Vol. 450, No. 6, pp. 673–681.
165 In recent years, light emitting diodes (LEDs) based on conjugated polymers have attracted considerable attention of researchers owing to their promising potential as newgeneration fullcolor flatpanel dis plays [1–6]. An important problem that still remains unsolved is improving of the external quantum effi ciency (EQE) of polymeric lightemitting diodes. The principle of operation of thinlayer LED structures is based on electroluminescence (EL) caused by radia tive decay of excitons arising upon recombination of the electrons and holes injected from opposite elec trodes into the polymer layer. Perfect conditions for producing intense EL and reaching high EQE values for a polymer are high electron–hole conductivity of the medium (that is, high mobility of charge carriers) and maintenance of the balance of injected charge carriers of different signs from the opposite electrodes into the material bulk. The mobilities of electrons and holes should not differ considerably. The main approach to the improvement of EQE for polymeric lightemitting diodes is development of luminescent bipolar (donor–acceptor) polymers [7, 8] where charge transport and light emission functions are con centrated within the same macromolecule. However, in donor–acceptor polymers, rather strong intramo lecular charge transfer (ICT) occurs along the back bone, resulting in a decrease in the luminescence intensity despite better charge transport [7, 8]. An alternative strategy to overcome this issue is develop ment of ptype polymers with ntype electronegative side groups [9]. According to this approach, a number of conjugated polymers with oxadiazole and quinoxa line side groups exhibiting promising electrolumines
cence properties were successfully developed [10–12]. However, the application of these materials was held up due to complexity of the synthesis.
Development of simple methods for the synthesis of ptype polymers with electronwithdrawing side groups is quite a topical problem. Meanwhile, pyridin 2ylimidazo[1,5a]pyridine arouses particular interest as an ntype electron transporting building block due to high electron affinity, good thermal stability, and easy synthesis. Presumably, more pronounced elec tronwithdrawing properties of the pyridin2ylimi dazo[1,5a]pyridine ring compared with quinoline or quinoxaline ring would enhance the electron transport in polymers. In turn, the introduction of tripheny lamine moieties having high hole conductivity and luminescent properties into conjugated polymer mac romolecules would improve the electron–hole balance. Polymers containing triarylamine (TAA) structures [13, 14] are among the most widely used holetrans port materials, because they are readily oxidized to give stable radical cations. The inclusion of TAA seg ments into a polymer chains may also improve their solubility and glass transition temperature. However, to our knowledge, donor–acceptor polymers with triphenylamine in the backbone and pyridin2ylimi dazo[1,5a]pyridine in the side chain have been unknown so far. These facts provide the opportunity to combine good processability and transport properties of charge carriers within the same macromolecule without deteriorating the luminescence efficiency.
This paper reports the synthesis of several new polyfluorene derivatives with triphenylamine moieties in the backbone and pyridin2ylimidazo[1,5a]pyri dine side groups having high thermal and oxidative stability and good mechanical and filmforming prop erties. The spectral and electroluminescence properties of LED structures of the obtained polymers were studied. For the preparation of new copolyfluorenes, bis(4 bromophenyl)[4'(1''pyridin2ylimidazo[1,5a]pyri din3yl)pheny]amine (3) was first synthesized. This
CHEMISTRY
New Conjugated Electroluminescent TriphenylamineContaining
Polymers with SideChain Pyridin2ylimidazo[1,5
a]pyridine
Groups for Polymer LightEmitting Diodes
M. L. Keshtova, FangChung Chenb, E. I. Maltsevc, D. V. Marochkina,
V. S. Kochurova, and Academician A. R. Khokhlova
Received March 11, 2013
DOI: 10.1134/S0012500813060050
a Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia
b National Chiao Tung University, 1000 University Road, Hsinchu, Taipei, 30010 Taiwan
c Frumkin Institute of Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia
compound contains triphenylamine groups, which enhance the hole transport, and bulky electronwith drawing bipyridyl analogue, namely, pyridin2ylimi dazo[1,5a]pyridine moieties, which increase the electron affinity of the target macromolecular struc tures. Compound 3 was synthesized by bromination of
commercially available 4(diphenylamino)benzalde hyde 1 and subsequent condensation of the resulting 4'bis[(4bromophenyl)amino]benzaldehyde 2 with 2,2'dipyridyl ketone and NH4OAc in glacial acetic acid to give target monomer 3 (Scheme 1).
Scheme 1.
The composition and structure of the intermediate compounds and target product 3 were confirmed by elemental analysis data and 1H and 13C NMR spec troscopy. In particular, the 1H NMR spectrum of 3 exhibits five doublets, two doublets of doublets, two triplets, and one multiplet in the lowfield region (δН= 8.7–6.6 ppm), which correspond to phenyl, pyridyl, and imidazopyridyl protons (Fig. 1a). In the 13C NMR spectrum of product 3, the range δ
С of
154.77 to 113.83 ppm contains nineteen signals, eight of these being due to the key quaternary carbon atoms (Fig. 1b).
The Yamamoto copolycondensation of the obtained bipolar aromatic dibromide 3 and 2,7 dibromo9,9dioctylfluorene catalyzed by Ni(COD)2 in a toluene–DMF mixture afforded new conjugated copolyfluorenes I–III with different contents of the monomer units: 10, 20, and 50 mol % (Scheme 2).
Scheme 2. N Br Br N N N N N O N H O Br Br N H O 1 2 3 Br2 CH2Cl2 NH4OAc 4 4' 3 2 1 1'' 2'' 1 4 4' N Br Br N N N Br Br H17C8 C8H17 x y 3 + N N N N H17C8 C8H17 I: x = 0.9, y = 0.1 II: x = 0.8, y = 0.2 III: x = 0.5, y = 0.5 x y n Br (1) Ni(COD)2 + COD toluene, DMF (2)
The structures of polymers I–III were confirmed by 1H NMR data. In the region δ
H = 8.75–6.73 of the 1H NMR spectrum, in addition to strong broadened signals for the fluorene aromatic protons, there are also wellresolved multiplets centered at δH = 8.30, 8.33, 7.97, and 6.72 ppm, which are due to the mono mer units of 3 present in the chain (Fig. 2). These results confirm that the triphenylamine and pyridin2 ylimidazo[1,5a]pyridine moieties were successfully introduced in the polymer chain. The δH = 2.11, 1.13, and 0.80 ppm signals correspond to the aliphatic pro tons of the octyl groups in position 9 of the fluorene unit. The integrated intensity ratio of the aromatic and aliphatic protons corresponds to the assumed struc tures for all polymers and confirms the required con tent of unit 3 in the backbone. The mole percents of monomer units 3 in polymers I, II, and III are 10.1, 20.8, and 51.1 mol. %, respectively, as found from the data of elemental analysis.
All polymers are soluble in common organic sol vents such as DMF, DMSO, THF, dimethylacet amide, toluene, and chloroform. Films with tensile strength of 83–90 MPa (Table 1) were obtained from the solutions. The number and weightaverage molecular weights and polydispersities of polymers I–
III determined by gel permeation chromatography
(elution with THF) vary in the ranges of (2.19–4.17) × 104 (M
n), (4.14–8.46) × 104 (Mw), and 1.89–2.03, respectively.
The thermal and thermooxidative characteristics of copolyfluorenes I–III were studied by thermome chanical (TMA) and thermogravimetric (TGA) anal yses; the results are summarized in Table 1. All poly mers exhibit high thermal stability. The glass transition temperatures of copolymers (Tg) found from TMA data are between 103 and 200°C. The temperatures of 10% weight loss (Т10%) determined by TGA in air and under argon are in the ranges of 401–422 and 409– 445°C, respectively. It follows from Table 1 that the introduction of bipolar heteroaromatic units into the polymer chain results in higher Tg and higher thermal stability of the copolyfluorenes compared with homopolyfluorene (HPF) (Tg = 66°C, Т10% = 410°C), and these values tend to increase with an increase in the molar content of monomer units 3 in the back bone. The introduction of 50 mol. % of monomer units 3 into the polyfluorene chain made the largest contribution to the increase in the glass transition temperature of polymer III, which becomes as high as 200°C with simultaneous maximum increase in the thermal stability with respect to other copolymers syn thesized.
The optical properties of polymers I–III were studied by UV and fluorescence spectroscopy; the
results are summarized in Table 2. The absorption spectra of copolymers I–III (Fig. 3) show similar pat terns, the peaks being in the range of λ = 370–387 nm. As the content of comonomer 3 in the backbone increases, the polymer absorption band shifts hypso chromically.
The forms and positions of the bands are appar ently related to lowenergy ππ∗ electron transitions in the polyconjugated macromolecular chain. The shortwavelength shift relative to that of HPF ( = 390 nm) is due to substituents that affect the polycon jugation length of copolymers I–III, particularly, somewhat decrease the polyconjugation length com pared with the parent polyfluorene.
All polymers I–III exhibit intense photolumines cence (PL), which is manifested as two bands in the blue region, which do not differ much from those in the fluorescence spectrum of HPF and have maxima at 417–419 and 433–442 nm (which are also the same as for HPF). Their structure does not practically change upon excitation of the system either at the absorption maximum or at a shorter wavelength, i.e., in the absorption region of substituents. These bands should be assigned to 0–0, 0–1, and 0–2 intrachain singlet transitions in HPF. Thus, nonradiative transfer of the excitation energy to the polyconjugated chain occurs in the macromolecular system, apparently, according to the Forster mechanism, because the absorption and PL spectra substantially overlap, which is true for all of copolymers I–III. The introduction of monomer unit 3 has almost no influence on the posi tions of the principal PL bands of the copolyfluorenes, and only a considerable increase in the intensity of the longerwavelength band compared to the shorter wavelength band occurs upon increase in the molar content of the monomer units 3.
From the onset of absorption spectra of I–III in solution ( ), the optical width of the forbidden gap ( ) was calculated; the results are presented in Table 2. The band gap of the polymers in question remains almost invariable upon changes in the struc ture and molar amount of comonomer 3 introduced in the polyfluorene backbone.
The electrochemical properties of I–III were stud ied by cyclic voltammetry; the results are also pre sented in Table 2. The cyclic voltammograms of the copolymers exhibit anodic oxidation peaks at positive voltage; no cathodic reduction peaks are present at negative voltage. From the potential of the onset of oxidation ( ), the energies of the highest occupied molecular orbital (HOMO) for I–III (Table 2) were found in the following way:
abs max λ ads ons λ opt g E ox ons λ
Fig. 1. (a) 1H NMR and (b) 13C NMR spectra of bis(4bromophenyl)[4'(1''pyridin2''ylimidazo[1,5a]pyridin3yl)phe nyl]amine 3 in CDCl3. 114 122 130 138 146 154 δС, ppm 20 18 3 7 2 6 17 19 10 12 11 13 9 5 1 16 8 15 14 4 6.4 7.2 7.6 8.0 8.4 8.8 δН, ppm 6.8 1.00 1.02 1.98 3.10 4.20 2.22 1.09 4.14 1.21 0.98 l e е, i d, h c a b g j CDCl3 k (a) (b) N Br Br N N N N Br Br N N N a b c d l k j i e f g h 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
where = –4.8 eV, = 0.21 eV, = 0.23 eV (found from cyclic voltammetry data for fer rocene used as the standard). Thus, = –4.36 eV + . The lowest unoccupied molecular orbital (LUMO)
energies were calculated as . It
follows from Table 2 that the HOMO energy of I–III gradually increases with increase in the content of monomer units 3.
In order to study the effect of monomer units 3 introduced into the polyfluorene backbone on the electroluminescence properties of the obtained copol ymers, LED structures with a hole injection layer based on a dispersion of polyethylenedioxythiophene stabilized by polystyrene sulfonate (PEDOT/PSS) and a thin electron injection layer based on lithium fluoride were fabricated. On voltage application on the LED structures, blue EL was observed (Fig. 3) with color coordinates given in Table 3 and in Fig. 4a. The EL spectra are shifted to longer wavelengths relative to PL bands, which is usual for organic materials.
EHOMO EFc HOMO e Eons ox Eons Fc( ) ox – –Eel ( ), + = HOMO Fc E Eel ox ons(Fc) E HOMO E ox ons eE
LUMO HOMO opt
g
E =E +E
The EL spectra of copolyfluorenes I and II are broad intense bands with peaks at 476 and 469 nm, respectively (Fig. 3), which corresponds to blue radia tion with color coordinates x = 0.191, y = 0.249 (for I, Table 3). An increase in the content of monomer units 3 in the macromolecule entails a slight hypsoflu oric shift of the principal EL band (by 7 nm for copol ymers I and II).
The brightness of devices based on copolymers I and II and on previously prepared HPF (used for comparison) was measured to characterize the emis sion properties of the LED structures. The depen dence of brightness on the applied voltage is presented in Fig. 4b.
It follows from the data that a device based on the active layer consisting of pure HPF has a brightness of 1.4 Cd/m2 at 10 V. Light emitting diodes based on copolymers I and II had brightnesses of 5.9 and 7.3 Cd/m2 at the same voltage. The introduction of 10 and 20 mol. % of monomer units 3 into the HPF struc ture increased the brightness of emission 4.2 and 5.2fold, respectively. 1 2 3 6 7 9 δH, ppm 8 5 4 9.0 8.5 8.0 7.5 7.0 6.5 Ar 7.40 8.70 8.33 7.97 7.47 7.34 6.72 30.66 Alk N N N N H17C8 C8H17 x y n I x : y = 0.9 : 0.1 0
Thus, we synthesized electroactive polymers of a new type having photo and electroluminescence properties incorporating donor–acceptor groups as functional substituents. The partial replacement of dialkylfluorene by the multifunctional monomer unit 3 in the copolyfluorene backbone gave rise to some advantages, in particular, higher thermal and chemical stability, higher electron affinity within the same mol ecule without deterioration of the emission properties of polyfluorenes, considerable improvement and extension of electrontransport and electrolumines cence properties. Moreover, replacement of alkyl sub stituents by bulky heteroaromatic moieties reduces the aggregation and phase separation, which are inherent in poly(dialkylfluorene) macromolecules in the pro duction of such devices. The electroactive conjugated copolyfluorenes based on bipolar monomers can serve as effective electroluminescence materials in poly meric LEDs. Moreover, pyridin2ylimidazo[1,5 a]pyridine moieties contained in copolymers I–III, like 2,2'bipyridine, exhibit chelating ability towards a
broad range of metal ions and are attractive as recog nition receptors. Thus, copolyfluorenes I–III can also be used as chemosensors for various analytes.
EXPERIMENTAL Synthesis of Monomers
4'Bis[(4bromophenyl)amino]benzaldehyde (2)
was prepared by a reported procedure [15]. Yield 5.38 g (72%). Mp = 152–154°C; lit. [15]: Mp = 156–157°C. 1H NMR (CDCl 3, 400 MHz, δ, ppm): 9.82 (s, 1H), 7.70 (d, J = 8.8 Hz, 2H), 7.43 (dd, J = 9.3, 2.5 Hz, 4H), 7.06–6.98 (m, 6H). 13C NMR (CDCl 3, 100 MHz, δ, ppm): 190.29, 152.22, 144.87, 132.80, 131.26, 129.99, 127.26, 120.29, 117.95.
For C19H13Br2NO anal. calcd. (%): C, 52.93; H, 3.04; N, 3.25; Br, 37.07.
Found (%): C, 52.72; H, 2.99; N, 3.32; Br, 37.25.
Bis(4bromophenyl)[4'(1''pyridin2''ylimi dazo[1,5a]pyridin3yl)phenyl] amine (3). 2,2'Bipy ridyl ketone (0.21 g, 1.16 mol) and glacial acetic acid (7.0 mL) were charged into a 25 mL threenecked roundbottom flask equipped with a reflux condenser, argon inlet, and a magnetic stirrer. The mixture was stirred at room temperature until the solid completely dissolved, and 4'bis[(4bromophenyl)amino]benzal dehyde 2 (1.0 g, 2.32 mmol) and NH4OAc (0.45 g, 5.80 mmol) were added. The reaction mixture was stirred at 80°C under argon for 1.5 h and cooled to Table 1. Molecularweight and thermal characteristics of copolyfluorenes I–III
Polymer Mn × 10–4 Mw × 10–4 Mw/Mn Тg, °C Т10%, °C* Film properties σ, MPa ε, % I 4.17 8.46 2.03 103 89 6 II 2.19 4.14 1.89 174 83 7 III 2.78 5.39 1.94 200 90 5
* In air, above the line, and under argon, below the line.
401 409 412 426 422 445
Table 2. Optical and electrochemical properties of copolyfluorenes I–III
Polymer , V EHOMO ELUMO* nm eV I 387 419; 440 415 1.28 –5.64 –2.65 2.99 II 382 419; 442 418 1.15 –5.51 –2.54 2.97 III 370 417; 433 420 1.10 –5.46 –2.51 2.95
* The LUMO values were found from the difference of and EHOMO.
lmax abs lmax PL lons abs Eonsox Eg opt Egopt
Table 3. Electroluminescence properties of copolyfluo renes I–III Polymer , nm Color coordinates x y I 476 0.191 0.249 II 470 0.206 0.309 III 462 0.213 0.348 lmaxel
room temperature. The mixture was poured into 100 mL of ice water and extracted with CH2Cl2. The organic layer was washed with doubly distilled water, dried with MgSO4, and concentrated on a rotary evap orator. The product was purified by column chroma tography (elution with toluene : ethyl acetate = 1 : 1) to give yellow crystals. Yield 0.57 g (82%). Mp = 174– 175°C. 1H NMR (CDCl 3, 400 MHz, δ, ppm): 8.70 (d, J =9.2 Hz, 1H), 8.61 (d, J = 4.2 Hz, 1H), 8.23 (dd, J = 7.4, 3.5 Hz, 2H), 7.70 (t, J =8.6 Hz, 3H), 7.38 (d, J = 8.7 Hz, 4H), 7.18 (d, J = 8.5 Hz, 2H), 7.12 – 7.05 (m, 1H), 7.00 (d, J = 8.7 Hz, 4H), 6.91 (dd, J = 9.0, 6.5 Hz, 1H), 6.64 (t, J = 6.7 Hz, 1H). 13C NMR (CDCl 3, 100 MHz, δ, ppm): 154.77, 148.82, 147.32, 145.91, 137.54, 136.14, 132.43, 130.04, 129.29, 125.87, 124.52, 123.81, 121.78, 121.48, 120.88, 120.34, 119.77, 116.13, 113.83.
For C30H20N4Br2 anal. calcd. (%): C, 60.42; H, 3.38; N, 9.40. Found (%): C, 60.69; H, 3.27; N, 9.07. 1.0 0 300 λ, nm Absorbance, rel.units 350 0.5 1.0 0.5 Electroluminescence, rel.units 0 400 450 500 550 600 1 2 3 2' 1' 3' 4 5 0.8 0 0.1 0.3 0.6 0.4 0.2 0.5 0.7 y x 20 5 15 Voltage, V Brightness, Cd/m 2 15 10 5 10 0 HPF II I (a) (b)
Fig. 4. (a) Color coordinates of a light emitting diode device based on polymer II and (b) brightness of the electroluminescence
of copolymers I and II and homopolyfluorene (HPF) vs. applied voltage.
Fig. 3. Absorption spectra of polymers (1) I, (2) II, and (3) III; photoluminescence spectra of (1') I, (2 ') II, and (3') III in chlo
Synthesis of Copolymers
Copolyfluorene I. A 25 mL threenecked flask
equipped with a reflux condenser, a magnetic stirrer, and argon inlet was charged with Ni(COD)2 (1.000 g, 3.636 mmol), 2,2'bipyridine (0.5600 g, 3.636 mmol), and 1,5cyclooctadiene (0.3933 g, 3.636 mmol). Dry DMF (5 mL) was added, and the mixture was stirred at 80°C for 30 min. A solution of 2,7dibromo9,9 dioctylfluorene (0.7898 g, 1.44 mmol) and bis(4bro mophenyl)[4'(1''pyridin2''ylimidazo[1,5a]pyridin 3yl)phenyl]amine (3) (0.095 g, 0.16 mmol) in a 4 : 1 toluene–DMF mixture (12.5 mL) was added through a septum. The mixture was stirred at 80°С for 48 h, and bromobenzene (0.06 g, 0.39 mmol) was added. The mixture was stirred for more 5 h, cooled to room tem perature, and poured into a 2 : 1 methanol–concen trated HCl mixture (250 mL). The precipitate was fil tered off, dissolved in chloroform, and reprecipitated with methanol. The polymer was purified by methanol and acetone extraction in a Soxhlet apparatus for 24 h and dried in vacuum at 70°C. Yield 75%.
1H NMR (CDCl
3, 400 MHz, δН, ppm): 8.75–7.73 (m, Ar H), 2.93–0.48 (m, Alk H).
For I anal. calcd. (%): C, 88.55; H, 9.66; N, 1.79. Found (%): C, 88.19; H, 9.71; N, 1.63.
Copolymers II and III were synthesized in a similar way.
Copyfluorene II. Yield 81%. 1H NMR (CDCl
3, 400 MHz, δН, ppm): 8.72–7.68 (m, Ar H), 2.97–0.52 (m, Alk H).
For II anal. calcd. (%): C, 88.06; H, 9.04; N, 2.90. Found (%): C, 87.82; H, 9.18; N, 2.78.
Copyfluorene III. Yield 77%. 1H NMR (CDCl
3, 400 MHz, δН, ppm): 8.73–7.75 (m, Ar H), 2.94–0.50 (m, Alk H).
For III anal. calcd. (%): C, 85.87; H, 7.27; N, 6.86. Found (%): C, 85.36; H, 7.36; N, 6.72.
REFERENCES
1. Tonzola, C.J., Alam, M.M., and Jenekhe, S.A., Adv.
Mater., 2002, vol. 14, no. 15, pp. 1086–1090.
2. Zhu, Y., Alam, M.M., and Jenekhe, S.A., Macromole cules, 2003, vol. 36, no. 24, pp. 8958–8968.
3. Keshtov, M.L., Stakhanov, A.I., Pozin, S.I., Mal’tsev, E.I., Petrovskii, P.V., and Khokhlov, A.R., Dokl. Chem.,
2011, vol. 436, part 2, pp. 39–42.
4. Chen, B.Z., Wu, Y.Z., Wang, M.Z., et al., Eur. Polym. J.,
2004, vol. 40, no. 6, pp. 1183–1191.
5. Keshtov, M.L., Mal’tsev, E.I., Marochkin, D.V., Pozin, S.I., Lypenko, D.A., Perevalov, V.P., Petro vskii, P.V., and Khokhlov, A.R., Dokl. Chem., 2011,
vol. 439, part 1, pp. 175–180.
6. Keshtov, M.L., Pozin, S.I., Marochkin, D.V., Pere valov, V.P., Petrovskii, P.V., Blagodatskikh, I.V., and Khokhlov, A.R., Dokl. Chem., 2012, vol. 442, part 2,
pp. 23–29.
7. Jenekhe, S.A., Lu, L., and Alam, M.M., Macromole cules, 2001, vol. 34, no. 21, pp. 7315–7324.
8. Yamamoto, T., Yasuda, T., Sakai, Y., and Aramaki, S.,
Macromol. Rapid Commun., 2005, vol. 26, no. 15,
pp. 1214–1217.
9. Kulkarni, A.P., Tonzola, C.J., Babel, A., and Jenekhe, S.A., Chem. Mater., 2004, vol. 16, no. 23,
pp. 4556–4573.
10. Shi, W., Wang, L., Zhen, H., Zhu, D., Awut, T., Mi, H., and Nurulla, I., Dyes Pigm., 2009, vol. 83, pp. 102–110.
11. Chen, S.Y., Xu, X.J., Liu, Y.Q., Qiu, W.F., Yu, G., Wang, H.P., and Zhu, D.B., J. Phys. Chem. C, 2007,
vol. 111, no. 2, pp. 1029–1034.
12. Hancock, J.M., Gifford, A.P., Zhu, Y., Lou, Y., and Jenekhe, S.A., Chem. Mater., 2006, vol. 18, no. 20,
pp. 4924–4932.
13. Shi, W., Fan, S.Q., Huang, F., Yang, W., Liu, R.S., and Cao, Y., J. Mater. Chem., 2006, vol. 16, pp. 2387–2394.
14. Xiao, H.B., Leng, B., and Tian, H., Polymer, 2005,
vol. 46, no. 15, pp. 5707–5713.
15. Lee, S.K., Hwang, D.H., Jung, B.J., et al., Adv. Funct. Mater., 2005, vol. 15, no. 10, pp. 1647–1655.
![Fig. 1. (a) 1 H NMR and (b) 13 C NMR spectra of bis(4bromophenyl)[4'(1''pyridin2''ylimidazo[1,5a]pyridin3yl)phe nyl]amine 3 in CDCl 3](https://thumb-ap.123doks.com/thumbv2/9libinfo/7689193.143053/4.918.95.807.87.938/fig-spectra-bromophenyl-pyridin-ylimidazo-pyridin-amine-cdcl.webp)
