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C: Energy Conversion and Storage; Energy and Charge Transport

Theoretical Investigations on Molecular Packing Motifs and Charge Transport Properties of a Family of Trialkylsilylethynyl Modified Pentacenes/Anthradithiophenes Ning-Xi Zhang, Ai-Min Ren, Li Fei Ji, Shou-Feng Zhang, and Jing-Fu Guo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06527 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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The Journal of Physical Chemistry

Theoretical Investigations on Molecular Packing Motifs and Charge Transport Properties of a Family of Trialkylsilylethynyl Modified Pentacenes/Anthradithiophenes

Ning-Xi Zhang a, Ai-Min Ren*a, Li-Fei Ji a, Shou-Feng Zhang a and Jing-Fu Guo b Correspondence to: Ai-Min Ren (E-mail: [email protected]) a

Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun, 130023, China

b

School of Physics, Northeast Normal University, Changchun 130024, P.R. China

1

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Abstract: A family of trialkylsilylethynyl (TAS) functionalized pentacenes (PENs) and anthradithiophenes (ADTs) are of immense interests due to their good solubility and air stability for uses in opto-electronic devices. Different TAS substituted PENs and ADTs would result in different crystal packing motifs and carrier transport properties. Quantum nuclear enabled hopping model combined with molecular dynamic (MD) simulations were performed to investigate the effects of the chemical modifications on the carrier transport properties. The disorder-free hole mobilities show that 6,13-bis(trialkylsilylethynyl)anthradithiophenes (TAS-ADTs)

own

better

intrinsic

hole

transport

behaviors

than

6,13-bis(trialkylsilylethynyl)pentacenes (TAS-PENs). The MD simulations show that in comparison with TAS-PENs, the thermal disorder effects are less significant for TAS-ADTs, this is probably due to the C-H…S hydrogen bonds, which are thought to stabilize the molecules in crystal environments. Furthermore, the syn-TAS-ADTs show more serious non-local electron-phonon interactions than the anti-TAS-ADTs which could be ascribed to the larger S…S overlap between neighboring molecules in the syn-TAS-ADTs. Additionally, symmetry-adapted perturbation theory and Hirshfeld surface analyses were performed to characterize the effects of non-covalent interactions on packing motifs. The results indicate that the C-H … π interaction, the balance relationship between electrostatic, induction, dispersion and exchange repulsion interactions and the C-H … S hydrogen bonds are responsible for the very different crystal packing motifs between these materials.

2


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1. INTRODUCTION Organic field-effect transistors (OFETs) and organic thin-film transistors (OTFTs) are cornerstones in the development of large-area, light-weight, flexible, and low-cost organic electronic devices such as large-area displays, electronic paper, sensors and electronic bar codes.1-4 In the past few decades, organic π-conjugated materials have attracted much attentions as candidates for applications in OFETs and OTFTs because of their outstanding electrical properties.3, 5-6 Among these materials, pentacene is the most representative p-type semiconductor with mobility of 35 cm2 V-1 s-1 for purified pentacene single crystal and field-effect mobility up to 8.62 cm2 V-1 s-1.7-8 It has also been a benchmark for small-molecular organic semiconductors in applications of OTFTs.9-11 Besides efficient charge transport properties, the ultimate success of OTFTs also requires organic semiconductors possess good stability under ambient air and solution-processable for low-cost fabrications such as drop casting and spin casting.12-14 In addition, the device performances depend strongly on the thin-film morphology and crystal structures of organic semiconductors. In particular, a high degree of intermolecular π-orbital overlap is believed to facilitate charge transport.15 However, despite the excellent charge transport properties, the applications of pentacene material are still limited by its poor oxidative stability and solubility.16-17 Furthermore, the edge-to-face herringbone packing in pentacene single crystal does not achieve the optimal intermolecular π-orbital overlap. To ameliorate the stability, solubility and packing arrangement of pentacene, functionalized

pentacene

6,13-bis(triisopropylsilylethynyl)pentacene

(TIPS-PEN)

was

designed and synthesized by Anthony et al.18 The bulky triisopropylsilylethynyl (TIPS) side 3



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groups make it readily soluble in organic solvents. Furthermore, it stacks in a two-dimensional (2D) brick-like arrangement in a solid state with significant π-orbital overlap of the adjacent pentacene rings, yielding excellent carrier transport properties.19 Up to now, its field-effect hole mobility has achieved 6.8 cm2 V-1 s-1 via superhydrophobic micropillar flow-coating method.20 Spurred by these delightful results, researchers applied this trialkylsilylethynyl (TAS) functionalization approach experimentally and theoretically to design and synthesize other fused systems with large and planar π surfaces, such as polycyclic aromatic hydrocarbons,15,

21-24

N-heteroacenes,12,

25-27

S-heteroacenes28-30 and

O-heteroacenes31. Among these materials, two classes of molecules attract our attention: (i) TAS functionalized pentacene (PEN) and (ii) TAS functionalized anthradithiophene (ADT). The structures can be seen in Figure 1. The former shows an effect of TAS group diameter and acene length on the molecular packing arrangement.32 Specifically, if the TAS group diameter is less than (TMS-PEN, TES-PEN) or slightly larger than (TNPS-PEN) half of the acene length, it will lead to a one-dimensional (1D) slipped π-stacking arrangement. For molecules with TAS group diameters closed to half of the acene length, the 2D brick-like π-stacking is preferred in solid state arrangement (TIPS-PEN). As the size of TAS group further increase, the packing motif turns back to the edge-to-face herringbone arrangement (TTMSS-PEN).33 Up to date, the hole mobility found from the 1D π-stacking material TES-PEN was less than 0.001 cm2 V-1 s-1 and no hole carrier transport characteristic was observed from TMS-PEN material.33 For the latter class of materials TAS-ADTs, unlike the functionalized pentacenes, a slight change in the size of the TAS group would have a dramatic effect on the solid state packing arrangement of TAS-ADTs. To be specific, 4


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TMS-ADT with trimethylsilylethynyl (TMS) group packs in a herringbone arrangement with no π-π stacking. Triethylsilylethynyl (TES) derivative TES-ADT adopts a 2D brick-like π-stacking arrangement similar to that of TIPS-PEN. TIPS derivative TIPS-ADT shows a 1D slipped stacking arrangement.28 Consequently, their field effect mobilities were significantly different. TMS-ADT displayed no transistor action. In contrast, TES-ADT formed high-quality thin films, yielding hole mobility of 1.0 cm2 V-1 s-1 28. By now the hole mobility of TES-ADT thin film has achieved 1.8 cm2 V-1 s-1 34. While for TIPS-ADT, the highest hole mobility measured by Anthony et al. was less than 10-4 cm2 V-1 s-1 owing to the amorphous morphology of thin films.28 However, K. Schulze et al. crystallized TIPS-ADT thin films with hole carrier mobilities up to 0.6 cm2 V-1 s-1 using spin coating processes.35 Here come the questions: Why do these similar molecules self-assemble into distinctly different packing motifs and what's the incentive factor? Besides the qualities of thin films, what roles do their intrinsic properties (electronic structures and solid-state packing arrangement) play in the course of charge transport? Previously, Thorley et al. reported that the electron density of the TAS group accounts for the very different solid state packing of two similar systems TIPS-PEN and TES-PEN through quantum-chemistry calculations. They also demonstrated that a balance relationship among three kinds of interactions: TAS-TAS, PEN-PEN and TAS-PEN must be achieved to control the molecular packing.36 Arulmozhiraja et al. evaluated the frontier orbital energies and hole reorganization energies of TMS and TES functionalized PEN and ADT, the results show that compared to their parent molecule PEN and ADT, TAS-substitution would increase hole reorganization energies by about 45 meV and 50 meV, respectively, and lower the 5



LUMO (lowest unoccupied molecular orbital) energies while almost have no effect on HOMO (highest occupied molecular orbital) energies.17 Wu et al. evaluated the intrinsic hole mobility of 0.622 cm2 V-1 s-1 for TIPS-ADT single crystal using Marcus theory. They also found that the hole transport of TIPS-ADT single crystal exhibit remarkable anisotropic behaviors.37 Troisi et al. and Geng et al. computed the non-local electron–phonon couplings of TMS-PEN and TIPS-PEN via molecular dynamic simulations, respectively. They found that the thermal disorders can dramatically affect and substantially modulate the transport properties of this class of materials.38-39 However, some fundamental issues regarding the functionalized PENs and ADTs families are still unclear. For example, systematic comparisons of the inherent hole mobilities between TAS-PENs and TAS-ADTs and the effect of thermal disorder on the transport behaviors for TAS-ADT materials have not been reported yet. Does thermal disorder still play significant roles in transport process for TAS-ADT materials? How do weak intermolecular interactions regulate and control the similar molecules to arrange in different packing motifs, herringbone or slipped π-stacking? In this paper, we systematically discuss the carrier transport properties of TAS-PENs (TMS/TES/TIPS-PEN) and TAS-ADTs (TMS/TES/TIPS-ADT) with the quantum nuclear enabled hopping model.40 Classical molecular dynamic (MD) simulations for all systems were carried out to evaluate the strengths of non-local electron-phonon coupling (or thermal disorder). The effects of thermal disorder on hole transport properties were also studied. In order to in-depth understand the intermolecular interactions and to reveal the different solid state packing of these molecules, Hirshfeld surface analyses were performed to visualize and 6


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characterize the intermolecular interactions in TAS-PENs and TAS-ADTs single crystals.41 Symmetry-adapted perturbation theory (SAPT)42 energy decomposition calculations were carried out to evaluate the intermolecular interaction energies and to gain physical insight into the intermolecular interactions.

Figure 1 The structures of TAS-PEN and TAS-ADT compounds investigated in this study and their corresponding single crystal packing motifs. The nearest-neighboring dimer was colored in red.

2. THEORETICAL METHODS 2.1. Transport mechanisms At present, two kinds of carrier transport mechanisms are widely identified in organic semiconductor materials: the coherent band-like mechanism and the incoherent hopping mechanism.26,

43

In general, at low temperatures, for highly ordered defect-free organic

crystals the band-like mechanism is employed.44 However, at high temperature (room temperature or above) the band-like mechanism is no longer suitable. Furthermore, π-conjugated molecules hold together via weak intermolecular interactions, thus the intermolecular electron couplings or transfer integral (V) are always weak. In this case, the carriers are thought to localize over single molecule.45 In addition, the thermal disorder of V 7



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would further localize the charge carriers.46 Here, all the compounds are π-conjugated organic materials and were investigated at 300 K, so the hopping regime was used to investigate the charge transport characters in this work. In the hopping regime, each carrier hopping event between two neighboring molecules is considered to be a non-adiabatic charge transfer process. In this paper, the charge transfer rate (kct) is expressed by the full quantum all-mode expression derived from Fermi Golden Rule under the harmonic oscillator approximation.40

=

|| & d exp ∆ − 2 + 1 − ℏ ℏ &

!"#

− + 1 !"#

%$Here, = 1/ ℏ"# /() * − 1 contributes the occupation number of the j-th intramolecular

vibrational mode, + is the frequency, kB is the Boltzmann constant, represents the

Huang–Rhys factor measuring the local electron–phonon coupling strength and V is the charge transfer integral (electronic coupling). One can see that the kct is predominantly determined by two important parameters: the local electron–phonon coupling (the overall strength is commonly expressed by reorganization energy λ) and the charge transfer integral V. In organic solids, the intermolecular λ are much smaller than the intramolecular one,6, 25 thus in this study only the intramolecular λ is considered. Here, the intramolecular reorganization energies are evaluated from both the adiabatic potential-energy surfaces (AP)26 and the normal-mode analysis (NM)5-6. The geometry optimizations of the neutral and charged states and the corresponding vibrational frequency analyses of all molecules were carried out using Gaussian 09 program package at the B3LYP/6-31G** level of theory.47 No imaginary frequency was found. The NM analyses were performed by the CTMP program package developed by our group.48 To assess the reduction and oxidation abilities of charged 8


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organic molecules, the adiabatic electron affinity EA(a) and adiabatic ionization potential IP(a) for all molecules were also studied at the B3LYP/6-311G** level (See the S4 part in the ESI for the selection of functionals). Based on the crystal structures download from the Cambridge Structural Database,49 the static V for all molecules studied here were calculated using the site-energy correction method at the PW91/TZ2P level.50-51 It has been proved that the thermal disorder of V is considered to be severe for charge transport in TAS-PEN molecular systems.38-39 Hence, it is necessary to investigate the effect of thermal disorder (non-local electron-phonon coupling) arising from the intermolecular vibrations. In this work, it was described by the time evolution of V (V(t)). MD simulations based on supercells of molecules were performed to achieve this goal using the Forcite module in the Material Studio package. The time-dependent transfer integral V(t) at each snapshot was calculated using the site-energy correction method at the PW91/DZP level (See the ESI for the selection of functional and basis set).50-51 All of the calculations of V were achieved using the Amsterdam density functional (ADF) program package.51-53 Finally, discrete Fourier transformations were performed to obtain the contributions of different phonon vibrations to the fluctuation of V. The discrete Fourier transformations and the time-dependent transfer integral V(t) can be expressed as:39 :/

:/

(;5

(;5

,- = 〈〉 + Re( cos,+( + 45 - + Im( sin,+( + 45 where N is the totality of MD snapshots (in this work N = 2000); ReV and ImV are the vibrational amplitudes of V based on the cosine and sine basis functions. There exists almost no correlation between transfer integral fluctuations of different dimers, thus the phase factor 9



45 can be chosen randomly. In the hopping mechanism, the average carrier mobility µ can be expressed by the Einstein-Smoluchowski equation < =

=

() *

> .54 In this equation, e denotes to the electronic

charge and D is the charge diffusion coefficient. Temperature T is set as 300 K in this work. D could be evaluated by kinetic Monte-Carlo (KMC) simulation employing the Brownian motion process.55 In this paper, 2000 KMC simulations were performed to obtain converged diffusion coefficients D. The magnitudes of mobility in the organic materials also relate to the direction of electric field applied in OFETs, thus the angular resolution anisotropic mobilities were also calculated. Detailed descriptions of all the computational formulas and expressions are given in the ESI. 2.2. Intermolecular interaction Although the configurations of the molecules studied in this paper are similar to each other, their packing motifs are obviously different. It is widely known that organic molecules combine through noncovalent interactions such as π-π interaction, hydrogen bonding and van der Waals forces.56 Therefore, to better understand the structure-packing-property relationships, intermolecular interactions in crystals were investigated. Hirshfeld surface analysis has been receiving much attentions in recent years because the Hirshfeld surface which is constructed according to the distribution of electron calculated from the sum of spherical atomic electron densities would provide unique information for a specific molecular crystal.41 Hirshfeld surface is useful and convenient for visualizing and analyzing intermolecular weak interactions in a molecular crystal. The normalized contact distance (dnorm) defined as ?@ABC =

DE FEGHI FEGHI

+

DJ FJGHI FJGHI

, enables identifying the regions of particular

10


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important intermolecular contacts, where rvdW is the atomic van der Waals radius, de and di are the distances from the Hirshfeld surface to the nearest atoms outside and inside the surface, respectively.57 The shape index (SI) defined as K = arctan P

L

QR SQT Q R QT

U can measure the shape

of the Hirshfeld surface (κ1 and κ2 are principal curvatures).58 The 2D fingerprint plots derived from the Hirshfeld surface could decode and quantify the intermolecular contacts in a crystal lattice by plotting the fraction of points on the surface as a function of (di, de).59 In this work, the dnorm and SI mapped on Hirshfeld surfaces and the corresponding 2D fingerprint plots for all the studied molecular crystals were generated by using CrystalExplorer 3.1.41 For a better understanding of the physical meanings to the intermolecular interactions, the interaction energies and the related energy decompositions of the nearest-neighboring dimers in the studied molecular crystals were calculated by symmetry-adapted perturbation theory (SAPT) at the SCS-SAPT(0)/jun-cc-pvdz level (SCS represents Spin-component scaling). The dimers were all extracted from the crystal structures. In the SAPT approach, the interaction energies are decomposed into four physically meaningful terms: electrostatic, induction, dispersion and exchange repulsion.42 SAPT calculations were achieved using the PSI4 program package.60 In addition, it is also necessary to point out that only isomerically pure anti-TAS-ADTs were detailedly considered and discussed in this main text, because the packing arrangements of anti isomer are more ordered and stable.61 The calculations and analyses about the syn-TAS-ADT cases are given in the S3 part of the ESI. To obtain isomerically pure anti-TAS-ADT single crystals, every TAS-ADT crystal structure download from the Cambridge Structural Database49 was post processed following two principles 1) keep the 11



symmetry of the crystal structure 2) minimize the sulfur contact between neighboring molecules. Detailed discussions on the second point are reported in Ref. 61. 3. RESULTS AND DISCUSSION 3.1. Frontier molecular orbitals, carrier injection and ionization potential At first, we investigate the structure-property relations at single molecular scale. After optimization, all the neutral and ionic molecular geometries exhibit rigid and planar skeletons. The relative values of HOMO and LUMO energies can qualitatively indicate the hole and electron injection abilities. IP(a) and EA(a) relate to the molecular reduction and oxidation capacities. Hence, it is essential to examine these intrinsic properties of the studied molecules. The calculated results are listed in Table 1. We only list the main conclusions here. Detailed and comprehensive discussions can be seen from the supporting information. Firstly, all these molecules are classified as potential p-type organic semiconductor materials with good antioxidation capacity due to their low hole injection energy barriers25 (Au electrode with 5.1 eV work function is taken as the source and drain electrodes in our case) and high IP(a)s. The high electron injection energy barriers and the very small EA(a) values determine their poor n-type behavior and the instability of their radical anions in ambient atmosphere. Secondly, TAS-ADTs are better in photostability and antioxidation capacity because of their wider HOMO-LUMO energy gaps and higher IP(a)s than TAS-PENs. Thirdly, molecules in one series possess parallel charge injection abilities, photostability, reduction and oxidation capacity owing to the similar HOMO/LUMO levels, IP(a)s and EA(a)s.

12


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Table 1 The calculated HOMO/LUMO energies (EHOMO/ELUMO in eV), energy gaps (Eg in eV), hole/electron reorganization energies (λh/λe in meV) evaluated by AP and NM approaches, adiabatic ionization potentials (IP(a) in eV) and adiabatic electron affinities (EA(a) in eV) at the B3LYP/6-311G** level for all molecules studied in this work.

TMS-PEN

TES-PEN

EHOMO

ELUMO

Eg

-4.88

-2.98

1.90

b

b

b

TMS-ADT

TES-ADT

λe(AP)

IP(a)

EA(a)

145.01

145.34

206.45

5.70

1.66

b

(-3.01 )

(1.89 )

(139 )

-4.89

-2.98

1.91

141.61

b

b

b

(1.89 )

(136 )

(-2.99 )

-4.89

-2.99

1.90

138.65

(-3.01a)

(1.89a)

(139a)

-5.03

-2.69

2.34

152.34

b

b

b

(5.95 ) 142.12

204.76

(-2.73 )

(2.31 )

(142 )

-5.03

-2.69

2.34

149.98

b

b

b

(2.31 )

(139 )

-5.04

-2.70

2.34

142.97

(-2.70a)

(2.34a)

(140a)

1.70

(5.91 ) 139.29

153.03

203.17

5.68

1.73

(197a)

(5.91a)

(2.04a)

248.04

5.90

1.38

b

(6.13 ) 148.38

246.17

b

(-5.04a)

5.69 b

b

(-5.05 )

(-2.72 )

b

b

(-4.90a)

(-5.03 ) TIPS-ADT

λh (NM)

(-4.89 )

(-4.88 ) TIPS-PEN

λh (AP)

5.88

1.41

b

(6.09 ) 144.18

a

Values obtained at the B3LYP/6-31++G**//B3LYP/6-31G* level of theory in Ref. 37.

b

Values obtained at the B3LYP/D95(d)+ level of theory in Ref. 17.

241.49

5.86

1.45

(232a)

(6.05a)

(1.72a)

3.2. Reorganization energy The reorganization energy λ originated from the geometrical relaxation is important to govern the charge transfer rate kct. Here, the intramolecular λ were calculated both by AP and NM methods at the B3LYP/6-311g(d, p) level. The values of λ (λh for hole and λe for electron) are collected in Table 1. The results show that there is little difference between the values evaluated by the AP and NM methods, indicating that the harmonic oscillator approximation is applicative in this system. Furthermore, the λes of TAS-PEN and TAS-ADT are much larger than their λhs. Combined with the analyses in the previous section, one can immediately conclude that all these molecules are more suitable for the applications as hole transport materials. This phenomenon has been observed from both experiments and theoretical 13



computations.20, 37, 62-63 As a result, we will only focus on the λhs in the following. To better comprehend the origin of the λh, the contribution of each normal mode to the λhs for these molecules are calculated and plotted in Figure 2. One can find that in the same series, the contributions in high frequency parts (> 1200 cm-1) are very similar, and different substituent groups mainly influence the normal modes in low frequency parts spread over 0-1000 cm-1, while replacing with thiophene rings obviously changes the contributions in high frequency parts within the range of 1200-1600 cm-1, which arise from the C-C stretching modes of the conjugated skeleton. To identify each normal mode more clearly, atoms in molecules and normal modes are labeled and the displacement vectors of each normal mode are provided in Figure S1. The values of λhs listed in Table 1 show that from TMS-PEN (TMS-ADT) to TIPS-PEN (TIPS-ADT), the λhs decrease gradually with the substituent groups increase. It is mainly attributed to the remarkable decreasing trend of contributions of normal modes 'b' and 'h' at ~750-900 cm-1, which arise from the Si-Csp stretching vibrations and the alkyl wagging vibrations (detailed analyses can be seen from the S2 part in the ESI). From Table 1, we can also see that in comparison with TAS-PENs, the corresponding molecules of TAS-ADTs with thiophene rings possess larger λhs by about 5-8 meV. Through summing the normal mode contributions, we find that the normal mode contributions come from high frequency part (1000-1600 cm-1) of TAS-ADTs decrease compared to that in TAS-PENs. Thus the increments of λhs for TAS-ADTs should attribute to the modes in the low frequency region (

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