Phenyl- and Thienyl-Ended Symmetric Azomethines and Azines as

Article pubs.acs.org/JPCC

Phenyl- and Thienyl-Ended Symmetric Azomethines and Azines as Model Compounds for n‑Channel Organic Field-Effect Transistors: An Electrochemical and Computational Study Barbara Vercelli Istituto CNR per l’Energetica e le Interfasi, Via R. Cozzi 53, 20125 Milano, Italy

Mariacecilia Pasini Istituto CNR per lo Studio delle Macromolecole, Via E. Bassini 15, 20133 Milano, Italy

Anna Berlin Istituto CNR di Scienze e Tecnologie Molecolari, Via C. Golgi 19, 20133 Milano, Italy

Juan Casado, J. Teodomiro López Navarrete,* and Rocío Ponce Ortiz* Department of Physical Chemistry, University of Malaga Campus de Teatinos s/n, 29071 Malaga, Spain

Gianni Zotti* Istituto CNR per l’Energetica e le Interfasi c/o Stati Uniti 4, 35127 Padova, Italy S Supporting Information *

ABSTRACT: The formation energy and stability of radical anions in a series of 12 phenyl- and 2-thienyl-ended linear, symmetric azomethines and azines were investigated by cyclic voltammetry. Replacing 1,4-phenylene with 2,5-thienylene cores and substitution with cyano or methyl moieties have allowed the lowering of lowest unoccupied molecular orbital energy levels even by 1 eV. Methyl capping stabilizes electron carriers (radical anions) toward dimerization, and the mechanism of such radical anion dimerization has been clarified by cyclic voltammetric kinetic analysis. The results have been compared with optical parameters and supported by density functional theory calculations.

introduction of fluoro substituents or cyano groups on the conjugated skeleton. Thus oligothiophenes functionalized with perfluoroalkyl, perfluoroaryl, or cyano substituents produce thermodynamically air-stable, high-performance n-channel semiconductors.3 Another strategy can be that of inserting in the polyconjugated chain nitrogen-containing, electron-poor groups such as azinic and azomethinic functionalities. This suggestion, combined with the fact that these moieties are

1. INTRODUCTION Organic conjugated semiconductors are actually investigated for application in organic field-effect transistors (OFETs), and to this scope n-channel (majority electron-transporting) materials are particularly requested.1−3 Electron transport in conjugated molecules occurs by a series of successive electron transfers between neutral and negatively charged (negative polaron) molecules.4 It is then clear that a basic requirement for an ideal electron-transport material is good stability of the negative polarons (radical anions) to water, oxygen, or other electronpoor agents. Functionalization with electron-withdrawing moieties lowers in fact the orbital energies of such systems. The most successful strategies for achieving n-channel transport involve the © 2014 American Chemical Society

Received: December 2, 2013 Revised: January 23, 2014 Published: February 4, 2014 3984

dx.doi.org/10.1021/jp411815w | J. Phys. Chem. C 2014, 118, 3984−3993

The Journal of Physical Chemistry C

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Chart 1

relatively robust and chemically stable, has prompted appropriate studies of azine and azomethine compounds.5,6 Structural studies are very important for OFET application as well since the physical properties of the materials are strongly dependent on the supramolecular architecture. To this regard, we must mention investigations of the mesomorphic behavior of symmetric and asymmetric azomethines with two imine groups7 and of a series of symmetric azine-type liquid crystals.8 Azomethines are compounds having the imine linkage (CHN) whereas azines contain in their backbone the linear azine moiety CN−NC. We will hereafter consider only symmetrical phenyl- or 2-thienyl-ended azomethines or azines. The imine linkage is not coplanar with a phenylene ring adjacent to the nitrogen atom,9 whereas the thiophene rings adopt an almost coplanar arrangement.10,11 Also the azine moiety, which is conjugated by itself, is not conjugated with benzene rings in arylazines because of significant twisting of the aryl groups out of the best plane of the azine moiety.12−15 In the light of these suggestions, we have compared benzene- and thiophene-based molecules. Moreover we have considered electron-acceptor substituents in the azomethine conjugated core, in order to favor a decrease of the energy levels. In particular we used cyano substitution, which may be helpful also because of structural effects. In fact the introduction of the electron-withdrawing cyano groups in n-type oligothiophene semiconductors has promoted molecular arrangements favorable to charge transport.16,17 Intermolecular interactions of different intensity, such as hydrogen bonding, π−π stacking, and donor−acceptor interactions, do in fact control the organization of the molecules in such materials.18 The present investigation addresses the energy of formation and the stability of n-type carriers (radical anions) in a series of selected model compounds. The study, which combines experimental (electrochemical and optical) and density functional theory (DFT) computational analyses, concerns the linear symmetric phenyl- and 2-thienyl-ended azomethines

(AM1−AM7) and azines (A1−A6) shown in Charts 1 and 2 respectively. Chart 2

Azomethines containing a progressive conjugation and electron-acceptor properties were investigated. The sequence starts from the p-phenylene core (AM1 and AM2); then the thienyl end is kept and the core changed to p-diphenylene (AM3) and 2,5-thienylene (AM4). AM4 was not synthesized, due to the difficulties in the synthesis of 2,5-diaminothiophene, and was considered only theoretically, whereas the 3,4dicarboxyethyl- (AM5) and 3,4-dicyano-substituted (AM6) 3985



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function generator. The working electrode for cyclic voltammetry (CV) was a platinum or glassy carbon minidisc electrode (0.003 or 0.06 cm2, respectively). Highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were calculated from the oxidation and reduction peak potentials, respectively. 2.3. Spectroscopy. UV−vis absorption spectra were run with a Lambda 900 Perkin-Elmer spectrometer. NMR spectra were recorded on Bruker Advance 400 and 270 spectrometer instruments equipped with a triple resonance gradient probe. Chemical shifts were calibrated using the internal CDCl3, deuterated dimethyl sulfoxide (DMSO-d6) resonance that were referenced to tetramethylsilane (TMS). FTIR spectra were taken on a Perkin-Elmer 2000 FTIR spectrometer. Mass spectra were collected on an ion trap Finningan Mat GCQ spectrometer operating in electron impact (EI) ionization mode. Each sample was introduced to the ion source region of GCQ via a direct exposure probe (DEP). The melting points were determined with a Büchi 510 apparatus. Elemental analyses were performed on an Elementar Vario EL. 2.4. DFT Calculations. DFT calculations were performed using the B3LYP functional24 and 6-31G** basis set25 as implemented in Gaussian 09.26 These calculations were performed as a guide for the analysis of the electrochemical and spectroscopic data. Optimal geometries were determined on isolated entities. From the resulting ground-state optimized structures, relevant molecular parameters such as bond lengths and angles, molecular orbital energies and topologies, and atomic charge distributions were estimated. Radical anions were treated as open-shell systems and computed using spin-unrestricted UB3LYP wave functions. For all of the molecules, the maximum value obtained for the spin contamination ⟨S2⟩ was ∼0.77−0.81, close to the 0.75 theoretically expected for a doublet, showing that spin contamination is almost absent. From the optimized radical anion structures, atomic charge distributions and spin densities were estimated. Although the 6-31G** basis set was used as a default in the radical anions calculations, few of the molecules were recalculated using a basis set including diffuse functions, 631++G**.

homologues were investigated. Finally the latter has been compared with the homologue (AM7) bearing the short 1,2dicyanoethylene conjugated core. Among azines, the phenyl-ended A1 and the corresponding 1,4-dimethyl-substituted A2 were compared with the 2-thienylended homologues A3 and A4. We have then introduced a progressive conjugation in A3 considering also thiophene-based bis-azines, namely, A5, functionalized with two methyl groups at the central azine moieties, and A6, 3,4-didodecyl-substituted at the central thiophene ring. Alkyl chains influence in fact the conformation of conjugated molecules, and in particular the presence of two alkyl chains onto adjacent aromatic rings provokes large distortions in polymeric backbones.19 In contrast A6 is highly conjugated20 as already observed in 3and 3,4-alkyl-substituted thienylenevinylene oligomers.21

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Analytical grade solvents, the supporting electrolyte tetrabutylammonium perchlorate (Bu4NClO4), the catalyst CF3COOH, reagents, and deuterated NMR solvents were purchased from Aldrich and used as received. Compounds A1 and A2 were reagent grade from Aldrich; A3, A4, A5, A6, AM1, AM2, and AM3 were prepared as described in the literature.22,23,20 AM6 and AM7 were prepared as described in the following text. 2.1.1. 2,5-Bis((E)-(thiophen-2-ylmethylene)amino)thiophene-3,4-dicarbonitrile (AM6). An excess of 2-thiophenecarboxaldehyde (0.7 mmol, 78 mg) was added to a stirred solution of 2,5-diamino-3,4-thiophenedicarbonitrile (0.3 mmol, 50 mg) dissolved in ethanol (15 mL). Some drops of trifluoroacetic acid were added to the solution that turns from yellow to red with precipitation of a red powder. The mixture was left at room temperature, under stirring, for 4 h, and then cooled in a freezer. The product, a red powder, was filtered and washed with ethanol (yield 80%); mp dec. ∼ 200 °C. FTIR (cm−1): 2240 (ν(CN)), 1560 (ν(CN), stretching). MS (EI): m/z 353 [M + 1]+.1H NMR (DMSOd6) δ 8.91 (s, H−CN, 2H), 8.07 (d, H thiophene, 2H), 7.92 (d, H thiophene, 2H), 7.30 (m, H thiophene, 2H). 13C (DMSO-d6): δ 103.39, 112.17, 129.35, 136.02, 138.09, 140.34, 157.30, 157.39. Elem. Anal. Calcd for C16H8N4S3 (%): C, 54.52; H, 2.29; N, 15.90; S, 27.29. Found (%): C, 54.97; H, 2.60; N 15.47; S 26.96. 2.1.2. 2,3-Bis((E)-(thiophen-2-ylmethylene)amino)maleonitrile (AM7). A procedure very similar to that used for AM6, starting from 2-thiophenecarboxaldehyde (5.6 mmol, 640 mg) and diaminomaleonitrile (2 mmol, 216 mg) dissolved in ethanol (25 mL), provides a red-orange product (yield 70%); mp 256 °C. FTIR (cm−1): 2231 (ν(CN)), 1557(ν(CN), stretching). MS (EI): m/z 297 [M + 1]+. 1H NMR (CDCl3): δ 8.88 (s, H−CN, 2H), 7.73 (d, H thiophene, 2H), 7.70 (d, H thiophene, 2H), 7.22 (m, H thiophene, 2H).13C (CDCl3): δ 110.54, 128.93, 129.06, 135.22, 136.15, 141.34, 157.61. Elem. Anal. Calcd for C14H8N4S2 (%): C, 56.74; H, 2.72; N, 18.90; S, 21.64. Found (%): C, 56. 93; H, 2.96, N, 18.54; S, 21.57. 2.2. Electrochemistry. Electrochemistry was performed at room temperature under nitrogen in three electrode cells using 0.1 M Bu4NClO4 as supporting electrolyte. The counter electrode was platinum; the reference electrode was silver/0.1 M silver perchlorate in CH3CN (0.34 V vs SCE, 4.77 V vs vacuum). The voltammetric apparatus (AMEL, Milan, Italy) included a 551 potentiostat modulated by a 568 programmable

3. RESULTS 3.1. Electrochemical Analysis. The cyclic voltammogram of the compounds, dissolved in CH3CN (or CH3CN/CH2Cl2 for A6 for solubility reasons), shows oxidation and reduction processes at peak potentials given in Table 1. The electrochemical gaps (EEC) were calculated from the difference between oxidation and reduction peak potentials, which bears an approximation due to kinetic shifts in irreversible processes. The calculated values are also summarized in Table 1. Oxidation is in general a one-electron irreversible process in which a terminal thiophene or benzene ring is involved. The irreversibility is due to fast follow-up reactions such as coupling or nucleophilic attack from agents (such as water) in the medium. The reduction step is also a one-electron process which may be reversible or irreversible (i.e., with follow-up reactions) depending on the type (azine or azomethine) and methyl substitution. 3.1.1. Azomethines. A typical example of CV for this series of compounds is shown in Figure 1. 3986



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potential, which confirms that an increased conjugation within the core is not effective on the overall conjugation. With AM5, reduction is strongly eased, confirming the increased conjugation allowed by the thiophene ring (and to some extent also by the electron-withdrawing action of the substituents). The effect is increased in AM6 by the introduction of the cyano substituents. Instead, making the dicyano-substituted conjugated core in AM7 shorter than the thienylene moiety in AM6 does in fact make the reduction potential less negative but by a 0.05 V amount only (and the electrochemical gap results to be the same). AM7 is in any case reduced at the earliest potential. Its oneelectron process, irreversible even at relatively high (up to 10 V s−1) scan rates, shows a backward oxidation peak at a potential ca. 0.7 V more anodic, which may be attributed to radical anion dimerization29 and back-reoxidation of the dimeric dianion.30 The same picture is shown by AM6 and reflects possibly a general behavior of all of the azomethines here investigated. 3.1.2. Azines. The CV of A4 is shown in Figure 1 as a typical example for this series of compounds. The oxidation potential decreses dramatically from A1 to A3, in contrast with what was observed for azomethines from AM1 to AM3. This result is accounted for by an easier oxidation of thiophene compared with a benzene ring in a better conjugated backbone. For the bis-azines A5 and A6 the oxidation potential is then stabilized at the same value of A3 and A4. Considering reduction, we first observed that the process for A1 becomes reversible at scan rates v > 1−2 V s−1. For A2 the process is fully reversible even at low scan rates, as reported in the literature.15 Dimer is in fact produced by one-electron reduction electrolysis of A1,31 whereas the radical anion is stable in A2, obviously due to the methyl substitution. The same picture is observed for the thienyl-ended azines A3 and A4. In the series formed by A4, A5, and A6, the reversible oneelectron reduction of the azine moiety is progressively shifted to less negative potentials so that two one-electron reversible reduction processes can be detected for A5 and A6 (Figure 2). We must also remark about the less negative reduction potential and lower potential difference between the two one-

Table 1. Oxidation and Reduction Potentials (Peak Potential Ep vs Ag/Ag+, at 0.1 V s−1) and Electrochemical Gap (EEC) in Acetonitrile and Maximum Absorption Wavelength (λmax) and Optical Gap (Eg) in CHCl3 compd

a

Epox/V

AM1 AM2 AM3 AM5 [73] AM6 AM7

0.95 0.93 0.97 0.8 1.09 1.21

A1 A2 A3 A4 A5 A6

1.49 1.23 1.10 1.06 1.00 1.00

Epred/V Azomethines −2.13 −2.07 −2.26 −1.7 −1.22 −1.16 Azines −2.20 −2.35a −2.10 −2.18a −1.80;a −2.00 −1.55;a 1.67a

EEC/V

λmax/nm

Eg/eV

3.1 3.0 3.2 2.5 2.3 2.3

352 369 365 440 463 430

3.5 3.4 3.4 2.8 2.7 2.9

3.7 3.6 3.2 3.2 2.8 2.5

315 314 342 340 388 459

3.9 3.9 3.6 3.6 3.2 2.7

Redox potential E0.

Figure 1. Cyclic voltammograms of (red) oxidation and (black) reduction of (A) AM7 and (B) A4 ca. 10−3 M in CH3CN + 0.1 M Bu4NClO4. Scan rate: 0.1 V s−1.

In AM1 and AM2 the oxidation potential changes scarcely, in spite of the easier oxidation generally found in thiophenes.27 This result may be accounted for by a leveling operated by the central benzene ring, not coplanar with the conjugated ends of the molecule (see DFT Calculations). Also the substitution of the phenylene core with a diphenylene moiety does not change the response, in spite of the well-known conjugative effect in 1,4-oligophenylenes.28 In contrast, substitution of the AM2 core with a 2,5thienylene ring in AM5 lowers the oxidation potential significantly, in spite of the electron-withdrawing ester groups, which indicates clearly a strongly increased conjugation along the molecular line. The oxidation in the case of AM6 and AM7 is shifted positively by the strong electron-withdrawing action of the dicyano moieties. Moreover it becomes a two-electron process, likely due to fast deprotonation of the radical cation (promoted by the electron-withdrawing cyano substituents) and oxidation of the resulting neutral radical. The reduction of the azomethines is in all cases an irreversible one-electron process. where the dimer is reported to be generally produced.29 Substitution of phenylene in AM2 with a diphenylene core in AM3 does not ease the reduction

Figure 2. Cyclic voltammograms of reduction of (a) A4, (b) A5, and (c) A6 ca. 10−3 M in CH3CN (CH3CN/CH2Cl2 for A6) + 0.1 M Bu4NClO4. Scan rate: 0.1 V s−1. 3987



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electron reduction processes of A6 compared with A5. These results are attributable to the absence in A6 of methyl groups close to the central thiophene ring (with consequent relief of distortion) and (to a lesser extent) to the presence in the ring of the alkyl moieties, which bring some extra coplanarization to the system.20 3.1.3. HOMO−LUMO Energy Levels and Gaps. Electrochemical HOMO−LUMO energy levels for the investigated compounds are illustrated in Figure 3. Comparing both series

Figure 4. Cyclic voltammograms at different scan rates (0.1, 0.2, 0.5, 1, and 2 V s−1) of reduction of A3 2 × 10−3 M in CH3CN + 0.1 M Bu4NClO4.

reversibility is provided by methyl capping of the azine moieties. The one-electron reduction of A1 in aprotic medium produces the dimer,31 and it is argued that the same reaction takes place also for the thiophene homologue A3. To clarify the reaction mechanism of the radical anion dimerization of such molecules, we have undertaken an analysis of the cyclic voltammetry (CV) response at various scan rates (Figure 4). The ratio of the backward oxidation current to the forward reduction current was analyzed according to the literature for follow-up first-order32 or second-order33 reactions. It was found that good constancy of rate constants evaluated at different scan rates was obtained only with a second-order analysis. Moreover the same values were obtained at different compound concentrations in the range of (0.5−2) × 10−3 M. This is in agreement with a follow-up dimerization process as the rate determining step. The rate constants k2 for A1 and A3 were (12 ± 2) × 103 and (5 ± 1) × 103 M−1 s−1, respectively. A typical analysis is shown in Table 2. Table 2. Typical CV Kinetic Analysis of Second-Order Dimerization of A3 Radical Anions in Acetonitrile at Room Temperature C/M −3

0.5 × 10

Figure 3. Electrochemical HOMO (blue) and LUMO (red) energy levels for the investigated (top) azomethines and (bottom) azines. Squares: calculated values.

1 × 10−3

of compounds, it can be observed that the HOMO energy levels are grossly the same for all of the compounds. Conversely, the LUMO is progressively lowered even by 1 eV. The electronic effect concomitant with the increased degree of conjugation thus narrows by the same amount (ca. 1 eV) the energy gap between the HOMO and the LUMO energy levels. The electrochemical gap values are corroborated by the spectroscopically measured and the DFT calculated values, as reported in subsequent sections. We can conclude that the possibility of tuning selectively the LUMO level, which is crucial for the development of n-type transistor, is clearly evidenced by these results. 3.2. Cyclic Voltammetry Kinetic Analysis of Azine Reduction. As pointed out before, for A2 and A4 the oneelectron reduction process is fully reversible. On the contrary, for A1 and A3 the reduction is irreversible, becoming reversible only at high scan rates (Figure 4). It is thus evident that

2 × 10−3

v/(V s−1)

K2/(103 M−1 s−1)

0.5 1 2 0.2 0.5 1 2 0.5 1 2 5 10

4.4 4.4 5.4 5.9 6.5 5.7 5.3 4.5 5.2 4.9 5.0 4.7

The suggested mechanism of dimerization, involving the acidic methine protons (see Scheme 1), is the following. The radical anion formed by reduction (1) undergoes free rotation along the newly formed single C−N bond, thus allowing two radical anions to form a face-to-face adduct in which the acidic methine protons face the negatively charged nitrogen atoms (2). In the resulting six-membered ring the negative charges are partially shielded thus lowering the electrostatic repulsion and 3988



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Scheme 1

then easing radical−radical dimerization (3). It is thus clear that methyl capping eliminates such stabilization therefore stabilizing the radical anion monomeric form. Dimerization is the rate determining step, after which further protonation by proton donors (typically moisture) in the medium (4) produces the final product, i.e., the dimeric hydrazone. In the light of this analysis the higher (ca. twice) dimerization rate of the phenylsubstituted azine, compared with the thienyl-substituted azine, radical anion may be accounted for by the more extended spin delocalization in the latter. The electron delocalization factor accounts also for the stability of more extended azine radical anions such as in A6, in spite of the absence of methyl capping of the azine moieties. 3.3. Optical Analysis. The UV−vis spectroscopic data on the compounds are summarized in Table 1 as maximum absorption wavelength (λmax) in CHCl3, and the optical gap (Eg) calculated accordingly. Compared with the electrochemical gaps, the optical gaps are systematically ca. 0.3 eV higher, which may only partially be attributed to electrochemical and optical solvent effects. In fact the deviation is mainly due to the evaluation from the peak instead of that from onset values, the latter procedure being a commonly used and theoretically proper but also a very illdefined method. Thus the electrochemical and optical gap onset values (from the x-axis intercept of the line of maximum slope) for our compounds are ca. 0.15 V and 0.45 eV lower than the corresponding peak values. As a consequence of these considerations, optical and electrochemical results are in quite satisfactory agreement. 3.3.1. Azomethines. For azomethines the gap is ca. 3.5 eV for the series AM1−AM3, but introduction of thiophene as the central group in AM534 and AM6 decreases the gap strongly. This can be attributed to increased coplanarity around the nitrogen-ring linkage. In contrast with the electrochemical gaps, which are the same, the optical gap for AM6 is 0.2 eV lower than for AM7. This result may be ascribed to some degree of charge-transfer character of the optical transition in AM6. The spectra of AM6 and AM7 solutions (Figure 5) are exceptions in the series being characterized by a clear vibronic structure, mostly evident in the

Figure 5. UV−vis spectra of (top left) AM6, (top right) AM7, and (bottom) (black) A4, (red) A5 and (blue) A6 in CHCl3.

case of AM6. The spectra in different solvents (odichlorobenzene, CHCl3, or DMSO) and at very different concentrations are the same, which is indicative that the vibronic features are due to the molecular structure and not to aggregate forms. The accentuation of the vibronic structure in AM6 confirms the rigid structure that we can assume is planar due to the high red shift. 3.3.2. Azines. Considering azines, in phenyl-ended members of the series the gap is higher than for the corresponding azomethines (ca. 3.9 vs 3.4 eV) but is lowered moderately by 3989



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the thiophene rings (to 3.6 eV) in A3 and A4 and strongly in bis-azines A5 and A6 (down to 2.7 eV). These results, clearly illustrated in Figure 5, confirm the progressively increased conjugation evidenced by the electrochemical data. 3.4. Computational Analysis. We have considered the performance of a computational analysis on the azomethine series from AM2 to AM7. In these D-A-D type compounds D is the terminal thienyl-imine moiety and A is a conjugated core with progressively higher conjugation and/or electron-acceptor properties. The structure of the virtual azomethine with nonsubstituted 2,5-thienylene as the A moiety (here named AM4) is also considered for comparison. Calculations were performed also on the radical anion forms to establish the charge and spin localization in such charge carrier forms. 3.4.1. Neutral Molecules. Figure 6 shows the optimized frontal and lateral molecular views at the DFT computational

Figure 7. B3LYP/6-31G** electronic density contours for the HOMO and LUMO frontier molecular orbitals of azomethines.

On comparing AM2 and AM4, a ca. 0.7 eV band gap decrease is observed on the latter, which is ascribed to both the more aromatic character of the benzene ring (which partially prevents interring conjugation) and again to the skeleton distortion found in AM2. The all-thiophene derivatives present a similar energetic band gap, with a value of approximately 2.7−2.8 eV; however the relative energies of the frontier orbitals differ depending on the lateral functionalization. Note that the substitution stabilizes both the HOMO and LUMO level to a similar extent, thus maintaining the band gap separation. Moreover, comparing AM4 with AM5 and AM6, it is clear that the downward shift caused by the cyano substituents is much higher (ca. 0.5 eV) than that promoted by the carboxylate moieties (ca. 0.2 eV). The topologies of the molecular orbitals indicate a similar delocalization over the whole conjugated skeleton for both HOMO and LUMO, and only small contributions of the electroactive groups are found in AM5−AM7. In those cases, we also observe practically the same contribution of the electroactive groups on both frontier molecular orbitals, which indicates that the one-electron HOMO−LUMO transistion does not involve a remarkable intramolecular charge transport. 3.4.2. Radical Anions. In order to analyze the structure of the radical anion form, we have performed DFT calculations on the negatively charged species. For all of the semiconductors, the 6-31G** basis set was used to optimize the radical anions; however two of the molecular systems were recalculated using a higher computational cost basis set including diffuse functions, 6-31++G**. No remarkable changes were found between calculations carried out using 6-31G** and 6-31++G** (see

Figure 6. Face (top) and edge (bottom) views of the B3LYP/6-31G** optimized azomethine structures.

level (B3LYP/6-31G**) for the investigated azomethine derivatives. As we expected, the all-thiophene derivatives adopt a basically coplanar arrangement, while AM2 and AM3, where either a phenylene ring or a biphenylene fragment is connected to the nitrogen atoms of the imine linkages, present a severely twisted configuration, with dihedral angles of ∼34− 35°. This is translated into a much less conjugated skeleton. We have also estimated the HOMO and LUMO energies in order to analyze the effect of the different imine substitutions and of the lateral functionalization with electron-acceptor groups on the frontier molecular orbital tuning, as shown in Figure 7. Frontier orbital and band gap energy values are summarized in Table 3, in comparison with the experimental (electrochemical and optical) values. Regarding the molecular orbital energies, the introduction of a second phenyl ring in AM3 (compared to AM2) slightly destabilizes both the HOMO and LUMO to a similar extent and, thus, the energy gap stays basically unaltered. Therefore, the inclusion of the second phenylene ring does not induce a higher conjugation in the conjugated core, probably due to the severe skeleton distorsions. 3990



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Table 3. Electrochemical (EC) and Theoretical (T) Energy Levels (eV vs Vacuum) and Electrochemical (EEC), Optical (Eg), and Theoretical (ET) Energy Gap Values azomethine

HOMO(EC)

LUMO(EC)

HOMO(T)

LUMO(T)

EEC/V

Eg/eV

ET/eV

AM2 AM3 AM4 AM5 [73] AM6 AM7

−5.70 −5.74

−2.70 −2.51

3.4 3.4

−3.10 −3.55 −3.61

−1.96 −1.91 −2.41 −2.63 −3.14 −3.11

3.0 3.2

−5.6 −5.86 −5.98

−5.42 −5.38 −5.15 −5.40 −5.85 −5.98

2.5 2.3 2.3

2.8 2.7 2.9

3.46 3.47 2.74 2.77 2.71 2.87

data in the Supporting Information), indicating that the use of diffuse functions is not strictly necessary in this case. Figure 8 shows how the extra charge is delocalized over the conjugated skeleton.

Figure 9. Isovalent surfaces (0.004 electron/bohr3) of spin electron density for the azomethine compounds. Atomic electron spin densities are given in electron.

4. DISCUSSION 4.1. Charge Transport in OFETs. The performance of OFETs critically depends on the effective transport of the carriers through the bulk of the material. In organic semiconductors, the constituting molecules are only weakly bound together so that band energy widths are small. Thus the mechanism of charge conduction implies that charge carriers overcome activated energy barriers. A carrier (a one-electron charged molecule, i.e., a positive or negative polaron) is initially localized at a particular energy site, confined approximately within a deep or shallow potential well. Upon receiving enough thermal energy, the carrier can overcome the potential barrier and hop over to a neighboring site (a neutral molecule). Localized states that trap mobile charges temporarily or permanently (immobile charges) are commonly observed in organic materials. They create shallow or deep trap energy states below the transport energy level so that charge transport within the bulk is hampered. In the case of negative polarons (radical anions) traps may be electron-poor molecules but also the polarons themselves (self-trapping) via, e.g., dimerization. 4.2. Stability of Azomethine Radical Anions. The electrochemical reduction of azomethines and azines occurs by charge transfer involving molecular orbitals centered on the bridge between the aromatic rings.36 Under aqueous conditions reduction has been shown to consist of an overall two-electron, two-proton transfer which converts the CN linkage to a CHNH group.36−38 Water thus acts as a very efficient proton donor vs the initially formed radical anion. In organic media radical anions of azomethines and azines have shown dimerization,29,31,39 but 1,4-dimethyl-substituted azines undergo successive reduction to their radical anion and

Figure 8. B3LYP/6-31G** calculated distribution of extra negative charge (in percent) in azomethine radical anions with respect to the neutral species.

Note that for the azomethine systems without functionalization with electron-withdrawing groups (AM2−AM4), the extra negative charge is delocalized over the whole conjugated skeleton but with a higher contribution on the external thiophene rings (∼27% each). On going from AM2 to AM4, we observe that the central benzene ring in AM2 bears less negative charge compared with the central thiophene ring in AM4, while the contrary occurs on the azomethine units. Upon functionalization with progressively stronger electronwithdrawing groups, the substituted central thiophene rings in AM5 and AM6 support the greater amount of charge, with 28% and 30% of the negative charge, respectively. Passing from AM6 to AM7 the overall charge distribution does not strongly change. However, while the central thiophene ring (excluding the cyano groups) bears 16% of the negative charge in AM6, only 10% goes to the ethylene group in AM7, while the cyano groups support 14% in the former versus 18% in the latter. These data indicate a stronger electron-withdrawing character of the cyano groups in AM7 compared with AM6. Finally, note that the azomethine units are the groups bearing the less amount of negative charge in all of the systems. We have also estimated the spin delocalization for all of the molecules under study. For all systems, the spin density is delocalized over the whole conjugated skeleton (Figure 9). However the carbon atoms of the azomethine units bear in all cases the highest spin density, thus supporting the observed higher reactivity of this molecular fragment to dimerization.35 3991



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then dianion.15 In such cases, where no acidic azomethine proton is present, the formation of the radical anion is completely reversible. In fact we have found that the presence of the azomethine protons makes the first reduction process irreversible for azomethines and reversible only at relatively high scan rates for azines. This result is accounted for by a higher charge and spin delocalization of the radical anion in azines, which makes it less reactive to dimerization. 4.3. Conjugation in Azines. Besides by the presence of acidic azomethine protons in the molecule, the radical anion form is destabilized also by charge localization as a consequence of, e.g., scarce conjugation, particularly for azines. The structure of aryl azines appears to be ideally suited for complete conjugation throughout the entire molecule because of the extended π-system, but this question is in fact debated. X-ray analyses and NMR, theoretical, and electrochemical studies suggest no apparent conjugation12−15 so that the C N−NC spacer was retained as a blocking element [53]. In contrast, semiempirical calculations and UV−vis spectra of the radical anions have suggested that the molecules adopt a more planar configuration and that the HOMO is more highly delocalized.15 Our electrochemical and optical analysis shows that effective conjugation is operative in thiophene-substituted analogous systems. The results here reported show that the presence of thiophene in place of benzene rings makes the electrochemical and optical properties strongly affected by conjugation. Therefore the process of electron transfer in such azine compounds is eased and the n-conduction of the relevant materials should be favorably influenced.

solubility and possibly enhanced structural properties, is actually under consideration.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing DFT//B3LYP/6-31++G** charge distributions and spin densities for AM4 and AM6 radical anions and text listing the complete ref 26. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: (34)952-132018. Fax: (34) 952-132000. *E-mail: [email protected]. Tel: (34)952-131863. Fax: (34) 952-132000. *E-mail: [email protected]. Tel.: (39)049-8295868. Fax: (39) 049-8295853. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Regione Lombardia through project “Tecnologie e materiali per l′utilizzo efficiente dell’energia solare” decreto 3667/2013. Research at University of Malaga was supported by MINECO (Grant CTQ201233733) and Junta de Andalucia (Grant P09-4708). R.P.O. thanks the MINECO for a “Ramón y Cajal” research contract.

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REFERENCES

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5. CONCLUSION Two series of phenyl- and 2-thienyl-ended linear, symmetric azomethines and azines were prepared via condensation reaction of benzaldehyde or thiophenecarboxaldehyde with hydrazine and selected conjugated diamines, respectively. Diamines with progressive conjugation and electron-acceptor properties were used. The progression of LUMO energy level decrease and radical anion reactivity to dimerization, basic issues for negative charge (radical anion) trapping, was monitored by electrochemical measurements and supported by optical data and DFT calculations. In azomethines the LUMO energy level was decreased by ca. 1 eV to −3.6 eV by substitution with thiophene rings and electron-withdrawing substituents. Modulation of conjugation in the azines via substitution with thiophene rings and suitable alkyl substitution has allowed a similar decrease of the LUMO energy to −3.4 eV. The conclusion is that three thiophene rings linked linearly with two azomethine or azine moieties display strongly stabilized LUMO energy levels. Negative carriers, which in such compounds may be trapped also by radical anion dimerization, may be stabilized by increased charge delocalization or methyl capping of reactive azomethine sites. Among the investigated compounds, thienyl-ended azomethines with a dicyano-substituted conjugated core, possibly with alkyl capping of the azomethine sites, appear to be the most favored candidates for efficient OFETs, both for their electronic and structural features. The compounds are in fact only model ones, also because their solubility does not allow the fabrication of devices. The use of suitable end substituents, with eased 3992



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Phenyl- and Thienyl-Ended Symmetric Azomethines and Azines as

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