Letter Cite This: Org. Lett. 2018, 20, 6606−6609
pubs.acs.org/OrgLett
Synthesis of Isomeric Perylenodithiophene Diimides Cheng Zeng,†,§ Dong Meng,† Wei Jiang,*,† and Zhaohui Wang*,†,‡ †
Org. Lett. 2018.20:6606-6609. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 10/19/18. For personal use only.
CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Two isoelectronic dithiophene-fused perylene diimides (PDTI-1 and PDTI-2) were synthesized via a “bayderivatization toward lateral extension” strategy. Single-crystal analysis unambiguously confirmed their unique structures and packing arrangements. The new PDTI system manifested significantly red-shifted absorptions with intense bands at 500− 700 nm. Further dimerization indicated the potential of these dithiophene-fused PDIs as new building blocks for the construction of versatile rylene dyes in optoelectronic devices.
T
he past decades have witnessed ever-increasing attention to the development of opto- and electro-active materials based on fused-ring systems for their applications in organic devices.1 Perylene diimide (PDI) is one of the most important prototype π architectures for the construction of n-type optoelectronic materials both in small molecular and donor− acceptor (D−A) polymeric forms applied in organic field-effect transistors (OFETs) and organic photovoltaic devices (OPVs).2 Fusion of donor moieties onto the PDI core provides more extended π-structures with grossly affected electronic properties.3 The conventional π-expansion of PDI systems is limited to Suzuki, Stille, or Sonogashira coupling of brominated species at bay (meta) regions with corresponding (hetero)carbocyclic rings and subsequent dehydrocyclization by cleavage of another bay-C-H bond.4 Because of the weaker reactivity of nonbay (ortho) regions and less synthetic methodologies, there are very few research studies on lateral-fused systems involving both bay- and nonbayfunctionalization,2a,5 however, they are of much importance to perylene materials that would lead to broad and red-shifted absorptions and helical structures due to the crowding effect of extended upper and lower rings (Figure 1). Herein, we first elucidate a facile synthesis of two structural isomers of dithiophene-fused PDI, namely, perylenodithiophene diimides ([2,1-b:8,7-b′]PDTI-1 and [2,1-b:5,6b′]PDTI-2) (Scheme 1). Both are formed due to the resulting mixture of bay-area-substituted dibrominated PDI precursors.6 The regioisomeric purity of PDI derivatives is highly demanding, which is sensitive to the fundamental properties, thus leading to a major influence on device performance.7 However, repetitive recrystallization can only be used to © 2018 American Chemical Society
Figure 1. Three synthetic strategies toward core-extended PDI systems.
remove the 1,6-isomeric form.8 Our method can lead to the facile formation of two isoelectronic PDI-fused D−A systems by annulation of two thiophene rings on the same or different naphthalene subunits through either 1,7- or 1,6-functionalization in one-pot. It is also the first example in which the bayarea-substituted derivative is further dehydrocyclized by cleaving an ortho-C-H bond instead of another bay-C-H bond to construct lateral extended rylenes. The two structural isomers can be used as novel dithiophene-fused PDI building blocks, which in turn provides a channel to be integrated into various π-conjugated systems by modification at vacant thiophene α-positions. The synthesis of two PDTI molecules is outlined in Scheme 1. Very recently, we successfully cyclized ortho-ethynyl PDI (nonbay-subtituted precursor) with cleavage of four bay-C-H bonds to generate tetrathiophene-fused PDIs with four S Received: September 18, 2018 Published: October 8, 2018 6606
DOI: 10.1021/acs.orglett.8b02983 Org. Lett. 2018, 20, 6606−6609
Letter
Organic Letters Scheme 1. Synthetic Route toward Perylenodithiophene Diimides (PDTIs)a
Conditions: (i) For 2a, TMSA, Pd(PPh3)4, CuI, i-Pr2NH, 40 °C, 30 min; for 2b, [(triisopropylsilyl)ethynyl]copper(I), DMSO, 90 °C. Yields: 90% for 2a; 80% for 2b. (ii) S8, 10/1 mixture of DMF/H2O, 90 °C, 12 h. Yields: 20% and 6% for 3a and 5a; 26% and 8% for 3b and 5b, respectively. (iii) For 4a and 6a, K2CO3 THF/MeOH, rt, 3 h; for 4b and 6b, Bu4NF, THF, rt, 1 h. Yields: 98% for 4 and 6. a
Figure 2. Single-crystal structures of two PDTI molecules: 4b (a) and (c), 6b (b) and (d) in top and side views, respectively. Packing arrangement of 4b (e) along the b-axis and 6b (f) in the a-c plane; the P,P- and M,M-isomers are shown in different colors. The dotted lines represent the intermolecular interactions in (e) and (f).
atoms facing the inside.5b Inspired by this efficient strategy, we used 1,7(6)-dibromo-PDIs (1) as the starting materials to undergo a standard Sonogashira coupling to produce diethynyl PDI mixture (2) in a high yield of 80−90%. Then, key intermediates 2 were directly reacted with S8 in DMF/H2O mixture9 at 90 °C to produce the dithienannulated products (3 and 5) with removal of another two less active nonbay-C-H bonds in total yields of 26% for 6-undecyl-substituted products and 34% for diisopropylphenyl-substituted products, respectively. After simple chromatographic purification and subsequent desilylation of the TMS or TIPS groups at room temperature,10 PDTI-1 (4) and PDTI-2 (6) with two S atoms facing the outside were achieved with quantitative yields. Because of the different molecular symmetry resulting from the two thienannulation positions, PDTI-2 (6a) in the 1H NMR spectrogram shows two sets of multiplets corresponding to the hydrogen atoms denoted as C−H of side chains and the adjacent methylene protons also split into separate signals. In addition, there are four and two carbonyl carbon peaks in 13C NMR spectra for 3a and 5a, respectively, owing to the nonplanar backbone and the difference in symmetry of the two isomers. Meanwhile, there are two peaks of 5a/6a for the side chains methyne. For their structures to be confirmed, the racemic single crystals of 4b and 6b are grown from chlorofrom/n-hexane or chloroform/methanol solutions, both of which have their two S atoms facing the outside. As indicated in Figure 2a and b, their single crystals show quasi
helical-shaped structures. The two molecules show slightly different dihedral angles between the terminal rings. The same helicity with two S-hetero[4]helicenes in a crystal is observed, and the meso form (P,M-isomer) was not identified. As viewed in Figure 2e and f, the heterochiral crystals of PDTI-1 (4b) with a central symmetry show ordered column arrangement along the b-axis, and the interlayer distances are 6.6 and 7.1 Å due to bulky diisopropylphenyl groups nearly perpendicularly orientated to the PDTI core. The P,P- and M,M-isomers are packed in an alternating fashion between layers and columns with multiple C−H···O (2.3 Å), C−H···π (2.8−2.9 Å), and O···π (3.1 Å) interactions. In contrast, the homoenantiomers in the racemic single crystals of PDTI-2 (6b) are also packing in columns along the b-axis with little overlap but forming close and layered structures in the a-c plane with dense hydrogen bonds (2.3−2.7 Å), C−H···π (2.8− 2.9 Å), S···O (2.9−3.0 Å), and O···π (3.0−3.1 Å) interactions. The P,P- and M,M-enantiomers within the layer are arranged in an alternating pattern, and the pairs of heterochiral crystals adopt an antiparallel manner. Furthermore, we described our endeavors to carry out the dimerization reactions of two PDTI molecules through Ullmann coupling via the active thiophene α-positions (Scheme 2). Starting from bis(trimethylsilyl)-substituted PDTIs (3a and 5a) instead of unsubstituted ones, the bromination process was conducted by treatment with N6607
DOI: 10.1021/acs.orglett.8b02983 Org. Lett. 2018, 20, 6606−6609
Letter
Organic Letters
in the range of 500−700 nm, largely red-shifted relative to pristine PDI (400−550 nm).12 The availability of dithiopheneand tetrathiophene-fused PDI series gives us the opportunity to assess the extent of electron delocalization across the entire core by comparing their optical spectra. The longest maxima are 527, 638, 654, and 700 nm for PDI, PDTI-1, PDTI-2, and PTTI,5b respectively, by considering the number and position of thiophene annulation. Remarkably, PDTI-2 6a showed bathochromic shifts of 16 nm relative to its isomer 4a. Both of the two dithiophene-fused PDIs feature more blue-shifted absorption maxima relative to dibenzo-fused ones (λmax = 697 nm).13 The absorption of dimerized products revealed redshifted and enhanced light-absorbing capability, and the maximum absorption coefficients are 127474 M−1 cm−1 at 705 nm for 8 and 104794 M−1 cm−1 at 708 nm for 10. Their molar extinction coefficients are largely enhanced with the increase in PDTI units, whereas PDTI-1 exhibited slightly higher absorptivity than PDTI-2. The optical energy gaps of 1.88, 1.83, 1.69, and 1.68 eV for 4a, 6a, 8, and 10, respectively, are consistent with the electrochemical HOMO−LUMO gaps (Table 1). The experimental conclusion is further supported
Scheme 2. Dimerization of Two PDTI Isomers through Ullmann Coupling
bromosuccinimide (NBS) in dichloromethane/acetic acid mixture solvents to give corresponding dibromides (7 and 9) in good yields. Then, the dibromides underwent conventional Ullmann coupling with nanosized copper powder as the catalyst in dry DMSO to directly afford their corresponding dimers with terminal bromo groups. The targeted products 8 as purple-black solids and 10 as purple solids were subsequently generated via hydrodehalogenation with PdCl2(dppf) as the catalyst in combination with the hydride source of NaBH4 and N,N,N′,N′-tetramethylethylenediamine (TMEDA) in THF with high yields.11 The newly synthesized PDTIs and dimer 8 have good solubility in common organic solvents, but dimer 10 only dissolved reasonably in halogenated solvents probably due to its larger linear conjugated length. Notably, the two PDTI compounds and their corresponding dimers show excellent thermal and oxidative stability with the 5% weight loss temperatures (Tdeg) around 430 °C (Table S1 and Figure S1). The photophysical properties of the two isomers (4a and 6a) and their corresponding dimers (8 and 10) were studied with UV/vis absorption spectroscopy and cyclic voltammetry (CV) measurements. As shown in Figure 3, the two structural PDTI isomers displayed characteristic π−π* transition bands
Table 1. Photophysical Properties of Two PDTI Isomers and Their Corresponding Dimers compd
λmax [nm]a
εmax [M−1 cm−1]a
ELUMO [eV]b
EHOMO [eV]c
Egelectro [eV]d
Egopt [eV]e
4a 6a 8 10
638 654 705 708
75023 60461 127474 104794
−3.87 −3.87 −4.00 −4.06
−5.68 −5.67 −5.62 −5.59
1.81 1.80 1.62 1.53
1.88 1.83 1.69 1.68
a Measured in dilute CHCl3 (1.0 × 10−5 M). bLUMO estimated by the onset of the reduction peaks and calculated according to ELUMO = −(4.8 + Eonsetre) eV. cHOMO estimated by the onset of the oxidative peaks and calculated according to EHOMO = −(4.8 + Eonsetox) eV. d Calculated according to Egelectro = (ELUMO − EHOMO) eV. eObtained from the edge of the absorption in CHCl3 solution according to Egopt = (1240/λonset).
by a comparison of the calculated absorption spectra of the two isomers, which was performed using time-dependent density functional theory (TD-DFT) (Figure S7). All of these compounds have weaker fluorescence than that of parent PDI resulting from the annulation of thiophene rings on the core (Figure S2). Combining with CV and differential pulse voltammetry (DPV) curves (Figures S3 and S4), the half-wave reduction and oxidation potentials versus Fc/Fc+ of four compounds are summarized in Table S1. The first half-wave potentials of the PDTI molecules and their dimers were progressively less negative compared with those of the parent PDI, indicating their enhanced electron-accepting abilities by lateral fusion with thiophene rings or further dimerization. In addition, the electrochemical gaps of two structural isomers are almost the same (∼1.8 eV) according to their estimated LUMO and HOMO energy levels by the onset of the reduction and oxidation peaks, whereas dimer 10 with bis(heterotetracene) substructure displayed slightly lower LUMO and high-lying HOMO levels that resulted in narrower electrochemical gap than that of dimer 8. The LUMO levels are located at −3.8 to 4.0 eV for all compounds, suggesting that they are rather PDIlike, and the annulation of electron-rich thiophene rings can greatly change their redox properties.
Figure 3. Room-temperature UV−vis absorption spectra of 4a (black), 6a (green), 8 (red), and 10 (blue) in dilute CHCl3 (1.0 × 10−5 M). 6608
DOI: 10.1021/acs.orglett.8b02983 Org. Lett. 2018, 20, 6606−6609
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Organic Letters
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In summary, we have developed a facile synthesis of two structural isomers of dithiophene-fused PDI in one pot from the robust mixture of bay-area-substituted dibrominated PDI precursors. This is also the first example through bayderivatization toward lateral extension on the PDI system. Single-crystal analysis unambiguously confirmed their unique structures and packing arrangements. The new PDTI system manifested significantly red-shifted absorptions with intense bands at 500−700 nm. Further dimerization of two PDTIs by utilizing the vacant thiophene α-positions has generated further red-shifted and largely enhanced light-absorbing capability, indicating the potential of dithiophene-fused PDIs as new building blocks for the construction of versatile rylene dyes in optoelectronic devices.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02983. Experimental procedures and spectral data for all new compounds (1H NMR, 13C NMR, HRMS, etc.) (PDF) Accession Codes
CCDC 1864647 and 1864658 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 Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Wei Jiang: 0000-0002-0153-7796 Zhaohui Wang: 0000-0001-5786-5660 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We grateful acknowledge Dr. Andong Zhang for his helpful absorption spectra calculations performed using time-dependent density functional theory (TD-DFT). This work was supported by the National Natural Science Foundation of China (NSFC) (No. 51673202), the National Key R&D Program of China (2017YFA0204701), and the Youth Innovation Promotion Association of Chinese Academy of Sciences.
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REFERENCES
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DOI: 10.1021/acs.orglett.8b02983 Org. Lett. 2018, 20, 6606−6609
