Letter pubs.acs.org/OrgLett
Synthesis and Properties of Conjugated Macrocycles Containing 2,7‑Bis(2-thienyl)‑9H‑fluoren-9-one Units Haresh Thakellapalli, Shuangjiang Li, Behzad Farajidizaji, Notashia N. Baughman, Novruz G. Akhmedov, Brian V. Popp, and Kung K. Wang* C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045, United States S Supporting Information *
ABSTRACT: Synthetic pathways to conjugated macrocycles containing one, two, or three 2,7-bis(2-thienyl)-9H-fluoren-9-one (TFOT) units in the macrocyclic frameworks bearing 10, 16, or 24 aromatic units were developed. The Diels−Alder reaction between (E,E)-1-(5-bromo-2thienyl)-4-(5-iodo-2-thienyl)-1,3-butadiene and dimethyl acetylenedicarboxylate produced the key Diels−Alder adduct for the subsequent macrocyclic ring formation. UV−vis and fluorescence spectra of the TFOT-containing molecules were recorded, and their electrochemical properties were investigated by cyclic and differential pulse voltammetry. Solvatofluorochromic properties were observed for the TFOT-containing molecules.
T
give 1,3-butadiene 5 (Scheme 1). Cycloaddition between 5 and dimethyl acetylenedicarboxylate (DMAD, 6) at 140 °C produced the Diels−Alder adduct 7 in 72% isolated yield. It is worth noting that the iodothienyl and bromothienyl groups in 7 are cis to each other exclusively, which is essential for the subsequent macrocyclic ring formation. The use of the Diels− Alder reaction to place the two haloaryl groups cis to each other is similar to the strategy for the synthesis of CPPs11 and furan-12 and thiophene-containing CPPs.5b Reduction of 7 with diisobutylaluminum hydride (DIBAL-H) to form diol 8 was followed by treatment with tertbutyldimethylsilyl chloride (TBSCl)/imidazole producing 9. The different reactivities of iodo and bromo substituents toward the Suzuki−Miyaura coupling reaction were exploited for the cross-coupling reactions with bisboronic ester 1013 (pin = pinacolato) to form 11a and 11b containing one TFOT unit as a 1:1 mixture of two isomers. Attempts to promote two consecutive Suzuki−Miyaura coupling reactions between 10 and 11a/11b to produce a macrocyclic system bearing two TFOT units were unsuccessful. Alternatively, cross-coupling with boronic ester 1214 was successful in producing 13a and 13b, which upon exposure to methyl iodide at 120 °C15 afforded diiodide 14a and 14b. The Ni(cod)2-mediated (cod = 1,5-cyclooctadiene) intramolecular homocoupling reaction16 of 14a/14b in the presence of 2,2′-bipyridyl (bpy) then produced a 1:1 mixture of syn-15 and anti-15 with syn-15 having the two sets of the TBSOCH2 groups on the same side of the macrocyclic ring and anti-15 having the two sets on the opposite sides. Oxidative aromatization with 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ)5b at 100 °C for 3 h then
he optical and electrochemical properties of 2,7-bis(2thienyl)-9H-fluoren-9-one (1, TFOT, Figure 1) and
Figure 1. Molecular structure of 2,7-bis(2-thienyl)-9H-fluoren-9-one (1, TFOT).
related compounds have received considerable attention in recent years.1 A variety of oligomers and polymers containing thiophene as a donor and 9-fluorenone as an acceptor were synthesized to allow the investigation of their optoelectronic properties and the assessment of their potential applications in electronic devices.1,2 Light-emitting copolymers derived from 9,9-dioctylfluorene and TFOT were prepared, and their optical and electroluminescence properties were studied.3 Only acyclic TFOTs and mainly linear oligomers/polymers have been synthesized. Incorporating donors and acceptors in a conjugated, macrocyclic structure is recognized to hold potential for tuning electronic properties and for designing new functional materials.4 Synthetic pathways to thiophenecontaining cycloparaphenylenes (CPPs),5 [4]cyclofluorene,6 and structurally related carbazole systems7 were reported recently. Our interest in the construction of macrocyclic structures led us to develop synthetic pathways for TFOTcontaining macrocycles. UV−vis and fluorescence spectra of these TFOT-containing molecules, and their electrochemical properties, were investigated. The synthetic pathway involved an initial Horner−Wadsworth−Emmons reaction between 28 and 39 to form 4 followed by selective iodination with N-iodosuccinimide (NIS)5b,10 to © 2017 American Chemical Society
Received: April 4, 2017 Published: May 11, 2017 2674
DOI: 10.1021/acs.orglett.7b01019 Org. Lett. 2017, 19, 2674−2677
Letter
Organic Letters Scheme 1. Synthesis of TFOT-Containing Macrocycles 15 and 16
gave 16 containing one TFOT unit in a macrocyclic system bearing 10 aromatic units. The Suzuki−Miyaura coupling reactions between 11a/11b and phenylboronic acid (17) produced 18a/18b, which upon treatment with DDQ produced 19 as an acyclic analogue of 16 (Scheme 2). The Ni(cod)2-mediated homocoupling reactions of 11a/11b followed by oxidative aromatization with DDQ furnished dimer 20 and trimer 21 in 7% and 14% isolated yields, respectively, over two steps (Scheme 3). Dimer 20 contains two TFOT units in a macrocyclic system bearing 16 aromatic units, and trimer 21 contains three TFOT units in a 24-membered macrocyclic system. The UV−vis absorption maxima (λabs) and the fluorescence maxima (λem) of TFOT (1),1a,b cyclic systems 15, 16, 20, and 21, and acyclic systems 18 and 19 in dichloromethane are summarized in Table 1. The absorption band of 1 at 348 nm can be attributed to the π−π* transition of the thiophene units.1a,d The broad and weaker band centered around 464 nm can be attributed to the n−π* transition of the carbonyl group of the 9-fluorenone unit or an internal charge-transfer (ICT) band arising from thiophene as the donor and 9-fluorenone as the acceptor.1a,d The n−π*/ICT transitions of the TFOTcontaining molecules in Table 1 exhibit red shifts from that of 1. Interestingly, the π−π* transition of 16, which is fully aromatized with a cyclic structure, shows a blue shift from that of the partially hydrogenated 15. On the other hand, a red shift was observed for the acyclic but fully aromatized 19 versus that of 18. In addition, 16 was blue-shifted from that of 19 by 30 nm. The higher ring strain in the fully aromatized and cyclic 16
Scheme 2. Synthesis of Acyclic TFOT-Containing 18 and 19
and its effects on the planarity of the aromatic units and the dihedral angles between the adjacent aromatic units may be responsible for the blue shift. The fluorescence spectrum of 1 in dichloromethane shows an emission maximum (λem) at 571 nm,1a,b whereas that of 16 2675
DOI: 10.1021/acs.orglett.7b01019 Org. Lett. 2017, 19, 2674−2677
Letter
Organic Letters Scheme 3. Synthesis of TFOT-Containing Macrocycles 20 and 21
Figure 2. UV−vis (solid line) spectrum of 16 in dichloromethane and fluorescence (dashed lines) spectra of 16 in various solvents.
on λem of 9-fluorenone and substituted derivatives were systematically investigated.17 The electrochemical properties of 11a,b and TFOTcontaining molecules were probed in dichloromethane using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) (Tables 1 and S1). A single quasi-reversible cathodic event was observed in all compounds. Small variations in E1/2 (1.0 V that can be attributed to newly formed 2,5-diphenylthiophene5b and/or 1,4-bis(2-thienyl)benzene groups.19 Larger macrocycles (20 and 21) each exhibit a single quasi-reversible anodic event at a potential consistent with a 5,5′-diphenyl-2,2′-bithiophene unit5b as well as anodic events consistent with 2,5-diphenylthiophene and/or 1,4-bis(2thienyl)benzene units. In conclusion, synthetic pathways leading to conjugated macrocycles 16, 20, or 21 containing one, two, or three TFOT units have been developed. Cycloaddition between 1,3butadiene 5 and dimethyl acetylenedicarboxylate (DMAD, 6) produces the Diels−Alder adduct 7 with the two thienyl groups cis to each other, which is necessary for the subsequent macrocyclic ring formation. Oxidative aromatization of the partially hydrogenated aromatic precursors with DDQ provides easy access to the fully aromatized macrocycles bearing TFOT units. The effects of cyclic and conjugated aromatic structures on UV−vis and fluorescence spectra of these TFOT-containing molecules were investigated, and their electrochemical properties were studied by cyclic and differential pulse voltammetry. The synthetic pathway to 16 is flexible and can be adopted to replace the biphenyl unit with either an electron-rich donor or an electron-poor acceptor.
Table 1. UV−vis Absorption Maxima, Fluorescence Emission Maxima, and Voltammetric Data λabs (nm) 1 15 16 18 19 20 21
π−π*
n−π*/ICT
348 348 340 362 370 372 368
464 482 470 475 470 473 476
λem (nm)
E1/2 (V vs Fc/Fc+)a
419, 425, 454, 429, 453, 596 597
−1.64, −1.67, −1.65, −1.67 −1.60 −1.64, −1.66,
571 591 573 595 594
0.76,b 0.99b 0.74 0.82,b 0.92b
0.67 0.76
a
Additional irreversible oxidation events were observed in the differential pulse voltammetry experiments (the Supporting Information). bIrreversible redox event.
exhibits two peaks of nearly equal intensity at 454 and 573 nm. The longer λem of all other TFOT-containing molecules in Table 1 are red-shifted more significantly from that of 1. Macrocycles 20, 21 also show weak emission bands between 380 and 510 nm. Recently, the 9,10-anthraquinon-2,6-ylene- and 2,6tetracyanoanthraquinodimethanylene-inserted CPPs were reported to display solvatofluorochromic properties due to their donor−acceptor characteristics.4a A modest solvatofluorochromic property was observed for 1, whereas those of 16 and 19 were more significant (Table 2). The fluorescence color of 16 changed from yellowish green in hexanes to reddish orange in chloroform (Figure 2). The presence of a polar 9-fluorenone unit in 1, 16, and 19 may also contribute to their solvatofluorochromic properties. The effects of solvent polarity Table 2. Solvatofluorochromic Properties of 1, 16, and 19 λem (nm)a 1 16 19 a
C6H14
Et2O
C6H6
CH2Cl2
CHCl3
554 545 534
553 560 555
554 569 562
571 573 594
577 597 580
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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.7b01019.
Excitation of 1 at 340 nm, 16 at 340 nm, and 19 at 360 nm. 2676
DOI: 10.1021/acs.orglett.7b01019 Org. Lett. 2017, 19, 2674−2677
Letter
Organic Letters
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Complete experimental details of new compounds; spectral data; UV−vis and fluorescence spectra; cyclic and differential pulse voltammograms (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Brian V. Popp: 0000-0001-6367-1168 Kung K. Wang: 0000-0001-7039-2984 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under CHE-1464026. The NMR spectrometer on which this work was carried out was supported by the Major Research Instrumentation (MRI) Program of the NSF (CHE-1228336). We thank Gregory C. Donohoe of West Virginia University for his assistance in obtaining the highresolution mass spectra.
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DOI: 10.1021/acs.orglett.7b01019 Org. Lett. 2017, 19, 2674−2677
