Fused alkyne molecules are important in organic semiconductors due to their desirable properties. Here, we report the design and synthesis of a new series of A–π–D molecules (III–VII) that can serve as mild electron acceptors to generate wide-bandgap p-type small compounds for use in organic field-effect transistors. The incorporation of donor units into fused isophorone frameworks can be used to tune the frontier molecular orbital energies. The electrochemical, optical, and thermal properties of the compounds were characterized. Compound VI, which has a fused phenyl-substituted alkyne moiety, had the highest occupied molecular orbital energy level as determined by optical and electrochemical analysis. Density functional theory calculations revealed that compounds VI and III had lower hole reorganization energy (λh) than the corresponding isophorone extended conjugated-based compounds (I–II). Conversely, compounds I and II had lower electron reorganization energy (λe) than the corresponding fused alkyne compounds. This is in line with the observed adiabatic ionization potential and electron affinity values. Consequently, devices fabricated with compound VI exhibited high mobility and low threshold voltage.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
1 Introduction
Organic semiconductors have garnered increased interest due to their unique properties, such as charge transport, light emission, photovoltaic capability, and thermoelectricity [1‐4]. These materials have the potential to replace silicon technology in low-cost, large-area, and flexible displays or sensors [5‐7]. In recent decades, extensive research has been focused on small π-conjugated optoelectronic materials as an alternative to non-organic optoelectronics, with promising applications such as organic photovoltaics, organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), flexible displays, and sensors [8, 9]. Among these, highly stretchable, lightweight, and low-power consumption organic field-effect transistors (OFETs) are particularly suited for use as sensors, artificial skins, drivers, transducers, and amplifiers [10, 11].
In organic compounds, two-electron affinities are observed: acceptors (A), which feature electron-withdrawing groups, and donors (D), which contain electron-donating groups. Some compounds exhibit both properties in different parts of the molecule. Alkyne derivatives are interesting candidates for use in various fields of organic electronics, including solar cells (especially dye-sensitized solar cell applications) [10‐12], OLEDs [13‐15], and OFETs [16‐18], whether used in linkage or at the terminal position. Prieto and co-workers [19] investigated alkyne derivatives of active semiconducting units, such as carbazole, benzothiophene, and N,N-diphenyl amino, for optical waveguides and OFETs. Their work demonstrated that alkyne derivatives possess ambipolar transport with electron and hole field-effect mobilities.
Anzeige
Isophorone skeleton plays a role as an important unit of various donor, π-conjugation and acceptor type analogues. Especially, isophorone-based dyes have garnered much attention from researchers due to their promising application in the field of optoelectronics. Structures containing isophorone as a π-bridge unit were synthesized by Chen et al., and their photophysical and electrochemical properties were examined, as well as their potential in dye-sensitive solar cell (DSSC) design [20]. They reported the conversion efficiency of the device fabricated for the structure with the phenothiazine donor moiety as 6.18%. Liu et al. designed a cell device in which an isophorone unit was selected as the p-cyclohexene bridge [21]. The researchers reported that results in an overall conversion efficiency of 7.41% on thin TiO2 film for new isophorone dye. Oh et al. synthesized a D–π–A type dye containing an isophorone moiety as an acceptor and observed changes in electrical resistance values depending on temperature due to its semiconductor properties of the compound. They also stated that it has a high potential to be applied in chemosensors with its sensitivity to other stimuli like anions and volatile organic compounds. In another paper, Oh et al. synthesized a D–π–A type dye containing an isophorone moiety as an acceptor and observed changes in electrical resistance values depending on temperature due to its semiconductor properties of the compound [22] another remarkable study. They emphasized that it has a high potential to be applied in chemosensors with its sensitivity to other stimuli like anions and volatile organic compounds.
In a similar study, our group used extended conjugated enone-based optoelectronic compounds in solar cells, achieving a moderate efficiency [23].
Herein, we reported five new simple A–π–D structured alkyne molecules (III–IV–V–VI–VII) with different terminal units as electron-donating groups. In this study, the derivatives have been synthesized to comprehend the structure–reactivity relationship of alkynyl groups in the design of organic field-effect transistors (OFETs), contingent upon the chosen substituents. Specifically, a hexyl group has been selected to observe the impact induced by inductive effects, and also phenyl group has been chosen due to its role in extended conjugation and its influence on the changes in transition energies in electron–hole transport. For comparison, positions with no substituents, containing only the alkyne group, and a silicon-substituted state have been planned. Furthermore, Compound VII has been synthesized to investigate the effects arising from the differences in energy levels associated with heteroatoms [24, 25]. We fabricated organic field-effect transistors (OFETs) using all-solution-processed thin films of title compounds. To investigate the semiconductor functionality, we described the synthesis, spectral characterization, and OFET properties of spin-coated films of alkyne derivatives of the enone system with a bottom-contact/top-gate device configuration. The semiconductor functionality was analyzed atomic force microscopy (AFM), ultraviolet–visible (UV–vis), fluorescence, and cyclic voltammetry (CV). The structural information was obtained via differential scanning calorimetry (DSC), thermogravimetric analysis-differential thermal analysis (TG–DTA), nuclear magnetic resonance (NMR), and mass spectrometry (MS) analysis. The frontier molecular orbital (FMO) energy levels, the values of adiabatic/vertical ionization potentials (AIP/VIP), electronic affinity (EA), reorganization energy parameters (λ), and non-linear optical (NLO) properties of core compounds (I and II) and alkyne series compounds (III–VII) were calculated based on density functional theory (DFT) calculations.
2 Materials and methods
2.1 Synthetic procedure and characterization data
2.1.1 General procedure of aldol reaction
The synthesis method, which our group has employed in previous studies, is a widely recognized technique in the literature [26]. To a solution of ethanol (50 mL) containing isophorone (7.24 mmol, 1 g) and 2-hydroxybenzaldeheyde/4-bromobenzaldehyde (7.93 mmol) under reflux was added dropwise solution of sodium hydroxide (10%, 10 mL) over 12 h in a nitrogen atmosphere. After cooling the reaction mixture was poured into water (200 mL) and added diluted asiaticoside to precipitate the crude product. Filtered crude product was purified by a crystallization of ethanol to afford the adduct I and II 78%, 65%, respectively.
2.1.2 General procedure of Sonogashira cross-coupling reaction
This procedure applies to compounds III, V, VI. To a flame dried and then oven dried resalable Schlenk tube was used. To an nitrogen flushed solution of I (1,24 mmol) in THF:triethylamine (1:1; 30 mL) were added trimethysilylacetylene/1-hexyn/phenylacetylene (3.1 mmol), Bis(triphenylphosphine) palladium(II) dichloride- PdCl2(PPh3)2 (0.062 mmol), Copper(I) iodide-CuI (0,124 mmol). Following by thin layer chromatography analysis, upon judged completion was filtered and purified by column chromatography.
2.1.3 Cleavage of TMS in compound III for obtained IV
Under nitrogen atmosphere, an oven dried roundbottom of flask were charged compound III (1.50 mmol) solution in newly dryed THF (20 mL) was added dried K2CO3 (5.5 mmol). The mixture was stirred 1 h, upon finished reaction was filtered and evaporated and the reaction mixture was purified column chromatography to afford desired product.
2.1.4 Synthesis method of the compound VII
To a stirred solution of compound II (1,1 mmol) in 10 mL dry-DMF were added freshly dried K2CO3 (1.8 mmol) After 10 min stirring at room temperature, the solution of the propargile bromide (2.5 mmol) in 5 mL DMF dropwisely. Upon completion reaction was extracted of brine: ethyle acetate mixture and collected organic phase dried MgSO4 and then evaporated, before purified via column chromatography.
2.2 Theoretical procedure
All theoretical analyses based on DFT for isophorone derivative structures were carried out with the Gaussian 09 electronic structure [27] and GaussView 5.0.9 graphical interface [28] programmes, employing B3LYP functional and 6–311 + + G(d,p) basis set. The energy values of the FMOs, namely the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and their electron density distributions were executed through optimized structures with time-dependent (TD-DFT) method in the gas phase. The reorganization energy (λ) parameters associated with the charge transport process in organic optoelectronic materials were evaluated by the adiabatic potential energy surface method (four-point approximation) using the equations given below [29];
where \({{\text{E}},\mathrm{ E}}^{\pm }\) represents neutral, cation and anion species in their neutral energy geometries; \({{{\text{E}}}^{*},\mathrm{ E}}^{{\pm }^{*}}\) are the neutral, cation and anion species in their ionic energy geometries, respectively. The formation energy of the adiabatic ionization potential (AIP), vertical ionization potential (VIP), and adiabatic electronic affinity (EA) are defined as [30]:
The NLO material behavior of the compounds was calculated based on parameters the mean isotropic polarizability ⟨α⟩ and first-order hyperpolarizability (βtot), in terms of x, y, z components, using the equations given following [31].
The alkyne chromophores of isophorone-based compounds films were prepared by spin coating compounds III, VI and VII solution in chloroform (2 mg/mL) on cleaned interdigitated Au substrates covered by that materials. To evaporate the remaining solution, the samples were dried and annealed at 110 °C for 10 min. The 100 nm thick top electrode was obtained by evaporating Ag onto the films covered by the bottom electrodes, forming conductive channels of 100 µm length and 30 mm width. (Fig. 1). The voltage was varied from the 0.0 to 100 V, resulting in an electric current of maximum 16 µA.
Fig. 1
Schematic OFET structure
×
3 Results and discussion
3.1 Synthesis
The core unit was generated by an aldol reaction between isophorone and 4-bromobenzaldehyde (I) and 2-hydroxybenzaldehyde (II). To improve the conjugation of the core, we started with the reaction of (E)-3-(4-bromostyryl)-5,5-dimethylcyclohex-2-en-1-one (I) and ethynyltrimethylsilane (III), 1-hexyne (V), and phenylethene (VI). As shown in Fig. 2, they were obtained via palladium-catalyzed Sonogashira coupling reaction and were characterized by 1H NMR, 13C NMR, 2D NMR, MS, and elemental analysis.
Fig. 2
Synthesis of compounds I–VII
×
The cleavage experiment of compound (III) was performed at K2CO3 in methanol and obtained compound IV in 75% yield. We began synthesis of (E)-5,5-dimethyl-3-(2-(prop-2-yn-1-yloxy)styryl)cyclohex-2-en-1-one with investigation of the etherification reaction with 3-bromoprop-1-yne. Another synthesis pathway of compound VII started with the reaction of bromoprop-1-yne with 2-hydroxybenzaldehyde (II). After purification of the etherification product, the second step was a generalized aldol reaction with isophorone. The novel compounds I–VII were synthesized facilely with a yield over 65%. The general and detailed synthetic methods are provided in the Supplementary File.
3.2 Optical and electrochemical properties
The UV–Vis absorption spectra for isophorone-based small molecules (I–VII), in case of solution are shown in Fig. 3. The absorption spectra for compounds I–VII in solution state were analysed UV–Vis spectroscopy using dimethylformamide (DMF) as the solvent. The maxima of the absorption peaks show π–π* electron absorption bands, which are red-shifted with conjugation from 325 to 352 nm. Additionally, groups attached to the cyclohexenone systems demonstrated a bathochromic effect in the range of 25–40 nm when achieving conjugation. We observed similar outcomes in the structures synthesized in our study [32‐34]. The main absorption peak of compound VI shows a 27 nm bathochromic shift compared to compound I and this peak shows vibronic shoulder (supplementary file), suggesting that compound VI is planar, induced by extended conjugation.
Fig. 3
a UV–Vis absorption spectra, b excitation spectra, c emission spectra of 10−5 M solutions of compounds I, II, III, V, VI, and VII in DMF
×
Lipper-Mataga solvatochromic method is based on rearranging vectorial dipole moments of the solvents depending on the behaviour of compounds in used solvents. In Fig. 4, Stokes’ shifts (Δν) were plotted against the orientation polarizability (Δƒ) values for compounds V–VI in the various solvents of n-hexane, n-heptane, 1,4-dioxane, chloroform, tetrahydrofuran, dichloromethane (DCM), dimethylsulphoxide (DMSO), dimethylformamide (DMF), acetone, ethanol and acetonitrile.
Fig. 4
Plots of Stokes shifts as a dependence of the solvent polarity for V (blue) and VI (red) in selected solvents as (1) n-Hexane, (2) n-Heptane, (3) 1,4-Dioxane, (4) Chloroform, (5) Tetrahydrofuran, (6) DCM, (7) DMSO, (8) DMF, (9) Acetone, (10) Ethanol and (11) Acetonitrile (Color figure online)
×
The stronger Stoke shift was observed in compound VI than V, due to the presence of extended conjugated units between alkyne and isophorone units. The findings obtained from Lippert–Mataga analysis in different solvent polarity implies on polar solvents especially DMF the higher charge transfer character is observed.
The electrochemical properties of all compounds were explored using cyclic voltammetry (CV) and HOMO–LUMO energy level calculated oxidation and reduction potential peaks as based on potential referenced to ferrocene. The HOMO energy level of the I–VII compounds dissolved in DMF (included 0.1 M TBAB) was measured from − 5.22 to − 5.62 eV and LUMO level between − 1.60 to − 2.58 with CV, and detailed data are summarized in Table 1.
Table 1
Summary of redox potential and calculation results of HOMO–LUMO via cyclic voltammetry
Compounds
Eox
(V)
Ered
(V)
HOMO (eV)
LUMO
(eV)
Egap
(eV)
I
1.08
− 2.81
− 5.48
− 1.60
3.88
II
1.04
− 2.78
− 5.44
− 1.62
3.82
III
0.98
− 2.32
− 5.38
− 2.08
3.30
IV
1.12
− 1.82
− 5.52
− 2.58
2.94
V
1.22
− 2.09
− 5.62
− 2.31
3.31
VI
0.82
− 1.87
− 5.22
− 2.53
2.69
VII
1.01
− 2.08
− 5.41
− 2.32
3.09
3.3 Thermal properties
The thermal stability and robustness of the organic semiconducting compounds were evaluated by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) under nitrogen atmosphere at heating rate 10 °C. The significance of the thermal properties of the title compounds for OFET devices is underlined by considerations related to OFET device performance and the thin film functionality during the device fabrication process. The annealing duration and temperature employed in the creation of the device’s thin film play a pivotal role in achieving optimal performance [35‐37]. Thermal decomposition temperature (Td) with 10% of weight loss of compounds I–VII were observed to be 225.58, 257.90, 237.79, 230.43, 307.15, and 235.98 °C, correspondingly these compounds estimated to appreciable for using as OFET materials. DSC analysis of compounds I and V revealed melting points as sharp endotherms at 123.81 and 123.29 °C, while exotherms as crystallization points were detected 106.91 and 106.29 °C as shown in Fig. 5.
Fig. 5
Thermogravimetric analysis (TG) of compounds I–VII
×
3.4 DFT-based analysis
The energy levels of FMOs provide predictions that can be considered as a reference for the efficiency of device designs with organic semiconductor components. Calculated energy values of HOMO, LUMO and band gap (ΔE) with the DFT approach for compounds I–VII are presented in Table 2. Distributions of electron density in HOMO and LUMO are given in Fig. S.1.
Table 2
Calculated energy values of FMOs, band gap (ΔE), and mean polarizability (< α >), first-order hyperpolarizability (βtot) values for compounds
Compounds
HOMO
(eV)
LUMO
(eV)
ΔE
(eV)
⟨α⟩
(× 10–24 esu)
βtot
(× 10–36 esu)
I
− 5.97
− 2.22
3.75
45.33
92.02
II
− 5.86
− 2.04
3.82
41.56
70.15
III
− 5.75
− 2.33
3.42
61.95
115.05
IV
− 5.57
− 2.11
3.46
30.14
20.15
V
− 5.75
− 2.25
3.50
59.06
160.69
VI
− 5.65
− 2.35
3.30
70.48
228.29
VII
− 5.78
− 2.06
3.72
46.94
66.88
As can be seen in Table 2, decreasing order of energies for HOMO and LUMO is IV > VI > III > V > VII > II > I and II > VII > IV > I > V > III > VI, respectively. Lower HOMO energy levels were obtained for core unit compounds of A–π–D system, and with the presence of donor substituents, HOMO energy levels tended to increase and hole transport capabilities were determined to be improved. The substituent effect was also observed in the variation of LUMO values, with a lower energy value obtained for compound VI containing a phenyl-substituted alkyne moiety. The calculated ΔE values for the compounds ranged from 3.30 to 3.82 eV, and they were determined in decreasing order as II > I > VII > V > IV > III > VI. The theoretical values indicate that compound VI has a narrower band gap among the designed structures, which is consistent with the results from CV measurements.
To gain a deeper understanding of the optoelectronic characteristics of compounds I–VII, reorganization energy (λ), ionization potential (IP) and electron affinity (EA) parameters were calculated with the DFT approach using Eqs. (1–4). The obtained results are tabulated in Table 3. Effective charge carrier mobility is correlated to a low value of reorganization energy for both electrons and holes. Conversely, a higher reorganization energy is indicative of reduced charge carrier mobility [38]. The calculated values of λe and λh are in the following order II < I < V < VII < IV < VI < III and VI < III < VII < V < IV < II < I, respectively. The calculated results indicate that alkyne attached type derivatization is a tool for increasing the mobility of hole and improving hole transfer capabilities. The calculated values of the hole transport energy are lower, as expected, due to the extended π-conjugation of the phenyl ring, similar to compounds with a comparable structure [39, 40]. The comparison of compounds IV and V reveals that the enriched alkyl chain in compound V has augmented the donor effect, resulting in a consequent reduction in hole transport energy [41, 42].
Table 3
Calculated values of for reorganization energy (λh; hole, λe; electron), vertical ionization potential (VIP), adiabatic ionization potential (AIP), electron affinity (EA), (all units in eV)
Compounds
λh
λe
VIP
AIP
EA
I
9.94
0.76
9.94
7.51
0.76
II
9.73
0.37
10.10
7.63
2.21
III
3.78
7.49
11.01
4.87
7.43
IV
7.62
4.37
10.88
8.04
4.51
V
6.75
3.50
6.75
9.49
3.50
VI
3.54
7.12
11.59
4.26
7.49
VII
4.65
4.04
7.40
6.17
5.50
IP and EA refer to the energy required to remove an electron from the system and the energy released upon the addition of an electron to the system, respectively. The performance of device designs consisting of organic semiconductor materials depends on the efficient injection of holes and electrons. IP is expected to be low enough to allow efficient hole injection, while EA is expected to be high enough to allow efficient electron injection [43, 44]. It can be stated that compounds III and VI are more suitable for hole injection into the HOMO of the donor with lower AIP values and for electron injection into the LUMO of the acceptor with higher EA values.
Organic functional materials combined with acceptor (A) and donor (D) groups connected by effective π-conjugation exhibit intramolecular charge transfer (ICT) ability due to the asymmetric polarization created by these moieties. Materials with effective ICT capabilities are also promising for NLO applications. The NLO behaviours of compounds with A–π–D type isophorone unit acceptor and alkyne substitute donor containing were evaluated with parameters of mean isotropic polarizability ⟨α⟩ and first-order hyperpolarizability (βtot) and computed values are given in Table 2. It has been determined that compound VI, with its higher ⟨α⟩ and (βtot) values, has the potential to be a better NLO material compared to others. It is noteworthy that nonlinear optical properties (NLO) are considered polarization induced in the material under an intense electric field. Although previous studies on the NLO properties and their first-order hyperpolarizability values of relatively small-size alkyne substituted organic systems show that they are found to be higher as compared to the results [45]. Between the core and substituted alkyne compounds was relatively higher first-order hyperpolarizability values compared to that of the oxazole as similar to our work.
3.5 Thin film AFM analysis
The results of AFM study showed that active layer has good morphology after annealing of the samples. Films that obtained from compounds III, VI and VII exhibit good morphological feature (Fig. 6). The compound III, VI and VII films in active layer exhibited root mean square (RMS) roughness value of 0.6 nm, 1.1 nm, 1.3 nm after annealing (at 100 °C).
Fig. 6
AFM image of the a compound III, b compound VI, c compound VII
×
3.6 OFET device fabrication and characterization
For the electrical characterization of OFETs, the Keithley 4200 semiconductor parameter analyzer was used. The current–voltage characteristics of the transistors were made in the form of measurements such as VDS–IDS, VGS–IDS (Table 4).
Table 4
Electrical parameters of OFETs
Samples
µmax
(cm2 V−1 s−1) × 10−3
VTh
(V)
Ion/off
Compound III
0.81
9
~ 10–2
Compound VI
10.80
11
~ 10–3
Compound VII
0.43
15
~ 10–2
According to the Table 4, the transistor structure with low threshold voltage, high mobility and high on/off ratio (the transistor with the best parameter values among the three transistor structures) is the transistor made with compound VI (Fig. 7). In case of good crystallization of small organic molecules with high solubility, they are effectively preferred in OFET production. It is known that they have higher charge carrier mobility than polymers due to intermolecular interactions [16]. Among the molecules we synthesized for device design, effects up to 1.17 cm2 V−1 s−1 mobility values were detected in those containing TMS [46].
Fig. 7
OFETs with a Compound III, b Compound VI, c Compound VII output and d transfer characteristics
×
4 Conclusion
In summary, we have developed an isophorone-core Pd-catalyzed Sonogashira coupling reaction for alkyne derivatives, aiming to achieve significantly enhanced hole mobility while maintaining their electron mobility. In the absorption and fluorescence spectra, VI showed an absorption band at 352 nm and lightly red emission. TG and DSC measurements indicated that VI and III have high thermal stability in the solid state. The results showed that VI and III had hole transport energy of 3.54 and 3.78 eV and the hole reorganization energy was lower, improving the charge transport ability of the molecule. Electrochemical measurements indicate that compound III possesses a relatively high HOMO energy level and a narrow HOMO–LUMO energy gap, primarily attributed to its favourable OFET design framework. Findings based on the DFT approach indicate that compound VI, with the lower ΔE, λh and AIP values, is more suitable for OFET device design. We then fabricated OFETs with compound VI exhibiting a hole mobility of 10.8 × 10−3 cm2 V−1 s−1. These results are relevant for OFETs for screen technology applications.
Acknowledgements
The authors gratefully acknowledge the Amasya University Scientific Research Unit (FMB-BAP-394/256/20-0455/21-0554/22-0572), for financial support. We appreciate Mr. S. Alper Akbaba for his valuable assistance with the graphics in the preparation of this article.
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.
Ethical approval
The authors declare that this article does not contain any studies involving human participants performed by any of the authors.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
