Tetrahedron Letters 53 (2012) 285–288

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Tetrahedron Letters

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Facile synthesis and coupling of functionalized isomeric biquinolines

Manisha Tomar a, Nigel T. Lucas b, Michael G. Gardiner c, Klaus Müllen d, Josemon Jacob a,⇑a Centre for Polymer Science and Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110016, Indiab Department of Chemistry, University of Otago, Dunedin 9054, New Zealandc School of Chemistry, University of Tasmania, Private Bag 75, Hobart 7001, Australiad Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 September 2011Revised 1 November 2011Accepted 3 November 2011Available online 17 November 2011

Keywords:Blue-emittersFluoreneBiquinolineOptical properties

0040-4039/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.tetlet.2011.11.008

⇑ Corresponding author. Tel.: +91 99 1106 1890; faE-mail address: jacob@polymers.iitd.ac.in (J. Jacob

A simple route toward functionalized biquinolines, namely 8,50-dibromo-5,80-biquinoline 1 and5,50-dibromo-8,80-biquinoline 2, was developed using Skraup syntheses. Both the dibromo compoundsundergo facile Suzuki coupling to afford fluorene-coupled products 3 and 4, respectively, an importanttransformation in designing conjugated materials based on these cores. X-ray structural analyses of 1and 4 provide insight into the mode of packing within materials containing these units.

� 2011 Elsevier Ltd. All rights reserved.

Br Br

N

N

Br Br

N

N

N

N

N

N

1 2

3 4

Chart 1. Structures of biquinoline isomers and model compounds.

Organic electronics based on semiconducting polymers has at-tracted much interest in recent years.1–3 N-Heterocyclic based poly-mers having n-type electrically conducting properties are ofparticular interest. However they are not readily synthesized andexamples include the compound classes like oxidazoles,4,5 quinoxa-lines,6,7 pyridines,8–10 and quinolines.11–13 Quinoline based materi-als have been widely studied for applications as electron-transportmaterial in light emitting diodes due to their electronic and opticalproperties.14–19 Polyquinolines possess n-type electrically conduct-ing properties along with good thermal, mechanical, and oxidativeproperties.20,21 There are many reports on quinoline based polymersbut limited work has been done on biquinoline based polymers.4,22–

30 The acid-catalyzed Friedlander condensation reaction is the mostcommonly used for the synthesis of polyquinolines and results inpolymers containing 6,60-biquinoline units.16,31–36 Incorporation of5,50- or 5,80-linked biquinoline units along a polymer backbone canlead to materials with extended conjugation. Herein we report thesynthesis of two isomers of biquinoline with different linkages,namely 8,50-dibromo-5,80-biquinoline 1 and 5,50-dibromo-8,80-biquinoline 2, with the bromo substituents ideally placed for furtherfunctionalization. For example, both the dibromides undergo facileSuzuki coupling to generate conjugated systems with extendedchromophores. Further to the synthesis, we present X-ray crystallo-graphic studies on two of these materials which give insight into themode of packing and significant solid state intermolecular contacts

ll rights reserved.

x: +91 11 2659 1421.).

in these systems. The structures of the two isomers and their cou-pled products are shown in Chart 1.

Scheme 1 illustrates the synthetic approach to the biquinolinemolecules and their coupled products. Both the isomers were syn-thesized from the corresponding diamino compounds. The precur-sors 4,40-dibromo-2,30-diaminobiphenyl 5 and 4,40-dibromo-2,20-diaminobiphenyl 6 were synthesized according to the litera-ture.37–39 Compounds 5 and 6 were then ring closed to the corre-sponding biquinoline molecules 1 and 2 via Skraup synthesis usingglycerol in the presence of sulfuric acid and iodine; yields for thesereactions were 52% and 35%, respectively. Use of this bromo-con-taining precursor approach helps to avoid undesirable and non-selective direct bromination on biquinoline. Coupling of 1 and 2 witha 2-boronate ester of 9,9-dimethylfluorene gave 8,50-di(9,9-dimeth-ylfluoren-2-yl)-5,80-biquinoline 3 and 5,50-di(9,9-dimethylfluoren-2-yl)-8,80-biquinoline 4 in 57% and 52% yields, respectively. All the

Figure 2. ORTEP representation of 1 showing the numbering scheme (for oneorientation only). Displacement ellipsoids are depicted at 50% probability.

Br Br

NH2

NH2

Br Br

NH2

NH2

2

6

5

1

4

3i ii

iii

Scheme 1. Synthesis and coupling reactions of dibromo biquinolines. Reagents: (i)glycerol, H2SO4, I2; (ii) 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dimeth-ylfluorene, Pd(PPh3)4, dioxane/2 M Na2CO3.

286 M. Tomar et al. / Tetrahedron Letters 53 (2012) 285–288

four molecules were purified by chromatography and then recrys-tallized from THF/methanol (5:1). Compounds 3 and 4 are solublein common organic solvents such as THF, CHCl3, CH2Cl2 at concen-trations >10 mg/mL. The synthesized molecules were characterizedby 1H and 13C NMR, elemental analysis, mass spectroscopy, UV–visabsorption, and emission spectroscopies and in two cases, by X-ray crystallography.

The optical absorption and photoluminescence (PL) emission of3 and 4 were measured in dilute THF solution and are shown inFigure 1. Compounds 1 and 2 show nearly identical absorptionspectra with absorption maxima at 308 and 307 nm, respectively.On the other hand, when 1 and 2 are coupled with fluorene unitsat either end, that is 3 and 4, the absorption spectra showed aslight dependence on the linkage between the two quinoline units.The absorption maximum for 3 is seen at 337 nm while for 4 theabsorption maximum shifts at 328 nm (extinction coefficients for3 and 4 are 8.5 � 104 and 2.3 � 104 M�1 cm�1, respectively). Theabsorption position for 3 at a slightly longer wavelength comparedwith that of 4 suggests that 3 adopts, on average, a more planarconformation in solution.

The photoluminescence behavior in THF solution also shows asmall shift in emission maximum between 3 and 4 (Fig. 1). Forthe fluorenyl-coupled compound with a 5,80-biquinoline unit, abathochromic shift of about 10 nm is observed compared to thecompound comprising a 8,80-biquinoline unit; both emit in theblue region in solution.

To gain further insight into the preferred conformations of thesebiquinoline compounds, crystals for X-ray diffraction studies weresuccessfully grown for 1 and 4. 8,50-dibromo-5,80-biquinoline 1crystallized from THF/methanol in the centrosymmetric C2/c spacegroup ( Fig. 2). The molecule sits on an inversion center at the

300 400 500 600

0.0

2.0x103

4.0x103

6.0x103

8.0x103 4 (328 nm) 3 (337 nm)

Wavelength (nm)

Ext

inct

ion

coef

fici

ent (

M-1cm

-1 )

0.0

0.2

0.4

0.6

0.8

1.0

4 (424 nm) 3 (433 nm)

Norm

alized Em

ission (a.u.)

Figure 1. UV–vis absorption and normalized emission spectra of model compounds3 and 4 in THF solution.

midpoint of the biquinoline bond, despite the fact that the 5,80-iso-mer does not possess inversion symmetry. This observation indi-cates 50:50 orientational disorder about molecular axis, such thatthe 1 and 4 positions of each quinoline ring are occupied by Nand C atoms in equal proportions in the best-fit crystallographicmodel. The biquinoline core of 1 adopts a transoid non-planar con-formation, with a dihedral angle of 107.3� between the two inter-secting least-squares quinolyl ring planes. The most notablesupramolecular interaction is p-stacking between each quinolineand a neighboring molecule (separation 3.401(6) Å), resulting incolumns of biquinolines, albeit with the aforementioned intramo-lecular twist. The bromines are not involved in any significantsupramolecular interactions.

Diffusion of methanol into a chloroform solution of the 8,80-biquinoline derivative 4 formed small crystals that diffracted suffi-ciently in synchrotron-sourced X-rays40 to enable a solid statestructure to be determined (Fig. 3). As is the case with 1, solventis not present in the structure of 4. A dihedral angle of 129.6� be-tween the quinoline rings places them in a non-planar, transoidconformation, while the fluorenyl groups are disposed to thebiquinoline core with angles of 40.9� (C5–C12) and 43.5�(C35–C42). Inversion-related pairs of molecules exhibit a face-to-facearrangement; quinolines N1–C10 with a separation of 3.642(3) Å,and quinolines N31–C40 by 4.000(3) Å, both suggesting little tono interaction between these groups (Fig. 4, red). Short N31���H55C(2.634(3) Å) contacts within pairs (green) and edge-to-face C–H���C(p) interactions (orange) appear to be a major stabilizingelement. The N1 atoms in 4 are also involved in intermolecularhydrogen bonds with a neighboring fluorene (N1���H16,2.733(3) Å) that appears to twist the quinoline rings from planarity.

No 5,80-linked biquinolines have been structurally characterized,the structure 1 herein being the first reported. Of the 15 structuralreports41 of 8,80-biquinoline and its derivatives, none is functional-ized in the 5/50-position(s), with 7/70-substitution the most com-monly investigated due to the possibility of atropisomerism.42–45

The parent 8,80-biquinoline adopts a transoid configuration in thesolid state, however is far from planar with the angle between thetwo halves being 96.8�.46

In conclusion, we have developed a new and efficient syntheticroute toward the synthesis of difunctional biquinoline compounds.The regiospecific incorporation of two bromo groups into 8,50-di-bromo-5,80-biquinoline and 5,50-dibromo-8,80-biquinoline pro-vides useful building blocks for new conjugated organicmaterials. Facile Suzuki coupling of the dibromo compounds to flu-orenyl groups provides a demonstration that these cores can bereadily partnered with other conjugated moieties to access newclasses of conjugated materials containing biquinolines.

Figure 4. Packing diagram of 4 showing quinoline face-to-face stacking (red), C–H���N (green) and C–H���C p (orange).

Figure 3. ORTEP representation of 4 showing the numbering scheme. Displacement ellipsoids are depicted at 50% probability.

M. Tomar et al. / Tetrahedron Letters 53 (2012) 285–288 287

Acknowledgments

The authors acknowledge Department of Science and Technol-ogy, India and Max Plank Society, Germany for generous financialsupport. M.T. acknowledges the research fellowship from IndianInstitute of Technology Delhi, India. Data for the structure of 4were obtained on the MX1 beamline at the Australian Synchrotron,Victoria, Australia.

Supplementary data

Crystallographic data (excluding structure factors) for thestructures in this paper have been deposited with the CambridgeCrystallographic Data Centre as supplementary publication nos.CCDC843080(1), 843081 (4). Copies of the data can be obtained,free of charge, from The Cambridge Crystallographic Data Centrevia www.ccdc.cam.ac.uk/data_request/cif.

Supplementary data (synthesis, NMR data) associated with thisarticle can be found, in the online version, at doi:10.1016/j.tetlet.2011.11.008.

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