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Letter Cite This: Org. Lett. 2018, 20, 7864−7868

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Buchwald-Hartwig Coupling at the Naphthalenediimide Core: Access to Dendritic, Panchromatic NIR Absorbers with Exceptionally Low Band Gap Jyoti Shukla, M. R. Ajayakumar, and Pritam Mukhopadhyay* Supramolecular and Material Chemistry Lab, School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India

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S Supporting Information *

ABSTRACT: The first successful Buchwald−Hartwig reaction at the naphthalenediimide core is reported, leading to the coupling of diverse secondary aromatic amines including dendritic donors. The G1-dendrimer-based donor exhibit blackish color, providing access to black absorbing systems. λonset values up to 1070 nm was achieved, which is the maximum from a single NDI scaffold. These dyes also manifest multielectron reservoir properties. A total of eight-redox states with a band gap of ∼0.95 eV was accomplished.

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diimides (cNDIs) having aliphatic and primary aromatic amines as donor groups are restricted to ∼2 eV and the absorption maxima is limited to 620 nm for the disubstituted and 640 nm for the tetrasubstituted cNDIs.12 We envisaged that the incorporation of diphenyl amine and its higher dendritic derivatives at the NDI core can manifest as truly panchromatic black absorbing dyes by virtue of efficient intramolecular charge transfer. Surprisingly, the Buchwald− Hartwig (B-H) reaction that can provide access to such attractive molecules has not been attempted for arylamine coupling at the NDI core, although there are various elegant synthetic protocols13 developed, e.g., Stille,14 Suzuki,15 and ArSN reactions.16 We also envisioned that the coupling of aromatic donors would provide entry to otherwise challenging stable electron reservoirs that simultaneously generate radical cations and anions. The aryl amination coupling reactions generally proceed through the palladium-catalyzed B-H reactions, which require a strong base for the deprotonation of the arylamines. This is one of the possible reasons why the seminal B-H reaction has not been attempted in aryleneimides or other π-conjugated imides. The other probable reason is the steric bulk of the arylamines. The new class of donor−acceptor−donor (D-A-D)

ynthesis of rationally designed band-gap engineered molecules have gained considerable attention, because of its wide applications in modern organic semiconducting and light-harvesting materials.1 Successful design of such molecular materials requires suitable preorganization of the donor− acceptor units, which can allow effective electronic coupling amongst them to suitably tune the frontier molecular orbitals (FMOs).2 One of the most sought-after goals has been to rationally construct black-absorbing molecules3 with true panchromatic behavior as their applications range from dyesensitized solar cell,4 near-infrared (NIR) absorbing and emissive dyes,5 and nonlinear optical systems6 to polarity sensors.7 Realization of such black absorbers has been a synthetic challenge and only very few that have been realized to date are based on organic2d and organic−inorganic hybrid systems.2e,f Aromatic amines have emerged as attractive donor scaffolds in low-band gap materials, because of their low oxidation potential, stability of generated radical cations, and low reorganization energy.8,9 On the other hand, naphthalenediimides (NDIs) have appeared as a promising class of electrondeficient scaffolds, because of its robust stability and selfassembly properties.10 The NDI scaffold, which has a band gap of 3.01 eV, shows a red-shifted absorption spectrum, as a result of functionalization with donor units at the core position.11 However, the band gap of the core-substituted naphthalene© 2018 American Chemical Society

Received: October 24, 2018 Published: November 30, 2018 7864

DOI: 10.1021/acs.orglett.8b03408 Org. Lett. 2018, 20, 7864−7868

Letter

Organic Letters

water, followed by acid workup gives a purple-colored product having a charge transfer band with λmax of 545 nm. We anticipated that the blue-shifted spectrum is possibly due to the presence of an open-ring intermediate diacid product (Scheme 2). The solid compound was dissolved in CHCl3 and stored in a closed vessel for 24 h to get the blue-colored monoanhydride product 9 with a yield of 70%. This new synthetic protocol thus allows us to generate a monoanhydride compound having donor substituents intact at the NDI core. To have an insight into the structural characteristics, we determined the X-ray crystal structure of compound 7 (Figure 1). The two crystallographically independent molecules A and

compounds 1−7 were synthesized by treating NDI-Br2 with various 2° arylamines through a palladium-catalyzed B-H approach, using 1,1′-ferrocenediyl-bis(diphenylphosphine) (DPPF) as the ligand and sodium tertiary butoxide as a strong base, in dry degassed toluene under glovebox condition (Scheme 1). After completion, the reaction mixture was Scheme 1. Molecular Structures of 1−7 and the Common Synthetic Procedure for Secondary Arylamine Coupling Reactions at the NDI Corea

Figure 1. X-ray crystal structure of 7 (only B is represented) ((a) top view and (b) side view) and the dihedral angle between donor amines (D) and the acceptor NDI unit (A).

a

Numbering of the amines denotes the corresponding compound number.

B are illustrated in Figure S2 in the SI. The diphenyl amine units are twisted out of the plane, with respect to the NDI scaffold, with dihedral angles of 52.93° in molecule A and 43.56° in molecule B (Table S1 in the Supporting Information). The shortening in the bond length of the NDI C atoms and the N atoms of the diphenyl amino group (1.402 and 1.403 Å) indicates strong conjugation between the donor and the acceptor units. The optical properties of the compounds 1−6 and monoanhydride compound 9 were investigated by UV-visNIR absorption spectroscopy (Figure 2 and Figure S2a in the SI). Compounds 1, 2, and 3 showed very weak fluorescence, while other compounds were nonfluorescent (Figure S2b). The corresponding UV-vis-NIR absorption data are collected in Table 1. The reference compound NDI-C6, without any

precipitated by the addition of an excess of hexane. The obtained precipitate was purified by repeated precipitation with chloroform and methanol, thus, bypassing the chromatographic purification step. Compounds 1−7 were synthesized in moderate yields of 35%−50% and characterized by diverse analytical techniques (see the Supporting Information (SI)). The presence of an acid functionality in these compounds are important as it provides anchoring sites during device fabrication. In order to explore such a possibility, we planned to synthesize 8 having ester groups at the NDI axial position (Scheme 2). However, the arylamine coupling reaction following the B-H synthetic protocol with the NDICOOCH3 did not generate the desired product. A modified protocol utilizing Cs2CO3 as the base and r-BINAP as the ligand gives the desired blue-colored product 8 with yield of 62%. The hydrolysis of compound 8 with NaOH in MeOH/ Scheme 2. Synthetic Procedure of the Ester Functionalized Compound 8 and Monoanhydride Compound 9

Figure 2. (a) Photographs of vials of 1−6 (1 mM in CHCl3), and (b) comparison of their absorption properties by plotting normalized UVvis-NIR absorption spectra in CHCl3 (0.2 mM). 7865

DOI: 10.1021/acs.orglett.8b03408 Org. Lett. 2018, 20, 7864−7868

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Organic Letters Table 1. Optical and Electrochemical (CV/DPV) Data for Compounds 1−6 and 9 in DCMa compd

λmax (abs)

λonset

NDI-C6 1 2 3 4 5 6 9

375 604 663 677 719 799 830 678

411 701 770 795 881 1006 1070 800

E1ox (V) +1.08 +1.00 +0.96 +0.75 +0.46 +0.24 +0.87

E2ox (V) +1.35 +1.19 +1.22 +0.95 +0.60 +0.44 +1.20

E3ox (V)

+1.42 +0.80 +0.68

E4ox (V)

+1.47 +1.68

E1red (V)

E2red (V)

HOMO (eV)

LUMO (eV)

Eg (eV)

−0.68 −0.54 −0.77 −0.79 −0.87 −0.78 −0.93 −0.59

−1.08 −1.03 −1.11 −1.09 −1.17 −1.06 −1.19 −0.93

−6.83a −5.35 −5.29 −5.20 −5.04 −4.74 −4.55 −5.11

−3.82 −3.96 −3.76 −3.77 −3.63 −3.76 −3.60 −3.97

3.01b 1.39 1.53 1.42 1.41 0.98 0.95 1.14

All potentials reported as E1/2 = [(Eap + Ecp)/2] V in CV (vs SCE), performed in DCM with 0.1 M TBAPF6. HOMO and LUMO = −[4.4 + E1red/ox onset (V)] eV, is calculated from E1ox onset and E1red onset given by DPV. bCalculated from the λonset value in the UV-vis-NIR data. a

donor substituents in the 2,6-core position shows absorption in the UV region with absorption maximum (λmax) at 375 nm and the λonset at 410 nm, attributed to the π−π* electronic transition from the NDI backbone. On the other hand, 1 having two carbazole units as the donor motif at the cNDI shows a sharp peak at 377 nm, followed by a broad intramolecular charge transfer (ICT) band with λmax located at 604 nm. The enhanced electron-donating character of arylamines in 1−6 results in significantly red-shifted ICT bands with λmax values reaching up to 830 nm, which are higher than the diindole annulated NDIs (λmax = 635 nm) or the heterocyclic acene NDIs (λmax = 740 nm).17 Interestingly, 5 and 6 appear to be dull blackish to blackish-green in color in solution, reflecting the exceptionally broad charge transfer band completely overlapping the visible region. The color shades of these compounds in solution varied according to the polarity of the solvents. Compound 6 having the NMe2 groups at the para-position of the diphenylamine (DPA) shows the maximally red-shifted ICT band with λmax at 830 nm covering the visible and NIR region (550−1070 nm). Compound 6 shows significantly more bathochromic shift, compared to the G1 dendrimer 5. This is due to the improved conjugation in 6, as a result of the smaller twist angle between the DPA and NMe2 group at para position, compared to 5, in which the phenyl rings disrupt the conjugation. A comparison of the HOMO surfaces validates the extent of conjugation (Figure S12 in the SI). Therefore, the HOMO−LUMO gap of the cNDI derivatives can be decreased to 0.95 eV, as a consequence of the elongated π-system and favorable ICT between the terminal donor units and the NDI acceptor. This is the lowest band gap achieved from a single NDI scaffold until date. Next, we examined the effect of axial substitution on the band gap of cNDI. Compound 9 in which the axial alkyl chains are replaced by methyl benzoate and an anhydride group shows a red-shift of 17 nm, compared to 2 (Figure S4 in the SI). The multiredox properties of 1−6 and 9 were evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in DCM solution at 298 K. The reference compound, NDI-C6 shows two reduction waves, at −0.68 and −1.08 V, with no oxidation wave at the anodic potential. The substitution of the aryl donors results in new oxidation as well as reduction waves for 1−9. Compound 1 exhibits two pseudoreversible oxidations at +1.08 and +1.35 V, corresponding to the formation of 1•+ and 12+, respectively. Moreover, 1 shows two well separated reversible reduction waves at −0.54 and −1.03 V corresponding to the generation of 1•̅ and 12̅ (Figure 3). Expectedly, 2 and 3 with similar donors exhibit almost similar oxidation and reduction waves (see Figure S5 in

Figure 3. CV and DPV results of 1 and 6 (0.5 mM) in degassed DCM with 0.1 M TBAPF6 using platinum wire as working and auxiliary electrode. Oxidation and reduction is plotted in the left and right panels, respectively.

the SI, as well as Table 1). In the case of methoxy-substituted DPA, the first two reversible waves (+0.75, + 0.95 V) could be attributed to the amine oxidation, while the third wave (+1.42 V) should be for the oxidation of methoxy group. In addition, 4 shows two reversible reductions with cathodically shifted potentials at −0.87 and −1.17 V. Interestingly, compound 6 exhibits five oxidation peaks in DPV for six-electron oxidation, because of the contribution from the six nitrogen redox centers (Figure S6 in the SI). The first three electrochemically reversible oxidations are expected for the oxidation of four NMe2 units at +0.24 V (1e−), + 0.44 V (1e−) and +0.68 V (2e−) (confirmed by peak fitting analysis, as the current area of peak 3 is almost double as compared to the other peaks), while the last two oxidations observed are determined for the oxidation of NDI-attached aromatic amine moieties at +1.10 V (1e−), +1.68 V (1e−) (Figure S7b). Compound 6 also shows two well-separated reversible reduction waves at −0.93 and −1.19 V vs SCE, implying the generation of the radical anion and the dianion states. Compound 5 has almost similar redox properties as 6 with comparable anodically shifted oxidation waves. Compound 9 shows similar redox properties as 2 having the same core structure with two quasi-reversible oxidations at +0.87 V and +1.20 V and two anodically shifted reduction waves at −0.59 V and −0.93 V (Table 1). These results validate the ambipolar characteristics of this new class of dye molecules. Thus, CV/DPV results confirmed the controlled multiredox properties of 5 and 6 with seven and eight electrochemically accessible redox states, respectively. Furthermore, the lower oxidation potentials of 5 and 6 indicate that these compounds can be potential hole-transporting materials. 7866

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aromatic amines including dendritic donors could be coupled at the rigid cNDI. Importantly, the G1-dendrimeric donors or the Bindschedler’s green leuco base scaffolds coupled with the cNDI shows blackish to blackish-green color providing systematic approach to black absorbing systems. We could also achieve the maximum red-shifted absorption in the NIR region as well as the largest number of redox states from a single NDI scaffold. Thus, NDI-based electron-rich, electron reservoirs can be realized in contrast to the electron-deficient electron sponges.16 Organic panchromatic systems18 are in demand and can be accomplished from hitherto unutilized combinations of donor−acceptor scaffolds.

Furthermore, chemical oxidation and reduction were performed to investigate the optical property of various redox states of 2, 5, and 6. Toward this, Cu(ClO4)2·6H2O was taken as the oxidizing agent and sodium sulfide (Na2S) as the reducing agent. On the basis of the redox potential of the Cu2+/Cu+ redox couple (+0.952 V vs SCE in MeCN), we anticipated that Cu(II) can oxidize 2 by one electron, while 5 and 6 can be oxidized by three electrons. The chemical oxidation and reduction processes of 2 and 5 are provided in Figures S8−S10 in the SI. Next, we turned our attention to the spectral change observed during the stepwise oxidation of compound 6 having six oxidizing centers. The stoichiometric addition of Cu(II) in the solution of 6 shows the spectral change with their corresponding colored solution (Figure 4a).

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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.orglett.8b03408. Experimental details, characterization data, NMR, and mass spectra for all new compounds (PDF) Accession Codes

CCDC 1871901 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

ORCID

M. R. Ajayakumar: 0000-0002-7041-7456 Pritam Mukhopadhyay: 0000-0002-3073-6719 Notes

The authors declare no competing financial interest.

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Figure 4. UV-vis-NIR spectra showing the spectral response of compound 6 (0.2 mM) in MeCN:CHCl3 solution (8:2 ratio): (a) with the gradual addition of Cu(ClO4)2·xH2O from 0 to 3.0 equiv and their corresponding colored solution under ambient condition with (i) 0 equiv, (ii) 1 equiv, (iii) 2 equiv, and (iv) 3 equiv of Cu(ClO4)2· xH2O (spectra of 6 and 6•̅ in DMSO) (b) with their corresponding colored solution for (v) neutral state and (vi) radical anion state generated with Na2S.

ACKNOWLEDGMENTS P.M. acknowledges financial support under the Swarna Jayanti Fellowship (No. DST/SJF-02/CSA-02/2013-14), DST-FIST, and DST-PURSE. P.M. thanks AIRF, JNU for the instrumentation facilities and acknowledges Dr. Ashwani Tripathi, SPS, JNU for the peak fitting analysis of DPV data.

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The addition of 1 equiv of Cu(II) gives a monocationic state, which has an intense IVCT band in the telecommunication range with the λmax at 1157 nm (ε = 17 367 M−1 cm−1) while the tricationic species (generated by the addition of 3 equiv of Cu(II)) shows absorption in the NIR region with a strong peak at 760 nm (ε = 44 910 M−1 cm−1). Each redox state of compound 6 has a different color from green to gray, to purplish, and to dark blue (Figure 4a). Consequently, we performed chemical reduction of compound 2, 5, and 6 which reduced to its radical anion by Na2S with the characteristics NDI radical anion peaks in UV-vis-NIR spectra (Figure 4b). ESR spectroscopic studies was also performed to confirm the paramagnetic nature of the chemically oxidized and reduced species of compound 6 as a representative example (Figure S11 in the SI). In conclusion, we report the first Buchwald−Hartwig (B-H) reaction at the NDI core. This is the first time that secondary

REFERENCES

(1) (a) Wang, M.; Wang, H.; Yokoyama, T.; Liu, X.; Huang, Y.; Zhang, Y.; Nguyen, T.-Q.; Aramaki, S.; Bazan, G. C. J. Am. Chem. Soc. 2014, 136, 12576−12579. (b) Zophel, L.; Enkelmann, V.; Mullen, K. Org. Lett. 2013, 15, 804−807. (2) (a) Takimiya, K.; Osaka, I.; Nakano, M. Chem. Mater. 2014, 26, 587−593. (b) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. Acc. Chem. Res. 2010, 43, 1396−1407. (c) Zheng, Z.; Zhang, S.; Zhang, J.; Qin, Y.; Li, W.; Yu, R.; Wei, Z.; Hou, J. Adv. Mater. 2016, 28, 5133− 5138. (d) Kage, Y.; Mori, S.; Ide, M.; Saeki, A.; Furuta, H.; Shimizu, S. Mater. Chem. Front. 2018, 2, 112−120. (e) Browning, C.; Hudson, J. M.; Reinheimer, E. W.; Kuo, F.-L.; McDougald, R. N., Jr.; Rabaâ, H.; Pan, H.; Bacsa, J.; Wang, X.; Dunbar, K. R.; Shepherd, N. D.; Omary, M. A. J. Am. Chem. Soc. 2014, 136, 16185−16200. (f) Banala, S.; Rühl, T.; Wurst, K.; Kräutler, B. Angew. Chem., Int. Ed. 2009, 48, 599−603. (3) (a) Yao, B.; Zhou, Y.; Ye, X.; Wang, R.; Zhang, J.; Wan, X. Org. Lett. 2017, 19, 1990−1993. (b) Zhang, J.; Yan, C.; Wang, W.; Xiao, Y.; Lu, X.; Barlow, S.; Parker, T. C.; Zhan, X.; Marder, S. R. Chem. Mater. 2018, 30, 309−313. 7867

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Organic Letters (4) Mayerhoffer, U.; Fimmel, B.; Würthner, F. Angew. Chem., Int. Ed. 2012, 51, 164−167. (5) (a) Iwanaga, T.; Ogawa, M.; Yamauchi, T.; Toyota, S. J. Org. Chem. 2016, 81, 4076−4080. (b) Zhang, Q.; Kuwabara, H.; Potscavage, W. J., Jr; Huang, S.; Hatae, Y.; Shibata, T.; Adachi, C. J. Am. Chem. Soc. 2014, 136, 18070−18081. (6) Poronik, Y. M.; Hugues, V.; Blanchard-Desce, M.; Gryko, D. T. Chem. - Eur. J. 2012, 18, 9258−9266. (7) Welch, G. C.; Coffin, R.; Peet, J.; Bazan, G. C. J. Am. Chem. Soc. 2009, 131, 10802−10803. (8) (a) Sakamaki, D.; Ito, A.; Tanaka, K.; Furukawa, K.; Kato, T.; Shiro, M. Angew. Chem., Int. Ed. 2012, 51, 8281−8285. (b) Ito, A.; Sakamaki, D.; Ichikawa, Y.; Tanaka, K. Chem. Mater. 2011, 23, 841− 850. (9) (a) Ito, A.; Yokoyama, Y.; Aihara, R.; Fukui, K.; Eguchi, S.; Shizu, K.; Sato, T.; Tanaka, K. Angew. Chem., Int. Ed. 2010, 49, 8205− 8208. (b) Fukui, N.; Cha, W.-Y.; Lee, S.; Tokuji, S.; Kim, D.; Yorimitsu, H.; Osuka, A. Angew. Chem., Int. Ed. 2013, 52, 9728−9732. (10) (a) Suraru, S.-L.; Würthner, F. Angew. Chem., Int. Ed. 2014, 53, 7428−7448. (b) Zhan, X.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S. R. Adv. Mater. 2011, 23, 268−284. (c) Sakai, N.; Mareda, J.; Vauthey, E.; Matile, S. Chem. Commun. 2010, 46, 4225−4237. (11) (a) Vollmann, H.; Becker, H.; Corell, M.; Streeck, H. Justus Liebigs Ann. Chem. 1937, 531 (1), 1−159. (b) Cai, K.; Xie, J.; Yang, X.; Zhao, D. Org. Lett. 2014, 16, 1852−1855. (c) Thalacker, C.; Röger, C.; Würthner, F. J. Org. Chem. 2006, 71, 8098−8105. (d) Alvey, P. M.; Ono, R. J.; Bielawski, C. W.; Iverson, B. L. Macromolecules 2013, 46, 718−726. (12) (a) Roger, C.; Würthner, F. J. Org. Chem. 2007, 72, 8070− 8075. (b) Ajayakumar, M. R.; Asthana, D.; Mukhopadhyay, P. Org. Lett. 2012, 14, 4822−4825. (c) Suraru, S.-L.; Würthner, F. J. Org. Chem. 2013, 78, 5227−5238. (13) (a) Ye, Q.; Chang, J.; Huang, K.-W.; Chi, C. Org. Lett. 2011, 13, 5960−5963. (b) Míšek, J.; Vargas Jentzsch, A.; Sakurai, S.-i.; Emery, D.; Mareda, J.; Matile, S. Angew. Chem., Int. Ed. 2010, 49, 7680−7683. (14) (a) Polander, L. E.; Romanov, A. S.; Barlow, S.; Hwang, D. K.; Kippelen, B.; Timofeeva, T. V.; Marder, S. R. Org. Lett. 2012, 14, 918−921. (b) Alvey, P. M.; Iverson, B. L. Org. Lett. 2012, 14, 2706− 2709. (15) Bhosale, S. V.; Kalyankar, M. B.; Bhosale, S. V.; Langford, S. J.; Reid, E. F.; Hogan, C. F. New J. Chem. 2009, 33, 2409−2413. (16) Shukla, J.; Ajayakumar, M. R.; Kumar, Y.; Mukhopadhyay, P. Chem. Commun. 2018, 54, 900−903. (17) (a) Li, C.; Xiao, C.; Li, Y.; Wang, Z. Org. Lett. 2013, 15, 682− 685. (b) Suraru, S.-L.; Burschka, C.; Würthner, F. J. Org. Chem. 2014, 79, 128−139. (c) Ye, Q.; Chang, J.; Huang, K.-W.; Shi, X.; Wu, J.; Chi, C. Org. Lett. 2013, 15, 1194−1197. (18) Mandal, A. K.; Diers, J. R.; Niedzwiedzki, D. M.; Hu, G.; Liu, R.; Alexy, E. J.; Lindsey, J. S.; Bocian, D. F.; Holten, D. J. Am. Chem. Soc. 2017, 139, 17547−17564.

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