Highly electron-deficient 3,6-diaza-9-borafluorene scaffolds for the construction of luminescent chelate complexes

The synthesis and characterization of two fluorinated 3,6-diaza-9-hydroxy-9-borafluorene oxonium acids featuring improved hydrolytic stability and the strong electron-deficient character of the diazaborafluorene core is reported. These boracycles served as precursors of fluorescent spiro-type complexes with (O,N)-chelating ligands which revealed specific properties such as delayed emission, white light emission in the solid state and photocatalytic performance in singlet oxygen-mediated oxidation reactions.


Introduction
Boracyclic compounds attract a considerable interest due to their numerous applications in organic synthesis, catalysis and materials chemistry.An important class of these compounds are dibenzo-fused derivatives comprising a central sixmembered boracyclic ring with incorporated another heteroatom such as oxa-, aza-, sila-, and thiaborins as well as diboraanthracenes. 1 Such compounds are usually more stable than diarylboron derivatives with a non-annulated boron atom.Modications within a boracycle or adjacent aromatic rings result in varying electron-acceptor properties stemming from the presence of the vacant 2p orbital on the boron atom.Importantly, boracyclic precursors can be easily converted to various chelate complexes featuring the spiro arrangement of a tetracoordinated boron center.In most cases, aromatic chromophore ligands (O,O-, O,N-, and N,N-) were used which enabled ne-tuning of the photophysical properties of respective products. 2ecently, the 9-borauorene scaffold has been extensively used for designing numerous boracycles and offers a useful alternative to its ring-expanded analogues. 3The presence of the ve-membered borole ring results in an increase of Lewis acidity which is benecial for the stability of respective chelate complexes.Further enhancement of the electron-acceptor character of the 9-borauorene scaffold can be achieved by uorination of aromatic rings or replacement of one of the benzene rings with the pyridine one. 4The obtained azabora-uorene derivative showed dual-uorescence behaviour promoted by the formation of a B-N four-coordinate adduct.However, it was prone to hydrolytic cleavage of the boracyclic ring.Herewith, we present a combined strategy involving (i) annulation of a central borole moiety with two pyridine rings and (ii) installation of uorine substituents as a tool for strong enhancement of electron-acceptor properties (Scheme 1).The designed uorinated 3,6-diaza-9-borauorenes feature strong Lewis acidity of the boron atom and thus they were isolated in the form of highly stable water adducts, which were further converted to luminescent (O,N) chelate complexes.
the synthesis of 5 starting with 2,2 ′ ,4,4 ′ -tetrauoro-6,6 ′ -diiodo-3,3 ′ -bipyridine 4. The products 2 and 5 were isolated as white powders soluble in DMSO and MeOH but insoluble in water, Et 2 O and acetone.They were characterized by multinuclear 1 H, 11 B, 13 C and 19 F NMR spectroscopy.A notable feature of 2 is a large through-space 19 F-19 F coupling constant of 82 Hz estimated from the simulation of the 13 C NMR multiplet of the uorine-bound carbon atom centered at 157.2 ppm, i.e., the "X" part of the ABX spin system (Fig. S8.3, ESI †).Taking into account the F/F distance of 2.579(2) Å, this J FF value is in agreement with the empirical correlation equation proposed by Ernst. 7 The single-crystal X-ray diffraction analysis of 5 conrmed that the boron atom is tetracoordinate due to the complexation of the water molecule. 8The molecules are assembled into the linear motifs held by very strong O-H/O hydrogen bond (HB) interactions (d O/O = 2.400(2) Å) formed between coordinated water (HB donor) and the B-OH group (HB acceptor) from a neighbouring molecule (Fig. 1a).In fact, the difference-Fourier density map indicates that the H atom is delocalized between two oxygen atoms (Fig. S2.1, ESI †).This was further conrmed by theoretical calculations (M062X/ 6-311++G(d,p)) showing that the proton can freely migrate between oxygen atoms (Fig. S3.7, ESI †).The estimated energy of this HB is −84 kJ mol −1 (calculation details are provided in the ESI †) and the amount of electron density at the bond critical point is r = 0.73 e$Å −3 which is comparable to the values found in very strong charge-assisted HBs. 9 The structure 5 is related to the oxonium acid structures of boronophthalide, 8a 3,4,5,6-tetrauorophenylene-1,2-diboronic acid 8c and 1-hydroxy-1H,3Hnaphtho [1,8-cd][1,2]oxaborinin-3-one, 10 which also feature comparably strong intermolecular HB interactions (d O/O = 2.424-2.486Å, Table S2.3,ESI †) correlating with high Brønsted acidity (pK a = 2-3).Indeed, pK a values for 2 and 5 are 2.4 and 1.4, respectively, as determined by potentiometric titration with 0.1 M aq.NaOH and pH-metric measurements of 0.02 M solutions in H 2 O/MeOH (1 : 1).In pure water, the pK a values should be lower by ca.0.3-0.5 units and thus 5 is signicantly more acidic than boronophthalide (pK a = 2.0).8a,10 However, it should be noted that 5 is poorly soluble in water which can be rationalized by its strong aggregation as the molecular chains are further interconnected by O-H/N interactions (d N/O = 2.833(2) Å) producing a very compact and highly symmetric HB network (Fig. 1b).The 11 B NMR spectra for 2 and 5 in DMSO-d 6 conrmed the presence of a tetracoordinated boron center with chemical shis of 7.9 and 6.0 ppm, respectively.This indicates that the water adduct observed in the crystal structure of 5 persists in solution which was unambiguously proved by ESI HRMS analysis of both 2 and 5. Finally, the phase purity of 5 was conrmed by PXRD analysis showing perfect overlap between experimental and simulated patterns (Fig S2 .6,ESI †).We were unable to grow single crystals of 2, whilst the PXRD pattern of a powder sample roughly resembles that of 5 albeit it shows strongly broadened peaks.Thus, it can be supposed that 2 is isostructural with 5 although the crystallinity of the former is very low.
The cyclic voltammetry measurements gave the reduction potentials of −1.71 eV (2) and −0.90 eV (5) vs. the FeCp 2 /FeCp 2 + pair (Fig. S5.1, Table S5.1 and ESI †), which correspond to the LUMO energy levels of −3.39 eV (2) and −4.20 eV (5).This conrms the strong electron-acceptor character of the dia-zaborauorene core in 5.It should be noted that the tetracoordinate nature of the boron atom in 2 and 5 must result in strong weakening of the electron-acceptor character compared to related putative systems 2-dehydr and 5-dehydr with the three-coordinate boron atom, i.e., lacking water molecules as donors.The Lewis acidity of 2-dehydr and 5-dehydr was further evaluated by DFT calculations (M062X/6-311++G(d,p) level of theory) of enthalpies of coordination of water molecules to the boron atom (Table 1).For comparison, 9-hydroxyborauorene (BF-OH) and the parent (non-uorinated) 9-hydroxy-3,6-diaza-9-Scheme 2 Synthesis of fluorinated 3,6-diaza-9-borafluorenes. borauorene (DABF-OH) were also studied.Overall, the results point out that the uorination of the diazaborauorene system systematically increases boron Lewis acidity.This is also re-ected in the shortening of B-OH 2 bond distances (Table 1).In general, for 3,6-diaza-9-borauorenes the oxonium acid species may equilibrate with the zwitterionic tautomer resulting from proton transfer to the pyridine nitrogen atom.According to calculations, the latter form dominates in the case of DABF-OH.In contrast, the oxonium acid tautomer is slightly more stable than the zwitterionic one for 2, however, since the DH B-H 2 O and DH zwitterion enthalpies are quite similar, it is expected that both forms equilibrate in solution.The structural lability of 2 could disrupt its crystal packing resulting in a partial amorphization.For 5, the proton transfer to the nitrogen atom is highly unfavourable which is in line with the strongly reduced basicity of pyridine nitrogen anked by two uorine substituents (see discussion in ESI, Table S3.2 †).Finally, the B-N coordination of the pyridine unit to the boron atom could be also considered 11 but DFT calculations indicate that the aggregation through B-OH 2 /N HB interactions is energetically more favoured for all studied diazaborauorenes (Table S3.4,ESI †).The TGA analyses performed for 2 and 5 showed that both systems apparently lose a water molecule at ca. 150-170 °C (Fig S7 .1 and S7.2, ESI †).It can be expected that this would be followed by network reorganization through pyridine-boron coordination.In the case of 2, the resulting material is stable up to 350 °C, while dehydrated 5 decomposes already at 200-300 °C suggesting that its stabilization through N-B coordination is not effective which is consistent with other experimental and theoretical results.
UV-Vis spectra of 2 and 5 showed the absorption maxima at l abs = 304 nm in EtOH solution (Fig. 2a).To ensure that the oxonium acid forms persist in solution, the measurements were performed upon the addition of a drop of conc.aq.HCl.According to B3LYP/6-311++G(d,p) calculations, the observed absorption band can be assigned to the p-p* transition (2: l cal- c abs = 320 nm, f = 0.109; 5: l calc abs = 328 nm, f = 0.067) occurring between HOMO and LUMO orbitals (Fig. 2b).
Diazaborauorene 2 exhibits intense sky-blue uorescence (l em = 450 nm, uorescence quantum yield QY F = 53%) in EtOH solution.Substitution with two additional uorine atoms in 5 enhances the uorescence intensity (QY F = 65%) and leads to the bathochromic shi of the emission band (l em = 472 nm).This is in agreement with TD-DFT calculations performed in the EtOH solvent eld (Tables S3.6 and S3.7, ESI †), however the origin of this effect is not fully clear; it may result from the stronger stabilization of the excited state of 5 due to its stronger interaction with the polar solvent.It should be noted that the uorescence spectra of 2 and 5 are somehow reminiscent of their 9-borauorene analogue, namely 9-(tert-butoxy)-9-bora-uorene.3g However, since the boron center is tetracoordinate, the absorption spectra of diazaborauorenes lack longer wavelength bands (l abs > 350 nm) of p-B(2p) transitions observed for various 9-borauorenes.3g Finally, it should be pointed out that uorescence was almost completely quenched in pure EtOH solutions, i.e., without HCl additive.This indicates that anionic forms of 2 and 5 are not luminescent.
In the next step, diazaborauorenes were employed for the preparation of a series of (O,N)-chelate complexes 3a-3g and 6a-6c with selected proligands including 8-hydroxyquinoline, 2-(2-pyridyl)phenol, two salicydeneaniline derivatives and three 2-(hydroxyphenyl)benzoheteroazoles (Het]O, S, NPh) (Scheme  Scheme 3 Synthesis of complexes 3a-3g and 6a-6c. 3).All compounds were obtained in reasonable yields (45-78%) as cream-white, pale yellow or intense yellow solids soluble in organic solvents such as CHCl 3 and acetone but in most cases insoluble in Et 2 O and hexane.They are stable in solution as their 1 H NMR spectra did not show any visible changes aer several weeks.This can be ascribed to the high Lewis acidity of the boron centre which strengthens coordination to the chelating ligands.In fact, 11 B NMR chemical shis are in the range of 4.0-11.0ppm, i.e., in agreement with the values reported for analogous organoboron complexes. 12he molecular structures of 3a-3f and 6c were determined by single-crystal X-ray diffraction.Overall, they feature the spiro geometry of boron with an orthogonal arrangement of dia-zaborauorene and ligand moieties (Fig. 3a).The B-N, B-O and B-C distances (Table S2.4,ESI †) are within a range typical of organoboron tetracoordinate complexes except for the remarkably short B-N dative bond in 6c (d B-N = 1.569(2)Å).A comprehensive analysis of all structures shows that the molecules remain quite rigid in the diazaborauorene plane, but regain some additional degree of exibility of the chelate ligand reected in the distortion of the B(O,N) heterocyclic ring and ligand in-plane or out-of-plane shiing (Fig. 3b).Such a behaviour was previously observed for crystal structures of related 9-borauorene chelate complexes. 12Concordantly with these studies, the B(O,N) chelate ring can adopt either at or halfchair conformations; the latter features boron and/or oxygen atoms distorting out of the ligand plane (Fig. 3c).According to DFT calculations, the conformers have similar electronic energies with low interconversion barriers (below 5 kJ mol −1 ).Thus, molecules should retain some conformational exibility in solution.
The supramolecular structures of the studied complexes are dominated by weak HB interactions mostly arranging pyridine nitrogen, chelating oxygen or uorine atoms as HB acceptors (Fig. 4).The propagation of these contacts results in two types of supramolecular arrangements, i.e., innite one-dimensional chains (structures 3a, 3c, 3d and 3f) or discrete dimeric motifs (structures 3b, 3e and 6c).The weak HB interactions are usually accompanied by C-H/C(p) interactions, although they are not very common as they were observed in the crystal structures of their 9-borauorene analogues. 12Conversely, diazaborauorene chelates more likely form p-stacking aggregates (Fig. 5), mainly through mutual interactions between ligands (3c and 3d) or alternating ligand-borauorene moiety stacking (3f).Notably, Jaggregate motifs, commonly encountered in spiro-organoboron compounds, 13 are solely observed for 3e (Fig. S2.4,ESI †).
The obtained complexes show the longest wavelength absorption bands with maxima in the range of 360-421 nm (CHCl 3 ) with molar extinction values ranging from 2680-16 500 M −1 cm −1 (CHCl 3 ) except for 3g showing much higher 3 = 109 000 M −1 cm −1 (Table 2).Their emission maxima vary in the range of 427-531 nm depending mainly on the ligand type and their luminescence colour can be further tuned by ligand functionalization.For instance, the introduction of the NEt 2 group in 3g naturally increases the HOMO energy level, but even more strongly elevates the LUMO (Fig. S3.8, ESI †), leading to an increased band gap and hypsochromic shi of the emission band with respect to 3b (Fig. 6). 14Interestingly, 3g shows the very narrow emission band in the solid state (FWHM = 1700 cm −1 ).Emission maxima are typically red-shied in the bulk solid-state (usually up to 20 nm).Exceptionally, complex 3b displays a signicant hypsochromic shi of the emission band in the solid state (by 28 nm; 1050 cm −1 ).This can be connected  In contrast, compound 3e exhibits substantial red-shi of the emission band in the bulk solid-state.The examination of the behaviour of 1 wt% and 5 wt% Zeonex thin lms revealed evidence that the emission is systematically shied as the concentration of the sample is increased.Thus it can be postulated that the observed behaviour is strongly affected by the formation of J-aggregates, which is consistent with the behaviour of other dyes displaying J-aggregate crystal motifs. 15Furthermore, a small shoulder in the emission band of 3e (powder) appears at a wavelength similar to that recorded for respective spectra in solution and Zeonex.This may point to the presence of a fraction of an amorphous or highly disordered phase of 3e in the powder sample.
All complexes are moderate to good emitters with quantum yields in the range of 22-73% (CHCl 3 ) and 16-66% (powder).Notably, in most cases the uorescence intensities are not affected by solid state aggregation effects.Exceptionally,   20%).Although this might be attributed to p-stacking aggregation, the oxazole analogue 3c is characterized by enhanced emission in the solid state (QY F solution = 36% / QY F powder = 50%) despite displaying similar p-stacking structural motifs (Fig. 5).The aggregate behaviour of the latter compound (and also its analogue 6c) is also strongly manifested by the appearance of additional intense bathochromically shied emission bands covering a wide range of the visible spectrum, responsible for net white emission.Even though the TD-DFT calculations may suggest that they result from the emission from the lowest charge transfer state (CT), the emission spectra in Zeonex thin lms (1 wt%) are generally retained from the CHCl 3 solution conrming the aggregation-caused origin of observed band broadening in the bulk solid-state.
Another interesting luminescent behaviour was observed for quinolate complexes 3a and 6a.The normalized emission spectra in solution, Zeonex thin lms and the bulk solid-state perfectly overlap indicating that the emission process is neither dependent on the environment nor on conformational effects.However, we have noted that emission amplies to some extent upon degassing the CHCl 3 solution (Fig. S4.11, ESI †).Furthermore, both systems exhibit biexponential uorescence decay in CHCl 3 with the shorter component attributed to the prompt uorescence (3a: s PF = 27.7 ns; 6a: s PF = 23.8ns) and the longer one characteristic for delayed uorescence (3a: s DF = 10.3 ms; 6a: s DF = 5.7 ms) (Fig. 7). 16The origin of the delayed uorescence is still not clear, i.e., it may originate either from thermally activated delayed uorescence (TADF) or triplettriplet annihilation (TTA).The latter mechanism was recently suggested for related quinolate complexes based on the 9-bor-auorene core. 17he DFT calculations (B3LYP/6-311++G(d,p)) of 3a-3g and 6a-6c revealed that the HOMO is localized on the diazabora-uorene scaffold whilst the LUMO is spread over the ligand (Fig. 8).Since HOMO−1 is localized on the ligand, the effective p-p* excitation can be described as the HOMO−1 / LUMO transition.This is further conrmed by TD-DFT calculations showing that the observed uorescence emission is attributed to the second ligand-localized singlet excited state { 1 LE 2 (Q)}, while the lowest laying singlet excited state possesses  a diazaborauorene-to-ligand charge transfer character ( 1 CT 1 ) and it is not visible due to its low oscillator strength (Table S3.7, ESI †).In accordance with the above results, the cyclic voltammetry (CV) measurements show that the red-ox processes occur solely on the ligand and are not strongly inuenced by the type of organoboron moiety (Fig. S5.1 and Table S5.1,ESI †).It should be noted that reduction and oxidation processes are irreversible, i.e., they are followed by the chemical reactions.
The calculations of triplet energy levels for 3a and 6a reveal the occurrence of the two lowest triplet excited states with quinoline-localized ( 3 LE 1 (Q), E = 1.70 eV) and charge transfer ( 3 CT 2 , E = 2.25 eV) nature, respectively.As initially postulated for boron dipyrromethene (BODIPY) compact donor-acceptor dyads, 18 the molecule can transfer to the lowest laying 3 LE 1 (Q) triplet state (E = 1.70 eV) due to direct conversion from the singlet 1 CT 1 state via the spin-orbit charge transfer intersystem crossing mechanism (SOCT-ISC, Fig. 9).The experimental and theoretical studies conrmed that the SOCT-ISC mechanism operates in a number of BODIPY dyads 19 as well as other organoboron complexes based on the borauorene core 2b and it is responsible for the formation of the long-lived triplet state of the molecule.
The interaction of the photoexcited triplet molecule with naturally abundant triplet oxygen ( 3 O 2 ) leads to the excitation of the latter species to its singlet state ( 1 O 2 ).Since singlet oxygen serves as a powerful oxidant for both small organic molecules and biological macromolecules, it is widely utilized in anticancer photodynamic therapy (PDT), 20 organic synthesis, 21 and water purication. 22Thus, in the next step we have decided to check the usability of studied diazaborauorene complexes 3a-3g and 6a-6c as singlet oxygen generators.The photocatalytic activity was quantied by tracking the singlet oxygen-mediated oxidation of 2-furoic acid (FA)a model reductant.All reactions were performed in CHCl 3 using 0.25 mol% photocatalyst loading and the irradiation wavelength was adjusted to respective absorption maxima.The samples were irradiated with a 365 nm (3b, 3c, 3e, 3f, 6b and 6c), 395 nm (3a, 3d and 6a) or 415 nm (3g) LED light source using our home-made reactor (Fig. S6.1, ESI †).All reactions were performed under air at 25 °C and their progress was monitored by 1 H NMR spectra analysis of the reaction mixture sampled aer a given time.The control experiments showed that the reactions do not proceed in the absence of light or a photocatalyst.We found that quinolate complexes 3a and 6a feature the highest activity with FA conversion reaching 98 and 90%, respectively, aer 10 h of irradiation (Fig. 10).Reaction proles for 3a and 6a show a continuous increase in oxidation product concentration indicating the high stability of photosensitizers under applied conditions.The photostability experiments performed under the same conditions but without FA demonstrate that 6a is characterized by higher stability (half-time decomposition t 1/2 = 16 h) with respect to its 3a analogue (t 1/2 = 9.2 h).In addition, both complexes are stable in the dark which means that they are not susceptible to chemical degradation (Fig S6 .2,ESI †), e.g., hydrolysis resulting from the presence of traces of water in the used solvent.

Conclusions
In summary, two uorinated diazaborauorenes 2 and 5 were obtained and characterized as stable water adducts due to the  strong Lewis acid properties of the boron atom.DFT calculations conrmed that the oxonium acid form is the most stable, although compound 2 may also equilibrate with its zwitterionic tautomer.Both compounds are characterized by intense blue uorescence in acidied EtOH solution.In the next step diazaborauorenes were converted to respective chelate complexes with various (O,N)-ligands.The structural analysis suggests that they are characterized by partial conformational exibility resulted from B(O,N) chelate ring inversion and ligand in-plane and out-of-plane movements.The molecules interact mainly through C-H/O and C-H/N hydrogen bonds as well as p-stacking intermolecular interactions, while C-H/C(p) contacts are rather avoided.All complexes exhibit moderate-to-good luminescence properties both in solution and the solid state.In most cases the luminescence is red-shied in the solid state compared to that in solution, but the photoluminescence quantum yields remain at a similar level.In the cases of 3c and 6c, the aggregation leads to the appearance of additional bands covering the wide range of the visible spectrum and resulting in white emission colour.The peculiar nature of electronic excitations and relaxation in quinolate complexes 3a and 6a, manifested by delayed emission and activity in photosensitized 1 O 2 generation, is the most appealing among other results regarding the optical properties of studied compounds.In fact, such a dual photophysical behaviour was not previously reported for organoboron quinolates.Thus, it seems that the use of proposed boracyclic scaffolds featuring a strong electronacceptor character can give rise to promising systems for potential diverse applications including organic electronics (electron transport and/or light-emitting materials), photoand organocatalysis and analytical chemistry (e.g., anion receptors).

Fig. 1
Fig. 1 (a) HB linear motif in the crystal structure of 5 (I-42d space group).Thermal ellipsoids were generated at the 50% probability level.Aromatic hydrogen atoms were omitted for clarity.(b) Packing diagram showing the formation of a symmetric tetragonal network based on O-H/N HB interactions.

Fig. 3
Fig. 3 (a) Molecular structures of diazaborafluorene complexes.Thermal ellipsoids were generated at the 50% probability level.Hydrogen atoms were omitted for clarity.(b) Overlay of the molecular structures of 3a-3f, 6c.(c) Two types of B(O,N) chelate ring conformations adopted by the studied complexes.
with the ligand conformational exibility resulting from the possible rotation of the phenyl group around the single C ar -N bond (s Ph , Fig.3) in the less strained solution environment.The TD-DFT calculations for single molecule 3b indicate that the ligand is attened upon excitation (s Ph = 36°), while in the crystal structure it remains twisted around the C ar -N bond by s Ph = 52(1)°resulting in weakening of p-electron conjugation.

Table 1
DFT-derived enthalpies of water coordination to the boron center and zwitterion formation for BF-OH, DABF-OH, 2-dehydr and 5-dehydr

Table 2
UV-Vis absorption and emission data for diazaborafluorene complexes in CHCl 3 solution and the solid state (powder and Zeonex)