Tuning the Crystal Packing and Semiconductor Electronic Properties

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C: Energy Conversion and Storage; Energy and Charge Transport

Tuning the Crystal Packing and Semiconductor Electronic Properties of 7,7’-Diazaisoindigo by Side-Chain Length and Halogenation Mónica Moral, Amparo Navarro, Andrés Garzón-Ruiz, and Eva M. García-Frutos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09633 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 9, 2018

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The Journal of Physical Chemistry

Scheme 1. Chemical structure of the studied compounds, showing the different substituents (X and R) and the numbering employed in the discussion about the molecular geometry. The compounds were called CR X where R is the number of carbon atoms of the side chains and X corresponds to hydrogen or halogen atom. The synthesized compounds are indicated with (exp.) while the modeled compounds are noted with (calc.). Nevertheless, theoretical crystals were also computed for the synthesized compounds for comparison porpoises. 79x90mm (600 x 600 DPI)

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Tuning the Crystal Packing and Semiconductor Electronic Properties of 7,7’-Diazaisoindigo by Side-Chain Length and Halogenation Mónica Moral,[b] Amparo Navarro,*[c] Andrés Garzón-Ruiz, [d] Eva M. García-Frutos*[a] [a] Instituto de Ciencia de Materiales de Madrid (ICMM), CSIC, Campus de Cantoblanco, 28049, Madrid, Spain [b]Instituto de Investigación de Energías Renovables, Universidad de Castilla-La Mancha, Paseo de la Investigación 1, 02071 Albacete, Spain [c]Departamento de Quí mica Física y Analítica, Facultad de Ciencias Experimentales, Universidad de Jaén, Campus Las Lagunillas, 23071 Jaen, Spain [d]Departamento de Química Física, Facultad Farmacia, Universidad de Castilla-La Mancha, 02071 Albacete, Spain

* E-mail: [email protected], Phone: (+34) 91-3349038, Fax: (+34) 91-3720623, [email protected]


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ABSTRACT. In the last years, the 7,7’-diazaisoindigo has emerged as a promising building block for semiconductor materials. In this work, we have studied different electronic properties which can be related to the semiconducting character of a family of 7,7’-diazaisoindigo derivatives. Concretely, we have analyzed the role of halogen substituents and different-length side chains on these properties calculated by means of the Density Functional Theory. In total, sixteen halogenated and non-halogenated diazaisoindigo derivatives were investigated. Four of these compounds were also synthetized and their X-ray structures were employed as starting points for the calculation of crystal structure of the rest of the novel compounds. In general, high electron transfer rate constants and electron mobilites were calculated for the studied 7,7’diazaisoindigo derivatives, especially for bromine derivatives and compounds with long-side chains. The origin of these high rate constants mainly resides in the strong electronic couplings found for diazaisoindigo crystals in the π-stacking direction.

INTRODUCTION In the last decades, developing new and more efficient π-conjugated organic compounds for molecular electronics applications is one of the main research topics in chemical and material science.1-4 Conjugated organic compounds are currently been employed in the fabrication of diverse electronic and optoelectronic devices such as organic field-effect transistors and organic thin-film transistors (OFETs and OTFTs), organic light-emitting diodes (OLEDs), and organic photovoltaic cells (OPVs), among others.5-7 Concretely, in the field of OFETs, great efforts toward the enhancement of carrier mobilities have led to the discovery of numerous highperformance p-type, n-type, and ambipolar polymeric semiconducting materials. The charge


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carrier mobilities of these materials are greater than the 1 cm2 V−1 s−1 of the presently used amorphous silicon.8 Our interest in the present work is focused on the study of the semiconducting properties of 7,7’-diazaisoindigo derivatives. Small molecules and polymers based on electron-deficient diazaisoindigo unit have recently been employed in the fabrication of electronic devices such as OFETs, OTFTs and OPVs.8-11 Isoindigo and diazaisoindigo are, in general, common building blocks of high-performance charge-transport polymers, used in OFETs,8 and of low-bandgap polymers, employed in OPVs.7 Thus, strong crystallinity and high hole mobility around 7 cm2 V-1 s-1 have been found for 7,7’-diazaisoindigo-based polymers showing the great potential of this novel building block in organic electronics.8 Interesting photophysical features have also been reported for diazaisoindigo derivatives suggesting possible light emitting applications.12,13 These extraordinary optoelectronic properties are related to the coplanarity of molecular platform which also favors crystal packing with strong orbital overlap enhancing charge transport. Side chain engineering is a common strategy to control the molecular packing and improves the charge transport in organic semiconducting materials.2,5 Functionalization with halogen atoms is another method usually employed to tune the semiconductor character of an organic material from p-type to n-type. For instance, the introduction of electron-withdrawing fluorine atoms has demonstrated to be an efficient method to improve electron injection (fluorination yields lower energy values of LUMO) but, in contrast, can worsen the charge transport properties (fluorination generally increases the reorganization energy).14-17 However, other halogen substitutions dealing with charge transport have been less explored so far.18-20 Isoindigo has been studied at different binding positions (5,5´; 5,6ánd 6,6′), evaluating how the type of substituent affects to the electronic properties.21-23 However, its diazaisoindigo


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analogues have only been deeply studied in the case of the 6,6´-derivative,24 while the 5,5´derivative has been less studied. The study of the 5,5´-substitution at the core have demonstrated to improve the carrier transport characteristics in comparison to the 6,6´-substitution.25 For this reason, the theoretical study of this position could help to improve the general understanding of the electronic properties and their effects on the diazaisoindigo properties. Besides, the study of this alternative 5,5´-position for future applications has the advantage of requiring cheaper starting materials for the preparation of the 5,5´-dibromodiazaisoindigo than those used for the preparation of the 6,6ánalogue. Thus, the first aim of our study was to analyze the effect of the length of the alkyl pendant chain, R, on the semiconducting properties of the family of 7,7’-diazaisoindigo derivatives shown in Scheme 1. As mentioned above, charge transport efficiency critically depends on the molecular arrangements in the crystal and, in turn, the type and length of side chains have a great influence on the supramolecular structure of conjugated compounds.2,5 In a second step, we aspire to improve the semiconducting properties of 7,7’-diazaisoindigo by analyzing the effect of the presence of halogen atoms in 5,5´-position (see Scheme 1).With these objectives in mind, we modeled the molecular and crystal structure of the series of halogenated and non-halogenated 7,7’-diazaisoindigo derivatives, CR-X, shown in the Scheme 1 by Density Functional Theory (DFT) calculations starting from the experimental crystal structures of some 7,7’-diazaisoindigo derivatives synthesized in this work, i.e. C4-H, C6-H, C8-H and C12-Br. As it well known, Computational Chemistry is a powerful tool to design new materials and predict the semiconducting properties of compounds from the isolated molecule level to the bulk material. Modeling of organic crystals by means of DFT periodic calculations as well as the calculation of


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electronic properties (descriptors) related to their semiconducting properties are common strategies followed by different research groups.26,35

R N

N O C2 X

C1

X

C3 C4 O N

N

R

X R H

F

Cl

Br

C4H9

C4-H (exp.)

C4-F (calc.)

C4-Cl (calc.)

C4-Br (calc.)

C6H15

C6-H (exp.)

C6-F (calc.)

C6-Cl (calc.)

C6-Br (calc.)

C8H17

C8-H (exp.)

C8-F (calc.)

C8-Cl (calc.)

C8-Br (calc.)

C12H25

C12-H (calc.)

C12-F (calc.)

C12-Cl (calc.)

C12-Br (exp.)

Scheme 1. Chemical structure of the studied compounds, showing the different substituents (X and R) and the numbering employed in the discussion about the molecular geometry. The compounds were called C R-X where R is the number of carbon atoms of the side chains and X corresponds to hydrogen or halogen atom. The synthesized compounds are indicated with (exp.) while the modeled compounds are noted with (calc.). Nevertheless, theoretical crystals were also computed for the synthesized compounds for comparison porpoises.

EXPERIMENTAL SECTION Theoretical Considerations. The performance of optoelectronic devices greatly depends on an efficient charge injection as well as high charge carrier mobilities. Both factors must be considered in the design of new semiconducting materials. In the crystal, the charge carrier


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mobility (μhop) was calculated by the Einstein-Smoluchowski relation which describes a diffusion process.5,36-39 This process is controlled by the charge-transfer rate (kCT) that can be obtained through the Marcus-Levich-Jortner (MLJ) equation.39-41 Charge-transfer rate, in turn, depends on two key parameters, i.e. the charge transfer integral (t), that reflects the strength of the electronic coupling between neighboring molecules,2,37,42 and the reorganization energy, λ. This last energy is composed by the sum of an intramolecular contribution, λi, and an external contribution, λe. In the present work, λi has been calculated through both the Adiabatic Potential (AP) approach (via Nelsen four-point method)43,44 and Normal Modes (NM) approach (via calculations of HuangRhys parameters).32,45,46 Two molecular descriptors are commonly used to evaluate the efficiency of charge injection, i.e. (i) the energy difference between EHOMO or ELUMO (energies of highest occupied molecular orbital or lowest unoccupied molecular orbital) and the work function (Φm) of the electrode; and (ii) the molecular ionization potential/electron affinity (IP/EA). The Gaussian09 package (Release D.01)47 has been employed for the computation of the molecular structure and electronic properties of the studied systems, employing for that the hybrid functional B3LYP48,49 together with Pople’s (6-31+G*) basis sets. The unknown crystal structures were modeled with the PBE50 exchange-correlation functional and a numerical doubleζ+ polarization atomic orbital basis set (DZP) using the SIESTA code.51 Crystallographic structures available for C4-H, C6-H, C8-H and C12-Br were employed as starting points for the calculation of the rest of crystals. DUSHIN program was used for the calculation of λi through the NM approach.52 t values were calculated at the B3LYP/6-31G* level for the different pairs of molecules extracted from crystal structures (experimental and modeled) using the electron transfer module implemented in J-from-g03 over each dimer.53,54


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More details on Theoretical Considerations can be found in ESI. Materials and Characterization. 1H and

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C NMR spectra were measured on a Bruker

AVANCE 200 and 300MHz spectrometer in CDCl3. UV absorption spectra studies were measured on a UV-2401 PC Shimazu spectrophotometer. The MALDI-TOF mass spectra were recorded on ULTRAFLEX III (MALDI-TOF / TOF) spectrometer from Bruker. All starting materials were acquired from Aldrich company. Column chromatography was carried out on silica gel 0.035-0.070 mm, 60 Å. Crystal X-ray diffraction data collections were done at 296(2) K on a Bruker Kappa Apex II diffractometer using graphite-monochromated Mo-Kα radiation (λ=0.71073 Å). The structure was solved and refined using the Bruker SHELXTL Software Package. Relevant data acquisition and refinement parameters are gathered in Figure S1 and Tables S1, S2, S3 and S4 in the ESI. The crystal structures have been deposited at the CSD with deposition number CCDC 1862008, 1862009 and 1862010.

RESULTS AND DISCUSSION Synthesis and characterization of compounds. The synthesis of N,N-dialkyl-7,7´diazaisoindigo derivatives, C4-H, C6-H, C8-H (Scheme S1) were previously described and the description of the synthesis and characterization of these compound can be found in our previous work.12,13 The synthesis of 5,5´-dibromo 7,7´-diazaisoindigo was carried out by condensation of the C5-bromooxindoles with the corresponding C5-bromoisatins using p-toluenesulfonic acid (PTSA) with toluene affording a red solid compound very insoluble in different solvents (synthetic details are reported in the ESI). The synthesis of 5,5´-dibromo-N,N-didodecyl-7,7´-diazaisoindigo (C12-Br) (Scheme 1) was carried out by alkylation of 5,5-dibromo-7,7´-diazaisoindigo with 1-iodododecane in the


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presence of K2CO3 as base in dimethylformamide (DMF), affording C12-Br with good yields (see Scheme S1, synthetic details are reported in the ESI).55 The alkyl chains are incorporated to improve the solubility for easy purification by column chromatography. The C12-Br was fully characterized by 1H and

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C NMR spectroscopy, UV-visible and mass spectrometry (MALDI-

TOF) (see ESI). Figure S2 shows the UV-vis absorption spectrum of the C12-Br in CH2Cl2 (2,62 x10-5 M), where three absorption bands centered at λmax=292, 334 and 502 nm are observed in the spectrum. Single crystal structure analysis and Solid State Molecular Structure. To achieve a proper understanding of the arrangement of the molecules in solid state, crystallization experiments of C4-H, C6-H, C12-Br were carried out. On the other hand, solvent-free crystals of C4-H and C6-H, were obtained by recrystallization from acetone. The crystals of C4-H, C6-H, were readily grown to form a red plate crystals by evaporating of the acetone solution at room temperature. The C4-H and C6H

both crystallize in the monoclinic P 21/c 1 space group. The single crystal of C8-H was

previously described and it crystallizes in the monoclinic P21/c space group with one half of the molecule in the asymmetric unit and an inversion center located in the middle of the C6-C6’ bond.13 On the other hand, the single crystal of C12-Br was obtained by slow evaporation of tetrahydrofurane solution. During the growth of a C12-Br single crystal we found that C12-Br molecules easily assembled into microwires. The detailed crystallographic data are given in the Supporting Information. As shown in Figure 1, experimental crystals of C4-H, C6-H, C8-H and C12-Br exhibit a herringbone disposition where the 7,7’-diazaisoindigo cores are planar and the plane-to-plane shifts ≤ 3.35 Å.


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Figure 1. Different views of the X-ray crystal structures of (a) C4-H and (b) C12-Br. Charge transport pathways are shown in different color.


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In the ESI Tables S1–S4 summarize the lattice constants of the unit cells along with the πstacking parameters between cofacial neighboring molecules. The calculated crystals are generally consistent with the experimental ones although the plane-to-plane shifts are ~0.1 Å higher in the modeled crystals. The lattice constants and π-stacking parameters of modeled crystals are also collected in Tables S1 – S4 except for the C8-F and C12-F crystals due to computation convergence problems. For an easy comparison, Table 1 shows the plane-to-plane distances between cofacial neighboring molecules. As it can be seen in the Table 1, fluorination does not significantly modify the separation between cofacial neighboring diazaisoindigo cores. Consequently, the lattice constants of CR-H crystals and their CR-F counterparts are also fairly similar. On the contrary, Cl and Br substitution increases plane-to-plane distances at least 0.04 Å with respect to CR-H compounds except for C12-X crystals, where the halogenation has a weak effect on the lattice constants and π-stacking parameters. Crystal packing also depends on the length of the side chains R. Plane-to-plane distances between cofacial CR-H and CR-Cl cores increase with the side-chain length up to -C8H17. Surprisingly, those distances are even shorter for C12-H and C12-Cl compounds than those calculated for C4-H and C4-Cl, respectively. A similar behavior was observed for the brominated derivatives but, in this case, the largest plane-to-plane distance was obtained for C6-Br compound. The plane-to-plane distances between C8-Br and C12-Br cores are similar, or even shorter, than that calculated for the smallest molecule of the series, C 4Br.

Therefore, the halogenation and the side-chain length have important effects, not only at molecular level, but also at supramolecular level. Electronic coupling and, in turn, the charge transfer rate strongly depends on the molecular arrangements in the crystal.


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Table 1. Plane-to-Plane Distances (in Å) between Cofacial Neighboring Molecules. X H F Cl Br a 3.381 3.442 3.461 C4H9 3.398 (3.309) 3.431 (3.345)a 3.426 3.474 3.495 C6H15 3.444 (3.335)a ‒b 3.500 3.464 C8H17 3.395 ‒b 3.407 3.410 (3.274)a C12H25 a Experimental values between parenthesis. b No available crystals due to computation convergence problems. R

Charge Injection. The charge injection barrier is defined as the difference between EHOMO or ELUMO and Φm of the metal injecting the charge (hole or electron) into the organic semiconductor material. If the barrier metal-organic interface is less than 0.3 eV, then charges can be injected into the semiconductor and the current will be controlled by the transport. 56 Figure 2 and Table 2 show the evolution of EHOMO and ELUMO as a function of the side chain length (R) and the type of halogen atom in X position (the shape of HOMO and LUMO of C4-X is shown in Figure S3). Methyl substituent in R position increases both EHOMO and ELUMO in about 0.2 eV with respect to C0-X derivatives, but the elongation of the side chain beyond one carbon atom has a negligible effect on the frontier orbitals energy (< 0.1 eV). On the other hand, halogenation lowers ELUMO in ~0.3 eV and EHOMO ~0.2 eV, while the type of halogen does not have a significant effect on those energies.


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Figure 2. Evolution of a) EHOMO and b) ELUMO as a function of the side chain length (R) and the type of halogen atom in X position.

For hole injection, an ohmic injection contact could be expected for all the studied compounds (CR-X; R = 4, 6, 8 and 12) with a large variety of metal oxide electrodes such as V2O5 (Φm = -7.0 eV), WO3 (Φm = -6.8 eV), MoO3 (Φm = -6.8 eV) and NiO (Φm = -6.3), among others.57,58 Unfortunately, non-ohmic contacts can be expected with the commonly used gold (Φm = -5.1 eV)59,60 and Indium Tin Oxide (ITO) (Φm = -4.7 eV)61 electrodes. Electrodes with high work functions such as Al, Ca and Mg are typically used to inject electrons into LUMO level of the organic semiconductor. Nevertheless, they are highly susceptible to oxygen and can form charge transfer complexes with semiconductors.5 All the studied compounds satisfy the condition of an electron-ohmic contact with calcium (Ca, Φm = 2.9 eV), sodium (Na, Φm = -2.6 eV) and cesium (Cs, Φm = -1.95 eV) electrodes.62 Halogenation reduces the electron-injection barriers and CR-F, CR-Cl and CR-Br compounds could also produce an ohmic contact with magnesium (Mg, Φm = -3.7 eV) electrode.62 In addition, halogenation increases adiabatic electron affinities (AEA) values, facilitating the electron injection, in contrast to hole injection.


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Table 2. Calculated Energy Values (in eV) for the HOMO, LUMO, Band Gap, AEA (Adiabatic Electron Affinity) and AIP (Adiabatic Ionization Potential). CR-X where R Is the Number of Carbon Atoms of the Side Chains and X Corresponds to Hydrogen or Halogen Atom (See Scheme 1).

Compound ELUMO (eV) EHOMO (eV) ∆EL-H (eV) C0-H -3.45 -6.45 3.00 C1-H -3.29 -6.27 2.98 C2-H -3.27 -6.23 2.97 C3-H -3.25 -6.22 2.97 C4-H -3.25 -6.21 2.96 C6-H -3.24 -6.20 2.97 C0-F -3.78 -6.67 2.89 C1-F -3.62 -6.49 2.87 C2-F -3.59 -6.44 2.85 C3-F -3.57 -6.42 2.85 C4-F -3.56 -6.41 2.85 C6-F -3.56 -6.40 2.85 C0-Cl -3.80 -6.67 2.87 C1-Cl -3.64 -6.49 2.85 C2-Cl -3.61 -6.45 2.84 C3-Cl -3.59 -6.43 2.84 C4-Cl -3.59 -6.42 2.84 C6-Cl -3.58 -6.41 2.83 C0-Br -3.80 -6.64 2.84 C1-Br -3.64 -6.47 2.83 C2-Br -3.61 -6.42 2.81 C3-Br -3.59 -6.40 2.81 C4-Br -3.59 -6.40 2.81 C6-Br -3.58 -6.39 2.81

AEA 2.20 2.10 2.10 2.10 2.10 2.10 2.53 2.42 2.41 2.41 2.41 2.41 2.59 2.49 2.47 2.47 2.47 2.48 2.60 2.50 2.49 2.49 2.47 2.48

AIP 7.76 7.54 7.47 7.44 7.42 7.41 7.98 7.75 7.68 7.64 7.62 7.60 7.93 7.70 7.63 7.60 7.58 7.56 7.87 7.66 7.59 7.56 7.54 7.52

Geometrical Changes upon Ionization. An important issue related to the possible applications of 7,7’-diazaisoindigo derivatives in molecular electronics is the analysis of the geometry modifications upon ionization. This is closely related to the internal reorganization energy, which will be analyzed in the next section. To discuss about the molecular geometry, we have selected some relevant molecular parameters related to the conjugation and the planarity of the central molecular core. Figure 3 and Table S5 show the length of the central C2=C3 bond, dC=C, of CR-X compounds in gas phase, as well as the changes in dC=C upon ionization (see Scheme 1 for numbering). Here, it was investigated both the effect of the halogenation and the


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side-chain length, only up to R = -C6H7, due to the geometrical structure of the conjugated core is not very dependent on the hydrocarbon side chains. The effect of the charge in the molecular geometry is much larger upon reduction than oxidation, e.i. dC=C is elongated 0.1 Å after reduction and 0.01-0.02 Å after oxidation showing the higher contribution of C2 and C3 atoms to HOMO than LUMO. The value of the dihedral angle, τ, involving C1-C2-C3-C4 atoms is shown in Figure 4 and Table S5. As a result, although neutral species are fairly planar τ~ 3.5 deg), large torsional deformations are produced upon ionization, except in the case of the bromine derivatives with the largest side-chain lengths (C4-Br and C6-Br) which remain planar.

Figure 3. Bond distance calculated for the C2=C3 central bond (dC=C) of neutral, anionic and cationic compounds. The length of the chain is represented in x-axis.


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The largest deformations were found when going from the neutral to the anionic state, with  ~ 15-20 degrees. In the case of hole injection, while the CR-H compounds undergo large deformations with  ~ 15 degrees, these are smaller for fluorine and chlorine derivatives ( ~ 8-9 degrees).

Figure 4. Dihedral angle involving C1-C2-C3-C4 atoms, in absolute value, (|τ|) of neutral, anionic and cationic compounds. The length of the chain is represented in x-axis.

The geometrical modifications upon ionization are not dependent on the side-chain length with the exception of the mentioned case of the bromine derivatives. In relation to this issue, the loss


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of planarity of the central π-conjugated core when going from ground state (S0) to the first excited electronic state (S1) has been thoroughly investigated in previous works.12,13 Thus, the torsion coordinate through the dihedral angle τ is crucial for enabling access to a conical intersection between potential energy surfaces of S1 and S0 states and favoring nonradiative decay channels in solution.12 However, when diazaisoindigo molecules are confined in the solid state, they undergo intermolecular interactions that block the twisting of the dihedral angle τ and favor the deactivation by fluorescence emission (aggregation-induced enhanced emission, AIEE).13 Thus, the central torsion in 7,7’-diazaisoindigo derivatives is a key geometry parameter which controls not only photophysical properties but also charge transport properties, as it will be described below. Charge Transport. The efficiency of charge transport inside the material was analyzed through the value of the internal reorganization energy and the electronic coupling between neighboring molecules in the crystal. Once known the value of both parameters, it possible to calculate the corresponding charge transfer rate and charge mobility. Here, only the reorganization energies of C4-X compounds were computed due to, as previously discussed, the side chain length does not have a significant effect on the electronic properties of the studied compounds. Table 3 shows the electron and hole internal reorganization energies calculated through the NM and AP approach (more details in ESI). Several conclusions can be drawn about the energies calculated using the AP by comparison with the reported values in literature. Thus, in all cases, i+ is a least 0.2 eV smaller than i‒ indicating a priori that the hole charge transport could be favored with respect to electron transport. In this case, the presence of halogen atoms, specially bromine, could improve the efficiency of hole transport. The value of i+ drops from 0.305 eV for C4-H to 0.226 eV for C4-Br.


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This last value is near the reorganization energies reported for some common p-type semiconductors such as rubrene (0.159 eV, calculated at the B3LYP/6-31G** level),63 some tetrathiofulvenes (0.071−0.234 eV, calculated at the B3LYP/6-31G** level),64 oligoacenes and oligothiophenes (0.077−0.182 eV, calculated at the B3LYP/6-31G** level).65

Table 3. Electron and Hole Internal Reorganization Energies Calculated Through the NM and AP Approaches. Effective Frequencies, ωeff, and Associated Huang-Rhys Factors, Seff, Employed in the Later Calculation of the Charge Transfer Rate Constants. Compound Property Approach C4-H C4-F C4-Cl C4-Br Electrons λi‒ (eV) AP 0.496 0.495 0.489 0.438 ‒ a λi (eV) NM 0.417 0.446 0.446 0.431 ωeff (cm-1)a NM 979.0 1020.3 983.6 1016.0 a Seff NM 3.433 3.523 3.655 3.419 Intramolecular λclass (eV)a NM 0.080 0.098 0.096 0.006 Total λclass (eV)a 0.085 0.103 0.101 0.011 Holes λi+ (eV) AP 0.305 0.284 0.259 0.226 λi+ (eV)a NM 0.244 0.270 0.233 0.203 -1 a ωeff (cm ) NM 853.7 887.0 847.1 896.0 a Seff NM 2.304 2.453 2.217 1.826 Intramolecular λclass (eV)a NM 0.061 0.031 0.026 0.020 a Total λclass (eV) 0.066 0.036 0.031 0.025 a Only frequencies above 250 cm-1.

The reorganization energy values reflect the geometry modifications after ionization process in such a way that large reorganization energy values imply significant geometry modifications. In this sense, it is relevant to analyze the origin of the large values found for the electron reorganization energy. To unravel this issue, the contribution of each vibrational mode to the total reorganization energy is pointed out in Figure 5. The stretching vibration of C 2=C3 bond, in the high energy region, shows the highest contribution to the reorganization energy, instead of the low energy modes associated to the twisting of the dihedral angle τ. Thus, similar


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reorganization energies could be expected for the four C4-X compounds because, in all of them, the central C2=C3 bond distance is elongated in a similar magnitude as shown in Figure 3. Nevertheless, a significant decrease in the value of λi was found for the bromine derivatives (see Table S6). The persistence of this result has also been checked for the series of C 6-X compounds (see Table S7). These discrepancies between brominated derivatives and the rest of the compounds can be explained on the basis of the Huang-Rhys (Si) parameters, dimensionless factors related to the displacement of the equilibrium positions of the nuclei between initial and final state of an electronic transition. Huang-Rhys parameters for hole and electron injection versus the wavenumber of the fundamental modes of the neutral states have been represented in Figure 6 (see more details in ESI). In contrast to the individual contributions of the vibrational modes to the total reorganization, the low energy modes present much higher Si values than the modes in the high energy region. Particularly high Si values were found for the vibrational modes appearing at 8 cm-1 for C4-H, 7 cm-1 for C4-F, and 10 cm-1 for C4-Cl. Interestingly, the atomic displacements of these modes are associated to the twisting of the dihedral angle τ (see ESI, Tables S8-S11). The exception is found for the bromine derivative, for which the Si associated to that twisting is significantly smaller compared to the rest of the compounds. This is consistent with the fact that C4-Br remains planar upon ionization. Low frequency vibrations (< 250 cm-1) were treated classically and were not considered in the calculation of λi through the NM approach (they are included in the λclass parameter of MLJ equation; see Theoretical Considerations in ESI for more details).32,45,46 Also, we must contemplate that the reorganization energy is calculated for an isolated free molecule but, in the crystal, the low energy modes associated to the twisting of the dihedral angle τ could be restricted and their contribution to the


18


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reorganization energy could be overestimated. Thus, the λi values calculated through the NM approach, excluding the contribution of the low energy frequencies, are in general significantly smaller than the counterpart values calculated with the AP approach. Particularly notorious is the case of λi‒ (NM) calculated for C4-H which is 0.08 eV smaller than its counterpart value, λi‒ (AP). In the other extreme, λi‒ (NM) ≈ λi‒ (AP) for C4-Br, in consistence with the small change observed for the dihedral angle τ upon the reduction. Electronic coupling between neighboring molecules in the bulk material is other key parameter that controls the charge transfer constant rate, according to the MLJ equation. The charge transfer integrals, t, were evaluated for all the neighboring molecular pairs from the experimental crystal structures. The different charge transfer pathways considered are shown in Figure 1. Noticeably high values of t were found for the cofacial dimers allowing us to neglect the contribution of the rest of the charge transfer pathways to the global constant rate (see t values calculated of all the charge transfer pathways in Table S12). Thus, for the entire series of modeled crystals, the calculation of t, and the subsequent charge transfer rate constant, was only carried out for the cofacial stacking direction. Table 4 compares the transfer integrals for holes and electrons calculated for cofacial dimers extracted both experimental and modeled crystal structures. t values calculated for the experimental crystals are always higher due to the closer proximity of the molecules with respect to the modeled crystal structure. Differences in the range of 0.01-0.04 eV were found for the t values calculated for both types of crystal structures which can modify the charge transfer rate constant by a factor of up to 3. This shows the extremely high sensitivity of the charge transfer rate constant to the crystal packing.


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Figure 5. Reorganization energy for a) holes (in red) and b) electrons (in blue) (in meV) versus normal modes wavenumber (in cm-1) calculated at the B3LYP/6-31+G* level of theory.


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Figure 6. Huang Rhys parameters for a) holes (in red) and b) electrons (in blue) versus normal modes wavenumber (in cm-1) calculated at the B3LYP/6-31+G* level of theory.

Several interesting conclusions can be extracted from the comparison of the t values calculated for the different modeled crystals. First, the halogenation, with few exceptions, reduces the values of the electronic coupling in the cofacial dimers. In many cases, the smallest values of t


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were found for CR-X compounds with the largest halogen atoms, i.e. -Cl and -Br. Nevertheless, in terms of charge transport rate, these smaller t values are sometimes compensated for the smaller reorganization energies calculated for the chlorinated and brominated derivatives in comparison to the rest of the compounds. Electronic coupling is also sensitive to the side-chain length but, here, no clear tendencies were found. In general, the values of t for electrons (t–) remain fairly constant from C4-X to C6-X, drop for C8-X, and reach their maximum for C12-X. The brominated derivatives, however, reach the maximum value of t– for C8-Br, although these values are more stable than in the case of CR-H and CR-Cl compounds (∆t– ≤ 0.020 eV). All t– values calculated for the studied diazaisoindigo derivatives are in general comparable or higher (≥ 0.110 eV) than the reported values for some common compounds used in molecular electronics such as pentacene (0.131 eV, calculated at the B3LYP/TVP level),66 pentacene trialkylsilylethynyl derivatives (0.001-0.129 eV, calculated at the PW91/TZ2P level),67 dimethylquaterthiophene (0.065 eV, calculated at the B3LYP/6-31G* level),67 tetrafluorotetracyanoquinodimethane derivatives (0.072-0.075 eV, calculated at the B3LYP/6-31G* level),67 antharacenepyromellitic dianhydride (0.086 eV, calculated at the B3LYP/6-31G* level),67 perfluoroarene-modified oligothiophenes (0.013-0.062 eV, calculated at the PW91/TZP level),15 perylene derivatives (0.026-0.064 eV, calculated at the PW91PW91/6-31G* level),68 and diimide derivatives studied by Chen et al. (0.022-0.088 eV, calculated at the PW91PW91/6-31G* level)69 and by Di Donato et al. (0.0740.096 eV, calculated at the B3LYP/3-21G level).70 The values of t for holes (t+) also are sensitive to the side-chain length but the specific tendencies as well depends on the type of halogen atom in X position. For instance, CR-H compounds reach the maximum values of t+ for R = -C4H9 and C8H17. For all the cases, the calculated value of t+ is smaller than the corresponding electron transfer integral. Despite of the smaller values calculated for the hole transfer integral, t+ is


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comparable or higher than the reported value for dimethylquaterthiophene (0.020 eV, calculated at the B3LYP/6-31G* level),71 tetrafluotetracyanoquinodimethane derivatives (0.028-0.041 meV, calculated at the B3LYP/6-31G* level),71 antharacenepyromellitic dianhydride (0.128 eV, at the B3LYP/6-31G* level),71 quaterthiophene and sexithiophene (0.040 and 0.036 eV, respectively, calculated at the PW91/6-31G* level),72 pentacene derivatives with the trialkylsilylethynyl groups (up to 0.082 eV, at the PW91/TZ2P level),67 or perfluoroarene-modified oligothiophenes (0.026−0.053 eV, calculated at the PW91/TZP level),15 among others.

Table 4. Calculated t Values for Cofacial Dimers of CR-X Compounds along with the Estimated Values for kCT (s-1) and μ (cm2 V−1 s−1). Both Experimental Crystal Structures (exp.) and Modeled Crystals (calc.) were Employed in the Calculation of kCT and μ. According to Equation 2, r (Å) is the Distance between the Centroids of two Neighboring Molecules (Considering only the Aromatic Core of each Molecule). r

CR-X a calc

t exp

calc.

exp.

Electrons kCT b calc. exp.

μ calc.

Holes kCT b

t exp.

calc.

exp.

calc.

C4-H 4.902 4.824 0.131 0.158 1.48×1013 2.16×1013 0.689 0.970 0.085 0.101 2.62×1013 0.122 9.02×1012 0.409 0.072 2.81×1013 C4-F 4.841 12 0.123 8.40×10 0.403 0.020 3.09×1012 C4-Cl 4.982 0.124 7.22×1013 3.518 1.9×10-4 4.84×108 C4-Br 5.021 13 13 C6-H 5.037 4.895 0.133 0.144 1.53×10 1.79×10 0.750 0.830 0.054 0.086 1.06×1013 0.129 1.01×1013 0.493 0.037 7.41×1012 C6-F 5.029 12 0.127 8.96×10 0.461 0.013 1.31×1012 C6-Cl 5.155 0.122 6.99×1013 3.633 0.026 9.61×1012 C6-Br 5.186 13 13 C8-H 5.002 4.867 0.119 0.138 1.22×10 1.65×10 0.592 0.754 0.069 0.095 1.73×1013 0.110 6.72×1012 0.339 0.009 6.27×1011 C8-Cl 5.108 13 0.130 7.93×10 4.058 0.020 5.36×1012 C8-Br 5.144 13 0.160 2.21×10 1.049 0.050 9.07×1012 C12-H 4.954 0.159 1.40×1013 0.676 7.9×10-5 4.83×107 C12-Cl 4.988 13 13 C12-Br 5.049 4.762 0.110 0.126 5.68×10 7.45×10 2.800 3.268 0.009 0.052 1.09×1012 a No available crystals for C8-F and C12-F due to computation convergence problems. b Calculated using λi values obtained through the NM approach.

μ exp.

calc.

exp.

3.70×1013

1.218

1.666

1.272 0.149 2.4×10-5 2.68×1013

0.519

1.244

0.363 0.068 0.500 3.28×10

13

0.836

1.501

0.032 0.274 0.431 2.3×10-6 3.63×1013

0.054

1.590

The high values of electronic coupling found for the studied diazaisoindigo derivatives are reflected, as well, in their high charge transfer rate constants (see Table 3). On the basis of the calculated charge transfer rate constants, we can infer that bromination seems to be a good strategy to improve the electron transfer rate. For CR-H compounds, high electron transfer rate


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constants (kCT–) were calculated in the range of 1.22×1013 to 2.21×1013 s-1. These rate constants are increased by 17 – 46% when they are calculated for experimental crystals. In general, the electron transfer rate constants obtained for CR-H derivatives are comparable to the rate constants calculated for typical n-type semiconductor compounds as naphthalene diimides. For instance, a kCT– value of 2.58×1013 s-1 was calculated for Canola and Negri through the π-stacking pathway in the crystal of a fluoro-alkylated naphthalene diimide using a similar methology.45 For brominated diazaisoindigo, still higher kCT– values were calculated within 5.7×1013 – 7.9×1013 s-1 while fluorination and chlorination reduce the electron transport rate. In parallel to the observations made on the behavior of electronic coupling, the side-chain length also has a dissimilar effect on charge transfer rate constants. Nevertheless, it seems that long side chains increase the charge transfer rate constant. For instance, kCT– reaches its maximum value for the hydrogenated derivative C12-H (kCT– = 2.21×1013 s-1), for the chlorinated derivative C12-Cl (kCT– = 1.40×1013 s-1) and for the brominated derivative C8-Br (kCT– = 7.93×1013 s-1). In consonance with the high electron transport rate constants obtained for the hydrogenated and brominated derivatives, high electron mobilities were also calculated for these compounds in the range of 0.59 – 1.05 cm2 V−1 s−1 for CR-H and 2.80 – 4.06 cm2 V−1 s−1 for CR-Br. These mobilities are generally higher than those calculated for different arylene diimides such as perylene diimide derivatives (μ– = 0.007-1.45 cm2 V−1 s−1),38 tetracarboxylic diimide derivatives (μ– = 0.08 and 0.34 cm2 V−1 s−1)73 and core-twisted chlorinated perylene diimide (μ– = 0.004−0.28 cm2 V−1 s−1)74 employing the MLJ formulation. In general, electron mobilities estimated for the studied diazaisoindigo derivatives are also higher than the experimental mobilities reported for common n-type organic semiconductors. For instance, experimental μ– values measured under vacuum for electron-deficient N,N´-substituted arylenediimides and


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perfluoroalkyl oligothiophenes turn out to be in the ranges of 0.02-0.35 and 0.03-1.7 cm2 V−1 s−1, respectively.75 Nevertheless, it must be noted that the comparison between experimental and theoretical mobilities is far from trivial because we have calculated the mobility only for the most favored pathway, i.e. π-stacking direction, and the `realórganic semiconductors exhibit an anisotropic behavior. In addition, impurities, disorders, charge traps, etc., are also factors that affect to the experimental mobility and are difficult to be taken into account in the theoretical model. As concern hole transport, we observed that the rate constant decreases as a function of the side-chain length for CR-H derivatives. Interestingly, kCT+ > kCT– for C4-H where kCT+ reaches its maximum value (2.62×1013 s-1), while kCT+ < kCT– for C12-H. The rate constants of CR-H compounds increases up to ~2.5 times when they are calculated for the experimental crystal (for instance, kCT+ = 3.70×1013 s-1 for C4-H). In general, halogenation reduces the hole transfer rate constants of diazaisoindigo derivatives and does not seem a wise strategy to improve their p-type semiconducting properties. Only fluorination and the specific brominated derivative C6-Br yielded kCT+ values comparable to CR-H derivatives. In literature, comparable or higher hole transfer rate constants can be found for some common p-type organic semiconductors which are, in general, more developed than their n-type counterparts. For instance, using a similar methodology, kCT+ values of 7.76×1012 s-1and 8.15×1013 s-1 has recently been reported for two thienoacene analogs of pentacene.46 Duan et al. have also been reported kCT+ values within 2.38×1013 - 2.85×1013 s-1 for a series of tetrathienoarene derivatives.76 All these rate constants were also calculated for the π-stacking direction. In view of these results, 7,7’-diazaisoindigo and their halogenated derivatives (specially brominated compounds) could have interesting applications as semiconductor materials in the


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field of molecular electronics. In particular, they could have a better performance as n-type semiconductors than as p-type ones. This is a promising result for the family of 7,7’diazaisoindigo compounds since n-type materials are less developed than their p-type counterparts due to different difficulties such as non-efficient electron injection, poor solubility, and difficult processability.46 In addition, although diazaisoindigo is common building blocks of polymers used in the fabrication of OFETs, there are still few research works focused on the study of small 7,7’-diazaisoindigo molecules as semiconductor materials and, in these works, diazaisoindigo derivatives are not generally presented as n-type semiconductors. This work may contribute to open new perspectives on the use of diazaisoindigo in molecular electronics. The synthesis of new 7,7’-diazaisoindigo derivatives will help to expand our knowledge on their electronic properties and their future applications.

CONCLUSIONS We have studied different the semiconducting character of a family of sexteen 7,7’diazaisoindigo derivatives from a theoretical perspective. It has been analyzed the effect of halogen substituents and different-length side chains on the crystal packing and semiconductor electronic properties. For this work, four compounds were synthetized and their X-ray structures were employed as starting points for the calculation of the rest of the crystal structures investigated which are consistent with the experimental ones. Both the experimental and modeled crystals exhibit a herringbone disposition where the diazaisoindigo cores are planar. Halogenation and side chains has important effects on the crystal packing, i.e. chlorination, bromination and long side-chains generally increases plane-to-plane distances.


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High electron charge transfers integrals, t –, were calculated in the π-stacking direction for all the modeled and experimental crystals. The highest values were generally reached for CR-H and CR-Br derivatives with long side chains. In contrast, 7,7’-diazaisoindigo derivatives show high intramolecular reorganization energy for electron transport, λi–, but this energy is significantly reduced when low frequency vibrations (< 250 cm-1) are treated classically and are not considered in the calculation of λi (NM approach). As a result, high electron transfer rate constants and mobilites, kCT– and μ–, in the π-stacking direction were in general calculated for 7,7’-diazaisoindigo derivatives in comparison to those reported in literature. It was observed that the presence of bromine atoms and long side chains could improve the electron mobility but their performance as semiconductors could be limited by electron injection. On the other hand, our calculations suggest that the studied diazaisoindigo derivatives would exhibit poorer performance as p-type semiconductor despite their smaller reorganization energies for hole transport, λi+. Hole transfer rate constants are smaller than those calculated for electron transport due to the weaker electronic couplings for hole transfer.

Conflicts of interest Dr. E. M. Garcia-Frutos has equity ownership in a company which is developing products related to the research here reported.

Acknowledgements This work is supported by the “Ministerio de Economía, Industria y Competitividad” of Spain through Project CTQ2017-84561-P. The authors thank “Centro de Servicios de Informática y Redes de Comunicaciones” (CSIRC, Universidad de Granada, Spain) and “Servicio de


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Supercomputación de la Universidad de Castilla-La Mancha” for providing the computing time. The authors also thank the “Consejería de Economía y Conocimiento, Junta de Andalucía (FQM337) and “Acción 1_PIUJA 2017-18” (Universidad de Jaén, Spain).

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Tuning the Crystal Packing and Semiconductor Electronic Properties

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