Predictability of Thermal and Electrical Properties of End-Capped

Article pubs.acs.org/cm

Predictability of Thermal and Electrical Properties of End-Capped Oligothiophenes by a Simple Bulkiness Parameter Andreas Kreyes,†,‡ Ahmed Mourran,§ Zhihua Hong,§ Jingbo Wang,§ Martin Möller,§ Fatemeh Gholamrezaie,∥ W. S. Christian Roelofs,∥ Dago M. de Leeuw,‡ and Ulrich Ziener*,† †

Institute of Organic Chemistry III, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany § Interactive Materials Research, DWI an der RWTH Aachen e.V. & Institute of Technical and Macromolecular Chemistry, Forckenbeckstraße 50, D-52056 Aachen, Germany ∥ Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡

S Supporting Information *

ABSTRACT: The branching topology of end groups attached to several series of oligothiophenes has a systematic effect on thermal and electrical properties of the oligomers. The series were synthesized in a modular approach and show a distinct drop of the melting point Tm on increasing bulkiness of the substituents. The same trend can be found for the dissociation temperatures Tdis of aggregates in solution. Similarly, monolayer OFET mobilities μFET are significantly decreasing with increasing bulkiness of the substituents. A simple geometric model is presented quantitatively correlating the transition temperatures and mobilities with the substituents’ structure based on a bulkiness parameter P, which allows predicting Tm, Tdis, and μFET of corresponding not yet synthesized oligomers with branched substituents. This model might be generally applicable for endcapped rod-like conjugated oligomers. KEYWORDS: transition temperature, oligothiophene, branched substituents, structure−property relationship, OFET mobility

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INTRODUCTION In recent years much effort was put on the molecular design of organic field effect transistors (OFETs) by chemical tailoring the active oligomers or polymers in order to control their supramolecular arrangement and finally the functional device.1 Two design strategies were followed, i.e., band gap engineering by adjusting the chemical structure of the conjugated moiety and steering the molecular packing by a broad variety of intermolecular interactions like van der Waals, hydrogen bonding, π−π interactions, etc. While a correlation between the supramolecular arrangement like thin film or bulk morphology of the electronic materials with the electrical properties of the devices is demonstrated in several cases,2 a direct line between molecular structure and electrical performance is difficult to reveal.1a,3 Even more, so far only qualitative structure−functionality correlations are reported for such materials but no quantitative relationships. On the other side, simple physical properties like solubility or phase transition temperatures determine the processability for manufacturing devices and the thermal working area of the final devices. Furthermore, only well-controlled fabrication processes guarantee reproducible and good electrical properties like charge carrier mobilities. In order to control those processes via molecular parameters of organic materials, substituents are introduced enhancing solubility and lowering thermal transitions like melting compared to the unsubstituted parent © 2013 American Chemical Society

compounds. It would be of utmost interest to tailor the molecules not just on a phenomenological basis but specifically in a predictable way. It dates back already to the 1940s that a model was established to predict the boiling points of pure paraffins by a simple geometric approach (Wiener Index).4 Further developments led to the prediction of melting points of alkanes,5 and many more approaches were worked out to predict melting points of various compounds via molecular descriptors.6 The melting point for a given class of organic materials can be steered most simply by introducing branched substituents as they offer a high structural variability to adjust the intermolecular interactions, molecular symmetry, and conformational degree of freedom of the molecules. Many reports are found in the literature where branched substituents were attached to the functional cores of the compounds like in oligothiophenes,7 phenylene thiophene co-oligomers,8 hexabenzocoronenes,9 perylene tetracarboxdiimides,10 and isoindigo-based conjugated polymers11 to steer solubility and thermal properties while still maintaining the desired materials properties like charge carrier mobility.7b,d,e,12 Despite the wealth of those reports to find the golden way between processability and materials properties there is no systematic Received: March 1, 2013 Revised: April 15, 2013 Published: April 30, 2013 2128

dx.doi.org/10.1021/cm400702t | Chem. Mater. 2013, 25, 2128−2136

Chemistry of Materials

Article

Temperature dependent absorption spectroscopy was performed on a Perkin-Elmer UV/vis spectrometer Lambda 16. The sample solutions were prepared from stock solutions, heated up, and stored at 5 °C for at least 12 h before measuring. Temperature dependent fluorescent spectroscopy was performed on a Horiba FluoroMax 3. The sample solutions were diluted from stock solutions, heated up, and stored at −20 °C for at least 12 h before measuring. DSC measurements were performed on a Perkin-Elmer DSC 7 under nitrogen atmosphere and with a heating rate of 10 K/min. Transistors were fabricated by direct spin coating the solution onto the electrodes at concentrations that give full monolayer surface coverage. Heavily doped silicon wafers, acting as the common bottom gate covered with a 200 nm layer of thermally grown SiO2, were used. The gold source and drain contacts were defined by conventional photolithography. Ti (10 nm) was used as an adhesion layer. To remove any organic contaminants, prior to spin-coating the surface was exposed to UV-irradiation under flux of oxygen for 12 min. The monolayer was applied by spin-coating from a toluene or chlorobenzene solution with spinning speed between 500 and 2000 rpm for 3 min. The solution was heated up to 60 °C to improve the solubility of the oligomer. The electrical characterization of the field effect transistors was carried out in a probe station under high vacuum (10−6 mbar) with a Keithly 4200-SCS Semiconductor Characterization System. A ring geometry was chosen to prevent parasitic leakage currents. The pristine transistors exhibited very little hysteresis. Scaling of the mobility with channel length shows only a small contact resistance, which is confirmed by the linear mobility that is comparable to the saturated mobility.

investigation of thermal transitions and charge carrier (OFET) mobility of conjugated oligomers on the geometry of branched substituents to the best of our knowledge. Recently, the dependence of hole mobilities on the branching position of the alkyl substituents in polymer thin film transistors was described.13 In the present contribution we report on in total five series of mono-end-capped R-4T-H, bis-end-capped R-7T-R and R-9TR, and precursor oligothiophenes R-7T*-R and R-9T*-R, respectively, and their thermal behavior in solid and solution state as well as monolayer OFET mobility with respect to the number of thiophene units and the chemical structure of the substituents R (Chart 1). For comparison, thermal transition Chart 1. Chemical Structures of the Oligothiophenes and the Carbosilane Substituents I−IX and the Corresponding Structural Parameters a1 and b1−b3 and Bulkiness Parameter P (see also Table 1)

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RESULTS AND DISCUSSION Synthesis. The synthesis of some of the compounds in Chart 1 was already described previously.7b,d,e,12b Schemes 1−3 display the reaction pathways including the long oligomers R11T-R (124) and R-13T-R (125) for completeness. In the first part of the synthesis the substituents were constructed and attached to thiophene. For this purpose, thiophene was alkylated with allylbromide, 6-bromohex-1-ene, and 11bromoundec-1-ene, respectively, to obtain olefins 1−3, in which the distance between branching point and oligomer core is predefined. For substituents I−VII, which bear one or two long alkyl chains, alkylsilanes 4−7 and dialkylsilanes 8 and 9 were synthesized by coupling in situ generated alkyl Grignard reagents to chlorodimethylsilane (I−V) and dichloromethylsilane (VI, VII), respectively.14 In the following step these silanes were added to the olefinic double bonds of 1−3 by hydrosilylation reactions catalyzed by Karstedt catalyst (Pd2(dvtms)3) to obtain monosubstituted thiophenes 10−16 (see Scheme 1). To get access to substituents with more of the long alkyl chains (VIII−IX) the strategy had to be slightly modified (Scheme 1). Monosubstituted thiophenes 19 and 20 could not be synthesized as described above. Hydrosilylations of 2 and 3 with the corresponding trialkylsilanes were not successful as no conversion of the starting materials took place. This we attribute to the steric demand of these silanes that hinder the coordination to the catalyst. Hence, olefins 2 and 3 were first hydrosilylated with trichlorosilane and subsequently alkylated with excess of Grignard agents. This worked very well for substituent VIII, but in the case of IX the undesired Markovnikov byproduct occurred, which could not be separated from the desired anti-Markovnikov product. This problem was overcome by application of carbene catalyst N,N′dicyclohexylimidazol-2-yliden-(divinyltetramethylsiloxan)Pt(0),15 which exclusively produced the anti-Markovnikov product.

data from solution and (OFET) mobility data of even longer oligomers (R-11T-R, R-13T-R)7b are included, too. In order to get access to a wide structural variety, we chose branched carbosilanes as side chains (Chart 1). The substituents own one branching point, whose distance to the aromatic core amounts to 3, 6, or 11 methylene groups. Additionally, the length (C6H13, C10H21, C14H29) and number (1 to 3) of long peripheral alkyl groups are varied. The substituents can be ordered according to their bulkiness parameter P, which will be described below.

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EXPERIMENTAL SECTION

Details of the synthesis of the oligothiophenes and precursor molecules is described in the Supporting Information or was reported earlier.7b,d,e,12b 2129



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Scheme 1. Synthesis of Monosubstituted Thiophenes 10−16, 19, 20, and 24 with One (I−V), Two (VI, VII), and Three Long Alkyl Chains (VIII, IX, see Chart 1) in the Periphery via Alkylation−Hydrosilylation

Scheme 2. Stille-Type Coupling Reactions to Monosubstituted Oligomers R-nT-H

In the following part the monosubstituted oligomers R-nT-H (see Scheme 2) and disubstituted odd-numbered oligomers R-

nT-R (Scheme 3) were built up according to common stannylation and Stille-type coupling reactions yielding the 2130



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Scheme 3. Synthesis of Disubstituted Oligothiophenes R-nT-R up to a 13-mer via Stille-Type Coupling to Soluble Diketal Precursors, Diketones, and Ring Closing

The length of the branches as well as the distance between the aromatic core and the branching point are crucial factors; e.g., for R-7T-R Tm varies from 127 to 290 °C (I > V > II > IV > III ≈ VI > VIII > VII > IX). This effect is expected and can be in the first instance attributed to the bulkiness of the substituents partially suppressing intermolecular attractions between the aromatic cores by steric hindrance (see below and Chart 1). Interestingly, this substitution dependent order of Tm is found not only for the symmetric oligomers R-7T-R and R-9TR but also for the monosubstituted series H-4T-R and even for the diketone precursor molecules R-7T*-R and R-9T*-R (Table 1). This suggests that the effect of the structure of the substituents on Tm follows a general principle. In order to obtain a deeper understanding of this principle and a more quantitative relation between Tm and the structure of R we introduce a bulkiness parameter P. As the effective bulkiness depends on both the absolute volume of R and R’s distance from the oligomeric core P is expressed by the approximated volume ratio of the chains behind the branching point (see Charts 1 and 2, red bonds) and the spacer between the

disubstituted diketals 72−89, in which the conjugation length of the end products is already given. The ketal groups then were removed with hydrochloric acid so that diketones 90−107 were obtained and converted to the final end-capped oligothiophenes R-7T-R (108−116), R-9T-R (117−122), R11T-R (123, 124), and IX-13T-IX (125) using Lawesson’s reagent. Thermal Transitions in Solid State. It can be expected that the chemical structure of the substituents R also plays a major role for the thermal properties, besides the influence on solubility, namely, the melting behavior of the oligothiophenes, which we investigated by differential scanning calorimetry (DSC). The thermograms of most of the compounds show several endothermic transitions owing to (thermotropic liquid crystalline) mesophases as proved by polarizing optical microscopy and X-ray (data not shown). In the present contribution, we will concentrate on the transition at the highest temperature to the isotropic phase, which shall be called melting temperature Tm for simplicity. Indeed, distinct differences of Tm are observed (see Table 1).

Chart 2. Schematic Illustration of Substituents and Their Spatial Demand

Table 1. Melting Points Tm [°C] of Oligothiophenes R-nT-R and Precursor Oligomers R-nT*-R with Substituents I−IX oligomeric corea R

4T

7T*

7T

I II III IV V VI VII VIII IX

149 137

242 214 189 199 217 196 133 163 92

290 260 241 254 282 239 177 205 127

130 71 96 52

9T*

289 233 269 176

bulkiness parameter 9T

P

347 270 319 224

1.09 3.67 9.33 6.67 4.00 12.00 66.67 42.00 366.67

branching point and the thiophene moieties (see Charts 1 and Chart 2, blue bonds). Thereby, we approximate the spatial demand of the red chains as a cuboid with the length of a side given by the number of backbone atoms per chain (b1, b2, and b3, respectively (Chart 1)) where the branching point (silicon atom) counts for the shortest of the three chains. The spacer’s volume is similarly approximated by a cuboid with two sides as unit length a0 (= 1) and the third side as number of backbone

a

Missing oligomers could either not be synthesized due to poor solubility (R-9T*-R, R-9T-R) or were not available with sufficient purity (R-4T-H). 2131



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anisotropic to the liquid state decreases with increasing P, which might be attributed to higher flexibility already in the anisotropic state and presumably decreased packing density compared to compounds with lower P. Those findings might apply to the other oligothiophene series as well but before evaluation a clear assignment of all transitions has to be undertaken. Comparing Tm,min with Tm in Table 1 reveals that the minimum is already reached with a bulkiness parameter P of ca. 370. Further reduction of Tm is not expected at least with the present type of branched substituents. Tm,max corresponds to Tm of oligomers with nonbranched (linear) substituents. Therefore, comparison to literature values should prove if the presented simple geometric approach is valid also for other analogous compounds. Indeed, H-4T-R with a linear C20H41 substituent with Tm = 148 °C7e matches perfectly the predicted value (149 ± 11 °C, Table 2) while R-7T-R with the same substituent C20H41 and Tm = 295 °C7e falls in the expected range (277 ± 25 °C, Table 2). Besides the potential of P to predict the limits of melting transitions Tm,min and Tm,max in a given series of oligothiophenes we assume that Tm of not yet synthesized oligothiophenes with substituents with one branching point can be predicted if the fit parameters of the series are already known. One example shall illustrate our assumption. The bulkiness parameter P of a branched pure hydrocarbon 2-octyldodecyl substituent amounts to 90 (b1 = 10, b2 = 9, b3 = 1, a = 1) leading to an expected value of Tm = 158 °C for a corresponding R-7T-R. The literature data is quite close with Tm = 171 °C.7e Thermal Transitions in Solution State. The thermal dissolution of aggregates of π-conjugated molecules in solution can be regarded as a microscopic analogue to macroscopic melting. Therefore, besides the melting properties in bulk, the aggregation and dissociation of aggregates in solution should also be controlled by the chemical structure of the substituents. Note that solvent-oligothiophene and solvent−solvent interactions will play a crucial role for the aggregation process in solution, besides the intermolecular interactions between oligothiophene molecules governing the bulk melting process. In the following we try to solve the question if there is also such a clear structure−property relationship for the aggregation process as for the melting (see above). Qualitative aspects of the self-assembly of the presented oligothiophenes were investigated by temperature dependent UV/vis spectroscopy. Figure 2 shows exemplary absorption spectra of the four oligothiophenes II-7T-II (109), II-9T-II (117), VII-9T-VII (120), and VII-11T-VII (123), and further spectra are found in Supporting Information (Figure 1S). The spectra were measured in TCE (1,1,2,2-tetrachloroethane, good solvent) or TCE/Isopar M mixtures. Isopar M is a nonpolar paraffinic mixture with C11 to C16 (bad solvent). The different solvents were used because of substantially different aggregation

atoms a1 (Chart 1). Thus, P increases when the ratio of the two volumes increases, by increasing the length of the branches and/or decreasing the length of the spacer (eq 1). P=

b1b2b3 a1a02

(1)

In good agreement with the experimental data, the order of P (I < II ≈ V < IV < III < VI < VIII < VII < IX Chart 1, Table 1) is almost perfectly reverse to the order of Tm with the substituents. More quantitatively, Figure 1 shows that there is a

Figure 1. Correlation between Tm of various R-nT-R and bulkiness P; symbols, experimental data; lines, fit according to eq 2.

clear exponential decay of Tm with P displaying good correlation coefficients between 0.9501 and 0.9905 (Table 2) following eq 2. Tm = Tm,min + (Tm,max − Tm,min) × e−kmP

(2)

with Tm,min is the minimum melting transition temperature for P = ∞, expressing the lowest achievable melting transition by a maximum bulkiness and depression of Tm; Tm,max is the maximum temperature achievable for P = 0; and km is a dimensionless constant. In order to elucidate the thermodynamic basis for the clear dependence between Tm and P we evaluated the transition enthalpies ΔHm and entropies ΔSm. Most of the thermograms of the oligothiophenes within one series show multiple and different number of transitions. In order to compare thermodynamic data of the single compounds we have chosen the series with the most homogeneous appearance of the DSC traces, i.e., the diketones R-7T*-R. Both variables ΔHm and ΔSm decrease strongly with increasing P (see Supporting Information Figure 3S). Thus, this dependency confirms the assumption above that increasing bulkiness lowers intermolecular attractions (ΔHm, e.g., π−π interactions). The decrease of ΔSm indicates that the change in chain order from the

Table 2. Fit Parameters for the Exponential Decay of Tm with P for the Different Series of Oligothiophenes R-4T-H, R-nT-R, and R-nT*-R oligomeric core 2

R Tm,min/°C Tm,max/°C km

4T

7T*

7T

9T*

9T

0.9905 51 ± 5 149 ± 11 0.021 ± 0.003

0.9501 93 ± 12 224 ± 24 0.018 ± 0.008

0.9593 129 ± 12 277 ± 25 0.018 ± 0.003

0.9807 173 ± 14 308 ± 34 0.011 ± 0.003

0.9652 222 ± 18 375 ± 49 0.014 ± 0.004

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Figure 2. Temperature dependent absorption spectra of (a) II-7T-II (109) (in TCE:IsoparM = 1:3 (v:v), c = 1 × 10−5 mol L−1), (b) II-9T-II (117), (c) VII-9T-VII (120), and (d) VII-11T-VII (123) (in TCE, b: c = 1 × 10−5 mol L−1; c: c = 4 × 10−5 mol L−1; d: c = 5 × 10−6 mol L−1 in TCE).

behavior of the oligomers. At high temperatures (no aggregation), broad and unstructured bands are observed. Upon cooling, the spectral shape changes due to the formation of aggregates. These changes depend considerably on the structure of R, the length of the oligomeric core, and to some extent on the nature of the solvent. While in the molecularly dissolved state the position of the maxima increases with increasing number of thiophene units n from R-7T-R (λmax = 457 to 451 nm depending on the solvent) to R-9T-R (λmax = 465 to 475 nm depending on the substituent), it does not further grow for R-11T-R and R-13T-R (λmax = 475 nm).7b This indicates that the effective conjugation length does not increase any more due to twisting of the thiophene moieties around the inter-ring single bond, which reduces intramolecular π-overlap. Upon aggregation it is found that the slightly branched substituents I−V (Chart 1) cause a significant hypsochromic shift of λmax up to around 2100 cm−1 (Δλmax ≈ 40 nm) and a vibronic fine structure. These features are very similar to those of oligothiophenes with linear alkyl α,ωsubstituents and are typical for H-aggregates.7e In contrast, the more bulky substituents VI−IX (Chart 1) lead to bathochromic shifts of λmax up to around 1000 cm−1 (Δλmax ≈ 20 nm). A uniform trend is found for the 0−0 transition in the aggregated state with increasing core length shifting from 537 nm (R-7TR) to 551−561 nm (R-9T-R), 570−575 nm (R-11T-R), and 585 nm (R-13T-R) fairly independent of the structure of R. These findings demonstrate that there is a dependence of the electronic absorption properties on the structure of R but too complex to be quantitatively correlated as found for the melting transitions in bulk (see above).

In contrast, the corresponding temperature dependent photoluminescence spectra display a much clearer picture. The position of the peaks is changed neither by temperature or concentration nor by the structure of R but the overall intensity increases substantially upon increasing temperature. Figure 3 shows exemplary photoluminescence spectra of I-7T-I (108). At high temperatures, when the compounds are molecularly dissolved, high fluorescence intensity and a distinct vibronic fine structure (up to 0−2) are observed. For all septithiophenes the 0−0 band occurs at around 536−538 nm and is red-shifted

Figure 3. Exemplary temperature dependent fluorescence spectra of septithiophene I-7T-I. Inset: normalized spectra at lowest and highest temperature, which show the change of relative intensities of the emission bands. 2133



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to 557−561 nm when the conjugated system is expanded to 9mers (see Supporting Information Figure 2S). Very similar to the absorption spectra at high temperatures, no significant further bathochromic shift is observed when additional monomer units are incorporated. Upon cooling fluorescence is quenched, accompanied by a change in relative intensities of the three vibrational bands (see inset in Figure 3). In a previous publication7b we showed for compounds IX-11T-IX (124) and IX-13T-IX (125) that at low temperatures the spectra are superpositions of weakly emitting aggregates and residual free molecules. This quenching of fluorescence intensity follows a sigmoidal progression,7b which we found for all oligothiophenes exemplarily shown for I-7T-I (108) in Figure 4. The inflection points of these S-shaped curves are defined as dissociation temperature Tdis of the aggregates, which depend on chemical structure, solvent, and concentration.

Figure 5. Dependence of the dissociation temperature of aggregates in solution Tdis, on the bulkiness parameter P in TCE and TCE:Isopar M 9:1 (v:v), respectively (c = 1 × 10−5 mol L−1). The solid line represents the fit curve for R-9T-R in TCE according to eq 3.

noted that this order could be derived only from three different series of measurements in the different solvents (TCE, TCE/ Isopar M, respectively) indicated by the triple slashes because of the big differences in solubility. Because of the limited accessible temperature range and solvent and concentration dependence of Tdis a clear quantitative relation between Tdis and P can be derived only for the nonathiophene series R-9T-R according to eq 3 (cf. eq 2) delivering the minimum dissociation temperature Tdis,min = 280.4 ± 5.2 K (for P → ∞), the maximum temperature Tdis,max = 351.5 ± 4.9 K (for P → 0), and the constant kdis = 0.047 ± 0.014 with R2 = 0.99034. Tdis = Tm,mis + (Tdis,max − Tdis,min) × e−kdisP Figure 4. Concentration dependent sigmoidal progression of photoluminescence intensity of I-7T-I (108) in TCE.

Comparing those results with the evaluation of Tm in bulk demonstrates that, expectedly, the dissociation temperatures Tdis are much lower than Tm, in addition to the dependence of Tdis on the solvent type. The additional interactions of the solvent molecules are also reflected by both the increased value of kdis in comparison with km and by the smaller differences ΔTdis between different oligomer lengths (ca. 50 K for the same substituent in the identical solvent, see Table 3, Figure 5) compared with ΔTm (ca. 100 K for the same substituent, see Figure 1 and Table 1). Monolayer Field Effect Mobilities. The ultimate goal of the presented compounds is to investigate their electrical properties and correlate them with the substituents’ structures. Such characteristics can be determined within devices like organic field effect transistors (OFETs) to evaluate mobilities. In a previous study we have shown that the influence of the substituents’ geometry (branching position) on the mobility is negligible for OFETs processed under standard conditions with thick films prepared by physical vapor deposition.12b Here, only little control over local variations in film thickness, interfacial structures, grain boundaries, etc. is given. Therefore, we decided to determine the mobilities of some oligomers in the present contribution from monolayer OFETs in accordance with a recently published protocol restricting the active layer to just a monolayer (≈ 5 nm) by direct casting the material onto a SiO2−dielectric surface.12a The spin coating process reproducibly results in the deposition of electrically connected monolayers. Representative AFM images of compounds II7T-II (109) and IX-7T-IX (116) with a small and large bulkiness parameter P (3.67 and 366.67, respectively) show

Plotting Tdis at a constant concentration (1 × 10−5 mol L−1) vs the bulkiness parameter P (definition see above) reveals that there is a monotonic decline of Tdis with increasing P for all oligomers R-nT-R (Table 3, Figure 5). Qualitatively, the following order of Tdis with the substituents R can be found: I > II > III /// V > IV > III > VI (derived from R-7T-R) /// II > III > VI > VIII > VII > IX (derived from R-9T-R). It has to be Table 3. Dissociation Temperatures Tdis in TCE or TCE/ Isopar M Mixture at c = 1 × 10−5 mol L−1 bulkiness parameter

oligomeric corea R

7Tb

I II III IV V VI VII VIII IX

291.1 279.8

7Tc

292.4 295.1 303.3 283.8

9Tb

11Tb

13Tb

P

344.1

1.09 3.67 9.33 6.67 4.00 12.00 66.67 42.00 366.67

338.7 330.0

320.0 285.0 288.4

334.7 303.8

(3)

a

Missing oligomers did either show no aggregates at all or not the full aggregation/dissolution process in the chosen solvents and technically accessible temperature range (ca. −10 to +80 °C). bSolvent: 1,1,2,2tetrachloroethane (TCE). cSolvent: TCE/Isopar M (1:9, v:v). 2134



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closed monolayers (Figure 6). AFM images of corresponding monolayers of the other septithiophenes are shown in Supporting Information (Figure 4S). We stress here the difficulties related to electrical contact between 5 nm thick oligothiophene layers and 100 or 50 nm thick gold electrodes. In order to get comparable results we had to choose appropriate oligomers from which reproducibly

defined monolayers can be formed from solution. It turned out that almost the full R-7T-R series (R = I−IV, VI, VIII, IX (108−111, 113, 115, 116)) are ideal candidates for this purpose; for unknown reasons, the missing oligomers did not show reproducible results, and thus their data were left out. The mobilities μ show a clear tendency with a drop by 4 orders of magnitude from 1.3 × 10−2 to 8 × 10−7 cm2 V−1 s−1 with increasing bulkiness parameter P (Table 4). Exemplary transfer Table 4. Monolayer FET Mobilities μ vs Bulkiness Parameter P for a Series of R-7T-R R I II III IV VI VIII IX a

μ [cm2 V−1 s−1] 8 1.3 6 2 1 1 8

× × × × × × ×

−3

10 10−2 10−3 10−2 10−4 10−5 10−7

ln(μ/μ0)a

P

−4.83 −4.34 −5.12 −3.91 −9.21 −11.51 −14.04

1.09 3.67 9.33 6.67 12.00 42.00 366.67

μ0: unit mobility

curves at a drain bias of −20 V are shown in Supporting Information (Figure 5S). The drop in the mobility correlates well with weakened intermolecular π−π interactions of the oligomeric cores because of increased bulkiness of the substituents. Besides the qualitative consideration, quantitative evaluation is also possible. If the natural logarithm of the mobilities ln(μ/ μ0) (μ0: unit mobility) is plotted against P an exponential decay according to eq 4 can be fitted (Figure 7). ln(μ/μ0 ) = ln(μmin /μ0 ) − ln(μmax /μmin )ekμP

(4)

Figure 7. Logarithmic saturated mobilities extracted from monolayer OFETs of R-7T-R oligomers vs bulkiness parameter P. The solid line represents the fitted curve according to eq 4.

The fit delivers ln(μmin/μ0) = −14.1, ln(μmax/μmin) = 10.7, kμ = 0.0347 ± 0.0170 and R2 = 0.8942 corresponding to a minimum and maximum mobility μmin = 8 × 10−7 cm2 V−1 s−1 and μmax = 3 × 10−2 cm2 V−1 s−1, respectively. It shows that μmin (P → ∞) is basically reached with the bulky substituent IX (see Table 4). μmax would be the mobility of a oligothiophene with P → 0 or linear substitutents. Indeed, alkyl endsubstituted oligothiophenes show mobilities of up to 0.1 cm2 V−1 s−1 and only in exceptional cases slightly higher values.3 Differences

Figure 6. AFM images of II-7T-II (109) (top) and IX-7T-IX (116) (bottom) showing closed monolayers with the profile taken at the white line and deposited by spin-coating from a solution with c = 0.67 (109) and 1 g L−1 (116), respectively. Scan size is 10 μm × 10 μm. 2135



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might be caused by the presence of the silicon atom in our substituents. It shall be noted that the corresponding monolayer mobility of a R-7T-R with a pure hydrocarbon 2octyldodecyl substituent (P = 90, see above) amounts to 1 × 10−2 cm2 V−1 s−1.12a From the larger P value compared to substituent VIII (P = 42, see Table 4) a mobility of around 1 × 10−6 cm2 V−1 s−1 is expected. This discrepancy might be caused by (i) the presence and absence, respectively, of the silicon atom in the substituents (see above) and (ii) the presence of diastereomers of the purely hydrocarbon substituted compound, which have unknown influences on the mobility. The quantitative correlation between molecular structure and mobility is quite astonishing as no information on monolayer structure of the different oligomers is employed. This suggests that all oligomers arrange in the same two-dimensional structure. A similar layered structure in bulk was already found for some of the compounds as shown in an earlier publication.12b More direct relations between the structure of the monolayer and the molecular structure and the mobility will be explored in the future.

CONCLUSION Several series of oligothiophenes with systematically varied branching topology of carbosilane end groups were synthesized in a modular approach. The oligomers show a clear dependence of field effect mobility and of thermal properties from the geometry of the substituents. Increasing bulkiness leads to a decrease of mobility μ, melting temperature in bulk Tm, and dissociation temperature of aggregates Tdis in solution. This finding can be easily understood as an effect of decreased intermolecular π−π interactions steered by steric repulsion due to the bulky substituents. The qualitative dependence is quantified by a simple geometric model based on an easily accessible bulkiness parameter P, allowing Tm and Tdis to be predicted very well, and is even able to describe μ. Furthermore, the applicability of P to the different series of oligothiophenes and even to the compounds with interrupted conjugation (diketone derivatives) suggests that it might be extended more generally to other rod-like conjugated oligomers with branched substituents. This has to be proven with further examples in the literature and would be of utmost interest for tailoring and fine-tuning the substituents of conjugated oligomers to meet specific materials thermal and mobility requirements. ASSOCIATED CONTENT

S Supporting Information *

Additional absorption and emission spectra; melting enthalpies and entropies; AFM images of monolayers; transfer curves of monolayer OFETs; and synthesis protocols. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support of the present work by the Deutsche Forschungsgemeinschaft (DFG) projects ZI567/4-1 and MO982/2-1. 2136


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