP-PSF with respect to the parent indole-PAF framework is in accordance with the formation of the SP pendants, which are bulkier and heavier than the indole moieties, thus occupying more volume in the material and leading to the increase of the molecular weight of the monomeric unit. 13 C{ 1 H} CP MAS spectra of indole-PAF (top), indolium-PAF (medium) and NSP-PSF (bottom) materials performed at 298 K at a spinning speed of 12.5 kHz with a contact time of 2 ms. The spectra were simulated by Lorentzian line-shape functions. The spectra are magnified 2-3 times in the regions 200 -175 ppm and 60 -5 ppm to highlight indole, indolium and SP typical 13 C NMR signals.

These solid-state functionality transformations were further confirmed by diffuse-reflectance UV/Vis (DR UV/Vis) and diffuse-reflectance infrared Fourier Transform (DRIFT) spectroscopies. The NSP-PSF material showed a clearly distinguishable spectrum compared to the predecessor material indolium-PAF and parent material indole-PAF (Figure 2c), indicating the step-wise transformation of functional groups during the post-synthetic solid-state transformations within the solid framework.

DRIFT spectroscopy displays the appearance of two additional intense bands for NSP-PSF material centered at 1530 cm -1 and 1340 cm -1 associated with the asymmetric and symmetric stretching of the nitro-group (Figure 2d), respectively. The two additional bands around 1100 cm -1 (Figure 2d, blue spectrum) were ascribed to the C-H out of plane asymmetric bending modes. [49] Together with a C spiro -O stretching bands centered at 1260 cm -1 , the combined data strongly support the successful formation of the NSP moiety. [50] Furthermore, the presence of characteristic 13 C resonances in 13 C CP MAS NMR allowed to unambiguously confirm the chemical structures of the framework at each stage of the solid-state synthesis (Figure 2e, Table S2-S7 and Figure S29 for the comparison of ss 13 C CP MAS NMR of indole-PAF with monomers). [51] The formation of indolium-PAF was supported by the presence of a signal at 43.8 ppm characteristic of methyl group adjacent to charged quaternary nitrogen (CH 3 -N + ) moiety (Figure 2e, middle spectrum). Accordingly, the disappearance of the resonance at 196.0 ppm ascribed to the iodolium carbon in 2-position (C a , see Figure 2e, middle panel) and emergence of the resonance at 106.7 ppm characteristic of quaternary spiro carbon (C a , Figure 2e, bottom panel) along with two distinct resonances for diastereotopic methyl groups (e and e') indicated the successful conversion to spiropyran derivative (NSP-PSF) (Figure 2e, bottom spectrum). Importantly, characteristic shifts of the diagnostic "C a " resonance (Figure 2e, see SI section 4.2 for further discussion) in the 13 C CP MAS NMR spectra showed quantitative conversion, within the sensitivity limit of the technique, at each stage of the transformation, indicating the remarkably high efficiency of this approach. These results support the fact that the reaction does not generate side products and residues which are irreversibly encapsulated within the pores. The exceptionally high efficiency of this solid-state SP synthesis most likely stems from the high porosity of the material and its hierarchical micro-/meso-porous structure which facilitates mass transport through the solid during the whole process. It should be emphasized that this post-solid-state synthesis strategy successfully proved the formation of NSP-PSF materials not accessible via direct synthetic methods. Thermal gravimetric analysis (TGA) revealed the high thermal stability of these porous materials, an initial weight loss is observed above 300°C due to the decomposition of the SP moiety, while the framework is stable up to 450°C (Figure S36). Powder X-ray diffraction (PXRD) data confirmed the amorphous nature of the materials (Figure S38) and scanning electron microscopy (SEM) images (Figure S39-41) showed that the porous materials comprise of submicrometer-size particles, common for the frameworks formed via irreversible reactions.

Extension of strategy for the synthesis of SP-PSF and SSP-PSF materials. SP motifs with various substituents at 6-position of the benzopyran part show dramatic differences in properties, leading to a variety of functions such as photoacidity, [52][53][54] actuation [18,[START_REF] Francis | Sensors Actuators[END_REF][START_REF] Li | [END_REF]55] and multi-stimuli responsiveness. [30,56,57] Therefore, we further extended the generality of our strategy to the fabrication of non-substituted and sulfonated SP functionalized materials (denoted as SP-PSF and SSP-PSF, respectively). By the condensation reaction of salicylaldehyde or sodium salicylaldehyde-5-sulfonate with indolium-PAF, we successfully constructed SP-PSF and SSP-PSF materials (Figure 3a), respectively. These two materials showed similar features as their sister NSP-PSF counterpart as both materials maintained high BET surface areas and pore volume of 1608 m 2 g -1 and 1.21 cm 3 g -1 for SP-PSF and 1440 m 2 g -1 and 0.76 cm 3 g -1 for SSP-PSF, respectively (Figure 3b). At higher partial pressure, both materials considerably differ regarding the polarity of the framework: the less polar SP-PSF displays a swelling behavior, [58] as confirmed by the wide hysteresis in the desorption curve upon nitrogen adsorption at 77 K, SSP-PSF showed much lower uptake and narrower hysteresis loop between adsorption-desorption branches (Figure 3b). Indeed, CO 2 adsorption isotherms collected at 195 K show a comparable behavior owing to stronger interactions of CO 2 with the frameworks which induce similar pore expansion in both frameworks, indicating that the swellability of these PSFs strongly depends on the polarity of the environment and adsorbate (Figure 3b, inset, see Figure S27 and discussion therein). DR UV/Vis spectra of both materials indicate that the responsive pendants are initially in the cyclized SP form as no bands characteristic of the opened MC moieties were observed in the visible region of the spectra (Figure 3c). In both cases, clear C spiro -O bond stretching bands were observed at 1260 cm -1 in the DRIFT spectra, indicating the successful grafting of the SP motifs in the materials (Figure 3d). An additional band at ∼1095 cm -1 characteristic of the SO 3 -group was observed in SSP-PSF, indicating the successful construction of the sulfonated spiropyran (Figure 3d, pink spectrum). [35,59] Furthermore, in the case of SSP-PSF, EDS spectroscopy proved the presence of both sulfur and sodium elements in the material with concomitant absence of iodine (Figure S35), indicating the complete transformation of functionalities. Finally, 13 C CP MAS NMR spectroscopy demonstrates the quantitative transformation of functionalities, within the sensitivity limit of the analysis (Figure S31). SEM images and PXRD (Figure S42-43 and S38) confirm that both SP-PSF and SSP-PSF materials have a similar morphology and are amorphous in nature. This article is protected by copyright. All rights reserved.

Stimuli-induced isomerization of SP-PSFs in the solid state.

The photochemical isomerization behavior of the solid NSP-PSF material was studied by DRUV/Vis and DRIFT spectroscopies (see Fig. S4-8 for the studies of NSP in solution). The initial DRUV/Vis spectrum (Figure 4b, blue spectrum) showed a small absorption band centered at 600 nm which was ascribed to a small amount of opened MC form present in the pristine sample of NSP-PSF material.

Upon the irradiation of the material at 365 nm light for 10 s, a dramatic color change from grey to bright blue was observed (Figure 4a), in line with the isomerization of closed SP form to opened MC ). The reversible changes in the DRIFT spectra were observed around ∼1260 cm -1 upon consecutive UV and Visible light irradiation which can be ascribed to the scission/reformation of C spiro -O bond, [33] further corroborating the isomerization between closed and open forms. Like the NSP photoisomerization behavior in solution (Figure S6 and S8), the NMC form in the solid state was found to be thermally unstable, and a fast equilibration of the NMC to NSP form was observed at ambient temperature (Figure S51a). The kinetics of the thermal isomerization of the NMC was followed by DRUV/Vis spectroscopy and found to be almost completed within a few hours at ambient temperature. (Figure S51b).

Similar to the typical 6-H SP behavior in solution, the SP-PSF material does not show any photoresponse under ambient conditions due to the low thermal stability of 6-H MC form that quickly thermalizes back to the SP form. [60] Instead, the switching behavior of SP-PSF material could be . Further experiments proved that the SP-PSF material could be also regenerated from MCH + -PSF in either vacuum or upon heating conditions, which is consistent with the volatility of hydrochloric acid (Figure S54). This pH-responsive behavior was also observed for the SSP-PSF material upon treatment with gaseous HCl (g) (Figure S53).

In addition to the pH-responsive behavior, the SSP-PSF material shows humidity-induced isomerization. This unique property stems from the SSP pendant, which is known to be stable in the opened MC form in aqueous media in the dark (see Fig. S17-21 for studies in solutions). [61] After storing under ambient conditions in a sealed vial in the dark for 4 weeks, the greyish SSP-PSF material changed color to purple, indicating isomerization of the SSP pendants (Figure 4e).

DRUV/Vis and DRIFT spectra supported the isomerization from SP to MC form in the solid state (Figure 4f, violet UV-Vis spectrum), both showing features characteristic of the MC isomer. [35] Due to the presence of the polar sulfonate side group in the material, moisture under ambient condition may This article is protected by copyright. All rights reserved.

enter the pores of the framework resulting in the increase of the polarity of the environment and therefore leading to the shift in the position of the SSP/SMC thermal equilibrium in favor of the stabilization of the SMC isomer. Indeed, after soaking of SSP-PSF in water, the 13 C CP MAS spectrum showed the complete disappearance of the signal at 106.6 ppm of C spiro -O (closed form) and the emergence of two new signals at 196.9 ppm and 183.9 ppm revealing the formation of both Z and E isomers of SMC, respectively (Figure S33). However, upon thermal treatment and water removal, only the Z-SMC reversed back quantitatively to SSP owing to the arrangement of the Z-SMC isomer which favors the formation of the closed form (Figure S33). Nevertheless, the N 2 adsorption isotherms collected at 77 K of SSP-PSF in its pristine state and after water soaking and another activation cycle were almost superimposable, indicating the reversibility of the water uptake without structural changes of the porous material (Fig. S28). This unique behavior of SSP-PSF may have potential applications in modulation of proton conductivity, [35] water-harvesting and purification or watersensing. [62][63][64][65] This article is protected by copyright. All rights reserved.

Multi-applications with SPs materials

To showcase the broad applicability of the PSF materials synthesized with this modular approach, a series of experiments testing the utility of these materials under various conditions were performed.

Due to the large pore-capacity, SP-PSF could find potential applications as acidic gas sensor or sorbent that could be used to neutralize gas mixtures (Figure 5a). Indeed, when dry gaseous HCl was diagnostic of the formation of E-MCH + isomer. [53] Vice versa, after N 2 -flow treatment the 13 C CP MAS NMR spectrum showed recovery of all the features of the closed form SP-PSF (Figure 5b, right). Thus, solid-state NMR spectroscopy highlights the quantitative (within the accuracy of the method) switching between the SP and the MCH + isomers, demonstrating the full accessibility of the SP switches decorating the pore surface. This process was followed by DR UV/Vis spectroscopy for two cycles which showed limited reversibility on account of the inhomogeneity in the gas flow and illumination of the material (Fig. S56). Nevertheless, this may be a question of experimental setup and in principle upon optimization of the setup this material can potentially be used as an acidic gas scavenger, which can be easily reactivated by light-and N 2 -assisted acidic gas removal or when acidified as a release agent for acidic gas upon light irradiation. [66] Due to the stability of the sulfonated MC form in polar media and the well-known capability of MC to bind metal ions, a metal ion binding experiment of the SMC-PSF material was investigated. [30] The In contrast, the control solution without SSP-PSF showed much higher concentration of the Cu 2+ This article is protected by copyright. All rights reserved.

SSP-PSF

visible light led to the release of the Cu 2+ ions (Figure S57, light orange spectrum), corroborating the reversibility of the process.

Conclusion

In summary, by Yamamoto type cross-coupling reaction, an unprecedented indole-functionalized porous aromatic framework with ultra-high pore capacity in both micropore and mesopore region was fabricated. Notably, the modular post-synthetic grafting to the pre-anchored indole sites allowed us to This article is protected by copyright. All rights reserved.

obtain SP functionalities as pendants in the nano-porous materials. Solid-state 13 C NMR confirmed the quantitative transformation of functionalities at each step during the solid-state synthesis and N 2 sorption data indicated the post-solid-state synthesis occurring in the entire framework without its collapse. Importantly, the high level of porosity maintained by this approach allows us to fabricate PSF based on SP, which is crucial for the responsive functions as well as capability of controlling material properties. It is worth to highlight that this is the first time that SP-based aromatic frameworks were constructed with remarkably high pore capacities. We furthermore demonstrate by DRUV-Vis, DRIFT spectroscopies and solid-state NMR studies, reversible photochromism of NSP-PSF, acidochromism of SP-PSF and SSP-PSF, ionochromism and humidity-responsiveness of SSP-PSF. The preservation of the multi-stimuli responsive behaviour of the SP in the constructed frameworks allowed us to show several areas in which these responsive materials can meet potential applications. Given the simplicity and efficiency of this modular synthetic approach, our work established an elegant strategy for the construction of highly porous yet chemically and structurally stable switchable materials with multi-stimuli responsive behavior. We also provide a more general synthesis strategy useful for other switchable materials with high pore capacity which cannot be accessed via de novo syntheses. Furthermore we envision that this strategy will allow to graft multiple functionalities within the same material, further increasing the functionality towards desired properties and applications. We are convinced that these switchable materials have potential to manifest more powerful functions beyond the examples illustrated herein transforming them into a crucial platform towards responsiveness and actuation on the molecular level in the solid state.

Figure 1 .

 1 Figure 1. Schematic representation of in situ generation of SP switches and their transformations in PSFs. Formation of Indole-PAF by direct insertion of indole moieties covalently connected to the framework (Indole-PAF), followed by post-

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Figure 2 .

 2 Figure 2. a. Top: synthesis of indole-PAF using Yamamoto-Ullmann cross-coupling reaction; bottom: post-synthetic functionalization to yield SP grafted porous switchable framework with nitro functional groups (NSP-PSF). b. Nitrogen adsorption isotherms collected at 77 K of indole-PAF (grey), indolium-PAF (yellow) and NSP-PSF (blue). c. Kubelka-Munk transformed DR UV/Vis spectra of indole-PAF, indolium-PAF and NSP-PSF. d. DRIFT-IR spectra of indole-PAF, indolium-PAF and NSP-PSF. e.13 C{ 1 H} CP MAS spectra of indole-PAF (top), indolium-PAF (medium) and NSP-PSF (bottom) materials performed at 298 K at a spinning speed of 12.5 kHz with a contact time of 2 ms. The spectra were simulated by Lorentzian line-shape functions. The spectra are magnified 2-3 times in the regions 200 -175 ppm and 60 -5 ppm to highlight indole, indolium and SP typical13 C NMR signals.

Figure 3 .

 3 Figure 3. a. Extension of strategy to the synthesis of the SP-PSF (left) and SSP-PSF (right). b. Nitrogen adsorption isotherms collected at 77 K of SP-PSF (orange) and SSP-PSF (pink). inset. Carbon dioxide adsorption isotherms collected at 195 K of SP-PSF (orange) and SSP-PSF (pink). c. Kubelka-Munk transformed DR UV/Vis spectra of SP-PSF (orange) and SSP-PSF (pink). d. DRIFT-IR spectra of SP-PSF (orange) and SSP-PSF (pink).

  This article is protected by copyright. All rights reserved. form via C spiro -O bond cleavage. This change was accompanied by the drastic increase of the intensity of the broad band centered at 600 nm, characteristic of the open MC-form (Figure 4b, green spectrum). Subsequent illumination at 617 nm for 15 min led to the recovery of the initial spectrum (Figure 4b, light blue spectrum), in line with the reversible photoisomerization between the two forms. DRIFT spectroscopy further confirms the isomerization behavior of NSP-PSF (Figure S45 and S46

  triggered upon spontaneous isomerization to the protonated merocyanine (MCH + -PSF) by exposure to gaseous or an aqueous HCl solution (1M). An obvious color change from grey to orange could be observed by the naked eye (Figure 4c), indicative of the formation of MCH + . DRUV/Vis and DRIFT spectroscopies indicate that illumination of the MCH + -PSF material at 365 nm or in the visible light region leads to the recovery of SP-PSF (Figure 4d, and Figure S52), likely with concomitant expulsion of the volatile acid from the framework. This behavior differs from the one in solution as 6-H spiropyrans typically open to Z-MCH + upon protonation and isomerize to E-MCH + upon irradiation at 365 nm (see Fig. S12-15)

Figure 4 .

 4 Figure 4. Multi-stimuli responsiveness of the NSP-PSF, SP-PSF and SSP-PSF materials. a. Schematic representation and visual changes of reversible photochromism of NSP-PSF. b. DR UV/Vis spectra showing the reversible photochromism of NSP-PSF. c. Schematic representation and visual changes of reversible acid-/photo-modulation of SP-PSF. d. DR UV/Vis spectra of reversible acid-/photo-modulation of SP-PSF. e. Schematic representation and visual changes of reversible moisture/heat modulation of SSP-PSF. f. DR UV/Vis spectra of reversible moisture/heat modulation of SSP-PSF.

  passed through the column packed with 10 mg of SP-PSF under N 2 flow, the dramatic color change from slight grey to deep orange was observed, indicating the formation of MCH + -PSF. Subsequent irradiation at 365 nm on the column induced the release of the gaseous HCl in N 2 flow and therefore regeneration of the pristine SP-PSF material. The ultimate confirmation of this structural transformation upon HCl (g) exposure from the close to open form was provided by 13 C CP MAS NMR spectra which showed the acid-induced opening of the SP-PSF as demonstrated by the disappearance of the resonance characteristic of the SP isomer at 106.2 ppm (C a , Figure 5b left) and the appearance of the new resonance at 182.3 ppm (C a , Figure 5b, middle) characteristic of the MCH + form, further corroborated by an emergence of a single resonance corresponding the chemically equivalent methyl groups (C e ) in flat MC (Figure 5b, middle). Notably, the downfield resonance at 182.3 ppm is

  material was found to spontaneously undergo thermal isomerization to the open form in a highly-polar acetonitrile suspension. The presence of SSP-PSF in the CuCl 2 acetonitrile solution led to a significant drop of the adsorption at 460 nm, and a visible color change of the solid, showing the successful removal of Cu 2+ from solution (Figure 5c, and Figure S57, grey spectrum). As observed from the UV/Vis spectra, the absorption band centered at 460 nm is related to the Cu 2+ in acetonitrile.

Figure 5 .

 5 Figure 5. a. Demonstration of the reversible uptake and release of HCl (g) by SP-PSF material for acidic gas removal. b. 13 C{ 1 H} CP MAS spectra performed at 298 K at a spinning speed of 12.5 kHz with a contact time of 2 ms of pristine SP-PSF (left panel), E-MCH + -PSF (medium panel) and SP-PSF material (right panel) after complete regeneration under nitrogen flow. c. Demonstration of copper ion induced ring opening of SSF-PSF for metal ion capture.

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(

  Figure S57, orange spectrum), proving the ability of SSP-PSF material to remove Cu 2+ and serve as a potential scavenger of transition-metal contaminants. Subsequent irradiation of the sample with 15214095, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adma.202305783 by Cochrane France, Wiley Online Library on [11/10/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

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This article is protected by copyright. All rights reserved.

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Acknowledgements

This work was supported from the following sources: China Scholarship Council (CSC PhD Fellowship No. 201808330459 to J.S.), Financial support from The Netherlands Organization for Scientific Research (NWO-CW), the European Research Council (ERC; advanced Grant No. 694345 to B.L.F.). The Ministero dell'Istruzione, dell'Università, PRIN (SHERPA) and om ardy Region for "Enhancing Photosynthesis" grant (2021-2023) are acknowledged for the financial support. W.D. is grateful for financial support from Marie Skłodowska-Curie Actions (Individual Fellowship No.

101027639). We thank prof. Wesley R. Browne for fruitful discussions.

This article is protected by copyright. All rights reserved.

Conflict of Interest:

There are no competing conflicts of interests to declare. This article is protected by copyright. All rights reserved.

Author contributions

with contributions from all coauthors. All of the authors participated in the discussion of data and commented on the manuscript.

A post-synthetic strategy of construction of highly porous aromatic frameworks is presented. This approach enables fabrication of series of materials with vide range of properties and applicability in diverse fields.


Ivano Caponigro

Andries Coetzee

Emily Clem

Veneeta Dayal

Donka Farkas

Daniel B Kane

Justin Royer

Radek Šimík

Harold Torrence

Michelle Yuan

Anamaria Fălăuș
email: anamaria.falaus@cnrs.fr

'What' clauses can and 'which' cannot: a Romanian puzzle

Keywords: wh-determiners, wh-words, free relative clauses, discourse-anaphoricity, interrogative clauses, light-headed relative clauses

A previously unnoticed puzzle is presented. It concerns the distribution of wh-determiners in free relative clauses in Romanian: while care 'which' + NP can never introduce free relative clauses, ce 'what' + NP does it productively, as all the other wh-words. New evidence is provided showing that care 'which' + NP in interrogative clauses in Romanian exhibits strong discourse-anaphoric requirements, unlike ce 'what' + NP. This feature of care 'which' + NP is suggested to be responsible for the puzzle by triggering a clash with the basic set-denoting function of a free relative clause, along the lines of what is observed in light-headed relative clauses.*

(henceforth, Max-FRs), which have been argued to semantically behave like definites, and Existential Free Relative Clauses (henceforth, Ex-FRs), which only occur as complements of a restricted class of predicates and have been argued to semantically behave like narrow scope indefinites. 2 Both wh-clauses are extremely productive in Romanian: they can be introduced by all the wh-words and wh-phrases that can introduce interrogative wh-clauses (see [START_REF] Grosu | Relative clause constructions and unbounded dependencies. A reference grammar of Romanian[END_REF], Caponigro & Fălăuș 2018, a.o.). This includes ce 'what' + NP, as shown in [START_REF] Link | The logical analysis of plurals and mass terms: a lattice-theoretical approach. Meaning, use and the interpretation of language[END_REF] and ( 4) with a singular count, a plural count, and a mass NP complement.

( There is only one exception: care 'which' + NP cannot introduce either Max-FRs nor Ex-FRs regardless of the kind of NP complement occurring with the wh-determiner, as shown in ( 5) and ( 6).

(5) Max-FRs introduced by ' Note that care 'which' + NP triggers obligatory clitic doubling when generated in object position, as shown in ( 5) and ( 6) above, while ce 'what' + NP does not. 3 This difference does not affect the 2 See [START_REF] Jacobson | On the quantificational force of English free relatives. Quantification in natural languages[END_REF][START_REF] Dayal | Locality in WH quantification: questions and relative clauses in Hindi[END_REF][START_REF] Caponigro | Free not to ask: on the semantics of free relatives and wh-words crosslinguistically[END_REF], 2004 for the semantics of Max-FRs. They all argue that Max-FRs are wh-clauses that denote the unique maximal individual of a set like definite DPs, borrowing from the analysis of definite DPs in [START_REF] Sharvy | A more general theory of definite descriptions[END_REF][START_REF] Link | The logical analysis of plurals and mass terms: a lattice-theoretical approach. Meaning, use and the interpretation of language[END_REF]. Their proposals differ in the semantics they assume for wh-words in Max-FRs. See Šimík 2020 for a detailed and thorough overview of generalizations and proposals about Max-FRs and FRs in general. See [START_REF] Izvorski | Non-indicative wh-complements of possessive and existential predicates[END_REF][START_REF] Caponigro | Free not to ask: on the semantics of free relatives and wh-words crosslinguistically[END_REF], 2004, 2021[START_REF] Caponigro | Headless relative clauses and the syntax-semantics mapping: evidence from Mesoamerica[END_REF][START_REF] Grosu | The syntax-semantics of modal existential wh-constructions. Balkan syntax and semantics[END_REF][START_REF] Šimík | Modal existential wh-constructions[END_REF], 2013 for Ex-FRs. Their analyses are significantly different: Izvorski treats them as interrogative wh-clauses syntactically; Caponigro treats them as a subgroup of FRs independent from Max-FRs; Grosu and Šimík, instead, argue that they can be clauses or smaller constituents and, therefore, prefer to label them "Modal Existential Construction". See Šimík 2017 for an overview of the work on this construction. See also [START_REF] Caponigro | Headless relative clauses in Mesoamerican languages[END_REF] for further generalizations and crosslinguistic evidence and comparison about Max-FRs and Ex-FRs from fifteen Mesoamerican languages. Here we follow Caponigro's 2021 terminology and basic syntactic assumptions about Max-FRs and Ex-FRs, although no crucial aspect of the puzzle we are presenting hinges on that. 3 An anonymous reviewer asked us to comment on this fact. See Pană Dindelegan 2013: ch. 3 for an overview on clitic doubling in Romanian and Comorovski 1996 for clitic doubling with wh-phrases.

puzzle we are investigating: the contrast between FRs introduced by care 'which' + NP and ce 'what' + NP also surfaces when the wh-phrase is generated as the subject, a syntactic position triggering no clitic doubling, as illustrated in ( 7) and ( 8).

( We suggest a line of explanation that links this puzzle to a meaning difference in D-linking or discourse anaphoricity between 'what' + NP and 'which' + NP in interrogative clausesa meaning difference that, we show, is even sharper in Romanian than in English and makes care 'which' + NP in Romanian resemble the partitive construction care dintre 'which of the' + NP. The fact that ce 'what' + NP can introduce Max-FRs and Ex-FRs follows as the default option for a language like Romanian that has a productive system of wh-clauses, where all wh-words and whphrases can undergo some systematic meaning change and be used in FRs as well, unless some extra factor intervenes and blocks this extension. We suggest that this is exactly the case for care 'which' + NP in Romanian. The strong presuppositional anaphoric requirement for its NP to refer to a set that has already been explicitly introduced in the discourse clashes with the inherently non-anaphoric nature of Max-FRs and Ex-FRs.

To the best of our knowledge, the contrast between the occurrence of 'what' + NP vs. the ban of 'which' + NP in Max-FRs and Ex-FRs has not been investigated in previous work on Romanian or any other language.5 There is at least one Romance language, namely Italian, where neither option is allowed in either construction. Germanic languages do not seem to have Ex-FRs at all. On the other hand, English has a productive system of Max-FRs, including those introduced by what + NP. The latter exhibit a behavior that is partially different from and less clear-cut than the behavior of their Romanian counterpart, though. We hope that the puzzle we introduce in this Research Report, the analytical tools that are used to investigate it, and the proposed account in terms of anaphoric properties will foster crosslinguistic work on the behavior of these two wh-determiners in interrogative and non-interrogative wh-clauses.

The remainder of the paper is structured as follows. In Section 2, we show the discourseanaphoric contrast between care 'which' + NP and ce 'what' + NP in interrogative clauses in Romanian building on insights from the literature on D-linking and Dayal 2017, 2020 (Section 2.1) and suggest ways to formalize it by adapting existing proposals about the meaning of what + NP and/or which + NP in English (Section 2.2). In Section 3, we use the findings from Section 2 to suggest an account for the contrast in Max-FRs and Ex-FRs illustrated in ( 3)-( 8). Section 4 concludes with some general remarks and outstanding issues.

'What' + NP versus 'which' + NP in interrogative clauses in Romanian

Empirical findings: a pragmatic contrast in anaphoricity

The literature on Romanian does not provide any systematic comparative investigation of ce 'what' + NP vs. care 'which' + NP in interrogative clauses in Romanian. Still, it seems to imply a pragmatic difference between the two wh-determiners: the wh-determiner care 'which', but not ce 'what', is assumed to come with a contextual salience/familiarity presupposition that is often cast in terms of Pesetsky's 1987 notion of D-linking (see [START_REF] Comorovski | Interrogative phrases and the syntax-semantics interface[END_REF][START_REF] Rațiu | De la syntaxe à la sémantique des propositions interrogatives[END_REF][START_REF] Giurgea | The syntax of determiners and other functional categories. A reference grammar of Romanian[END_REF], Giurgea & Grosu 2019, a.o.). Accordingly, care 'which' can only be used "when the domain of entities from which values are to be selected is contextually available" (Farkas 2013:220).

In this section, we probe the D-linking/discourse-anaphoric properties of care 'which' + NP and ce 'what' + NP in Romanian by applying some of the tests that [START_REF] Dayal | Deconstructing the D in D-linking[END_REF][START_REF] Dayal | WH-scope taking in questions[END_REF] develops to distinguish between which + NP and phrasal what (without an NP complement) in English. We conclude that care 'which' + NP in Romanian carries a strong presuppositional requirement: the set denoted by its NP must have been explicitly introduced in the discourse in a way that makes it akin to the partitive construction care dintre 'which of' + NP, as suggested in Comorovski 1996:11. On the other hand, ce 'what' + NP carries no presuppositional requirement whatsoever. 11b) by care 'which' + NP, and the one in (11c) by the partitive care dintre 'which of' + NP. We investigated the felicity of each interrogative clause when uttered in two different contexts. In Context 1, the set denoted by the NP complement of care 'which' or ce 'what' is not mentioned explicitly but it can be inferred from other pieces of information that are provided. Ana's mentioning her trip to the nearby bookstore to buy a book for Lia makes it clear that the book in question is a member of the set of books in that bookstore. Still, this contextual information is not enough to license the use of care 'which' + NP or the closely related partitive construction, whereas ce 'what' + NP is fully acceptable. A similar point is made by the contrast in (12), modeled after an example discussed in [START_REF] Kroch | Amount quantification, referentiality, and long wh-movement[END_REF], as well as Dayal 2020. This time the interrogative clauses are used in a situation where the referent of the wh-phrase is contextually salient. This is not sufficient to make interrogative clauses felicitous when introduced by care 'which' + NP, as in (12b), or the partitive care dintre 'which of' + NP, as in (12c). On the other hand, interrogative clauses introduced by ce 'what' + NP are fully felicitous in this context, as shown in (12a).

(12) CONTEXT: A man walks into an apartment building in front of Maria and Ana, who While ce 'what' + NP is unaffected by these context differences, care 'which' + NP is only felicitous in Context 3, in which the full list of nominees had been introduced in the discoursethe same behavior that is exhibited by the partitive care dintre 'which of' + NP. Notice that introducing a list of three nominees that is clearly partial and doesn't cover all the members of the set denoted by the NP actriţă 'actress' is not enough to license care 'which' + NP, as shown by the infelicity of (13b) in Context 2. Once again, this is the same pattern as the one observed for partitive care dintre 'which of' + NP, as shown in (13c). This contrast also shows that care 'which' imposes anaphoric constraints not just on a subset of the set denoted by its NP complement, but on the whole set, to which standard domain restrictions for NP complement of determiners are expected to apply (e.g. actriţă 'actress' in ( 13) is not expected to denote the set of all the actresses in the world, but, given the discourse context, is restricted to the set of actress that are nominees for the Oscar for Best Actress that year). 7To sum up, we have shown that care 'which' + NP in interrogative clauses in Romanian is subject to very strict discourse conditions: the set denoted by its NP is presupposed to have been previously introduced in the discourse explicitly. 8 This presuppositional requirement also holds for embedded interrogative wh-clauses that are introduced by care 'which' + NP. In the example in (2), care 'which' + NP can be used felicitously only if the discourse makes it clear that Ana has a certain set of books (or food) in mind and the speaker is aware of that. On the other hand, ce 'what' + NP doesn't exhibit this restriction and can be used regardless of the availability of a previously introduced set.

Formalizing the empirical findings

We now formalize the empirical findings in Section 2.1 by adapting a popular sematic analysis for wh-phrases in English-the one in [START_REF] Karttunen | Syntax and semantics of questions[END_REF], outlined in his ex. (32) and ex. (47). According to this approach, what + NP and which + NP receive the same semantic analysis: they both denote existential generalized quantifiers, exactly like some + NP. They differ in how they semantically combine with their sister constituent, that is the remainder of their clause. While some + NP combines with its sister constituent to return a truth value (or a proposition) by standard function application, a special rule is postulated to combine a wh-phrase in an interrogative clause with its sister constituent and produce a set of propositions (Karttunen 1977: ex. (33) and ex. ( 45b)). If we adapt Karttunen's analysis of wh-determiners to Romanian by incorporating our findings about the pragmatic import of care 'which', the wh-determiner ce 'what' and the wh-determiner care 'which' can receive the logical translations in ( 14) and ( 15), respectively.

(14) ce INTERR 'what' ⤳ λQλP∃y[Q(y) ∧ P(y)] (15) care INTERR 'which' ⤳ λQλP: Q ∈ D REF . ∃y[Q(y) ∧ P(y)]
The wh-determiner ce 'what' in ( 14) semantically behaves like a plain indefinite determiner, returning the value true if and only if the intersection between the set of individuals Q denoted by its NP complement and the set of individuals P denoted by the rest of its clause is non-empty. The wh-determiner care 'which' in (15) adds to the semantic contribution of ce 'what' the presuppositional requirement that the set Qthe denotation of its NP complementbe already a member of the set of discourse referents; if this condition is not satisfied, care 'which' returns no semantic value. 9 An anonymous reviewer and our editors encouraged us to consider also Rullmann and Beck's (1998) analysis for which-phrases in interrogative clauses in English. Their analysis takes Groenendijk andStokhof's (1982, 1984) criticism of Karttunen's proposal into consideration and treats which + NP as a (Fregean) singular definite DP: it refers to an individual in the set denoted by its NP and presupposes the uniqueness of such an individual. As shown in ( 16), [START_REF] Rullmann | Presupposition projection and the interpretation of which-questions[END_REF] treats which as a determiner that combines with the set denoted by Q (its NP complement) to output the individual that is in the set and is identical to the individual denoted by 8 A possible diachronic source for this presuppositional component may be the fact that in old Romanian care is the only wh-word that incorporates the enclitic definite determiner-carele (M.SG), carii (M.PL), carea (F.SG), carele (F.PL) (Pană Dindelegan 2016). 9 The logical translation in (15) assumes that a set and its intensional counterpart (a property) can act as referents in the discourse (see [START_REF] Nouwen | Plural pronominal anaphora in context: dynamic aspects of quantification[END_REF]. Also, we are assuming some form of quantifier domain restriction on the denotation of the NP complement of care, as it is usually done for the NP complement of any (quantificational) determiner. Syntactic, semantic, and pragmatic analyses have been proposed for quantifier domains restrictions (see [START_REF] Stanley | On quantifier domain restriction[END_REF] for an overview). As far as we can tell, our proposal in ( 15) is compatible with any of those approaches. the variable xi. Such an individual is presupposed to exist and be unique. 10 The variable over individuals xi is then existentially bound by the interrogative complementizer.11 (16) which INTERR ⤳ λQιy[Q(y) ∧ y=xi] So for example a question like Which actress won? presupposes that the winner is an actress and denotes a set of propositions of the form 'The actress x won', as in {The actress Ingrid Bergman won, the actress Bette Davies won, the actress Anna Magnani won, …}. Note however that, although presuppositional, the denotation in ( 16) does not impose any requirement that the set denoted by the NP must have been previously mentioned. In other words, Rullmann and Beck's (1998) discussion of the presuppositional properties of which + NP does not make any reference to its discourse anaphoricity, a meaning component which we have shown to be crucial for Romanian care 'which'. They also do not compare which + NP to what + NP, nor do they provide an analysis for what + NP. Most of the arguments motivating their analysis of which + NP apply to what + NP as well, meaning that what + NP should receive a similar analysis. Accordingly, insofar as we can tell, their proposal cannot capture the contrasts between care 'which' + NP and ce 'what' + NP we discussed in the previous section.

In the next section, we investigate whether the semantic analyses we just sketched for ce 'what' + NP in ( 14) and care 'which' + NP in (15) can be adopted as such or can at least be adapted to account for the two varieties of FRs under investigation, Max-FRs and Ex-FRs, and the puzzle presented in Section 1. In particular, we want to explore whether the "Karttunenian" analysis of the wh-determiner care 'which' in (15) provides reasons why care 'which' may be banned in Max-FR and Ex-FRs.

Before concluding this section, a cautionary crosslinguistic remark is needed. Some of the contexts that do not license care 'which' + NP in interrogative clauses in Romanian may, instead, be appropriate for which + NP in English (e.g. Context 1 in (11)). Similarly, the contexts licensing ce 'what' + NP may not be exactly the same as those licensing what + NP (e.g. Context 2 in (11)). These possible crosslinguistic differences deserve further investigation. Here we just want to raise the general issue that, even if the repertoire of complex wh-phrases in a language includes some form of 'what' + NP and 'which' + NP, it should not be taken for granted that the way these two complex wh-phrases are sensitive to discourse conditions and the way they differ from each other in this respect are necessarily identical across languages. Consequently, they should be carefully investigated in each language.

Back to the puzzle about free relative clauses

Let us now return to the puzzle we started with concerning the interrogative clauses in (1)-( 2) vs. the non-interrogative wh-clauses in ( 3)-( 8). In this section, we suggest a solution in light of the observations and conclusions in Sections 1 and 2. We first provide an explicit semantics for ce 'what' + NP in Max-FRs and Ex-FRs in Romanian and then speculate on why a similar move does not extend to care 'which' + NP.

As mentioned in Section 1, Max-FRs behave like definite DPs semantically, while Ex-FRs resemble narrow scope indefinites (see references in fn. 2). For the sake of simplicity, we assume that they are both fully clausal (CPs) and share most of their semantic derivation, including the meaning of their wh-phrases. If Ex-FRs are smaller than CPs, our core observations and suggestions would still hold. Let us look at the concrete example in ( 17), showing the semantic derivation of the CP of the Max-FR in ( 3) and the Ex-FR in (4) with lexical entries in English and the verbal form 'take' standing for both the indicative form in the Max-FR and the subjunctive form in the Ex-FR in the Romanian original clauses. The wh-determiner ce FR in (18) semantically behaves like a purely functional/logical element: it applies to two sets of individuals to return their intersection. Once ce FR combines with the set denoted by its NP complement, the whole DP turns into a set restrictor: it applies to a set of individuals Q to return the subset of Q containing those members that are in the denotation of the NP complement as well, as shown in ( 19). In the semantic derivation in ( 17), ce FR combines with 'book' to return the function that restricts whatever set it applies to next to its subset containing books. When the wh-DP combines with the C' denoting the set of individuals that Maria took or can take, it returns the subset of those individuals that are books, as shown by the logical translation of CP in ( 17). This set is existentially closed by the matrix existential predicate in an Ex-FR, while it is type-shifted into its maximal individual in a Max-FR (see references in fn. 2 for details).

If we compare the semantic contribution of ce FR in FRs in (18) to the one of ce INTERR in ( 14), we can see that both wh-words apply to two sets and trigger their intersection. ce FR doesn't do anything else semantically, while ce INTERR existentially quantifies over the intersection. In other words, ce INTERR semantically behaves like a plain existential determiner (e.g. a/some in English or un(M)/o(F) in Romanian). This meaning change from existential quantification to λabstraction/set-formation applies not just to ce FR , but to all the wh-words occurring in FRs.

If it is so natural for wh-words in Romanian to occur in FRs, what prevents care from joining all its wh-colleagues in FRs? In other words, why doesn't Romanian allow for a care FR with a logical translation like the one in (20)?

(20) *care FR 'which' ⤳ λQλPλy: Q ∈ D REF . [Q(y) ∧ P(y)]
The denotation in (20) contains exactly the same presuppositional discourse-requirement as care INTERR in (15) minus the existential quantifier, that is the meaning change we assume to take place with all other wh-words in Romanian when they occur in an FR. But care FR is not an actual lexical item of Romanianthis is why it is starred in (20).

Although we do not have a full-fledged answer to this question, we provide evidence that the lack of care FR should be analogized to a broader phenomenon: the incompatibility between headless relative clauses, that is relative clauses lacking a nominal head they modify, and an anaphoric/deictic determiner-like element. The semantic role of headless relative clauses is to denote a set by using all the richness and articulation of a clause. The core property of a discourseanaphoric element is to refer back to a referent that has already been introduced in the discourse and, therefore, needs not to be described. These two meaning functions seem to be at odds.

For instance, the personal pronoun he refers back to the only contextually salient male, while the demonstrative pronouns that and those refers back to the only contextually salient object or objects. All these pronouns can introduce headless relative clauses in English that differ from FRs and are known as light-headed relative clauses [START_REF] Citko | On headed, headless, and light-headed relative clauses[END_REF][START_REF] Caponigro | Introducing Headless Relative Clauses and the findings from Mesoamerican languages. Headless relative clauses in Mesoamerican languages[END_REF]. This use of he, that, and those matters for our investigation because it shows that, when followed by the rich content of a headless relative clause, these pronouns lose their anaphoric properties: both those and he in (21) and that in (22) introduce a bracketed wh-headless relative clause and none of them triggers reference to any contextually-salient individual. Notice that the option of losing anaphoric features doesn't generalize to the whole class of anaphoric pronominals but crucially depends on the individual lexical item. For instance, it is permitted with he, although sounding archaic or proverb-like, and fully acceptable with those and that, as shown in ( 21) and ( 22), but it is completely banned with anaphoric pronouns like it, they, this, and these, as shown in ( 23) and ( 24).

(23) * They/These [who laugh last], laugh best.

(24) * A very small part of acting is it/this [which/that takes place on the stage]! Analogous facts hold for Romanian. Whatever mechanism is responsible for the loss of anaphoric features of some pronominals when they introduce headless relative clauses and the complete unacceptability of others in the same constructions, the very same mechanism can be used to account for the ban on the wh-determiner *care FR 'which' as well. More generally, this mechanism would predict two options: (i) either care retains its anaphoric features and cannot occur at all or (ii) care has to lose its anaphoric features to be able to occur in a non-interrogative wh-clause. If existential 'have' existentially quantifies over its set-denoting complement triggering the assertion of its non-emptiness, it would trigger infelicity if combined with a complement like care that anaphorically refers to a set of individuals, which is already well-established as a discourse referent and, as such, non-empty.

The second option-wh-determiner care without its anaphoric features-is attested in non-interrogative wh-clauses other than Max-FRs and Ex-FRs. For instance, the bracketed clause in (26), taken from Brașoveanu 2008:49, is a correlative clause.

(26) [Pe care om l-a interogat Securitatea], în acela ACC which person CL.ACC.3SG-has interrogated Security-the in DEM.3M.SG nu mai am încredere. not anymore have.1SG trust 'I don't trust any person interrogated by the secret police anymore.' It occurs left dislocated, while Max-FRs and Ex-FRs occur clause-internally, as arguments or adjuncts. Also, it licenses the italicized demonstrative pronoun in the matrix clause, which can be optional in subject position (Romanian is subject pro-drop). Last, the bracketed clause in (26) cannot be replaced by nor be interpreted as a definite nor an indefinite DP. Its semantic contribution adds a universal or free choice component to the interpretation of the whole sentence (see [START_REF] Brașoveanu | Uniqueness effects in correlatives[END_REF].

The bracketed clause in ( 27), instead, is a kind of wh-clause originally discussed in Rudin 1986 and recently described in Caponigro & Fălăuș 2020. It obligatorily involves a wh-clause with more than one wh-phrase, each of which is related to an argument of both the matrix and the embedded predicate. For instance, in ( 27) the children eating are the ones that want to eat and the things they eat are the things they want to eat. Crucially, care 'which' + NP can only be replaced by a universally quantified DP or the free choice DP oricare/orice copil 'any child' in order to obtain a sentence that is roughly truth-conditionally equivalent to the original one.

The free-choice/quantificational interpretation of the bracketed wh-clauses introduced by care 'which' in ( 26) and ( 27) exhibits no anaphoric component. Why can't that care introduce Max-FRs or Ex-FRs as well then? The syntactic and semantic differences between the constructions in ( 26) and ( 27) on the one hand, and Max-FRs and Ex-FRs on the other indicate that we are dealing with different constructions and possibly different care 'which', each of which has to be specifically licensed for that construction. If care 'which' in ( 26) and ( 27) contributes to the free choice/quantificational interpretation those constructions receive, then that care 'which' would not be able to occur in Max-FRs and Ex-FRs, whose interpretation is incompatible with free-choice or universal quantification.

The acceptability of the wh-determiner care 'which' in ( 26) and ( 27) raises various questions on the lack of anaphoricity displayed in these wh-clauses. Why and how does the anaphoric component of careINTERR 'which' (described in Section 2) disappear in correlative clauses for example? Is this a systematic difference between wh-determiners in interrogative vs. correlative clauses across languages or are there languages where the semantic/pragmatic properties of whdeterminers are identical in interrogative clauses, correlative clauses and FRs? These are open questions, which can only be addressed based on thorough crosslinguistic investigation. Typologically, it is common to observe variation across non-interrogative wh-constructions within the same language with respect to the subset of wh-words introducing them [START_REF] Caponigro | Free not to ask: on the semantics of free relatives and wh-words crosslinguistically[END_REF][START_REF] Caponigro | Introducing Headless Relative Clauses and the findings from Mesoamerican languages. Headless relative clauses in Mesoamerican languages[END_REF]. We are not aware of any general account of this variation within a language and the crosslinguistic variation in the use of wh-words would disfavor such a general approach. Language specific or idiosyncratic restrictions are likely to be at play.

In the specific case of Romanian, if the wh-determiner care had undergone a meaning change in order to be licensed in Max-FRs and Ex-FRs, it would have become fully identical to ce FR 'what' + NP in (18). Some form of blocking may disfavor a lexical item from assuming exactly the same meaning as another lexical item with exactly the same distribution. 14 Also, care 'which' would have to change its meaning along two dimensions (loss of existential quantification and loss of anaphoricity) in order to be able to occur in Max-FRs and Ex-FRs, while ce 'what' loses its existential strength only. Although neither ban is absolute, the combination of both may be harder to overcome and explains the lack of careFR. Interestingly, care 'which' + NP in the non-interrogative wh-clauses in (26) cannot be replaced by ce 'what' + NP: the result is judged degraded and only a kind reading is marginally available. This pattern may be taken as showing blocking as well, although in the reverse direction: the meaning switch that has taken place with the care 'which' occurring in these constructions blocks ce 'what' from doing exactly the same.

Conclusions

We have presented a previously unnoticed and puzzling contrast between the unconstrainted productivity of ce 'what' + NP in Max-FRs and Ex-FRs and the complete ban on care 'which' + NP in the same two constructions in Romaniana language in which both types of FRs are highly productive and can be introduced by all the other wh-words and wh-phrases that are attested in interrogative clauses. We have also advanced and supported the novel observation that care 'which' + NP in interrogative clauses in Romanian exhibits strong discourse-anaphoric requirements, unlike ce 'what' + NP. We have connected the two findings by suggesting that a clash may be triggered if discourse anaphoricity and set denotation are directly combined by means of just a clause, without a nominal that the clause modifies. Lexical blocking and further restrictions on the complement of existential predicates may play a role as well.

The constraints we have invoked (and their interplay) are neither absolute nor exhaustive. Some languages may apply further constraints and ban both 'what' + NP and 'which' + NP from introducing FRs. This would be the case for Italian, as mentioned earlier. Others may ban 'which' + NP from FRs and allow 'what' + NP in FRs, but with restrictions, rather than as freely as in Romanian. This would be the case for English, if Andrews' 1975 generalizations hold under further scrutiny: 'what' + NP in Max-FRs in English would apply to two sets to return not their intersection, but rather a kindthe kind that is instantiated by the members of the intersection of those two sets. 15 Because these constraints are "soft", there may be languages that overcome the ban on complex anaphoric descriptions and/or semantic lexical blocking and allow for 'which' + NP in FRs. We have no clear evidence of any language of this kind yet. 16Overall, our findings contribute to the growing body of crosslinguistic evidence showing that wh-words occurring in non-interrogative clauses need to be licensed not only construction by construction but also wh-word by wh-word in each language. While 'which' + NP can never introduce a Max-FR nor an Ex-FR in Romanian, it can introduce other non-interrogative wh-constructions like correlative clauses, as seen in ( 26), or Rudin's sentence, as seen in ( 27) above, in the very same language. The full nature of this licensing is one of the main open issues deserving further investigation.

We hope that our paper will encourage further language-specific and crosslinguistic investigation on clauses introduced by 'what' + NP and 'which' + NP, with special attention to the semantic/pragmatic properties that these wh-phrases exhibit in interrogative clauses and the way these properties transfer to FRs and non-interrogative wh-constructions in general.

  6.3.2 for a thorough discussion and morphosyntactic and semantic arguments supporting our claim that the bracketed clauses in (9a,b) and in the Romanian examples in (3) and (4) are FRs, rather than internally headed relative clauses or correlative clauses. Let us start by considering the three Romanian examples in (11): the interrogative clause in (11a) is introduced by ce 'what' + NP, the one in (

  12

  ⤳ λy[book'(y)∧take'(m,y)] DP1 ⤳ λPλy[book'(y) ∧ P(y)]C' ⤳ λx1.take'(m,x1)D NP C IP ⤳ take'(m,x1) what ⤳ λQλPλy[Q(y) ∧ P(y)] book ⤳ λx2.book'(x2) λ1 Maria take t1 ⤳ x1The logical translation of C' in (17) denotes the set of objects that Maria took or can take. This is the standard semantic contribution of a wh-clause in general right before combining with its whphrase. The logical translation of the NP is standard as well, denoting a set of individuals. What characterizes FRs-regardless of whether Max-FRs or Ex-FRs-is the meaning of its wh-word and the whole wh-phrase. The logical translation of the wh-determiner ce 'what' in an FR (henceforth, ce FR ) is given in (18), while the resulting translation for the whole wh-phrase is given in its general form in (19), with NP' standing for the logical translation of the NP complement of ce 'what'. (18) ceFR 'what' ⤳ λQλPλy[Q(y) ∧ P(y)] (19) [DP ceFR + NP] ⤳ λPλy[NP'(y) ∧ P(y)]

( 21 )

 21 Those/He [who laugh(s) last, laugh(s) best]. (22) A very small part of acting is that [which takes place on the stage]! (T.S. Eliot, The Sacred Wood)

(

  27) A mâncat [care copil ce a vrut ]. has eaten which child what has wanted 'Every child ate whatever they wanted.'

  3) Max-FRs introduced by 'what' + NP Ana a luat [ ce { carte/ cărți/ mâncare} a luat și Maria]. Ana has taken what book books food has taken also Maria 'Ana took the book/books/food Maria took as well.'

	(4) Ex-FRs introduced by 'what' + NP
	Maria are [ ce { carte/ cărți/ mâncare} să	ia].
	Maria has what book books food	SBJV take.3SG
	'Maria has a book/books/food to take.'

  Andrews 1975 reports them as acceptable with a plural or mass NP complement, but unacceptable with a singular count NP complement, as shown in (9a,b). 6 (9) a. I drank [{what beer}/{*what glass of milk} was provided]. b. Fred hid [{*what weapon was}/{what weapons were} on the table].

	(10) % Ana took [what {book/books/food} Maria took].
	The English native speakers we asked confirmed Andrews' judgments, but found (10)the
	English equivalents of the fully acceptable Romanian examples in (3)either all degraded or all
	acceptable, regardless of the NP complement. This variation in acceptability of Max-FRs
	introduced by what + NP depending on the examples and the speakers needs a study of its own.
	On the other hand, FRs introduced by which + NP, which Andrews 1975 doesn't mention, are all
	less acceptable than their what + NP counterpart without any kind of variation, at least according
	to our consultants.

  If we slightly change the scenario as in Context 2, and have Ana explicitly mention a list of books Maria is familiar with, then care 'which' + NP and the partitive construction join ce 'what' + NP in being felicitous, as shown in (11a-c). Neither (11b) nor (11c) would be acceptable if Ana made reference to a list of books Maria is not familiar with.

			luat?	√ (Context 1)
	what book have.2SG taken	√ (Context 2)
	'What book did you get?'
	b. Care carte ai	luat-o?	# (Context 1)
	which book have.2SG taken-CL.3SG	√ (Context 2)
	'Which book did you get?'
	c. Care dintre cărți ai	luat-o?	# (Context 1)
	which of	books have.2SG taken-CL.3SG	√ (Context 2)
	'Which of the books did you get?'

(11) CONTEXT 1: Ana says: "I went to the nearby bookstore and got a book for Lia." Maria asks:

CONTEXT 2: Ana says: "I went to the nearby bookstore and got a book for Lia out of the list you recommended." Maria asks:

a. Ce carte ai

  Let us now consider the set of examples in (13), which differ with respect to the wh-phrases introducing them as above: ce 'what' + NP in (13a), care 'which' + NP in (13b), and the partitive care dintre 'which of' + NP in (13c). Ana just said that the Academy Award (aka Oscar) for Best Actress was announced. The list of nominees has not been introduced in the discourse. Maria asks: CONTEXT 2: Ana just said that the Oscar for Best Actress was announced after she had mentioned the names of two nominees and made it clear that there were others. Maria asks: CONTEXT 3: Ana just said that the Oscar for Best Actress was announced after she had listed the names of all the nominees. Maria asks:

	c. # Care dintre bărbați a intrat?
	which of	men	has entered
	'Which of the men entered?'
	(13) CONTEXT 1: a. Ce actriță a câștigat premiul Oscar anul	acesta?	√ (Context 1)
	what actress has won	award-the Oscar year-the this	√ (Context 2)
	'What actress won the Oscar this year?'	√ (Context 3)
	b. Care actriță a câștigat premiul	Oscar anul acesta?	# (Context 1)
	which actress has won	award-the Oscar year-the this	# (Context 2)
	'Which actress won the Oscar this year?'	√ (Context 3)
	c. Care dintre actrițe a câștigat premiul Oscar anul	acesta?	# (Context 1)
	which of	actresses has won	award-the Oscar year-the this	# (Context 2)
	'Which of the actresses won the Oscar this year?'	√ (Context 3)
					are
	conversing on the sidewalk. Maria asks:
	a. √ Ce bărbat a intrat?
	what man has entered
	'What man entered?'
	b. # Care bărbat a intrat?
	which man has entered
	'Which man entered?'

This time, we investigate the felicity of each interrogative clause in three different contexts. In all contexts, Ana says that the Oscar for Best Actress was announced, and then Maria asks (13a-c). Both Ana and Maria know that every year there is a list of nominees for the Oscar for Best Actress. The contexts vary in the amount of knowledge of this list right before Ana's announcement and Maria's question: none in Context 1, a partial one in Context 2, and complete one in Context 3.

  Romanian illustrates either option in different constructions. The first option is at the center of the puzzle we are discussing: *care FR 'which' is banned in both Max-FRs and Ex-FRs. Ex-FRs may add further evidence against the anaphoric care FR in (20). Remember that Ex-FRs very naturally occur as the complement of the existential predicate a avea 'to have'. It looks like care INTERR cannot be base-generated in this syntactic position, as shown in (25a). On the other hand, ce INTERR exhibits no such restriction while triggering a 'kind' interpretation, as shown by the acceptability of the interrogative clause in (25b) and the 'kind' answer it licenses.

	(25) a. * [ Care prieteni] îi	ai __? 13
	which friends CL.3PL have.2SG
	('Which friends do you have?')
	b. [ Ce prieteni] ai __?
	what friends have.2SG
	'What kind of friends do you have?'
	(ANSWER 1: # 'The friends you met yesterday.'; ANSWER 2: √ 'Friends that I
	can fully trust.')

Both ce and care can be used without NP complements (see e.g. Pană Dindelegan 2013), much like English what or the relativizer which. In this paper, we focus exclusively on the semantic and pragmatic properties of ce and care as wh-determiners with an overt NP complement and leave possible connections with other uses for future investigation.

This contrast can be inferred from the description of Romanian FRs in[START_REF] Grosu | Relative clause constructions and unbounded dependencies. A reference grammar of Romanian[END_REF]

and the fact that care is missing from this list: "The relative elements that can head a DP (i.e., relative determiners) are ce 'what', orice 'whatever' and oricare 'whichever'." 6 Example (9a) is adapted fromAndrews 1975: 76, ex. (76b,c) and example (9b) fromAndrews 1975: 76, ex. (77b,c).[START_REF] Andrews | Studies in the syntax of relative and comparative clauses[END_REF] calls the bracketed clauses in 9a,b "internally headed relative clauses." See de Vries 2002: ch. 2, sec.

We would like to thank one of the editors for remarks that made us reflect further on this aspect.

See also den Dikken & Giannakidou's 2002:41 remarks that which + NP in interrogative clauses in English is "existential" and "presuppositional".

The logical translation for which in (16) is inferred from Rullmann & Beck 1998:39 and related discussion.[START_REF] Rullmann | Presupposition projection and the interpretation of which-questions[END_REF] formulate their proposal intensionally. We are providing an extensional version in order to facilitate its comparison with the analysis in (15).

The Ex-FR in 4 has a null subject that is obligatorily coreferential with the matrix subject Maria. This is a very common feature of Ex-FRs, as discussed in the literature we refer to in fn. 2. For the sake of simplicity, we are assuming the subject to be overt for the Ex-FR in our representation in (17).

Our consultants find (i) acceptable, with care 'which' + NP obligatorily preceded by the preposition de 'of'. As made clear by the translation of the interrogative wh-clause and by its only possible answer, care 'which' in this use only allows for a 'kind' interpretation without any discourse anaphoricity. This is the same semantic behavior as the interrogative clause with ce 'what' in (25b). (i) De care prieteni ai ? of which friends have.2SG 'What kind of friends do you have?' (ANSWER 1: # 'The friends you met yesterday.'; ANSWER 2: √ 'Friends that I can fully trust.')

See Embick & Marantz 2008 and references therein for the general notion of blocking and Chierchia 1998 for blocking applied to semantic rules and type shifting.

[START_REF] Heim | Where does the definiteness restriction apply? Evidence from the definiteness of variables. The representation of[END_REF] suggests that what + NP in interrogative clauses in English licenses a variable ranging over subkinds of the kind denoted by its NP complement, while which + NP is inherently partitive and licenses a variable ranging over individuals.

[START_REF] Royer | Headless relative clauses in Chuj. Headless relative clauses in Mesoamerican languages[END_REF] reports that in Chuj (Mayan) Max-FRs can be introduced by both 'what' + NP and 'which' + NP(Royer 2021: ex. 49 and 50); the only example with 'which' seems to be heavily discourse dependent, based on its translation, but it lacks an overt NP complement.