Role of the Alkylation Patterning in the Performance of OTFTs: The Case of Thiophene-Functionalized Triindoles

Abstract

Organic semiconductors have emerged as potential alternatives to conventional inorganic materials due to their numerous assets and applications. In this context, the star-shaped triindole core stands as a promising system to design new organic materials with enticing charge-transporting properties. Herein, we present the synthesis of three thiophene-containing triindole derivatives that feature N-alkyl chains of different lengths, from methyl to decyl. The impact of the alkylation patterning on the crystallinity of the thin films and their resultant performance as semiconductor have been analyzed. All derivatives displayed p-type semiconductor properties, as demonstrated via both TOF measurements and integration in organic thin-film transistor (OTFT) devices. The attachment of longer alkyl chains and the functionalization of the silicon substrate with octadecyltrichlorosilane (OTS) prompted better OTFT characteristics, with a hole mobility value up to 5 × 10⁻⁴ cm² V⁻¹ s⁻¹. As elucidated from the single crystal, this core is arranged in a convenient cofacial packing that maximizes the π-overlapping. The analysis of the thin films also corroborates that derivatives possessing longer N-alkyl chains confer a higher degree of order and a more adequate morphology.

1. Introduction

The current upsurge in organic electronics acknowledges the potential of organic semiconductors as a realistic alternative to conventional inorganic materials [1,2]. Even though the state-of-the-art technology in this area still cannot compete with the performance of crystalline silicon in terms of charge mobility, several materials have already surpassed the milestone of 10 cm² V⁻¹ s⁻¹ [3,4]. Furthermore, the aim of organic materials is not substituting inorganic ones in high-performing applications but rather granting desired features in next-generation devices, such as flexibility or transparency. Their many advantages also encompass uses from the fabrication of large-area displays with a lower production cost to the modulation of their properties via facile synthetic methods [2,5,6,7].

The π-conjugated backbone of an organic compound rules fundamental properties, such as the electronic profile, the arrangement in the solid state, the air stability, and the optical characteristics [8,9,10]. Since the structural design represents the cornerstone of an organic semiconductor, the investigation of this topic has led to countless molecular structures developed from a wide array of building blocks [3,6,11,12,13,14,15]. Apart from the key role of the main aromatic nucleus, however, additional structural features can be equally decisive in governing the semiconductor characteristics of a material. A prime example of this is the inclusion of flexible alkyl chains, which not only improve the solubility but also influence the intermolecular interactions in the solid state [16,17,18,19,20]. Therefore, the alkylation patterning in a particular core should be carefully addressed to extract its full potential in terms of charge carrier mobility.

In this way, 9H-carbazole emerges as a highly promising nucleus that can supply several characteristics needed in novel materials, e.g., hole-transporting properties with stability against oxidative doping by atmospheric oxygen [13,21] and high fluorescence [22,23]. Additionally, it embeds nitrogen as a heteroatom that provides a suitable point to insert alkyl chains. The case of the diindolo[3,2-a:3′,2′-c]carbazole core, also known as triindole, represents a particularly enticing system constructed from carbazole that features a star-shaped π-extension [24,25]. The successful integration of triindole-based structures in assorted devices, such as OSCs [26,27,28], OLEDs [29,30,31], and, more recently, OTFTs [18,32,33,34], clearly reinforces their potential.

Herein, we report the synthesis of a set of new triindole derivatives functionalized with thiophene moieties and their integration in OTFT devices. The inclusion of sulfur-containing heterocycles, such as thiophene, represents a well-known strategy in the search for novel materials within organic electronics [35,36,37,38,39,40]. The advantages of attaching sulfurated moieties, such as 5-methylthien-2-yl and benzothien-2-yl, to the triindole core, instead of aromatic hydrocarbons, have been already demonstrated in our research group [33]. In this particular study, the 3,8,13-tri(thiophen-2-yl)-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c] carbazole nucleus has been characterized as a p-type semiconductor and structurally ameliorated considering both the molecular and the device design. Synthetically, this core has been N-alkylated with chains of different lengths, from methyl to decyl, as shown in Figure 1. Regarding the architecture of the device, the focus has been placed on optimizing the interface between these semiconductors and the Si/SiO₂ substrate in a bottom-gate top-contact OTFT. In fact, the presence of a passivation coating or anchored groups in the interface can greatly influence the growth and features of a vacuum-deposited thin film [41,42,43,44,45]. Specifically, the SiO₂ surface has been either functionalized with octadecyltrichlorosilane (OTS) as an aliphatic self-assembled monolayer (SAM) or coated with polystyrene (PS) as an aromatic polymeric layer. The effects of both the N-alkylation patterning and the passivation layer over the performance of this core have been correlated with the degree of order and morphology of the thin films, confirming the relevance of an adequate structural design.

2. Materials and Methods

2.1. Synthesis and Characterization

The commercially available chemicals were employed as received. Chemicals and reagents were as follows: 5-bromoisatin (Alfa Aesar, 91.2%), sodium hydride (Aldrich, 60% dispersion in mineral oil), methyl iodide (Acros Organics, 99%), 1-bromohexane (Aldrich, 98%), 1-bromodecane (Aldrich, 98%), hydrazine hydrate (Sigma-Aldrich, reagent grade, 55% N₂H₄), phosphorous(V) oxychloride (Acros Organics, 99%), 2-thienylboronic acid (TCI, 98.7%), and Pd(PPh₃)₄ (Acros Organics, 99%). Anhydrous DMF (Thermo Scientific) was kept under nitrogen atmosphere over a molecular sieve. Dichloromethane (VWR) was distilled from CaH₂. Flash chromatography was carried out over commercial silica gel (VWR, 40–63 µm). All synthetic procedures were carried out in open-air atmosphere unless otherwise stated.

2.2. Instrumentation and Methods

Here, ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were collected in a Varian Mercury spectrophotometer (Varian Inc., Palo Alto, CA, USA). In the case of compound 1a, the ¹³C NMR (100 MHz) spectrum was recorded in a Bruker 400 MHz Avance III. The analysis of the NMR spectra was achieved using MestRec Nova software (version 14.2.0). The solvent signal was used to reference all the spectra. Absorption spectra were registered on a Varian Cary UV–Vis–NIR 500E spectrophotometer (Palo Alto, CA, USA) and emission spectra were registered using a PTI fluorimeter (Birmingham, AL, USA). 1,4-Bis(5-phenyl-2-oxazolyl)benzene (POPOP) was selected as the standard (λ_Ex = 300 nm, Φ = 0.93 in cyclohexane) for the analysis of the fluorescence quantum yields, as suggested in the literature protocol [46]. Fluorescence quantum yields of thin films were determined by means of an integrating sphere. Cyclic voltammograms were registered in a cylindrical three-electrode cell using the following electrodes: an Ag/Ag⁺ electrode (1 mM AgNO₃ in acetonitrile) as the reference electrode, a glassy-carbon electrode as the working electrode, and a platinum wire as the counter electrode. All voltammetric curves were recorded with a microcomputer-controlled potentiostat/galvanostat Autolab with PGSTAT30 equipment (Metrohm Autolab BV, Utrecht, The Netherlands) and GPES software (version 4.9) under quiescent conditions, at a scan rate of 100 mV s⁻¹ and under an argon atmosphere. The solutions were prepared in distilled dichloromethane (1 mM) with tetrabutylammonium hexafluorophosphate (TBAP) as the supporting electrolyte (0.1 M). The potentials were referred to the Fc⁺/Fc redox couple. The ionization potentials (IP) were calculated from the onset of the first oxidation peak as IP = ^oxE_onset + 5.39, where 5.39 eV stands as the formal potential of the Fc⁺/Fc couple in the Fermi scale [47]. The electron affinities (EA) were estimated as EA = IP − E_gap. The optical gap energies (E_gap) were estimated from the absorption spectra (λ_onset). Ionization potentials (IP) in the solid state were measured by the photoelectron emission method in air. Thin films for IP measurements were prepared by vacuum thermal evaporation (10⁻⁶ mbar) of the organic compounds on glass slides coated with fluorine-doped tin oxide. A negative voltage of 300 V was applied to the sample substrate. A deep UV deuterium light source (ASBN-D130-CM) and a CM110 1/8m monochromator (Spectral Products, Putnam, CT, USA) were used for illumination of the samples with monochromatic light. A 6517B Keithley electrometer (Keithley, Solon, OH, USA) was connected to the counter electrode for the photocurrent measurement, which was flowing in the circuit under illumination. An energy scan of the incident photons was performed while increasing the photon energy. Thermogravimetric analyses (TGA) were recorded at a heating rate of 20 °C min⁻¹ under nitrogen atmosphere using a TA Instruments Q50 (New Castle, DE, USA). The extraction of the charge drift mobility was performed by means of the TOF technique. The organic compounds were vacuum-deposited (10⁻⁶ mbar) on pre-cleaned indium tin oxide (ITO)-coated glass substrates, and then 80 nm of an aluminum layer was also deposited via thermal vacuum evaporation using a mask (area = 0.06 cm²). Photo generation of charge carriers was performed by a light pulse through the ITO. A Keithley 6517B electrometer was used to apply external voltages with a pulsed third-harmonic Nd:YAG laser EKSPLA NL300 (pulse duration of 3–6 ns, λ = 355 nm). A Tektronix TDS 3032C digital storage oscilloscope was used to record the TOF transients. The transit times (t_t) were calculated using the kink on the curve of the transient in the log–log scale. The drift mobilities were calculated as μ = d²/Ut_t, where d is the layer thickness and U is the surface potential at the moment of illumination. Zero-field mobilities (μ₀) and field dependence parameters (α) were extracted from μ = μ₀$e^{\alpha E^{1/2}}$. The single crystal was analyzed using a D8 Venture System (Bruker AXS, Karlsruhe, Germany) equipped with a multilayer monochromator and a Mo microfocus (λ = 0.71073 Å). The frames were integrated with the Bruker SAINT software package (version SAINT V8.38A) via a narrow-frame algorithm. The structure was elucidated and refined using the Bruker SHELXTL software package. Out-of-plane GIXRD measurements were performed on vacuum-deposited thin films (semiconductor thickness = 75 nm) with a PANalytical X’Pert PRO MRD diffractometer (Almelo, the Netherlands) possessing a PIXcel detector, a parabolic Göbel mirror at the incident beam, and a parallel plate collimator at the diffracted beam (Cu Kα radiation (λ = 1.5418 Å), with a work power of 45 kV × 40 mA). The optimized angle of incidence used was 0.20° (1a) or 0.18° (1b,c). The morphology of the layers, analyzed by means of atomic force microscopy (AFM), was profiled using an AFM Dimension 3100 system connected to a Nanoscope IVa electronics unit (Bruker, Billerica, MA, USA).

2.3. OTFT Fabrication and Characterization

OTFTs were constructed in a bottom-gate top-contact geometry using thermally-oxidized crystalline-silicon wafers with a SiO₂ layer as the gate dielectric. The gate side of the substrates was partially unprotected with ammonium fluoride. Then, the wafers were cleansed by subsequent ultrasonic treatments in acetone, isopropyl alcohol, and water, dried by a nitrogen blow, and heated at 100 °C for 5 min. The SiO₂ surface was then either functionalized with octadecyltrichlorosilane (OTS) or coated with polystyrene (PS). The functionalization with OTS SAMs [48,49] was achieved by immersing the substrates in a solution of OTS in toluene (2 mM) for 24 h at room temperature. Then, the substrates were cleaned by subsequent ultrasonic treatments in toluene, acetone, and isopropyl alcohol, and finally dried by a nitrogen blow and heated at 100 °C for 5 min. The coating with PS was carried out with a solution of PS in toluene (4 mg mL⁻¹), which was spin-coated onto the wafer. The substrate was spun at 1500 rpm for 5 s and 2500 rpm for 33 s with a P6700 spin-coater (Specialty Coating System, Indianapolis, IN, USA). The organic semiconductor was deposited by thermal evaporation in a vacuum system with a base pressure below 10⁻⁶ mbar. The temperature was manually controlled to ensure a stable deposition rate of 0.3 Å s⁻¹ until a thickness of ca. 75 nm was obtained. Then, the substrates were transferred to a different vacuum chamber to deposit the metallic contacts. Gold was chosen as the metal for the drain and source electrodes, which were defined with a metallic mask possessing a channel length (L) of 80 μm and width (W) of 2 mm. The OTFTs were electrically characterized in the dark under ambient conditions using a Keithley 2636A source meter (Solon, OH, USA). The charge carrier mobility was extracted in the saturation regime (µ_sat) from Equation (1), as follows:

$$I_{\mathrm{D}}=\frac{W C_{\mathrm{ox}} \mathsf{\mu }}{2 L}{\left(V_{\mathrm{G}}{-V}_{\mathrm{th}}\right)}^2$$

(1)

where W and L are the channel width and length, respectively, and C_ox is the unit dimensional dielectric capacitance of the gate insulator.

3. Results and Discussion

3.1. Synthesis and Characterization

The synthesis towards the final compounds 1a–c, which started with the commercially available 5-bromoisatin, is presented in Scheme 1. The alkylation of the starting material was conducted under standard conditions, using NaH and CH₃I to obtain 2a or K₂CO₃ and the corresponding 1-bromoalkane in the case of 2b and 2c. Then, the reduction via the Wolff–Kishner reaction led to the 5-bromooxindole derivatives 3a–c. The cyclocondensation towards the brominated triindole systems, accomplished with POCl₃ under reflux, prompted a higher yield for the derivatives featuring shorter alkyl chains. Finally, the attachment of the thiophene moieties was achieved through the Suzuki–Miyaura cross-coupling reaction [50]. It should be mentioned that the severe insolubility of the methylated precursor 4a required the use of microwave irradiation to provide the desired compound 1a in a comparable yield.

3.2. Physical Characterization

The thermal and optical properties in solution and in the solid state of compounds 1a–c are compiled in

Table 1. Thermal and optical properties in solution and in the solid state of derivatives 1a–c.

		Solution 2			Solid State 3
Compound	Td (°C) 1	λabs,max (nm)	λem,max (nm)	Φf 4	λabs,max (nm)	λem,max (nm)	Φf 4
1a	476	317, 344	395, 414	0.14	315, 353	432	0.02
1b	433	318, 344	397, 415	0.15	316, 350	424	0.03
1c	427	318, 344	397, 415	0.15	313, 352	424	0.03

¹ Onset decomposition temperature obtained from TGA. ² Optical properties determined from a 10 μM solution in CH₂Cl₂. ³ Optical properties determined from a vacuum-evaporated thin film of the compound deposited over quartz. ⁴ Fluorescence quantum yield (Φ_f) obtained with an integration sphere (λ_ex = 330 nm).

. Remarkably, all compounds are suitable for the vacuum-evaporation process, with decomposition temperatures (T_d) that surpass 425 °C (TGA scans are depicted in the Supporting Information).

In terms of the optical properties, derivatives 1a–c display similar characteristics regardless of the length of the N-alkyl chains. All of them emit into the UV–blue region, peaking at 415 nm in CH₂Cl₂ and displaying a considerable fluorescence quantum yield of ca. 0.15. In the solid state, the emission spectra are slightly red-shifted and broadened, with a decay of the fluorescence quantum yield to ca. 0.03, which is associated to the aggregation of the triindole systems. The absorption and emission spectra of derivatives 1a–c are represented in the Supporting Information.

The electrochemical properties of 1a–c, analyzed by means of cyclic voltammetry in CH₂Cl₂, are listed in

Table 2. Electrochemical characterization of derivatives 1a–c.

Compound	Egap (eV) 1	oxEonset (V) 2	IP (eV) 3	EA (eV) 4	IP (eV) 5
1a	3.22	0.18	5.58	2.36	5.18
1b	3.22	0.21	5.60	2.38	5.11
1c	3.22	0.27	5.66	2.44	5.11

¹ Optical energy gap (E_gap) estimated from the absorption spectrum (λ_abs,onset) in CH₂Cl₂. ² Onset oxidation potential (^oxE_onset) vs. Fc⁺/Fc determined from CV in dichloromethane (1 mM). ³ Ionization potential (IP) estimated as IP = ^oxE_{onset vs. Fc+/Fc} + 5.39. ⁴ Electron affinity (EA) estimated as EA = IP − E_gap. ⁵ Ionization potential (IP) in the solid state, determined via the photoelectron emission technique.

. All compounds exhibited a first quasi-reversible oxidation process and a second irreversible one, whereas no reduction process could be observed. Consequently, the ionization potentials could be estimated. Their values are slightly conditioned by the length of the N-alkyl chains, ranging from 5.58 to 5.66 eV in derivatives 1a and 1c, respectively. These values are translated into quite low-lying HOMO energy levels, conferring stability against atmospheric oxygen to the structure while also being suitable for hole injection. The energy of the optical band gaps (E_gap) was estimated to be 3.22 eV in all three cases, which is smaller than that of the bare triindole system (3.53 eV [20]) because of the more extended π-conjugation of the system. The ionization potentials were also estimated in the solid state though the photoelectron emission technique as a closer approximation of the device conditions. The featured values, which go from 5.11 eV (1b,c) to 5.18 eV (1a), should ensure an optimal injection process due to the closeness to the gold work function (5.1 eV). The resulting energy levels are represented in Figure 2. The cyclic voltammograms and the photoelectron emission spectra are compiled in the Supporting Information.

The charge-transporting properties of the final compounds 1a–c were analyzed via the time-of-flight (TOF) technique as a first approximation (

Table 3. Charge carrier mobility values of derivatives 1a–c obtained from TOF measurements.

Compound	μh (cm2 V−1 s−1) [E (V cm−1)] 1	μ0 (cm2 V−1 s−1) 2	α ((cm V−1)1/2) 3
1a	7 × 10−5 [7 × 105]	4 × 10−6	0.0034
1b	1 × 10−3 [2 × 105]	6 × 10−4	0.0017
1c	1 × 10−3 [2 × 105]	2 × 10−4	0.0053

¹ Hole mobility (μ_h) at the specified electric field. ² Zero-field mobility (μ₀). ³ Field dependence (α). The measurements were performed under ambient conditions at room temperature. The layers were prepared under vacuum evaporation, with thicknesses ranging from 0.56 to 1.4 μm.

). The graphics containing the TOF transients and the dependence of the hole drift mobilities are collected in the Supporting Information. All three compounds showed a non-dispersive behavior and anticipated promising p-type semiconductor properties. As observed, the extracted mobility values are highly conditioned by the N-alkylation patterning. Derivative 1a, featuring the shortest N-alkyl chain, displays a maximum μ_h of 7 × 10⁻⁵ cm² V⁻¹ s⁻¹ that is clearly outdone by its analogs 1b,c (1 × 10⁻³ cm² V⁻¹ s⁻¹). This enhancement, which goes up to an order of magnitude, manifests the preference of longer N-alkyl chains in this particular core. Nevertheless, a further elongation from hexyl to decyl did not show a substantial modification using this technique.

3.3. Organic Thin-Film Transistors

Considering the appropriate hole-transporting properties in conjunction with the adequate energy levels of compounds 1a–c, all of them were tested as p-type semiconductors in OTFT. The characteristics of the OTFT devices fabricated from 1a–c over OTS- and PS-treated Si/SiO₂ substrates are compiled in

Table 4. OTFT characteristics of devices integrating compounds 1a–c over Si/SiO₂ substrates passivated with either OTS or PS.

	SiO2/OTS		SiO2/PS
Compound	μh (cm2 V−1 s−1)	Ion/Ioff	μh (cm2 V−1 s−1)	Ion/Ioff
1a	7 × 10−5	103	6 × 10−5	102
1b	2 × 10−4	104	1 × 10−4	103
1c	5 × 10−4	104	2 × 10−4	104

.

As observed, the length of the N-alkyl chains appears as the main factor to modulate the hole mobility. In fact, the substitution of the N-methyl by the N-decyl implies an increase in the μ_h up to an order of magnitude over OTS-treated substrates, from 7 × 10⁻⁵ to 5 × 10⁻⁴ cm² V⁻¹ s⁻¹. The trend displayed by OTFT devices is, therefore, consistent with the results collected via TOF measurements. The effect of the N-alkylation on the performance of triindole also surpasses that of the nature of the aromatic moieties. Specifically, the attachment of alternative sulfurated scaffolds (i.e., 5-methylthien-2-yl and benzothien-2-yl) or aromatic hydrocarbons (i.e., phenyl and naphtyl) could only modulate the μ_h from 2 × 10⁻⁴ to 4 × 10⁻⁴ cm² V⁻¹ s⁻¹ [33]. The I_on/I_off ratios follow the same tendency, with higher and more suitable values of ca. 10⁴ in the case of the N-decylated 1c. Considering the effect of the passivation layer, OTS-containing devices slightly outperform their PS counterparts. This effect is also more significant with derivatives featuring longer N-alkyl chains. Another point to highlight is that devices fabricated with the N-methylated derivative 1a display very linear saturation characteristics and low threshold voltage. Contrarily, derivatives 1b,c exhibit a kink, so the charge mobility values were extracted from the region at higher V_G, as suggested in the literature [51,52]. The OTFT characteristics of devices fabricated from compounds 1a–c over OTS-treated substrates are illustrated in Figure 3, whereas those of their PS counterparts can be found in the Supporting Information. The evolution of the hole mobility through time was also monitored to evaluate the air stability of this core, a feature that is highly coveted in organic electronics. As shown in Figure 3d, compounds 1a–c exhibit minimal fluctuation of the hole mobility throughout a month. Their notorious air-stability was also corroborated by a shelf lifetime surpassing 100 days (Figure S7).

3.4. Solid-State Characterization

3.4.1. Crystallographic Data

The crystal structure of derivative 1a could be elucidated by means of single-crystal X-ray diffraction. The ORTEP projection and the molecular packing detailing the main intermolecular interactions are depicted in Figure 4. Compound 1a crystallized in space group P–1 of the triclinic system, with dimensions a = 7.2546 (7) Å, b = 14.2460 (14) Å, c = 14.4915 (15) Å, α = 95.662 (5)°, β = 100.412 (4)°, and γ = 98.766 (4)°, and a total volume of 1443.5(3) ų. The intermolecular packing could be classified as a β-type or sheet [53,54], featuring slightly displaced cofacial interactions. This type of arrangement is often considered as optimal for charge transport due to the strong π-overlap between molecules [54,55]. Specifically, the π–π interactions show conveniently short distances as close as 3.34 Å (Figure 4c). The methyl chains also assist and reinforce the packing with CH···π interactions. It should be noted that the triindole backbone is not entirely planar, since the peripheral benzene rings display a deviation of ca. 9° with respect to the central one. The thiophene moieties also show torsion angles with respect to the triindole nucleus that go from 13.4 to 40.8° (Figure 4a). In spite of this, the arrangement of derivative 1a is prone to facilitating the charge transport throughout the material.

3.4.2. Order and Morphology of the Thin Films

In order to correlate the characteristics of the OTFT devices based on derivatives 1a–c with the analyzed structural variations, the crystallinity and disposition within the thin films was evaluated by means of grazing incidence X-ray diffraction (GIXRD). The GIXRD patterns, illustrated in Figure 5a,b, are consistent with the tendency exhibited by the OTFT devices. The longer the N-alkyl chains, the sharper and more intense the diffraction peaks, which indicates a more prominent degree of order and crystallinity in the thin films. Comparing the nature of the passivation layer, the outperforming OTS-treated devices also provide more crystalline films than their PS analogs in derivatives 1b,c.

The N-methylated 1a provides a rather amorphous arrangement regardless of the passivation layer, agreeing with the lower OTFT performance and the small difference between OTS and PS. Nevertheless, the availability of the single-crystal structure of 1a permitted a closer insight into the arrangement of this material. From the simulated powder diffractogram, the diffraction signal peaking at 2θ = 5.52° could be assigned as the plane (010), which stands parallel to the substrate. As represented in Figure 5c, the π-system of 1a adopts a perpendicular disposition with respect to the substrate. Consequently, the π-stacking direction lays parallel to the substrate, ensuring a proper pathway for charge transport. The morphology of the vacuum-deposited thin films of 1a–c over OTS- and PS-treated substrates was analyzed by means of atomic force microscopy (AFM). The AFM images, depicted in Figure 6, reveal quite homogeneous layers with small protrusions or hillocks.

The structural differences in derivatives 1a–c are mainly translated into distinct grain sizes. The methylated derivative 1a presents a quite uniform surface with undefined domains of small grain sizes from 0.03 to 0.06 μm over PS, coinciding with the notoriously low degree of order detected via GIXRD. The deposition over OTS constitutes a more irregular layer but with slightly larger grains (0.1 μm). On the other hand, derivatives 1b,c exhibit surface profiles with more defined regions and larger grain sizes from 0.15 to 0.3 μm over both OTS and PS, agreeing with the higher degree of order of the films. In addition, the minor roughness and morphology of 1c films are again accordant to the superior hole mobilities extracted from the OTFTs.

4. Conclusions

Three thiophene-functionalized triindoles possessing different N-alkylation patterning (1a–c) were successfully synthesized in a facile four-step route. The incorporation of the thiophene moieties placed the ionization potential of the triindole nucleus at 5.11 eV in the solid state, making it perfectly suitable for hole injection while still preserving stability against oxidation. Particularly, the N-decylated derivative 1c reached a hole mobility up to 1 × 10⁻³ and 5 × 10⁻⁴ cm² V⁻¹ s⁻¹ based on TOF and OTFT measurements, respectively. In fact, the presence of longer N-alkyl chains not only endowed the thin films with a higher degree of order and crystallinity, but also granted a more adequate morphology. The deposition of the thin film and its characteristics were also ameliorated by functionalizing the Si/SiO₂ surface with OTS in comparison with the PS coating. All these factors directly contributed to promoting the hole mobility in the respective OTFTs. Structurally, the strong π–π interactions and the favorable β packing found in the single crystal correlate with the good OTFT performance. Finally, the potential of these materials in organic electronics is supported by their outstanding air stability, which implies a minimal decrease in the OTFT performance and a shelf lifetime surpassing 100 days.