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Abstract
This article delves into the functionalisation of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) to perylene diimide (PDI) derivatives, focusing on the impact of various functional groups on solubility, optical bandgap, and electronic structure. The study systematically investigates how electron-donating and electron-withdrawing groups at the imide and bay positions influence the HOMO-LUMO energy levels and solubility of PDI derivatives. Key findings include the significant enhancement of solubility with branched alkyl groups and the modulation of electronic properties with electron-withdrawing NO2 groups. The article also explores the structure-property relationships, providing a framework for designing PDI derivatives with tailored properties for optoelectronic applications. The conclusion highlights the potential of targeted functionalisation to optimise the performance of organic electronic devices.
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Abstract
The transformation of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) to perylene diimide (PDI) and further derivatives offers a promising strategy to tailor the solubility and electronic properties of organic semiconductors for optoelectronic applications. In this study, a series of PDI derivatives with targeted substituents at the imide and bay positions was synthesised and systematically characterised. Structural integrity and functionalisation were confirmed by solid-state NMR, FTIR, and XPS. Solubility measurements revealed that branched alkyl or cyclohexyl groups at the imide position significantly enhance solubility, while diamide polyether chains can induce crosslinking and reduce processability. UV/Vis-spectroscopy, Ultraviolet Photoelectron Spectroscopy (UPS), and Reflection Electron Energy Loss Spectroscopy (REELS) were used to determine optical and fundamental bandgaps as well as HOMO levels. Electron-donating substituents at the imide position increased the HOMO level and narrowed the bandgap, whereas electron-withdrawing NO2 groups at the bay position lowered the HOMO and widened the bandgap. The difference between optical and fundamental bandgaps highlights the significant exciton binding energy in these materials. Overall, this work demonstrates structure–property relationships in PDI derivatives, providing a framework for the rational design of next-generation organic electronic materials with optimised solubility and electronic structure.
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1 Introduction
Organic electronics have attracted considerable attention for use in optoelectronic applications, including organic photovoltaics (OPVs), organic field-effect transistors (OFETs), and light-emitting diodes (OLEDs) [1‐6]. Devices based on organic materials offer several advantages, such as low weight, suitability for flexible solar modules [7, 8], and the potential for semi-transparent designs, making them ideal for integration into solar glass and windows [8, 9]. Additional benefits include cost-effective fabrication and straightforward processing. Nevertheless, challenges persist, particularly regarding relatively low efficiencies and limited operational lifetime.
Among the diverse range of organic semiconductors, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) highlights as a robust n-type semiconductor characterised by strong π˗π stacking interactions due to its planar aromatic structure [2, 6]. PTCDA is widely regarded as a benchmark compound in organic electronics because of its excellent photostability, efficient electron transport properties, and amenability to chemical modification. These attributes render it suitable for a variety of device architectures. However, the practical application of PTCDA is limited by its poor solubility and restricted tunability of its HOMO–LUMO energy levels. To address these challenges, attention has shifted towards perylene diimides (PDIs), which are derivatives of PTCDA and offer improved solubility and greater flexibility in tuning electronic properties. As a result, PDI derivatives have already demonstrated relevance in applications such as photovoltaics [10, 11] and other optoelectronic devices, owing to their high electron affinity, thermal and chemical stability, and strong absorption in the visible spectrum. Key electronic parameters for organic semiconductors include the optical bandgap, the fundamental bandgap, and charge transport behaviour. The ability to tune these electronic properties makes PDI derivatives attractive as acceptor materials in various organic electronic applications. The transformation of PTCDA into perylene diimide (PDI) can be achieved by imidization at the anhydride group in the peri position, with further functionalisation possible at the bay or ortho positions of the PDI core [12‐14].
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Consequently, a significant amount of research has focussed on the structural modification of PDIs to tune their HOMO–LUMO bandgap [6, 10, 15‐17]. By introducing electron-donating or electron-withdrawing groups at the imide or bay position, energy levels can be adjusted [5, 10, 15, 17‐19]. For example, electron-donating groups, such as alkyl or aryl substituents, increase the electron density of the perylene core, resulting in a higher HOMO level [20]. In contrast, electron-withdrawing groups, including halogens, cyano or nitro groups, stabilize the LUMO, lower the overall energy level, and result in a narrow bandgap [21]. Additionally, core extension through dimerization or polymerization has been shown to further reduce the bandgap [22].
Beyond the electronic properties, the introduction of various functional groups also affects the self-assembly and structural arrangement of the PDIs, which in turn influences device performance [3‐5, 17]. The aromatic core of PTCDA and PDI derivatives without any functional groups attached (unfunctionalized PDI derivatives) exhibits strong π˗π stacking interactions, significantly reducing solubility and thus limiting solution-based processing methods such as dip coating or spin coating [13]. Introducing bulky substituents, such as alkyl or alkoxy chains at the imide positions, can impede strong π˗π stacking, reduce crystallinity, and enhance solubility in organic solvents. Similarly, electron-withdrawing groups in the bay region can reduce stacking interactions by polarising the core. They may, however, also limit solubility in nonpolar solvents. Optimising PDI derivatives therefore requires balancing improved solubility with the risk of charge transport degradation from excessive side chains, while also considering the impact of functional group polarity in a high dielectric environment [23‐25]. Despite progress, challenges remain in fully understanding how specific functional groups affect the electronic properties and behaviour of PDIs [14, 26].
This study aims to elucidate structure-property relationships governing the tuning of HOMO-LUMO energy levels and the enhancement of solubility to improve processability and address current limitations. We hypothesise that the introduction of substituents with varying electron-donating or electron-withdrawing characteristics will modulate both solubility and energy levels. Herein, we present a systematic investigation of these two parameters in parallel, representing the first comprehensive study of their combined effects. While previous research has addressed these factors individually, our work uniquely elucidates their interplay and mutual impact, offering new insights and advancing understanding in this area.
Derivatives of perylene diimide (PDI) bearing functional groups at the imide position were obtained by imidazation reaction of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) with aniline, aminophenol, Jeffamine ED-600, cyclohexylamine, and 1-ethylpropylamine, respectively (Fig. 1, step 1). In a typical synthesis, PTCDA (5 g, 12.7 mmol), Zn(OAc)2 (1.9 g, 7.65 mmol), and the respective amine (38.1 mmol), were dissolved in 100 ml of imidazole under argon atmosphere and stirred for 24 h at 130 °C under reflux. After cooling to 80 °C, the mixture was diluted and precipitated in 200 ml methanol. The obtained solid was first washed with an excess of water and dried overnight at 80 °C before stirring with 100 ml 2% NaOH solution at 25 °C for 1 h to remove unreacted PTCDA. The residue was filtered and washed with 100 ml 2% NaOH solution until the green coloration disappeared. Finally, the crude perylene diimide products were washed with an excess of water and dried over night at 80 °C under vacuum.
Fig. 1
Synthesis procedure for PDI derivatives; step 1: Imidization of PTCDA with appropriate amines, yielding respective imide-functionalized PDI derivatives 1, 2, 3a, and 4a, and step 2: Nitration of imide-functionalized PDIs (3a, 4a) to introduce NO2 groups in bay positions, yielding perylene functionalized PDI derivatives 3b and 4b
Bay-region functionalisation of the PDI core was achieved via nitration using fuming nitric acid (HNO3) (Fig. 1, step 2). Briefly, dried PDI (1.6 mmol) was stirred in 150 ml anhydrous DCM, while fuming HNO3 (1.32 ml, 32 mmol) was added dropwise. The reaction mixture was stirred at room temperature for 48 h. After complete monomer conversion (monitored by thin layer chromatography (TLC), the reaction mixture was poured into water, and the product was extracted with DCM. After washing three times, the organic phase was collected, and the solvent was removed. The resulting product was precipitated in methanol and filtered to obtain a red solid.
2.2 Methodology
Solid-state NMR experiments were performed on a Bruker 500 MHz solid-state NMR spectrometer (Bruker Corporation, Billerica, Massachusetts, United States) by using cross-polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) technique. All spectra were referenced to Tris(trimethylsilyl)silane (TTMSS) as a secondary reference for 13C at 25 kHz spinning speed and 25 °C bearing gas.
FTIR spectra were recorded using an ALPHA II (Bruker Corporation, Billerica, Massachusetts, United States) with a platinum-ATR unit in the range of 4000–400 cm⁻1 at room temperature with a resolution of 2 cm−1 and a scan time of 32 scans.
Absorption spectra were recorded on a Multiskan GO spectrophotometer (Thermo Fisher Scientific, Waltham, Massachusetts, United States) in the range of 200–1000 nm.
Solubility was assessed by absorbance measurements of PTCDA and PDI derivatives in chloroform, toluene, and methanol with concentrations of 0.1, 0.5, 1, 2, 5, and 10 mg/ml. Regression plots of absorbance versus concentration were constructed to determine the solubility by comparing the absorbance of the saturated solutions. Triplicate measurements were carried out, and standard deviations were derived accordingly.
The optical properties of PTCDA and PDI derivatives were investigated using UV/Vis-spectroscopy. Determining the absorption maxima and optical bandgap of the synthesised PDI derivatives by analysing the UV/Vis absorption spectra using Tauc plots and Tauc equation (Eq. 1) [27, 28].
$$\left( {\alpha \cdot h \cdot \nu } \right)^{n} = A \cdot \left( {h \cdot \nu - E_{g} } \right)$$
(1)
The Tauc method involves plotting (αhν)n versus photon energy hν, where α is the absorption coefficient, A is a proportionality constant, Eg is the optical bandgap, and n = 1/2 for an indirect and n = 2 for a direct allowed transition. The linear portion of the plot was extrapolated to intercept the energy axis, resulting in the optical bandgap of the material. Each analysis was repeated at least three times, and the resulting data were used to calculate standard deviations.
The highest occupied molecular orbital (HOMO), the fundamental bandgap, and the elemental composition of the samples were characterised using Ultraviolet Photoelectron Spectroscopy (UPS), Reflection Electron Energy Loss Spectroscopy (REELS), and X-ray Photoelectron Spectroscopy (XPS), respectively. All three measurements were conducted on an ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts, United States). For UPS, a helium discharge lamp providing He I radiation (21.2 eV) was used, with a pass energy of 1 eV. The measurements were performed with an applied bias of 5 V. Thermal vapour deposition was employed to deposit thin, homogeneous films of 200 nm thickness of PTCDA and PDI derivatives on Si wafers (Si-Mat, Silicon Materials e.K., Kaufering, Germany) at vacuum pressure of 3 × 10–5 mbar. The setup featured a boat configuration, with the organic powder loaded into a tungsten boat, which was resistively heated by applying a current of 53 A for 6 min. The substrates were positioned inverted 15 cm above the boat. REELS measurements utilized the same film samples and were conducted with a primary electron energy of 1 keV, generated by a coated tungsten emitter, a pass energy of 2 eV and an energy step size of 0.02 eV, recording the energy loss spectrum to determine the fundamental bandgap. XPS measurements were performed using the same ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts, United States), equipped with a monochromatic AlKα source (1486.7 eV, 650 µm spot). Survey spectra (150 eV pass energy) and high-resolution line spectra (20 eV pass energy, 0.45 eV resolution) were acquired in electrostatic mode at 0° emission angle. Surface neutralisation used low-energy electrons and Ar+ ions. Data were processed in CasaXPS v2.3.25, referencing the C1s peak at 285.0 eV, and fitted with 70/30 Gaussian/Lorentzian line shapes. The analysis focussed on elemental composition and chemical states of the films. At least three measurements were performed, and the standard deviations were calculated based on these measurements.
3 Results and discussion
3.1 Structural characterisation of functionalized PDI derivatives
PTCDA and imide functionalized PDI derivatives 1 and 2 displayed insufficient solubility for liquid NMR. Therefore, the structural integrity and successful functionalization of PTCDA and PDI derivatives 1, 2, 3a, 3b, 4a, and 4b were confirmed by 13C solid-state NMR (see Supporting Information, Figure S1). For all derivatives, 13C-NMR spectra depict peaks for the imide carbon at 160–165 ppm and perylene carbon atoms at 120–135 ppm, respectively. For perylene functionalized PDI derivatives 3b and 4b, additional peaks at 145–150 ppm confirmed NO2 group functionalization of the respective bay positioned carbon atoms.
FTIR ATR spectra of powdered samples show that PTCDA exhibits strong anhydride absorption bands between 1730 and1780 cm−1. In contrast, all PDI derivatives display new, strong absorption bands in the range of 1690–1715 cm−1 and 1620–1670 cm−1, corresponding to imide stretching vibrations (Fig. 2). The characteristic anhydride band of PTCDA at 1730–1780 cm−1 is reduced or absent in all PDI derivatives, indicating partial or complete imidisation. For the bay functionalized PDI derivatives 3b and 4b, a shift and change in intensity of the imide bands at 1630–1720 cm−1 is observed, corresponding to the presence of additional electron withdrawing NO2 groups. Together with NMR data, the FTIR spectra confirm the successful synthesis and the presence of the targeted functional groups.
Fig. 2
FTIR spectra of the synthesised compounds in powder form showing the most relevant area from 1800 to 1600 cm−1 for PTCDA, 1, and 2 a and 3a, 3b, 4a, and 4b b
To further elucidate the chemical composition, the thin film materials were analysed by X-ray photoelectron spectroscopy (XPS). The XPS data provide information on the chemical states of carbon (C1s, Supporting Information, Figure S2), oxygen (O1s, Supporting Information, Figure S3), and nitrogen (N1s, Supporting Information, Figure S4).
For PTCDA and all PDI derivatives, the C1s XPS spectra exhibit a primary peak at 284.8 eV corresponding to C–C and C–H bonds, and a secondary peak at 287.7 eV, assigned to C=O functionalities (Figure S2). Additionally, PTCDA shows a shoulder at 289.6 eV attributed to the shake-up satellite of the electronic interactions between the anhydride group and the aromatic system (Fig. 3) [29, 30]. PDI derivative 2 exhibits an additional peak at 286.2 eV, attributed to the C–O bonds of the attached polyether chain. The O1s XPS spectrum of PTCDA shows a double peak at 531.4 eV and 533.3 eV, corresponding to the C=O and the C-O bonds of the anhydride carbonyl groups, respectively (Fig. 3). In comparison, PDI derivative 1 displays a primary peak at 531.2 eV corresponding to N–C=O groups, and a secondary peak at 532.9 eV, which can be attributed to residual anhydride PTCDA or adsorbed water due to ambient exposure [31]. PDI derivative 2 shows a primary peak at 532.6 eV attributed to the C-O bonds of the attached polyether group, and a secondary peak at 530.9 eV corresponding to the N–C=O groups. PDI derivatives 3a and 4a exhibit a primary peak around 531.1 eV corresponding to the N–C=O groups. In analogy to PDI derivative 1, a small secondary peak at 532.9 eV is observed, corresponding to residual anhydride PTCDA or adsorbed water due to ambient exposure [31]. In comparison to 3a and 4a, NO2 functionalized PDI derivatives 3b and 4b show a strong peak at 530.9 eV, attributed to the N–C=O groups, and a second strong peak at 532.7 eV, attributed to the oxygen atoms in the NO2 substituents (Figure S3). As expected, the N1s spectrum of PTCDA shows no nitrogen signal. All PDI derivatives display a primary peak at 399.9eV, corresponding to imide nitrogen (Figure S4). Additionally, PDI derivatives 3b and 4b display a strong peak at 405.5 eV, assigned to the nitrogen of the NO2 functional group. The observed XPS line shapes and fitted chemical components were in general agreement with the molecular composition of the PTCDA derivatives.
Fig. 3
High-resolution XPS spectra of a C1s and b O1s core levels for PTCDA and PDI derivative 4b thin film materials
The introduction of functional groups significantly influences the solubility of PDI derivatives, which is crucial for solution-based processing and device fabrication. Solubility was determined by absorbance measurements at various concentrations of PTCDA and PDI derivatives in chloroform, toluene, and methanol. Regression plots of absorbance versus concentration were constructed to determine the solubility by comparing the absorbance of the saturated solutions (Fig. 4).
Fig. 4
Solubility of PTCDA and the PDI derivatives 1, 2, 3a, 3b, 4a, and 4b in chloroform (polar, aprotic), toluene (non-polar, aprotic), and methanol (polar, protic)
The classification is based on the USP solubility classification system [32], with our specific definition as follows: basically insoluble < 2 mg/ml, soluble 2–20 mg/ml, and very soluble > 20 mg/ml. Chloroform, toluene, and methanol were selected as representative solvents to assess the impact of PDI functionalisation in different polar/non-polar and protic/aprotic environments.
PTCDA and PDI derivatives 1 and 2 are insoluble in all tested solvents. Cyclohexyl substituted PDIs 3a and 3b show increased solubility in chloroform, with 3b being soluble in toluene and methanol. Ethylpropyl substituted PDI derivatives 4a and 4b are very soluble in chloroform, with 4b also showing moderate solubility in toluene (5 mg/ml) and in methanol (2 mg/ml). This demonstrates that functionalisation, especially with bulky or polar groups, significantly enhances solubility in organic solvents (Table 1)
Table 1
Solubilities of PTCDA and PDI derivatives in solvents chloroform, toluene, and methanol based on solubility classification
Sample name
Chloroform / mg/ml
Toluene / mg/ml
Methanol / mg/ml
Solubility classification
PTCDA
< 2
0
0
No solubility
1
< 2
0
0
No solubility
2
0
0
0
No solubility
3a
8
< 2
< 2
Soluble in chloroform
3b
10
3
3.5
Soluble in all
4a
92
0
0
Very soluble in chloroform
4b
21
5
2
Very soluble
The comparison between PTCDA and PDI derivative 1 reveals similar solubility profiles, which is consistent with their structural similarity, as PDI derivative 1 contains solely a phenyl substituent. In contrast, PDI derivative 2 is insoluble in all tested solvents, contrary to the expectations that the introduction of hydrophilic polyethylene glycol chains would enhance solubility. This unexpected result may be due to steric hindrance from the long polyether chains. XPS analysis shows only N–C= O signals and no residual NH2 signals (Figure S4, N1s scan), indicating that crosslinking has occurred, rather than the attachment of a single polyethylene glycol chain originating from Jeffamine ED-600 at the imide position. This suggests that multiple perylene cores are interconnected via Jeffamine linkages, accounting for the observed lack of solubility.
Cyclohexyl functionalized PDI derivative 3a, with its aliphatic and hydrophobic substitution, demonstrates good solubility in chloroform but is practically insoluble in toluene and methanol. This behaviour, which deviates from expectations, may be attributed to increased steric hindrance from the cyclohexyl group. In comparison, PDI derivative 3b exhibits overall higher solubility than 3a, likely due to the presence of additional polar NO2 groups.
The ethylpropyl functionalized PDI derivatives 4a shows the highest solubility in chloroform, attributed to its aliphatic chain, but is insoluble in toluene and methanol. This is unexpected for a hydrophobic molecule and may also be due to steric effects. PDI derivative 4b, which contains additional NO2 groups, displays slightly reduced solubility in chloroform (21 mg/ml) but improved solubility in toluene and methanol compared to 4a, likely due to the increased polarity and altered molecular interactions introduced by the NO2 substituents.
3.3 Molecular energy levels and optoelectronic properties
Optical properties measured by UV/Vis-spectroscopy reveal distinct variations in the absorption spectra among the different derivatives (Fig. 5 (a)). PTCDA displays one broad peak between 450–580 nm that can be assigned to π-π* transitions [33, 34]. In contrast, PDI derivatives with functionalized imide and perylene groups exhibit several separated peaks. Notably, PDI derivatives 3b and 4b, which contain NO2 groups in the bay position, show pronounced peaks around 700 nm, indicating significant changes in the UV/Vis spectra. The strong electron-withdrawing nature of the NO2 group induces a highly polarised electronic structure, facilitating low-energy intramolecular charge-transfer transitions from the PDI π-system to the NO2 group, resulting in a red-shifted peak [1, 35]. Additionally, these derivatives exhibit a strong absorption around 400–430 nm, likely corresponding to higher-energy π-π* transitions that are less prominent in PDI derivatives 1, 2, 3a, and 4a [33, 34]. The absence of peaks at 480–500 nm and the reduced intensity of the 530 nm peaks further reflects the perturbation of the π-π* transitions, likely due to lowering of the LUMO energy level by the NO2 group. These results highlight the significant impact of NO2 functionalisation on the electronic and optical properties of the PDI derivatives [1, 34, 35].
Fig. 5
a normalised absorption graphs and b Tauc plot of PTCDA and the PDI derivatives in solvent CHCl3. The arrows indicate the extrapolation that leads to the x-axis intercept
The optical bandgap was determined using the Tauc plot method by plotting (αhν)1/2 versus photon energy hν. The resulting plots (Fig. 5 (b)) provide clear data suitable for indirect bandgap calculations [36]. PTCDA exhibits an optical bandgap of 2.1 eV, consistent with literature values [37]. PDI derivatives 1, 2, 3a, and 4a display similar Tauc plots, with optical bandgaps of 2.29 eV, 2.28 eV, 2.32 eV, and 2.31 eV, respectively, indicating that conversion from anhydride to imide has minimal effect on the optical bandgap. In contrast, NO2 functionalized PDI derivatives, 3b and 4b, show reduced optical bandgaps of 2.22 eV and 2.23 eV, respectively, demonstrating that NO2 functionalization leads to a decrease in the optical bandgap.
Upon photon absorption, a bound electron–hole pair (exciton) is created. The optical bandgap is the minimum photon energy needed to form an exciton, but this energy may not be enough to separate the electron and hole into free charge carriers due to their Coulomb attraction (exciton binding energy). This effect is especially pronounced in organic molecular materials, where intermolecular interactions broaden energy levels into bands. To study the electronic properties in detail, Ultraviolet Photoelectron Spectroscopy (UPS) and Reflection Electron Energy Loss Spectroscopy (REELS) measurements were conducted. UPS provides information on the occupied electronic states and helps determine the HOMO level, while REELS reveals the energy loss process, providing information on the fundamental (or electronic) bandgap.
Representative UPS and REELS dataset for PTCDA is shown in Figs. 6, 7, with corresponding spectra for the PDI derivatives that are available in the Supporting Information (Figure S5 and Figure S6). The analytical procedure for UPS and REELS applied to PTCDA were used identically for all PDI derivatives [38‐41].
Fig. 6
UPS spectrum of PTCDA, displaying the full spectrum in the middle graph. The left graph exhibits the SECO in greater detail, and the right graph displays the lower binding energy region in the spectrum
Figure 6 displays the UPS spectrum for PTCDA, where the HOMO level was determined by identifying the onset of the secondary electron cut-off (SECO) and the valence band edge. The SECO was identified by linear extrapolation of the high-energy edge, while the valence band onset was determined by extrapolating the low-energy edge relative to the Fermi level. The HOMO level was calculated by subtracting the width from the initial photon energy of the He I lamp (21.2 eV) [38, 41‐43]. For PTCDA, the HOMO level was determined to be − 6.8 eV, consistent with literature [40] values.
For the PDI derivatives, the influence of functional groups on the HOMO levels is evident. The PDI derivatives 1, 2, 3a, and 4a, which are modified only at the imide position, show increased HOMO levels compared to PTCDA (− 6.8 eV): − 6.6 eV (1), − 6 eV (2), − 5.9 eV (3a), and − 6.2 eV (4a). This increase is attributed to the electron-donating inductive (+ I) effects of the alkyl and aryl substituents, which enhance the electron density in the π-system. Specifically, PDI derivative 1, bearing a phenyl group, exhibits a moderate -I and a notable + M (mesomeric) effect resulting in a modest HOMO shift of 0.2 eV. PDI derivative 2 with polyether chains demonstrates a stronger + I effect, raising the HOMO level to −6 eV. PDI derivative 3a, with a cyclohexyl group, also has a strong + I effect, resulting in a HOMO level of − 5.9 eV. In contrast, PDI derivative 4a, which contains an ethylpropyl group, displays a moderate + I effect, raising the HOMO level to − 6.2 eV.
Conversely, PDI derivatives 3b and 4b, which include the electron-withdrawing NO2 groups, exhibit lower HOMO levels of − 7.1 eV and − 7.2 eV, respectively, compared to their parent derivatives 3a and 4a. This decrease is due to the strong ˗M and ˗I effect of the NO2 group, which withdraws electron density from the conjugated π-system and stabilises the HOMO. Thus, the presence of NO2 groups significantly lowers the electronic density in the π-system, substantially influencing the electronic properties of the respective PDI derivatives [44]. Overall, the trend of the HOMO energy level adaptation correlates with the electron-donating or electron-withdrawing strength of the substituents, demonstrating that the choice of functional groups enables fine-tuning of the electronic properties, which is essential for optimising performance in organic electronic applications.
For the characterisation of the molecular energy level structure, the HOMO energy level derived from UPS measurements was evaluated in combination with the fundamental bandgap determined from Reflection Electron Energy Loss Spectroscopy (REELS). The fundamental bandgap is identified as the energy at which the energy loss intensity begins to increase significantly, representing the minimum energy required for electronic excitation.
The REELS spectrum for PTCDA is displayed in Fig. 7. The onset was determined by fitting a baseline and extrapolating the leading edge of the energy loss feature to this baseline [40, 42, 45]. The analysis revealed two peaks: a higher-energy peak at ~ 5.8 eV, corresponding to π-π* transitions and a second peak ~ 2.1 eV. The onset of the higher-energy peak yields a fundamental bandgap of 4.2 eV for PTCDA, consistent with the literature values [40]. The lower-energy peak ~ 2.1 eV is attributed either to defect-related states within the bandgap or, more plausibly, to sub-bandgap energy loss features, specifically the optical bandgap dominated by Frenkel excitons. This interpretation is supported by the optical bandgap of 2.1 eV derived by Tauc plot analysis, reflecting PTCDA’s strong excitonic character. Compared to PTCDA, the fundamental bandgaps of the PDI derivatives are reduced (Table 2). PDI derivatives 1 and 2, and 3a and 4a, show fundamental bandgaps of approximately the same bandgap value of 3.7 eV and 3.4 eV, respectively. Upon NO2 functionalisation, the fundamental bandgap increases: for 3a to 3b, and from 4a to 4b, from 3.4 eV to 3.6 eV and 3.7 eV, respectively. This demonstrates the influence of the NO2 functional groups on the electronic structure of PDI derivatives 3b and 4b.
Fig. 7
REELS energy loss spectrum of PTCDA including a Shirley background correction [42]
Overview of optical bandgap (Tauc Plot), fundamental bandgap, HOMO, and LUMO (UPS and REELS) for PTCDA and PDI derivatives 1, 2, 3a, 3b, 4a, and 4b, including their standard deviations (St.Dev)
Sample name
Optical bandgap in solvent / eV
St.Dev
Fundamental bandgap exp
/ eV
St.Dev
HOMO exp. / eV
St.Dev
LUMO calc. / eV
St.Dev
PTCDA
2.11
0.16
4.2
0.13
− 6.8
0.19
− 2.6
0.23
1
2.29
0.15
3.7
0.14
− 6.6
0.13
− 2.7
0.19
2
2.28
0.14
3.7
0.36
− 6
0.16
− 2.3
0.39
3a
2.32
0.11
3.4
0.10
− 5.9
0.14
− 2.6
0.17
3b
2.22
0.12
3.6
0.11
− 7.1
0.14
− 3.1
0.18
4a
2.31
0.11
3.4
0.12
− 6.2
0.16
− 2.8
0.2
4b
2.23
0.13
3.7
0.16
− 7.2
0.16
− 3.1
0.23
The combination of the UPS and the REELS dataset results in an energy level diagram, presented in Fig. 8 and summarised in Table 2, along with their standard deviations (St.Dev) calculated from at least three measurements and including a systematic device error of 0.1 eV. The analysis of the HOMO level and the fundamental bandgap confirms the impact of functional groups on the electronic structure of PTCDA and its PDI derivatives.
Fig. 8
graphical representation of the energy levels of PTCDA and the PDI derivatives 1, 2, 3a, 3b, 4a, and 4b, obtained by UPS and REELS measurements
Comparison of optical and fundamental bandgap reveals that the fundamental bandgap is consistently 0.5–2 eV larger for PTCDA and all the PDI derivatives. The optical bandgap is the energy needed to create a bound electron–hole pair (exciton) by photon absorption, while the fundamental (transport) bandgap represents the energy to create free charge carriers. The difference between them is the exciton binding energy, which is significant in π-conjugated organic materials as PTCDA and PDI derivatives, due to their low dielectric constant and strong interactions [46].
A detailed comparison of the different functionalisation reveals a structure–property relationship for the studied PDI derivatives. PDI derivative 1 with a phenyl group substitution at the imide position did not improve solubility but yielded a modest increase in the HOMO level and a narrower bandgap. Potential crosslinking in case of PDI derivative 2 with polyether glycol groups at the imide position prevented solubility. However, the HOMO increased by 0.8 eV compared to PTCDA, and the bandgap narrows, indicating strong electronic effects despite poor processability. Cyclohexyl (3a) and ethylpropyl (4a) substitution at the imide position both increase the HOMO level and result in a narrower bandgap for the PDI derivatives. Adding a NO2 group at the bay position (3b and 4b) lowers the HOMO level and widens the bandgap, highlighting the strong electron-withdrawing effect. In both cases, solubility is affected but remains good.
A comparison of PDI derivatives shows that branched alkyl groups (like ethylpropyl) at the imide position improve solubility. Electron-donating groups raise the HOMO level and narrow the bandgap, while electron-withdrawing groups (like NO2) lower the HOMO level and can widen the bandgap. Polyether glycol groups led to crosslinking and poor solubility but still caused strong electronic effects.
4 Conclusion
In summary, the results demonstrate that the targeted transformation of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) into perylene diimide (PDI) and its derivatives enables precise tuning of both solubility and electronic properties. Electron-donating groups (e.g., alkyl, cyclohexyl, polyether) at the imide position generally increase the HOMO level and can enhance solubility, especially with branched alkyl chains. In contrast, electron-withdrawing groups (e.g., NO2) at the bay position lower the HOMO level, and can further modulate solubility depending on the overall molecular structure. The combination of these effects allows for the rational design of PDI derivatives with tailored properties for specific optoelectronic applications. Overall, the study highlights the structure–property relationship in PDI derivatives and provides a framework for optimising their performance and manufacturing of next-generation electronic devices with optimised properties.
Declarations
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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