Volume 29, Issue 46 e202301639
Research Article
Open Access

Extended π-Conjugation and Structural Planarity Effects of Symmetrical D-π-A-π-D Naphthalene and Perylene Diimide Semiconductors on n-type Electrical Properties

Sergio Gámez-Valenzuela

Sergio Gámez-Valenzuela

Department of Physical Chemistry, University of Malaga Campus de Teatinos s/n, Malaga, 29071 Spain

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Dr. Iván Torres-Moya

Corresponding Author

Dr. Iván Torres-Moya

Department of Inorganic, Organic Chemistry and Biochemistry University of Castilla-La Mancha-IRICA, Faculty of Science and Chemical Technologies, Ciudad Real, 13071 Spain

Department of Organic Chemistry, University of Murcia Campus of Espinardo, Murcia, 30005 Spain

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Abelardo Sánchez

Abelardo Sánchez

Department of Inorganic, Organic Chemistry and Biochemistry University of Castilla-La Mancha-IRICA, Faculty of Science and Chemical Technologies, Ciudad Real, 13071 Spain

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Beatriz Donoso

Beatriz Donoso

Department of Inorganic, Organic Chemistry and Biochemistry University of Castilla-La Mancha-IRICA, Faculty of Science and Chemical Technologies, Ciudad Real, 13071 Spain

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Prof. Juan Teodomiro López Navarrete

Prof. Juan Teodomiro López Navarrete

Department of Physical Chemistry, University of Malaga Campus de Teatinos s/n, Malaga, 29071 Spain

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Prof. M. Carmen Ruiz Delgado

Prof. M. Carmen Ruiz Delgado

Department of Physical Chemistry, University of Malaga Campus de Teatinos s/n, Malaga, 29071 Spain

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Prof. Pilar Prieto

Corresponding Author

Prof. Pilar Prieto

Department of Inorganic, Organic Chemistry and Biochemistry University of Castilla-La Mancha-IRICA, Faculty of Science and Chemical Technologies, Ciudad Real, 13071 Spain

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Prof. Rocío Ponce Ortiz

Corresponding Author

Prof. Rocío Ponce Ortiz

Department of Physical Chemistry, University of Malaga Campus de Teatinos s/n, Malaga, 29071 Spain

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First published: 02 June 2023

Graphical Abstract

Efficient D-π-A-π-D naphthalene and perylene diimide n-type semiconductors were designed and synthesized following the principles of Sustainable Chemistry, achieving electron-field-effect mobilities up to 0.3 cm2 V−1 s−1. A comprehensive physical chemistry analysis was approached to elucidate structure-charge transport properties relationships in these new materials.

Abstract

A series of donor-π-acceptor-π-donor (D-π-A-π-D) compounds based on naphthalendiimide (NDI) and perylenediimide (PDI) central cores combined with triphenylamine and phenylcarbazole donor groups have been synthesized, characterized and tested in top-contact/bottom gate organic field-effect transistors (OFETs). The results showed high electron mobilities, up to 0.3 cm2 V−1 s−1, in the case of NDI derivatives and moderate values of around 10−3 cm2 V−1 s−1 for PDI-based semiconductors. Quantum chemical calculations were performed in order to support the experimental data. The results suggest that adequate molecular characteristics and larger crystalline domains in NDI vs. PDI semiconducting films may be the reasons behind the enhanced electrical properties of NDI derivatives. Furthermore, when the lateral donor substituents are triphenylamine groups, the mobilities were slightly higher in comparison to phenylcarbazole donor groups due to an improved electron-donating character. Other characterization techniques, such as AFM, X-ray diffraction or spectroelectrochemistry, among others, have been performed to analyze supramolecular order, charge carriers’ nature and stability, parameters closely related to charge transport characteristics.

Introduction

During the last decades, the development of new organic semiconductors, based on π-conjugated materials, has gained an enormous interest for applications in low cost, light weight, and flexible optoelectronic devices.1 In these materials, their electric performance in devices is highly influenced by both molecular and intermolecular properties, as well as by their purity; being these properties readily tuned by rational modifications of their molecular structures. In this sense, studies directed to understand and elucidate the structure-charge transport property relationships of organic semiconductors are required to rationally design new and improved materials.

Among organic materials, π-conjugated fused-ring systems, such as polycyclic aromatic dicarboximides (PADI)2 have demonstrated to be excellent candidates for optoelectronic applications.3 Particular interest has been shown by the community in naphthalene (NDI) and perylene diimides (PDI) derivatives, due to their stable and robust conjugated skeletons, and their marked electron-deficient character due to the imide groups. In addition, alkyl functionalization at the nitrogen atom of the imide groups allows fine modulation of processability, which is highly desired for low cost devices fabrication.4 Therefore, NDI and PDI semiconductors have been widely used as efficient n-type materials in field-effect transistors.5 In addition, other molecular properties such as large fluorescence quantum yields, high molar absorptivity, excellent thermal and photostability, chemical inertness, high π-conjugation degree, etc. make these materials appropriate for applications in organic photovoltaics,6 biosensors,7 organic light emitting diodes (OLEDS),8 optical switches,9 molecular wires,10 and as electrodes in batteries,11 among others.

One of the ways of improvement for NDI and PDI derivatives entails lateral π-conjugation extension, following strategies involving either the enlargement of the fused skeleton12 or the lateral substitution with electro-active conjugated groups.2a, 13 In fact, the substitution of molecular conjugated frameworks with donor or acceptor groups, aiming at the tuning of their molecular and electronic properties, is one of the most widely used strategies for molecular functionalization.14 These electro-active groups are commonly attached to the main conjugated core via single bonds; however, extra conjugation and electronic modulation can be achieved by the use of connecting ethylene and/or ethynyl groups.15 In this sense, we have previously found that the ethynyl connecting group is very helpful to direct and enhance intermolecular π-π interactions,16 which is fundamental to obtain ordered molecular films for efficient charge transport. Representative examples of alkynyl NDI and PDI samples with interesting optoelectronic properties can be found in the literature.17

All the above considered, in this work we present the synthesis and characterization of a new series of D-π-A-π-D semiconductors having NDI and PDI as central acceptor cores. Lateral substitution, through an ethynyl bond, with different donor groups (i. e. triphenylamine and phenylcarbazole groups) has been studied. These molecules allow us to analyze the (i) effect of π-conjugation extension of the PADI building block and (ii) the effect of π-conjugation extension via introduction of lateral electron rich groups on the molecular and electronic properties of the analyzed materials.

Results and Discussion

Synthesis of NDI and PDI derivatives

The synthesis of the new D-π-A-π-D naphthalendiimide derivatives (NDI) was performed by a three-step procedure. The first step was the bromination of the 1,4,5,8-naphthalenetetracarboxylic dianhydride with 1,3-dibromo-5,5-dimethylhydantoin (DBH).18 Then, the imide formation took place, following the procedure described by Ren et al.,19 ending with a Sonogashira cross-coupling reaction with the aryl ethynyl donor fragments by a procedure which is widely employed in our research group for other similar heterocycles,20 using microwave irradiation as energy source and a reusable catalyst (Pd-Encat TPP30) to reduce the environmental impact of the process obtaining the desired NDI compounds (Scheme 1).

Details are in the caption following the image

Synthetic procedure for the preparation of D-π-A-π-D NDI derivatives.

In the case of the perylenediimide derivatives (PDI) one less reaction step was required than in the case of NDI derivatives because the bromoderivative is commercially available. In this way, the first step was the imide formation19 from 1,7-dibromo-3,4,9,10-tetracarboxylic acid dianhydride followed by the Sonogashira coupling reaction between the dibromo derivative and the arylethynyl donor fragments by the same procedure use for the NDI derivatives, obtaining the desired compounds PDI1 and PDI2 (Scheme 2).

Details are in the caption following the image

Synthetic procedure for the preparation of D-π-A-π-D PDI derivatives.

General procedure for NDI: A mixture of 4,9-dibromo-2,7-dioctylnaphthalene-1,3,6,8-naphthalenediimide (0.100 g, 0.155 mmol), the corresponding arylethynyl derivative (0.310 mmol), DBU (0.050 g, 0.310 mmol), CuI (1.45×10−3 g, 7.75×10−3 mmol) and Pd-EncatTM TPP30 (0.014 g, 5.4×10−3 mmol) was charged under argon to a dried microwave vessel. Then, 1 mL of CH3CN was added. After that, the vessel was closed and irradiated at 130 °C for 20 min. The obtained crude reaction product was purified by chromatography, eluting with hexane/ethyl acetate to give analytically pure products NDI. NMR and MS spectra can be found in Supporting Information.

4,9-bis((4-(diphenylamino)phenyl)ethynyl)-2,7-dioctyl-1,3,6,8-naphthalenediimide (NDI1): From 4-ethynyl-N,N-diphenynaniline (0.083 g), derivative NDI1 was obtained as a dark purple solid (0.056 g, 35 %), eluting with hexane/ethyl acetate (20/1). M.p.: 169–170 °C. 1H NMR (CDCl3, 500 MHz) (δ, ppm): 8.76 (s, 2H, H-NDI), 7.34 (d, J=8.9 Hz, 4H, o-Ph(A)), 7.30–7.26 (m, 8H, o-Ph(B)), 7.13–7.06 (m, 12H, m, p-Ph(B)), 6.94 (d, J=8.9 Hz, 4H, m-Ph(A)), 4.19 (t, J=7.7 Hz, 4H, N−CH2), 1.78–1.71 (m, 4H, −CH2), 1.45–1.23 (m, 20H, −CH2), 0.88 (t, J=7.1 Hz, 6H, −CH3). 13C NMR (CDCl3, 125 MHz) (δ, ppm): 162.9, 148.6, 146.9, 133.4, 120.9, 129.5, 126.8, 126.7, 125.3, 123.9, 121.5, 114.1, 97.6, 93.0, 41.0, 31.8, 29.3, 29.2, 28.1, 27.1, 22.6, 14.1. MS calculated for C70H64N4O4: 1025.31 g/mol. MS: (MALDI-TOF) m/z: 1026.76 [M]⋅+.

4,9-bis((4-(9H-carbazol-9-yl)phenyl)ethynyl)-2,7-dioctyl-1,3,6,8-naphthalenediimide (NDI2): From 9-(4-ethynylphenyl)-9H-carbazole (0.082 g), NDI2 derivative was obtained (0.060 g, 40 %) as a dark purple solid by chromatography, eluting with hexane/ethyl acetate (20/1). M.p.: 115–116 °C. 1H NMR (CDCl3, 500 MHz) (δ, ppm): 8.76 (s, 2H, H-NDI), 8.13 (d, J=7.7 Hz, 4H, H1), 7.98 (d, J=8.5 Hz, 4H, o-Ph(A)), 7.67 (d, J=8.5 Hz, 4H, m-Ph(A)), 7.51–7.42 (m, 8H, H3, H4), 7.34–7.29 (m, 4H, H2), 4.19 (t, J=7.7 Hz, 4H, N−CH2), 1.78–1.71 (m, 4H, −CH2), 1.45–1.23 (m, 20H, −CH2), 0.88 (t, J=7.1 Hz, 6H, −CH3). 13C NMR (CDCl3, 125 MHz) (δ, ppm): 162.8, 152.1, 140.4, 138.3, 137.2, 133.7, 130.9, 128.7, 127.7, 126.8, 126.7, 126.6, 126.1, 125.5, 123.6, 122.7, 122.0, 120.3, 109.8, 100.0, 97.3, 41.0, 31.8, 30.9, 29.2, 27.9, 26.9, 22.6, 17.5. MS calculated for C70H60N4O4: 1021.27 g/mol. MS: (MALDI-TOF) m/z: 1022.46 [M]⋅+.

General procedure for PDI: A mixture of 1,7-dibromo-N,N′-di[2,6-dioctyl]perylene-3,4,9,10-perylenediimide (0.100 g, 0.130 mmol), the corresponding arylethynyl derivative (0.260 mmol), DBU (0.039 g, 0.260 mmol), CuI (1.2×10−3 g, 6.5×10−3 mmol) and Pd-EncatTM TPP30 (0.012 g, 4.55×10−3 mmol) was charged under argon to a dried microwave vessel. Then, CH3CN was added. After that, the vessel was closed and irradiated at 130 °C for 20 min. The obtained crude reaction product was purified by chromatography, eluting with hexane/ethyl acetate to give analytically pure products PDI. NMR and MS spectra can be found in Supporting Information.

1,7-bis((4-(diphenylamino)phenyl)ethynyl)-2,6-dioctyl-3,4,9,10-perylenediimide (PDI1): From 4-ethynyl-N,N-diphenylaniline (0.070 g), derivative PD1 was obtained as a dark purple viscous oil (0.049 g, 33 %) by chromatography, eluting with hexane/ethyl acetate (20/1). 1H NMR (CDCl3, 500 MHz) (δ, ppm): 9.50 (d, J=8.2 Hz, 2H, H-PDI), 8.93 (s, 2H, H-PDI), 8.72 (d, J=8.2 Hz, H-PDI), 7.49 (d, J=8.8 Hz, 4H, o-Ph(A)), 7.38–7.32 (m, 8H, o-Ph(B)), 7.22–7.16 (m, 12H, m, p-Ph(B)), 7.10 (d, 4H, m-Ph(A)), 4.19 (t, J=7.7 Hz, 4H, N−CH2), 1.78–1.71 (m, 4H, −CH2), 1.45–1.23 (m, 20H, −CH2), 0.88 (t, J=7.1 Hz, 6H, −CH3). 13C NMR (CDCl3, 125 MHz) (δ, ppm): 162.3, 149.3, 146.6, 137.9, 137.1, 133.0, 132.8, 132.7, 129.9, 129.7, 129.4, 129.1, 128.6, 128.4, 126.8, 125.8, 125.7, 124.5, 124.4, 123.1, 122.7, 121.1, 121.0, 120.7, 98.6, 94.1, 40.8, 31.8, 30.9, 28.0, 26.8, 22.6, 17.5, 13.7. MS calculated for C80H68N4O4: 1148.45 g/mol. MS: (MALDI-TOF) m/z: 1149.86 [M]⋅+.

1,7-bis((4-(9H-carbazol-9-yl)phenyl)ethynyl)phenyl)ethynyl)-2,6-dioctyl-3,4,9,10-perylenediimide (PDI2): From 9-(4-ethynylphenyl)-9H-carbazole (0.069 g), PDI2 derivative was obtained as a dark purple solid by chromatography (0.051, 34 %), eluting with hexane/ethyl acetate (20/1). M.p.: 196–198 °C. 1H NMR (CDCl3, 500 MHz) (δ, ppm): 9.50 (d, J=8.1 Hz, 2H, H-PDI), 8.93 (s, 2H, H-PDI), 8.71 (d, J=8.1 Hz, 2H, H-PDI), 8.15 (d, J=7.9 Hz, 4H, H1), 7.79 (d, J=8.3 Hz, 4H, o-Ph(A)), 7.60 (d, J=8.3 Hz, 4H, m-Ph(A)), 7.47–7.40 (m, 8H, H3, H4), 7.32 (t, J=7.2 Hz, 4H, H2), 4.19 (t, J=7.7 Hz, 4H, N−CH2), 1.78–1.71 (m, 4H, −CH2), 1.45–1.23 (m, 20H, −CH2), 0.88 (t, J=7.1 Hz, 6H, −CH3). 13C NMR (CDCl3, 125 MHz) (δ, ppm): 162.4, 140.4, 138.6, 138.0, 134.1, 133.0, 129.3, 128.5, 127.0, 126.8, 126.1, 123.7, 123.2, 122.8, 120.8, 120.4, 99.6, 95.1, 40.8, 31.8, 29.3, 29.2, 28.1, 27.1, 22.6, 14.1. MS calculated for C80H64N4O4: 1144.49 g/mol. MS: (MALDI-TOF) m/z: 1145.25 [M]⋅+.

Structural features

It is well known that geometric structure of semiconductors exerts important influences on their intermolecular interactions, playing a key role in their stacking motifs and optoelectronic performance in solid state.21 Therefore, to gain information on the molecular ground-state geometry, density functional theory (DFT)-based calculations were performed at the B3LYP/6-31G** level of theory. For comparative purposes, the M06-2X functional was also used and the results, which are comparable to those obtained at the B3LYP level, are shown in the Supporting Information. In order to explore the impact of long-range corrections on the optical properties of these systems, the ωB97X-D functional was also used and similar trends were also predicted. In this sense, the results obtained using both theoretical approaches indicate a totally coplanar π-conjugated backbone for the NDI central moiety, while the extension of the π-conjugated core on PDI derivatives lead to the twisting of the two naphthalene halves of the central unit due to steric effects with torsion angles about 17° (Figure 1 and Figures S13–S14). Note that similar distortion effects on the PDI skeleton have been previously reported.21b

Details are in the caption following the image

Top and lateral views of the DFT optimized geometries of NDI and PDI derivatives calculated at the B3LYP/6-31G** level.

On the other hand, the attached peripheral donor groups are largely distorted by ∼46–50° respect to the π-conjugated backbone, independently of the nature of the π-conjugated central platform.

Electronic Properties

The photophysical properties of these compounds were studied by using UV-vis absorption and fluorescence spectroscopies (Figure 2) and rationalized with DFT calculations (Figures S15–S17). Figure 2a shows the recorded absorption spectra of all compounds under study in CH2Cl2 at a concentration of 10−3 M, which result in moderately complicated spectral profiles with a vast number of high energy electronic transitions (with a π-π* nature) and the presence of one broad and low intense band at lower energies, which is basically absent for PDI-2. This is in agreement with previously published results of a perylenediimide substituted at 1,7 bay positions with phenylcarbazole group.22 This lowest energy band (S0→S1) is theoretically ascribed to the HOMO→LUMO one electron excitation. As indicated by the frontier molecular orbital (FMOs) topologies shown in Figure 3 and Figure S18, this electronic transition entails an intramolecular charge transfer (ICT) character, with electron density redistribution from the peripheral donor units (HOMO) to the electron deficient NDI or PDI cores (LUMO). Note however, that NDI-1 and PDI-1 show slightly more intense ICT absorption bands than their phenylcarbazole-based homologous, suggesting a stronger donor-acceptor interaction for these derivatives.

Details are in the caption following the image

(a) Normalized UV-vis absorption and (b) emission spectra of NDI and PDI derivatives.

Details are in the caption following the image

DFT-calculated FMOs energies and topologies for all the compounds under study at the B3LYP/6-31G** level of theory. H-1, H, L and L+1 refer to HOMO-1, HOMO, LUMO and LUMO+1, respectively.

This fact is in agreement with the observed solvatochromic behaviour, where solvent polarity has a higher influence in the photophysical properties of the triphenylamine substituted NDI-1 and PDI-1 (see Figure S22 and Table S1), confirming the more accentuated ICT character of the low energy absorption band for the compounds substituted with the strong triphenylamine donor moiety. Note that the solvent-dependent behavior is more notorious for the fluorescence spectra, with emission bands shifting bathochromically as the solvent polarity is increased from hexane (406 and 534 nm for NDI-1 and PDI-1, respectively) to dimethylformamide (448 and 549 nm for NDI-1 and PDI-1, respectively). This behavior suggests a more polarized first excited S1 state when compared to the S0 ground state, in line with the higher DFT-calculated molecular dipole moment in the first excited state when compared to the ground state (see Table S2).

Interestingly, thermally accessible rotational energy barriers (smaller than 3 Kcal/mol) between the lateral groups and the central naphthalene core of NDI-1 and NDI-2 have been theoretically predicted, which suggest that these compounds possess several rotamers at room temperature (see Figure S19). In this context, a notable reduction of the oscillator strength corresponding to the lowest energy band has been observed by progressively increasing the intramolecular distortion, as a consequence of an inefficient donor-acceptor intramolecular interaction with a low wavefunction overlap between HOMO and LUMO orbitals.

Furthermore, a comparison between NDI and PDI derivatives shows a bathochromic shift of the absorption spectra, going from 572 nm in NDI-1 to 640 nm in PDI-1, in line with the longer extension of the π-conjugated core in PDI-based compounds. This decrease in the HOMO-LUMO gap, found when comparing NDI-1 vs. PDI-1, and NDI-2 vs. PDI-2, is nicely reproduced by DFT-calculations, as shown in Figure 3 and Figure S18. This can be ascribed to a stabilization of the LUMO orbital after the elongation of the arylene unit, with minimal change of the HOMO orbital energy. On the contrary, the lateral substitution with electro-active groups has a significant impact on both HOMO and LUMO energy levels, which is similar in both NDI and PDI systems. In particular, the change of the lateral substituents from triphenylamine to phenylcarbazole groups provokes a noticeable stabilization of both HOMO and LUMO energy levels, which is slightly more pronounced in the latter. This result highlights the more electron-donating character of the triphenylamine group.

Concerning the photoluminescence properties (Figure 2b), practically superimposed emission spectra for the PDI-based semiconductors are registered, independently of the lateral donor substituents, upon excitation at 500 nm, which suggests that the emission comes primarily from the perylenediimide core with a null contribution of the lateral groups. This is in line with the highly emissive nature of extended aromatic arylene diimides cores in solution.23 In marked contrast, remarkable changes on the emission spectra for the NDI-based compounds were obtained, showing the key role that the peripheral donor groups play on the photoluminescence properties. In fact, NDI derivatives, with more limited aromatic cores, are found to be less emissive either in solution or in the solid state. Strategies to enhance fluorescence emission in solution include the lateral substitution with electron donating groups, as in our approach.13a In addition, intermolecular interactions in less emissive NDIs are able to enhance their emissions, which is attributed to the formation of aggregation excimers.24 In this sense, changes similar to the ones registered in our compounds have been previously ascribed to different aggregates formation due to lateral substitution with electron-donating groups.25

Thereby, the photophysical properties of these compounds were evaluated as a function of temperature and concentration in order to elucidate their tendency to aggregate (Figures S23 and S24, Supporting Information). On the basis of the obtained results, the presence of similar UV-Vis absorption spectral profiles and practically superimposed fluorescence spectra for the PDI derivatives on increasing concentration suggest no aggregation at the concentration range we work. Likewise, although PDI-based compounds showed blue-shifted and weakened fine-structures in the absorption spectra with increasing temperature due to the increased conformational freedom, no aggregation effects were neither seen. However, that is not the case for the NDI derivatives, where discernible variations were observed on their photophysical properties by increasing concentrations and temperatures (specially for NDI-2). In addition, similar UV-Vis absorption spectral profiles were recorded for NDI-based compounds in solution and as thin-film, pointing to a great tendency to form molecular aggregates (Figure S25, Supporting Information). This behaviour also becomes evident by the comparison of the UV-vis absorption and emission spectra recorded in presence of 30 μL of trifluoroacetic acid (TFA) in 10 mL of 10−3 M solutions of NDI and PDI derivatives (Figure S26, Supporting Information), which is widely used to avoid aggregation effects.26 In this context, the remarkable spectral changes observed upon addition of TFA might be ascribed to the breakdown of the aggregates. The possibility of aggregation in this kind of systems might explain the non-conventional photophysical behaviour observed in some of them (i. e. the dual emission recorded for NDI-2), which in fact affects their photophysical properties.27

To get more insight into the ability for π-dimer formation and π-π aggregates, we carried out DFT calculations for NDI-1 and NDI-2 dimer systems. Since PDI derivatives have a limited ring planarization and low tendency to π-stacking, these systems were not taken further into consideration. The negative free energy of formation values reveal the favourable formation of π-π aggregates in both NDI-1 and NDI-2 systems (Figure S27, Supporting Information). Curiously, π-dimers of NDI-2 are found to be 10 kcal/mol more stable than those of NDI-1, further probing the higher tendency to form molecular aggregates in the former.

Spectroelectrochemical studies: charged species characterization

In order to analyze charged species, Figure 4 shows the evolution of the UV-Vis absorption spectra obtained by the progressive spectroelectrochemical reduction of NDI-1, NDI-2, PDI-1 and PDI-2 within an Optically Transparent Thin Layer Electrochemical (OTTLE) cell in dichloromethane solution with the presence of 0.1 M (n-Bu)4NPF6 supporting electrolyte (see Supporting Information for more details). Focusing on the case of NDI-1 (Figure 4a), when the potential increases in the negative direction the initial UV-Vis absorption spectrum corresponding to the neutral specie (black curve) progressively evolves to a series of absorption bands recorded at 478, 612, 674 and 747 nm (red curve), which are ascribed to the one electron injection on the naphthalenediimide fragment and the sequent formation of radical anion species. Further electrochemical reduction provokes the vanishing of this spectral profile and the appearance of two new absorption peaks centred at 548 and 595 nm, corresponding to the second electron injection on the naphthalenediimide fragment and the formation of dianion species (blue curve). Clear isosbestic points can be observed. These spectral changes are supported by quantum chemical calculations at TD-DFT level (Figure S28, Supporting Information). As we expected, considering the LUMO topology shown in Figure 3, the injected negative charge is mainly stabilized over the naphthalenediimide fragment. DFT calculations of NDI-1 charges species predict the accommodation of around an 80 % of each injected charge (−0.78 e and −1.58 e for the radical anion and dianion states, respectively) by the naphthalenediimide unit (Figure S29, Supporting Information). A similar scenario has been previously reported for naphthalenediimide derivatives.13d, 28 The same behavior is also observed for NDI-2, with practically superimposed spectral profiles for the charge species with respect to NDI-1, what is in agreement with the stabilization of the negative charges by the central NDI core. This fact also supports the ability of the naphthalenediimide central core to accommodate negative charges, independently of the side groups attached to it. On the other hand, the extension of the π-conjugation in PDI-1 allows to accommodate a third negative charge on the perylenediimide unit and thus, the formation of trianion species (Figures 4c and 4d). In this case, DFT calculations of PDI-1 charges species predict the accommodation of around an 70 % of each injected charge (−0.68 e, −1.54 e and −1.91 e for the radical anion, dianion and trianion states, respectively) by the perylenediimide unit (Figure S30). This percentage is slightly lower than the predicted for the NDI molecules.

Details are in the caption following the image

In situ UV-Vis spectral changes upon electrochemical reduction of NDI-1 (a), NDI-2 (b), PDI-1 (c) and PDI-2 (d). The black, red, blue and orange curves represent the absorption bands for the neutral, radical anion, dianion and trianion state, respectively.

In line with this, an increment of negative charge localization on the ethynyl group of PDI-1, in comparison with that of NDI-1 in their radical cation (c.a. 20 %) and dication (c.a. 38 %) states, is predicted. Therefore, while the stabilization of the negative charges basically lies on the naphthalimide unit in NDI systems, in PDI molecules the lateral ethynyl groups play a role in charge stabilization. This fact may also suggest a higher electronic communication between the electron-withdrawing PDI unit and the lateral donor groups with respect to NDI compounds, probably due to the twisting of the two naphthalene half units of PDI-based derivatives. Furthermore, lateral substitution, with electron donor groups of different strengths, shows a basically negligible impact on the ability of these systems to accommodate negative charges.

The evolution of the UV-Vis absorption spectra upon progressive spectroelectrochemical oxidation of NDI-1 and PDI-1 was also registered and is shown in Figure S31. In this case, only the formation of radical cation species is observed for both molecules. On the contrary, NDI-2 and PDI-2, with less electron-donating phenyl carbazole lateral groups, do not stabilize any oxidized species.

Electrical characterization and charge transport parameters

To characterize the charge transport properties of the NDI and PDI-based semiconductors, bottom-gate top-contact OFETs were fabricated. The transistors performance was fully optimized by systematically investigating the effect of the substrate treatment and annealing conditions on the charge transport characteristics of these semiconductors (see Supporting Information for the exact OFETs fabrication details). As shown in Figure 5, all devices exhibited typical unipolar n-channel characteristics with excellent output and transfer curves measured under vacuum conditions. The main device performance parameters: electron charge carrier mobilities (μe), intensities ratios (Ion/Ioff) and threshold voltages (VT), were extracted from the saturated region in transfer curves and summarized in Table 1 and Table S3–S6 Supporting Information.

Details are in the caption following the image

Typical thin-film transistors (TFT) transfer (left) and output (right) characteristics of NDI-1 (a), NDI-2 (b), PDI-1 (c) and PDI-2 (d) semiconductors. The transfer characteristics were measured at a source-drain voltage (VDS) of 80 V. The output curves were measured at gate voltages (VG) from 0 to 80 V in intervals of 10 V.

Table 1. OFET electrical data for NDI and PDI-based semiconductors measured under vacuum. Average and the best (in parenthesis) values are shown. The average values were obtained from at least 6 devices for each material.
image

The highest electron mobility was 0.3 cm2 V−1 s−1, registered for NDI-1 semiconductor, which is 3-fold higher than that of devices based on NDI-2 (0.1 cm2 V−1 s−1). In addition, the electrical performances of the NDI-based semiconductors are substantially higher than those of the PDI-based materials (1.5×10−3 and 4.0×10−3 cm2 V−1 s−1 for PDI-1 and PDI-2, respectively) which can be ascribed to the more twisted molecular structure of perylenediimide scaffold, inefficient overlap of LUMO orbitals and thus lower electronic coupling, which is generally detrimental for efficient charge transport in OFETs. Thermal annealing of the thin films results in a decrease in OFET performances, with mobility values reaching, for devices fabricated on OTS-treated Si/SiO2 substrates, 0.1, 4.9×10−3 and 2.4×10−4 cm2 V−1 s−1 for NDI-1, PDI-1 and PDI-2, respectively, after a thermal treatment at 120 °C for 2 h (Tables S2–S5). Note that the electrical performance of NDI-2 devices is lost after the thermal treatment.

The comparison of the electrical performances of semiconductors with varying lateral groups shows that the substitution with bulky electron-rich triphenylamine groups renders devices with mobilities more than 3-fold higher than those substituted with phenylcarbazole groups. This result is surprising considering that: (i) the more electron-rich group (i. e. triphenylamine) destabilizes the LUMO energy level, increasing the energetic barrier with gold electrodes Fermi level. (ii) triphenylamine is bulkier than phenylcarbazole, which could be detrimental for molecular packing. However, spectroelectrochemical analysis of charged species indicate that the inclusion of electron-rich groups is not an obstacle to electron injection and stabilization.

Thin-film morphology characterization

In order to better understand the remarkable difference in electron mobility between NDI and PDI-based OFETs, thin film molecular crystallinity and morphology have been measured with grazing incidence X-ray diffraction (GIXRD) and atomic force microscopy (AFM). The GIXRD patters of the semiconducting films prepared under the optimal device fabrication conditions are shown in Figure 6. Moderate crystalline films for all of the semiconductors were observed, although the triphenylamine-substituted semiconductors show slightly sharper diffraction peaks. An intense characteristic (100) reflection at 2θ=4.04° in NDI-1, NDI-2 and PDI-1 and at 2θ=3.78° in PDI-2, which correspond with an interlayer spacing of d100=21.85 Å and d100=23.36 Å respectively, suggest that all the semiconductors under study self-organize into a lamellar crystalline structure. We note that the lamellar packing distance d100 is almost identical than expected from the C8 alkyl chain length, and thus, we conclude that there is an almost total interdigitation of these alkyl chains. Similar trends have been observed for NDI-based compounds.14a, 29 As displayed in Figure 7, AFM height images of NDI-1 and NDI-2 show morphologies that are different to those of the perylenediimide-based samples.

Details are in the caption following the image

GIXRD patterns of NDI-1, NDI-2, PDI-1 and PDI-2 thin films prepared under the optimal device fabrication conditions.

Details are in the caption following the image

Tapping-mode AFM images of height (left) and phase (right) in 5×5 μm2 scan size of NDI-1 (a), NDI-2 (b), PDI-1 (c) and PDI-2 (d) thin films prepared under the optimal device fabrication conditions. The root-mean-square roughness (Rq) values are also shown.

In this sense, larger crystalline domains on the submicron scale can be observed on the surface of NDI-1 and NDI-2 thin films, while PDI-1 and PDI-2 thin films exhibit smoother textures accompanied by smaller grain sizes. These differences in morphology indicate that NDI-1 and NDI-2 samples exhibit stronger aggregation and nucleation compared to their PDI-based counterpart. Note that a more pronounced contrast appears to outline the contours of the crystalline grains in the NDI-1 phase image, while a more homogenous image can be observed for the rest of samples. It also agrees with the largest grain size and much higher rougher surface values of root-mean-square (Rq) roughness found for NDI-1 (Rq=9.3 nm) in comparison with the rest of semiconductors (Rq values of 1.4–3.3 nm). Taken all these facts into account, it seems reasonable to conclude that the larger crystalline domains found in NDI based thin films may significantly contribute to the higher carrier mobilities of NDI-1 and NDI-2 materials in OFETs.

Conclusions

We have synthesized and characterized a new series of donor-π-acceptor-π-donor (D-π-A-π-D) compounds based on naphthalendiimide (NDI) and perylenediimide (PDI) cores, laterally substituted with triphenylamine and phenylcarbazole donor groups through an ethynyl bond. The syntheses were performed following the principles of Sustainable Chemistry, employing microwave irradiation as energy source and a minimum amount of solvent. The physical chemistry study indicates that while the extension of the arylene fragment in PDI derivatives provokes a downshift of the energy gap, the loss of the π-conjugated skeleton planarity acts in detrimental to the materials charge transport properties. Even so, electrochemical results show that negative charge stabilization is enhanced in PDI molecules, due to the role of the ethynyl linkers in charge stabilization, probably due to the twisting of the two naphthalene half units of PDI-based derivatives. On the contrary, the extension of the arylene fragment does not have a significant effect of hole injection and stabilization, being the lateral electron-donating groups the ones dictating the ability to stabilize positive charges in the studied molecules. Both NDI and PDI derivatives were tested in top-contact/bottom gate organic field-effect transistors (OFETs), showing good performance as n-type semiconductors, with mobilities up to 0.3 cm2 V−1 s−1 in the case of NDI-1. The semiconductors characterization, including DFT calculations, AFM, X-ray diffraction and spectroelectrochemistry, revealed that adequate molecular characteristics and larger crystalline domains in NDI vs. PDI semiconducting films explain the enhanced electrical properties of NDI derivatives. On the other hand, lateral substitution with the more electron-donating triphenylamine group resulted in more efficient charge transport characteristics.

Experimental Section

The experimental and theoretical methodology are available in the Supporting Information.

Supporting Information

In this section, we can find information about synthetic details, 1H NMR and 13C NMR spectra of all the compounds under study, DFT-Calculations performed at the B3LYP/6-31G**, M06-2X/6-31G** and ωB97X-D/6-31G** levels of theory, solvatochromic effects, spectroelectrochemical oxidations of NDI-1 and PDI-1, transistors performance optimization and morphologic characterization. Additional references cited within the Supporting Information.30, 31, 32, 33, 34, 35-38

Acknowledgments

This research was funded by JCCM-FEDER (project SBPLY/21/180501/000114) and MICINN (project PID2020-119636GB-I00) at University of Castilla-La Mancha and by Junta de Andalucía (project P18-FR-4559) and MICINN (project PID2019-110305GB-I00) at University of Málaga. I. Torres-Moya is indebted to the Junta de Comunidades de Castilla-La Mancha for a post-doctoral grant (SBPLY/19/180501/000346) and Juan de la Cierva Formación 2020 FJC2020-043684-I of the assistance financed by MCIN/AEI/10.13039/501100011033 and for the Unión Europea NextGeneration EU/PRTR. S. G.-V. thanks the MINECO for a FPU predoctoral fellowship (FPU17/04908) and B.D for FPU fellowship (FPU16/05099). The authors would like to thank Pablo Fernández for his assistance in the purification of the products and the work lab. Computer resources, technical expertise and assistance provided by High Performance Computing Service of the University of Castilla-La Mancha and the SCBI (Supercomputing and Bioinformatics) centre of the University of Málaga are gratefully acknowledged. We thank the Vibrational spectroscopy (EVI), XRD and AFM labs of the Research Central Services (SCAI) of the University of Málaga. Funding for open access charge is acknowledged to University of Malaga/CBUA.

    Conflict of interest

    The authors declare no conflict of interest.

    Data Availability Statement

    The data that support the findings of this study are available from the corresponding author upon reasonable request.