Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis and Isolation of a Stable Perylenediimide Radical Anion and Its Exceptionally Electron-Deficient Precursor Yogendra Kumar, Sharvan Kumar, Deepak Bansal, 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 synthesis and isolation of an ambient stable perylenediimide radical anion is reported, and its precursor is established as one of the strongest electron acceptors. The radical anion shows absorption up to 1400 nm and is stable in mixed aqueous solution. Interestingly, the radical anion can organize two electron-deficient molecules over its surface to form a π-stacked array. Calculations revealed weak spin polarization via noncovalent interactions. Such interactions are of significance for magnetic exchange and catalysis.
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ynthesis and stabilization of organic radical ions is a topic of immense interest.1,2 These spin, charge-bearing systems find applications as organic conductors, magnets, NIR probes, etc. and have mostly initiated from the radical anions of 7,7,8,8, tetracyanoquinodimethane (TCNQ) and its congeners.3−5 Their one-electron oxidized precursors are known as the strongest acceptors. Radical anions, with the exception of TCNQ and a few others, are known to be inherently reactive and unstable. Therefore, synthetic efforts to expand the repertoire of stable radical anions and electron acceptors beyond the TCNQs is highly desired. In this context, arylenediimides are attractive for their intrinsic π-acidic surface and synthetic versatility.6 In particular, perylenediimides (PDIs) find applications in solar cells,7 optoelectronic devices,8 elegant self-assembled materials,9 and as novel photoreductants.10 Some of these functionalities depend on their ability to accept electrons and form radical anions. However, isolation of the crystalline PDI radical anion and X-ray structural investigation remain elusive. The only report is the isolation of a PDI radical in zwitterionic form, which bestows an imidazolium ring at the ortho position.11 The PDI radical anions have been mostly obtained by electrochemical or photochemical methods,12 whereas chemical generation of most of the PDI radical ions involves hazardous or toxic agents such as hydrazine, TBACN, etc.13 Recently, Würthner et al. demonstrated the use of palladium on activated charcoal (Pd/C−H2) and sodium hydrogen carbonate in DMF to obtain the first ambient stable PDI dianion.14a Subsequently, Zhang et al. reported the synthesis of an ambient isolable PDI radical anion salt with excess potassium carbonate in DMF.14b In addition, there is huge interest toward the synthesis of highly electron-deficient PDI molecules and their congeners given the advances in anion−π interactions and semiconductors.15−17 It is well accepted that realization of molecules with a LUMO close to that of TCNQ (−4.90 eV) is a challenge because such electron-deficient molecules with a LUMO ≤ −4.5 eV suffer from intrinsic reactivity.16 © XXXX American Chemical Society
Tetrabromo/tetracyano-substituted PDIs with R = C4H9 [1] and C8H17 [2] were synthesized from the corresponding octabromo-substituted PDI (PDI-Br8)18 with CuCN in anhydrous DMF under an inert atmosphere (Scheme 1). [1] Scheme 1. Synthesis of Compounds [1] and [2] and Their Radical Anions [1]•− and [2]•−
and [2] form one of the strongest electron-deficient PDI molecules. To perform the electron transfer (ET) reaction, tetraphenylphosphonium iodide (PPh4I) was used as a mild reducing agent in DCM at room temperature to attain the corresponding radical anions in 80% yields. The radical anions [1]•− and [2]•− having the phosphonium countercation could be isolated in crystalline form (vide infra). The dianions [1]2− and [2]2− were prepared utilizing sodium sulfide as the reducing agent. The clean ET reactions ensure one of the most straightforward ways to synthesize and isolate PDI radical anions and dianions in high yields. The UV−vis−NIR absorption spectroscopy of [1], [2], [(1•−)·PPh4+], and [(2•−)·PPh4+] exhibited interesting optical features. Compound [1] exhibited UV−vis absorption bands at 457, 502, and 538 nm, whereas [1]•− showed NIR bands at 822, 951, 1052, and 1240 nm (Figure 1). The λmax of [1]•− and [2]•− Received: February 6, 2019
A
DOI: 10.1021/acs.orglett.9b00490 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Figure 1. Normalized UV−vis−NIR absorption spectra of [1] and [1]•− in DCM.
Figure 3. (a) ORTEP representation of [(1•−)·PPh4+] (ellipsoids are drawn at 50% probability). The H atoms and solvent molecules have been removed for clarity. (b) Showing π−π stacking with 1,2,3trifluorobenzene molecules.
appears at ∼1240 nm, which is one of the highest red-shifted λmax recorded for a PDI radical anion. This indicates significant stabilization of the orbitals responsible for the doublet D0 → Dn transitions. Importantly, [1] and [2] could be transformed into [1]•− and [2]•− and the two-electron reduced dianions [1]2− and [2]2− with mild reducing agents through straightforward ET reaction and could be followed by UV−vis−NIR spectroscopy (Figure S1) as well as by spectroelectrochemical methods (Figure S3). The dianions [1]2− and [2]2− exhibited strong absorption bands at 804 nm followed by weak bands in the 600−620 nm region. These dianions are significantly red-shifted compared to the nonsubstituted PDI dianions synthesized in aqueous solution.19 The spectral features of the radical anions remained intact even after keeping the isolated crystalline radical anions for months under ambient conditions, which corroborate their high stability. We also examined the solution state stability of [(1•−)·PPh4+]. The radical anion showed high stability in DCM as well as in THF/H2O (90:10) solution (Figure S2). The remarkable stability of the radical anion in mixed aqueous solution shows its ability to exist in discrete form without undergoing aggregation. The single-crystal X-ray structure analysis of [2] demonstrated a nonplanar PDI backbone. The dihedral angle between the planes of BCF and DEG was found to be 39.29° (Figures 2
positions C(4)−C(5)−C(6′)−C(7′) and C(4′)−C(5′)− C(6)−C(7) were found to be −40.04° (Figure 3a). The carbonyl imide (CO) and cyano (CN) bond distances were found to be 1.219−1.221 and 1.144−1.151 Å, respectively. This clearly shows that the added electron effects the lengthening of these bonds as compared to the neutral form as a result of electronic delocalization. The O···CN noncovalent interactions were found to be 2.639−2.656 Å. The structural parameters derived from theoretically optimized structures are in line with the crystal data (Table S1). Thus, the cyano groups provide interesting milieu of nonconventional bonding possibilities as realized in TCNE and TCNQ.21 The carbonyl imide O atoms form C−H···O H-bonding interactions (C···O, 2.990 Å) with the ortho hydrogen atoms of the phenyl rings of the PPh4+ group. Likewise, the cyano groups also exhibited C− H···N H-bonding interactions (C···N, 3.256 Å) with the alkyl groups of neighboring radical anion (Figure S5). Interestingly, the PDI scaffold aligns two 1,2,3-trifluorobenzene solvent molecules over its large electron-rich surface to form an infinite π-stacked array. The shortest C···C distance was found to be 3.517 Å. In addition, these molecules form F···F interaction with distances of 2.353 Å, which is significantly smaller than the sum of the van der Waals radii of 2.94 Å (Figure 3b). FT-IR spectroscopy revealed large shifts in the symmetric and asymmetric stretching frequencies of the imide carbonyls from 1710 and 1668 cm−1 in [1] to 1664 and 1640 cm−1 in [1]•−, suggesting greater single-bond character of the carbonyl groups in the radical anion. The cyano groups in [1]•− exhibited a stretching frequency at 2229 cm−1, whereas in [1], it was 2221 cm−1. The same trend was seen in [2] and [2]•− (Figure S7). [1]•− and [2]•− were further characterized by EPR spectroscopy (Figure 4 and Figures S9 and S10). The g value was found to be 2.0052 for [1]•− and 2.0061 for [2]•− with peak-to-peak width of 18 G. The g values are higher than those of a free electron spin (2.0023), suggesting spin−orbit coupling. The reduction potentials of [1] and [2] were determined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (Figure 5). The CV of [1] and [2] showed two reversible
Figure 2. Pov-ray representation of [2]. The H atoms and solvent molecules have been removed for clarity.
and S4). The torsion angles for C(3)−C(4)−C(5)−C(6) and C(11)−C(12)−C(13)−C(14) were found to be 39.29 and 41.84°, respectively. In comparison, the dihedral and torsion angles for PDI-Cl8 were reported to be 35.58 and 37.80°, respectively.20 The carbonyl imide (CO) and cyano (CN) bond distances in [2] were found to be 1.191−1.215 and 1.126− 1.139 Å, respectively (Table S1). The carbonyl imide O atoms and the CN groups exhibited noncovalent interactions with O··· C≡N distances of 2.588−2.641 Å, which is shorter than the sum of the van der Waals radii of O and C atoms (3.2 Å). The single-crystal structure of [1]•− validated the anionic state and its structural composition as [(1•−)·PPh4+]. Compared to its neutral state, the perylene backbone in [1]•− demonstrated a smaller twist with a dihedral angle of 38.90° between the planes of rings BCF and DEG (Figure 3). The torsion angles at the bay
Figure 4. EPR spectrum of [(1•−)·PPh4+] in DCM at 295 K. B
DOI: 10.1021/acs.orglett.9b00490 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
molecular contacts confer enhanced electronic delocalization and stability. In summary, we reported the synthesis, isolation, and first Xray structural analysis of a stable PDI radical anion. Its precursor is established as one of the strongest electron acceptors. Iodide has been used as an eco-friendly reductant for the first time to realize a PDI radical anion circumventing the use of toxic reducing agents. The radical anion shows intense NIR bands extending to 1400 nm. Importantly, the PDI radical anion drives spin polarization in the counterion and π-stacked molecules. Such weak interactions are of significance in catalysis and magnetic exchange. The cyano groups provide noncovalent intramolecular electronic paths for charge delocalization, an aspect that can be utilized for new radical ion design. Also, ionic assemblies can be modulated in the presence of stimuli23 and therefore have possibilities galore with these open-shell systems.
Figure 5. CV and DPV of (a) [1] and (b) [2] in DCM; working and auxiliary electrode, Pt with 0.1 M Bu4NPF6 at 298 K.
reduction peaks, viz. E1red and E2red, at −0.16 and −0.51 V vs Fc/ Fc+ in DCM. A LUMO of −4.94 eV for [1] and [2] was estimated from ELUMO = −[5.1 + (E1red)]. Notably, LUMO of [1] is 40 meV lower than that of the tetrachloro/tetracyanosubstituted PDI molecule.19b Therefore, [1] and [2] are the strongest electron-deficient PDI-based molecules. The bromine atoms at the bay positions play a significant role in lowering the LUMO level, an aspect recently recognized in other halogenated electron-deficient systems.16a The HOMO and LUMO energy of the neutral molecule was found to be −7.54 and −5.10 eV, respectively. The HOMO and LUMO surfaces were found to be delocalized over the PDI moiety (Figure S11). The ESP diagram of [1] showed a high positive potential along the π-scaffold and the imide region and a relatively lower positive potential at the cyano N atoms (Figure S12). In contrast, ESP of [1]•− demonstrated a high negative potential uniformly distributed over the entire radical anion, validating an effective electronic delocalization. Further, the spin density calculations and the SOMO orbital diagrams of [1]•− showed spin distribution over the PDI scaffold and weakly spin-polarized countercation (Figure 6 and Table
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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.9b00490. Experimental details, characterization data, NMR, and mass spectra for all new compounds (PDF) Accession Codes
CCDC 1883885−1883886 contain 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
Corresponding Author
*E-mail:
[email protected]. ORCID
Pritam Mukhopadhyay: 0000-0002-3073-6719 Notes
The authors declare no competing financial interest.
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Figure 6. (a) Spin density (isovalue = 0.0001 e/au ) and (b) SOMO (isovalue = 0.005 e/au3) of [(1•−)·PPh4+]. Geometry optimized using DFT CAMB3LYP and LANL2DZ. Notice the spin-polarized atoms near the carbonyl groups (orange arrows).
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 Prof. S. Nagendran and Mr. Dharmendra Singh, IIT Delhi, for extending their laboratory facilities. Y.K. thanks CSIR for a research fellowship.
S2). The calculations also revealed spin-polarized 1,2,3trifluorobenzene solvent molecules (Figure S13). The spin polarization in one of the rings of the tetraphenylphosphonium ion and the solvent molecules is driven by H-bonding and πstacking interactions, respectively. The calculations also revealed that the carbonyl imide O atom in proximity to the countercation has a spin density larger than that of rest of the carbonyl groups. Finally, atoms in molecule analysis22 suggested intramolecular electronic distribution in [1] and [1]•− as a result of the overlap of the nonbonding orbitals of the carbonyl O atoms (nO) and antibonding orbitals of the cyano group (π*CN) (Figures S14 and S15 and Table S3). The electron density (ρb) at the bond critical points of [1] was found to be 0.0162 au, which increased to 0.0170 au in [1]•−, suggesting greater electronic delocalization through this intramolecular bond in the radical anion. Such multiple intra-
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REFERENCES
(1) (a) Mas-Torrent, M.; Crivillers, N.; Rovira, C.; Veciana, J. Chem. Rev. 2012, 112, 2506−2527. (b) Morita, Y.; Suzuki, S.; Sato, K.; Takui, T. Nat. Chem. 2011, 3, 197−204. (2) (a) Ji, L.; Friedrich, A.; Krummenacher, I.; Eichhorn, A.; Braunschweig, H.; Moos, M.; Hahn, S.; Geyer, F. L.; Tverskoy, O.; Han, J.; Lambert, C.; Dreuw, A.; Marder, T. B.; Bunz, U. H. F. J. Am. Chem. Soc. 2017, 139, 15968−15976. (b) Berville, M.; Richard, J.; Stolar, M.; Choua, S.; Le Breton, N.; Gourlaouen, C.; Boudon, C.; Ruhlmann, L.; Baumgartner, T.; Wytko, J. A.; Weiss, J. Org. Lett. 2018, 20, 8004−8008. (c) Suzuki, S.; Takeda, T.; Kuratsu, M.; Kozaki, M.; Sato, K.; Shiomi, D.; Takui, T.; Okada, K. Org. Lett. 2009, 11, 2816− 2818. C
DOI: 10.1021/acs.orglett.9b00490 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters (3) (a) Ferraris, J.; Cowan, D. O.; Walatka, V.; Perlstein, J. H. J. Am. Chem. Soc. 1973, 95, 948−949. (b) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1987, 26, 287−293. (4) Ajayakumar, M. R.; Mandal, K.; Rawat, K.; Asthana, D.; Pandey, R.; Sharma, A.; Yadav, S.; Ghosh, S.; Mukhopadhyay, P. ACS Appl. Mater. Interfaces 2013, 5, 6996−7000. (5) Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 2508−2525. (6) (a) Rademacher, A.; Markle, S.; Langhals, H. Chem. Ber. 1982, 115, 2927. (b) Langhals, H. Heterocycles 1995, 40, 477−500. (c) Lee, S. K.; Zu, Y.; Herrmann, A.; Geerts, Y.; Mullen, K.; Bard, A. J. J. Am. Chem. Soc. 1999, 121, 3513−3520. (7) (a) Huang, C.; Barlow, S.; Marder, S. R. J. Org. Chem. 2011, 76, 2386−2407. (b) Würthner, F.; Saha-Möller, C. R.; Fimmel, B.; Ogi, S.; Leowanawat, P.; Schmidt, D. Chem. Rev. 2016, 116, 962−1052. (8) (a) Zhong, Y.; Trinh, M. T.; Chen, R.; Wang, W.; Khlyabich, P. P.; Kumar, B.; Xu, Q.; Nam, C.-Y.; Sfeir, M. Y.; Black, C.; Steigerwald, M. L.; Loo, Y.-L.; Xiao, S.; Ng, F.; Zhu, X.-Y.; Nuckolls, C. J. Am. Chem. Soc. 2014, 136, 15215−15221. (b) Jiao, Y.; Liu, K.; Wang, G.; Wang, Y.; Zhang, X. Chem. Sci. 2015, 6, 3975−3980. (9) Krieg, E.; Shirman, E.; Weissman, H.; Shimoni, E.; Wolf, S. G.; Pinkas, I.; Rybtchinski, B. J. Am. Chem. Soc. 2009, 131, 14365−14373. (10) (a) Ghosh, I.; Ghosh, T.; Bardagi, J. I.; König, B. Science 2014, 346, 725−728. (b) Ghosh, I.; Marzo, L.; Das, A.; Shaikh, R.; König, B. Acc. Chem. Res. 2016, 49, 1566−1577. (c) Gong, H.-X.; Cao, Z.; Li, M.H.; Liao, S.-H.; Lin, M.-J. Org. Chem. Front. 2018, 5, 2296−2302. (11) Schmidt, D.; Bialas, D.; Würthner, F. Angew. Chem., Int. Ed. 2015, 54, 3611−3614. (12) (a) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielewski, M. R. J. Phys. Chem. A 2000, 104, 6545−6551. (b) Ahrens, M. J.; Fuller, M. J.; Wasielewski, M. R. Chem. Mater. 2003, 15, 2684− 2686. (c) Tauber, M. J.; Kelley, R. F.; Giaimo, J. M.; Rybtchinski, B.; Wasielewski, M. R. J. Am. Chem. Soc. 2006, 128, 1782−1783. (d) Sharma, V.; Puthumana, U.; Karak, P.; Koner, A. L. J. Org. Chem. 2018, 83, 11458−11462. (13) (a) Che, Y.; Datar, A.; Balakrishnan, K.; Zang, L. J. Am. Chem. Soc. 2007, 129, 7234−7235. (b) Ajayakumar, M. R.; Yadav, S.; Ghosh, S.; Mukhopadhyay, P. Org. Lett. 2010, 12, 2646−2649. (14) (a) Seifert, S.; Schmidt, D.; Würthner, F. Chem. Sci. 2015, 6, 1663−1667. (b) He, E.; Wang, J.; Liu, H.; He, Z.; Zhao, H.; Bao, W.; Zhang, R.; Zhang, H. J. Mater. Sci. 2016, 51, 9229−9238. (15) (a) Roznyatovskiy, V. V.; Gardner, D. M.; Eaton, S. W.; Wasielewski, M. R. Org. Lett. 2014, 16, 696−699. (b) Yue, W.; Jiang, W.; Bockmann, M.; Doltsinis, N. L.; Wang, Z. Chem. - Eur. J. 2014, 20, 5209−5213. (16) (a) Kumar, S.; Shukla, J.; Kumar, Y.; Mukhopadhyay, P. Org. Chem. Front. 2018, 5, 2254−2276. (b) Miros, F. N.; Matile, S. ChemistryOpen 2016, 5, 219−226. (c) Dawson, R. E.; Hennig, A.; Weimann, D. P.; Emery, D.; Ravikumar, V.; Montenegro, J.; Takeuchi, T.; Mareda, M. J.; Schalley, C. A.; Matile, S. Nat. Chem. 2010, 2, 533− 538. (d) Kumar, Y.; Kumar, S.; Mandal, K.; Mukhopadhyay, P. Angew. Chem., Int. Ed. 2018, 57, 16318−16322. (17) (a) Zhao, Y.; Beuchat, C.; Domoto, Y.; Gajewy, J.; Wilson, A.; Mareda, J.; Sakai, N.; Matile, S. J. Am. Chem. Soc. 2014, 136, 2101− 2111. (b) Chifotides, H. T.; Dunbar, K. R. Acc. Chem. Res. 2013, 46, 894−906. (c) Ballester, P. Acc. Chem. Res. 2013, 46, 874−884. (d) Zhao, Y.; Cotelle, Y.; Liu, L.; López-Andarias, J.; Bornhof, A.-B.; Akamatsu, M.; Sakai, N.; Matile, S. Acc. Chem. Res. 2018, 51, 2255− 2263. (18) Kumar, Y.; Kumar, S.; Kumar Keshri, S.; Shukla, J.; Singh, S. S.; Thakur, S. T.; Denti, M.; Facchetti, A.; Mukhopadhyay, P. Org. Lett. 2016, 18, 472−475. (19) (a) Shirman, E.; Ustinov, A.; Ben-Shitrit, N.; Weissman, H.; Iron, M. A.; Cohen, R.; Rybtchinski, B. J. Phys. Chem. B 2008, 112, 8855− 8858. (b) Gao, J.; Xiao, C.; Jiang, W.; Wang, Z. Org. Lett. 2014, 16, 394−397. (20) Gsänger, M.; Oh, J. H.; Könemann, M.; Höffken, H. W.; Krause, A.-M.; Bao, Z.; Würthner, F. Angew. Chem., Int. Ed. 2010, 49, 740−743. (21) Miller, J. S.; Novoa, J. J. Acc. Chem. Res. 2007, 40, 189−196.
(22) Popelier, P. Atoms in Molecules: An Introduction; Pearson Education: Harlow, U.K., 2000. (23) Asthana, D.; Pandey, R.; Mukhopadhyay, P. Chem. Commun. 2013, 49, 451−453.
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DOI: 10.1021/acs.orglett.9b00490 Org. Lett. XXXX, XXX, XXX−XXX
