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. 2025 Jan 21;147(4):3459–3467. doi: 10.1021/jacs.4c14775

Molecular Orbital Tuning of Pentacene-Based Organic Semiconductors through N-Ethynylation of Dihydrodiazapentacene

Li Zhang a,b, Yujie Zhao d,e,f, Jiasheng Li g, Yuang Fu c, Boyu Peng d,e,f,*, Jun Yang g,h, Xinhui Lu c, Qian Miao a,b,*
PMCID: PMC11783513  PMID: 39835460

Abstract

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This study explores the concept of molecular orbital tuning for organic semiconductors through the use of N,N′-diethynylated derivatives of 6,13-dihydro-6,13-diazapentacene (2a and 2b). These novel molecules maintain the same molecular geometry and π–π stacking as their parent pentacene derivatives (1a and 1b), as confirmed by X-ray crystallography. However, they exhibit altered frontier molecular orbitals in terms of the phase, nodal properties, and energy levels. Theoretical calculations based on crystal structures indicate that 2a and 2b could significantly enhance the hole mobilities of the parent compounds by improving the hole transfer integral. Organic field-effect transistors (OFETs) of 1a and 2a were fabricated by using dip-coating and bar-coating methods. Both types of devices for 2a demonstrated a hole mobility exceeding 1 cm2 V–1 s–1, more than twice that of the respective devices for 1a. Additionally, unlike its pentacene parent, 2a is transparent to visible light and exhibits significantly enhanced environmental stability against light and air, making it a promising candidate for broader applications in organic electronic devices.

Introduction

Charge transfer integral and reorganization energy are crucial factors in determining the rate of charge transport in organic semiconductors,1 as described by Marcus theory,2,3 which outlines electron transfer processes. Consequently, a higher charge transfer integral and a lower reorganization energy lead to increased charge carrier mobility in organic field-effect transistors (OFETs),4 which are fundamental components of organic integrated circuits, playing an important role in low-cost, flexible, and wearable organic electronics.5−7 The charge transfer integral is governed by electronic coupling between neighboring semiconductor molecules, in a way that intimately depends on both the relative positions of interacting molecules and the phase and nodal properties of π-orbitals.8 Thus, it can, in principle, be enhanced by optimizing the arrangement of π-backbones or by tuning the phase and nodal characteristics of the π-orbitals. Significant efforts have been made to fine-tune the molecular packing of organic semiconductors in the solid state for high-performance OFETs.9−13 Such a strategy is also known as crystal engineering,14 which typically involves attaching different substituting groups to the same π-backbone to modify its packing in the crystals. Notably, the introduction of substituents usually does not substantially alter the phase and nodal properties of frontier π-orbitals. One of the best examples of crystal engineering for organic semiconductors in OFETs is the addition of triisopropylsilylethynyl groups to pentacene, resulting in 6,13-bis(triisopropylsilylethynyl)pentacene (1a in Figure 1a),15 a solution-processed organic semiconductor with high hole mobility.16 As illustrated in Figure 1a, this modification transforms the herringbone packing of pentacene into a two-dimensional (2D) π-stacking with a brickwork arrangement.

Figure 1.

Figure 1

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(a) Structures of pentacene and 1a and their molecular packing in the crystals (the triisopropylsilylethynyl groups are shown with wire models, and other atoms are shown with ball and stick models). (b) Structure of 6,13-DHDAP and its molecular packing in the crystal. (c) Structure of DPP and DP-6,13-DHDAP and their molecular packing in the crystals.

Herein, we introduce a novel strategy called molecular orbital tuning that modifies the frontier molecular orbitals of an organic semiconductor without changing its shape and molecular packing in the crystal structure. One promising candidate for this strategy is 6,13-dihydro-6,13-diazapentacene (6,13-DHDAP, shown in Figure 1b),17 which maintains the same planar geometry as pentacene but differs in the phase and nodal properties of its highest occupied molecular orbital (HOMO) due to the presence of two additional π-electrons.18 However, a detailed analysis of the crystal structures of both 6,13-DHDAP and pentacene reveals that neighboring 6,13-DHDAP molecules exhibit significant displacement along the long molecular axis relative to pentacene (Figure 1b), despite both adopting similar herringbone packing. This displacement, presumably due to the avoidance of direct overlap of the electron-rich dihydropyrazine units, disqualifies 6,13-DHDAP from effective molecular orbital tuning of pentacene. Further analysis of the crystal structures of reported derivatives of 6,13-DHDAP19,20 and 5,14-dihydro-5,14-diazapentacene (5,14-DHDAP) also found them unsuitable for molecular orbital tuning of pentacene-based organic semiconductors. For instance, N,N′-diphenyl-6,13-dihydro-6,13-diazapentacene (DP-6,13-DHDAP in Figure 1c) and 6,13-diphenylpentacene (DPP in Figure 1c) exhibit essentially the same molecular packing due to dominating edge-to-face interactions between the phenyl substituents and the pentacene core.21 However, their pentacyclic π-planes are separated by distances of up to 4.9 Å, resulting in poor semiconductor properties with low charge carrier mobility. Additionally, 6,13-bis(triisopropylsilylethynyl)-5,14-dihydro-5,14-diazapentacene adopts a 2D π-stacking with a brickwork arrangement similar to 1a.22 However, the difference in overlap between the adjacent pentacyclic π-planes is not negligible, and more importantly, the disorder in the arrangement of molecules introduces uncertainty to the molecular packing (Figure S10 in the Supporting Information).

In this study, we demonstrate molecular orbital tuning using N,N′-diethynylated 6,13-DHDAPs (2a/b, shown in Figure 2a). These compounds are distinct from previously reported ethynylated N-heteroacenes,23−27 as the ethynyl groups are bonded to nitrogen atoms rather than carbon atoms. As shown in Figure 2a, compound 2a maintains the same two-dimensional π-stacking with a brickwork arrangement as 1a, driven by the bulky triisopropylsilylethynyl groups. On the other hand, the frontier molecular orbitals of 2a differ from that of 1a in both phase and nodal properties as shown in Figure 2b due to the presence of two additional π-electrons. Herein, we report the synthesis, electronic properties, and crystal structures of 2a/b, along with experimental and computational studies of their semiconductor properties compared to the corresponding ethynylated pentacenes. Notably, 2a and 2b demonstrated field-effect mobilities in OFETs that are more than double those of 1a and 1b, respectively, highlighting the effectiveness of the molecular orbital tuning strategy.

Figure 2.

Figure 2

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(a) Structures of 2a/b and the molecular packing of 2a in the crystal (the triisopropylsilylethynyl groups are shown with wire models, and other atoms are shown with ball and stick models). (b) Frontier molecular orbitals of 1a and 2a calculated at the B3LYP level of density functional theory (DFT) with the 6-311++G(d,p) basis set.

Results and Discussion

Scheme 1 illustrates the synthesis of compounds 2a and 2b, which builds on the previously reported method for ynamides using trichloroethylene as a two-carbon synthon.28 Unlike the original ynamide synthesis, which used a weak base such as Cs2CO3, our approach employed sodium hydride to deprotonate 6,13-DHDAP and eliminate HCl from trichloroethylene, generating dichloroacetylene in situ. The subsequent addition of the 6,13-DHDAP anion to dichloroacetylene produced compound 3 in a yield of 73%. The E configuration of the C–C double bonds in 3 was confirmed by X-ray crystallography. Dehalogenation of compound 3 with n-BuLi, involving the elimination of HCl and halogen-lithium exchange, resulted in an alkynide intermediate. This intermediate was then trapped with trialkylsilane chloride, yielding compounds 2a and 2b as white solids. Both 2a and 2b demonstrated good solubility in common organic solvents such as hexane, toluene, CH2Cl2, and CHCl3.

Scheme 1. Synthesis of 2a and 2b.

Scheme 1

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The electronic structures and properties of compounds 1a and 2a were studied by using both computational and experimental methods. Figure 2b compares the frontier molecular orbitals of 1a and 2a, calculated at the B3LYP level of density functional theory (DFT) with a 6-311++G(d,p) basis set, highlighting the distinct phase and nodal properties of the two molecules. The HOMO of 1a exhibits C2h symmetry, while that of 2a displays C2v symmetry. The lowest unoccupied molecular orbital (LUMO) of 1a spans the entire π-backbone with C2v symmetry, whereas the LUMO of 2a, characterized by C2h symmetry, is localized on the two naphthalene moieties. The calculated HOMO energy levels of 1a (−4.91 eV) and 2a (−5.20 eV) differ by 0.29 eV, suggesting that both of them can function as p-type semiconductors, while the LUMO energy level of 1a (−2.95 eV) is significantly lower than that of 2a (−1.39 eV) by 1.56 eV. In the test window of cyclic voltammetry (CV), 1a exhibited one reversible and one pseudoreversible oxidation wave, while 2a showed one reversible oxidation wave. Based on the first oxidation potentials, the HOMO energy levels of 1a and 2a are estimated to be −5.47 and −5.64 eV, respectively. The lower HOMO energy level of 2a is consistent with DFT calculations. Figure 3a compares the UV–vis absorption and photoluminescence spectra of 1a and 2a in toluene solutions. The solution of 2a is essentially transparent to visible light, with the longest-wavelength absorption maximum at 389 nm, significantly blue-shifted by 1.26 eV relative to 1a. This observation agrees with the DFT-calculated HOMO–LUMO gaps. The solution 2a in toluene exhibits strong blue luminescence with a quantum yield of 53% when excited at 370 nm, while that of 1a exhibits red luminescence with a quantum yield of 34% when excited at 590 nm.

Figure 3.

Figure 3

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(a) UV–vis absorption (solid line) and photoluminescence (dashed line) of 1a and 2a in toluene. (The concentration was 10 μM for absorption and 1 μM for photoluminescence; the excitation wavelength was 590 nm for 1a and 370 nm for 2a.) (b) Relative absorbance as a function of time as measured from the toluene solutions of 1a and 2a (10 μM). (The absorbance was measured at the longest-wavelength absorption: 642 nm for 1a, 389 nm for 2a.)

UV–vis spectroscopy was also used to monitor the stability of 1a and 2a in air-saturated toluene exposed to ambient air and light. Figure 3b compares the relative absorbance of 1a and 2a over time under the same conditions, revealing that the characteristic absorption of 1a disappeared after 9 days, while that of 2a decreased by only 4.2% after 49 days. The instability of 1a is known as a result of self-sensitized photooxidation via a Diels–Alder reaction between the diene moiety in pentacene and singlet oxygen molecule.29,30 Therefore, the stability of 2a can be attributed to its transparency to visible light, which inhibits self-sensitization, and its 6,13-DHDAP backbone, which lacks a diene moiety for the Diels–Alder reaction.

A key aspect of molecular orbital tuning is ensuring that the geometry and molecular packing of semiconductor molecules remain unchanged or exhibit only negligible variations when the frontier molecular orbitals are altered. However, achieving this retention of the same crystal structure is challenging and has rarely been realized in the field of organic semiconductors. The crystal structure of 1a was reported by Anthony in 2001.15 To confirm that N,N′-diethynylated 6,13-DHDAP and the corresponding 6,13-diethynylated pentacene meet this criterion, single crystals of 2a and 2b as well as 1b were grown by the slow diffusion of methanol vapor into their CH2Cl2 solutions and subjected to X-ray crystallography. Both 2a and 2b exhibit a planar diazapentacene backbone, with their acetylene moieties remaining in the same plane as the diazapentacene core. The crystal structures of 1a and 2a have nearly identical unit cell parameters, as summarized in Table S2 in the Supporting Information. Figure 4 compares the crystal structures of 1a and 2a. The side view in Figure 4a shows the 2D π–π stacking of 1a with a brickwork arrangement, which involves two slightly different distances between the adjacent π-planes. The shorter π–π distance (3.319 Å) is associated with a small overlap between the terminal benzenoid rings, as demonstrated with molecules M1 and M3 in the top view. The longer π–π distance (3.403 Å) is associated with a larger overlap between two and a half hexagonal rings, as demonstrated with molecules M2 and M3. Figure 4b illustrates the 2D π–π stacking of 2a, which is essentially identical to that of 1a except for slightly different π–π distances, 3.358 and 3.362 Å.

Figure 4.

Figure 4

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2D π-stacking of 1a (a) and 2a (b) in the crystals (hydrogen atoms are removed for clarity, triisopropylsilylethynyl groups are shown as wires in the side view, and other atoms are shown as ellipsoids set at 50% probability).

Similar to 6,13-bis(trialkylsilylethynyl)pentacene,31 the molecular packing of N,N′-ethynylated 6,13-DHDAP is controlled by the size of the trialkylsilyl group. The crystal structures of compounds 1b and 2b, unlike those of 1a and 2a, exhibit one-dimensional (1D) π–π stacking with an offset due to their smaller triethylsilyl groups compared to the triisopropyl groups in 1a and 2a. The side view in Figure 5a shows the 1D π–π stacking of 1b with a π–π distance of 3.458 Å, which corresponds to an overlap between two and a half hexagonal rings, as illustrated by molecules M1 and M2 in the top view. Adjacent π-stacks of 1b interact through the triethylsilyl group and the terminal benzenoid ring. Figure 5b illustrates the 1D π–π stacking of 2b, which is essentially identical to that of 1b, except for a slightly shorter π–π distance of 3.422 Å.

Figure 5.

Figure 5

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1D π-stacking of 1b (a) and 2b (b) in the crystals (hydrogen atoms are removed for clarity, triethylsilylethynyl groups are shown as wires in the side view, and other atoms are shown as ellipsoids set at 50% probability).

To calculate the charge transfer integral for holes, different dimers within the crystal structures of compounds 1a/b and 2a/b were identified, as illustrated in Figures 4 and 5, respectively. These dimers define the direction of the charge transfer. The distances between the molecular centroids and the hole transfer integrals for each dimer are summarized in Table 1. Additionally, the reorganization energy for holes was calculated for each molecule at the 6-311++G(d,p) level of DFT. These values, along with the hole transfer integrals, were then used to calculate the Marcus hopping rate and hole mobility, all of which are included in Table 1. The hole mobility of compound 1a calculated in this study is comparable to the reported value of 1.49 cm2 V–1 s–1 obtained using DFT calculations at the B3LYP/6-31G(d,p) level.32 However, it is lower than the reported value of 2.25 cm2 V–1 s–1, which was obtained with thermal disorder taken into consideration.33 It is found that although compound 1a adopts 2D π–π stacking, its hole transport is essentially one-dimensional along the direction of the π–π stacking between M2 and M3. The hole transfer integral of 1a, and consequently its Marcus hopping rate and hole mobility between M1 and M3, is much smaller than those between M2 and M3. In contrast, the hole transport of compound 2a is two-dimensional, with more balanced transfer integrals along two directions: 22.8 meV between M4 and M6, and 50.7 meV between M5 and M6. Both of these values are larger than the corresponding values for 1a. As a result, the calculated hole mobility of 2a is higher than that of 1a in both directions, despite 2a having a higher reorganization energy than 1a. The enhanced hole transport in 2a compared to that in 1a is attributed to the different phase and nodal properties of the HOMO of 2a, showcasing the effectiveness of molecular orbital tuning. Both 1b and 2b crystals exhibit one-dimensional hole transport due to their 1D π–π stacking. The hole transport between adjacent π-stacks is negligible compared to that within the π-stack. Compound 2b has a hole transfer integral of 56.3 meV along the π–π stacking direction, which is higher than that of 1b (43.7 meV). Consequently, the calculated mobility of 2b is higher than that of 1b, despite 2b having a higher reorganization energy.

Table 1. Calculated Reorganization Energy, Hole Transfer Integral, Marcus Hopping Rate, and Hole Mobilities.

molecule reorganization energy (meV) dimer distance (Å)a transfer integral (meV)b Marcus hopping rate (meV) hole mobility (cm2 V–1 s–1)c
1a 136.51 M1–M3 10.21 4.3 0.15 0.001
M2–M3 7.57 32.5 8.42 1.387
2a 158.11 M4–M6 10.27 22.8 3.12 0.164
M5–M6 7.45 50.7 15.45 2.093
1b 138.89 M1–M2 7.25 43.7 14.74 2.209
M2–M3 10.27 3.9 0.12 0.00025
M3–M1 13.38 6.3 0.31 0.003
2b 164.16 M4–M5 7.24 56.3 17.63 2.697
M5–M6 10.14 4.0 0.09 0.00013
M6–M4 13.34 0 0 0
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a

Measured between the centroids of the two molecules.

b

Hole transfer integral is calculated at the 6-311++G(d,p) level of DFT.

c

The computational methods are detailed in the Supporting Information.

To compare the semiconductor performance of compounds 1a/b and 2a/b in OFETs, two solution-based fabrication methods—dip coating and bar coating—34−36were employed to fabricate bottom-gate top-contact devices. For dip coating, films of these compounds were prepared on a silicon substrate layered with thermally grown silica, solution-processed alumina,37 and either 12-cyclohexyldodecylphosphonic acid (CDPA)38 or 12-methoxydodecylphosphonic acid (MODPA)39 as composite dielectric materials. The top-contact electrodes were formed by the vacuum deposition of gold through a shadow mask so that the resulting conduction channel was roughly parallel to the film growth direction. The solvent, pulling speed, and solution concentration used in the dip-coating process were optimized, resulting in films of 1a and 2a composed of aligned microribbons or fibrous crystallites (Figure 6a and Figure S13). However, the dip-coated films of 1b and 2b consisted of narrower microribbons or fibrous crystallites with poorer alignment, as observed in the polarized light micrographs (Figure S13) and AFM images (Figure S15). For bar coating,40 solutions of the four compounds were applied to SiO2/Si substrates coated with a polymer of divinyltetramethyldisiloxane bis(benzocyclobutene) (BCB). The solvent, solution concentration, substrate temperature, and substrate moving speed used in the bar-coating process were optimized, resulting in thin films of 1a and 2a with millimeter-sized domains, full coverage, and uniform thickness (Figure 6a and Figure S14). However, the bar-coated films of 1b and 2b were composed of microribbons with varied thickness, as observed in the polarized light micrographs (Figure S14) and AFM images (Figure S16).

Figure 6.

Figure 6

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(a) Polarized light micrographs for the dip-coated and bar-coated films of 2a. (b) Typical transfer I–V curves measured from the OFETs of 2a and 1a (the W/L is 13.8 for the dip-coated OFET of 2a, 5.9 for the dip-coated OFET of 1a, 8.4 for the bar-coated OFET of 2a, and 8.3 for the bar-coated OFET of 1a. Ci is 28 nF/cm2 for the dip-coated OFETs and 11 nF/cm2 for the bar-coated OFETs. The I–V characteristics of dip-coated OFETs were measured in ambient air, and those of bar-coated OFETs were measured in a N2 atmosphere).

The molecular arrangement and morphology of these films were analyzed using X-ray diffraction (XRD), grazing incidence wide-angle X-ray scattering (GIWAXS),41−43 and atomic force microscopy (AFM). The out-of-plane X-ray diffraction patterns for dip-coated and bar-coated films of 1a and 2a (Figure S17 and S18) all showed three peaks corresponding to the (001), (002), and (003) diffractions derived from the single-crystal structures. Meanwhile, the diffraction patterns for films of 1b and 2b showed only (001) and (002) peaks, suggesting lower crystallinity in these films compared to those of 1a and 2a. The GIWAXS patterns of the dip-coated and bar-coated films of 1a/b and 2a/b all exhibited (001) diffraction along the qz (out-of-plane) axis, consistent with the out-of-plane X-ray diffraction patterns. This indicates that the (001) crystal plane is parallel to the substrate surface, and thus, the π-backbones adopt an edge-on orientation on the substrate surface. The π-planes of 1a and 2a form angles of 69.6 and 70.4°, respectively, with the substrate surface, while those of 1b and 2b form angles of 52.4 and 53.2°, respectively, with the substrate surface (Figure S19). When the incident X-ray beam was perpendicular to the film growth direction of 1a and 2a, the GIWAXS patterns exhibited (10l) diffractions and higher-order (20l) diffractions, similar to the reported GIWAXS patterns of blade-coated films of compound 1a.44 This suggests that the film growth direction is roughly along the a-axis of the crystal unit cell (Figures S21 and S22). The GIWAXS patterns of the dip-coated films of 1b and 2b, unlike those of 1a and 2a, exhibited small arcs, indicative of less oriented domains, where the crystallites are oriented over a range of angles. On the other hand, the GIWAXS patterns of the bar-coated films of 1b and 2b exhibited dots corresponding to (01l) diffractions when the incident X-ray beam was parallel to the film growth direction. This suggests that the major film growth direction is roughly along the a-axis of the crystal unit cell (Figures S24 and S26).

Typical AFM images of the dip-coated films of 1a and 2a (Figure S15) revealed flat surfaces of the microribbons, with section analysis showing steps of 4 to 6 nm between adjacent microribbons, corresponding to two to three molecular layers. Meanwhile, typical AFM images of the bar-coated film of 1a and 2a (Figure S16) revealed uniform thicknesses of 12 and 24 nm, respectively. These thicknesses correspond to about seven molecular layers for 1a and 14 molecular layers for 2a. On the other hand, AFM images of the dip-coated films of 1b and 2b (Figure S15) showed deep and wide gaps between microribbons, while those of the bar-coated films of 1b and 2b (Figure S16) exhibited rougher surfaces compared to those of 1a and 2a.

Figure 6 presents the typical transfer I–V curves for OFETs fabricated from compounds 1a and 2a using two different methods. The field-effect mobility for holes in the saturated regime was extracted using the equation IDS = (μWCi/2L)(VGS–VT),2 where IDS is the drain current, μ is the field-effect mobility, Ci is the capacitance per unit area for the corresponding dielectric, W is the channel width, L is the channel length, and VGS and VT are the gate and threshold voltages, respectively. As summarized in Table 2, the average field-effect mobility of 1a (0.72 ± 0.13 cm2 V–1 s–1) is comparable to the values reported for drop-cast films of 1a (0.65 ± 0.35 and 0.73 ± 0.06 cm2 V–1 s–1),45,46 although it is lower than those observed in single-crystal arrays (1.54 ± 0.26 cm2 V–1 s–1).47 However, the highest field-effect mobility of 1a so far obtained from lattice-strained single crystalline films (8.1 ± 1.2 cm2 V–1 s–1) may be overestimated, as it was extracted from transfer I–V curves exhibiting apparent double-slope nonideality.48 The average field-effect mobilities of 2a in both dip-coated and bar-coated films exceed 1 cm2 V–1 s–1. The bar-coated films, in particular, exhibited a higher mobility, reaching up to 1.98 cm2 V–1 s–1, which is close to the calculated value (2.09 cm2 V–1 s–1). The on/off ratio is >1 × 106 for the dip-coated OFETs of 2a, and >1 × 109 for the bar-coated OFETs of 2a. Similarly, the bar-coated OFETs of 1a showed higher mobility compared to that of the dip-coated OFETs of the same material. This increase in mobility is attributed to the single-crystal nature of the large-sized domains in the bar-coated films. The mobilities of 2a are more than double those of 1a, aligning qualitatively with the calculated values (Table 1). Since the thin films of 1a and 2a fabricated using the same method exhibit similar crystallinity and morphology, the significantly enhanced mobility of 2a compared to 1a should be attributed to the molecular structure itself, confirming the effectiveness of molecular orbital tuning.

Table 2. Hole Mobilities for OFETs of 1a/b and 2a/b.

molecules fabrication method μ (cm2 V–1 s–1)a
1a dip coating 0.55 ± 0.13
highest: 0.76
bar coating 0.72 ± 0.13
highest: 0.94
2a dip coating 1.29 ± 0.20
highest: 1.62
bar coating 1.64 ± 0.17
highest: 1.98
1b dip coating (5.8 ± 3.8) × 10–3
highest: 1.46 × 10–2
bar coating 0.21 ± 0.06
highest: 0.34
2b dip coating (6.0 ± 4.3) × 10–3
highest: 1.62 × 10–2
bar coating 0.43 ± 0.09
highest: 0.57
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a

The average mobilities were measured from 47 independent dip-coated OFETs for 1a, 70 for 2a, 28 for 1b, and 30 for 2b in ambient air, and from 16 independent bar-coated OFETs for each compound of 1a/b and 2a/b in a N2 atmosphere.

In comparison to 1a and 2a, the triethylsilyl-substituted molecules 1b and 2b in dip-coated and bar-coated films exhibited lower field-effect mobilities, as shown in Table 2. This reduction in mobility for 1b and 2b can be attributed to the films consisting of ribbon or fiber-like domains with lower ordering and poorer orientation than those of 1a and 2a, as observed from the polarized light micrographs, XRD, GIWAXS, and AFM images. The charge transport in the dip-coated films of 1b and 2b was hindered by deep and wide grain boundaries between these microribbons or fibrous crystallites, resulting in much lower measured mobilities than the intrinsic values predicted by theoretical calculations. Compared to the dip-coated films, the bar-coated films of 1b and 2b exhibited mobilities higher by 1 order of magnitude, closer to the intrinsic values predicted by theoretical calculations. In the bar-coated films with similar crystallinity and morphology, the mobility of 2b approximately doubles that of 1b, qualitatively aligning with the theoretically calculated values and thereby supporting the effectiveness of molecular orbital tuning.

Conclusions

In conclusion, this study demonstrates the concept of molecular orbital tuning of organic semiconductors using N,N′-diethynylated 6,13-DHDAPs (2a and 2b). The two new molecules retain the same molecular geometry and π–π stacking as the parent pentacene derivatives (1a and 1b), as revealed by X-ray crystallography, but they alter the frontier molecular orbitals in terms of phase, nodal properties, and energy levels. Theoretical calculations based on the crystal structures suggest that 2a and 2b have the potential to improve the hole mobilities of the parent compounds (1a and 1b) by enhancing the hole transfer integral. This prediction was supported by the OFETs fabricated using dip- and bar-coating methods. Both types of the devices for 2a exhibited hole mobility exceeding 1 cm2 V–1 s–1, more than twice that of the respective devices for 1a. The field-effect mobility of 2b in bar-coated OFETs also doubled that of 1b, although the mobilities of 1b and 2b are both lower than those of 1a and 2a due to their films having lower ordering and poorer orientation. Additionally, unlike its pentacene parent, 2a is not only transparent to visible light but also exhibits significantly enhanced environmental stability toward light and air, making it more promising for wider application in organic electronic devices.

Molecular orbital tuning, which can alternatively be termed molecular orbital engineering, involves designing and producing frontier molecular orbitals for organic semiconductors. Specifically, it modifies the frontier molecular orbitals of an organic semiconductor without altering its shape and molecular packing in the crystal structure by substituting atoms in its π-backbone. This modification changes the charge transfer integral, leading to an increase or decrease in the rate of charge transport in organic semiconductors. While such modifications do not always guarantee an enhanced charge transport rate, the concept of molecular orbital tuning provides a novel strategy for designing new organic semiconductors. It allows for the prediction of transfer integral and charge carrier mobility before the synthesis of new molecules based on the crystal structure of known organic semiconductors. If the predicted charge transfer integral and charge carrier mobility are higher than those of the known organic semiconductor, the new molecule becomes a promising candidate for improved semiconductor performance. Research on new organic semiconductors designed using this strategy is in progress in our laboratory.

Acknowledgments

We are grateful to Ms. Hoi Shan Chan (the Chinese University of Hong Kong) and Dr. Xuebing Leng (Shanghai Institute of Organic Chemistry) for the single-crystal crystallography and to Ms. Xiuyan Liu (Shanghai Institute of Organic Chemistry, the Chinese Academy of Sciences) for measurement of photoluminescence quantum yield. We acknowledge the financial support provided by the Research Grants Council of Hong Kong (GRF 14300323) and the State Key Laboratory of Synthetic Chemistry.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c14775.

  • Details of synthesis, characterization of new compounds, X-ray crystallography, DFT calculations, and fabrication and characterization of thin films and field-effect transistors (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja4c14775_si_001.pdf (3MB, pdf)

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