Four-Coordinate Monoboron Complexes with 8-Hydroxyquinolin-5-Sulfonate: Synthesis, Crystal Structures, Theoretical Studies, and Luminescence Properties

8-Hydroxyquinolin-5-sulfonic acid (8HQSA) was combined with 3-pyridineboronic acid (3PBA) or 4-pyridineboronic acid (4PBA) to give two zwitterionic monoboron complexes in crystalline form. The compounds were characterized by elemental analysis, single-crystal X-ray diffraction studies, and IR, 1H NMR, UV-Visible, and luminescence spectroscopy. The analyses revealed compounds with boron atoms adopting tetrahedral geometry. In the solid state, the molecular components are linked by charge-assisted (B)(O−H· · ·−O(S) and N+−H· · ·O(S) hydrogen bonds aside from C−H· · ·O contacts and π· · ·π interactions, as shown by Hirshfeld surface analyses and 2D fingerprint plots. The luminescence properties were characterized in terms of the emission behavior in solution and the solid state, showing emission in the bluish-green region in solution and large positive solvatofluorochromism, caused by intramolecular charge transfer. According to TD-DFT calculations at the M06-2X/6-31G(d) level of theory simulating an ethanol solvent environment, the emission properties are originated from π-π * and n-π * HOMO-LUMO transitions.


Introduction
The last few decades have featured the design and synthesis of a large number of luminescent organic and organometallic compounds with outstanding electronic and optical properties [1,2], and a broad spectrum of applications in various fields such as organic light-emitting diodes (OLEDs) [3], laser dyes [4], fluorescence imaging probes [5], solar cells [6], sensors [7], and photodynamic therapy [8], In the past decade, much research progress and important discoveries have been achieved with π-conjugated three-and fourcoordinate boron compounds [9,10]. Frequently, such organoboron compounds are targeted to study the influence on the electronic and photophysical properties of the ligand and substituents attached at different positions, because they can exhibit tunable fluorescence emission ranging from green to orange in the solid state [11,12]. Furthermore, zwitterionic four-coordinate organoboron compounds with rigid π-conjugated structures are promising

General
All reagents and solvents were commercially available from Sigma-Aldrich and used as received without further purification. All preparative methods were performed under normal ambient conditions without use of inert atmosphere.
IR spectra were measured on a Bruker Alpha Tensor 27 spectrophotometer using KBr pellets in the 4000-500 cm −1 region. 1 H NMR spectra were recorded in CD 3 OD at 400 MHz with a BrukerAvance III spectrometer at 30 • C unless otherwise specified. Chemical shifts are reported in ppm and were referenced based on residual solvent resonances. Complexes exhibit low solubility in organic solvents, and therefore the signals referred to carbon and boron atoms were not detected in the corresponding 13 C NMR and 11 B NMR spectra, even after prolonged acquisition. UV-Vis absorption spectra were recorded on a Shimadzu UV-1800 UV spectrophotometer. Emission spectra in solution and the solid state were obtained on a PerkinElmer LS-55 fluorescence spectrophotometer. Elemental analyses were performed on a Vario Micro (Elementar) spectrometer. Melting points were determined with a Büchi B-540 digital apparatus.

Preparation of Boronic Ester Complex 2
Compound 2 was prepared according to the methodology described for 1, except that 4-pyridineboronic acid (0.03 g, 0.244 mmol) was utilized instead of 3-pyridineboronic acid. After three weeks of slow solvent evaporation, yellow crystals suitable for X-ray diffraction analysis had formed. Yield

Crystallography
Single-crystal X-ray diffraction analyses of complexes 1 and 2 were carried out on an Agilent Technologies SuperNova diffractometer equipped with a CCD area detector (EosS2) using Cu-K α radiation (1.54184 Å) from a microfocus X-ray source and an Oxford Instruments Cryojet cooler. Data for compounds 1 and 2 were collected at T = 100 K. The measured intensities were reduced to F 2 and corrected for absorption using spherical harmonics (CrysAlisPro) [35]. Structure solution, refinement, and data output were performed with the OLEX2 [36] program package using SHELXTL [37] for the structure solution and SHELXL-2014 [38] for the refinement. Nonhydrogen atoms were refined anisotropically. C-H hydrogen atoms were placed in geometrically calculated positions using the riding model. O-H and N-H hydrogen atoms in 1 and 2 were located from difference Fourier maps and refined with distance restraints (AFIX 147 for the (B)O-H, AFIX 43 for the N-H and  DFIX 0.85 for the O w -H hydrogen atoms). Hydrogen-bonding interactions in the crystal lattice were calculated with the MERCURY program package [39] DIAMOND was used for the creation of figures [40].  Phase purity was examined by powder X-ray diffraction (PXRD) analysis performed at 295 K, using a BRUKER D8-ADVANCE diffractometer equipped with a LynxEye detector (λ CuKα1 = 1.5406 Å, monochromator: germanium). The equipment was operated at 40 kV and 40 mA, and data were collected at room temperature in the range of 2θ = 5-50 • . The powder XRD patterns of 1 and 2 at room temperature match well with the simulated XRD patterns based on the respective crystal structure (see, Figures S8 and S9), in terms of the peak positions, confirming that the powder samples are single phases, which can be used for the investigation of the photoluminescence properties described in Section 3.5.

Computational Details
Hirshfeld surface analyses and fingerprint plots of 1 and 2 were generated based on the crystallographic information files (CIFs) using Crystal Explorer 3.1 [41][42][43]. The Hirshfeld surface (d norm ), shape index and curvedness were mapped over the range -0.7 to +1.8, -1.0 to +1.0 and -4.0 to +4.0, respectively. Quantum chemical calculations for compounds 1 and 2 were performed by density functional theory [44,45] with the GAUSSIAN09 package [46]. Visualization of calculated parameters was performed by the GaussView molecular visualization program [47]. Minimum energy structures were calculated and confirmed through a frequency calculation (without imaginary frequencies). Transitions between the different orbitals were evaluated with time-dependent density functional theory (TD-DFT) [48,49] at the calculation level M06-2X/6-31G(d) [50,51]. The effects of a solvated environment were evaluated with the integral equation formalism for the polarizable continuum model (IEF-PCM) and the implementation of the nonequilibrium solvation model [52]. The solvent considered for this analysis was ethanol.

Results and Discussion
Organoboron complexes 1 and 2 were obtained from reactions of 8-hydroxyquinoline-5-sulfonic acid (8HQSA) with 3-pyridineboronic (3PBA) or 4-pyridineboronic acid (4PBA) using a solvent mixture EtOH/H 2 O/DMF (15:4:1 v/v/v) (Scheme 1). The novel compounds were obtained in good yields (64 and 73%) as yellow solids, which are slightly soluble in common polar organic solvents. The products were characterized by elemental analysis, powder, and single-crystal X-ray diffraction analysis, as well as IR, 1 H NMR, UV-vis and luminescence spectroscopy (Figures S1-S9, in Supplementary Materials). v/v/v). The compounds crystallized as monoboron complexes in the monoclinic and triclinic space groups P2 1 /c and P-1, respectively. The asymmetric unit of 1 contains the boron complex and a water molecule; meanwhile, the asymmetric unit of 2 is occupied only by the boron complex, without solvent. Figures 1 and 2 show the molecular structures of 1 and 2 with atom labeling. Tables S1-S4 contain selected interatomic distances and bond angles. Geometries for intermolecular hydrogen-bonding interactions and other contacts are given in Table S5 (in Supplementary Materials).  The solid-state structures of the title compounds resemble each other in several aspects but exhibit also significant differences. In both boronic ester compounds, a B←N coordination bond is present giving place to the formation of four-coordinate complexes, in which a fivemembered C 2 BNO ring is joined to the quinoline skeleton. The B←N distances are similar [1.6422(19)] and 1.665(3) Å for 1 and 2, respectively] and in good agreement with the B←N distances (1.568-1.681 Å) reported previously for related boron complexes [33,53] , which is attributed to ring strain in the five-membered chelate ring. The B-C bond lengths are 1.614(2) Å in 1 and 1.621(3) Å in 2, respectively, similar to compounds based on ortho-phenylenediboronic acid and 8-hydroxyquinoline [18,54]. The bond angles around the boron atoms correspond to distorted tetrahedral coordination geometries having values ranging from 97.69 (13) to 114.78(16) • . The variation of the bond angles around the boron atoms induces significant distortion from ideal tetrahedral geometry (τ 4 = 0.95 for 1 y τ 4 = 0.94 for 2) [55], which is seen also from the tetrahedral character values (THC) [56] of 75.8% and 71.7 % for compounds 1 and 2, respectively (see , Tables S6 and S7,  The boron atoms in crystals of 1 and 2 deviate from the quinoline mean planes by 0.118 Å and 0.076 Å, respectively. Because of the tetrahedral geometry, in both compounds the pyridinium and hydroxyl groups are located above and below the boron-bridged π-ring plane of the quinolato moieties. The proton transfer from the sulfonic-acid group in the starting ligand 8HQSA to the pyridyl substituent in complexes 1 and 2 was evidenced by difference Fourier map analysis during the refinement of the crystal structures, and is also confirmed by analysis of the S−O bond lengths in the sulfonate groups ranging from 1.4469(11) to 1.4630(11) Å, exhibiting a difference of less than 0.02 Å (see, Tables S1 and S3, in Supplementary Materials) [57,58].  (Table S5, in Supplementary Materials) [62,63]. Interestingly, motif A also exhibits π· · · π [64,65] interactions among the antiparallel-oriented quinoline residues, which in compound 1 accomplish the entire quinolone skeleton, while in compound 2 only half the quinolone moieties (atoms C4−C9) are involved. The centroid· · · centroid distances are 4.112 Å in 1 (Cg1· · · Cg2, see Table S5 and 3.529 Å in 2 (Cg2· · · Cg2, see Table S5). To the best of our knowledge, so far there are no reports on molecular organoboron crystals with this synthon.

Analysis of the Hirshfeld Surface
To accomplish the description of the supramolecular connectivity in the crystal structures of 1 and 2, Hirshfeld surface analyses were realized. Maps of the Hirshfeld surface, shape index, and curvedness of the complexes are shown in Figure 5. In the Hirshfeld surface maps, areas marked in blue reveal the longest contacts, while the depressions in red color are indicative of the zones of strong donor-acceptor interactions [70].  The shape index is a more sensitive method to evidence subtle changes of the electron density surrounding the molecules. Pairs of red and blue triangles on planar sections in the shape index map are typical for π· · · π interaction sites (C···C contacts) in the supramolecular structure and the percentage contributions can be extracted by fingerprint analysis (Table S8, in Supplementary Materials) [73,74]. For compound 2, the percentage contribution of the C···C contacts (8.2%) is higher compared to 1 (6.2%). The presence of various red and blue triangles in the shape index of 1 and 2 indicates diverse π-π aggregations in the crystal structures ( Figure 5), as corroborated in the fragments shown in Figures 3 and 4. In the curvedness diagrams given in Figure 5c, the green patches with blue outlines represent interactions with adjacent molecules. Curvedness maps are used to identify packaging and the provisions are flat where π-stacking occurs [70][71][72][73][74]. Boronic esters 1 and 2 both exhibit blue coloration in the regions of the aromatic quinoline and pyridine rings, indicating that the three-dimensional networks are developed mainly by π· · · π interactions aside from C−H···O contacts (vide infra).
The hydrogen bonds in complexes 1 and 2 are represented in the 2D fingerprint plots shown in Figure 6. The fingerprints around 1.8 (d i , d e ) vary from a blue tone to a slightly green color and are associated with the C···C contacts from the π· · · π interactions [42,[75][76][77]. The greenish coloration in the central part of the fingerprints corresponds to the stronger π· · · π contributions in the solid-state structures, illustrating that π· · · π interactions are more dominant in 2, as corroborated by the overall shorter centroid-centroid distances of the π· · · π contacts (Table S5, Figure 7 and Table S8 provide an overview of the percentage contributions of the above-mentioned intermolecular interactions and short contacts. The largest contributions correspond to the diverse O· · · H/H· · · O (38.3-40.4%) hydrogen bonds, but also to van der Waals contacts (H· · · H, 28.9-31.3%). π-interactions are less abundant, with contributions of 12.6-14.9% for C· · · H/H· · · C and 6.2-8.2% for C· · · C contacts.

Analysis of the Molecular Electrostatic Potential Maps
Molecular electrostatic potential (MEP) mapping enables to visualize the distribution of the electron density in molecular structures and provides a very useful tool for the location of electrophilic and nucleophilic reaction sites, as well as the identification of donors and acceptors for hydrogen-bonding interactions [78] MEP scans can be generated by superimposing the van der Waals radii of all atoms in the molecule [79].
The electrostatic potential maps on the isodensity surface of the organoboron complexes 1 and 2 were evaluated based on the optimized geometries obtained by DFT calculations at the level M06/6-31G(d) shown in Figure    The characterization of complexes 1 and 2 was complemented by NMR spectroscopic analysis (Figures S3 and S4, in Supplementary Materials). The 1 H NMR spectra of compound 1 shows nine well-defined signals for the aromatic hydrogens in the region 9.5-7.2 ppm. On the contrary, for compound 2 only six broad 1 H NMR signals were observed at δ = 9.7-7.1 ppm.

DFT Calculations and Evaluation of the Photophysical Properties
Molecular orbital theory has been successful in explaining and predicting the chemical behavior of molecular systems. HOMO orbitals usually act as electron donors and LUMO orbitals as electron acceptors. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is an important parameter for determining the electrical and optical properties of organic materials [87]. In photoactive materials, the HOMO and LUMO orbitals are frequently located in different sections within the same molecule, thus enabling intramolecular HOMO-LUMO charge transfer. To examine the nature of the electronic transitions in complexes 1 and 2, DFT calculations were performed at the M06-2X/6-31G(d) level [88]. The initial geometric parameters for 1 and 2 used in the calculations were extracted from the crystal data and used for subsequent geometry optimizations.
Since the electronic excitations crucial for understanding photophysical processes occur from the highest occupied molecular orbitals (HOMO) to the lowest unoccupied molecular orbitals (LUMO), it is essential to form efficient charge-separated states with the HOMO localized on the donor unit and the LUMO on the acceptor unit. The isodensity plots of the frontier molecular orbitals (FMO) in 1 and 2 reveal that intramolecular charge transfer of the donor orbital HOMO to the LUMO acceptor can occur (see Figures 9 and S5). The HOMO and LUMO energies are −5.18 and −3.55 eV for 1; −5.11 and −3.50 eV for 2, respectively ( Figure 9). The energy gap of 1.63 and 1.61 eV in 1 and 2, respectively, indicates low chemical hardness and high reactivity, since it is energetically favorable to promote electrons from the HOMO to the LUMO orbital. As illustrated in Figure 9 and Figure  S5, in both compounds the HOMO orbital is mainly concentrated at the sulfonate group, while the LUMO orbital is located at the pyridinium group (pyNH + ) and the boron atom. Additionally, Figure S5 shows that the HOMO-1, LUMO+2, and LUMO+3 orbitals are spread over the entire quinoline group. Figure 10 shows the experimental and calculated UV-Vis absorption spectra of 1 and 2, of which the experimental spectra were obtained from a 1.0 × 10 −4 M solution in EtOH at room temperature and the theoretical spectra calculated at the TD-DFT/M06-2X/6-31G(d) level of theory. The oscillator strength (f ) is a parameter quantifying the probability of electron transitions. The results of the TD-DFT calculation for 1 reveal three major signals in the UV region, i.e., two low-intensity bands at 278 and 336 nm due to the HOMO→LUMO+1 and HOMO→LUMO (n-π * transitions), respectively, and one additional intense band (f = 0.1886) at 226 nm for the π-π * transition excitations, which was assigned to the HOMO→LUMO+3 and HOMO-2→LUMO transitions ( Figure S5 and Table S10, in Supplementary Materials). The calculated bands are consistent with the three bands found experimentally in the UV region (<350 nm), centered at 242 (ε = 24,634 M −1 ·cm −1 ), 259 (ε = 3230 M −1 ·cm −1 ) and 319 nm (ε = 2945 M −1 ·cm −1 ). The wavelength of the HOMO→LUMO transition (336 nm) implies that intramolecular charge transfer takes place [15,89].  The calculated UV-Vis spectrum of 2 displays similar bands to 1, of which the most intense band at 227 nm (f = 0.2139) is due to the HOMO→LUMO+3 and HOMO-2→LUMO transitions. This π-π * transition band is consistent with the broad band centered at 242 nm (ε = 17,696 M −1 ·cm −1 ) in the experimental spectrum. The calculated spectrum displays two additional bands at 287 and 339 nm, which are assigned to HOMO→LUMO+1 and HOMO→LUMO transitions. These bands have their equivalents at 259 (ε = 1981 M −1 ·cm −1 ) and 322 nm (ε = 1986 M −1 ·cm −1 ) in the experimental spectrum and correspond to n-π * transitions. Detailed assignments of the spectra obtained by the TD-DFT calculations in terms of FMO are included in the Supplementary Materials ( Figure S5 and Table S8).
Considering the excellent luminescent properties of organoboron compounds [10,90] and with the aim to explore the potential application of monoboron complexes 1 and 2 as luminescent materials, the solid-state fluorescent properties of the title compounds were studied in comparison to the starting reagents ( Figures S6 and S7 in Supplementary Materials). Upon excitation (λ ex = 393 nm), solids 1 and 2 display bluish-green luminescence bands centered at 494 and 498 nm, respectively (Figure 11), which can be attributed to S 1 →S 0 transitions from the lowest vibrational level of the first excited singlet electronic state (S 1 ) to the vibrational levels of the singlet ground state (S 0 ) [91]. The Stokes shifts for 1 and 2 with values of 101 nm (12.28 eV) and 105 nm (11.81 eV), respectively, are indicative of charge transfer, which is an important feature for fluorophores suitable for use in materials science [92][93][94]. The Commission Internacionale d'Eclairage (CIE) coordinates [95] for these emissions are (0.1625, 0.4388) and (0.1638, 0.4072), respectively. The profiles and positions of the emission spectra of 1 and 2 are very similar (Figure 11), indicating that essentially only the sulfonate (distribution of the HOMO) and the PBA groups (distribution of the LUMO) are involved in the excitation and emission processes. In comparison with the PBA starting materials ( Figures S6 and S7 in Supplementary Materials), the emission maximum of 1 and 2 is red-shifted by 24 nm and 28 nm, respectively, indicating that the luminescent behavior of the monoboron complexes is influenced by the coordination of the ligand 8HQSA to the pyridineboronic acid. The bonding of the lone-pair electrons at the nitrogen atom in the ligand to the boron atom reduces the energy gap between the HOMO-LUMO orbitals of the ligand. A similar increase in the emission wavelength was observed also in previous studies of other four-coordinate monoboron complexes with N,O-chelating ligands [96][97][98].
The emission spectra of crystalline solids are significantly different from phases where the molecular components are dissociated (e.g., in solution or in amorphous phases), evidencing that the supramolecular organization in the solid state plays a significant role for the optoelectronic properties [99][100][101]. In order to examine if there are changes in the emission properties upon grinding, pristine solids of 1 and 2 were thoroughly ground in an agate mortar for 10 min. This resulted in a reduction in the emission efficiency by 1.1-fold for 1 and 1.5-fold for 2 (from J 0 /J = 1.1 in 1 and J 0 /J = 1.5 in 2, where J and J 0 are the emission efficiencies determined by integration of the area under the curve- Figures S8 and S9, respectively). However, the samples had not turned amorphous since the powder XRD patterns of the ground samples recorded at room temperature indicated crystallinity. In addition, the crystal structure is conserved, because the PXRD patterns still match well with the simulated XRD patterns based on the respective crystal structure in terms of the peak positions (see, Figures S8 and S9 in Supplementary Materials), indicating that the molecular components are strongly linked via noncovalent bonds, which could not be disrupted even by mechanical grinding. The observed decrease in the emission bands can be attributed to the effect of a less beneficial packing in the powders obtained after grinding [102,103]. The high crystallinity is useful for a practical application of complexes 1 and 2, because no additional procedures or treatments would be necessary to show luminescent activity at room temperature [103].
Having explored the fluorescence characteristics of solid samples of 1 and 2, the photophysical properties in solution using solvents of different polarity were also investigated. The spectra recorded for 1.0 × 10 −5 M solutions are shown in Figures 12a,b, S10 and S11. The corresponding photophysical data are collected in Table S11 (Supplementary Materials). The photophysical properties reveal a clear solvatochromic effect as different fluorescence colors emerge under the UV lamp (Figure 12c,d). Figure 12a shows the solvent-dependent photoluminescence (PL) spectra of 1. In nonpolar solvents, such as hexane and toluene, the fluorescence spectra of 1 exhibit vibrational structures, indicating two excited states of similar energy, so it is deemed that the excited-state contribution to the emission in hexane and toluene originates from a localized excitation [104]. With increasing polarity of the solvents, the fluorescent emission bands become significantly red-shifted. For example, in hexane, the fluorescent emission bands of 1 are located at 422 nm and 438 nm; meanwhile, for THF and CHCl 3 solutions, the emission bands are bathochromically shifted to 489 nm and 493 nm, respectively. In very polar solvents, such as MeOH and DMF, the bands at 524 nm and 529 nm, respectively, are in addition broadened and have larger Stokes shifts ( Figure S10, Table S11 in Supplementary Materials). The large bathochromic shifts in the emission spectra in these solvents suggest an increase of the molecular dipole moment in the excited state compared to the ground state [105]. The Stokes shifts (e.g., 6737 cm −1 and 0.83 eV in DMF), the band broadening, and the red-shift of the emission bands indicate that in polar solvents, intramolecular charge transfer (ICT) takes place in the excited state [104,105]. The emission maxima, shift differences among absorption and emission spectra, and Stokes shifts for solutions of complex 2 follow similar tendencies (see, Figures 12b and S11, Table S11, in Supplementary Materials).
In conclusion, these data are indicative of positive solvatofluorochromism for organoboron complexes 1 and 2. For a more profound comparison of the solid-state and solution fluorescence properties, the emission spectra of boron complexes 1 and 2 in MeOH (1.0 × 10 −4 M, T = 298 K) were compared to the solid-state spectra. In both the solution (1, 507 nm; 2, 505 nm) and the solid state (1, 494 nm; 2, 498 nm) the samples show intense bluish-green fluorescence (Figures S12 and S13, in Supplementary Materials). However, in solution, a drastic three-fold decrease in the luminescence intensity is observed compared to the solid-state emission spectra, suggesting Aggregation Induced Emission Enhancement (AIEE) characteristics [105][106][107][108]. In addition, the solid-state emission maxima of complexes 1 and 2 are blue-shifted by about 10 nm compared to the solution spectra, which is different to what was observed by Cui and Wang for a related N,O-chelated monoboron compound based on 8-hydroxyquinoline [109]. The observed blue shift and increased intensity for the emissions from the solid-state samples might be the result of energetically more favorable intermolecular interactions in the bulk samples. As shown in Figures S14 and S15 (in Supplementary Materials), the solid-state packing of compounds 1 and 2 involves multiple π· · · π interactions with centroid-centroid distances in the range 3.529-4.112 Å. Previous studies have shown that π-stacking reduces the rotational freedom of the aromatic rings involved, thus reducing ISC [106][107][108].
In order to examine if the monoboron complexes 1 and 2 in solution show AIEE characteristics, fluorescence spectra were recorded in the binary solvent system MeOH-THF varying the fractional composition ( Figure 13) [110,111]. Considering the zwitterionic character of the boron complexes, the luminogens should aggregate and might even precipitate in solvent mixtures with high THF fractions (f THF ). During the experiment, the concentration of 1 and 2 in the solution was kept constant at 1.0 × 10 −4 M. With increasing THF content (f THF ) from 0% to 15%, the fluorescence intensity (λ ex = 324 nm) becomes weaker for 1, possibly due to the formation of intermolecular solvent-solute interactions between THF and the dye. However, for f THF in the range of 15-70%, the fluorescence intensity is gradually increased, reaching a maximum at 70%, where the relative emission intensity is approximately two-fold higher than in pure MeOH, which might be due to the restriction of molecule rotation as a result of π· · · π interactions and other hydrophobic molecule-molecule contacts, alleviating the radiationless relaxation channel [110,111]. With further increasing THF amount, the fluorescence is weakened again, and a significant blue shift is observed. Complex 1 emits at 507 nm in dilute MeOH solution (1.0 × 10 −4 M) and the fluorescence emission shifts to 492 nm at f THF = 70%. Precipitation was not observed under these conditions. Similarly, for a dilute solution of 2 in pure MeOH (1.0 × 10 −4 M), only weak emission is observed. Upon addition of THF, in this case the fluorescence intensity increases significantly as soon as the THF fraction exceeds 5%. The plot of luminescence intensity versus THF fraction in the MeOH/THF solvent mixture is shown in Figure 14. Similar to 1, the maximum emission was achieved for the 1:3 (v/v) ratio of the solvent mixture (f THF = 75%), where the relative emission intensity is approximately 2.1-fold higher than in pure MeOH. At the same time, a slight blue shift of the fluorescence occurs, and a new absorption band appears around 430 nm, manifesting the eventual formation of π· · · π interactions among aromatic rings in solution. Nevertheless, at the final stage of the experiment, fluorescence quenching was observed (at f THF = 95%), in contrast to 1. Furthermore, precipitation was not observed under these conditions. Since for the solid-state structures of the boron complexes the luminescence intensity is increased significantly and accompanied by a blue shift of the emission maximum compared to a dilute solution in MeOH, AIEE might be indicated [109,[112][113][114]. For the case of intermolecular π-interactions, it has been proposed that a restriction-of-intramolecularvibrations mechanism is responsible for the AIEE effect [112,113]. In accordance with the SCXRD analysis (vide supra) it is suggested that THF activates intermolecular aggregation through π· · · π interactions between the aromatic components (quinoline and/or pyridinium rings) in solution. The overlap between π-orbitals of adjacent molecules in close-packed head-to-head arrangements can/could facilitate the delocalization of excitons and increase the charge-carrier mobility [115]. Figure S12c,d illustrate that boron complexes 1 and 2 exhibit a high-contrast photochromic effect. Because of this, the compounds might be adopted for applications as luminescent materials, in particular for anticounterfeiting applications (security inks) [114,116,117]. To verify this hypothesis, ethanol solutions with 1 and 2 ink were prepared at different concentrations (1.0 × 10 −3 , 1.0 × 10 −4 and 1.0 × 10 −5 M) and employed for writing on a watercolor paper, and then allowed to dry at room temperature. For all samples, under sunlight no coloration was observed; meanwhile, under UV light of 254 nm and 365 nm, the samples generated strong bluish-green fluorescence with increasing intensity as the concentration was incremented ( Figure 15). These results suggest that compounds 1 and 2 could indeed be used as fluorescent security inks.

Conclusions
In the present study, two new molecular organoboron complexes were synthesized in good yields from 8-hydroxyquinolin-5-sulfonic acid and 3-or 4-pyridineboronic acid. X-ray structure analysis confirmed that in both complexes, the boron atoms are embedded in a five-membered chelate ring formed with the N,O-ligand donor atoms, adopting a distorted tetrahedral geometry. In addition, proton transfer from the sulfonic-acid group to the pyridine substituent occurred, giving overall zwitterionic molecular structures. The crystal structure analysis further revealed the presence of charge-assisted hydrogen bonds of the N + −H···O − /O and O−H···O − /O type, with the difference being that in compound 1, water molecules were included in the crystal lattice. In both complexes, these hydrogen bonds were accomplished by π· · · π interactions and C−H···O contacts to yield three-dimensional networks. The locations and percentage contributions of the different intermolecular interactions were also analyzed by means of Hirshfeld surface analysis and fingerprint plots. DFT calculations established HOMO-LUMO gaps of 1.61 eV for 1 and 1.63 eV for 2. In addition, the nature of the electronic intramolecular transitions was calculated by means of TD-DFT calculations at the M06-2X/6-31G(d) level using ethanol as solvent. Comparison of the experimental and calculated UV spectra showed good agreement, observing only small variations. Fluorescence emission of solid crystalline samples of complexes 1-2 occurs in the bluish-green region upon excitation at λ = 393 nm. The compounds were also fluorescent in solution and showed moderate solvatochromism when the solvent was changed from hexane to DMF. The fluorescence emission of 1 and 2 was also analyzed in dilute methanol/THF solvent mixtures. Dilute solutions in methanol gave only weak emissions, but the fluorescence increased significantly in the presence of THF, giving maximum emission for a solvent mixture of 70% (v/v) and 75% (v/v) for 1 and 2, respectively. The increase in the fluorescence intensity might be originated by molecular aggregation of the fluorophores under the category of AIEE. Based on the results, the photophysical properties of 1 and 2 are attributed to intra and/or intermolecular π· · · π interactions enabling π-π * and n-π * HOMO→LUMO transitions. The intense photoluminescence, high-contrast photochromic effect, and good stability of inks (at least for five months) prepared from complexes 1 and 2 indicate potential for use as fluorescent dopants in security inks.
Supplementary Materials: The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/cryst12060783/s1: Tables S1 and S3, bond distances of 1 and 2; Tables S2 and S4, bond angles of 1 and 2; Table S5, hydrogen-bonding geometries of complexes 1 and 2, calculation of τ4 for complexes 1 and 2, calculation of THC for complexes 1 and 2; Table S6, Calculation of τ4 for complexes 1 and 2; Table S7, Calculation of THC for complexes 1 and 2; Table S8, fingerprint plots of compound 1 and 2; Figures S1 and S2, IR spectra of 1 and 2, Table S9 IR spectroscopic data of 1 and 2; Figures S3 and S4, 1H-NMR spectra of 1 and 2; Figure S5, HOMO and LUMO frontier orbital plots of compounds 1 and 2; Table S10, electronic excited states calculated through TD-DFT; Figures S6-S9, solid-state emission spectra and PXRD data of 1 and 2; Figures S10 and S11, normalized absorption and emission spectra of 1 and 2; Table S11, photophysical properties of the solutions of complexes 1 and 2 in different solvents; Figures S12 and S13, emission spectra of 1 and 2 in the solid state and in methanol solution; Figures S14 and S15: packing diagrams of the crystal structures of 1 and 2.