Polybrominated BOPHY Dyes: Synthesis, Reactivity, and Properties

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Polybrominated BOPHY Dyes: Synthesis, Reactivity and Properties Xiaokang Lv, Tingting Li, Qinghua Wu, Changjiang Yu, Lijuan Jiao, and Erhong Hao J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b02415 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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The Journal of Organic Chemistry

Polybrominated BOPHY Dyes: Synthesis, Reactivity and Properties Xiaokang Lv, Tingting Li, Qinghua Wu, Changjiang Yu,* Lijuan Jiao, and Erhong Hao*

The Key Laboratory of Functional Molecular Solids, Ministry of Education; Anhui Laboratory of Molecule-Based Materials (State Key Laboratory Cultivation Base); School of Chemistry and Materials Science, Anhui Normal University, Wuhu, China 241000.

*To whom correspondence should be addressed. E-mail: [email protected]; [email protected]

Abstract Graphic

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Abstract A series of mono- to hexa-brominated BOPHY dyes have been regioselectively synthesized in 41%-96% yields from bromination of parent BOPHY, bromination of hydrazine linked bispyrrole intermediate, or brominated 2-formylpyrrole precursors in moderate to excellent yields. The reactivities of these polybrominated BOPHY dyes were further studied via regioselective nucleophilic substitution or Suzuki/Stille cross-coupling reactions, from which a series of 5- or 5,5’-substituted BOPHYs with various nucleophiles such as amine, pyrrole, thiophene and phenyl groups were obtained in moderate to high yields of 37%-94%. The regioselectivities of both the bromination and nucleophilic substitution reaction, and these resultant BOPHY dyes are confirmed by NMR, HRMS, and crystal structures. The spectroscopic properties of these resultant BOPHYs were studied, and most of them showed strong absorbance and bright fluorescence with maximum wavelengths centered at the between range of 430 and 660 nm. The absorption and emission spectra of them were red shifted for each bromine atom incorporated. The positions in which bromines or substituents are attached modulate the photophysical properties of the resulting BOPHY dyes.


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INTRODUCTION Small organic fluorescent dyes have attracted ongoing attention because of their diverse applications in biomedicals, materials, and related fields.1 The development of simple and low molecular weight dyes that can be further modified is thus of great interest. Among the synthetic fluorophores studied, boron complexes of π-conjugated chelates often have very promising features, such as high quantum yields, fine-tuned absorption/emission wavelengths, and good photostability.2,3 For example, extensive research on 4,4’-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) has been received because of its excellent photophysical properties and high photostability.4-6 Recently, a BF2-complexed

hydrazine-Schiff

base

linked

bispyrrole,

bis(difluoroboron)-1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine (BOPHY, Figure 1), has been developed as a novel small organic fluorescent dye.7,8 This new fluorophore has excellent spectroscopic properties, including high quantum yields of fluorescence, high molar extinction coefficients, highly tunable absorption/emission profiles, and excellent photostability. In addition, BOPHY dyes have larger Stokes shifts and higher solid-state fluorescence than those of classical BODIPY dyes which generally suffer from small Stokes shifts2a,6a-b and relatively weak solid-state fluorescence.

Figure 1. The chemical structures of BOPHY and its post-functionalized derivatives. Because of their excellent photophysical properties and extremely easy syntheses from 2-formylpyrrole and hydrazine, BOPHY dyes have quickly found applications7-17 such as energy-transfer cascades,11 cell imaging,12 photosensitizers for solar cells13 and photodynamic 3 ACS Paragon Plus Environment


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therapy14,15, and fluorescence sensors16,17 in the last three years. As a result, synthetic modifications which can modulate or improve the photophysical and photochemical properties of BOPHY dyes to achieve the desired fluorophores are needed to further spread their application. Until now there have not been many synthetic alterations on BOPHY dyes. Reported functionalizations of BOPHY, such as halogenation10-12,14, Knoevenagel condensation7c,8a,11,13, and formylation17, have been developed (BOPHY 1-3, Figure 1). In these reactions, 3,3’,5,5’-tetramethyl substituted BOPHY was used and no regioselectivity was involved due to the existence of blocking methyl groups. Modification from the starting pyrroles provides another strategy to achieve functionalized BOPHYs, from which ring-fused, red shifted BOPHYs and 4,4’-dihalo substituted BOPHYs8b,9,10-12 were synthesized. Among those, halogenated BOPHYs are highly versatile starting materials for installing further functionalities to achieve desired dyes. For example, further derivations on these 4,4'-diiodoBOPHYs based on the Sonogashira and Suzuki coupling reaction have been explored by several groups.10,11 However, there is still not a definitive research on the regioselectivity in these reactions for the parent BOPHY.

Figure 2. The natural population analysis (NPA) charges of carbon atoms in BOPHY (C2 symmetry), electrostatic potential map (ESP) of BOPHY and chemicals structures of brominated BOPHYs 1-6.


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Inspired by our recent work on regioselective brominations of BODIPYs and regioselective nucleophilic substitution of polybrominated BODIPYs,18 herein, we report the regioselective synthesis of a series of newly brominated BOPHYs bearing one to six bromine atoms at different positions on the parent BOPHY (Figure 2, 1-6) and the subsequent application of these resultant bromoBOPHYs for the regioselective nucleophilic substitution and Palladium-catalyzed coupling reactions. The spectroscopic properties of these resultant BOPHYs were studied.

RESULTS AND DISCUSSION Theoretical study To better understand the regioselectivity of the electrophilic bromination of the BOPHY, which would prefer to occur at the least positive charged positions of the BOPHY core, we first conducted density functional theory (DFT) calculations on BOPHY. DFT calculations suggested C2-symmetry (both boron atoms deviating from planarity toward one side) and Ci-symmetry (boron atoms deviating from planarity toward different sides) structures are the stationary points on the potential energy surfaces.7a,19 The electrostatic potential and the natural population analysis (NPA) of the C2 and Ci geometries are very similar to each other. The charge distribution map indicates that the parent BOPHY chromophore is most susceptible to electrophilic attack at the 4- and 4’-positions with the highest negative charge of -0.317 (Figure 2, Tables S1-2 in the supporting infromation (SI)). This data is consistent with the 1H NMR spectroscopy results (Figure S1, SI), in which protons at 4,4’-position of BOPHY showed upfield signal at 6.65 ppm (δ = 7.32, 7.74 and 8.26 ppm in CDCl3 for the protons attached at 3,3’-, 5,5’- and 6,6’-positions, respectively). Thus, by carefully controlling the reaction conditions, it may be possible to achieve regioselective 4-bromination of BOPHY. Syntheses of brominated BOPHYs

To test these hypotheses, we have investigated the bromination reactions of BOPHY by using 5 ACS Paragon Plus Environment


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different amount of the bromine at different reaction times. The products obtained in the bromination reactions are shown in Scheme 1, from which the mono-, di-, tri-, tetra-, penta-, hexa-brominated derivatives were regioselectively synthesized by direct bromination of the unsubstituted BOPHY with different amounts of liquid bromine, respectively.

Scheme 1. Regioselective bromination of BOPHY. In contrast to the bromination of BODIPY, BOPHY showed lower reactivity toward bromine. Treatment of BOPHY with 1 equiv of bromine in dichloromethane at room temperature gave the mixture of unreacted BOPHY and mono-bromoBOPHY 1. 1H-NMR of 1 only showed one proton signal at 6.75 ppm in d6-DMSO. The calculated 1H-NMR (Figures S2-3, SI) is in good agreement with the experimental results, indicating the proposed regioselective 4-bromination of BOPHY. We found that 3 equiv of bromine were needed to completely consume the BOPHY and gave the mono-bromoBOPHY 1 in 60% yield, together with small amount of dibromoBOPHY. Increasing the amount of liquid bromine to 6 equiv, the reaction still gave the above two products, and 4,4’-dibromoBOPHY 2a turned into the main product, with the isolated yield of 54%. The 6 ACS Paragon Plus Environment

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symmetrical structure of 2a was confirmed by the three sets of proton signals in its H1 NMR spectrum, and the signal of two protons at 4,4’-positions disappeared. Alternatively, since the corresponding 4-bromo-2-formylpyrrole P1 was readily available by regioselective bromination of 2-formylpyrrole, 4,4’-dibromoBOPHY 2a was efficiently synthesized by the condensation between P1 and hydrazine followed by BF2 complexation with boron trifluoride ether in the presence of triethylamine (Scheme 2).

Scheme 2. Syntheses of brominated BOPHYs from formylpyrroles. Further increasing the amount of bromine to 12 equiv, the bromination reaction afforded a major product, which was isolated to be tri-bromoBOPHY 3 in 62% yield (Scheme 1) from the 1H NMR spectrum, which showed two proton signals at 7.33-7.31 (3,3’-positions) and one at 7.67 ppm (5-position). Interestingly, 30 equiv of bromine exclusively afforded one product, which was confirmed to be 4,4’,5,5’-tetra-bromoBOPHY 4 in 90% yield. The highly symmetrical structure was evident from its NMR signals in d6-DMSO. The third and fourth brominated positions in 1H NMR spectrum showed the difference with the calculated results of electrostatic potential map and NPA distributions on BOPHY, which is similar to that of BODIPY. And the regiochemistry of 4 was further confirmed by the condensation between known 4,5-dibromo-2-formylpyrrole P2 and hydrazine followed by BF2 complexation (Scheme 2). It is worth to note that the third and fourth brominated positions differed from the calculated results, where the 3- and 3’-positions of the 7 ACS Paragon Plus Environment


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parent BOPHY chromophore have higher negative charge of than those of 5- and 5’-positions (Figure 2). However, similar inconsistency was observed during the halogenation of BODIPY dyes.18,19 The further bromination at 3,3’-positions of BOPHY 4 required a large excess amount (160 equiv) of bromine and exclusively gave penta-bromoBOPHY 5 in 91% yield (Scheme 1). Hexa-bromoBOPHY 6 was obtained in 93% yield with 300 equiv of bromine, and no further bromination product was obtained. Considering the large amount of bromine used for the direct bromination of BOPHY, especially for penta-bromoBOPHY 5 and hexa-bromoBOPHY 6, we then investigated the bromination of hydrazine linked bispyrrole (HBP), the synthetic precursor for BOPHY (Scheme 3). Unlike the dipyrrin for the synthesis of BODIPY, HBP is stable and suitable for functionalization. Indeed, Ziessel and coworkers recently briefly reported the synthesis of 5,5’-dibromoBOPHY 2b from HBP in THF in one-pot reaction. In this bromination of HBP, the regioselectivity was different from the bromination of BOPHY and the bromination of 2-formylpyrrole.

Scheme 3. Syntheses of brominated BOPHYs from regioselective bromination of HBP. With slight modification, we isolated the dibrominated H2b in 89% yield by using 2.1 equiv of 8 ACS Paragon Plus Environment

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NBS in ethyl acetate (Scheme 3). Bromine instead of NBS for this bromination gave similar results. Attempts to synthesize mono-brominated HBP by carefully controlling the reaction temperature and amount of bromine were failed. These reactions always gave mixtures of starting HBP, mono-brominated HBP and di-brominated HBP. Separation of these products was very difficult. Unlike the bromination of BOPHY, HBP is highly reactive toward bromine. The desired pentaand hexa-brominated hydrazine linked bispyrroles H5 and H6 were regioselectively obtained in 87% and 92% yields using only 10 and 20 equiv of bromine, respectively. Brominated BOPHYs 2b, 5 and 6 were synthesized after boron complexation in toluene. The structures of all the intermediates and the brominated BOPHYs were fully characterized with 1H NMR,

13

C NMR, as well as

high-resolution mass spectroscopies. Selective functionalization The easily obtained polybrominated BOPHY derivatives are possible valuable synthetic precursors for selective functionalization allowing the development of a variety of symmetric and asymmetric BOPHY compounds that are difficult to obtain by alternative procedures. It is well known that 3,5-dihalogenated BODIPY derivatives can undergo SNAr reactions by a wide range of nucleophiles. Furthermore, the reaction is stepwise and the reaction conditions can be adjusted to obtain mono-, di- or even tetra-substituted products. Inspired by these works on BODIPYs20-25, we have carried out SNAr reactions of polybrominated BOPHYs with n-butylamine, 4-tert-butylaniline, diethylamine and pyrrole.



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Scheme 4. Regioselective nucleophilic substitution of bromoBOPHYs with n-butylamine in 1,2-dichloroethane (DCE). First, we studied the reactivities of polybrominated BOPHYs with n-butylamine by nucleophilic substitution reactions. 5,5’-Dibrominated BOPHY 2b and 6 equiv n-butylamine in DCE at 70 ℃ gave mainly mono-substituted BOPHY 7 in 86% yield (Scheme 4). Extending the reaction time, increasing the amount of n-butylamine and increasing the reaction temperature, the di-substituted product was still not isolated in decent amount. However, tetra-bromoBOPHY 4 with different amounts of n-butylamine gave mono-substituted BOPHY 8 and di-substituted BOPHY 9 in 83% and 85% yields, respectively (Scheme 4), which indicated that tetra-bromo 4 has higher reactivity than di-bromo 2b. Similarly, hexa-brominated BOPHY 6 was combined with 3 equiv of n-butylamine, affording mono-substituted BOPHY 10 in 93% yield. However, while increasing the amount of n-butylamine, the reaction unexpectedly gave only one product 11 in 94% yield. The reaction between mono-substituted BOPHY 10 and excess n-butylamine also gave compound 11, instead of expected di-substituted BOPHY derivative. The X-ray structure of 11 (Figure 3) indicated that the hexa-brominated BOPHY core was broken. It might be reasoned that n-butylamine regioselectively 10 ACS Paragon Plus Environment

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attacked the hexa-bromoBOPHY 6 at the meso-position which may have higher activity than that of 5-position. Further attempts to prepare di-substituted products from 6 were failed.

Scheme 5. Syntheses of BOPHYs 12-15 through regioselective nucleophilic substitution of bromoBOPHYs. Next, a less reactive nucleophile 4-tert-butylaniline was attempted to react with these brominated BOPHYs (Scheme 5). This reaction between hexa-brominated BOPHY 6 and 4-tert-butylaniline gave mainly mono-substituted BOPHY 12 in 73% yield, while mono-substituted BOPHY 15 was obtained in 41% yield from the reaction of 2b and 4-tert-butylaniline (Scheme 5). The corresponding disubstituted products were not obtained in these reaction conditions. Similarly, hexa-brominated 6 reacted with pyrrole in refluxing toluene also only gave mono-pyrrole substituted BOPHY 13 in 37% yield (Scheme 5). Furthermore, unsymmetrical derivative 14 was obtained in 78% yield in one pot by first reaction with 4-tert-butylaniline and subsequent reaction with diethylamine from the hexa-bromoBOPHY 6 (Scheme 5). The structure of 14 was confirmed by the X-ray structure (Figure 4). In this case, the high nucleophilicity of secondary amine (diethylamine) allows the second SNAr reaction. Finally, although 5,5’-dibromoBOPHY 2b has been reported to be unreactive towards metal 11 ACS Paragon Plus Environment


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catalyzed reactions,11 however, we found 2b was successful towards Suzuki and Stille reactions (Scheme 6). The usual Suzuki coupling reaction conditions23-27 were used to obtain 4-dimethylaminobenzene substituted BOPHY 16 in 83% yield. Similarly, the Stille coupling reaction afforded thiophene substituted BOPHY 17 in 86% yield. The structures of both 16 and 17 were confirmed by the X-ray structure analysis (Figure 5).

Scheme 6. Syntheses of BOPHYs 16 and 17 from 2b. X-ray structure analysis The crystals of 10, 11, 14, 16 and 17 suitable for X-ray analysis were obtained from slow diffusion of hexane into dichloromethane at room temperature. Their ortep polts are given in Figures S4-5 (SI). Selected geometrical parameters are summarized in Tables S3-5 (SI). The crystal structure of amine substituted BOPHY 10 shows the usual planar BOPHY core of two pyrrole units at the periphery and two BF2-containing six-membered rings in the center (Figure 3), while for extended BOPHYs 16 and 17, the additioal aryl and thiophene rings at 5,5’-positions are also coplanar with the BOPHY core. The dihedral angles between the BOPHY core and the phenyl or thiophene ring in 16 and 17 were observed to be less than 9.0°(Figure 5). Dihedral angle between pyrrole ring and the neighbor BF2-containing six-membered ring in 11 is 2.9°, indicating an almost planar conformation of half BOPHY core (Figure 3d). These results indicate that no steric feature of the amino substituents causes structural irregularities. Extended packing forces and structural motifs that exist in the solid state can account for the small dihedral angles noted for these dyes. 12 ACS Paragon Plus Environment

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Figure 3. Top (a, c) and side (b, d) views of X-ray structure of 10 (a) and 11 (c) with thermal ellipsoids at 50% probability level. C, light gray; N, blue; B, yellow; F, bright green; Br, dark yellow; All hydrogen atoms are omitted for clarity.

Figure 4. Top (a) and side (b) views of X-ray structure of 14 with thermal ellipsoids at 50% probability level. C, light gray; N, blue; B, yellow; F, bright green; Br, dark yellow; All hydrogen atoms are omitted for clarity.

Figure 5. Top (a, c) and side (b, d) views of X-ray structure of 16 (a) and 17 (c) with thermal ellipsoids at 50% probability level. C, gray; N, blue; B, yellow; F, bright green; S, green; All hydrogen atoms are omitted for clarity. In contrast, 5,5’-diamino substituted BOPHY 14 showed highly twisted conformation of the BOPHY core in solid state (Figure 4), with the dihedral angles of 33.2°between two pyrrolic rings and 26.3°between two BF2-containing six-membered rings, a serious deviation from the previously largely planar arrangements. Further analysis of 14 illustrates that this dihedral angle generates the 13 ACS Paragon Plus Environment


formation of a “boat” conformation of the BOPHY core. This behavior demonstrates the significant steric hindrance introduced by the two amino substituted groups at 5,5’-positions. Such significant distortion that breaks the planarity of the BOPHY core has not been observed previously.

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Figure 6. Overlaid absorption (a) and normalized fluorescence emission (b) spectra of BOPHYs B1-B6 and the parent BOPHY in dichloromethane at room temperature. UV-vis and fluorescence spectra of all the synthesized BOPHYs were examined in dichloromethane as shown in Figures 6-8 and Figures S6-8 (SI). Their photophysical properties were summarized in Table 1. Similar to the absorption and emission maximum of the parent BOPHY, each dye exhibited a strong absorption band with a shoulder in the region of 430-470 nm. The emission spectra of 1-6 are also similar to that of the parent BOPHY, displaying emission maxima in the range of 484-513 nm with relatively large Stokes-shift around 50 nm (Table 1). Similar to previous reported BOPHY derivatives,7-18 most of these BOPHY dyes exhibit the double absorption and emission maxima in their absorption and emission spectra. For example, 1 exhibits the double absorption and emission maxima at 433, 451 nm and at 484, 508 nm, respectively. Similar phenomenon was observed for other dyes such as BOPHYs 2-6. The double absorption bands 14 ACS Paragon Plus Environment

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observed in the visible range could be attributed by HOMO to LUMO excitation and the splitting of the strong bands belongs to a vibronic progression of the same excited state.7a And the splitted highand low-energy emission bands are possible 0-0 and 0-1 transitions of the same vibronic progression.10 The absorption and emission spectra of the brominated BOPHYs 1-6 were red shifted for each bromine atom incorporated to the parent BOPHY (Figure 6) due to the electron-withdrawing effect of the electronegative bromine atom. Table 1. Photophysical properties of BOPHYs 1-17 in dichloromethane at room temperature. BOPHYs λmax(nm) λem(nm) lgε Stokes-shift (nm) Φa BOPHYb 468, 495(sh) 4.09 2273 1.00 423, 442(sh） 1 433, 451(sh) 484, 508(sh) 4.39 2434 0.53 2a 439, 455 488, 515(sh) 4.58 2287 0.20 2b 444, 467 492, 522(sh) 4.49 2197 0.59 3 450, 467(sh) 498, 530(sh) 4.37 2142 0.51 4 455, 477(sh) 505, 537(sh) 4.30 2176 0.61 5 462, 485(sh) 511, 541(sh) 4.69 2076 0.65 6 465, 488(sh) 513, 546(sh) 4.64 2012 0.69 7 487 546, 576(sh) 4.36 2219 0.02 8 497 558, 587(sh) 4.66 2200 0.01 9 510, 540 564, 594(sh) 4.70 1877 0.33 10 508 568, 601(sh) 4.48 2079 0.02 11 522(sh), 552 580 3.79 1916 0.24 12 518 608 4.53 2858 0.004 13 557 630 4.67 2080 0.05 14 545 590 4.75 1399 0.14 15 499 574 4.43 2618 0.03 16 573 657 4.43 2231 0.17 17 516, 538 572, 613(sh) 4.65 1928 0.57 a The relative fluorescence quantum yields (Φ) in dichloromethane were calculated using the parent BOPHY (Φ = 1.0 in dichloromethane) as the standard for 1, 2a and 2b (excitation at 430 nm), Fluorescein (Φ = 0.9 in 0.1 M NaOH) as reference for 3-15 (excitation at 450 nm for 3-6, 470 nm for 7-15), Cresyl Violet perchlorate (Φ = 0.54 in methanol ) for 16 (excitation at 550 nm), and Rhodamine B (Φ = 0.49 in ethanol) for 17 (excitation at 500 nm). The standard errors are less than 10%. b Photophysical properties of BOPHY were obtained from the literature of ref. 8a.

All of these BOPHYs with bromine atoms are colorful to eyes and most of them are brilliant upon 365 nm hand-hold UV lamp irradiation condition with fluorescence quantum yields ranging from 0.14 to 0.69. The bromination of the BOPHY core should induce a decrease in the fluorescence 15 ACS Paragon Plus Environment


capacity due to the heavy atom effect, which increases the intersystem crossing probability. 27 This influence is observed for the mono-bromo and the di-bromo derivatives 1 and 2a, respectively.

The

mono-brominated 1 exhibited the two main absorption bands of 433 nm and a shoulder of 451 nm which gave around 10 nm red shifts in dichloromethane, comparing to these of parent BOPHY. The fluorescence was also red shifted with 16 nm and gave a decreased but moderate quantum yield (53%), where the parent BOPHY has a fluorescence quantum yield close to unity. The di-brominated BOPHY 2a exhibited the absorption maximum of 439 nm and a shoulder of 455 nm and the emission maximums of 488 nm with a fluorescence quantum yield of 20%. In contrast, di-brominated 2b with bromines at the 5,5’-positions gave the absorption maximum of 444 nm and a shoulder of 467 nm and the emission maximums of 491 nm with a much higher fluorescence quantum yield of 59%. Surprisingly, further addition of bromine atoms to 2a leads to an increase in the fluorescence emission. The fluorescence quantum yields for polybrominated BOPHYs 3, 4, 5

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and 6, is 0.51, 0.61, 0.65 and 0.69, respectively, much higher than the value of 0.20 for 2a (Table 1).


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Figure 7. Overlaid absorption (a) and normalized fluorescence emission (b) spectra of BOPHYs 4, 8, 9 and 14 in dichloromethane at room temperature. In comparison with the unsubstituted BOPHY, the functionalized BOPHYs 7-15 exhibited obvious red shifts of up to 134 and 162 nm in their absorption and emission spectra, respectively 16 ACS Paragon Plus Environment

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(Table 1). The gradual red-shift of the absorption and emission has been observed with the different numbers and kinds of amine substituents. As shown in Figure 7, mono-amine substituted 8 absorbs/emits at 497/558 nm; di-amine substituted 9 absorbs/emits at 540/565 nm and 14 with the two different amine substitutions absorbs/emits at 545/590 nm. Among those dyes of 7-15, the mono-pyrrole substituted 13 gave the longest absorption and emission wavelength, exhibiting the absorption and emission maxima in dichloromethane centred at 557 nm and 630 nm (Figure 8), respectively. All the mono-amine-substituted BOPHYs 7-8, 10 and 12 exhibited very weak fluorescence with the fluorescence quantum yields in the range of 0.004-0.02. Similar phenomenon was observed for the mono-pyrrole substituted 13 with the fluorescence quantum yield of 0.04 in dichloromethane, which may be due to the efficient intra-molecular charge transfer due to the participation of amine or pyrrole. However, the di-amine substituted BOPHY 9 and 14 still give relatively bright red fluorescence, with the fluorescence quantum yields of 0.33 and 0.14, respectively. To our surprise, the unexpected broken compound 11 exhibited the emission

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wavelength maximum at 580 nm and the fluorescence quantum yield of 0.24 in dichloromethane.


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Figure 8. Overlaid absorption (a) and normalized fluorescence emission (b) spectra of the parent BOPHY, 13, 16 and 17 in dichloromethane at room temperature.


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Figure 9. Absorption (a) and fluorescence (b) titration spectra of 16 (4.5 μM) in toluene with the addition of TFA, excited at 470 nm. As expected, after the installation of electron-donating substituents (NMe2) at the p-phenyl position of the chromophore, obvious 150 and 189 nm bathochromic shifts were observed in the absorption and emission maximum of 16 with respect to those of the parent BOPHY in dichloromethane, as shown in Table 1 and Figure 8. Similarly, the thiophene substituted 17 in dichloromethane shows a red shift of 120 nm of the absorption wavelength maximum and 98 nm in the emission wavelength maximum compared to the parent BOPHY. Both of these two dyes showed a gradual decrease of the fluorescence quantum yields with the increase of the polarity of the solvents (Figures S9-10, SI). As summarized in Table S6, the fluorescence quantum yield for 16 was 0.39 in hexane, which was gradually reduced to 0.12 (in toluene), 0.09 (in dichloromethane), and 0.04 (in tetrahydrofuran). With the titration of trifluoroacetic acid (TFA, Figure 9) in toluene, a stepwise disappearance of the absorption bands at 590 nm with the simultaneous appearance of a new band at 495 nm was observed for 16 in toluene (Figure 9a). The sharp isosbestic point at 510 nm indicates the formation of a diprotonated specie (16-2H+), which was confirmed by the NMR spectrum of 16 in the presence of excess amount of TFA (SI). Similarly, a blue-shift of the emission band was also observed from 634 nm to 530 nm, and 8-fold of the fluorescence intensity was increased (Figure 9b) due to the inhibition of intramolecular charge transfer (ICT) process from the dimethylamine (NMe2) 18 ACS Paragon Plus Environment

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moiety to the BOPHY chromophore. Very similar results were observed in acetonitrile/water solution of different pH values (Figure S12). It shows very low fluorescence in low acidic condition, while it is highly fluorescent in acidic condition. Especially, change of the pH values from 4.0 to a strong acidic condition of 2.0 results in significant increases of fluorescence intensity. This result indicates that 16 could be used as a potential ratiometric “turn-on” pH probe up further modification.

Conclusion

We have demonstrated that it is possible to control the degree of bromination in the BOPHY core, which allows the synthesis of various mono-, di- and polybrominated derivatives in moderate to excellent yields. Three different synthetic methods for brominated BOPHY derivatives have been developed, including regioselective bromination of parent BOPHY, regioselective bromination of hydrazine linked bispyrrole intermediate, or condensation of brominated 2-formylpyrrole precursors. These are the first examples of this type of dyes with more than two halogen atoms in the BOPHY core. These polyhalogenated compounds can be valuable synthetic precursors for the selective incorporation of the desired functional groups in a specific position of the BOPHY, and in this study were applied for further functionalization via nucleophilic substitution and metal-catalyzed coupling reactions, from which a series of BOPHYs 7-17 were obtained in 37%-94% yields. Nucleophilic substitutions occurred first at the 5,5’-positions, while the 4,4’-bromo sites were unreactive under these conditions. The regioselectivities of both the bromination and nucleophilic substitution reactions were confirmed by X-ray crystallography. Moreover, the position in which bromine or substituent is attached modulates the photophysical properties of the resulting BOPHY. Bromine atoms at 4- or 4’-positions lead to an important reduction of the fluorescence quantum yield owing to 19 ACS Paragon Plus Environment


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the heavy atom effect, which activates the intersystem crossing processes. The functionalized BOPHYs 7-17 with substituents at 5,5’-positions exhibited obvious red shifts spectra (up to 134 and 162 nm in their absorption and emission spectra, respectively). These regioselective stepwise bromination and its applications for the regioselectively nucleophilic substitution reaction presented in this work provide another dimension for the regioselective functionalization of BOPHY derivatives.

Experimental Section General Methods Reagents and solvents were used as received from commercial suppliers unless noted otherwise. All reactions were performed in flame-dried or oven-dried glassware, and were monitored by thin layer chromatography (TLC). Flash column chromatography was performed using silica gel (300−400 mesh). 1H NMR and 13C NMR spectra were recorded at 300 MHz or 500 MHz NMR spectrometer in CDCl3 or d6-DMSO. Chemical shifts (δ) are given in ppm relative to CDCl 3 (7.26 ppm for 1H and 77 ppm for

13

C) or internal TMS (δ = 0 ppm) as internal standard. Data are reported as follows:

chemical shift, multiplicity, coupling constants and integration. High resolution mass spectra (HRMS) of all the new compounds except 1 and 3 were obtained using APCI-TOF or ESI-TOF in positive mode. HRMS was obtained using ESI-TOF for 1 and MALDI for 3 in negative mode. UV–vis absorption spectra and fluorescence emission spectra were recorded on commercial spectrophotometers (190–900 nm scan range) with a quartz cuvette (path length = 1 cm). The relative fluorescence quantum yields (Φ) in dichloromethane were calculated by comparing the areas under the corrected emission spectrum using the parent BOPHY (Φ = 1.0 in dichloromethane)8a as 20 ACS Paragon Plus Environment

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the standard for BOPHYs 1, 2a and 2b (excitation at 430 nm), Fluorescein (Φ = 0.9 in 0.1 M NaOH)28 as reference for BOPHYs 3-15 (excitation at 450 nm for 3-6, 470 nm for 7-15), Cresyl Violet perchlorate (Φ = 0.54 in methanol )29 for 16 (excitation at 550 nm), and Rhodamine B (Φ = 0.49 in methanol )30 for 17 (excitation at 500 nm). Non-degassed, spectroscopic grade solvents and a 10 mm quartz cuvette were used. Dilute solutions (0.01 300 ℃; 1H NMR (300 MHz, CDCl3) δ 8.16 (s, 2H);

13

C NMR (125 MHz, CDCl3) δ 183.0, 127.1,

124.6, 120.6, 113.2. HRMS (APCI) Calcd. for C10H2B2Br6F3N4 [M - F]+: 734.5477, found 734.5460. Synthesis of BOPHY 7: To BOPHY 2b (50 mg, 0.11 mmol) in 12 mL of DCE was added butylamine (38.8 mg, 0.531 mmol). The reaction mixture was stirred at 70 ℃ for 75 min, poured into water and extracted with CH2Cl2 (3 × 60 mL). Organic layers were dried over Na2SO4, and evaporated to dryness under vacuum. The crude product was purified by silica gel column chromatography (petroleum ether/dichloromethane = 4/1, v/v) to give a dark orange powder in 86% yield (66 mg). mp 186.3-187.2 ℃; 1H NMR (300 MHz, CDCl3) δ 7.87 (s, 1H), 7.50 (s, 1H), 7.21 (d, J = 4.5 Hz, 1H), 7.02 (d, J = 3.6 Hz, 1H), 6.54 (d, J = 3.9 Hz, 1H), 6.05 (d, J = 4.5 Hz, 1H), 5.57 (s, 1H), 3.37-3.35 (m, 2H), 1.72-1.63 (m, 2H), 1.44 (q, J = 7.2 Hz, 2H), 0.98 (t, J = 7.2 Hz, 3H);

13

C

NMR (125 MHz, CDCl3) δ 157.9, 134.0, 133.9, 129.2, 125.3, 124.6, 121.0, 119.1, 118.7, 105.7, 44.4, 32.0, 19.8, 13.7. HRMS (APCI) Calcd. for C14H16B2BrF3N5 [M - F]+: 412.0727, found 412.0728. 26 ACS Paragon Plus Environment

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Synthesis of BOPHY 8: 8 was obtained from 4 (50 mg, 0.08 mmol) and butylamine (38.9 mg, 0.53 mmol) at 70 ℃ using the above procedure for 7 as a red powder in 85% yield (88 mg). mp 188.5-188.9 ℃; 1H NMR (500 MHz, d6-DMSO) δ 7.76 (s, 1H), 7.46 (s, 1H), 6.73 (d, J = 7.0 Hz, 1H), 3.59 (q, J = 7.0 Hz, 3H), 1.59-1.53 (m, 2H), 1.35-1.28 (m, 2H), 0.89 (t, J = 7.5 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 151.1, 131.9, 129.7, 119.0, 92.6, 43.6, 33.0, 19.6, 14.1. HRMS (APCI) Calcd. for C14H14B2Br3F3N5 [M - F]+: 569.8917, found 569.8914. Synthesis of BOPHY 9: 9 was obtained from 4 (200 mg, 0.337 mmol) in 35 mL of ClCH2CH2Cl and butylamine (147.6 mg, 2.042 mmol) at 70 ℃ using the above procedure for 7 as a red powder in 83% yield (162 mg). mp 193.2-194.7 ℃; 1H NMR (500 MHz, d6-DMSO) δ 7.76 (s, 1H), 7.46 (s, 1H), 6.73 (d, J = 7.0 Hz, 1H), 3.59 (q, J = 7.0 Hz, 3H), 1.59-1.53 (m, 2H), 1.35-1.28 (m, 2H), 0.89 (t, J = 7.5 Hz, 3H). 13C NMR (75 MHz, CDCl3) δ 151.1, 131.9, 129.7, 119.0, 92.6, 43.6, 33.0, 19.6, 14.1. HRMS (APCI) Calcd. for C18H24B2Br2F3N6 [M - F]+: 563.0547, found 563.0547. Synthesis of BOPHY 10: 10 was obtained from 6 (50 mg, 0.067 mmol) and butylamine (14.67 mg, 0.20 mmol) at 40 ℃ for 25 min using the above procedure for 7 as a dark orange powder

in 93%

yield (46 mg). mp 209.7-210.9 ℃; 1H NMR (300 MHz, CDCl3) δ 7.94 (s, 1H), 7.57 (s, 1H), 5.51 (s, 1H), 3.80 (q, J = 6.6 Hz, 2H), 1.76-1.66 (m, 2H), 1.51-1.44 (m, 3H), 1.00 (t, J = 7.2 Hz, 2H). 13C NMR (75 MHz, CDCl3) δ 151.7, 150.5, 132.7, 129.2, 127.4, 121.4, 119.3, 117.5, 95.8, 93.6, 43.7, 32.9, 19.6, 14.1. HRMS (APCI) Calcd. for C14H12B2Br5F3N5 [M - F]+: 727.7107, found 727.7114. Synthesis of BOPHY 11: 11 was obtained from 6 (50 mg, 0.067 mmol) and butylamine (48.9 mg, 0.67 mmol) at 40 ℃ for 1 h using the above procedure for 7 as a dark orange powder in 94% yield (30 mg). mp 166.1-166.7 ℃; 1H NMR (300 MHz, CDCl3) δ 7.94 (s, 1H), 7.57 (s, 1H), 5.51 (s, 1H), 3.80 (q, J = 6.6 Hz, 2H), 1.76-1.66 (m, 2H), 1.51-1.44 (m, 3H), 1.00 (t, J = 7.2 Hz, 2H); 13C NMR 27 ACS Paragon Plus Environment


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(75 MHz, CDCl3) δ 197.3, 150.2, 128.6, 121.9, 117.9, 95.3, 44.1, 32.9, 19.6, 13.7. HRMS (APCI) Calcd. for C10H12BBr2ClFN4 [M - F + Cl - Br]+: 412.9174, found 412.9189. Synthesis of BOPHY 12: 12 was obtained from 6 (20 mg, 0.027 mmol) in 15 mL of toluene and 4-tert-butylaniline (40 mg, 0.27 mmol) at 80 ℃ using the above procedure for 7 to give a dark orange powder in 73% yield (16 mg). mp 245.4-246.7 ℃; 1H NMR (300 MHz, CDCl3) δ 7.96 (s, 1H), 7.77 (s, 1H), 7.42 (d, J = 7.8 Hz, 2H), 7.13 (d, J = 8.1 Hz, 2H), 1.35 (s, 9H);

13

C NMR (125

MHz, CDCl3) δ 150.7, 150.3, 134.2, 133.1, 130.9, 126.9, 126.2, 125.0, 124.2, 121.8, 119.7, 115.9, 111.2, 34.7, 31.4. HRMS (APCI) Calcd. for C20H16B2Br5F3N5 [M - F]+: 803.7420, found 803.7438. Synthesis of BOPHY 13: To 6 (55 mg, 0.126 mmol) in a 25 mL Schlenk tube was added toluene (1.0 mL) and pyrrole (1.0 mL). The reaction mixture was heated at 110 °C for 12 h under Ar. The cooled reaction mixture was concentrated under vacuum and the residue was purified by silica gel column chromatography to give 13 in 37% yield (34 mg) as a red powder. mp 273.5-274.2 ℃; 1H NMR (300 MHz, CDCl3) δ 9.90 (brs, 1H), 8.12 (s, 1H), 8.06 (s, 1H), 7.59 (brs, 1H), 7.18 (brs, 1H), 6.47 (brs, 1H); 13C NMR (125 MHz, CDCl3) δ 142.9, 136.1, 135.1, 125.0, 124.5, 124.4, 123.3, 120.3, 119.0, 118.4, 118.0, 112.4, 111.5, 109.6. HRMS (APCI) Calcd. for C14H7B2Br5F4N5 [M + H]+: 741.6700, found 741.6697. Synthesis of BOPHY 14：To BOPHY 4 (200 mg, 0.337 mmol) in 15 mL of ClCH2CH2Cl was added 4-tert-butylaniline (151 mg, 1.01 mmol). The reaction mixture was stirred at 80 ℃. After the starting material was consumed completely, diethylamine (37 mg, 0.51 mmol) was added into the reaction mixture and the reaction was stirred for 4 h. The mixture was poured into water (100 mL) and was extracted with CH2Cl2 (3 × 60 mL). Organic layers were combined, dried over Na2SO4, and evaporated to dryness under vacuum. The crude product was purified by silica gel column 28 ACS Paragon Plus Environment

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chromatography (petroleum ether/ dichloromethane = 2/1, v/v) to give a dark orange powder 14 in 78% yield (172 mg). mp 168.2-186.4 ℃; 1H NMR (300 MHz, CDCl3) δ 9.90 (s, 1H), 8.12 (s, 1H), 8.06 (s, 1H), 7.59 (s, 1H), 7.18 (s, 1H), 6.47 (s, 1H);

13

C NMR (125 MHz, CDCl3) δ 142.9, 136.1,

135.1, 125.0, 124.5, 124.4, 123.3, 120.3, 119.0, 118.4, 118.0, 112.4, 111.5, 109.6. HRMS (APCI) Calcd. for C24H29B2Br2F4N6 [M + H]+: 659.0922, found 659.0921. Synthesis of BOPHY 15: 15 was obtained from 2b (25 mg, 0.057 mmol) in 15 mL of toluene and 4-tert-butylaniline (85 mg, 0.57 mmol) at 85 ℃ using the above procedure for 7 to give a red powder A9 in 41% yield (12 mg). mp 272.4-273.6 ℃; 1H NMR (300 MHz, CDCl3) δ 7.95 (s, 1H), 7.64 (s, 1H), 7.43 (d, J = 8.4 Hz, 2H), 7.24 (d, J = 4.8 Hz, 1H), 7.18 (d, J = 8.4 Hz, 2H), 7.07 (d, J = 3.6 Hz, 1H), 6.57 (d, J = 3.9 Hz, 1H), 6.34 (d, J = 4.5 Hz, 1H), 1.34 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 154.5, 149.1, 135.1, 134.8, 133.3, 130.9, 126.7, 125.4, 121.8, 120.9, 119.8, 119.5, 107.1, 98.9, 34.6, 31.3. HRMS (APCI) Calcd. for C20H20B2BrF3N5 [M - F]+: 488.1040, found 488.1048. Synthesis of BOPHY 16: To a dry a dry Schlenk tube (10 mL) loaded with BOPHY 2b (30 mg, 0.068 mmol), 4-dimethylaminobenzene boronic acid (34 mg, 0.204 mmol), Pd(PPh3)4 (9 mg, 7.8 μmol) and Na2CO3 (30 mg, 0.286 mmol), toluene (2 mL) and H2O (0.2 mL) was were added through a syringe into the mixture. Freeze-pump-thaw cycle was carried out three times. After that, the mixture was warmed to 90 oC under argon and stirred for 10 h. After cooling to room temperature, the reaction mixture was extracted with ethyl acetate and dried over anhydrous Na2SO4. Organic layers were combined, dried over Na2SO4, and evaporated to dryness under vacuum. The crude product was purified by silica gel column chromatography to give a violet powder 16 in 83% yield (29 mg). mp > 300 ℃; 1H NMR (300 MHz, CDCl3) δ 8.02 (s, 2H), 7.80 (d, J = 8.7 Hz, 4H), 7.22 (d, J = 4.2 Hz, 2H), 6.79 (d, J = 8.1 Hz, 4H), 6.74 (d, J = 4.2 Hz, 2H), 3.06 (s, 12H). 29 ACS Paragon Plus Environment

13

C NMR (125


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MHz, CDCl3) δ 153.3, 151.8, 137.9, 131.0, 130.5, 126.5, 118.6, 118.5, 112.4, 58.8, 55.7. HRMS (APCI) Calcd. for C26H27B2F4N6 [M + H]+: 521.2419, found 521.2420. Synthesis of BOPHY 17: To a dry Schlenk tube (10 mL) loaded with BOPHY 2b (55 mg, 0.126 mmol), 2-(tributylstannyl)thiophene (197 mg, 0.52 mmol), Pd(PPh3)4 (7 mg, 6.3 μmol), toluene (2 mL) was were added through a syringe into the mixture. Freeze-pump-thaw cycle was carried out three times. After that, the mixture was warmed to 90 oC under argon and stirred for 5 h. After cooling to room temperature, the reaction mixture was extracted with ethyl acetate and dried over anhydrous Na2SO4. Organic layers were combined, dried over Na2SO4, and evaporated to dryness under vacuum. The crude product was purified by silica gel column chromatography to give a purple powder 17 in 86% yield (48 mg). mp 235.5-236.7 ℃; 1H NMR (500 MHz, CDCl3) δ 8.14 (s, 2H), 7.91 (d, J = 4.0 Hz, 2H), 7.50 (d, J = 5.0 Hz, 2H), 7.29 (d, J = 4.5 Hz, 2H), 7.20 (t, J = 4.0 Hz, 2H), 6.88 (t, J = 4.0 Hz, 2H).

13

C NMR (125 MHz, CDCl3) δ 144.4, 139.4, 132.4, 130.6, 130.2, 129.6,

128.6, 125.9, 118.3. HRMS (ESI) Calcd. for C18H13B2F4N4S2 [M + H]+: 447.0704, found 447.0699. DFT calculation: The ground state geometry was optimized by using DFT method at B3LYP/6-31G (d,p) level. The same method was used for vibrational analysis to verify that the optimized structures correspond to local minima on the energy surface. The natural population analysis (NPA) was used based on the optimized ground state geometry at the same level to be calculated. All of the calculations were carried out by the methods implemented in Gaussian 09 package.35 Supplementary Information (SI) available: Crystal structure data and CIF files, additional photophysical data and spectra, copies of NMR spectra, high resolution mass spectra and additional computational data for all new compounds could be available free of charge via the Internet at http://pubs.acs.org. 30 ACS Paragon Plus Environment

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Acknowledgements This work is financially supported by the National Nature Science Foundation of China (Grants Nos. 21672006, 21402001 and 21372011), Nature Science Foundation of Anhui Province (Grants No. 1508085J07), and Doctoral up Starting Foundation of Anhui Normal University (Grants No. 2017XJJ28). The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of The University of Science and Technology of China.

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