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. 2025 Aug 1;10(31):34308–34320. doi: 10.1021/acsomega.5c01652

Computational Investigation of the Structural and Electronic Effects of Phenyl, Alkyl, and Halogen Fully Substituted Acenes

Viraj V Jain 1, Georgia Grace Tully 1, Peter W Jarvis 1, Emily A A Jarvis 1,*
PMCID: PMC12355298  PMID: 40821517

Abstract

Acenes are a class of polycyclic aromatic hydrocarbons that may hold promise as organic semiconductors (OSCs) in solar cells and electronic devices. Their instability and poor solubility present challenges that may be improved by replacing the hydrogens with phenyl, halogen or alkyl substituents to sterically induce a helical twist to these otherwise planar molecules. This twisted structure also impacts the optical properties of these molecules. We employ time-dependent density functional theory (TD DFT) to investigate acenes spanning from naphthalene to heptacene. We focus on the structural and electronic effects of fully substituting these molecular backbones to create seven distinct substituent series, many of which have been previously synthesized. The end-to-end intramolecular twist increases linearly with acene length for all series, however the degree of twist varies significantly depending on the specific substituent. All series display similar trends of increasing red shifts in the estimated highest occupied molecular orbital–lowest-unoccupied molecular orbital (HOMO–LUMO), fundamental, and optical gaps as the number of fused rings along the polycyclic backbone increases. Despite the similarity of measured gaps, features distinguishing the series from one another are more apparent in their near ultraviolet–visible-near infrared (UV–vis-NIR) absorption spectra. Furthermore, halogen and alkyl substituents display local minima for two other structural configurations in addition to the twisted structure. Gibbs free energy calculations show these three distinct configurations are likely energetically competitive at room temperature. One novel nonhelical geometry shows significant reductions in excitation energies, while the other displays similar values to the twisted acene structures. The structural and electronic trends of these series offer insights that can guide the use of these and similar acenes as functional materials.

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1. Introduction

A myriad of polycyclic aromatic hydrocarbons have been synthesized for integration into semiconducting materials thanks to their narrow band gaps resulting from electron delocalization. − Pentacene, a polyacene comprised of five linearly fused benzene rings, can be a useful small organic semiconductor (OSC) for incorporation into organic field-effect transistors (OFETs) because of its favorable charge carrier mobility and transport. , The success of pentacene and its derivatives has sparked interest into synthesizing polyacenes with backbone lengths even larger than five rings to further extend the π-orbital systems. However, extending the backbone length encounters the challenges of increased instability and decreased solubility of those molecules. ,

Introducing an intramolecular twist into the acene backbone is one means of addressing these stability and solubility limitations as discussed by Pascal, Tait et al. and others. , The beneficial effects of distortion from planarity have been supported by several experimental and theoretical studies. Norton et al. used DFT to analyze a highly twisted pentacene derivative, demonstrating that key electronic properties, including end-to-end delocalization, remain largely intact under significant distortion. Bedi et al. further built upon the molecular understanding of twisted acenes (twistacenes) beyond these early computational predictions by varying the twist of a series of anthracenes by attaching carbon tethers of several lengths and exploring absorbance and fluorescence. Their results demonstrated that the most twisted analogue exhibited significantly reduced photooxidation. Further investigations by Bedi et al. on acenes ranging from two to five fused rings, with varying twist angles, revealed only a modest reduction in the highest occupied molecular orbital–lowest-unoccupied molecular orbital (HOMO–LUMO) gap and decreased fluorescence quantum efficiency, presumably due to increased spin–orbit coupling. Collectively, these studies highlight that twisted acenes preserve the desirable electronic attributes of their planar counterparts while exhibiting enhanced solubility and stability. ,

The simplest way to induce a twist in acenes is through substituting hydrogen with bulky substituents. This initially was achieved using phenyl substituents because of their size and synthetic accessibility. − Additionally, McGarry et al., Liu et al. and Matsuoka et al. indicate phenyl can extend electron delocalization, as exemplified by rubrene, whose phenyl-substituted tetracene core exhibits enhanced carrier mobilities in organic electronic devices. − Beyond simple substitution, combining benzannulation with bulky phenyl groups has proven particularly effective in generating significant distortion from planarity. This approach has led to the formation of twistacenes with large twist angles (much greater than 100° for the larger acenes). The remarkable stability conferred by substitutions of this kind has allowed the synthesis of polyacenes up to decacene. −

Sterically induced intramolecular twist has also been achieved through full halogenation and alkylation of naphthalene, dating back to the 1980s. − However, the challenges in synthesizing similarly crowded higher-order acenes initially hampered deeper investigation into their twisting characteristics. More recently, partial halogenation strategies have been pursued for tetracenes and pentacenes with the goal of optimizing their performance in organic field-effect transistors (OFETs). − Electronegative substitution can modulate the HOMO–LUMO energies − and fine-tune charge carrier mobilities, in part by altering intermolecular stacking interactions. − In particular, fluorination has been extensively studied due to its capacity to enhance both the thermal and photochemical stability of acenes, while maintaining electron delocalization. − Notably, Bao and co-workers demonstrated that the bulkier chlorine substituents could confer stability on par with, or even superior to, fluorinated analogues; and these chlorinated acenes exhibited improved charge carrier mobilities in OFET architectures. , Despite these advances, the effect of halogen substitution on inducing and controlling intramolecular twist remains underexplored for higher acenes beyond naphthalene and anthracene.

While previous studies have provided valuable insight into isolating the effect of intramolecular twisting, there has yet to be a comprehensive investigation comparing the effects of the substituents used to achieve this twist. Here, we focus on acenes with backbones ranging from naphthalene to heptacene. For these, we investigate three classes of phenyl substituted acenes, including the fully phenyl substituted acenes (Series 1), partially phenyl substituted acenes (Series 2), and the alternately fused phenyl substituted series synthesized by Clevenger et al. (Series 3) as well as series fully substituted with chlorine, bromine, and ethyl, respectively. We provide a quantitative exploration of their structural and electronic properties as well as qualitative analysis of their absorbance spectra.

2. Experimental Section

2.1. Geometry Optimization

We optimized the geometries of all molecules from initially planar structures using the hybrid B3LYP exchange-correlation functional , with empirical dispersion (GD3BJ) and employing the 6–311G basis set as implemented within Gaussian 16. We investigated fully substituted acenes containing two to seven linearly fused benzene rings (naphthalene to heptacene backbones) for six distinct series. These include three series of phenyl functional groups (which differ in end structure) as well as chlorine, bromine, and ethyl, fully substituted molecules as illustrated in Figure with the example of pentacene. Starting from a planar geometry, each molecule was optimized to its lowest energy conformation.

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Planar geometries of fully substituted acenes with backbone lengths of n = 5 before optimization; (a) a pentacene with Series 1 phenyl substituents; (b) a pentacene with Series 2 phenyl substituents; (c) a pentacene with Series 3 phenyl substituents; (d) a pentacene with Cl, Br and ethyl (Et) substituents.

For a smaller set of molecules, we also investigated the impact of including long-range corrections employing CAM-B3LYP and the conductor-like polarizable continuum model with cyclohexane to investigate the impact of these and compare with experimental results. Figure shows that these variety of computational approaches for obtaining the wave function and optimized structures resulted in similar predicted trends, with our chosen method of B3LYP with empirical dispersion (GD3BJ) in close agreement with experimental values. ,, We also explored the impact of adding polarization functions to our employed basis set as well as using alternative exchange-correlation functionals. Those calculations exhibited similar trends to those reported here even while individual values differed slightly. These additional trends as well as the calculated HOMO and LUMO energies and Cartesian coordinates and images of the optimized molecular structures are included as Supporting Information for interested readers. Supporting Information Figure 1 displays the impact on calculated optical gaps for these variety of calculation modifications across Series 1. Supporting Information Figure 2 shows the impact of additional polarization functions added to the basis set on the optimized structures of Series 1. This impact was minor but did result in slightly smaller twist angles across the series. In the case of the bromine substituted pentacene, the effect was very minimal showing less than 1 degree modification in the twist angle.

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Optical gap calculations for acenes of backbone lengths ranging from naphthalene (2) to heptacene (7) substituted with phenyl groups at each hydrogen position (Series 1) at different levels of DFT computation for the wave functions and the optimized structures and compared to experimentally measured values. (1) B3LYP, (2) CAM-B3LYP with CAM-B3LYP optimized structures, (3) CAM-B3LYP with B3LYP optimized structures, (4) experimental values in cyclohexane, (5) cyclohexane implicit solvent with structures optimized without implicit solvent, (6) cyclohexane implicit solvent with structures optimized with implicit solvent, (7) B3LYP with GD3BJ empirical dispersion used in this study.

Previously, similar methods have been demonstrated to accurately predict the ground-state properties of PAHs.

2.2. Electronic Properties

We explored the electronic properties of our π-conjugated molecular systems by analyzing the excitation energies in several different ways. For each molecule, we calculated the difference in energy between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) on the neutral, ground-state molecules from our DFT calculations. The HOMO–LUMO gap approximates the fundamental gap, which is the difference between the ionization potential (IE) and electron affinity (EA), rigorously defined by eq . ,

Efund=IE−EA=(EN−1−EN)−(EN−EN+1) 1

The anion and cation energies were calculated in the ground state using the previously optimized neutral geometries – i.e., the vertical ionization potential and electron affinities. The optical gap was calculated from the wavelength associated with the lowest absorption electronic transition (S0 to S1) from the TD-DFT energy calculation of each molecule. For these, we employed the ground state optimized structure and calculated a minimum of 25 states to obtain their excitation energies and associated oscillator strengths to simulate the near ultraviolet–visible-near infrared (UV–vis-NIR) spectra of each molecule using B3LYP with empirical dispersion (GD3BJ) and the 6–311G basis, implemented within Gaussian 16 as mentioned earlier. Figure displays the relationship of the optical and fundamental gaps, with the difference between them being the exciton binding energy, E B.

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Inspired by Bredas’ Mind the Gap! this energy diagram displays the relationship between the various excitation energies explored in this study.

3. Results and Discussion

3.1. Degree of Twist

Upon optimization, acenes with each functional group display varying degrees of end-to-end molecular twist. We quantify this twist by measuring the torsion angle between the terminal carbons of the acene backbonespecifically, ∠1234 or ∠2143, depending on the twist direction (Figure ). All structures displayed in Figure are rendered from the optimized structure coordinates using PyMOL. The degree of twist increases linearly with increasing backbone length for all molecular series from an average angle of 30° (n = 2) to 207° (n = 7). Figure shows the degree of twist versus backbone length for all series; the degree of twist varies significantly among the different substituents. At a backbone length of n = 5, bromine substituted acenes exhibit the highest twist angle of 162°, followed by ethyl (148°) and chlorine (137°). In contrast, phenyl-substituted acenes show lower twist angles, with Series 3 at 132°, Series 1 at 123°, and Series 2 at 112° (Figure ). We highlight the pentacene (n = 5) results when comparing the properties across series throughout this study both because it is a relevant backbone length for experimental studies, as described earlier, and since it is generally representative of the acenes' behavior in the many linear trends we observe across the backbone lengths.

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Twisted geometries of fully substituted acenes with backbone lengths of n = 5 after optimization; (a) a pentacene with Series 1 phenyl substituents; (b) a pentacene with Series 2 phenyl substituents; (c) a pentacene with Series 3 phenyl substituents; (d) a pentacene with bromine substituents.

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Twist angle vs backbone length. The degree of end-to-end molecular twist for fully substituted acenes with increasing backbone length from naphthalene to heptacene and varied substituents.

Solid-state X-ray diffraction data are available for n = 2–4 of Series 1 , (Table ), n = 3–6 of Series 3 − (Table ), and the halogenated naphthalenes (Table ). Our DFT-optimized geometries generally capture the values and trends observed for the experimental structures of Series 1, Series 3 and the halogenated species. Variations between the gas-phase optimized and solid-state experimental structures are expected due to intermolecular interactions in the crystal lattice. The one instance where our computational results differ markedly from the experimental 3° twist angle reported for octaphenylnaphthalene may result from competing symmetries and crystal packing effects in the physical system as previously discussed in detail by Qiao et al.

1. Series 1 Calculated Twist Angles Compared with Experimental X-ray Diffraction Data ,

backbone length B3LYP-GD3BJ/6–311G X-ray diffraction
2 26 3
3 60 63
4 91 97
5 123 --
6 150 --
7 179 --
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2. Series 3 Calculated Twist Angles Compared with Experimental X-ray Diffraction Data − .

backbone length B3LYP-GD3BJ/6–311G X-ray diffraction
2 37 --
3 68 66
4 100 105
5 132 144
6 163 184
7 195 --
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3. Calculated Twist Angles of Halogenated Naphthalenes Compared with Experimental X-ray Diffraction Data .

molecule B3LYP-GD3BJ/6–311G X-ray diffraction
octachloronaphthalene 27 24
octabromonaphthalene 35 31
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The twist angles observed for halogen and alkyl substituted acenes are primarily determined by steric hindrance and are a reflection of the size of the substituents’ atomic radii. However, it is somewhat counterintuitive that substitution with the bulkier phenyl substituents results in acenes with lower twist angles. An analysis of the molecular structure reveals that π–π stacking, rather than steric hindrance, defines the degree of twist in Series 1–3. Figure a shows the Series 1 optimized molecular structure for n = 5 and Figure b displays the electrostatic potential surface plot (isovalue of 0.01) showing the π–π stacking for the structure in the same orientation.

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(a) Series 1 optimized molecular structure for n = 5, and (b) electrostatic potential isosurface (for a 0.01 value) displaying the π–π stacking for this molecule in the same orientation.

Figure shows ideal π–π stacking geometry as well as the π–π interactions resulting from the optimized twisted structures for phenyl series of this study. π–π stacking interactions between benzene rings are known to be strongest when they are in an offset stacked conformation, roughly 3.4–3.6 Å apart and laterally offset by 1.6–1.8 Å (Figure a). In Series 1–3, the consecutive phenyl substituents along the length of the backbone are nearly parallel and have separation distances ranging from 3.1–3.4 Å with lateral offsets of 1.0–1.4 Å (Figure b). The similarity between these parameters and those of the optimal arrangement suggests that π–π stacking is the main interaction determining the degree of separation between adjacent phenyl substituents.

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Benzene ring alignment in (a) the optimal offset stacked conformation, in (b) Series 2 at n = 5, and in (c) a model showing the terminal end of Series 2 and (d) a model showing the terminal ends of Series 3. The separation distance in A and B is measured from the center of one phenyl ring to the nearest carbon on the phenyl adjacent to it. C and D show the distances between the two closest atoms on each pair of rings. The 90° rotation of the terminal benzene in Series 3 increases steric strain and results in a greater twist angle.

The differences in twist angles among Series 1–3 can be explained by both the number of substituted phenyl rings and their orientations. Series 1 has two additional terminal phenyl groups on each end of the backbone compared to Series 2, producing a 12° larger twist angle in the pentacene backbone (n = 5). These extra phenyl substituents participate in π–π stacking interactions with their neighboring rings, thus stabilizing a more twisted conformation. Series 3 is unique in that the terminal ends of the backbone are benzannulated. If these added benzenes are counted as substituents, then Series 3 has the same number of substituents as Series 2, while producing a 20° greater twist angle. The reason for this lies in the orientation of the benzannulated rings, which are rotated by 90° relative to the adjacent phenyl substituents due to their fused structure (Figure d). This orthogonal alignment deviates from the optimal offset stacking, which weakens the π–π interactions. It also orients its bonded hydrogen closer to its neighbors. Since the benzenes are directly fused to the backbone, the resulting steric hindrance significantly increases the twist angle of Series 3.

3.2. Excitation Energies

Figure shows the HOMO–LUMO, fundamental, and optical gap energies for all series of twisted acenes as a function of backbone length to explore the relationship of these excitations to substituent type and molecular length. Experimentally, the fundamental gap, as a measure of the difference between the ionization potential and electron affinity, can be measured by combining the results of gas-phase UV photoelectron spectroscopy with those of electron attachment spectroscopy; while the optical gap is the energy of the lowest energy electronic excitation that can be achieved through single photon absorption (except in the case where those are optically forbidden). To quantify the trends, we find the average value of all substituted acenes at each backbone length. The average HOMO–LUMO gap decreases by 61% from 4.03 eV for naphthalene (n = 2) to 1.52 eV for heptacene (n = 7) backbones. Similar reductions are seen for the fundamental gap (50%) and optical gap (65%). This decrease in excitation energies with increasing backbone length is expected for a π-conjugated system of increasing size. While the gap energies do not show dramatic variation by substituent, there are some exceptions. Series 3 exhibits significantly higher excitation energies compared to the other substituents, particularly for backbone lengths ranging from n = 3 to n = 5 (Figure ). The structures for this series are more structurally rigid due to the fused end groups having steric interaction with each other without the free rotation as in the phenyl substituents and displays somewhat anomalous behavior along the excitation trends particularly for the n = 2. Compared to the average values of the other substituents, Series 3′s HOMO–LUMO gap is higher by 25.2, 28.4, and 25.1% at n = 3, 4, and 5, respectively. Similarly, Series 3 shows elevated values for the optical gap (30.6%) and fundamental gap (12.5%) for n = 4. For illustrative comparison, the values for the planar parent acenes (denoted Hydrogen) are shown in Figure , but their calculated gaps are not included in the above-mentioned averages. Generally, the twisted structures have lower HOMO–LUMO, fundamental and optical gaps than the planar, unsubstituted parent acenes with the exception of Series 3. This effect is most pronounced for naphthalene and is much less pronounced by heptacene.

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Excitation energies as a function of backbone length for all molecular series in this study and compared with the parent (hydrogen) acenes. The calculated excitation energies of twisted acenes decrease with increasing backbone length and vary by substituent; (a) the HOMO–LUMO gap, (b) the fundamental gap, and (c) the optical gap. The planar parent acenes with hydrogen substituents are shown for comparison.

The energies of the HOMO and LUMO states for the unsubstituted, planar acenes and the three twisted phenyl-substituted series for n = 2 through n = 7 are shown in Figure . The greater gap for Series 3 is apparent. Interestingly, the HOMO values closely track those of the planar acene (with the exception of n = 2) but the LUMO states are higher for Series 3. Series 1 and 2 display very similar higher HOMO values than the parent acenes but have LUMO energies close to those of the acenes, especially for n = 4 and higher. The HOMO and LUMO energies for all molecules in this study are reported in the Supporting Information Tables I and II.

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HOMO and LUMO energies as a function of backbone length for the unsubstituted acenes and the three phenyl-substituted series in this study.

The optical gaps of Series 1 (Table ) ,, and Series 3 (Table ) − show close agreement with the available experimental values measured from the onset of absorption in UV–vis spectra (recorded in cyclohexane or chlorinated solvents). On average, the theoretical and experimental optical gaps differ by only 4.4%, with the largest deviation (8.2%) observed at n = 2 for Series 1. Additional HOMO–LUMO gaps determined by cyclic voltammetry for Series 1 (n = 2–3) and Series 3 (n = 6) differ from our calculated values by an average of 6.2% (Table ).

4. Series 1 Optical Gaps Compared with Experimental Data ,,

backbone length calculated (eV) experimental (eV)
2 3.55 3.26
3 2.72 2.57
4 2.08 2.07
5 1.62 --
6 1.30 --
7 1.07 --
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5. Series 3 Optical Gaps Compared with Experimental Data − .

backbone length calculated (eV) experimental (eV)
2 3.63 --
3 3.23 3.00
4 2.78 2.70
5 2.18 2.30
6 1.78 1.80
7 1.48 --
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6. Series 1 and 3 HOMO-LUMO Gaps Compared with Prior Studies ,

molecule calculated (eV) cyclic voltammetry (eV)
octaphenylnaphthalene 3.99 3.69
decaphenylanthracene 3.12 2.79
9,10,11,12,21,22,23,24-octaphenyltetrabenzo[a,c,n,p]hexacene 2.13 2.14
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While substituent type causes some variation in the excitation energies, their impact is small relative to the dominant influence of the number of fused rings comprising the molecular backbone. This effect appears to compound with increasing length; for instance, the range of optical gaps among the substituents (the difference between the largest and smallest values) narrows from 0.84 eV at n = 2 to 0.42 eV at n = 7 (Figure c). Consequently, the variation in excitation energies due to different substituents is overshadowed by the trend for increasing length of the π-conjugated backbone. A closer look at the absorbance spectra more clearly distinguishes the optical behavior induced by each substituent.

3.3. Absorption Spectra

Figure shows the near UV–vis-NIR absorption spectra of each twisted pentacene (n = 5), with the unsubstituted planar pentacene (Hydrogen) shown for comparison. Twisted pentacenes for all substituent types have primary absorption peaks (λmax) between 361 and 411 nm, significantly red-shifted relative to the 283 nm λmax of the planar pentacene. While most substituents exhibit similar levels of maximum absorbance, the ethyl substituted pentacene is significantly more intense.

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Absorbance as a function of wavelength for twisted pentacene structures as well as the planar unsubstituted pentacene indicated with dashed line for comparison. Each twisted molecule displays a maximum absorbances within the range of 350–400 nm. The substituted Ethyl series exhibits the highest absorbance of the series. Further, the halogen substituted molecules as well as Series 3 show notable broad peaks with lower intensity at wavelengths above 500 nm.

Broad absorption bands appear adjacent to the primary peak, with varying degrees of separation and intensity. Ethyl, Series 1, and Series 2 have low intensity secondary peaks at 727, 763, and 761 nm, respectively, whereas both halogen-substituted pentacenes show more intense maxima at 593 (chlorine) and 657 (bromine) nm. These broad secondary peaks are seen for halogen substituted acenes of all backbone lengths. Series 3 also has a high energy shoulder at 553 nm, but of much lower intensity.

The impact of increasing backbone length on these spectral features is illustrated in Figure . We use Series 1 to represent the general trends which are shared by all substituent types (Figure a). λmax increases linearly as a function of backbone length, from 318 nm at n = 3 to 444 nm at n = 7, with growing intensity. This linear trend is also seen in the experimental UV–vis spectra (recorded in chlorinated solvents) of Series 3 − from n = 3–6, with λmax of 324, 365, 393, and 421 nm, respectively. The adjacent broad absorption bands undergo a similar redshift (from 448 nm at n = 3 to 940 nm at n = 6), although their intensities generally diminish. Interestingly, halogen-substituted acenes deviate from this pattern. As their broad absorption bands redshift with increasing backbone length, their intensities increase. This is seen for the brominated heptacene (Figure b) in its intense shoulder at 720 nm.

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UV–vis spectra of acenes with increasing backbone length n = 2 to n = 7. (a) Series 1 phenyl-substituted acenes show a progressive redshift with longer backbones; (b) brominated acenes exhibit a similar redshift of λmax, but the intensity of its low energy broad absorption in the red-near IR region increases with larger backbone lengths.

These spectra show that the substituent effects extend beyond simple modulation of the twisted geometry. While the twisted configuration consistently emerges as a minimum energy structure for all substituted series investigated here, it is important to consider whether other stable conformations might exist. To explore this possibility, we further investigate the conformational landscape.

3.4. Competing Geometries

Several of the fully substituted acenes also adopt nonhelical configurations upon optimization. Instead of twisting, the molecules with halogen and ethyl substituents can relieve steric strain by alternating up and down along each edge of the nominally planar acene backbone. The cis configuration is when the two substituents bonded to each fused ring along the backbone are oriented in the same direction and alternating with the neighboring ring, (Figure a). Conversely, the trans configuration is when the two substituents on a fused ring orient in opposite directions from each other (Figure b). Both the line drawings and ball and stick models shown in Figure are depicted from the optimized coordinates of each molecule.

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Competing geometries of fully brominated pentacenes after optimization shown both in line drawings and ball and stick models; (a) The cis configuration; (b) The trans configuration. Pentacenes with chlorine and ethyl substituents also display local minima in both of these structural configurations.

Acenes with chlorine, bromine and ethyl substituents achieve local minima in both cis and trans configurations for all backbone lengths in addition to the twisted configuration already discussed. Interestingly, none of the three phenyl series display similar geometries. The series with phenyl substituents exhibit the aforementioned π–π interactions between the phenyl groups along the twisted backbone and hence favor this configuration rather than the cis or trans competing geometries adopted to relieve steric strain by the series with nonphenyl substituents. Thus, we obtain three distinct structural configurations for chlorine, bromine and ethyl substituted acenes. Their Gibbs free energies are, on average, within 2.9 kcal mol–1 per ring. The magnitude of the energy difference (between the most and least stable form) is heavily influenced by the length of the backbone, ranging from 0.9 kcal mol–1 per ring between chlorinated naphthalenes to 5.5 kcal mol–1 per ring between ethylated heptacenes. The cis configuration consistently ranks as the most energetically stable geometry, although condensed phase and solvent effects might impact this in the physical system. It is worth noting that at n = 2, there is no distinction between the twisted and trans geometries; hence, that length is omitted from these comparisons.

For the helical geometries, it was observed that the HOMO–LUMO, optical, and fundamental gaps decrease with increasing backbone length. This behavior persists in both the cis and trans configurations. However, the degree of reduction differs across the three structures. At n = 3, the average excitation energies for the bromine, chlorine, and ethyl series are similar in all three configurations (Figure ), as shown by the fundamental gap values of 5.64 ± 0.17 (twisted), 5.32 ± 0.26 (cis), and 5.25 ± 0.16 (trans) eV. By n = 7, these average gaps decrease by 39% (to 3.44 ± 0.15 eV) in the twisted geometry, by 37% (to 3.37 ± 0.07 eV) in the cis structure, and by 48% (to 2.74 ± 0.10 eV) in the trans geometry. The degree of reduction is much larger in the trans configuration than in either the twisted or cis configurations (Figure ). This pattern is even more apparent in the average HOMO–LUMO gaps, which decrease by 49% (twisted), 47% (cis), and 69% (trans), and for the average optical gap, which decrease by 54% (twisted), 52% (cis), and 80% (trans).

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Average excitation energies by structural configuration. Gap values with chlorine, bromine, and ethyl substituted acenes are averaged and compared in each structural configuration for (a) the fundamental gap, (b) the HOMO–LUMO gap, and (c) the optical gap. The trans configuration deviates from the twisted and cis geometries with increasing backbone length.

Consequently, the excitation energies in the twisted and cis configurations are similar while the trans structure is much lower (Figure ). At n = 5, the average HOMO–LUMO gap is 2.11 ± 0.06 eV in the twisted configuration and 1.97 ± 0.05 eV in the cis configuration, while the average trans configuration has a much lower HOMO–LUMO gap of 1.51 ± 0.08 eV (Figure ). As can be seen in Supporting Table 2, this is attributed both to increases in the HOMO energies as well as corresponding decreases in the LUMO energies for these series. The same pattern is seen in the average optical gaps, with values of 1.74 ± 0.04 (twisted), 1.67 ± 0.05 (cis) and 1.15 ± 0.08 (trans) eV, and fundamental gaps, with values of 4.18 ± 0.05 (twisted), 4.08 ± 0.08 (cis), and 3.62 ± 0.11 (trans) eV (Figure ).

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Excitation energies by structural configuration. The HOMO–LUMO, optical, and fundamental gaps are compared in the twisted, cis and trans configurations at n = 5. The trans configuration shows lower excitation energies for all substituents.

The properties of each structural configuration can be further distinguished via their absorption spectra at n = 5 (Figure ). Using the Bromine substituted pentacene as an example, λmax appears at the shortest wavelength in the cis geometry (360 nm), while the twisted and trans peaks are at longer wavelengths of 411 and 423 nm, respectively. The primary peaks in the twisted and trans configurations do not exhibit a consistent ordering among the three substituents (Table ). Additionally, cis configured acenes substituted with bromine (of all backbone lengths) show considerably lower intensity in their primary maxima69% below the average value of the other substituents at n = 5 (Table ).

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Absorption spectra of pentacenes by structural configuration. Chlorine, bromine, and ethyl substituted acenes are compared at n = 5 in (a) the twisted configuration, (b) the cis configuration, and (c) the trans configuration. The cis configuration displays a blue shift to λmax.

7. λmax for Substituted Acenes by Structural Configuration at n = 5.

  twisted
 cis
trans
substituent λmax (nm) Abs λmax (nm) Abs λmax (nm) Abs
chlorine 380 76,099 338 72,881 377 70,780
bromine 411 67,898 360 22,292 423 45,351
ethyl 370 98,557 327 71,301 359 78,966
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The differences in the properties of the broad low intensity bands are more dramatic. As previously described, notable absorption regions appear between 593–657 nm for halogenated pentacenes in the twisted configuration. This intensity is lost in the cis geometry, where they closely resemble the 727 nm peak with ethyl substituents (Table ). In the trans configuration, the broad bands for each substituent are drastically diminished in intensity and red-shifted to 1037–1181 nm (Table ). This finding is consistent with the lower optical gaps mentioned earlier (Figure c).

8. Maxima of the Broad Low Energy Absorption Region Adjacent to λmax at n = 5.

  twisted
 cis
trans
substituent λ (nm) Abs λ (nm) Abs λ (nm) Abs
chlorine 593 9168 727 2629 1037 1418
bromine 657 8195 765 2588 1181 1171
ethyl 727 2070 727 2684 1045 1511
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The distinct optical and electronic properties of the cis and trans geometries, compared to the twisted forms, introduce additional complexity in the design of halogenated and alkyl-substituted acenes. The significantly lower excitation energies of the trans configuration, and distinct spectral signatures across all three geometries, suggest opportunities to tailor acene properties by controlling the structural configuration.

4. Conclusions

We computationally investigated the structural and electronic properties of fully substituted acenes from naphthalene to heptacene. Our findings reveal that complete halogenation and alkylation effectively induces a helical twist in the acene backbone with a larger torsion angle than with phenyl substituents. This appears to be a consequence of stabilizing π–π interactions between consecutive phenyl groups. Substitution induces discernible red shifts in the HOMO–LUMO, fundamental, and optical gaps across all series, although variations among substituents are generally modest compared to the dominant effect of the number of fused rings comprising the molecular backbone. Nonetheless, distinct features are readily observed in the absorption spectra. Bromine and chlorine substituents display broad absorption bands adjacent to their primary peaks which are uniquely intense between 500 and 700 nm. Future studies investigating effects of spin–orbit coupling and frontier orbitals may lend additional quantitative and qualitative insights.

Interestingly, for the halogen- and ethyl-substituted derivatives, alternative nonhelical local minimanamely, the cis and trans geometriesare energetically competitive with the helical conformation at smaller backbone lengths. The trans geometry exhibits considerably lower excitation energies than the twisted and cis structures, and this trend intensifies at longer backbone lengths. Additionally, λmax is blue-shifted in the cis configuration relative to other geometries.

The clear trends in electronic and structural properties for all series, as well as the emergence of multiple stable configurations for certain substituent types, suggest that controlling the synthesisand the solid-state packing environmentcould offer multiple avenues to tune material properties for applications in organic electronics or photonics. By providing quantitative assessments of structure–property relationships, these findings offer valuable guidelines for designing novel acene derivatives poised for functional materials development.

Supplementary Material

ao5c01652_si_001.pdf (3.4MB, pdf)

Acknowledgments

The authors acknowledge summer research support through SURP and SOAR programs and computational resources from Seaver College of Science and Engineering at Loyola Marymount University, Los Angeles as well as the LMU Honors Program for which Georgia Grace Tully submitted a portion of this work for fulfilment of her honors thesis.

Glossary

Abbreviations

TD DFT

time-dependent density functional theory

OSC

organic semiconductor

OFET

organic field-effect transistors

HOMO

highest occupied molecular orbital

LUMO

lowest unoccupied molecular orbital

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c01652.

  • Optical gap versus backbone length for Series 1 molecules under a variety of exchange-correlation functionals and with polarization functions added to the basis set; twist angle versus backbone length for Series 1 molecules with and without added polarization functions; HOMO–LUMO Energies for all phenyl substituted molecules in this study; HOMO–LUMO Energies and Gibbs Free Energies for competing geometries of all the halogen and ethyl-substituted series; optimized ball and stick structures and Cartesian coordinates of all molecules (PDF)

†.

Stanford University, Department of Chemistry, 337 Campus Drive, Stanford, California 94305, United States

‡.

V.V.J. and G.G.T. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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