Molecular Tailoring of New Thieno(bis)imide-Based Semiconductors

Article pubs.acs.org/cm

Molecular Tailoring of New Thieno(bis)imide-Based Semiconductors for Single Layer Ambipolar Light Emitting Transistors Manuela Melucci,*,† Laura Favaretto,† Massimo Zambianchi,† Margherita Durso,† Massimo Gazzano,† Alberto Zanelli,† Magda Monari,‡ Maria G. Lobello,§ Filippo De Angelis,§ Viviana Biondo,∥ Gianluca Generali,∥ Stefano Troisi,⊥ Wouter Koopman,⊥ Stefano Toffanin,⊥ Raffaella Capelli,*,∥,⊥ and Michele Muccini*,∥,⊥ †

Istituto per la Sintesi Organica e la Fotoreattività (CNR-ISOF), Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy ‡ Dipartimento di Chimica, ‘G. Ciamician’, via Selmi 2, 40138 Bologna, Italy § Istituto di Scienze e Tecnologie Molecolari (CNR-ISTM), Consiglio Nazionale delle Ricerche, via Elce di Sotto 8, 06123 Perugia, Italy ∥ E.T.C. s.r.l., via Gobetti 101, 40129 Bologna, Italy ⊥ Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN), Consiglio Nazionale delle Ricerche, via Gobetti 101, 40129 Bologna, Italy S Supporting Information *

ABSTRACT: Organic molecular semiconductors are key components for a new generation of low cost, flexible, and large area electronic devices. In particular, ambipolar semiconductors endowed with electroluminescent properties have the potential to enable a photonic field-effect technology platform, whose key building blocks are the emerging organic light-emitting transistor (OLET) devices. To this aim, the design of innovative molecular configurations combining effective electrical and optical properties in the solid state is highly desirable. Here, we investigate the effect of the insertion of a thieno(bis)imide (TBI) moiety as end group in highly performing unipolar oligothiophene semiconductors on the packing, electrical, and optoelectronic properties of the resulting materials. We show that, regardless of the HOMO−LUMO energy, orbital distribution, and molecular packing pattern, a TBI end moiety switches unipolar and nonemissive oligothiophene semiconductors to ambipolar and electroluminescent materials. Remarkably, the newly developed materials enabled the fabrication of single layer molecular ambipolar OLETs with optical power comparable to that of the equivalent polymeric single layer devices. KEYWORDS: organic semiconductors, ambipolarity, electroluminescence, oligothiophenes, organic light emitting transistors

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INTRODUCTION Small molecule organic semiconductors are currently a matter of intense research for use in charge transport-based devices spanning from field-effect transistors1 to solar cells.2 Despite the numerous structures realized so far (i.e., of different size, substitution, shape, etc.),1d−f materials combining charge transport and efficient electroluminescence are still a synthetic challenge,3a as these properties typically exclude one another in the solid state. 3b−e In particular, materials combining ambipolarity and electroluminescence would be of great interest to realize single layer molecular ambipolar lightemitting transistors (OLETs)4 that are a new class of highly integrated organic devices, which combine the light emission capability with the switching function of an organic field effect transistor (OFET).5 While the practical use of light emitting diodes, OLEDs, e.g., in active matrix electroluminescent displays, requires transistors to control their luminance, in OLETs the optoelectronic characteristics are intrinsically controlled without the use of any additional devices. Moreover, © 2013 American Chemical Society

differently from OLEDs, in ambipolar OLETs the emitting zone can be placed within the transistor channel to be optically decoupled from the metal electrodes, which are one of the main sources of optical losses in electroluminescent devices. Although, multilayer geometries having several function-specific layers have been successfully developed,4a,h,6 ambipolar OLETs based on a single semiconducting and emissive material are more attractive for both their simplified architecture and processing cost issues. Solution processed poly(9,9-dioctylfluorenealt-benzothiadiazole) (F8BT) has recently led to high performance ambipolar single layer polymer OLETs.7 However, molecular materials offer the advantages of higher structure and purity reproducibility from batch to batch, better control of the molecular order and packing in thin films, and higher field-effect charge Received: October 5, 2012 Revised: February 11, 2013 Published: February 11, 2013 668

dx.doi.org/10.1021/cm303224a | Chem. Mater. 2013, 25, 668−676

Chemistry of Materials

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Scheme 1. Molecular Structure of the Newly Developed Non-Symmetric TBI Ended Quaterthiophenes and of the Reference Unipolar Oligothiophene Semiconductors (HT4H, FT4F)

mobilities in OFETs.8 Among ambipolar molecular materials (that are still poorly developed),1d,9 only tetracene,10 rubrene, and some phenyl (BSBP)11 or phenyl-thiophene co-oligomers (DHCO4T,12 BP1T,13 BP3T,14 TPTPT15) are also electroluminescent and have allowed the fabrication of working ambipolar OLETs based on a single material, the best performing ones being based on single crystals rather than on the more technologically attractive thin films.6b,11,14 Noteworthy, in single crystal OLETs the application of a voltage of up to 200 V at the drain−source electrodes (VDS) is needed4,5 in order to achieve a detectable electroluminescence signal. It should be noted that in OLETs the drain−source voltage determines the electrical power dissipation of the device,10a and thus low VDS values are highly desirable. Therefore, new design strategies to achieve molecular ambipolar and electroluminescent semiconductors are urgently needed for the development of practical OLET devices. We recently reported new thiophene based materials characterized by end substitution with two strong electronwithdrawing thieno(bis)imide (TBI, N) moieties.16 In particular, NT4N (Scheme 1) exhibited ambipolar electrical behavior with major n-type contribution to charge transport and weak light emission. Here, we report on a significant improvement of the electrical and electroluminescent characteristics of this material achieved by optimizing the thin film processing conditions that allows achieving state-of-the-art optical power for single layer OLETs. Moreover, starting from this first case, we demonstrate that the insertion of only one TBI moiety as oligothiophene end17 is a valuable strategy to achieve materials combining ambipolar charge transport and electroluminescence. For this study, we consider dihexylquaterthiophene (HT4H, Scheme 1)18 and perfluorohexylquaterthiophene (FT4F, Scheme 1)19 that are, respectively, the most successful p-type and n-type (FT4F) semiconductors for multilayer molecular OLET fabrication,4a and we replace one alkyl/perfluoroalkyl chain by a TBI group (Scheme 1, HT4N, FT4N). T4N (Scheme 1) having an unsubstituted thienyl end ring opposite to the TBI end was also realized for comparison. In-depth analysis of the structure−property relationship for the newly synthesized materials reveals TBI end insertion as a valuable design approach to switch unipolar and nonelectroluminescent thiophene-based materials, to ambipolar and electroluminescent ones. Moreover, by using the newly developed TBI-based molecules we realize single layer ambipolar OLETs with optical

power comparable to that of state-of-the-art single layer polymeric OLET devices.7

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RESULTS AND DISCUSSION Synthesis. The target compounds were prepared by a Stille coupling reaction based sequence starting from the brominated TBI building block in 55%, 68%, and 74% yield, respectively (Scheme 2).

Scheme 2. Synthetic Route to Oligomers T4N, HT4N, and FT4Na

a

(i) Stille coupling: toluene, reflux temp., in situ Pd(AsPh3)4, (ii) NBS, CH2Cl2/acetic acid.

The newly synthesized TBIs were highly soluble in common organic solvents, this allowing easy purification through conventional chromatography and crystallization methods. Thermal characterization by differential scanning calorimetry (DSC, Figure SI_1−3, Supporting Information) and hot stage polarized optical microscopy (POM, Figure SI_4, Supporting Information) revealed thermotropic liquid crystalline properties at temperatures below 200 °C for all the newly synthesized compounds. Optical and Electrochemical Properties. UV−vis absorption spectra in CH2Cl2 diluted solution (Figure 1) display structured spectra with maximum peaks at similar energetic positions (440−449 nm) regardless of the type of ωend substituent, in good agreement with the calculated values (Table 1). The photoluminescence spectra (PL, Figure 1) are broad, almost unstructured, and strongly red-shifted with respect to the absorption spectra. In vacuum-sublimed thin films (Figure 1, right) the absorption spectra are blue-shifted with respect to the solution ones with a convoluted profile exhibiting at least two absorption peaks suggesting the possible formation of aggregates. 669



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Figure 1. Absorption and emission spectra of (a) T4N, (b) HT4N, and (c) FT4N compounds in CH2Cl2 solution (left) and vacuum sublimed film (30 nm thick, right).

Table 1. Optical and Cyclic Voltammetries Data for TBI Compounds in Scheme 1 comp.

λabsa (nm) (theor)

T4N HT4N FT4N NT4N HT4H

441 (442) 447 (450) 433 (437) 449 (450) 406d

FT4F

417d

λPLa (nm) 606 631 580 572 470, 500, 535d 466, 500, 529d

Φa (%)

Egopt (eV)

8 17 11 13 19

2.81 2.77 2.83 2.76 3.05

16

2.97

λabs filmb (nm) 344 340 361 430 267, 337, 456 350, 413, 450

λPL filmb (nm)

Φ filmb (%)

HOMOc (eV) (theor)

LUMOc (eV) (theor)

Egec (eV)

−3.47 (−2.43) −3.42 (−2.31) −3.42 (−2.47) −3.47 (−2.62) −2.40f

2.27 2.31 2.57 2.53 2.87

−3.30f

2.88

603 584/611 580 603 523/562e

1 1.2 1.8 1.5 0.6

−5.74 −5.73 −5.99 −6.00 −5.5f

525/563/614e

7

−6.2f

(−5.25) (−5.14) (−5.43) (−5.51)

In CH2Cl2, 10−5 M. b30 nm thick film. cEHOMO = e(4.68 − E°ox); ELUMO = e(4.68 − E°red).20,21 dIn o-chlorobenzene. eSpectra in Figures SI_5 and SI_6, Supporting Information. fFrom ref 4a.

a

The thin film PL spectrum of FT4N shows almost identical features to that in solution, while a blue-shifted maximum emission wavelength was found for HT4N and T4N with respect to the solution. HT4N shows a shoulder before the emission maximum resembling the structured spectra observed for NT4N, FT4F, and HT4H compounds (Figure 6e and Figures SI_5 and SI_6, Supporting Information, respectively) and different from those of T4N and FT4N. Moreover, HT4N emission is slightly red-shifted with respect to that of T4N and FT4N. These features suggest the role of the hexyl chain in promoting H-motif-like packing. The PL quantum yield values in solution are in the range 8−17%, i.e., slightly lower those of HT4H and FT4F. On the other hand, the quantum yield of

thin films was higher than that of HT4H (1−1.5% vs 0.6%) but markedly lower than that of FT4F (∼7%). Cyclic voltammetry (CV) curves of compounds T4N, HT4N, and FT4N are shown in Figure 2. Compound T4N shows a quasi-reversible reduction wave at E°red = −1.22 V, related to the TBI moiety, and an oxidation wave at E°ox = 1.10 V whose current is enhanced by the electrochemical−chemical−electrochemical kinetics of the dimerization at the terminal thiophene group.22 The reverse wave is split in two current peaks, the lower one (1.05 V) due to the reduction of the radical cation, and the sharper one (0.73 V) to the reduction of the dimer. 670



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calculations are in good agreement with experimental absorption maxima, allowing the assignment of the main optical transition to a HOMO → LUMO one. Single Crystal and Thin Films Morphology. The crystal structures of compounds T4N and FT4N are shown in Figure 4. The two structures have in common a nearly planar backbone (Figure 4a,b). In the crystal packing of T4N (Figure 4c), the molecules are aligned in an antiparallel way with respect to each other with the long molecular axis oriented along the a axis and adopt a herringbone-like arrangement with slipped π−π stacking (interplanar distance ca. 3.39 Å, Figure 4e). In FT4N one of the two independent molecules shows the usual anti−anti−anti orientation of the S atoms whereas the second one exhibits an uncommon syn−anti−anti conformation (Figure 4d). In the crystal packing of FT4N the two conformers are aligned face-to-face with the long molecular axis almost parallel to the diagonal of the ac plane and establish slipped π stackings (slip distance between two parallel molecules along the long molecular axis ≅ 5.10 Å, interplanar distance 3.48 Å). Looking down the long molecular axis a herringbone-like pattern is noticed (Figure 4f). Intermolecular C−H···O and C−H···F interactions connect the π−π stacks. Interestingly, while compounds T4N and FT4N show herringbone-like packing, as generally observed for linear alkyl end substituted oligothiophenes,23 NT4N having two TBI end moieties showed a slipped π−π-stacking packing motif (interplanar distance ca. 3.51 Å).16a The XRD of the vacuum evaporated films (Figures SI_7−9, Supporting Information) suggests a molecular organization with the long molecular axis almost perpendicular to the surface as generally observed for oligothiophenes. The morphology of the same films was investigated by atomic force microscopy (AFM, Figure SI_10, Supporting Information). All compounds show highly three-dimensional and poorly interconnected fiberlike crystal structures. Underneath these structures, a more uniform terraced morphology was visible, this suggesting a good suitability of these compounds for thin film transistors applications. Electrical Characterization and Molecular Structure− Charge Transport Relationships. Electrical characterization in field-effect configuration (sketch of the device geometry in Figure 5a) was performed to analyze the charge transport properties in thin films of the newly synthesized molecules. All TBI compounds displayed ambipolar charge transport. Indeed, all the electrical transfer characteristics, obtained by sweeping the gate voltage at fixed drain−source bias, show the v-shaped behavior typical of ambipolar transistors (Figure 5b−e). Data in Table 2 reveal major n-type behavior for NT4N and FT4N in accordance to the unipolar n-type behavior observed for oligothiophene bearing electron-withdrawing ends such as FT4F. On the contrary major hole charge transport was observed for HT4N and T4N in accordance to the unipolar ptype-only behavior of unsubstituted quaterthiophene and HT4H. The reason of such inversion is difficult to explain in

Figure 2. CVs of 1.1 mmol L−1 compounds T4N (a), HT4N (b) and FT4N (c) at 100 mV/s in CH2Cl2, 0.1 mol L−1 (C4H9)4NClO4.

The voltammogram of HT4N shows two oxidation waves at E°ox1 = 1.05 V and E°ox2 = 1.28 V. On the other hand, it shows a quasi-reversible reduction wave at E°red1 = −1.24 V and an irreversible reduction wave at E1/2red2 = −1.75 V due to the oligothiophene moiety (Figure 2). Finally, the quasi-reversible reduction of compound FT4N occurs at the same potential of compound HT4N, with the irreversible one at E1/2red2 = −1.58 V that is 0.17 V less negative than that of HT4N. This potential shift is due to the electronwithdrawing effect of the perfluorehexyl chain opposite to the TBI, which also influences the oxidation potential. Indeed, the oxidation potential shifts to more positive potentials, i.e., E°ox1 = 1.29 V and E°ox2 = 1.63 V, enhancing the energy gap of the frontier orbitals (Eg = 2.57 eV vs 2.31 eV). Overall, the electrochemical data show that the number of end TBI units has a minor effect on the LUMO energy value (Figure 2). Indeed, compounds T4N, HT4N, and FT4N have LUMO values similar to that of NT4N. On the contrary, the HOMO energy values of T4N and HT4N are higher than those of NT4N and FT4N. Finally, compound NT4N has a HOMO energy value between those of FT4F and HT4T (−6.00 eV, −6.2 eV, and −5.5 eV, respectively). DFT calculations22 were carried out to gain insights on the frontier orbital distribution. They show that in all TBI-based compounds the HOMO is delocalized over the entire molecule (Figure 3a−d, bottom) while the LUMO is delocalized along the whole molecule only for symmetric NT4N (Figure 3d, top). On the contrary, for nonsymmetric compounds the LUMO is mainly localized on the TBI moiety independently on the type of substitution opposite to the TBI, explaining why the LUMO energy level is insensitive to the type of substitution. Therefore, electrochemical and theoretical data highlight that in TBI endsubstituted compounds it is possible to tune the HOMO level maintaining an unaltered LUMO level simply by changing the type of end substituent opposite to TBI. TDDFT excited state

Figure 3. Isodensity plots of the HOMOs (down) and LUMOs (top) for NT4N, T4N, HT4N, and FT4N. 671



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Figure 4. Crystal structure of T4N (a) and FT4N (b). (c, d) View down the b axis of the crystal packing of T4N (top) and FT4N (down), respectively. (e, f) View along the long molecular axis of T4N (top) and FT4N (down), respectively, showing the herringbone-like packing. The H atoms, perfluorohexyl, and n-butyl chains in FT4N have been removed for clarity.

Table 2. Electrical Parameters for Compounds in Scheme 1 Measured under the Same Experimental Conditionsa comp. T4N HT4N FT4N NT4N HT4H FT4F a

μh (cm2 V−1 s−1) 1.2 × 3.1 × 1.4 × 7.0 × 0.12

10−4 10−3 10−5 10−3

VTP (V)

Ion/ Ioff

μe (cm2 V−1 s−1)

VTN (V)

OLET EP (nW)

−20.7 −17.5 −39.5 −26.7 −15.0

103 104 104 105 106 106

2 × 10−6 2.8 × 10−5 0,011 0.55

10.0 10.9 19.1 10.0

b 8 50 180

0.48

38

Film thickness 30 nm. bValue below the detection limit.

calculation results (reported in Supporting Information, section 6) do not justify the observed differences in charge mobility values. However, our evidence clearly highlights a crucial role of the molecular packing on the electrical properties of these materials. The direct comparison between NT4N and FT4N is strongly indicative. Indeed, if the HOMO−LUMO energy levels (see Table 1) and the thin film morphology (see AFM, Figure SI_10, Supporting Information) of these two compounds are almost identical, their charge mobilities are markedly different (two and one order of magnitude for the p- and n-type mobilities, respectively). A significant difference between NT4N and FT4N is their crystal packing, i.e., herringbone-like for FT4N and π-stack-like for NT4N. The latter packing type has been already related to enhanced charge transport capability in molecular materials,23 and indeed NT4N shows the best p- and n-type charge mobility with respect to all nonsymmetric compounds. OLETs Realization and Characterization. During the electrical characterization of OFETs, intense electroluminescence emission from the devices was also measured, highlighting the potential of these ambipolar materials for single layer ambipolar OLETs devices. In Figure 6a−c we report the optoelectronic transfer curves of single layer OLET devices based, respectively, on HT4N, FT4N, and NT4N compounds. The device architecture is invariant for all devices (see Figure 5a and Experimental Methods) to enable a direct comparison of the electrical and optoelectronic properties. The peaked electroluminescence corresponding to the maximum electron−hole balance (VGS ∼ 1/2VDS) shown in Figure 6 is a clear fingerprint of the light generation inside the OLET channel, occurring at the spatial position where electron and hole currents meet. Indeed, in an unipolar OLET, in which only one type of charge is effectively transported through the

Figure 5. (a) Sketch of the device architecture used in this work. Electrical characteristics for T4N (b), HT4N (c), FT4N (d), and NT4N (e) based OFETs. The left graphs show the OFETs output curves while in the right graphs are reported the OFETs transfer curves.

terms of different HOMO−LUMO energy levels, being their values very close one another. Calculations carried out by using the Markus approach (which is currently the state-of-the-art approach to explain the charge transport in organic materials) did not help us to explain the observed trend. Indeed, the 672



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Figure 6. Opto-electronic transfer curves of an OLET device based on (a) FT4N, (b) HT4N, and (c) NT4N molecules. The blue lines represent the measured OLETs drain current, and the purple lines are the emitted photons power. (d) EQE efficiency curve for NT4N based OLET. The blue line is the drain current, and the purple line is the EQE. (e) Electroluminescence spectrum of OLET based on NT4N (red line) compared to the PL spectrum (blue line). (f) Optical microscope image of a working FT4N OLET. (g) High resolution inverted microscope image of a working NT4N OLET highlighting the emissive stripe motion within the device channel by decreasing the gate voltage. The device channel size is 70 μm.

a video file taken by a high-resolution inverted microscope is in the Supporting Information. The emissive stripe within the transistor channel is closer to the drain electrode as a consequence of the difference between hole and electron mobilities. However, the electroluminescence generation area is well separated from the drain edge, preventing optical coupling of the emitted light with the metal electrodes. Taking into account the specific geometry used for our devices (same channel width, PMMA dielectric layer, and gold electrodes) and comparing it with that of F8BT-polymer based OLETs (state of the art ambipolar OLET),7b it emerges that the total light power of our single layer molecular OLETs

channel, the light is generated at the drain electrode following the minority carrier injection and the electroluminescence intensity increases monotonically with the applied V GS potential. Figure 6d shows the plot of the external quantum efficiency for NT4N showing the highest device efficiency. The maximum EQE relevant value is about 0.2%, at 50 V (where the EL intensity is still significant). In Figure 6e the EL spectrum compared to the PL of NT4N is shown. The spectra are very similar, confirming that the EL emission originates from NT4N. Images of working FT4N and NT4N based devices, showing the typical ambipolar OLET emission inside the channel, controlled by the gate potential, are reported in Figure 6f,g, and 673



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compares favorably with it.7b Even if the comparison of the maximum observed EQE is still unfavorable,7b it should be noted that our devices have not yet been optimized to maximize the light outcoupling. Moreover, the intrinsic quantum efficiency of our materials in film is much lower than that of the F8BT polymer, and the hole and electron charge mobility are still quite unbalanced. Nevertheless, these results demonstrate the suitability of the small molecules approach for the development of truly ambipolar single layer OLET devices, thereby widening the scope of the integrated photonic field-effect technology platform.

apparatus where the melting process was observed with the aid of a microscope. General Stille Coupling Procedure for the Synthesis of Compounds T4N, HT4N, and FT4N. To a refluxing toluene solution of in situ prepared Pd(AsPh3)4 (8 mol %) under N2 atmosphere and bromo-TBI, the appropriate tributylstannyl derivative (compounds 5− 7, Scheme 1) diluted in toluene was added dropwise. The solution was refluxed for 8 h, then the solvent was removed under vacuum, and the crude purified by chromatography on silica gel followed by crystallization. 2-(2,2′:5′,2″-Terthiophene-5-yl)-5-butyl-5H-thieno[3,2-c]pyrrole4,6-dione, T4N. Flash chromatography on silica, eluent: DCM, then crystallization from toluene. Orange powder (55% yield). Mp 203° (K → LC), 226 °C. MS (70 eV, EI): m/z 455 (M + 1). 1H NMR (CD2Cl2, TMS/ppm): δ 7.34 (s, 1H), 7.30 (m, 2H), 7.25 (dd, 3J = 3.6 Hz, 4J = 1.2 Hz, 1H), 7.19 (d, 3J = 3.6 Hz, 1H), 7.18 (d, 3J = 4.0 Hz, 1H), 7.15 (d, 3J = 4.0 Hz, 1H), 7.07 (dd, 3J = 4.8 Hz, 3J = 4.8 1H), 3.60 (t, 2H), 1.63 (m, 2H), 1.36 (m, 2H), 0.96 (t, 3H). 13C NMR (CDCl3, TMS/ppm): δ 163.9, 162.8, 149.8, 145.3, 139.0, 137.5, 137.1, 136.7, 134.7, 133.6, 128.0, 126.8, 125.3, 125.0, 124.5, 124.4, 124.1, 116.3, 38.3, 30.8, 20.0, 13.6. Anal. Calcd for C22H17NO2S4 (455.64): C, 57.99; H, 3.76. Found: C, 57.93; H, 3.81. 2-(5″-Hexyl-2,2′:5′,2″-terthiophene-5-yl)-5-butyl-5H-thieno[3,2c]pyrrole-4,6-dione, HT4N. Flash chromatography on silica gel, eluent: pet. eth./ethyl acetate 95:5 to 0:100. Orange powder (68% yield). Mp 178° (K → LC), 260°. MS (70 eV, EI): m/z 539 (M + 1). 1 H NMR (CDCl3, TMS/ppm): δ 7.28 (s, 1H), 7.22 (d, 3J = 3.6 Hz, 1H), 7.10 (d, 3J = 3.6 Hz, 1H), 7.09 (d, 3J = 4.0 Hz, 1H), 7.01 (d, 3J = 4.0 Hz, 1H), 7.00 (d, 3J = 3.6 Hz, 1H), 7.69 (d, 3J = 3.6 Hz, 1H), 3.60 (t, 2H), 2.80 (t, 2H), 1.66 (m, 4H), 1.35 (m, 8H), 0.92 (m, 6H). 13C NMR (CDCl3, TMS/ppm): δ 163.9, 162.8, 149.9, 146.2, 145.3, 139.2, 138.2, 137.0, 134.0, 133.3, 126.7, 125.2, 124.9, 124.2, 123.8, 123.7,116.2, 38.3, 31.5, 30.9, 30.2, 28.7, 22.6, 20.0, 14.1, 13.6. Anal. Calcd for C28H29NO2S4 (539.80): C, 62.30; H, 5.42. Found: C, 62.24; H, 5.49. 2-(5″-(Perfluorohexyl)-2,2′:5′,2″-terthiophene-5-yl)-5-butyl-5Hthieno[3,2-c]pyrrole-4,6-dione, FT4N. Flash chromatography on silica, eluent: DCM, then crystallization from toluene. Orange powder (74% yield). Mp 182° (K → LC), 320 °C (LC → I). MS (70 eV, EI): m/z 773 (M + 1). 1H NMR (CDCl3, TMS/ppm): δ 7.36 (d, 3J = 3.6 Hz 1 H), 7.31 (s, 1H), 7.25 (d, 3J = 4.4 Hz, 1H), 7.17 (m, 4H), 3.61 (t, 2H), 1.63 (m, 2H), 1.36 (m, 2H), 0.95 (t, 3H). 13C NMR (CDCl3, TMS/ppm): δ 163.9, 162.7, 149.5, 145.3, 141.8, 138.3, 137.5, 136.7, 135.1, 134.3, 131.1, 126.8, 126.1, 125.4, 125.0, 123.7, 116.5, 38.4, 30.8, 20.0, 13.6. 19F NMR (CDCl3, C6H5F/ppm): δ −80.2 (t, 3F), −100.8 (m, 2F), −121.0 (m, 4F), −122.3 (broad singlet, 2F), −125.6 (m, 2F). Anal. Calcd for C28H16F13NO2S4 (773.67): C, 43.47; H, 2.08. Found: C, 43.52; H, 2.02. 2-([2,2′-Bithiophen]-5-yl)-5-butyl-4H-thieno[2,3-c]pyrrole4,6(5H)-dione, 3. To a refluxing toluene solution (8 mL) of compound 1 (82 mg, 0.28 mmol) and in situ-prepared Pd(AsPh3)4 (8 mol %, 11 mg of Pd2dba3 and 27 mg of AsPh3) under N2 atmosphere was added dropwise 5-(tributylstannyl)-2,2′-bithiophene, 2 (135 mg, 0.3 mmol) in toluene (0.5 mL). The solution was refluxed for 6 h, then the solvent was removed under vacuum, and the crude product was purified by flash chromatography on silica gel by using a solution of pet. eth./DCM/AcOEt = 90:5:5 as eluent. Compound 3 was isolated as an orange powder (84 mg, yield 80%) and used for the following step without further purification. MS (70 eV, EI): m/z 373 (M + 1). 1H NMR (CDCl3, TMS/ppm): δ 7.29 (s, 1H), 7.28 (dd, 3J = 5.2 Hz, 4J = 1.2, 1H), 7.23 (m, 2H), 7.12 (d, 3J = 3,6 Hz 1H), 7.05 (dd, 3 J = 5.2 Hz, 3J = 4.8, 1H), 3.60 (t, 2H), 1.63 (m, 2H), 1.36 (m, 2H), 0.95 (t, 3H). 13C NMR (CDCl3, TMS/ppm): δ 163.9, 162.8, 149.9, 145.2, 139.3, 137.1, 136.1, 133.6, 128.1, 126.7,125.5, 124.7,124.5, 116.3, 38.3, 30.8, 20.0, 13.6. 2-(5′-Bromo-[2,2′-bithiophen]-5-yl)-5-butyl-4H-thieno[2,3-c]pyrrole-4,6(5H)-dione, 4. Compound 3 (90 mg, 0.24 mmol) was dissolved in 8 mL of a 1:1 mixture of dichloromethane and acetic acid solution. NBS (50 mg, 0.28 mmol) was added at 0 °C, and the reaction mixture was stirred at room temperature overnight in

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CONCLUSIONS The insertion of the TBI group into linear thiophene oligomers promotes ambipolar charge transport and electroluminescence in the resulting semiconductors. When in the p-type semiconductor HT4H a hexyl group is replaced by a TBI moiety, the resulting material becomes ambipolar, with major p-type character, and electroluminescent. Analogously, ambipolarity, but with major n-type contribution, and consequent electroluminescence are observed for NT4F derived from the n-type FT4F. Our data show that substitution with one TBI end moiety can be exploited for the following purposes: (i) tuning the HOMO energy values of oligothiophenes while maintaining unchanged the LUMO energy, (ii) localizing the LUMO distribution on the oligomer periphery, (iii) mastering the charge transport (ambipolar with predefined major p- or n-type behavior) by a proper TBI opposite end substitution, and (iv) enabling thin film electroluminescence in combination with ambipolar charge transport. Indeed, end substitution by an electron-donating moiety promotes hole charge transport (i.e., T4N or HT4N) while an electron-withdrawing moiety as TBI opposite end favors electron charge transport (i.e., FT4N). To the best of our knowledge this class of molecular materials is the first one reported so far showing such features. Given the stringent need for electroluminescent molecular semiconductors, these results are relevant for a deeper understanding of the structure−property relationship in such materials, which is of great relevance and still a matter of debate. Remarkably, by using the newly developed ambipolar and electroluminescent TBI materials we realized thin film single layer ambipolar OLETs with optical power comparable to that of the equivalent polymeric devices. These results open the perspective of enabling high power emitting devices exploiting the high mobility values typical of small molecules. The inclusion of fused heteroaryl cores is currently under investigation in our lab to enhance and balance the electron and hole charge mobility values as well as to improve the electroluminescence properties.

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EXPERIMENTAL METHODS

Synthesis. Compounds 1,16 6,24 and 725 were prepared according to already reported procedures. Compounds 2 and 5 are commercially available. All 1H, 13C, and 19F NMR spectra were recorded with a Varian Mercury 400 spectrometer operating at 400 MHz (1H). Chemical shifts were calibrated using the internal CDCl3, acetone-d6, or CD2Cl2 resonance which were referenced to TMS. For 19F NMR spectra, fluorobenzene was added as the internal standard. Mass spectra were collected on a ion trap Finningan Mat GCQ spectrometer operating in electron impact (EI) ionization mode. Each sample was introduced to the ion source region of GCQ via a direct exposure probe (DEP). Melting points (uncorrected) were determined on a “hot-stage” 674



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sublimation. The channel length (L) is equal to 70 μm, while the channel width (W) is equal to 12 mm. All opto-electronic measurements were carried out in a MBraun nitrogen glovebox using a standard SUSS Probe Station equipped with a Hamamatsu photodiode for light detection. For the EQE measurements the emitted photons were collected through an Avantes AVA-SPHERE 50-IRR integrating sphere and measured by an Avantes AVA-SPEC 2048 calibrated spectrometer. The EQE was calculated directly as the ratio between the total emitted photons and the charge flow that formed the drain current. Electroluminescence spectra of devices biased using a B1500A Agilent semiconductor device analyzer were acquired by a CS200 Konica Minolta spectro-radiometer. Images of Figure 6f have been collected using an Nikon optical microscope, while the zoom of the emissive stripe shift reported in Figure 6g has been collected with a Nikon inverted microscope implementing a 60× magnification objectives and a Hamamatsu high-resolution digital camera.

darkness. The yellow solution so obtained was then diluted with 10 mL of water, extracted with dichloromethane, and washed with 10% NaHCO3 and water. The organic phase was dried over anhydrous sodium sulfate and evaporated and the crude product purified by flash chromatography on silica gel by using pet. eth./AcOEt 94:6 to 80:20 as eluent. Compound 4 was isolated as an orange powder (97% yield). Mp 153 °C, MS (70 eV, EI): m/z 451453 (M + 1). 1H NMR (CDCl3, TMS/ppm): δ 7.29 (s, 1H), 7.21 (d, 3J = 4.0 Hz, 1H), 7.05 (d, 3J = 3.6 Hz, 1H), 7.00 (d, 3J = 4.0 Hz, 1H), 6.96 (d, 3J = 4.0 Hz, 1H), 3.60 (t, 2H), 1.62 (m, 2H), 1.36 (m, 2H), 0.95 (t, 3H). 13C NMR (CDCl3, TMS/ppm): δ 163.9, 162.7, 149.5, 145.2, 138.1, 137.6, 137.4, 134.0, 130.9, 126.7, 124.8, 124.7, 116.5, 112.3, 38.3, 30.8, 20.0, 13.6. Optical Characterization. Photoluminescence spectra were collected in transmission mode by a Hamamatsu multichannel optical analyzer (PMA11) after excitation of the device active area with the 375 nm emission of an Oxius laser diode. Quantum yield measurements of both thin films and solutions were carried out inside a 6 in. integrating sphere optically coupled to a Hamamatsu PMA optical multichannel analyzer by exciting photoluminescence at different wavelengths (325 nm, 375 nm, 440 nm) according to absorption maxima of the samples. Cyclic Voltammetries (CVs). CV measurements have been performed at room temperature, after Ar purging, with an AMEL 5000 electrochemical system in CH2Cl2 (Carlo Erba RPE, distilled over anhidrous P2O5 and stored under Ar pressure) and 0.1 M (C4H9)4NClO4 (Fluka, puriss. crystallized from methanol and vacuumdried). The electrochemical cell was in three compartment shape, with the Pt semisphere electrode (diameter 2 mm), Pt wire counter electrode, and aqueous KCl saturated calomel electrode (SCE = 0.47 V vs ferrocene/ferricinium).26 The concentrations of the compounds were 1.1 mmol L‑1. Single Crystal XRD. X-ray data were collected using a Bruker SMART Apex II CCD area detector diffractometer with Mo Kα (λ = 0.71073 Å) as the incident radiation. Structures were solved using SIR 97 and were refined by full-matrix least-squares on Fo2 using SHELXL97. Crystal Data for T4N. C22H17NO2S4, M = 455.61, monoclinic, P21/ c, a = 23.489(3), b = 5.6217(8), c = 15.651(2) Å, β = 96.705(2), V = 2052.6(5) Å3, Z = 4, Dc = 1.474 g cm−3, μ = 0.483 mm−1, T = 293 K, λ(Mo Kα) = 0.71073 Å, data/parameters = 4941/262, converging to R1 = 0.0530, wR2 = 0.1536 (on 4941 I > 2σ(I) observed data); R1 = 0.0653, wR2 = 0.1619 (all data), residual electron density 1.022 e Å3. Crystal Data for FT4N. C28H16F13NO2S4, M = 773.66, monoclinic, C2/c, a = 109.92(2), b = 5.7796(11), c = 19.480(4) Å, β = 97.866(3)°, V = 12259.(4) Å3, Z = 16, Dc = 1.677g cm−3, μ = 0.419 mm−1, T = 293 K, λ(Mo−Kα) = 0.71073 Å, data/parameters =10695/874, converging to R1 = 0.1401, wR2 = 0.3803 (on 4941 I >2σ(I) observed data); R1 = 0.2049, wR2 = 0.4287 (all data), residual electron density: 1.299 e Å3. Theoretical Calculations. The geometry of compounds was optimized in their ground, oxidized, and reduced state in vacuum and CH2Cl2 using B3LYP27 exchange-correlation functional and 6-31g* basis set,28 using the Gaussian 0329 (G03) program package. All the single points to the computed the HOMO and LUMO bandwidths are performed in a vacuum using B3LYP exchange-correlation functional and 6-31g* basis set. TDDFT calculations of the lowest singlet−singlet excitations were performed for all the species in vacuum and CH3CN solution using MPW1K functional, which incorporates a fixed amount of Hartree− Fock exchange, (ca. 42%) and represents a practical and efficient choice to describe the excited state of this class of organic compounds30 and the 6-31g* basis set, using the CPCM31 nonequilibrium version as implemented in G03. To simulate the optical spectra, the eight lowest spin-allowed singlet−singlet transitions were computed on the ground state geometry. OFET/OLETs Fabrication. The device platform consists of 420 thick PMMA dielectric film, on top of a glass/ITO gate electrode, and gold made drain and source top contacts. The PMMA film was spin coated, while the gold electrodes have been evaporated by high vacuum sublimation through a shadow mask process. OLET devices are based on 30 nm thick active organic films, grown by high vacuum

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ASSOCIATED CONTENT

S Supporting Information *

Differential scanning calorimetry (DSC), hot stage polarized microscopy (POM), PL spectra of thin films of HT4H and FT4F, thin film XRD, AFM, oFET fabrication and electrical characterization, theoretical calculations, and video file of a NT4N working OLET taken by high-resolution inverted microscope showing the stripe motion within the device channel (PDF, AVI). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M. Melucci); r.capelli@ bo.ismn.cnr.it (R. Capelli); [email protected] (M. Muccini). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of this work by the Consorzio MIST-ER through project FESR-tecnopolo AMBIMAT, by the EU through project FP7-ICT- 248052 (PHOTO-FET), and by the Italian MSE through project Industria 2015 (ALADIN).

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REFERENCES

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