Taking Advantage of a Luminescent ESIPT-Based Zr-MOF for Fluorochromic Detection of Multiple External Stimuli: Acid and Base Vapors, Mechanical Compression, and Temperature

Luminescent materials responsive to external stimuli have captivated great attention owing to their potential implementation in noninvasive photonic sensors. Luminescent metal–organic frameworks (LMOFs), a type of porous crystalline material, have emerged as one of the most promising candidates for these applications. Moreover, LMOFs constructed with organic linkers that undergo excited-state intramolecular proton-transfer (ESIPT) reactions are particularly relevant since changes in the surrounding environment induce modifications in their emission properties. Herein, an ESIPT-based LMOF, UiO-66-(OH)2, has been synthesized, spectroscopically and photodynamically characterized, and tested for detecting multiple external stimuli. First, the spectroscopic and photodynamic characterization of the organic linker (2,5-dihydroxyterephthalic acid (DHT)) and the UiO-66-(OH)2 MOF demonstrates that the emission properties are mainly governed by the enol → keto tautomerization, occurring in the organic linker via the ESIPT reaction. Afterward, the UiO-66-(OH)2 MOF proves for the first time to be a promising candidate to detect vapors of acid (HCl) and base (Et3N) toxic chemicals, changes in the mechanical compression (exercised pressure), and changes in the temperature. These results shed light on the potential of ESIPT-based LMOFs to be implemented in the development of advanced optical materials and luminescent sensors.


Index
DMF is a single band with its maximum centered at 370 nm, while its emission spectrum is a broad band (FWHM = 3230 cm -1 ) with its maximum located at ~525 nm (Figure S1A).The large Stokes shift (∆νStokes = 7980 cm -1 ) indicates the occurrence of an ESIPT process, and thus, the emission can be attributed to the keto tautomer.Similar emission band has been previously observed for this linker and was attributed to the emission of a keto structure formed through an ESIPT reaction in the excited enol species. 1,2 owever, as the linker contains two −OH functional groups, we suggest that two different keto tautomers might be formed upon excitation of the enols to the S1 state, mono− and di−keto tautomers (see Scheme 1). 3,4 shed more light on the spectroscopic properties of the possible conformers of DHT linker, we added 0.972 g of HCl (37%) to the DMF solution to stabilize the enol species.
The absorption spectrum of DHT in the DMF/HCl solution is comparable to that found in pure DMF, however, the emission spectrum is different and consists of a combination of 2 bands (Figure S1B).The red-shifted band is similar to that observed in pure DMF, with its maximum at ~525 nm, and therefore, can be assigned to the emission of the keto tautomers.However, the new blue emission band has its intensity maximum located around 450 nm (Figure S1B), being the Stokes shift just 4805 cm -1 .][7] The presence of HCl might partially protonate the C=O groups of the carboxylic acids of DHT molecules, hindering the ESIPT reaction, and stabilizing the enol tautomers.
The steady-state spectroscopic properties of DHT in DMF solution and in presence of to the formation of an anionic specie, which is emitting at wavelengths (570 nm, Figure S1C) longer than the keto tautomer (525 nm, Figure S1A).Moreover, the FWHM of this band is also very broad, with a value of ~ 4328 cm -1 , suggesting the emission of different anionic species, as there are many functional groups that can be deprotonated (Scheme 1).However, we cannot fully discard that even in presence of Et3N, some DHT molecules might undergo an ESIPT reaction leading to the formation of keto tautomers.
Finally, the excitation spectra of DHT in the different environments are very similar to the absorption ones independently on the observation wavelengths (Figure S1), indicating a common origin of the excited species.S1 shows the values of the time constants, amplitudes, and contributions obtained from the multiexponential global fit analysis of the decay traces.Firstly, we focus on the results obtained for DHT in DMF (Figure S2A).The emission decays of DHT in DMF were accurately fitted using a biexponential function, giving two time constants of τ1= 2.6 ns and τ2 = 8.6 ns.As explained above, the broad emission spectrum observed for DHT in DMF indicates the presence of different species at the first electronically excited state (S1) that can be ascribed to mono− and di−keto tautomers, and therefore, we suggest that the observed time components correspond to the emission lifetimes of these species.We propose that the longest component (τ2) would correspond to the emission lifetime of the di-keto tautomer, since this structure will be more stabilized, while τ1 would be the emission lifetime of the mono-keto tautomer (Scheme 1).
The photodynamics of DHT in presence of HCl (DMF/HCl mixture) is rather different, showing a dependency with the observation wavelength (Figure S2B and Table S1).In the bluest region (425 to 475 nm), the decays present a biexponential behavior with time constants of τ1 = 1.1 ns and τ2 = 4.3 ns, however, at lower energies (500 -650 nm) the shorter time component (τ1) vanishes.Since the emission of the enol tautomer appears at higher energies, we attribute the shorter component (τ1), which contributes more to the signal in the bluest spectral region, to the emission lifetime of the enol species, while the longest one (τ2) is the emission lifetime of the keto one.Note that the value of τ2 is a mean value of the two lifetimes observed for DHT in pure DMF.
Finally, the photodynamics of DHT in presence of Et3N (DMF/ Et3N mixtures) exhibits a much more complex behavior, giving three-time constants of τ1= 411 ps, τ2 = 3.1 ns, and τ3= 8.3 ns (Figure S2C and Table S1).As explained above, the broad emission spectrum of DHT in DMF/Et3N reflects the coexistence of different anionic and keto tautomers at the S1 state.Since τ2 and τ3 values are very similar to those found for DHT in pure DMF, we attribute them to the emission lifetime of the mono-and di-keto tautomers, while τ1 (not observed in pure DMF) could be the emission of an anionic specie.However, we cannot rule out that other anionic structures might have similar emission lifetimes to that of the keto tautomers, and therefore, τ2 and τ3 components could be a result of the combination of the emission lifetimes of keto and anionic species.
Once the spectroscopic and photodynamical properties of the DHT linker were unraveled, we will now focus on the UiO-66-(OH)2 structural, chemical, spectroscopic, and photodynamical properties.where I(T) and I0 are the fluorescence intensities at Tn and T0 (303 K), respectively, A is the preexponential Arrhenius factor, kB is the Boltzmann constant and ΔEa is the activation energy barrier of non-radiative processes.

0
.602 g of Et3N were also explored to get information on the possible existence of anionic species.As shown in FigureS1C, the absorption spectrum of DHT in DMF/Et3N is blue shifted (absorption maximum at 355 nm) while the emission spectrum is red shifted (emission maximum at 570 nm) when compared to the spectra obtained in pure DMF (370 and 525 nm, respectively).It is well-known that ESIPT molecules might present Hbond interactions in their ground-state which stabilize the structure, shifting the absorption maximum towards longer wavelengths.[8][9][10]Hence, in presence of Et3N, the −OH groups of DHT linker can interact with the Et3N molecules, breaking the preexistent intramolecular H-bond interactions in DHT molecule, and inducing a blue shift in the absorption maximum.Upon photoexcitation, the DHT molecule will transfer the H atoms to the Et3N molecules (excited state intermolecular proton transfer, ESPT) leading

Figure S1 .Section 1 . 2 :
Figure S1.Normalized absorption (black solid line), excitation (dotted lines) and emission spectra (dashed lines) of DHT linker in A) DMF, B) DMF/HCl and C) DMF/Et3N solutions.The different excitation and observation wavelengths are indicated in the figure.

Figure S2 .
Figure S2.Magic-angle emission decays of DHT in A) DMF, B) DMF/HCl and C) DMF/Et3N.The sample was excited with a 371 nm pulsed laser and probed at the indicated wavelengths.The solid black lines correspond to the best fit of the decays using a multiexponential function, while the IRF is the instrumental response function.

Figure S6 .
Figure S6.Normalized absorption, diffuse reflectance (converted to K-M function, solid lines) and emission spectra (dashed lines) of A) DHT and UiO-66-(OH)2 in DMF and B) UiO-66-(OH)2 in DMF and solid state.The excitation wavelengths are indicated in the inset.

Figure S8 .
Figure S8.Normalized diffuse reflectance (converted to K-M function, black solid line) and emission spectra (dashed lines) of DHT in powder form.The excitation wavelengths are shown in the figure.

Figure S9 .
Figure S9.Normalized excitation spectra of UiO-66-(OH)2 in powder form after its exposure to a saturated atmosphere of HCl and Et3N vapors.A) Observation at 430 nm and B) at 570 nm.

Figure
Figure S10.A) Emission spectra of UiO-66-(OH)2 after different times of exposition to an atmosphere of 85% of humidity.The sample was excited at 380 nm.B) Comparison of the emission intensity (recorded at 465 nm) of UiO-66-(OH)2 after its exposure to a saturated atmosphere of HCl (black squares), and to an atmosphere with an 85% of humidity (red dots).

Figure
Figure S11.A) PXRD patterns and B) FTIR spectra of UiO-66-(OH)2 before and after its exposure to saturated atmospheres of HCl (7 h), Et3N (5 h), and to an atmosphere with an 85% of humidity (7 h) respectively.The graph in A) includes the simulated PXRD pattern of UiO-66 (CCDC 837796).

Figure S12 .
Figure S12.Normalized A) excitation and B) emission spectra of UiO-66-(OH)2 pellets compressed at different pressures (indicated as inset).The excitation and observation wavelengths are indicated in the graphs.

Figure
Figure S13.A) PXRD patterns of UiO-66-(OH)2 pellets after being compressed at different pressures (1 ton, 2 tons, 4 tons, 10 tons), respectively.B) Normalized to 1 (at 525 nm) emission spectra of UiO-66-(OH)2 in powder form (black line), in the form of a pellet after bein applied 10 tons of pressure (red line) and in powder form after grind the pellet (blue line).The samples were excited at 380 nm.

Figure S14 .
Figure S14.Normalized excitation spectra of UiO-66-(OH)2 in powder form at different temperatures (indicated in the inset) recording the signal at A) 440 and B) 570 nm.