Cyclopentadithiophene-Based Organic Semiconductors: Effect of

LETTER pubs.acs.org/JPCL

Cyclopentadithiophene-Based Organic Semiconductors: Effect of Fluorinated Substituents on Electrochemical and Charge Transport Properties J. Sreedhar Reddy,† Tejaswini Kale,† Ganapathy Balaji, A. Chandrasekaran, and S. Thayumanavan* Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States

bS Supporting Information ABSTRACT: Thiophene-based semiconductors are often hole conductors that have been converted to electron-transporting materials by incorporation of electron-withdrawing groups at terminal positions, such as fluorinated substituents. This conversion of an otherwise p-type material to n-type material is often attributed to the lowering of the lowest unoccupied molecular orbital (LUMO) energy level due to the increased electron affinity in the molecule. Yet, it is not clear if lowering of LUMO energy level is a sufficient condition for yielding n-type material. Herein, we report small-molecule semiconductors based on cyclopentadithiophene (CPD), which can be orthogonally functionalized at two different positions, which allows us to tune the frontier orbital energy levels. We find that simply lowering the LUMO energy level, without inclusion of fluoro groups, does not result in conversion of the otherwise p-type material to n-type material, whereas incorporation of fluorinated substituents does. This indicates that charge transport behavior is not an exclusive function of the frontier orbital energy levels. SECTION: Electron Transport, Optical and Electronic Devices, Hard Matter

S

olution processable organic semiconductors have generated tremendous interest because of their applicability in organic microelectronics, light-emitting diodes, and photovoltaics.1-6 Both p-type and n-type materials, with commensurate mobilities, are required for effective functioning of many of these devices. Significant progress has been made in the development of materials possessing high charge mobilities with materials based on heteroaromatic molecules, especially thiophenes. Whereas thiophene-based materials are often p-type materials,7,8 it has been shown that the incorporation of fluorinated substituents as terminal functionalities causes these materials to exhibit n-type characteristics.9-15 This behavior has often been attributed to the lowering of LUMO energy level, making electron injection easier and hence imparting n-type character to the molecule.11,13 However, it is not clear whether the factors underlying stabilization of frontier orbital energy levels have correlations to the transport of the injected charges. We were interested in testing this issue with a molecular scaffold that satisfies the following design criteria: (i) changing the hydrocarbon substituent to a fluorocarbon substituent should have minimal effect on the frontier orbital energy level and (ii) the scaffold allows for tuning the frontier orbital energy level independent of the fluorocarbon substituent. For this purpose, we report here the design, syntheses, and charge transport characteristics of solution-processable systems based on cyclopenta[2,1-b:3,4-b0 ]dithiophene. The cyclopenta[2,1-b:3,4-b0 ]dithiophene, referred here as simply cyclopentadithiophene (CPD), unit can be said to be a combination of structural motifs found in fluorenes and oligothiophenes. The r 2011 American Chemical Society

rigid fused ring structure in this molecule lowers the reorganization energy, a parameter that strongly affects the rate of intermolecular charge hopping and therefore the charge carrier mobility in organic semiconductors.16 The CPD unit has two locations where substituents can be incorporated: (a) the R-positions in the thiophenes, where hydrocarbon or fluorocarbon substituents can be incorporated, and (b) the bridgehead position, where substituents can be incorporated to alter significantly the frontier orbital energy levels. In fact, it has been shown previously that variations at these positions in such fused thiophene structures can be used to tune the frontier orbital energy levels.17-19 The molecules that satisfy our design criteria and potentially address the issue in hand are shown in Chart 1. The CPD core is functionalized at the bridgehead position with the electronwithdrawing carbonyl or dicyanomethylene functionality, whereas the R-positions are substituted with either phenyl or pentafluorophenyl moieties. Installation of electron-withdrawing functionalities, such as carbonyl group, at the bridgehead position of the CPD leads to lowering of bandgap, which has been attributed to the stabilization of the quinoid form.20,21 The oxidation potential of cyclopenta[2,1-b:3,4-b0 ]dithiophen-4-one has been reported to be only slightly higher (40-60 mV) than that of bithiophene, indicating that the carbonyl group has only a moderate effect on the energy level of the HOMO.20 A stronger Received: January 26, 2011 Accepted: February 17, 2011 Published: March 03, 2011 648

dx.doi.org/10.1021/jz200126s | J. Phys. Chem. Lett. 2011, 2, 648–654

The Journal of Physical Chemistry Letters

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Chart 1. Molecular Structures of Small Molecule Semiconductors

Scheme 1. Synthesis of Diphenyl Substituted Molecules 1-3 and Crystal Structures of 2 and 3

[2,1-b:3,4-b0 ]dithiophene, 9, which was synthesized following a previously reported procedure.22 This was brominated using N-bromosuccinimide to obtain 2,6-dibromo-4,4-ethylenedioxy-4H-cyclopenta[2,1-b:3,4-b0 ]dithiophene, 10, in 89% yield. The key step in synthesizing the diphenyl-functionalized molecule was the C-C coupling of the R-position of thiophene ring with the phenyl functionality. Suzuki coupling conditions were used to achieve this bond formation. Accordingly, palladium-catalyzed reaction of 10 with phenyl boronic acid yielded 1 in 70% yield (Scheme 1). This was hydrolyzed using hydrochloric acid and acetic acid to obtain 2 in 72% yield. Knoevenagel condensation of 2 with malononitrile yielded 3 in 69% yield. Whenever possible, single-crystal X-ray diffraction was used to confirm the molecular structure, in addition to NMR and mass spectrometry.

electron-withdrawing functionality, the dicyanomethylene functionality, leads to further lowering of the bandgap.17 This is consistent with theoretical calculations comparing the effect of the dicyanomethylene and carbonyl groups on the electronic states of these molecules and their polymers.21 These calculations indicate that the dicyanomethylene group extends the conjugation outside the conjugated bithiophene. We envisaged that with the installation of such strong electron-withdrawing functionalities at the bridgehead position, incorporating fluorinated substituent at the R-positions should have hardly any effect on the position of the LUMO. Thus, this will provide an avenue to delineate the effect of incorporation of fluorinated substituents and lowering of LUMO energy level on charge transport properties of these molecules. To synthesize the diphenyl-substituted molecules 1-3, the common intermediate was 4,4-ethylenedioxy-4H-cyclopenta649

dx.doi.org/10.1021/jz200126s |J. Phys. Chem. Lett. 2011, 2, 648–654


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Scheme 2. Synthesis of Bis(pentafluorophenyl)-Substituted Molecules 4-6 and Crystal Structure of 6

Figure 1. Absorption and emission spectra of (a) 1-3 and (b) 4-6.

To synthesize the bis(pentafluorophenyl)-substituted compounds 4-6, 4,4-ethylenedioxy-4H-cyclopenta[2,1-b:3,4-b0 ]dithiophene was reacted with n-butyl lithium, followed by hexafluorobenzene to obtain 4 in 42% yield (Scheme 2). This was hydrolyzed using hydrochloric acid and acetic acid to obtain 5 in 62% yield. Knoevenagel condensation of 5 with malononitrile yielded 6 in 39% yield. Absorption and emission spectra of phenyl derivatives 1-3 and pentafluorophenyl derivatives 4-6 are shown in Figure 1. All spectra were recorded in dichloromethane to evaluate the effect of structural modifications on their optical properties.

The optical band gap is determined from the onset of absorption spectra. Table 1 summarizes the opto-electronic properties of all compounds in dichloromethane. A red shift in the absorption spectra was observed upon conversion of bridgehead carbon from sp3 to sp2 center. For instance, the S0-S1 transition peak shifted by 144 from 409 nm in the case of 1 to 553 nm in the case of 2. Similarly, a red shift of 109 nm was observed in the case of bis(pentafluorophenyl)-substituted molecules (λmax: 398 nm in case of 4 as compared with 507 nm for 5). Interestingly, the absorption spectra of the dicyanomethylenefunctionalized molecules appear to be significantly blue-shifted with 650



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the most prominent peak appearing at 333 nm in the case of 3 and 330 nm in the case of 6. In donor-acceptor copolymers containing fluorene and 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b0 ]dithiophene units, this phenomenon was attributed to the forbidden nature of the S0-S1 transition arising because of the apparent meta linkage of the dicyanomethylene group and the conjugated backbone of the remainder molecule.23 A similar phenomenon may be responsible for the low oscillator strength of the said transition in the case of the carbonyl-containing molecules 2 and 5. The comparison of absorption spectra of the diphenyl- and bis(pentafluorophenyl)-substituted CPD molecules is also interesting. In all molecules, irrespective of the functional groups at the bridgehead position, a slight blue shift was observed in the absorption maxima upon replacing phenyl groups with pentafluorophenyl groups (Figure 1). This can be attributed to the electron-withdrawing nature of the pentafluorophenyl units, which decreases the donor-acceptor character in the molecules by weakening the donor component. The emission spectra of all molecules were recorded in dichloromethane. Except the dioxolane derivatives 1 and 4, all other molecules were found to be nonemissive. As in case of absorption spectra, a blue shift in emission maxima in the dioxolane derivatives is observed (1: 476 nm, 4: 454 nm) upon replacing phenyl groups with electron-withdrawing pentafluorophenyl groups. The absence of emission properties in ketoneand dicyanomethylene-containing molecules further indicates that the S0-S1 transition, that is, the π-π* transition, is forbidden.23

The electrochemical properties of CPD derivatives 1-3 and 4-6 were investigated using cyclic voltammetry to determine the redox properties of these molecules. The cyclic voltammograms are shown in Figure 2. Dioxolane derivatives (1 and 4) showed one quasi-reversible oxidation peak. Upon introducing the carbonyl group, CPD derivatives (2 and 5) showed one reversible oxidation and one reversible reduction peak. Furthermore, upon introduction of dicyanomethylene functionality, the CPD derivatives (3 and 6) showed one reversible oxidation and two reversible reduction peaks. In the diphenyl-functionalized molecules, upon conversion of dioxolane to carbonyl functionality and further to dicyanomethylene group, the first oxidation potential increased from 0.46 (1) to 0.72 (2) to 0.78 V (3), whereas the first reduction potential decreased from -1.56 (2) to -1.07 V (3), consistent with the increasing electron-withdrawing nature of the bridgehead substituent. Similar trends were observed in the bis(pentafluorophenyl)-substituted molecules. No reduction was observed in dioxolane derivatives (1 and 4) within the electrochemical window of the solvent. The electrochemically derived HOMO and LUMO energy levels and bandgap are summarized in Table 2. The electrochemical band gaps in 2 and 5 were found to be about 0.3 eV higher than their optical energy gap (Table 1). The electrochemically estimated HOMO-LUMO gaps in 3 and 6 were found to be much smaller than the optical energy gaps because the lower energy peak was not observed in the absorption spectra, presumably due to the forbidden π-π* transition. The lower LUMO levels and smaller band gaps in 3 and 6, compared with 2 and 5, respectively, indicate that the highly electron-withdrawing dicyanomethylene functionality has a greater effect on the LUMO energy level and leads to a greater π-electron delocalization due to the apparent donor-acceptor character of the molecules.21 Replacing phenyl groups with the electron-deficient pentafluorophenyl groups at the R-position of the CPD backbone leads to lowered HOMO energy level and interestingly slightly raised LUMO energy level, leading to a higher band gap in the pentafluorophenyl CPD derivatives. In contrast, terminal perfluoro-substituted oligothiophenes have been known to exhibit lowering of both the HOMO and LUMO levels upon replacement of hydrocarbon substituent with fluorocarbon substituent.11,13 This observation supports our initial assertion that the strong electron-withdrawing groups at the bridgehead position would dampen the effect of the substituents at the R-position.

Table 1. Optical Properties of 1-8 in Dichloromethane λmax (nm), compound

[ε ( 104 M-1 cm-1)]

1

409 [5.0]

2 3

313, 353 [8.5], 553 333 [7.2]

4

398 [3.3]

5

309, 347 [7.5], 507

6

330 [9.0]

7

405 [6.7]

8

310, 353 [1.5], 526

optical bandgap λem (nm)

Eg (eV)

476

2.61 1.77 3.02

454

2.69 2.03 3.10

470

2.66 1.90

Figure 2. Cyclic voltammograms of (a) 1-3 and (b) 4-6. 651



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Table 2. Electrochemical and Charge Transport Properties of Molecules 2, 3, 5, and 6

a

μ (cm2/(V s)) (type of charge)

HOMO (eV)a

LUMO (eV)a

Eg (eV)

-1.56

-5.70

-3.59

2.11

8.8 10-5 (hole)

0.78

-1.07, -1.64

-5.76

-4.10

1.66

1.9 10-3 (hole)

5

1.10

-1.45

-5.77

-3.46

2.31

5.4 10-5 (electron)

6

1.16

-0.96, -1.54

-5.84

-3.93

1.91

5.9 10-5 (electron)

compound

Eox (eV)

2

0.72

3

Ered (eV)

EHOMO/LUMO = -(4.8 þ qEox/red) eV.

Scheme 3. Synthesis of Unsymetrically Substituted Molecules 7 and 8

We were concerned that a hydrocarbon solvent such as dichloromethane may not be sufficiently solvating the pentafluorophenyl substituents and if this was leading to a higher apparent LUMO energy level. To test this possibility, we carried out cyclic voltammetry experiments in trifluoromethyl toluene to address this issue because this solvent is known to solvate both fluorocarbon- and hydrocarbon-based molecules well.24 We found that the solvent change yielded no significant difference in the redox potentials of these molecules. (See the Supporting Information.) We hypothesize that the higher LUMO energy level may be due to the reduced donor-acceptor nature of the molecule, which is known to cause a decrease in the HOMOLUMO gap. Next, we investigated the charge carrier mobilities of these molecules. This was measured by fabricating bottom contact field effect transistors by spin coating thin films (∼100 nm) on prefabricated transistor substrates. Charge mobility in the dioxolane derivatives could not be measured because of their limited film forming capability. In the case of 2 and 3, only hole mobility was observed. In compound 2, hole mobility was found to be 8.8 10-5 cm2/(V s). Upon changing the electron-withdrawing functionality from carbonyl to dicyanomethylene functionality in molecule 3, hole mobility increased by over an order of magnitude and was found to be 1.9 10-3 cm2/(V s). It is interesting to note that increasing the strength of the electron-withdrawing functionality in the molecule did drop the LUMO energy level by about 500 meV. However, the hole mobility increased significantly, and no measurable electron mobility was observed. This provides the preliminary indication that the nature of charge that is transported through a molecule is not an exclusive function of the frontier orbital energy levels. When pentafluorophenyl groups were incorporated at Rpositions, in molecules 5 and 6, a reversal in charge transport

characteristics was indeed observed in our systems as well. These molecules were found to be n-type materials with no measurable hole mobility. The electron mobility in 5 was found to be 5.4 10-5 cm2/(V s), whereas that in 6 was 5.9 10-5 cm2/(V s). No significant effect of varying strength of electron-withdrawing functionality at the bridgehead position was observed in this case. From the electrochemical properties of these molecules, as previously discussed, the LUMO energy levels of fluorinated molecules are slightly higher than the corresponding phenyl derivates. Yet the fluorinated molecules exhibit n-type behavior, whereas the phenyl derivatives are p-type. Therefore, in this case, the incorporation of fluorinated substituents imparts n-type character to the molecules without lowering their LUMO energy level. This further indicates that determining the frontier orbital energy levels is not a sufficient condition to understanding the charge transport properties of a molecule. We were further interested in investigating if one terminal fluoro substituent would be sufficient to impart n-type character to the molecule. Therefore, we synthesized unsymetrically substituted monophenyl-monopentafluorophenyl derivatives 7 and 8, as shown in Scheme 3. This molecule contains a pentafluorophenyl group as one of the terminal substituents and a phenyl group as the other. We hypothesized that the unsymmetrical substitution could impart significantly different characteristics, as compared with the symmetrically substituted molecules. To synthesize molecule 7, 2-bromo-4,4-ethylenedioxy-4H-cyclopenta[2,1-b:3,4-b0 ]dithiophene, 11, was sequentially reacted with phenyl boronic acid under Suzuki coupling conditions, followed by n-butyllithium and hexafluorobenzene (Scheme 3). Hydrolysis of 7 using HCl/ acetic acid yielded 8 in 64% yield. The optical properties of these molecules were studied in dichloromethane. (See the Supporting Information.) We were unable to record cyclic voltammograms for 8 because of 652



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solubility issues. We also studied the charge transport properties in 8. Surprisingly, we found that 8 was an exclusively p-type material, with significantly lower hole mobility as compared with 2; the hole mobility was found to be 5.5 10-7 cm2/(V s). No measurable electron mobility was observed. In conclusion, we have designed and synthesized small molecule organic semiconductors containing a CPD core, which are differentially functionalized at the bridgehead position with electron-withdrawing functionalities of varying strengths and at the R-positions with phenyl or pentafluorophenyl groups. The optical, electrochemical, and charge transport properties of these molecules were measured. We find that the functionality at the bridgehead position affects the LUMO energy level more significantly than the HOMO energy level. The dicyanomethylene group lowers the LUMO energy levels significantly, yet the molecule remains a hole-conducting material. Also, we found the LUMO energy level of pentafluorophenyl derivatives at the Rposition to have a very small effect on the frontier orbital energy levels. Yet, the fluorocarbon modification clearly changed the p-type material to an n-type material. The exact reason for the observed change in the charge carrier behavior remains unanswered. However, our results do clearly indicate that defining the frontier orbital energy levels in the molecule is not a sufficient condition to understanding the change in the nature of the charge carried through a material.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Listing of synthetic and characterization details of all molecules, CIF files of the crystal structures, and mobility measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions †

These two authors contributed equally to this work.

’ ACKNOWLEDGMENT We thank the U.S. Department of Energy for support through the EFRC at UMass Amherst (DE-SC0001087) and the U.S. Army Research Office (54635CH) for support through the Green Energy Center. ’ REFERENCES (1) Brabec, C. J.; Sariciftci, N. S. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15–26. (2) Schueppel, R.; Schmidt, K.; Uhrich, C.; Schulze, K.; Wynands, D.; Bredas, J. L.; Brier, E.; Reinold, E.; Bu, H. B.; Baeuerle, P.; et al. Optimizing Organic Photovoltaics Using Tailored Heterojunctions: A Photoinduced Absorption Study of Oligothiophenes with Low Band Gap. Phys. Rev. B 2008, 77, 085311/14. (3) Shirota, Y.; Kageyama, H. Charge Carrier Transporting Molecular Materials and Their Applications in Devices. Chem. Rev. 2007, 107, 953–1010. (4) Zhan, X.; Zhu, D. Conjugated Polymers for High-Efficiency Organic Photovoltaics. Polym. Chem. 2010, 1, 409–419. (5) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bredas, J. L.; Ewbank, P. C.; Mann, K. R. Introduction to Organic Thin Film 653



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(22) Yen, W.; Pal, B.; Yang, J.; Hung, Y.; Lin, S.; Chao, C.; Su, W. Synthesis and Characterization of Low Bandgap Copolymers Based on Indenofluorene and Thiophene Derivative. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 5044–5056. (23) Pal, B.; Yen, W.; Yang, J.; Chao, C.; Hung, Y.; Lin, S.; Chuang, C.; Chen, C.; Su, W. Substituent Effect on the Optoelectronic Properties of Alternating Fluorene-Cyclopentadithiophene Copolymers. Macromolecules 2008, 41, 6664–6671. (24) Ogawa, A.; Curran, D. P.; Benzotrifluoride, A Useful Alternative Solvent for Organic Reactions Currently Conducted in Dichloromethane and Related Solvents. J. Org. Chem. 1997, 62, 450–451.

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Cyclopentadithiophene-Based Organic Semiconductors: Effect of

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