Tunable Charge Transport Using Heterocycles-Flanked Alkoxyphenanthrenes for High-Performing OFETs

Abstract

A series of new heterocycles-flanked alkoxyphenanthrenes with D′-D-D′ and A-D-A architecture was synthesized for high-performance solution-processable p-channel, n-channel, and ambipolar organic field-effect transistors. The impact of electron-donating and -accepting abilities of the sulfur- and nitrogen-containing heteroaromatics on photophysical, electrochemical, and semiconducting properties was analyzed. The presence of heteroaryl rings improves the extended conjugation, two-dimensional lattices of π–π stacks, and increased molecular interaction of the functionalized phenanthrenes (PN) to allow better self-assembly. The electronically dynamic PN self-assembles into continuous microdomains, forming percolation channels for holes, electrons, or both reliant on functionalization. The low-lying LUMO levels of the compounds enabled ambipolar transport and reduced energy levels for charge injections. Spin-coated devices fabricated using functionalized PN with sulfur-containing heteroaryl substituted PN exhibited the highest hole mobility of 0.85 cm²/(V s) with 10⁸ on/off current ratio. Compounds with A-D-A architecture showed n-channel/ambipolar charge transport, especially napthalimide acceptor substituted PN exhibited n-channel electron mobility of 0.78 cm²/(V s) and an on/off ratio of 10⁶. X-ray diffraction and scanning electron microscopy studies further delineate the impact of efficient packing in the film. Quantum chemistry calculations combined with Marcus–Hush electron transfer theory interpret the transport parameters, and heteroatoms are established to impact the charge mobility intensely.

Introduction

Organic field-effect transistors (OFETs) have attracted increasing research consideration from academic and industrial communities owing to their potential applications in integrated circuits (ICs), flexible displays, and low-cost electronic devices.¹⁻³ In recent years, significant progress has been made in developing new small molecular semiconductors and device fabrication techniques to improve OFET performance.⁴⁻⁶ Solution-processable conjugated organic small molecules continue to captivate extensive interest in OFETs due to their potential for large-area, lightweight, and facile fabrication.⁷⁻⁹ Organic small molecules also possess tailorable chemical structures,^10,11 adjustable functional groups, and feasible synthetic techniques, making them next-generation electronic materials.^12,13 Many organic small molecule-based semiconductors were reported with p-type, n-type, and ambipolar OFETs, but their performance and stability under ambient conditions still need improvement.¹⁴⁻¹⁶ Because semiconductors with n-type and ambipolar OFETs can form complementary ICs, which exhibit lower power consumption and larger noise margins than unipolar ICs.^17,18 In addition, nowadays, OFET-based memory devices, light-emitting diodes, and sensor-like dual applications are lining up. For that, there is a considerable need to improve photophysical and electrochemical properties with efficient morphological arrangements for small molecules.^19,20 Molecular engineering of organic semiconducting molecules can solve the above concerns while making desired OFET materials.²¹ Progression of designs in organic semiconductors with high pi-orbital planarity and outstanding inter- and intramolecular charge carrier transport could lead to the realization of high-performance OFETs as the molecular structure and packing directly influence the OFET performances.²²⁻²⁴ The most efficient way toward air-stable ambipolar OFETs is to fabricate devices with a single semiconductor material with high charge mobility for both hole and electron transfer. In practice, however, unipolar transport dominates when organic semiconductors are applied in transistors because of the charge traps and the presence of the injection barriers between electrodes and molecular frontier orbitals.²⁵ Consequently, it is necessary that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) must fulfill comparatively suitable conditions to attain air stability. The inadequate advances in this case are affected by challenges related to stabilizing and delocalizing the LUMO of π-conjugated molecules. Stabilization of the LUMO denotes the increase of the electron affinity, which can be comprehended by materials that comprise electron-deficient units. The LUMO delocalization can be improved by backbone planarization and intramolecular interaction.^26,27

Functionalizing p-type semiconductors with heteroatoms or electron-withdrawing groups has been discovered as one of the most active approaches in enhancing performance and tuning the semiconducting property from p-type to n-type or ambipolar semiconductors.²⁸ It is observed that the introduction of a heteroatom greatly affected the physical properties such as crystal structures, energy levels, and charge transport mobilities by introducing heteroatom-containing polycyclic aromatic hydrocarbons with alkyl and aryl substitutions. Recently, Wong and co-workers²⁰⁰ developed a novel series of heteroatom-substituted napthodithiophene and dithienylbenzothiadiazoles, including fluorine, oxygen, or nitrogen. Incorporating fluorine can reduce the LUMO energy level, and nitrogen or oxygen atoms stabilize the LUMO energy level. The effect of heteroatom on the photovoltaic and charge mobility properties of organic solar cells (OSC) and OFETs was witnessed with high performance. Jiang and co-workers²⁶ proved a series of oligothienoacenes and their heteroatomic and halogen-substituted derivatives have been investigated computationally to elucidate their potential as semiconducting materials for OFETs. Unsubstituted oligothienoacenes are found to work only as p-type semiconductors due to their high electron injection barrier on metal electrodes. Heteroatomic substitution improves their semiconducting properties for holes as well as electrons, leading to the alteration of p-type oligothienoacenes to ambipolar boron-substituted oligothienoacenes as active semiconductor layers for OFETs.

Motivated by the fact that the heteroaromatic substitution on organic small molecules would be an effective method for obtaining different optical, electrochemical, well-structured morphological, and OFET characteristics.³⁰ Herein, we have targeted one of the remarkable polyaromatic hydrocarbons, phenanthrene (PN), as the conjugate core and introduced heteroatoms (N and S) containing aromatic rings. Phenanthrene, known as [3]-Phenacene, comprises three fused benzene rings with a curved structure and more stability than its linear isomer, anthracene. Owing to their strong π–π stacking, phenanthrene can form rigid and planar structures, which may give rise to high charge carrier mobilities in OFET devices. In 2006,³¹ Wang and co-workers designed and synthesized two novel phenanthrene-based conjugated oligomers as p-channel material, which had shown mobility up to 2.5 × 10^–2 cm²/(V s) and outstanding stability. By employing pi-extended phenyl and styryl functionalized 2,7-phenanthrenes OFETs were fabricated and resulted in mobility up to 1.6 cm²/(V s) with a 10⁸ on/off ratio. Thus, functionalizing the PN backbone with different aromatic/heteroaromatic rings can be utilized as a strategy to establish the relationship between the electronic properties and their molecular structures.³² The hexyloxy chains are introduced at 9 and 10 positions of PN to improve solubility and encourage humid-resistivity and noncovalent inter- and intramolecular interactions, leading to an extended molecular ordering with improved morphologies.^33,34 The target molecules are designed in D′-D-D′ and A-D-A architectures by introducing the standard heteroaromatic rings containing sulfur and nitrogen, which have well-known literature in OFET devices.^8,35,36 In this work, six molecules were synthesized and characterized, and all of the molecules were systematically investigated for OFET application. Our interest is to establish the relationship between the electronic properties of the molecules and their structures. Further, this work reveals the effects of varying heterocycles on the performance of alkoxy phenanthrene-based OFET devices. It may provide useful knowledge for the design of high-performance phenacenes-based materials.

Results and Discussion

Design and Synthesis

The molecular design is achieved by flanking heterocycles with donor and acceptor capabilities to the PN backbone and understanding their impact on photophysical, electrochemical, and related properties, along with OFET performances. Incorporating oxygen atoms via hexyloxy chains will act as an electron-pushing side chain to improve the p-channel characteristics and solution processability of the PN backbone.³⁷ It also provides a molecular fastener effect with minimal π–π stacking disruption leading to strong intermolecular interactions, which has been found to enhance significantly the molecular ordering in the thin film.³⁸ In this work, phenyl, sulfur-containing electron-donating thiophene and benzo(b)thiophene, nitrogen-containing electron-deficient pyridine, isatin, and naphthalimide have been chosen as the end-caps for alkoxy phenanthrenes which were designed to have D′-D-D′ (7a–c) and A-D-A (7d–e) architectures. Thiophene and benzothiophene fused heterocyclic derivatives have been studied due to their large planarity, high stability, and hole mobility. Specifically, Yu et al.²⁹ synthesized a series of ladder-type thienoacenes based on them, which showed versatile optical properties and p-type performance. The presence of exposed lone pairs at the nitrogen atoms of 4-pyridine enhances the intermolecular interactions, thus facilitating carrier transport. Pyridine acts as an electron-withdrawing group when incorporated with tetrathiafulvalene,³⁹ showing high mobilities. Isatin is a weak electron-withdrawing moiety and consists of two oxygens and nitrogen with a planar structure. Recently, researchers synthesized isatin-based monomers with elongated π-conjugation for the OFET devices, which showed p-type or ambipolar charge transport⁴⁰ electron mobility up to 0.024 cm²/(V s). 1,8-Napthalimide (NI) derivatives are multipurpose fluorescent compounds and an interesting class of electron-deficient organic materials owing to NIs. They have unique photophysical properties and high electron affinity and are widely utilized in small molecules and polymers for OFETs. The exploration of NI was motivated by the fact that an already low-lying LUMO can be further lowered by the incorporation of phenanthrene to obtain air-stable n-type OFETs. D-π–A based highly emissive 6-arylalkynyl-1,8-naphthalimides were synthesized and used for OFETs and optical waveguides.⁴¹

They exhibited ambipolar charge transport with good mobilities and stabilized the LUMO energy levels. All these heterocycles substituted PNs could have induced weak intramolecular interaction between S---S/S---O/N---H and N---O, making the core more rigid and eventually inducing molecular crystallinity toward high-performing OFETs. The structures of the synthesized compounds (7a–f) and the schematic routes are given in Figure 1 and Scheme 1, respectively. 2,7-Diiodophenanthrene-9,10-dione (1) was synthesized from 9,10-phenanthroquinone as per the literature. Phenanthroquinone was allowed to react with Na₂S₂O₄ using a phase-transfer catalyst to give Compound 2. 1-Hexyl 5-bromoisatin (3) was prepared by reacting 5-bromoisatin and 1-bromohexane. Compound 5 was prepared by the N-alkylation of 1,8-naphthalic anhydride by reacting with dodecylamine. Alkylated bromo compounds 3 and 5 were reacted with bis(pinacolato)diboron to yield precursors 4 and 6 in 70 and 62% yield, respectively. The target compounds that resulted through the Suzuki cross-coupling between diiodinated alkoxy phenanthrene (2) and the corresponding boronic acids/boronates afforded compounds 7a–f with good yield. Compound 2 was substituted with a series of aryls and heteroaryl, namely, phenyl (7a), thiophene (7b), benzo(b)thiophene (7c), pyridine (7d), isatin (7e), and napthalimide (7f) rings. The compounds are soluble in common organic solvents, such as dichloromethane, chloroform, tetrahydrofuran (THF), and dimethylformamide (DMF). The molecular structure of the synthesized compounds is determined by NMR and high-resolution mass spectral techniques (Figures S1–S16).

Figure 1.

Molecular structure of compounds 7a–f.

Scheme 1. Synthetic Route to Compounds 7a–f.

Photophysical Properties

To understand their photophysical properties, the phenyl- and heterocyclic-functionalized phenanthrene triads were analyzed by UV–vis absorption and fluorescence spectroscopic techniques. The absorption (10^–5 M) and emission (10^–7 M) spectra were recorded in dichloromethane as the solvent, as shown in Figure 2a,b. The relevant data are compiled in Table 1. The UV–vis spectra of compound 7a having a simple phenyl ring substitution on the PN backbone resulted in a maximum absorbance of 291 nm. For compounds 7b–7f, the sharp absorption maxima from 292 to 358 nm correspond to the π–π* transition of the conjugated backbone and bathochromic shifts observed bathochromic shifts observed than compound 7a, likely the conjugation of the heteroatom lone pair. Compounds 7b and 7c with S atoms showed the maximum absorbance at 292 and 356 nm, respectively. Compound 7c was observed with a redshift in absorbance maxima of about 65 nm from compound 7a, attributed to the electron-donating ability and extended π-conjugation by the fused benzo(b)thiophene.⁴¹ Among the nitrogen-based heterocycles, compound 7f has shown a maximum absorbance of 358 nm with an absorbance coefficient of 64,000 dm³/mol cm, which is attributed to extended π-conjugation with high planarity.⁴¹ All of the compounds were excited to obtain emission spectra with their absorption maxima.

Figure 2.

(a) Absorption and (b) emission spectra (excited with their corresponding λ_max) of compounds 7a–f in dichloromethane.

Table 1. Optical Properties of Compounds 7a–f in Dichloromethane Solution.

compounds 7	absorption	emission	stokes shift (nm)	absorption coefficient (103 × dm3 mol–1 cm–1)
	λmax (nm)	λmax (nm)
a	291	413	122	70
b	292	416	124	58
c	311, 356a	436	80	35
d	291	407	116	26
e	295	419	124	90
f	284, 358a	434	76	64

^aAbsorption coefficient calculated.

The compounds 7a–f showed a broad emission spectrum with λ_max around 407–436 nm, and it is observed in the following order: 7c > 7f > 7e > 7b > 7a > 7d. The compounds, particularly 7c–7f with extended conjugation due to fused phenyl over the heteroaryl ring, increase their emission wavelength. Among these compounds, 7c with an electron-donating substituent showed the highest λ_max of 436 nm. The PN-based compounds resulted in high Stokes’ shift ranging from 76 to 124 nm. To understand the origin of the unique spectroscopic properties, time-dependent-DFT calculations were performed on 7a–f. The optical bands of these compounds arise from the combinations of multiple HOMO – n and LUMO + m transitions (Table S4), which are quite in agreement with the experimental results. The DOS states were utilized to calculate the number of available states in the systems, which can be used as a basis for the transport model (Figure S18).⁴² The highest density of states was available for compounds 7c, 7e, and 7f, whereas it was relatively low for other compounds.

Electrochemical Properties

Cyclic voltammetric (CV) measurements were carried out to estimate the electrochemical properties and FMO energy levels of compounds 7a–f. All the experiments were performed using 0.1 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as a supporting electrolyte with a scan rate of 100 mV/s. The CVs were recorded vs SCE potential, which was calibrated using the (Fc/Fc⁺) redox couple. The HOMO and LUMO energy levels of the compounds were calculated from the first onset oxidation (E_Ox) of CV curves (Figure 3). To eliminate the effect of oxygen, the system was purged with nitrogen for 15 min. E_HOMO = −(E_ox + 4.8 – E_fc/fc+) was used to obtain the HOMO levels of the compounds with reference to ferrocene.

Figure 3.

Cyclic voltammograms of compounds 7a–f (in dichloromethane solution, 100 mV/s scan rate, TBAHFP as the supporting electrolyte).

For all the compounds 7a–f, LUMO lies around −2.68 to −3.84 eV, and HOMO levels are between −5.81 and −5.64 eV, respectively (Table 2). The substitution of various heterocycles on the central phenanthrene is exhibited to be favorable for stabilizing the LUMO level and increasing the HOMO level, respectively. The oxidation potentials of the compounds decreased with electron-donating substituents and increased with electron-accepting substituents. Compound 7a with a simple phenyl ring has shown (E_ox = 1.43 V) with a HOMO level of −5.81 eV. Compounds 7b and 7c with electron-donating thiophene rings exhibited the highest HOMO level of −5.63 eV, which may facilitate the charge injection by reducing the charge injection barrier and ensuring efficient p-channel characteristics for fabricating OFET devices. Compounds 7d–f with electron-accepting units, estimated from the onset reductive potentials, have resulted in low-lying LUMO levels of −3.84 to −3.62 eV, which can show good electron transporting ability and can deliver ambipolar/n-channel OFETs.^43,44 The HOMO and LUMO energy levels suggest that they are air-stable materials with electrochemical reproducibility.⁴⁵ The HOMOs/LUMOs of these PNs estimated from the CV measurements show a trend similar to those provided by DFT calculations.

Table 2. Electrochemical Properties of Compounds 7a–f.

7	Eox (V)	Ered (V)	experimental			computational
			HOMO (eV)	LUMO (eV)	band gap Ega (eV)	HOMO (eV)	LUMO (eV)	bandgap (eV)
a	1.43		–5.806	–2.726b	3.08	–5.521	–1.266	4.26
b	1.37		–5.710	–2.770b	2.94	–5.500	–1.282	4.22
c	1.29		–5.636	–2.676b	2.96	–5.402	–1.716	3.69
d	1.41	–0.52	–5.756	–3.840c	3.00	–5.822	–1.761	4.06
e	1.44	–0.74	–5.786	–3.620c	3.01	–5.964	–2.507	3.46
f	1.46	–0.61	–5.806	–3.740c	2.90	–6.048	–2.780	3.27

^aE_g calculated from the onset of the absorption spectra (E_g = 1240/λ_onset).

^bE_LUMO = [E_HOMO – E_g].

^cE_LUMO = −(4.8 + E_red – E_Fc+/Fc).

Thin Film Morphologies

The thin film microstructure is vital for individual OFET, and larger-scale IC performance. For small molecule-based solution-processable devices, efficient self-assembly is a prerequisite for reproducible performance. The surface morphologies of the fabricated thin films were analyzed by scanning electron microscopy. To control the self-assembly of the films, the spinning time, speed, solvent choice, and postdeposition methods were optimized. The devices (7a–f) were fabricated by a spin-coating method using CHCl₃ as a solvent and annealed at 80 °C for 45 min to eliminate the residual solvent and to get well-defined self-ordering and crystallinity of the molecules. The presence of different end groups resulted in diversely self-assembled films with unique morphologies (Figure 4a). Compound 7a with simple phenyl substitution resulted in microbundles arrangements. Thiophene containing 7b resulted in square-shaped grains throughout the film. The benzo(b)thiophene substituted 7c, resulting in rod-like morphologies with a well-connected network to support efficient transportation of charge carriers. The pyridine-flanked compound 7d resulted in a closely packed cuboid structure in a well-ordered manner. Compounds 7e and 7f with isatin and naphthalimide substitutions reveal flower- and petal-like hierarchical arrangements, respectively. The grain size plays a vital role in determining the performance of the OFET device by enhancing carrier mobility.⁴⁶ All of the thin films are made of homogeneous grains with an average diameter of 3 μm. The largest average grain size of 6 μm was observed for compound 7f. This larger grain size is attributed to the naphthalimide substituent with a large planar π-conjugation, which induces better molecular ordering. A grain size of less than 5 μm was observed for the remaining compounds. SEM images reveal that the thin film has covered the complete surface with a good charge carrier network.⁴⁷

Figure 4.

(a) SEM images of thin films of compounds 7a–f. (b) Thin film GIXRD patterns of compounds 7a–f.

X-ray diffraction studies were performed to explore further the crystallinities and molecular orientations of 7a–f in a thin film. The thin film XRD patterns are given in Figure 4b. All of the compounds have formed highly crystalline films. Crystalline nature is one of the significant criteria for reaching good charge carrier mobility in thin film transistors. In all films, the first-order reflection is intense, and multiple orders of reflection indicate the films to be well-ordered, layered microstructures. The compound’s interplanar d-spacing was calculated from the thin film XRD data. Compounds 7a–f, the strong reflective XRD pattern at 2θ = 7.6–9.1° corresponds to a d-spacing of 0.98 nm. An additional peak around 2θ = 20–25° resulted in a d-spacing value of 0.44–0.36 nm of compounds 7a–f corresponding to the π–π stacking distances.⁴⁸ From the analysis, compounds 7a–f present better crystallization capability owing to their larger π-conjugation, noncovalent interactions between heteroatoms and hydrogen, and better molecular planarity, which are ultimate requirements for high-performing OFETs. Interplanar distances in nanoscale regimes result from the tendency of the strong noncovalent interactions between heteroatoms.

Computational Studies

Density functional theory (DFT) studies were carried out to understand the theoretical aspects of heteroaryl functionalized PNs. It is well-known that the electronic structure of semiconductors is a critical factor in the transistor performance. To obtain more information about the molecular structure of compounds 7a–f, all the compounds’ geometry was optimized by DFT using the Gaussian 03 program at the B3LYP/6-31 (d) level. To examine the impact of solvent correction, van der Waals dispersion correction was used, such as the role of noncovalent interactions using VASP calculation, where DFT-B was used. The optimized structures were utilized for HOMO–LUMO energy distributions, as shown in Figure 5. The HOMO/LUMO surface plots of compounds 7b and 7c suggest that the HOMO is uniformly spread and delocalized all over the molecules, and LUMOs are not continuously delocalized due to the more electron-donating nature of thiophene and benzo(b)thiophene substituents. For compound 7d–f, the electronic clouds on the HOMO energy levels are mainly located on the PN backbone. The LUMO orbitals are delocalized and dense on acceptor moieties such as pyridine, isatin, and naphthalimide, suggesting they are n-channel materials. It is beneficial to have heteroatoms in compounds 7a–f since the lone pair electrons can be released toward the aromatic rings and can be delocalized through the conjugated system. The optimized geometries and the dihedral angles suggested that the PN backbone remains planar and substituted heteroaryl substituent, tilted with a dihedral angle of 36.69°, the maximum for compound 7a with a simple phenyl ring. Low dihedral angles of compounds 7b–f suggest that heterocycle functionalized PN, do not change their complete planarity due to the functionalization. Instead, they will have conformational locking encouraged by noncovalent interactions, which makes rigid and planar structures that will assist OFET performance.⁴⁹ The molecular length of compounds 7a–f predicted by Gaussian ranges from 1.56 to 4.91 nm. The larger size indicates compound 7f can serve as a “small molecular wire” for high-performing electronic material.⁵⁰ Well-defined three-dimensional self-assembly on the nanometer scale can have applications in thin film technology. The crystallographic values obtained from VASP were used to assign the crystal parameters and set up packing patterns for Discover Studio software. The packing pattern was modeled with various group symmetry elements, and only those that have plausible parameters and hopping values were computed and are presented in Table S2. These values were used to build up the polycrystals of the respective molecules. All of the molecules pertained to a simple orthorhombic system. The interatomic distances are indicated by lines in the individual packing, as shown in Figure S17. The interatomic distances of the molecules were obtained by considering the volume and space groups. The main advantage of having heteroatoms in a molecule is that it induces noncovalent interactions, which leads to better molecular ordering and orientation for charge hopping. These interactions are observed between the H atom and the O, N, and S atoms of the adjacent molecule, and there are possibilities for N---N, O---N, and O---S interactions within the same molecule. Compounds 7b–c have an S’ atom, which makes an S–S linkage with a minimum interactive distance of 0.235 and 0.219 nm, respectively. These intra- and intermolecular packing distances in nanoscale and the most planar structures propose good environments for intra- and intermolecular π–π orbital interaction to attain the highest charge transport results in the solid state.⁵¹⁻⁵³

Figure 5.

Optimized geometry, HOMO and LUMO of compounds 7a–f.

OFET Characteristics

The OFET properties of all six synthesized compounds 7a–f were investigated by using the Keithley 4200A semiconductor characterization system. OFET devices were fabricated using bottom-gate top contact (BGTC) architecture over a heavily n⁺² doped/p⁺² doped silicon wafer under air and atmospheric conditions. Thin films of 7a–f were spin-coated, followed by annealing at 80 °C for 45 min. In addition, the wafer is thermally annealed at 90 °C for 30 min as a postdeposition treatment to attain better self-assembly. The transfer and output curves were measured, and the average charge transport performance of five devices and data are summarized in Table 3. The average field-effect mobilities obtained were measured in the saturated region. The OFET characterization of compounds 7a–f has been plotted into output and transfer curves (Figure 6). The width and length of the channels were 3 mm and 150 μm, respectively. The OFET parameters were extracted by considering the kink observed in the transfer curve.^54,55 The D′-D-D′ and A-D-A molecular architects with different donor/acceptor substituents have resulted in both unipolar (p-type, n-type) and ambipolar OFETs. The transistors exhibited moderate to high hole and electron transport mobilities.

Table 3. OFET Properties of Compounds 7a–f.

compounds 7	μha (cm2/V s)	on/off ratio	threshold voltage (V)	μea (cm2/V s)	on/off ratio	threshold voltage (V)
a	0.20 ± 0.02	105	–12
b	0.69 ± 0.05	105	–5
c	0.85 ± 0.05	108	–11
d	0.21 ± 0.04	104	–10	0.22 ± 0.05	105	8
e	0.43 ± 0.08	104	–4	0.60 ± 0.07	106	9
f				0.78 ± 0.05	106	5

^aError bar calculated from five devices, μ_h—hole mobility, μ_e—electron mobility.

Figure 6.

OFET characteristics of compounds 7a (a,b), 7b (c,d), 7c (e,f), 7dp-type (g,h), n-type (i,j), 7ep-type (k,l), n-type (m,n), and 7fn-type (o,p).

The PN is a flat planar backbone, performing as an efficient donor unit with electron-pushing hexyloxy groups in 9,10-positions. Compound 7a with simple phenyl substituents has resulted in p-channel behavior with mobility of 0.20 cm²/(V s) and an on/off ratio of 10⁵. The compounds 7b and 7c have thiophene and benzo(b)thiophene as the substituents, both exhibited p-channel behavior. Compound 7c had resulted in maximum hole mobility of 0.85 cm²/(V s) and 10⁸ on/off ratio, and the electron-donating substituent with a fused aromatic ring of thiophene and benzene is found to be an effective donor to extending the π-electron delocalization of a molecule without destabilizing the HOMO energy level. In particular, the thiophene and fused-ring thiophene were proposed to have good OFET performance due to various intra- and intermolecular interactions, including weak hydrogen bonding, π–π stacking, and sulfur–sulfur interactions originating from the high polarizability of sulfur electrons in the thiophene rings.²⁶ Moreover, it is believed that the large atomic radius of sulfur leads to strong intermolecular interactions, and high electron density in the HOMO leads to an adequate overlap between the HOMO of neighboring molecules in the solid state.⁵⁶ Compounds 7d–f have nitrogen heteroatoms and exhibit ambipolar and n-channel characteristics. Nitrogen is a versatile heteroatom and can be used in various hybridization forms. Compounds 7d and 7e with pyridine and isatin substituents, which are electron acceptors, have resulted in ambipolar materials with a high electron mobility of up to 0.60 cm²/(V s).

The lone pair electrons of pyridine-nitrogen do not contribute to the π–π-electron system, and the nitrogen atom has a stronger electronegativity. The resulting semiconductors often have low HOMO and LUMO energy levels, and thus could exhibit ambipolar characteristics.⁵⁷ Mainly, 7e resulted in balanced hole and electron mobilities, an important prerequisite for an ambipolar small molecule. Compound 7f with a naphthalimide substituent has resulted in n-channel behavior with high electron mobility of 0.78 cm²/(V s). This proposes that the accepting ability of the naphthalimide group is much more than the donating ability of the PN unit, making the compound a unipolar n-channel material. The PN backbone is a π-spacer/bridge between two naphthalimide units. Compounds 7e and 7f with diimide groups are composed of pyrrole-nitrogen and display n-type/ambipolar characteristics, which can be attributed to the stronger electronegativity of oxygen atoms, making the diimide group act as a strong electron-withdrawing unit.²⁷

For a better understanding of the charge transport ability of various organic semiconducting layers (7a–f), it can be explained according to the reorganization energy (λ_h,e) and transfer integral (A_{+,_}), which have been calculated using Marcus–Hush theory (Table S1). The transfer integral, which describes the degree of the adjacent molecular orbital, strongly depends on the π–π distance, orientation, and the relative displacement distance. In addition, reorganization energy is defined as the energy change associated with geometry relaxation during charge transfer and is related to molecular degrees of freedom. In theory, large transfer integral and small reorganization energy are favorable for high charge carrier mobility.⁵⁸ By comparing reorganization energy values, it is observed that with the introduction of electron-donating heterocycles, hole transport property increased as well, and by introducing electron-accepting heterocycles, electron transport property increased.⁵⁹ This observation is valid with the trend of transfer integral values (Table S1). From the transfer integral, compounds 7b and 7c containing sulfur produce intermolecular short-range interactions that lead to large intermolecular orbital overlaps (increased transfer integral for hole) and suppress molecular motions in the film state (low reorganization energy for hole). Compounds 7d and 7e exhibited ambipolar characteristics with substantially similar reorganization energies for n- and p-type conduction. Compound 7f is more suitable for the n-type material and has a relatively large electron mobility. The magnitude of the hole reorganization energy (λ_h) for the molecule is profoundly larger than its corresponding electron reorganization energy (λ_e), which supports the idea that it could be a good electron transport material. In general, all compounds 7a–f consist of symmetrical functionalizations, which have lower reorganization energies than unsymmetrical molecular functionalizations.^60,61

These molecules are good hole and electron transport materials and can be adjusted upon a suitable substitution. The OFET results are also well supported by SEM and GIXRD analysis. The nanoscale inter- and intramolecular interactive distances support the efficient self-assembly in thin films. The grain size and grain boundaries played vital roles in the field-effect performance through continuous charge movement. The compounds 7c and 7f with high hole and electron mobilities have shown maximum grain sizes. The tuning of the p-channel (7a–c) behavior to n-channel behavior (7d–f) of compounds is due to the presence of nitrogen atoms containing heterocycles, which reduce the LUMO energy levels and energy barrier for electron injection and increase the energy barrier for hole injection from metal electrodes. Besides, the minimal noncovalent interactive distances predicted from computational analysis also support the high-performing OFETs. In addition, PN backbone, isatin, and napthalimide substituents were incorporated with an optimum length of alkyl substituent(s) onto the π-conjugated system, not only enriching solubility and solution processability of the consequential materials but also enabling close molecular packing in the solid state, which is beneficial for better charge transport, resulting in high performance of the devices. This improvement is due to a decrease in the injection barrier and an increase in the charge transfer mobility for electrons. However, without lowering their semiconducting performance for holes, it leads to the tunability of charge transport properties from p-type PNs to n-type and ambipolar PNs and is competent with the literature.^62,63

To understand the linearity of the mobility values, the V_g dependence of the mobility was plotted for compounds 7a–7f, and the plots are given in the Supporting Information (Figures S17 and S23). For most of the compounds, mobility initially increases with V_g and tends to decrease at high V_g. The decrease in the mobility values with respect to V_g has been revealed due to the current reduction in source and drain contact resistance.⁶⁴

Conclusions

A new series of sulfur- and nitrogen-containing heteroaryl-incorporated alkoxy phenanthrenes were synthesized for OFET applications. The compounds were synthesized from a Suzuki coupling reaction with a moderate to good yield. The low-lying LUMO level of nitrogen-based heteroaryl compounds resulted in high electron mobility for the fabricated devices. Electron-donating thiophene units substituted compounds have exhibited p-channel behavior with maximum mobility of 0.85 cm²/(V s) and a high ON/OFF ratio 10⁸. SEM and DFT analyses support the high performance of the devices. Thin film XRD proved to be efficient molecular packing through self-assembly. The reorganization of energy and transfer of integral values comply with the OFET results. The OFET characteristics of compounds 7a–f indicate that heteroatom substitution of the p-type phenacenes is a reasonable step toward producing good ambipolar/n-channel OFET materials and opens the doors for a wide range of electronic applications.

Experimental Section

Materials and Methods

9,10-Phenanthroquinone, N-iodoscuccinimide, trifluoromethanesulfonic acid, 5-bromoisatin, 4-bromo-1, 8-naphthalic anhydride, dodecylamine, 1-bromohexane, bis (pinacolato)diborane, dioxane phenylboronic acid, pyridine boronic acid, 3-thiopheneboronic acid, benzo(b)-thiophene boronic acid, Na₂S₂O₄, Bu₄NBr, K₂CO₃, KOH, Pd(dppf)Cl₂, and Pd(PPh₃)₄ were used as received from the commercial sources. THF, ethanol, and DMF solvents were also used as received. ACS-grade solvents were used for the spectroscopic analysis.

¹H and ¹³C NMR spectra were recorded in a Bruker 400 MHz spectrometer using tetramethylsilane as an internal standard and CDCl₃ as a solvent. High-resolution mass spectra were obtained from Thermo Exactive Plus UHPLC-MS. Absorption and emission spectra were recorded using the JASCO UV-NIR spectrophotometer and PerkinElmer LS 55 spectrophotometer. Electrochemical studies were performed in a CHI electrochemical workstation (CHI 6035D). A conventional cell setup containing three electrodes was used with glassy carbon (working electrode), a standard calomel electrode (reference), and a platinum wire as the counter electrode in an anhydrous dichloromethane solvent with tetrabutylammonium hexafluorophosphate (Bu₄NPF₆) as a supporting electrolyte. The experiments were performed in an inert atmosphere with scan rates of 100 mV/s. The system was standardized externally by using Fc/Fc⁺. SEM measurements were performed with a VEGA 3 TESCAN microscopy. An XPERT-PRO X-ray diffractometer was used for Grazing incidence X-ray diffraction in the reflection mode (CuKα radiation). DFT studies were employed to analyze the geometry and energy levels of the molecules. OFET characterizations were carried out using a Keithley 4200A semiconductor parameter analyzer at ambient conditions.

Experimental Procedure

Compound 2

Phenanthrene-9,10-dione (6.0 g, 28.8 mmol, 1.0 equiv) in trifluoromethanesulfonic acid (30 mL) was cooled to 0 °C. N-Iodosuccinimide (19.5 g, 86.4 mmol, 3.0 equiv) was added slowly to the reaction mixture for 30 min. Then, it was allowed to settle to room temperature, and the content was added over H₂O/ice to induce the precipitation. The orange solid 1 was recrystallized from CHCl₃ with a yield of 65%.¹ A mixture of compound 1 (5 g, 24.0 mmol), Na₂S₂O₄ (22.78 g, 144 mmol), and Bu₄NBr (4.64 g, 14.4 mmol) in 200 mL of THF:H₂O (1:1, v/v) was stirred for 15 min at room temperature. To this mixture, hexyl bromide (17.95 g, 72 mmol) followed by aqueous KOH (20 g, 360 mmol, in 100 mL of H₂O) was added slowly and allowed to stir for a further 48 h. The reaction mixture was diluted with 150 mL of water and then extracted with ethyl acetate (200 mL × 2). The combined organic layer was washed with water and brine, and then, the solvent was removed under low pressure. The crude recrystallized from methanol to yield compound 2 as a white solid (yield, 75%)^2,31H NMR (400 MHz, CDCl₃) δ 8.58 (s, 2H), 8.27 (d, J = 8.8 Hz, 2H), 7.86 (dd, J = 8.8, 2 Hz, 2H), 4.19–4.16 (t, J = 6.8 Hz, 4H), 1.92–1.85 (m, 4H), 1.42–1.37 (m, 8H), 1.30–1.24 (m, 4H), 0.96–0.92 (m, 6H). ¹³C NMR (100 MHz, CDCl₃) δ: 142.51, 134.54, 131.39, 127.16, 123.96, 93.28, 73.61, 31.60, 30.33, 29.68, 25.85, 22.72, 14.10.

Compound 3

A mixture of 5-bromoisatin (1.0 g, 4.42 mmol) in DMF and potassium carbonate (1.22 g, 8.8 mmol) was stirred at 0 °C for 1 h. Bromohexane (0.73 g, 4.42 mmol) was added to the reaction mixture and stirred at room temperature for 12 h. The resultant mixture was poured over water and extracted with dichloromethane. The combined organic phase was dried over anhydrous Na₂SO₄, and the solvent was removed under reduced pressure. The crude product was purified by column chromatography on silica gel (EtOAc:hexane) to afford 3. Red crystals (91%). ¹H NMR (400 MHz, CDCl₃) δ 7.73–7.67 (m, 2H), 6.81 (d, J = 7.9 Hz, 1H), 3.74–3.68 (m, 2H), 1.71–1.64 (m, 2H), 1.38–1.30 (m, 6H), 0.89 (t, J = 7.0 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ: 182.52, 157.41, 149.77, 140.51, 128.26, 118.76, 116.43, 111.85, 40.43, 31.35, 27.13, 26.53, 22.50, 13.99.

Compound 4

Compound 3 (1 equiv, 0.79 mmol) in dioxane (10 mL) was taken under a nitrogen atmosphere, followed by PdCl₂(dppf) (5 mol %) and potassium acetate. bis(pinacolato)diborane (2 equiv) was introduced after 20 min, and the mixture was refluxed for 24 h. After being washed with brine solution, the organic phase was separated using dichloromethane, and the solvent was removed under reduced pressure to afford compound 4 as red liquid (70%). ¹H NMR (400 MHz, CDCl₃) δ 8.08–7.97 (m, 2H), 6.89 (d, J = 7.9 Hz, 1H), 3.72 (t, J = 7.3 Hz, 2H), 1.71–1.67 (m, 2H), 1.35–1.29 (m, 18H), 0.87 (d, J = 7.1 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ: 183.47, 158.39, 153.16, 145.05, 131.93, 117.24, 109.30, 84.06, 40.39, 31.38, 29.71, 27.19, 26.54, 24.85, 22.44, 13.98.

Compound 5

A mixture of 4-bromo-1,8-naphthalic anhydride (5.54 g, 20 mmol), and dodecylamine (10 mL, 24 mmol) in ethanol (100 mL) was refluxed under nitrogen for 12 h until it became clear. The solution was cooled to room temperature, and the formed crystals were separated by filtration and washed with cold ethanol to obtain compound 5. Off-white solid (90%). ¹H NMR (400 MHz, CDCl₃) δ 8.66 (d, J = 7.3 Hz, 1H), 8.57 (d, J = 8.5 Hz, 1H), 8.41 (d, J = 7.9 Hz, 1H), 8.04 (d, J = 7.9 Hz, 1H), 7.87–7.80 (m, 1H), 4.19–4.13 (m, 2H), 1.71 (m, 2H), 1.58 (s, 4H), 1.36 (m, 14H), 0.87 (t, J = 6.8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃) δ: 163.51, 133.13, 131.94, 131.35, 131.09, 130.76, 130.23, 129.05, 128.07, 123.22, 122.36, 40.68, 31.88, 29.63, 29.62, 29.59, 29.54, 29.36, 29.33, 28.19, 26.98, 22.67, 14.13.

Compound 6

Compound 5 (0.79 mmol, 1 equiv) in dioxane was taken under a nitrogen atmosphere, followed by PdCl₂(dppf) (5 mol %) and potassium acetate. Bis(pinacolato)diborane (2 equiv) was introduced after 20 min, and the mixture was refluxed for 24 h. After being washed with brine solution, the organic phase was separated using dichloromethane, and the solvent was removed under reduced pressure to afford compound 6 as a pale yellow solid (62%).

General Procedure for Suzuki Coupling Reactions

Compound 2 and Pd(PPh₃)₄ (10 mol %) were dissolved in anhydrous THF. The reaction mixture was then deoxygenated with nitrogen gas for 20 min, followed by the addition of 2 M aqueous Na₂CO₃. Then, the boronic acid (2 mmol) was added, and the reaction mixture was refluxed for 5–24 h. After the completion of the reaction, the crude reaction mixture was brought to room temperature; the solvent was removed under reduced pressure, and the residue was diluted with DCM. The organic layer was extracted with DCM and dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure, and the residue was purified by column chromatography using a hexane/DCM solvent system.⁴

Compound 7a–f

Compound 2 (0.79 mmol, 1 equiv) was allowed to react with corresponding arylboronic acids and arylboronates (2 equiv) using the general procedure for the Suzuki reaction for 5–24 h to afford compounds 7a–f, respectively.

Compound 7a

White solid (70%). ¹H NMR (400 MHz, CDCl₃) δ 8.71 (d, J = 8.6 Hz, 2H), 8.49 (s, 2H), 7.88 (dd, J = 8.5, 1.9 Hz, 2H), 7.80 (d, J = 7.3 Hz, 4H), 7.52 (t, J = 7.6 Hz, 4H), 7.41 (t, J = 7.3 Hz, 2H), 4.27 (t, J = 6.6 Hz, 4H), 1.93 (m, 4H), 1.69–1.62 (m, 4H), 1.44–1.34 (m, 8H), 0.95–0.85 (m, 6H). ¹³C NMR (100 MHz, CDCl₃) δ: 143.76, 140.70, 139.46, 130.13, 128.57, 127.83, 127.32, 125.09, 123.30, 120.28, 109.91, 101.60, 99.30, 73.81, 31.86, 29.76, 25.79, 22.75, 14.15. HRMS (ESI) (m/z): 530.3170 [M]; calculated: 530.3170 [M].

Compound 7b

Pale yellow solid (75%). ¹H NMR (400 MHz, CDCl₃) δ 8.48 (d, J = 10.9 Hz, 2H), 8.39 (d, J = 1.9 Hz, 2H), 7.73 (d, J = 8.5 Hz, 2H), 7.55 (m, 2H), 7.51 (m, 2H), 7.37 (m, 2H), 4.17 (t, J = 6.6 Hz, 4H), 1.93–1.79 (m, 4H), 1.70–1.41 (m, 4H), 1.41–1.04 (m, 8H), 0.84 (t, J = 7.1 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃) δ: 143.60, 142.37, 134.05, 129.91, 127.40, 126.47, 126.40, 124.35, 123.32, 120.54, 119.39, 73.71, 32.00, 30.18, 26.41, 22.65, 14.23. HRMS (ESI) (m/z): 542.2269 [M]; calculated: 542.2260 [M].

Compound 7c

Pale white solid (69%).¹H NMR (400 MHz, CDCl₃) δ 8.58 (d, J = 1.7 Hz, 2H), 8.53 (d, J = 8.7 Hz, 2H), 7.91–7.84 (m, 4H), 7.80–7.73 (m, 2H), 7.69 (s, 2H), 7.35 (m, 4H), 4.28 (t, J = 6.5 Hz, 4H), 2.01–1.92 (m, 4H), 1.73–1.64 (m, 4H), 1.47–1.41 (m, 8H), 0.95 (t, J = 6.8 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃) δ: 144.35, 143.76, 140.82, 139.72, 132.64, 130.14, 128.04, 124.61, 124.50, 123.97, 123.70, 123.45, 122.33, 120.00, 119.90, 73.55, 31.72, 29.88, 25.47, 22.69, 13.94. HRMS (ESI) (m/z): 642.2585 [M]; calculated: 642.2580 [M].

Compound 7d

Pale yellow solid (70%). ¹H NMR (400 MHz, CDCl₃) δ 9.06 (s, 2H), 8.75 (d, J = 8.6 Hz, 2H), 8.66 (d, J = 4.0 Hz, 2H), 8.50 (d, J = 1.9 Hz, 2H), 8.08 (d, J = 7.9 Hz, 2H), 7.86 (d, J = 6.6 Hz, 2H), 7.45 (dd, J = 7.8, 4.8 Hz, 2H), 4.28 (t, J = 6.7 Hz, 4H), 1.98–1.89 (m, 4H), 1.65–1.55 (m, 4H), 1.43–1.34 (m, 8H), 0.91 (t, J = 7.0 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃) δ: 148.81, 143.74, 136.29, 134.51, 130.24, 127.86, 124.75, 123.74, 120.60, 73.75, 31.73, 30.50, 26.06, 22.71, 14.06. HRMS (ESI) (m/z): 533.3178 [M]; Calculated: 533.3178 [M].

Compound 7e

Red solid (65%). ¹H NMR (400 MHz, CDCl₃) δ 8.70 (d, J = 8.6 Hz, 2H), 8.42 (d, J = 1.9 Hz, 2H), 8.01 (dd, J = 10.8, 2.7 Hz, 4H), 7.79 (dd, J = 8.5, 1.9 Hz, 2H), 7.05 (d, J = 8.1 Hz, 2H), 4.27 (t, J = 6.7 Hz, 4H), 3.79 (t, J = 7.3 Hz, 4H), 2.03–1.88 (m, 4H), 1.88–1.70 (m, 4H), 1.67–1.23 (m, 8H), 1.23–1.11 (m,16H), 0.91 (t, J = 6.9 Hz, 12H). ¹³C NMR (100 MHz, CDCl₃) δ: 183.71, 158.11, 150.26, 143.65, 137.34, 136.94, 130.32, 127.45, 124.30, 123.93, 123.52, 119.75, 118.04, 110.69, 73.26, 39.88, 31.84, 31.38, 30.50, 29.71, 27.27, 26.57, 26.02, 22.74, 22.55, 14.08, 14.02. HRMS (ESI) (m/z): 836.4730 [M]; calculated: 836.4732 [M].

Compound 7f

Yellow solid (58%).¹H NMR (400 MHz, CDCl₃) δ 8.87 (d, J = 8.6 Hz, 1H), 8.75 (d, J = 7.4 Hz, 3H), 8.69 (d, J = 7.2 Hz, 1H), 8.65 (d, J = 7.1 Hz, 2H), 8.45 (d, J = 1.8 Hz, 1H), 8.40 (d, J = 9.5 Hz, 1H), 7.88 (d, J = 7.5 Hz, 1H), 7.78 (s, 2H), 7.68 (d, J = 8.5 Hz, 2H), 7.62 (d, J = 7.2 Hz, 2H), 4.25 (dd, J = 15.5, 7.9 Hz, 6H), 1.89–1.75 (m, 6H), 1.54–1.27 (m, 46H), 0.90–0.78 (m, 12H). ¹³C NMR (100 MHz, CDCl₃) δ: 164.03, 163.86, 146.90, 142.82, 142.77, 137.20, 132.07, 131.57, 130.81, 130.51, 128.87, 128.41, 127.51, 123.26, 123.17, 118.51, 73.86, 40.14, 38.86, 37.11, 36.74, 35.20, 34.88, 34.43, 31.94, 31.45, 30.25, 29.65, 29.60, 29.23, 28.19, 27.10, 25.69, 24.79, 23.67, 22.71, 14.15. HRMS (ESI) (m/z): 1102.4063 [M]; calculated: 1102.7062 [M].

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c01166.

• NMR, HRMS, device characterization, and TD-DFT calculations are presented (PDF)

Author Contributions

The manuscript was written with the contributions from all authors. All authors have approved the final version of the manuscript.

The authors declare no competing financial interest.