Bifuran-imide: a stable furan building unit for organic electronics

Sandip V. Mulay a, Benny Bogoslavky b, Idan Galanti a, Ehud Galun c and Ori Gidron *a
aInstitute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem, Israel. E-mail:
bThe Laboratory for Molecular Structure Analysis, The Institute of Chemistry, The Hebrew University of Jerusalem, The Edmond J. Safra Campus, Jerusalem, Israel
cHead of Materials Division, R&T Base Unit, DDR&D IMOD, Hakirya, Tel Aviv, Israel

Received 13th June 2018 , Accepted 9th August 2018

First published on 9th August 2018

Oligo- and polyfurans hold many advantages over their thiophene analogues, yet their low environmental stability greatly limits their applications. In this work, we introduce a new building unit, bifuran-imide (BFI), which displays many of the advantageous properties observed for oligofurans, such as strong fluorescence, high solubility, significant quinoid character, and planarity, while also being significantly more stable (both thermally and photochemically) than parent oligofurans. Oligomers and polymers containing BFI show good solid state packing, strong fluorescence, and solubility, which makes them ideal candidates for active materials in organic electronics.

The field of organic electronics depends on the discovery of new materials whose π-conjugated backbones endow them with desired properties.1–4 Good candidate materials are those that demonstrate small HOMO–LUMO gaps, strong fluorescence, good solid-state packing, and a planar backbone resulting in good charge delocalization and mobility. These criteria are met by only a limited number of π-conjugated backbones, with oligo- and polythiophenes (nT, Chart 1) and their derivatives currently dominating the field.5–10
image file: c8tc02908c-c1.tif
Chart 1 Structures of nT, nF, nBTI and nBFI discussed in this work.

We have previously introduced a new family of organic electronic materials, oligofurans (nF, Chart 1), which are the oxygen-containing analogues of oligothiophenes.11–16 Oligofurans display significant advantages over oligothiophenes, such as high rigidity/planarity, strong fluorescence, extensive conjugation, good field-effect mobility, and high solubility.17–23 These properties make oligofurans attractive candidates for devices such as organic light emitting diodes (OLEDs),24–26 organic light emitting transistors (OLETs), and organic field-effect transistors (OFETs).27–30 Unlike nT, which are inert toward Diels–Alder cycloaddition, nF can serve as synthons for long oligoarenes.31 However, oligofurans can undergo photooxidation under ambient conditions.11 Their low environmental stability must be overcome if oligo- and polyfurans are to become useful as active materials in organic electronic devices.

Recently, bithiophene-imide (BTI, Chart 1) was introduced by the Marks group as an n-type semiconductor.32,33 This building unit was successfully applied in OFETs and organic solar cells.34,35 Ladder oligomers and polymers of BTI units were shown to have electron mobilities of up to 3.71 cm2 V−1 s−1 and high environmental stabilities.36,37 Rupar's group recently reported bridged bifuran complexes stabilized by p-block elements that have a lower LUMO level through σ*–π* conjugation.38 Considering these factors, we sought to stabilize furans by introducing the bifuran-imide (BFI, Chart 1) building unit.

Here, we report the synthesis of the BFI unit, and its corresponding oligomers and polymers (nBFI) bearing hexyl (nBFI-H) or 2-octyldodecyl (nBFI-OD) substituents (Chart 1). We find that nBFI are significantly more stable (both thermally and photochemically) compared with α-oligofurans while displaying good solid-state packing, strong fluorescence, and good solubility, which is important for their processability. These properties allowed us to obtain the high molecular weight polymer, poly(BFI-OD), which was also found to be soluble in a wide range of organic solvents.

The synthesis of the BFI monomer, dimer, trimer, and polymer is depicted in Scheme 1. In the first step, dibromobifuran (1)15 was converted to bifuran-dicarboxylic acid (2) by lithiation with n-butyllithium followed by addition of dry ice. Next, bifuran-dicarboxylic anhydride (3) was obtained in 81% yield from 2 by condensation/cyclization in refluxing acetic anhydride. The key building units, bifuran-imides BFI-H (88%) and BFI-OD (79%), were synthesized from anhydride 3 by reaction with hexylamine or 2-octyldodecylamine, respectively, followed by in situ treatment with SOCl2.

image file: c8tc02908c-s1.tif
Scheme 1 Synthetic procedure for bifuran-imide building units, dimer, trimer, and homopolymer. Reagents and conditions: (a) n-butyllithium, dry ice, −70 °C, 7 h; (b) acetic anhydride, reflux, 12 h; (c) (i) RNH2 (R = n-hexyl or 2-octyldodecyl), CH2Cl2, 40 °C, 12 h, (ii) SOCl2, 40 °C, 15 h; (d) Br2, FeCl3 (cat.), CH2Cl2, room temperature; (e) lithium diisopropylamide, tributyltin chloride, tetrahydrofuran, −85 °C, 3 h; (f) Pd(PPh3)4 (i.e., Pd[(C6H5)3P]4), toluene, 90 °C; (g) Pd(PPh3)4, dimethylformamide, 90 °C.

To synthesize 2BFI-H and 3BFI-H, BFI-H was brominated with Br2 and FeCl3 to yield the monobromobifuran-imide 4a (56%), and stannylated by lithiation with lithium diisopropylamide (LDA) followed by treatment with tributyltin chloride to give 5 (52%). The dibromobifuran-imide 4b (97%) and 4c (96%) were obtained using an excess of Br2. The Stille coupling of 5 with monobromobifuran-imide (4a) and dibromobifuran-imide (4b) delivered the dimer 2BFI-H (61%) and trimer 3BFI-H (45%), respectively.

Attempts to synthesize the homopolymer of BFI-H by Stille polymerization of 4b resulted in an insoluble polymer. In contrast, when the distannyl BFI-OD (6) was coupled with the corresponding dibromide 4c, the resulting polymer, poly(BFI-OD), was found to be considerably more soluble than the hexyl analogue. This was first evident from the NMR spectrum of poly(BFI-OD), which could be recorded in CDCl3 with no heating. For comparison, the NMR spectrum of the corresponding thiophene-based homopolymer, poly(BTI-OD) was recorded at 120 °C in 1,2-tetrachloroethane.33 Gel permeation chromatography (GPC) was used to determine the polymer molecular weight versus polystyrene. Poly(BFI-OD) was found to have a molecular weight (Mw) of 27[thin space (1/6-em)]000 Da and a polydispersity index (PDI) of 2.1, corresponding to a mean chain length of ∼60 monomer units (∼120 furan rings). This is significantly longer than for the previously reported thiophene analogue poly(BTI-OD), which consists of only ∼15 monomer units.32 This difference in mean chain length reflects the better solubility of poly(BFI-OD), as greater solubility permits the formation of longer polymer chains. Thermogravimetric analysis (TGA) showed poly(BFI-OD) to have good thermal stability with less than 5% weight loss up to 350 °C (Fig. S4, ESI). Glass phase transitions were not observed by differential calorimetry measurements for either nBFI-H or poly(BFI-OD) (Fig. S1–S4, ESI).

The single X-ray crystal structures of 3, BFI-H, and 2BFI-H were obtained by slow diffusion of hexane into dichloromethane at room temperature. Both the anhydride 3 and imide BFI-H (Fig. 1a and b) display planar conformations with the C–C bond length between two furan rings measured at 1.425 Å and 1.418 Å, respectively. These distances are slightly shorter than the C–C bond length (1.430 Å) previously reported phosphorus bridged bifurans.38 The dimer 2BFI-H displays even shorter C–C bond lengths between the two furans in the same imide unit (1.403 Å and 1.407 Å) and an interunit bond length of 1.425 Å (Fig. 1c). Additionally, no torsion is observed between the two BFI units, which is an indication of the strong quinoid character of this π-conjugated backbone. One factor which contributes to the stronger quinoid character of 2BFI-H is the lower aromaticity of furan compared with its thiophene analogues.22 In addition, the larger exo angles in furan often associated with the planar its backbone, and consequently with increased conjugation.16BFI-H and 2BFI-H show π–π interactions in a parallel-displaced manner with interplanar distances of 3.31–3.51 Å and 3.20–3.36 Å, respectively (Fig. 2).

image file: c8tc02908c-f1.tif
Fig. 1 ORTEP representation of (a) 3; (b) BFI-H; and (c) 2BFI-H.

image file: c8tc02908c-f2.tif
Fig. 2 Solid state packing of (a) BFI-H; and (b) 2BFI-H.

The absorption and emission spectra of nBFI-H, BFI-OD, and poly(BFI-OD) in chloroform are displayed in Fig. 3a and b with the thiophene analogue, BTI-OD,32 for comparison. While all the furan-containing analogues display clear vibronic shoulders in the absorption spectra, the featureless appearance of the BTI-OD spectrum indicates its more flexible backbone. An additional indication of the greater rigidity of the furan-containing analogues is their lower Stokes shift (0.52 eV vs. 0.66 eV for BFI-OD and BTI-OD, respectively). The dimer and trimer display a smaller Stokes shift of 0.26 eV for both (Table 1). The polymer, poly(BFI-OD), also displays similar features, with λabs = 532 nm and λem = 537 nm, indicating a rigid backbone, despite the high solubility (Table 1). No significant solvatochromic shift was observed for the absorption spectra upon increasing solvent polarity from hexane to acetonitrile (Fig. S5, S6, S8–S11 and Table S1, ESI), indicating that the lowest energy transition (π–π*) has an apolar nature. The optical energy gap (Eg(opt)) ranges between 3.44 eV for BFI-H to 2.26 eV for poly(BFI-OD) (Table 1). The solid state (film) absorption and fluorescence spectra experience a bathochromic shift of up to 0.35 eV, and Eg(opt) is lower by 0.34–0.1 eV compared with the values in measured chloroform (the film Eg(opt) ranges between 3.10 eV for BFI-H to 2.16 eV for poly(BFI-OD), Fig. S7 and Table S1, ESI).

image file: c8tc02908c-f3.tif
Fig. 3 Normalized (a) UV-Vis absorption and (b) fluorescence spectra of BFI-H, 2BFI-H, 3BFI-H, BFI-OD, BTI-OD, and poly(BFI-OD) in CHCl3 at room temperature; (c) photograph of fluorescence emission of BFI-H, 2BFI-H, 3BFI-H, and poly(BFI-OD): (top) in CHCl3 solution and (bottom) in the solid state at 365 nm; (d) emission colors of BFI-H, 2BFI-H, 3BFI-H, and poly(BFI-OD) on the CIE 1931 chromaticity diagram.
Table 1 Photophysical data of bifuran-imide-based compounds
Compounds λ abs (nm) λ em (nm) Stokes shift (eV) ε (M−1 cm−1) Φ f Φ f (solid) τ F (ns) k F (ns−1) k NR (ns−1) E g(opt) [eV] HOMO [eV]
a Measured in the CHCl3. b Calculated according to the equation kF = Φf/τF. c Calculated according to the equation Φf = kF/(kF + kNR). d E gap = 1242/λonset; image file: c8tc02908c-t1.tif.39
BFI-H 330, 343 383 0.52 13[thin space (1/6-em)]714 0.80 0.18 4.14 0.19 0.05 3.44 −6.05
2BFI-H 396, 415 432, 459 0.26 37[thin space (1/6-em)]353 0.81 0.06 1.74 0.46 0.11 2.85 −5.71
3BFI-H 365, 435, 457 478, 513 0.26 57[thin space (1/6-em)]600 0.65 0.06 1.78 0.36 0.20 2.57 −5.69
BFI-OD 331, 344 385 0.52 10[thin space (1/6-em)]318 0.77 4.08 0.19 0.06 3.44 −5.94
BTI-OD 348 427 0.66 11[thin space (1/6-em)]670 0.78 5.83 0.13 0.04 3.22 −6.03
Poly(BFI-OD) 492, 532 537, 584 0.02 0.71 0.05 1.61 0.44 0.18 2.26 −6.30

Fig. 3c shows the fluorescence images of BFI-H, 2BFI-H, 3BFI-H, and poly(BFI-OD) in solution (CHCl3) and in the solid state at 365 nm. The fluorescence quantum efficiencies (Φf) range between 0.65 and 0.81 for all oligomers, and remain high for the polymer poly(BFI-OD), with Φf = 0.71 (Table 1). Fig. 3d places BFI-H, 2BFI-H, 3BFI-H, and poly(BFI-OD) on the ICE 1931 chromaticity diagram in solution (CHCl3) and in the solid state. BFI-H shows relatively high solid-state quantum efficiency with Φf = 0.18, despite having strong π–π interactions (Fig. 2a), and the combination of these factors with ICE coordinates of (0.15, 0.07), close to those of the European Broadcasting Union (EBU) blue standard (0.15, 0.06), renders it a good candidate for blue light emitting devices. The fluorescence rate (kf) and non-radiative decay rate (kNR) do not change dramatically upon backbone elongation, with poly(BFI-OD) (kf = 0.44 and kNR = 0.18 ns−1) and 2BFI-H (kf = 0.46 and kNR = 0.11 ns−1) displaying similar values. This is another indication of the backbone rigidity of the polymer (Table 1).

As oligofurans are prone to photooxidation, we examined the ambient photostability of BFI-containing materials in comparison with quaterfuran (4F) measured under the same conditions. While 4F undergoes rapid photobleaching, with less than 50% of the initial absorption remaining after 2 h (Fig. 4, and Fig. S12, ESI), BFI-H and 2BFI-H (having similar λabs values and extinction coefficients as 4F) show negligible bleaching, even after prolonged (24 h) exposure. It is important to note that both 2BFI-H and 4F have similar λabs values and extinction coefficients. Longer oligomers show more significant bleaching, but are considerably more stable than 4F, with less than 10% bleaching after 24 h for poly(BFI-OD). Our previous efforts to obtain polyfurans by chemical polymerization always resulted in materials that undergo rapid photobleaching. To the best of our knowledge, poly(BFI-OD) is the first chemically prepared polyfuran homopolymer stable under such conditions.

image file: c8tc02908c-f4.tif
Fig. 4 Photostability of solutions of 4F, BFI-H, 2BFI-H, 3BFI-H, and poly(BFI-OD) in 1,4-dioxane in ambient light at room temperature.

As the HOMO levels of nF are higher than those of the corresponding nT by approximately 0.35 eV, nF are considered p-type materials. However, high HOMO levels greatly contribute to their ambient instability. The calculated (B3LYP/6-31G(d)) HOMO and LUMO levels of nBFI are depicted in Fig. 5. Unlike the corresponding BTI, which can be considered n-type materials, the HOMO level remains high for BFI, with 3BFI having HOMO and LUMO levels (−5.58 and −2.73 eV respectively) similar to those of sexithiophene (6T). The calculated (PBC/B3LYP/6-31G(d)) band gap of poly(BFI-OD) is 2.38 eV, similar to that of parent polyfurans calculated at the same level of theory. However absolute energy of the HOCO and LUCO levels of poly(BFI-OD), being −5.48 and −3.10 eV respectively, are 0.9 eV lower, which accounts for their high stability (both thermally and photochemically) compared with the corresponding polyfurans. The HOMO levels were also estimated using the oxidation potential measured by cyclic voltammetry (Table 1 and Fig. S34, ESI), which revealed a good match to the calculated values.

image file: c8tc02908c-f5.tif
Fig. 5 (top) Frontier molecular orbitals of 3BFI and (bottom) calculated (B3LYP/6-31G(d)) energy levels of the HOMO (bottom values) and LUMO (top values), and the HOMO–LUMO gaps (middle) of the bifuran-imides discussed (in eV).


In summary, we have introduced a new building unit, bifuran-imide (BFI), and its corresponding oligomers and homopolymer, as a new family of π-conjugated backbones. The polymer poly(BFI-OD) displayed high solubility and has a higher molecular weight compared with its thiophene analogues. The strong quinoid character of nBFI is evident from the short interring bond in their X-ray structures, the vibronic features in their absorption spectra, and their low Stokes shifts. The obtained oligomers and polymers exhibit strong fluorescence with quantum yields reaching up to 81% in solution. The monomer, BFI-H, also demonstrates strong blue emission in the solid state. Compared with their parent oligofurans, bifuran-imides are significantly more stable under ambient conditions. Overall, this study establishes BFI as a stable furan building unit for organic electronics.

Conflicts of interest

There are no conflicts to declare.


The work is supported by the German-Israeli Foundation for Scientific Research and Development. We thank Dr K. S. N. Raju, for his assistance in GPC measurements.

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Electronic supplementary information (ESI) available: Experimental procedures, computational and electrochemistry details, NMR spectra, CIF files and additional data. CCDC 1839512–1839517. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8tc02908c

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