Thiophene-Fused 1,10-Phenanthroline and Its Conjugated Polymers

Publication Date (Web): May 20, 2016. Copyright © 2016 American Chemical Society. *E-mail [email protected] (Y.C.)., *E-mail [email protected] (Z.B...
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Thiophene-Fused 1,10-Phenanthroline and Its Conjugated Polymers Tingting Wang,†,§ Hongyan Wang,†,§ Guangwu Li,‡ Mengwei Li,†,§ Zhishan Bo,*,‡ and Yulan Chen*,†,§ †

Department of Chemistry, Tianjin University, Tianjin 300072, P. R. China Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China § Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, P. R. China ‡

S Supporting Information *

ABSTRACT: A novel type of π-extended 1,10-phenanthroline, specifically with fused thiophene groups at the less exploited 3-, 4-, 7-, and 8positions of the phenanthroline ring, and its conjugated polymers were designed and synthesized. The current developed route is based on the Bischler−Napieralski cyclization of the (1,2-phenylene)diamide precursor, which offers a facile and versatile strategy for preparing soluble and well-defined 1,10-phenanthroline derivatives and their analogues. High molecular weight poly(phenanthroline-co-fluorene)s with good solubility in common organic solvents or water were prepared by palladiumcatalyzed Suzuki−Miyaura−Schlüter polycondensation. The optical responsive properties of these thiophene-fused 1,10phenanthroline-containing polymers have demonstrated these polymers could be a good candidate for potential applications as luminescent chemosensor materials thanks to the specific repeating unit along the backbone.

A

challenge, even though theoretical calculations already suggest that the most intense electronic transition is polarized along the 3,8-positions of the Phen skeleton,29 and multifused (hetero)aromatic compounds are expected with spectacular properties, such as strong fluorescence and high charge carrier mobility resulting from their flat and rigid frameworks. In these scenarios, two critical stepsthe alkylation of the 2- and 9positions of the Phen ring and the dihalogenationusually require special and elaborate reaction conditions.21 In our previous work, Bischler−Napieralski cyclization was utilized as the key step to phenanthridine derivatives and azomethine-bridged ladder-type poly(p-phenylenes), highlighting its capability in the facile synthesis of nitrogen-containing heterocycles.30,31 Herein, we reasoned that if the amide groups were designed to locate at the adjacent position of a phenyl group, Bischler−Napieralski cyclization would furnish the target π-fused 1,10-phenanthroline with alkyl substitutions (Scheme 1). To this end, we apply this reaction to 1,10-phenanthroline derivatives and present herein a successful strategy to synthesize new thiophene-fused 1,10-phenanthroline and its conjugated polymers. Photophysical property studies illustrated that the polymers are a class of promising chemosensor materials. As shown in Scheme 2, the synthetic route to the target molecule, 2,9-dibromo-4,7-di-tert-butyldithieno[3,2-c:2′,3′-i][1,10]phenanthroline (6), is straightforward. Thiophene herein was chosen as the fused group for extended π-conjugated

s one of the most important building blocks for luminescent molecules, photosensitizers, and metal complexes, 1,10-phenanthroline (Phen) has unique features of a rigid geometry with three aromatic rings being substantially coplanar and the two nitrogen atoms located at juxtaposition, giving rise to distinct electronic and optical properties as well as outstanding chemosensing capability.1−5 Such properties originate, at least in part, from its planarity, aromaticity and chelating capability, endowing 1,10-phenanthroline an attractive material used in the fields such as optoelectronics, chemical sensors, pharmaceuticals, catalysis, and analytical probes.6−16 Up to date, 1,10-phenanthroline derivatives have been tailored to provide chemical products for the above-mentioned versatile purposes. A fruitful strategy involves the chemical functionalization at the various ring positions or the incorporation into polymers. For example, many attempts have focused on substituted 1,10-phenanthroline with various functional groups attached at the 2-, 9-, 5-, 6-, 3-, 8- or 4-, 7positions, leading to derivatives with a good coordination capacity.17−20 Besides, 1,10-phenanthrolines have been incorporated into different conjugated polymer backbones which showed excellent sensitivity toward specific metal cations because of the molecular wire effect.12,13,21−25 Rather than periphery substitution, π-extended 1,10-phenanthrolines are less exploited due to the lacking of sophisticated synthetic strategies. Synthesis of π-fused 1,10-phenanthrolines is by no means straightforward. Substantial efforts have been made to prepare 2-D-type 1,10-phenanthroline derivatives with aromatic rings fused at the 5- and 6-positions;26−28 however, the synthesis of π-fused 1,10-phenanthrolines, particularly extending at the 3-, 4-, 7-, and 8-positions, and therefore the corresponding soluble conjugated polymers remains a big © 2016 American Chemical Society

Received: February 1, 2016 Revised: May 12, 2016 Published: May 20, 2016 4088

DOI: 10.1021/acs.macromol.6b00236 Macromolecules 2016, 49, 4088−4094

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Macromolecules Scheme 1. Synthesis of N-Containing Six-Membered Arenes via Bischler−Napieralski Cyclization

Scheme 2. Synthetic Route to the Thiophene-Fused 1,10-Phenanthroline (6)

systems due to its higher electron density and abundant functionalization possibility than benzene ring. tert-Butyl groups were attached at the 2- and 9-positions of phenanthroline to increase the solubility of monomer 6 and its conjugated polymers. First, commercially available 4,7-dibromobenzo[c][1,2,5]thiadiazole (1) was treated with NaBH4 to generate the corresponding 3,6-dibromobenzene-1,2-diamine (2) in a yield of 89%. 3,6-Dibromo-1,2-benzenediamide (3) was then prepared by the coupling of 2 with pivaloyl chloride in THF/ TEA for the formation of amide linkage in a yield of 87%. Suzuki−Miyaura cross-coupling of 3 and thiophene 2-bronic ester was carried out by using Pd(0) as the catalyst precursor, affording the diamide precursor 4 in a yield of 68%. The conversion of the resulting intermediate 4 to thiophene-fused 1,10-phenanthroline (5) was accomplished with Bischler− Napieralski cyclization, using POCl3 as the solvent and P2O5 as the catalyst under refluxing with a yield of 40%. 5 can be successfully brominated by treatment with LDA and followed by quenching the formed anions with CBr4 to afford the dibromo-substituted 1,10-phenanthroline derivatives 6 in a yield of 60%, an essential monomer for building up the corresponding conjugated polymers. All the compounds were obtained readily and characterized unambiguously with 1H and 13 C NMR spectroscopy and mass spectroscopy. To the best of our knowledge, this is the first case of 1,10-phenanthroline derivative with fused groups at the 3-, 4-, 7-, and 8-positions of the Phen ring. Single-crystal structure of 6 from X-ray diffraction is depicted in Figure 1, revealing a rigid and almost planar skeleton of the target molecule which is packed into the typical herringbone structure with intermolecular interactive short contacts and with the dihedral angle between the molecular planes of 70.1°. Therefore, this approach is applicable to thiophene-fused 1,10-phenanthroline.

Figure 1. (a) Molecular structure. (b) Packing structure for 6.

The π-extended 1,10-phenanthroline was subsequently incorporated into conjugated polyarenes via palladium(0)catalyzed Suzuki−Miyaura−Schlü t er polycondensation (SMSPC). High molecular weight phenanthroline-alt-fluorene polymers P1 and P2 were prepared using 6 and 9,9-dioctyl-2,7diboronic ester−fluorene or 9,9-di(6-bromohexyl)-2,7-diboronic ester−fluorene as the monomers, respectively (Scheme 3). Both polymers could be readily dissolved in common organic solvents, such as dichloromethane, chloroform, and THF. The weight-average molecular weights (Mw) for P1 and P2, determined by GPC against polystyrene standards, were found up to 74 and 79 kg/mol, respectively. For the purpose of expanding biological applications of such a novel building block, a water-soluble polymer bearing thiophene-fused 1,10phenanthroline 6 in the backbone was designed and synthesized. For example, cationic water-soluble polymer P3 was subsequently obtained in high yield by reaction of P2 with trimethylamine in THF/H2O. These polymers were characterized with 1H and 13C NMR spectroscopy. The thermal properties of the polymers were examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), which exhibited good thermal stability. P1 showed less than 5% weight loss up to 295 °C and a residual 4089

DOI: 10.1021/acs.macromol.6b00236 Macromolecules 2016, 49, 4088−4094

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Macromolecules Scheme 3. Synthesis of Phenanthroline-alt-Fluorene Polymers P1−P3

Figure 2. Normalized absorption and PL spectra of (a) 5, P1, and P2 in chloroform and P3 in methanol (10−6 M for repeating unit); (b) P1−P3 as films.

Figure 3. (a) Cyclic voltammograms of monomer and polymers: 5 in dichloromethane (0.5 mM), P1 and P2 in dichloromethane, and P3 in acetonitrile (2.5 mM for repeating unit). Potentials are reported vs the Fc+/Fc redox couple as an internal standard and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte; scan rate = 100 mV/s. (b) Frontier molecular orbitals and their energy levels of 5 calculated by density functional theory (DFT) calculations in Gaussian 09 software at the B3LYP functional with the 6-31G(d,p) basis set level.

reflects that such a π-fused phenanthroline polymer is rigid. The current polymer P1 thus emitted blue-green light in solution under UV irradiation. In comparison with those optical spectra of the reported poly(1,10-phenanthroline-alt-fluorene) by Crayston et al. in which pristine 1,10-phenanthroline was selected as one of the building blocks (λmax = 378 nm, λem = 411 nm in CHCl3),34 both the absorption and emission maximum peaks of P1 in CHCl3 solution bathochromically shifted by 49 and 31 nm, respectively, due to the increased conjugation of phenanthroline unit with the fused thiophene groups. The optical features of P2 and P3 in solution resemble those of P1, with emission peaks at 446 and 444 nm, respectively. Compared to the absorption spectra in solution, the absorption peaks of polymers P1−P3 in the films are slightly broadening and shift bathochromically as shown in Figure 2b. The spin-coated films of P1−P3 exhibited featureless and red-shift emission bands centered at about 506, 488, and 501 nm, respectively, indicating a tendency for

weight percentage of about 60% at 800 °C under a nitrogen atmosphere; as for P2, the losing of alkyl chains started at temperature of ca. 230 °C (Figure S1). No distinct transition was observed from 50 to 250 °C in their DSC curves, which suggested that they were amorphous.32 The optoelectronic properties of monomer 5 and polymers P1−P3 were investigated by UV−vis absorption and photoluminescence (PL) spectroscopies as well as cyclic voltammetry. In dilute chloroform solution, 5 exhibited intense absorption in the ultraviolet region ranging from 250 to 370 nm with a maximum at around 275 nm. The PL spectroscopic analysis showed 5 emitted purple-blue fluorescence with a maximum peak at 386 nm and a shoulder at 407 nm. P1 displayed well-resolved spectra with absorption and emission maximum at 427 and 442 nm, respectively (Figure 2a and Figure S2). The absorption band appears in the range of 350− 450 nm which is attributed to the π−π* transition of the conjugated backbone.33 A very small Stokes shift of 15 nm 4090

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Table 1. Summary of UV−Vis Absorption and PL Maxima (λ), Optical Band Gaps (Eg), and HOMO and LUMO Levels of Monomer 5 and Polymers P1−P3 λabs (nm) (ε (104 M−1 cm−1)) in solution 5 P1 P2 P3

275 406 399 398

in film

a

(1.7) (7.9), 427 (8.2)a (5.0)a (2.3)b

407, 427 404, 426 409, 432

λPL (nm) in solution 386, 442, 446, 444,

in film

Eg (eV)c

HOMO (eV)d

LUMO (eV)e

506 488 501

3.47 2.77 2.82 2.79

−5.40 −5.46 −5.52 −5.17

−1.93 −2.69 −2.70 −2.38

a

407(sh) 470 (sh)a 472 (sh)a 469 (sh)b

a

Measured in chloroform. sh: shoulder peak. bMeasured in methanol. cEstimated from absorption onset. dEstimated from onset potentials, determined by cyclic voltammetric measurements in 0.1 M solution of TBAPF6 vs Fc+/ Fc (EHOMO = −(4.80 + Eox)).35,36 eEstimated from ELUMO = EHOMO + Eg.

Figure 4. Changes in (a) UV−vis absorption and (b) PL spectra of P1 (10−5 M for repeating unit) in chloroform at various concentrations of TFA. Inset shows the fluorescence images (I) before and (II) after the titration of TFA and (III) recovery of optical property when the protonated P1 solution was treated with TEA (taken under natural light and under the illumination with 365 nm UV light).

aggregation in their solid films. The molecular energy levels of the monomer and polymers were estimated by cyclic voltammetry (Figure 3a). The cyclic voltammogram of 5 in dichloromethane displayed two irreversible 1e− oxidation steps, whose HOMO energy level was thus estimated from its oxidation onset potential (Eox) to be −5.40 eV (EHOMO = −(4.80 + Eox)).35,36 The optical band gap (Eg) was determined from the onset of the UV−vis absorption spectrum. According to the equation ELUMO = EHOMO + Eg, the LUMO energy level was calculated to be −1.93 eV for 5. To gain more insights into the electronic properties of 5, we performed density functional theory (DFT) calculations in Gaussian 09 software at the B3LYP functional with 6-311G (d, p) basis set level.37 As depicted in Figure 3b, the HOMO and LUMO are well delocalized over the phenanthroline scaffold. The difference between the values deduced from optoelectronic measurements and calculation implied that the monomer could have complex interactions between the molecule and solvent.26 As for the polymers P1−P3, they all owned decreased band gaps (2.77 eV for P1, 2.82 eV for P2, and 2.79 eV for P3) compared to that of the monomer 5 and exhibited one oxidation peak with onset oxidation potential (versus Fc+/Fc) as 0.66 eV for P1, 0.72 eV for P2 in dichloromethane solution, and 0.37 eV for P3 in acetonitrile solution. All the optical and electrochemical data are summarized in Table 1. Phenanthroline nitrogen groups are usually identified as efficient receptor for protons and cations.33 To examine the sensing potential of these polymers both in organic solvent and in water, we first performed the titration experiment of P1 in chloroform with trifluoroacetic acid (TFA). Figure 4 traces the changes of UV−vis absorption and photoluminescence upon

addition of TFA from 0 to 0.25 mM. A red-shift of the absorption peak was observed from 433 to 452 nm. Meanwhile, the intensity of the blue-green emission peak decreased dramatically and finally vanished, while a new weak and long wavelength emission band peaked at approximately 534 nm appeared and its intensity increased with the increase of TFA. These changes can be clearly discerned by the naked eye as illustrated in the insets of Figure 4, which showed the pronounced color changes of the P1 solution from colorless to yellow under nature light and blue to green light emission under UV irradiation upon the protonation of phenanthroline groups in the polymer main chains with TFA. Similar results were observed for P2 and P3. As for P3, its UV−vis absorption and PL emission spectroscopy in Britton−Robinson buffer displayed pH-dependent optical features. The blue emission of P1−P3 could be recovered when the protonated solution was treated with triethylamine (TEA) (Figure S3). The UV−vis absorption spectra of the monomer 5 upon addition of several metal ions in chloroform−methanol are characterized by the appearance of low-energy bands (ranging from 420 to 350 nm) and a red-shift of the shoulder peak at around 314 nm with intense absorption, especially for Fe3+, Fe2+, and Al3+ (Figure S4). These changes indicated the interactions of metal ions with the monomer 5 and inspired us to evaluate the metal ion responsive behaviors of the corresponding polymers. Different metal salts (Mn2+, Co2+, Cu2+, Al3+, Fe2+, and Fe3+) which were dissolved in methanol were screened.38 P1 in chloroform and P3 in acetonitrile both exhibited metal ion-dependent photophysical properties (Figure 5 and Figure S6). As displayed in Figure 5, Mn2+ led to weak blue fluorescence without the influence to the emission 4091

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Figure 5. PL response of (a) P1 in chloroform and (b) P3 in acetonitrile (10−5 M for repeating unit) to metal ions (10−3 M). The following metal salts were used: MnCl2, CoCl2, CuCl2, AlCl3, FeCl2, and FeCl3. Insets show the pictures of polymer solutions treated with metal ions (taken under natural light and under the illumination with 365 nm UV light).

peaks of P1. Other metal ions, such as Cu2+, Al3+, and Co2+, induced a more significant decrease in PL intensity with a redshifted emission peak appeared at around 504 nm. Under identical conditions, Fe3+ and Fe2+ caused complete quenching of PL, which was reasonable not only due to the wellrecognized coordinating ability of 1,10-phenanthroline unit toward Fe2+ and Fe3+,39−42 but also benefited from the amplification effect through molecular wire of conjugated backnone.43,44 Concentration-dependent quenching behavior of P1 was assessed by UV−vis absorption and PL spectroscopy (Figure S7), which showed the emission intensity was diminished to 4% of the initial value at a low concentration below 5 ppm for Fe3+. Notably, P3 exhibited more remarkable ionochromic effect: for instance, as for Co2+, the color of the resulted polymer solution changed from colorless to blue, and as for Al3+, PL was clearly observed with a bathochromic-shift of emission position (λem = 522 nm). As a result, the corresponding P3 solution emitted intense yellow light. The ionochromic effect may be attributed to an energy transfer from the photoactivated conjugated polymer main chain to the metal complex part.33,45 Our investigation has illustrated that these polymers were able to determine a group of ions, potentially suitable for applications as optical chemosensors. In conclusion, we have found that Bischler−Napieralski cyclization facilitated the formation of 1,10-phenanthroline derivative with fused thiophene groups at the particular 3-, 4-, 7-, and 8-positions of the Phen ring. This approach presented herein is expected to offer a flexible tool for the extension of other aromatic rings, such as pyrrole, furan, pyridine, etc. Organic- and water-soluble polymers containing the new 1,10phenanthroline unit have been synthesized by Suzuki−

Miyaura−Schlüter polycondensation. Increased rigidity and conjugation of the phenanthroline repeating units led to the bathochromic shift in UV−vis absorption and photoluminescence peaks. Owing to the phenanthroline units in the polymer backbone, significant changes in the spectroscopic signature of the polymer induced by acid and metal ions make it a useful candidate for chemosensor and actuator applications. We envisioned that this synthetic strategy can be a promising method for the facile synthesis of other π-extended phenanthroline derivatives and analogues with applications far beyond the scope of this study, and further investigations are in progress.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00236. Synthetic procedures, experimental details, TGA data, more optical responsive data, crystal data of 6, and 1H NMR data (PDF) Structure of 6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (Y.C.). *E-mail [email protected] (Z.B.). Notes

The authors declare no competing financial interest. 4092

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ACKNOWLEDGMENTS We thank Prof. Y. Z. Tan for single-crystal X-ray diffraction analysis. Financial support by the National Natural Science Foundation of China (Grants 51503142 and 21522405), the Thousand Youth Talents Plan, and the Open Project of the State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201505) is gratefully acknowledged.

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DOI: 10.1021/acs.macromol.6b00236 Macromolecules 2016, 49, 4088−4094

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DOI: 10.1021/acs.macromol.6b00236 Macromolecules 2016, 49, 4088−4094