Aggregation-Induced Emission Enhancement and Solid-State Photoswitching of Crystalline Carbazole N-Salicylidene Anilines

The development of fluorescent stimuli-responsive organic materials has attracted substantial interest due to their increasing optoelectronic applications. This study systematically introduces fluorine atoms on one end of carbazole-based N-salicylidene anilines 5a–5f to elucidate the impact in their solution and solid-state photophysics. The addition of fluorine atoms at one end of the molecule induced significant changes, for example, a reduction in the quantum yield (QY) fluorescence emission in solution, going from QY near unity in compound 5a (QY ∼ 100%) to a negligible emission in 5f (QY < 1%). Similarly, compound 5a showed a very strong aggregation-induced enhancement emission behavior, whereas compounds with a higher fluorine content were almost quenched. Furthermore, the crystalline solid-state photoisomerization in N-salicylidene anilines is not trivial, and only compounds with three (5e) and five fluorine atoms (5f) exhibited reversible solid-state photoisomerization under 405 nm light source irradiation. We propose that the presence of the arene-perfluoroarene interaction in the crystalline array facilitates the latter behavior. Our findings present a comprehensive study of crystal engineering for the obtention of photoswitchable crystalline materials and adjustable photophysics response, paving the way for its implementation in other systems.


■ INTRODUCTION
The design and synthesis of pure organic luminescent materials are the subject of intense research given their numerous optoelectronic applications, including light-emitting diodes, 1 organic field-effect transistors, 2 chemical sensors, 3 materials for bioimaging, 4 or stimuli-responsive materials. 5To this end, continuous efforts are directed toward developing simple yet functional turn-on fluorescent materials, such as compounds based on a Schiff-base moiety. 6,7The materials based on that structure offer several advantages, such as low toxicity or relatively direct synthetic methods.They can also be structurally modified with a wide range of functional groups, making them ideal candidates for the development of new smart materials. 8-Salicylidene anilines are well-known photoactive molecules that exhibit various phenomena in both solution and the crystalline solid state, such as thermochromism, 11 or photochromism, 9 or a combination of both. 10Thermochromism is attributed to modifications in ground state keto−enol tautomerism equilibria, 11 and photochromism can be due to excited state intramolecular proton transfer 12 or photoisomerization. 13It has been suggested that the "dihedral angle rule" dictates whether a compound exhibits a photochromic response.However, this rule states that the dihedral angle primarily influences the intensity of the photochromic effect rather than determining its existence. 14Regulating the balance between thermochromism and photochromism in the crystalline solid state is still very challenging.Some strategies to control their responsive behavior have been developed, such as cocrystallization 15 or incorporation in metal−organic frameworks as linkers 16 or guests 17 with applications in various fields, including molecular photoswitching 18,19 or data storage. 20chieving densely packed photochromic crystals remains an exciting challenge, mainly when using common groups such as Schiff bases that require significant changes in volume upon isomerization within crystals.To address this issue, bulky surrounding groups may facilitate the isomerization process, thus increasing the probability of the phenomenon occurring in the crystalline solid state. 21onsidering the reported photochromic behavior of Nsalicylidene anilines, 22 in this work, we explored the synthesis of Schiff bases with progressively fluorinated phenyl rings at one end.The N-salicylidene anilines were linked to a carbazole component to explore their resulting emissive behavior (Figure 1a).Fluorinated derivatives have been reported to change the supramolecular array through new noncovalent interactions.In some cases, perfluorination may significantly improve the mechanical strength of nanotubes, 23 the change of the photophysical properties in solution, 24 or the modulation of the fluorescence in cocrystals. 25We envisioned that adding an increasing number of halogen atoms in the periphery of the aniline precursor of the Schiff base might result in (1) changes in the fluorescence response in solution as in aggregates due to the electron-withdrawing nature of the fluorine atoms and (2) changes in the crystalline array perhaps leading to solid-state photochromism (Figure 1b).We have found that all compounds are aggregation-induced emission enhancement luminogens (AIEEgens). 26Furthermore, such behavior is more pronounced in compound 5a with no fluorine atoms, showing an excellent solid-state quantum yield.Conversely, we switch from a nonphotochromic crystal, 5a, to two fluorinated crystals, 5e and 5f, with reversible photochromism, possibly due to the arene-perfluoroarene interactions in their crystal packing.

■ EXPERIMENTAL SECTION
Synthesis of all compounds was carried out in a two-step methodology with a C−N Ullmann reaction followed by an imine condensation.All the compounds were crystallized by slow evaporation from CH 2 Cl 2 and structurally characterized by single crystal X-ray diffraction.Detailed crystallographic analysis was carried out with Crystal Maker and Mercury software.Dissolution steady-state absorption and emission spectra were recorded with a concentration of 4 μM in different dissolvents, with every sample excited at the absorption maxima; for aggregation experiments, the media were gradually changed from 100% THF to 100% water.The luminescence lifetime was measured using the standard timecorrelated single-photon counting technique using a picosecond pulsed diode laser of wavelength 405 nm from Edinburg Instruments (EPL-405).For photoisomerization experiments, a diode laser at 405 nm (57 mW) was employed.DFT or TDDFT calculations were carried out with the CAM-B3LYP functional and the 6-311++g(d,p) basis set.The detailed experimental section can be found in the Supporting Information.
■ RESULTS AND DISCUSSION Synthesis of Carbazole-N-salicylidene Anilines.The preparation of compounds 5a−5f was carried out by performing a C−N Ullmann-type coupling reaction between compounds 1 and 2, as outlined in Scheme 1.To this end, compound 2 was synthesized according to the literature protocol. 27An Ullmann reaction with commercially available carbazole was used to afford derivative 3 with a 26% yield.Better results were achieved for the obtention of compound 3 when the reaction was conducted using microwave irradiation (100 W for 2 h at 150 °C), affording 54% yield.We also included a chlorine atom in 5d.However, its crystalline array is isostructural with compound 5c having two fluorine atoms.The −Cl atom does not play a significant role in the emissive properties.Subsequently, the condensation reaction between 3 and different substituted anilines 4a−4f produced Nsalicylidene anilines 5a−5f with yields ranging from 51 to 79% (Scheme 1).All of the compounds were characterized by 1 H and 13 C solution NMR, FTIR, and high-resolution mass spectrometry (Section S2).
Structural Characterization by Single-Crystal X-ray Diffraction.All synthesized compounds were crystallized by slow evaporation from CH 2 Cl 2 , and suitable specimens were studied by using single-crystal X-ray diffraction.Synchrotron radiation was employed in the case of 5f due to its very small crystal size.For illustrative purposes, in this section, we discuss only the crystalline array of compounds 5a and 5f in detail.The relevant crystallographic data of all compounds and a detailed description of the crystal packing of compounds 5b to 5e are provided in Sections S3 and S5 in the Supporting Information.
To provide a systematic description, we established some parameters that include the angle between the planes formed by the carbazole and central phenyl ring (ϕ), the angle between the planes of the central and outer phenyl rings (φ), the angle formed by the carbazole and outer phenylene (θ), and geometric parameters of the internal OH•••N bond (Figure 2).Detailed information about these parameters can be found in Table 1.
The structure of compound 5a was solved in the P2  3b).By comparison, compound 5f, which was also solved in the P2 1 /c space group although is not isostructural to 5a (Figure S6), exhibited the most considerable O−H distance in the intramolecular O−H••• N hydrogen bonds.Furthermore, the θ angle shows a quasicoplanar conformation facilitating the arene-perfluoroarene stacking in a head-to-tail fashion (Figure 3d) through the [001] direction, and a CH•••F interaction propagates the lattice through the [010] direction (Figure 3e).These interactions are responsible for the crystal packing of the structure (Figure 3f, Table S2).
Steady-State Fluorescence Studies in Solution and the Crystalline Solid State.Generally, Schiff base derivatives show a low emission intensity in solution and moderate emission in the solid state, and they are good bidentate groups that can chelate different metal ions.This aspect makes them excellent candidates to be used as turn-on/off sensors of Zn(II), 28,29 Cu(II), 30 Al(III), 31 drug delivery, 32 or for developing new AIEgens. 33In addition, there are a few Nsalicylidene anilines in the literature with good emission in the solid state.For instance, Tong et al. reported a Schiff base derivative with a QY= 24.3%, 34 and Wang et al. described a compound with a QY = 91.7%,both in the solid state. 35t has been described that the intermolecular interactions between the solvent and the Schiff bases can modify the hydrogen−oxygen distance in the ground state. 36In favorable cases, UV−vis light irradiation is energetic enough to promote an excited state intramolecular proton transfer (ESIPT). 37−41 To determine if ESIPT fluorescence occurs and whether the formation of the keto form in compounds 5a−5f follows an ESIPT fluorescence relaxation pathway, we evaluated the emission behavior by collecting the steady-state fluorescence (PL) response in 13 solvents with different polarities, which could stabilize either the enol or keto excited form.These solvents ranged from the apolar solvent (hexane, ε = 1.88) to the most polar solvent (DMSO, ε = 46.7),using the dielectric constant, ε, as a comparison parameter.It was observed that enol emission is favored in dioxane, while in other solvents, the keto form emission is present, as exemplified in Figure 4b for compound 5a.The behavior was observed in all compounds, with a more noticeable contribution of the keto form in the compounds with a higher number of fluorine atoms (5a < 5b < 5e < 5f) attributable to the increasingly Carbazole rings were labeled as R1, R2, and R3.The inner and outer phenyl rings were designated as R4 and R5, respectively.The ϕ angle is defined between the plane of carbazole ring R2 and the plane of ring R4, the φ angle lies between the planes of rings R4 and R5, and the θ is the angle between the planes of rings R2 and R5.lower basicity of the nitrogen atom.Their complete emission spectra can be found in Section S5a.Despite showing emission in the explored solvents, the PL intensity of all compounds could be better in solution, which motivated us to explore their AIEE properties.We carried out the aggregation experiments by measuring the photolumines-cence under different mixtures of THF:H 2 O (f w = 0−100%) as a well-documented strategy to promote the formation of aggregates reducing the solubility of the studied molecule. 42ur studies show a subtle increase in the emission intensity on going from f w = 0 to 60%.However, upon reaching f w = 70%, an abrupt increment in the emission intensity was observed, especially for compound 5a (Section S5), indicating that at higher water fractions, the compounds create highly emissive aggregates.In fact, in compound 5a, the emission increase was observable by the naked eye (Figure 5a), with fluorescence measurements revealing an increment of 9 orders of magnitude (from 1.4 × 10 4 to 1.7 × 10 13 a.u.), as compared to that carried out in THF (Figure 5b).All of the compounds show AIEE behavior; nevertheless, in compound 5f, the increase in fluorescence was only moderate (Figure 5c).Detailed information on AIEE behavior for compounds 5b−5e can be found in Section S5b.
Subsequently, we further investigated their fluorescence behavior in the crystalline solid state.We measured the crystalline solid-state fluorescence spectra, the fluorescence quantum yield (QY), and their fluorescence lifetimes (τ) (Figure 6).Fluorescence lifetime (FLT) measurements were performed on imines using the time-correlated single photon counting technique.The complete UV−vis and fluorescence spectroscopy data is available in Section S5.Table 2 summarizes the average FLT ( = ) for imines in solid state where τ 1 and τ 2 are referred to fast and slow constants, while A 1 and A 2 are the amplitude constants.Imines show τ values at the nanosecond time scale; fluorophores present multiexponential decays commonly observed for organic fluorophores in the solid state and are determined by intermolecular interactions and dynamics of excited states. 43he experimental photophysical data are summarized in Table 2. Compound 5a, which does not possess fluorine atoms, exhibited the highest quantum yield (near 100%), and its fluorescence lifetime was adequately adjusted with a biexponential decay with τ 1 and τ 2 values of 11.57 and 26.06 ns, respectively.For compound 5b, with a quantum yield of 30%, the lifetime was adjusted with τ 1 = 9.50 ns and τ 2 = 19.35ns, whereas for compound 5c, with a quantum yield of 24%, the lifetimes were τ 1 = 10.35 ns and τ 2 = 22.46 ns.In contrast, it was impossible to determine their fluorescence lifetimes for compounds 5e and 5f that present photochromism phenomena due to our instrument's meager quantum yields and low resolution.
To understand this photophysical process better, we calculated the energy of the frontier molecular orbitals using Density Functional Theory (DFT) with the hybrid functional CAM-B3LYP and the 6-311++g(d,p) basis set.The increase in the number of fluorine atoms resulted in a slight reduction of the HOMO−LUMO energy gap, with a difference of 5.99 eV for compound 5a to 5.66 eV for compound 5f and oscillator strengths of 0.829 and 0.783 respectively (Table 2, Section S4).Results show that fluorescence QY undergoes a more drastic change.The QY reduction in 5f concerning 5a suggests additional relaxation pathways originated from replacing C−H for C−F.Such substitution implies an increased spin−orbit coupling, 44 and electron-withdrawing effects of the C−F bonds, could stabilize nonradiative decay pathways, such as intersystem crossing, internal conversion, and vibrational relaxation. 45These factors collectively favor nonradiative transitions over fluorescence, leading to a decreased quantum yield.Another relaxation pathway that can occur is photoisomerization.To probe this idea, we decided to explore the photoisomerization behavior of these compounds through different strategies, as mentioned in the following section.
Solid-State Photochromic Response.N-Salicylidene anilines are studied by their switching performance. 13It is known that the aggregation phase influences the photochromic behavior of the N-salicylidene anilines, and it has been demonstrated that the photochromic behavior can take place in solution, 46 thin films, 47 after the introduction into a MOF matrix, 17 or in crystalline but not in the amorphous solid state. 38Then, as a first approach to evaluate the photochromic behavior of compounds 5a−5f, we used poly(methyl methacrylate) (PMMA) as a matrix to prepare the thin films.Despite numerous attempts, no discernible photochromic response was observed in the thin films.Furthermore, we tried melting the compounds between two quartz plates to obtain a vitreous solid and subsequently evaluated its photochromic response, but no photochromic response was observed.Finally, polycrystalline samples were placed between two quartz plates and irradiated with a 405 nm light source, revealing a photochromic response only for compounds 5e (Figure S25) and 5f (Figure 7).Therefore, the photochromic response for such compounds can only be facilitated by its crystalline array, supported by the fact that the φ angle is higher than 30°( Table 1).We focused on sample 5f for further evaluation and subjected it to light irradiation.Before irradiation, the samples exhibited an absorption maximum at 416 nm, which was attributed to the absorption of the enol form (Figure 7b).Upon irradiation with a 405 nm laser diode, a secondary band emerged at 516 nm, suggesting the presence of the trans-keto form.Remarkably, this form proved to be stable in the dark for at least 20 min and exhibited reversibility upon exposure to a 532 nm laser (Figure 7c).
Considering the X-ray diffraction data described above, it was possible to assess the potential photochromicity of the Nsalicylidene anilines in their crystalline solid state.It has been reported that as the torsional angle φ deviates from coplanarity, the propensity for photochromism increases. 22,48,49As the data compiled in Table 1 show, the structures 5e and 5f exhibit the most pronounced φ angles, with 34.3 and 43.7°, respectively.In addition, for 5f, there are no CH-acceptor adjacent interactions.Therefore, it is reasonable to expect that the photoisomerization process in 5f could occur in the excited state. 50o get a more detailed understanding of the photoisomerization process, we calculated the frontier molecular orbital energies for the 5f-enol, 5f-cis-keto, and 5f-trans-keto (Figure 8), implied in the photoisomerization process as  illustrated in Figure 7a.Furthermore, we also calculated the UV−vis spectra through TD-DFT for such forms to investigate if the observed photoproduct of 5f was the keto-trans species (Figure S10).Our computations indicate that the calculated absorbance for the 5f-keto-cis is different from the 5f-keto-trans, with a π−π* additional shoulder that agrees well with the solid-state absorption experimental data (Figure 7b).The stability of the 5f-keto-trans form in darkness could be attributed to the high energy barrier in the ground state to reverse from the 5f-keto-trans form to 5f-keto-cis, with a calculated value of 43.4 kcal•mol −1 (Figure S11).

■ CONCLUSIONS
Five carbazole N-salicylidene anilines 5a−5f were synthesized using a two-step methodology involving sequential Ullmann coupling and imine condensation.The fluorescence experiments in solution revealed a gradual decrease in the emission intensity with the inclusion of fluorine atoms, going quantum yields near unity for compound 5a, to an almost quenched fluorescence in compound 5f, suggesting that the fluorine substitution opens an alternative relaxation pathway.Additionally, all the compounds show excited state intramolecular proton transfer (ESIPT) fluorescence.Last, the aggregationinduced emission enhancement (AIEE) properties of all compounds were explored, revealing that the nonfluorinated compound shows a stronger increase in emission intensity, with a diminished fluorescence response observed with the introduction of fluorine atoms.
The inclusion of fluorine atoms is advantageous because it drives the arene-perfluoroarene intermolecular interactions, thus influencing the crystallization and supramolecular array of the compounds reported here.In addition, only trifluorinated 5e and pentafluorinated compound 5f exhibited a photochromic response in the crystalline solid state when irradiated with a 405 nm laser diode, attributed to the enol-cis to ketotrans photoisomerization.The 5f-keto-trans form can be reversed to the 5f-enol form by using a 532 nm laser source.Our X-ray and computational data indicate that this response is possible due to the high torsional angle between the central phenylene ring and the aniline.
Our findings provide a detailed examination of the effect of fluorine substitution in the emission response of this series, shedding light on compounds with potential applications such as ESIPT, AIEEgens, or solid-state photoswitches, leading the way to the design and optimization of crystalline materials with tailored photophysical properties.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c04764.Synthesis conditions, 1 H NMR, 13 C NMR, IR spectra data and HRMS of each compound, X-ray collection   details, DFT calculations and UV−vis and Fluorescence data in solution and the solid state (PDF)

Figure 1 .
Figure 1.(a) Components of the carbazole N-salicylidene anilines reported in this work.(b) Different envisioned states for compound 5f as a photoswitch.
1 /c space group, with one molecule per asymmetric unit.This structure exhibited the smallest O•••H distance for intramolecular hydrogen bonds.The crystal lattice of 5a propagated through the [100] direction via a combination of π•••π stacking and CH•••O interactions (Figure 3a).Additionally, the array was further propagated through the [001] and [010] directions by π•••π and CH•••π interactions (Figure

Figure 2 .
Figure 2. General structure of the N-salicylidene anilines in this work.Carbazole rings were labeled as R1, R2, and R3.The inner and outer phenyl rings were designated as R4 and R5, respectively.The ϕ angle is defined between the plane of carbazole ring R2 and the plane of ring R4, the φ angle lies between the planes of rings R4 and R5, and the θ is the angle between the planes of rings R2 and R5.

Figure 4 .
Figure 4. (a) Four-level photochemical cycle diagram for the ESIPT process.(b) Emission spectra of 5a in 13 different solvents with different polarities.

Figure 5 .
Figure 5. (a) Solution fluorescence response for 5a in different THF:H 2 O mixtures.Aggregation-induced emission enhancement behavior comparison of compounds in different THF:H 2 O mixtures: (b) 5a and (c) 5f.

Figure 7 .
Figure 7. (a) Reversible enol−keto photochromic behavior of compound 5f.(b) Formation and stability of 5f-trans-keto form after irradiation with 405 nm and its stability in darkness.(c) Reversibility of the process after irradiation with 532 nm at different times.

Table 1 .
Conformational Parameters for the Asymmetric Units of Each Crystalline Structure a a Labels i and ii were added where two molecules were resolved per asymmetric unit.

Table 2 .
Photophysical Properties of the Synthesized Compounds, Including Absorption and Emission Maxima, Quantum Yield, Fluorescence Lifetimes, Energy Difference of Frontier Molecular Orbitals, and Oscillator Strength a-means not determined.