Synthesis and Regioselective Functionalization of Tetrafluorobenzo-[α]-Fused BOPYPY Dyes

The synthesis of a new bis-BF2 tetrafluorobenzo-[α]-fused BOPYPY dye from 4,5,6,7-tetrafluoroisoindole and 2-hydrazinopyrazine is reported. The regioselectivity of nucleophilic substitution reactions at the periphery of the tetrafluorinated BOPYPY and its α-bromo derivative were investigated using N-, O-, S-, and C-based nucleophiles. Among the aromatic fluorine atoms, the F2 atom is consistently regioselectively substituted, except when the α-position contains a thiophenol group; in this case, F4 is substituted instead due to stabilizing π–π-stacking between the two aromatic groups. The α-bromo BOPYPY derivative also reacts under Stille cross-coupling reaction conditions to produce the corresponding α-substituted product. The spectroscopic properties of these new fluorinated BOPYPYs were investigated and compared with nonfluorinated analogs.


INTRODUCTION
−6 The diverse applications of these molecules demand unique optoelectronic properties and desirable chemical, biological and physical features.As a result, many useful organic fluorophores have been developed in pursuit of new molecules with desired properties. 3,7,8Among the most recently explored dyes, the 4,4-difluoro-4-bora-3a,4adiaza-s-indacene or boron dipyrromethene dyes (abbreviated BODIPY, Figure 1) have attracted intense research interest owing to their favorable photostability, easy synthesis and structural modifications, and their tunable photophysical properties. 5,9In particular, BODIPY derivatives bearing two BF 2 -complexed groups were recently explored and observed to exhibit near unity fluorescence quantum yields and enhanced photostability as a result of their rigid planar structures. 9,10−14 However, some BOPHYs show self-quenching effects and limited solubility as a result of their rigid symmetric cores, leading to reduced fluorescence quantum yields. 9To address this challenge, unsymmetric bisBF 2 derivatives, known as BOPPY 15 and BOPYPY 16 (Figure 1), were recently synthesized and observed to display large Stokes shifts and high fluorescence quantum yields both in solution and in the solid state.These unsymmetric bisBF 2 fluorophores are readily prepared in moderate to good yields from condensation of an α-formylpyrrole ring with 2-hydrazinylpyridine (in the case of BOPPY) or with 2-hydrazinylpyrazine (in the case of BOPYPY). 15,16Both BOPPY and BOPYPY dyes in general show large molar absorptivities, dual absorptions and emissions, and high photostability.In addition, these dyes are reported to exhibit strong solid-state emissions and twophoton absorptions in the near-IR region. 15Due to the presence of the additional N atom in the pyrazine ring, the absorption and emission of BOPYPY dyes are red-shifted by about 40−60 nm compared with those of the BOPPY analogs. 16To achieve bathochromic shifts in the absorption and emission bands, benzo-fused BOPPYs with extended πsystems have been recently reported; 17 however, benzo-fused BOPYPY dyes have not yet been investigated.
Herein, we report the synthesis of new unsymmetric tetrafluorobenzo-[α]-fused-BOPYPYs 1 and 2, and their regioselective functionalization by nucleophilic substitution reactions (S N Ar) at both the halogenated α-position and the aromatic fluorine sites.The electron-withdrawing fluorine atoms in these molecules further stabilize this type of chromophore relative to the currently known compounds. 18n addition to improving stability, advantages of introducing a tetrafluorobenzo-fused unit in the chromophore include (1)  extending the chromophore π-system leading to longer wavelength absorptions and emissions, (2) allowing direct attachment of molecules via nucleophilic substitutions of one or more fluorine atoms, (3) allowing for 19 F-radiolabeling for dual fluorescence and MRI imaging, and (4) improving solubility. 19,20In addition, a Stille coupling reaction was investigated on the α-bromo-BOPYPY derivative 2, demonstrating the versatility of regioselective functionalization of this type of chromophore.
The structures of BOPYPYs 1 and 2 were characterized by 19 F, 11 11 B NMR spectrum, two sets of triplets at 1.02 and 3.02 ppm were observed, corresponding to the two BF 2 groups on the 6membered and the 5-membered rings, respectively, based on previous studies. 19The 1 H NMR of BOPYPY 1 showed five aromatic protons at 9.16, 8.34, 8.33, 8.26, and 7.86 ppm; the αpyrrolic proton at 8.34 ppm disappeared upon bromination, to afford BOPYPY 2. Crystals of BOPYPY 1 and its α-bromo derivative 2 suitable for X-ray analysis were obtained by slow diffusion of hexane into dichloromethane (Figure 2).The structures of 1 and 2 (as the substitutionally disordered crystal with the Br atom 67% occupied) have distinct differences and clearly show the asymmetry in the BOPYPY chromophore.In 1, the molecule has a slightly bowed shape, with the central 6membered C 2 BN 3 ring being planar to within an average deviation of 0.02 Å, the F 4 Ph group forming a dihedral angle of 2.9°with it, and the pyrazine ring forming a 9.2°dihedral angle with it.The two boron atoms differ in their coordination geometry, with the one in the 6-membered ring having slightly shorter B−N distances (av.1.556 Å) and slightly longer B−F distances (av.1.378 Å) than those in the 5-membered ring (1.573 and 1.363 Å, respectively).On the other hand, the bowed shape in 2 is more pronounced than in 1.The central 6membered ring is nonplanar, with the boron atom lying 0. introduction of various types of nucleophiles, including S-, N-, O-, and C-centered examples, onto a chromophore to modulate its chemical and photophysical properties. 22,23In particular, the introduction of aromatic fluorines provides a useful strategy for direct conjugation of molecules via S N Ar reactions, under mild conditions.These reactions normally occur via a concerted two-step addition−elimination mechanism by nucleophilic attack on the electron-deficient benzo ring.We have previously reported that a symmetric bis-(tetrafluorobenzo-[α]-fused) BOPHY derivative reacts in the presence of S-centered nucleophiles giving the corresponding substituted products with high regioselectivity. 19Therefore, we investigated the reactivity of unsymmetric BOPYPY 1 in the presence of the reactive 4-methoxythiophenol as the nucleophile (Scheme 2).The reaction occurred smoothly at room temperature overnight with high regioselectivity, giving BOPYPY 3 as the sole product in 68% yield.Minor byproducts containing two nucleophiles were also produced, albeit in low yield, as detected by mass spectrometry.The structure of 3 was confirmed by 19 F, 11 B, 1 H NMR, and 2D 1 H− 19 F HOESY spectroscopy (see Figures S7−S9,S49) and HRMS (Figure S70).In the 19 F NMR of 3, the three aromatic fluorines appear at −114.66 (F 1 ), −135.30(F 3 ), and −145.84 ppm (F 4 ) and the two BF 2 groups on the 6-and 5-membered rings at −140.37 and −143.44 ppm, respectively.In the 2D 1 H− 19 F HOESY, cross-peaks between F 1 and the meso-H, and between F 4 and the α-H were observed.The chemical shifts of the α-pyrrolic and meso-protons of BOPYPY 3 did not change significantly upon introduction of the p-methoxythiophenol substituent.In the 11 B NMR spectrum, the boron atoms appear as triplets at 1.04 and 2.98 ppm.
We have previously observed similar regioselectivity in the S N Ar reactions of a tetrafluorobenzo-[α]-fused BOPHY in the presence of thiols, which occurred exclusively at the F 2 position. 19To further understand the experimentally observed regioselectivity, we used DFT calculations to evaluate the charges on the tetrafluoro aromatic carbon atoms using two different schemes: NPA (natural population analysis) and MK (Merz−Singh−Kollman).However, neither of these schemes produced results that were able to explain the experimentally observed regioselectivity.As shown previously, 24,25 the molecular electrostatic potentials (MESPs) at atomic nuclei might be better reactivity descriptors, since these are calculated directly from the electron densities without any additional approximations.These calculations showed that for BOPYPY 1 (see Figure S63), the least negative MESPs are located on the carbons attached to F 1 and F 2 , suggesting that these are the most susceptible to nucleophilic attack, with the F 2 carbon being slightly more reactive.In agreement with this result, our calculations indicate that the F 2 -substituted BOPYPY 3 and its F 1 -substituted analog have similar energies, with BOPYPY 3 being slightly more stable by approximately 0.5 kcal/mol.
Since increasing the concentration of nucleophile and the reaction temperature resulted in mixtures of products in low yields, we next investigated the regioselectivity of S N Ar reactions on the α-bromo BOPYPY 2, in the presence of S-, N-, O-, and C-centered nucleophiles (Scheme 3). 26As expected, these reactions occurred with high selectivity at the brominated carbon atom bearing the best leaving group.
The corresponding mono-α-substituted BOPYPYs 4 were isolated after purification by silica gel column chromatography followed by recrystallization.The reaction conditions and the yields obtained in these reactions are summarized in Table 1 and reflect the reactivity of the nucleophile.The most reactive S-and N-centered nucleophiles readily reacted at room temperature in chloroform, while the less reactive O-and Ccentered nucleophiles required heating in toluene, affording the corresponding products in moderate to high yields.4-Methoxythiophenol reacted the fastest with BOPYPY 2 giving BOPYPY 4b in 78% isolated yield, after 1 h at room temperature in chloroform solution.4-Methylthiophenol and diethylsulfide gave similar high yields of 4c and 4d upon heating at 50 °C for 1 or 3 h, respectively.On the other hand, diethylamine reacted smoothly with BOPYPY 2 at room temperature overnight, affording BOPYPY 4e in 88% yield, while 4-methoxyphenol required higher temperature (80 °C), giving BOPYPY 4a in 54% isolated yield.The carbon nucleophiles 3-ethyl-2,4-dimethylpyrrole and 2,4-dimethylpyrrole also reacted with BOPYPY 2 in toluene at 80 °C for 3−5 h  to afford the corresponding BOPYPY derivatives 4f and 4g in 63 and 79% yields, respectively.All the monosubstituted derivatives 4 were stable under light, humidity, and air, and their structures were characterized by 19 F, 11

B, and 1 H NMR spectroscopy (see Figures S10−S30) and HRMS (Figures S71−S77
).In all the 19 F NMR spectra of BOPYPYs 4, the four distinct aromatic fluorines appeared consistently between −138.03 and −160.79 ppm, and the two BF 2 groups at around −136.10 and −145.09ppm, with the exception of 4f and 4g; in these cases, due to the hydrogen bond interaction between the pyrrolic NH and the neighboring BF 2 , the two fluorines on the 6-membered ring BF 2 group show distinct chemical shifts in 19 F NMR spectra.In the 11 B NMR spectra, the two distinct triplets appeared at around 0.80 and 3.06 ppm for all the compounds.Slight shifts were observed in the 1 H NMR chemical shifts of the meso-protons.Furthermore, the X-ray crystal structures for BOPYPYs 4a, 4c, 4d, and 4e were obtained and are shown in Figure 3.
Interestingly, the 4a molecule has a bowed shape that differs from those of 1 and 2. Its central C 2 BN 3 ring is planar to within an average deviation of 0.01 Å, the F 4 Ph group forms a dihedral angle of 10.7°with it, and the pyrazine ring is nearly coplanar with it, forming a dihedral angle of only 1.1°.The methoxy substituent is distinctly twisted with respect to the core of the molecule, with a N−C−O−C torsion angle 125.2(7)°.Furthermore, the B−N and B−F distances in 4a do not show the asymmetry seen in 1, with mean values of 1.550 and 1.370 Å, respectively.The shape of the BOPYPY ring system of the 4c molecule is like that of 1, except that the central C 2 BN 3 ring in this case is nonplanar, with the B atom 0.29 Å out of the plane of the other five atoms.The dihedral angles between the central ring and F 4 Ph, and between the central ring and the pyrazine are 0.9°and 9.7°, respectively.The N−C−S−C torsion angle to the ethylthio substituent is 114.9(6)°, and the B−N and B−F distances are not asymmetric, with average values of 1.574 and 1.364 Å, respectively.The C−S distance from the ring system to the S atom is 1.752(6) Å.The crystal structure of 4e is unusual in that there are 13 molecules in the asymmetric unit.The largest differences among them are the different conformation of the ethyl groups of the diethylamine substituent.In a typical molecule, the central C 2 BN 3 ring is nearly planar, with average deviation of 0.05 Å.The 5-ring system is closer to planarity than the previously described molecules, with the F 4 Ph group forming a dihedral angle of 3.7°with the central ring and the pyrazine forming a dihedral angle of 5.4°.Although the precision is not high for this structure, there does not appear to be any difference between the B−N and B−F distances for the two boron atoms.
The regioselectivity of fluoride substitution on select BOPYPYs 4d, 4e, and 4g was then investigated in the presence of the most reactive 4-methoxythiophenol, as shown in Scheme 4. When BOPYPYs 4d, 4e, and 4g were reacted with a slight excess of the nucleophile at room temperature in dichloromethane solution, the corresponding BOPYPYs 5a, 5b, and 5c were isolated in 72, 92, and 84% yields, respectively.These BOPYPYs could also be prepared from 3 by first performing α-bromination followed by nucleophilic substitution using the appropriate nucleophile, albeit in lower overall yield.The structures of BOPYPYs 5a, 5b, and 5c were investigated using 19 F, 11 B, 1 H NMR spectroscopy and 2D 1 H− 19    which indicate that the F 2 site is the most reactive toward nucleophilic substitution, except in the case of 4e.Indeed, the performed calculations for compounds 4b, 4d, 4e, and 4g (see Figure S64) indicate that the α-substituent on BOPYPY 4 influences the values of the carbon MESPs.Interestingly, the effect is more pronounced on the bis-BF 2 and pyrazine moieties; however, the tetrafluoro aromatic carbon MESPs also change upon α-substitution.In the case of 4e, the F 4 site bears the least negative MESP, but the proximity of the diethylamine group and its involvement in a hydrogen bonding interaction with F 4 (Figure S64) reduce the reactivity at this site, making the F 2 position the most favored site for reaction, in agreement with the experimental findings.It is also worth noting that our calculations indicate that the N-substituents tend to increase the regioselectivity for F 2 substitution compared to the Ssubstituents.
As shown in Figure 4, the central C 2 BN 3 ring in 5a is slightly nonplanar, with the boron atom lying 0.25 Å out of the plane of the other five atoms.The fluorinated phenyl group is almost coplanar with it, forming a dihedral angle of 1.4°.The pyrazine ring is tilted by 8.7°from the central ring.In 5b, the BOPYPY ring system is similar to that in 4e, not deviating much from coplanarity.The central 6-membered ring has a mean deviation 0.07 Å, the phenyl ring forms a dihedral angle of 1.7°with it, and the pyrazine forms a dihedral angle of 5.7°with it.The B− N and B−F distances do not differ between the two boron atoms, and the C−S distance to the ring system is 1.775(3) Å.The precision of the structure determination of 5c is lower, but it shows that the BOPYPY ring system is bowed.The central C 2 BN 3 ring has a dihedral angle of 8.7°with the phenyl ring and a dihedral angle of 12.2°with the pyrazine ring.The dihedral angle between the BOPYPY core and the dimethylpyrrole substituent is 45.6°, and the N−H group forms an intramolecular hydrogen bond with the neighboring BF 2 fluorine atom, having a N•••F distance of 2.904 Å.
Interestingly, different regioselectivity resulted when BOPY-PY 4b reacted with 4-methoxythiophenol at room temperature, as shown in Scheme 5.In this case, F 4 was substituted instead, giving BOPYPY 6 as the only product, isolated in 97% yield.Similar results were also obtained when BOPYPY 2 was treated with an excess (3 equiv) of 4-methoxythiophenol; in this case, 6 was obtained in slightly lower yield (Scheme 5).A possible explanation for the observed regioselective substitution of F 4 are the stabilizing π−π stacking interactions between the 4methoxythiophenol molecules, which place the nucleophile in 4b in very close proximity to F 4 leading to its substitution, rather than at the more electronically favored F 2 .Indeed, we have observed similar regioselectively in the multisubstitution of a tetrafluorobenzo-[α]-fused BODIPY using an aromatic thiophenol as the nucleophile. 20Although the calculated MESPs for BOPYPY 4b indicate that the F 2 site is the most susceptible to nucleophilic substitution, compound 6 was found to be the most stable product among all possible regioisomers using the ωB97X-D DFT potential (Figures S64  and S65). 27Since the ωB97X-D method accounts for dispersion interactions, this result is in agreement with the hypothesized stabilizing π−π stacking interactions that lead to the formation of BOPYPY 6.

Stille Coupling Reaction.
The Stille cross-coupling reaction is an attractive methodology for the introduction of a variety of substituents under mild conditions onto halogenated BODIPYs and their derivatives. 26,28BOPYPY 2 reacted with an excess of 2-(tributylstannyl)thiophene in the presence of 5 mol % of chloro(tricyclohexylphosphine-2−2'aminobiphenyl) palladium [Pd(PCy 3 )G 2 ] as the catalyst, giving BOPYPY 7 in 53% yield (Scheme 6).The structure of 7 was confirmed by 19 F, 11 B, 1 H NMR spectroscopy (see Figures S43−S45), HRMS (Figure S82), and X-ray crystallography, confirming the regioselectivity of the reaction.In BOPYPY 7 (Figure 4), the 20-atom main ring system of is nearly planar, with mean deviation of only 0.034 Å.The thiophene plane makes a dihedral angle of 56.5°with it, and the B−N and B−F bond distances are like those of 4a.
2.4.Spectroscopic Properties.The spectroscopic properties of the new fluorinated BOPYPYs were evaluated in dichloromethane and toluene solutions.The results obtained from these studies are summarized in Table 2, and the spectra are shown in the Figures S56−S62.For comparison purposes, the previously reported BOPYPY 8 16 and BOPPY 9 15,17 (see Figure 5) were synthesized, and their spectroscopic properties evaluated under the same conditions.
As expected, BOPYPY 1 showed dual absorptions in both solvents with maximum absorption peaks at 457 and 474 nm in dichloromethane, which are 6−14 nm red-shifted from those of previously reported compound 8. Larger red shifts of up to 110 nm were observed for the α-substituted BOPYPYs, as previously observed, due to the extension of the π-conjugation  S1).The observed red shift is influenced by the smaller dihedral angle between the pyrrolic substituent and the BOPYPY core compared with 7, as a result of hydrogen bonding between the pyrrolic NH and the adjacent BF 2 group (see Figure 4).The split absorption bands of the BOPYPYs in both solvents are likely due to vibronic progressions of the same excited state, as previously suggested. 15Indeed, in agreement with these previous studies, our DFT calculations confirm a significant shortening of the N−N bond length upon excitation (see Table S1), varying from 0.032 to 0.058 Å.This is consistent with the more N−N antibonding character of the HOMO compared to the LUMO (see Figure 6).The strengthening of the N−N bond length in the current series of BOPYPYs is accompanied by weakening of the adjacent B−N bond lengths in both the 5-membered and the 6-membered rings and corresponding shortening of the neighboring B−N bond length.The vibronic progression explanation is consistent with the experimentally observed difference in the absorption and emission spectra in both dichloromethane and toluene.The increased interactions that occur in the more polar solvent result in smearing of the double peaks and the appearance of shoulders for several of the BOPYPYs, while the peaks are sharper and the split bands more easily observable in nonpolar toluene.
BOPYPY 1 shows a broad emission in dichloromethane with a maximum wavelength at 545 nm, red-shifted by about 23 nm relative to that previously reported for compound 8. 16 However, the fluorescence quantum yields of BOPYPY 8 in both solvents are very low, in contrast with the previously reported values. 16Indeed, the replacement of the pyridine ring of BOPPY with a pyrazine ring in BOPYPY results in large red shifts in the wavelengths of absorption and emission and also has a dramatic effect on the fluorescence quantum yields of the compounds.To further investigate these observations, we performed TD-DFT calculations of the ground and the excited states of the entire series of compounds and analyzed their molecular orbitals.For all studied compounds, the leading transition is between HOMO and LUMO.The experimentally observed red shifts in the absorption and emission wavelengths are due to stabilization of both HOMO and LUMO, with significantly greater effect on LUMO (Table S1).This results in a significantly smaller HOMO−LUMO gap in BOPYPY 8 compared to BOPPY 9 and therefore explains the observed red shift.One major difference between the electronic structure of the pyridine ring in BOPPYs and the pyrazine ring in BOPYPYs is the existence of the pyrazine N-lone pair MO that is absent in pyridine.This MO is present in the entire     series of BOPYPYs studied.For example, Figure 6 shows the molecular orbital diagram in the case of BOPYPY 1.The pyrazine N-lone pair molecular orbital appears as HOMO−4 and is localized mainly on the pyrazine.Just a small part of the 6-membered BF 2 ring contributes to this MO, leaving the majority of the BOPYPY core with little to no electron density, resulting in a charge-transfer quenching of the fluorescence.Our TD-DFT calculations show that this charge-transfer transition occurs between HOMO−4 and LUMO and LUMO+1 and corresponds to excitation to S 3 .We hypothesize that the lower observed fluorescence in BOPYPYs compared to BOPPYs is the result of an internal conversion from S 1 to the optically dark state (S 3 , in the case of BOPYPY 1).This is in agreement with previously published detailed experimental and theoretical study of pyrazine, 29 suggesting an internal conversion process involving a similar dark state.To further investigate this hypothesis, we performed TD-DFT calculations of the first 10 excited states for compounds 1, and 3−9.We found that similar charge-transfer transitions between the pyrazine N-lone pair molecular orbital and LUMO and LUMO +1 are observed for the entire series of BOPYPYs studied.As shown in Figure S67, the dark excited state appears as S 3 , S 4 , or S 5 , depending on the BOPYPY substituent(s).Such an excited state does not exist in the case BOPPY 9.As expected, the energy of the dark excited state is relatively independent of the substituent on the isoindole moiety because the electron density is localized mainly on the pyrazine ring.For all BOPYPY compounds studied (1, 3−8), we observed the existence of multiple excited states that are very close in energy, thus supporting the internal conversion hypothesis.The substituted BOPYPYs 3, 4, and 5 derivatives displayed even lower fluorescence quantum yields compared with BOPYPY 1. Indeed, the substituents introduce additional excited levels that are very close in energy to the dark state, as shown in Figure S67.Our results are also in agreement with previous studies on 8-halo-BODIPYs and their 8-(C, N, O, S)substituted analogs. 30nother characteristic of BOPYPY dyes is their large Stokes shifts, in the order of ca.3000 cm −1 .These values are larger than those reported for symmetric BODIPY systems and their derivatives, such as for aza-BODIPYs and BOPHYs. 31,32The performed TD-DFT calculations (Table S1) also demonstrate larger Stokes shifts for unsymmetric BOPYPYs compared with symmetric BOPHYs. 19The reason for the large Stokes shifts might be the significant change in the N−N and associated B− N bond lengths upon excitation.Except for 4f, 4g, and 5c, the Stokes shifts in these compounds were similar to those observed for BOPYPY 1.
In toluene, small red shifts were observed in the absorption spectra for most of the BOPYPYs synthesized, as previously observed for BODIPY derivatives.The relative fluorescence quantum yields of the BOPYPYs also increased in the nonpolar toluene relative to dichloromethane, as previously observed in the case of BOPPY dyes, 15,17 due to the stronger interactions with the more polar solvent.
Although BOPYPYs display low fluorescence quantum yields, their strong absorptions (ε = 14300−53,700 M −1 .cm−1 ) in the visible region of the optical spectrum warrant their investigation as potential optical sensors.Interestingly, we observed that upon addition of an excess of DBU (>50 equiv) to a dilute solution (ca. 10 −5 M) of each BOPYPYs caused immediate loss of color.We hypothesize that the base attacks the BOPYPYs' electron deficient meso-position leading to disruption of conjugation, similar to that observed in the case of BOPHYs. 19Studies on the potential application of these BOPYPYs as base sensors are currently underway in our laboratory.

CONCLUSIONS
We report the synthesis of a new series of fluorinated benzo-[α]-fused BOPYPY dyes with extended π-conjugated systems from those previously reported.These unsymmetric bisBF 2 compounds display nearly planar or slightly bowed structures, with the two BF 2 moieties having distinct coordination geometries in a 6-or 5-membered ring.The reactivity of αbromo-BOPYPY 2 was investigated in nucleophilic substitutions with N-, O-, S-, and C-centered nucleophiles, and in a Pd(0)-catalyzed Stille cross-coupling reaction.The regioselectivity of nucleophilic substitution reactions at the aromatic fluorine atoms was investigated on the α-free BOPYPY 1 and on the α-substituted BOPYPYs 4b, 4d, 4e, and 4g.With the exception of 4b, the F 2 position is electronically and/or sterically favored, giving the corresponding product with high regioselectivity.In the case of BOPYPY 4b, the F 4 is the favored substitution site due to stabilizing π−π-interactions between the two aromatic groups.
The new BOPYPYs show dual absorptions due to vibronic progressions in the same excited state, as suggested by DFT calculations, and 6−55 nm red-shifted from those of previously reported compounds.They also show broad emissions and large Stokes shifts, in the order of 3000 cm −1 , which are attributed to changes in the N−N and associated B−N bond lengths upon excitation.The relative fluorescence quantum yields of the BOPYPYs are lower than those observed for the related BOPPYs because of an internal conversion transition to a dark excited state due to the presence of the pyrazine N-lone pair molecular orbital.Our results show for the first time the drastic effect of presence of the N-lone pair on the pyrazine ring of BOPYPYs on the fluorescence properties of this type of dye.Nevertheless, BOPYPYs might find applications as optical sensors, for example for bases such as DBU.
4.4.Spectroscopic Analyses.UV−vis absorption and emission spectra were collected at room temperature on a Varian Cary 50 spectrophotometer or on a PerkinElmer LS55 spectrophotometer, respectively.Dilute solutions (1.0 × 10 −6 to 5.0 × 10 −6 M) using spectrophotometric grade solvents in quartz cuvettes (1 cm path length) were used to minimize reabsorption effects.The relative fluorescence quantum yields (Φ F ) were calculated using rhodamine 6G (Φ F = 0.86 in MeOH) as reference using the following equation: 33 Φ X = Φ ST × Grad X /Grad ST × (η X /η ST ) 2 , where the Φ X and Φ ST are the quantum yields of the sample and standard, Grad X and Grad ST are the gradients from the plot of integrated fluorescence intensity vs absorbance, and η represents the refractive index of the solvent.
4.5.Theoretical Calculations.All structures were optimized without symmetry constrains, and the stationary points were confirmed with frequency calculations.Solvent effects were taken into account using the polarized continuum model (PCM). 34The reported energies, atomic charges, and molecular electrostatic potentials were evaluated at the ωB97X-D/6-311++G(d,p) level to include empirical dispersion. 27The atomic charges were calculated using the NPA 35 and MK schemes. 36The UV−vis absorption data were calculated using the TD-DFT method 37 at the M06-2X/6-31+G(d,p) level 38 as recommended in the literature. 39,40All calculations were performed using the Gaussian 09 program package. 41ASSOCIATED CONTENT * sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c00499.
1D and 2D NMR data (PDF) for all BOPYPYs, absorption and emission spectra (PDF), and X-ray data (CIF).The Supporting Information is available free of charge.CIFs for ) and HRMS (Figures S68−S69).The 19 F NMR of BOPYPY 1 showed the four aromatic fluorines at −144.31, −147.83,−152.76, and −160.06 ppm, and the two distinct BF 2 groups appear at −141.53 and −144.40 ppm.In the

Scheme 5 .
Scheme 5. Regioselective S N Ar Reactions with Different Regioselectivity

Table 1 .
S N Ar Reaction Conditions and Yields for α-