Electric Field Promotes Pentacene Dimerization in Thin Film

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Electric Field Promotes Pentacene Dimerization in Thin Film Transistors Micaela Matta, Fabio Biscarini, and Francesco Zerbetto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03405 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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The Journal of Physical Chemistry

Electric Field Promotes Pentacene Dimerization in Thin Film Transistors Micaela Matta1†*, Fabio Biscarini,2 Francesco Zerbetto1 1 Dipartimento di Chimica “G. Ciamician”, Università di Bologna, via F. Selmi 2, 40126 Bologna, Italy 2 Dipartimento di Scienze della Vita, Università degli Studi di Modena e Reggio Emilia, Via Campi 103, 41125 Modena, Italy

Present Addresses † Institut des Sciences Moléculaires, Université de Bordeaux, Bâtiment A12, 351 Cours de la Libération, 33405 Talence, France Corresponding Author Micaela Matta, Institut des Sciences Moléculaires, Université de Bordeaux, Bâtiment A12, 351 Cours de la Libération, 33405 Talence, France Email: [email protected] Phone: +33-0540002888

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ABSTRACT

Density functional theory (DFT) calculations were used to assess the effect of electric fields of varying magnitudes and directions on the molecular structure of pentacene and other acenes. The aim is to understand the response of acenes in organic field effect transistors, specifically the structure of the first monolayer(s) deposited on the gate dielectric, where the transversal electric field and the charge carrier density are largest and charge transport occurs. Pentacene cycloaddition can be enhanced by the application of electric fields oriented along the direction of the forming bonds. Dimerization is likely to occur in low-density, disordered domains, such as grain boundaries or terrace edges. Together with other factors, dimerization could affect device performance leading to an irreversible decrease of mobility due to the creation of new trap states.

INTRODUCTION Pentacene is a polycyclic aromatic hydrocarbon that belongs to the family of acenes. It consists of five linearly arranged and fused phenyl rings. The extended π-electron conjugation is able to sustain the substantial charge carrier mobility required in organic field effect transistors, OFETs. However, its reactivity might hamper its applications in molecular electronics.1,2 The reactivity of acenes increases proportionally to their length,3,4 yielding photo-oxydation5,6 or cycloaddition products. The formation of dimers is more exothermic in higher acenes7 and, concomitantly, the polymerization process becomes more favored. Indeed, starting from heptacene, dimerization goes further than the dimer, yielding acene-based polymers.8 Dimerization – either thermally or photochemically activated – appears in the solution chemistry of pentacene and its substituted derivatives.9-11 Coppo et al.12 observed the photodimerization of a substituted pentacene in solution, and in amorphous and semicrystalline thin films upon exposure to sunlight. The degree of crystallinity of the sample was 2 ACS Paragon Plus Environment

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proportional to its photostability. In tightly packed domains, dimerization (and also oxidation) was prevented. At grain boundaries, where molecules have fewer neighbors, there was a higher probability of degradation. According to Woodward-Hoffmann theory, pentacene dimerization is a [4+4] (concerted) cycloaddition that is thermally forbidden and photochemically allowed. The extended πelectron conjugation, though, blurs the accuracy of this definition. The thermal reaction pathway was studied by Density Functional Theory (DFT) calculations, which showed the presence of two transition states and a biradical intermediate.13 The symmetric 3,3’ "butterfly dimer" (5P3,3’ in Figure 1) obtained via cycloaddition of the central rings was found to be the favored product, while the asymmetric 2,3’ and 1,3’ dimers are less stable.

Figure 1. Structures of all dimers studied in this work.



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The reaction coordinate is characterized by the presence of two transition states of similar energy, TS1 and TS2, for the formation of each one of the sp3-sp3 carbon bonds, which are separated by a local minimum or intermediate, Min. The formation of TS1 is the highest barrier, which thus determines the reaction rate. In more detail, the preferred reaction path starts by syn addition in the π-π van der Waals complex, which represents the most stable configuration for two interacting pentacene molecules (-18.4 kcal mol-1). The first transition state, TS1, is 17.3 kcal mol-1 above the π-π van der Waals complex. This biradicaloid species has an incipient bond length of 2.04 Å. TS1 then evolves towards an intermediate, Min, which is a local minimum (-8.2 kcal mol-1). The newly formed C-C bond is 1.62 Å long, while the associated dihedral C-C-C’-C’ is 52°. The reaction then terminates by crossing a second transition state, TS2, where the second C-C bond is formed. The final 3,3’ dimer is 34.5 kcal mol-1 more stable than its isolated precursors. All biradical species involved in the reaction path are singlets, as predicted by theory.14 The results of the DFT calculations imply a reaction rate that can be estimated, using transition state theory, as ~110 s-1 at room temperature. Inside the crystalline domains of pentacene films this rate is hardly achieved, as molecules are tightly packed in a herringbone fashion and the π-π cofacial configuration (van der Waals dimer) is not stable. To further complicate the chemistry of pentacene dimerization, one can consider the presence of an electric field, as in a thin film transistor subject to a bias voltage. It is fair to say that the effect of electric fields on reactivity has received relatively small attention, also because of experimental difficulties.15-20 The electrostatic potential associated with an electric field changes the energies of the molecular orbitals by modifying the energy of the reference “vacuum” level.21 The field also directly interacts with (and changes the energy of) molecular orbitals, proportionally to their dipole moment. From the reactivity point of view, changes of molecular orbital energies can make allowed previously symmetry forbidden reaction pathways. Quantum chemical calculations, based on Density Functional


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Theory at M06-2X/6-31G* level,22 can be used to assess the effect of electric fields on the orbitals and the process of dimerization. The aim of the present investigation is to determine the energetics of the thermal cycloaddition pathway of pentacene under the influence of external electric fields of different intensities and directions, namely 1, 2 or 3 V/nm along each Cartesian axis. In order to generalize our findings, also tetracene and anthracene dimers, together with pentacene asymmetric dimers and substituted pentacenes have been investigated. The structures of all studied molecules are reported in Figure 1.

From the point of view of molecular electronics, the calculations are relevant to the first monolayer(s) of acene deposited on the gate dielectric of a bottom-gate field effect transistor, which is where the transversal field is largest and charge transport occurs. In general, charge transport pathways are sensitive to defects of the first monolayers, which deplete the population of charge carriers.21 The lack of transport in layers higher than the third is due to the screening of the gate voltage by the organic molecules.23,24 In a linear approximation, for a gate voltage of 3 V that decays to zero within 1 nm length scale, the associated field is of the order of the largest one investigated in this work.

THEORETICAL METHODS DFT simulations were performed at level M06-2X/6-31G*22 using the Gaussian09 software suite.25 For the calculation of biradical intermediates Min, TS1 and TS2, the unrestricted form UM06-2X/631G* was used instead. All intermediates were fully optimized without any constraints, and frequency calculations were performed to confirm that the located stationary point corresponds to a minimum in the potential energy surface. The starting structures were generated using data in ref. 13 as a reference; finally, the energies of the stationary points were compared to the results in ref. 13. The transition states were first optimized using the “Opt=TS” keyword of the Gaussian suite of programs and a frequency 5 ACS Paragon Plus Environment


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calculation was carried out to confirm their nature, then an QST3 calculation was performed using the previously characterized minima as reactants and products, and the previously optimized transition state as a starting point.

RESULTS AND DISCUSSION Table 1 illustrates the effect of nine electric fields on the energies of pentacene HOMO and LUMO (see also Supporting Information, Figures S6-S8).

Pentacene and its dimer belong to the same

symmetry group, D2h. The irreducible representations of their HOMO and LUMO orbitals are, respectively, B2g and B1u for pentacene and Ag and Au for the 3,3’ dimer. When the electric field is oriented along the long molecular axis, pentacene LUMO interacts directly with the field and HOMOLUMO gap tends to decrease. If symmetry does not allow the field to interact directly with the molecular orbitals, their energies remain unaffected, as in the case of an electric field along the dimer’s short axis, Z. The effect of an electric field along X or Y on the dimer orbitals is to decrease their energy gap by stabilizing the LUMO while destabilizing the HOMO.

Table 1. Energies (eV) of HOMO, LUMO and associated HOMO-LUMO gap values for two individual pentacene molecules and their van der Waals complex as a function of the electric field (V/nm). The structures and orientation of the individual molecules were taken from the π-π van der Waals complex. The values in bracket refer to the second molecule in the complex (non-equivalent with respect to the applied electric field).


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field/ orientation (V/nm)a 0

X

Y

Z

pentacene HOMO

LUMO

-5.70

-1.73

vdW complex

3,3’ dimer

Gap HOMO LUMO Gap

HOMO LUMO Gap

3.97

-5.48

-1.67

3.80

-6.75

-0.36

6.39

1

-5.57 (-5.69) -1.63 (-1.75) 3.94

-5.41

-1.65

3.75

-6.62

-0.46

6.16

2

-5.41 (-5.66) -1.56 (-1.80) 3.86

-5.29

-1.66

3.63

-6.41

-0.65

5.76

3

-5.23 (-5.60) -1.51 (-1.88) 3.72

-5.16

-1.69

3.47

-6.19

-0.87

5.32

1

-5.82 (-5.52) -1.85 (-1.55) 3.97

-5.38

-1.71

3.67

-6.68

-0.45

6.23

2

-5.95 (-5.34) -1.98 (-1.38) 3.97

-5.25

-1.77

3.47

-6.54

-0.60

5.93

3

-6.07 (-5.17) -2.10 (-1.20) 3.97

-5.11

-1.85

3.26

-6.37

-0.78

5.59

1

-5.85 (-5.69) -1.88 (-1.72) 3.97

-5.53

-1.77

3.76

-6.75

-0.36

6.39

2

-6.00 (-5.69) -2.03 (-1.72) 3.97

-5.56

-1.88

3.68

-6.75

-0.36

6.39

3

-6.07 (-5.69) -2.10 (-1.72) 3.97

-5.59

-1.99

3.59

-6.75

-0.36

6.38

a

X corresponds to the pentacene long axis, Y to the direction of the sp3 bonds, and Z to the short axis.

We investigated the reaction pathway for the formation of the symmetric 3,3’ “butterfly dimer” in the presence of different electric fields. The energies of the reaction intermediates for electric fields of different magnitude and directions are reported in Table 2. If the field is oriented along X, the long molecular axis, the stabilization is greater. The largest energy variation occurs for a single molecule of pentacene. As the reaction proceeds, the number of unsaturated, i.e. polarizable atoms, decreases and accordingly does the stabilization induced by the field applied along X. The stabilization energy is also quite high for the TS2 intermediate, as its C-C-C’-C’ dihedral is lower and the incipient dimer is more parallel to the X axis. Instead, If the field is oriented along the Y direction, corresponding to the new C(sp3)-C(sp3) bonds, the energies of the intermediates TS1, TS2 and Min are lowered, while the van der Waals dimer is destabilized. It can therefore be expected that the rate of reaction will be faster for this orientation. Finally, the field applied along the Z direction stabilizes the intermediates TS1, TS2, 7 ACS Paragon Plus Environment


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Min and the final product, having one or two (incipient) sigma bonds; in particular, the most asymmetric ones (TS1 and Min). By inspection of Table 2 and Figure 2, it appears that the stabilization effect of the electric field is related to the molecular polarizability along the direction of the applied field: the higher the induced dipole, the greater the effect on the energy.

Figure 2. Dipole moments (in Debye units) of the different reaction intermediates and product of pentacene dimerization at 3,3’. calculated at level (U)M06-2X/6-31G*, in kcal mol-1.

Table 2.

field/orientation (V/nm)a

X

Y

Z

pentacene

vdW complex

TS1

Min

TS2

3,3' dimer

1

-1.23

-0.38

-0.94

-0.33

-1.24

-0.81

2

-5.12

-3.74

-3.88

-3.28

-4.71

-3.24

3

-11.63

-9.37

-8.86

-7.73

-9.85

-7.32

1

-0.15

0.50

-0.41

0.27

-0.61

-0.37

2

-0.78

-0.35

-1.60

-0.70

-1.72

-1.50

3

-1.80

-1.81

-3.01

-2.18

-3.55

-3.45

1

-0.48

0.26

-0.56

-0.04

-0.64

-0.81


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2

-2.11

-1.18

-2.39

-2.17

-2.09

-1.82

3

-3.31

-3.57

-5.50

-5.41

-4.27

-4.09

a


for the dimerization path starting from the π-π van der Waals

Table 3.

complex, calculated at level (U)M06-2X/6-31G*, in kcalmol-1. field/ orientation (V/nm)a

TS1

Min

TS2

3,3' dimer

0

17.23

10.19

18.24

-16.15

1

16.68

10.25

17.39

-16.57

2

17.09

10.66

17.28

-15.65

3

17.75

11.84

17.77

-14.10

1

16.32

9.96

17.13

-17.02

2

15.99

9.84

16.87

-17.30

3

16.04

9.83

16.51

-17.78

1

16.42

9.90

17.34

-17.21

2

16.02

9.20

17.33

-16.79

3

15.31

8.36

17.55

-16.67

X

Y

Z

a


Table 3 and Figure 3 present the energies of the stationary points of the reaction path that leads to the formation of the 3,3’ dimer in the presence of different electric fields. The energy variations of the stationary points are non-trivial. Some systems are stabilized by the presence of a field in one direction and destabilized when the orientation is in a different direction. In some cases, a plateau is reached at 2 V/nm, such as for TS1 and Min for the field oriented along the direction of the forming bonds. U9 ACS Paragon Plus Environment


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shaped behavior of the energy is also observed, for instance for TS2, if the field is oriented along the long molecular axis. The increase of the field does not necessarily correspond to a linear variation of the energy of the stationary point. The rate determining transition state, TS1, can be lowered by almost 2 kcal mol-1. In terms of kinetic constant, for an Arrhenius-type mechanism, it entails an increase of the rate of 20 times.

Figure 3. Pentacene dimerization pathway in the absence (black) and the presence of electric fields of 3 V/nm. X (red) corresponds to the long molecular axis, Y (green) to the direction of the sp3 bonds, and Z (blue) to the short molecular axis. The numbers correspond to the relative energies (with ZPVE) of each species compared to the van der Waals complex, in kcal mol-1, in the same color code.


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The orientation of pentacene molecules in crystalline domains of thin films forming the semiconductor layer in OFETs is such that the long molecular axis is almost normal to the film plane. The tilt angle, defined as the angle between the long molecular axis and the normal to the film surface, can vary from almost zero to 15.4° according to the crystalline form, the number of molecular layers, and the dielectric.26 With respect to our calculations, we can identify the X axis to the OFET normal axis and the Y and Z directions as lying in the thin film plane, as shown in the upper left of Figure 3. In our reference system, therefore, the electric field directed along X would correspond to the electric field generated by the gate electrode, while the source-drain bias effect would be identifiable with an electric field directed along Y and Z. It is important to point out that the orientation we just described is relative to the crystalline domains. Molecules located at grain boundaries possess more degrees of freedom to reorient according to several factors, such as the orientation of neighboring molecules, available space, temperature and also the direction of the local electric field.

In addition to the “butterfly” dimer, a series of acene dimers was also investigated and their dimerization energies were calculated with respect to the applied electric field. Pentacene asymmetric cycloaddition products (2,2’ and 2,3’) and the corresponding tetraphenyl substituted dimers at the central ring (3,3’_4Ph, 2,3’_4Ph and 2,2’_4Ph) were considered. Also tetracene (syn and anti products) and anthracene (2,2’ dimer) were studied, since their dimerization reaction, while less favored than that of pentacene, is still slightly exothermic. Figure 4 shows dimerization energies (ZPVE corrected) for the whole series for electric fields directed, as previously, parallel the three molecular symmetry axes. In general, dimers having similar polarizabilities (see Figure S9), symmetry and substituents behave in a similar way. The effect of the applied field is more pronounced for molecules with higher polarizability (see also Figure 5). In the case of an electric field along X (long axis of the dimers), only unsubstituted pentacene dimers are stabilized, while the cycloaddition of the other acenes is less 11 ACS Paragon Plus Environment


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exothermic. The effect of an electric field along Y has a destabilizing effect for all acenes, and this is more pronounced for unsubstituted pentacenes, which are less polarizable along this direction. Finally, an electric field along Z can make the cycloaddition more exothermic for all species, and markedly so for 5P2,2’_4Ph and the previously investigated “butterfly” dimer.

Figure 4. Dimerization energies (ZPVE corrected) with electric fields for an acene series including: anthracene (3P2,2’, smaller graphs on the right), tetracene syn and anti dimers (4Panti and 4Psyn, with circles), pentacene dimers (5P3,3’, 5P2,3’ and 5P2,2’, with squares) and its tetraphenyl substituted analogues (5P3,3’_4Ph, 5P2,3’_4Ph and 5P2,2’_4Ph, with diamonds). For ∆G values, see Figure S8 in


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the Supporting Information file. Dimerization energies were calculated with respect to the corresponding acene under the same applied electric field.

Figure 5. Changes in dimerization energies with electric fields for an acene series including: anthracene (3P2,2’), tetracene syn and anti dimers (4Panti and 4Psyn, with circles), pentacene dimers (5P3,3’, 5P2,3’



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and 5P2,2’, with squares) and its tetraphenyl substituted analogues (5P3,3’_4Ph , 5P2,3’_4Ph and 5P2,2’_4Ph, with diamonds).

CONCLUSIONS In conclusion, acene cycloaddition can be enhanced by the application of electric fields as in the case of OFETs. The results obtained for shorter acenes confirm the generality of the observed trends. In the case of substituted pentacenes, it is shown that the stabilizing effect of the electric field on the dimer form can be even higher than for the unsubstituted molecule. The electric field along X, which would originate from the gate bias in our reference system, seems to have a minor role in this degradation path, while the source-drain bias could be identified as responsible for enhancing dimerization. As previously suggested,12 pentacene dimerization may occur in disordered domains and at grain boundaries or at terraces of polycrystalline thin films. Dimerization may also be favored by strong local electric fields generated by the presence of adsorbed ions, charged defects or dielectric surfaces. Dimerization in pentacene thin film transistors could affect the device performances in different ways. Firstly, it would result in an irreversible loss of possible charge carriers (since the dimer would not be able to participate in the charge transport). Secondly, the HOMO of the dimer would act as a trap state, thus lowering the charge mobility. Finally, dimerization at grain boundaries or interfaces would result in a loss of crystallinity, which could further enhance the reactivity and allow oxidation of newly exposed molecules. In a previous study, such deterioration of the OFET performances was detected even in the absence of oxygen and light.27 Once again, this study points out the importance of morphology control in semi-crystalline films for electronic devices. 28-30 The same approach used to prevent oxidation, improve crystallinity and limit defects at the semiconductor-dielectric interface can also prevent unwanted cycloaddition reactions in acene-based devices. 14 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information available: electrostatic potential maps; HOMO and LUMO orbitals; reaction pathways for electric fields of 1 and 2 V/nm; details of the calculations including geometry data, polarizabilities, dipole moments, absolute energies, ZPVE corrections, Gibbs free energies and spin contamination () values. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was funded by the EU 7th Framework Programme [FP7/2007–2013] under Grant Agreement No. 280772, “Implantable Organic Nanoelectronics (I-ONE-FP7)” project.

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(4) Bettinger, H. F. ; C. Tönshoff, C. The longest acenes. Chem. Rec., 2015, 15, 364–369. (5) Reddy, A. R.; Bendikov, M. Diels-Alder reaction of acenes with singlet and triplet oxygen -theoretical study of two-state reactivity. Chem. Commun. (Camb). 2006, 11, 1179–1181. (6) Northrop, B. H.; Houk, K. N.; Maliakal, A. Photostability of pentacene and 6,13-disubstituted pentacene derivatives: a theoretical and experimental mechanistic study. Photochem. Photobiol. Sci. 2008, 7, 1463–1468. (7) Zade, S. S.; Bendikov, M. Reactivity of acenes: mechanisms and dependence on acene length. J. Phys. Org. Chem. 2012, 25, 452–461. (8) Zamoshchik, N.; Zade, S. S.; Bendikov, M. Formation of acene-based polymers: mechanistic computational study. J. Org. Chem. 2013, 78, 10058–10068. (9) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T. Photochemical stability of pentacene and a substituted pentacene in solution and in thin films. Chem. Mater. 2004, 16, 4980– 4986. (10) Li, S.; Li, Z.; Nakajima, K.; Kanno, K.; Takahashi, T. Double homologation method for substituted soluble pentacenes and dimerization behaviours of pentacenes. Chem. Asian J. 2009, 4, 294–301. (11) Chan, S. H.; Lee, H. K.; Wang, Y. M.; Fu, N. Y.; Chen, X. M.; Cai, Z. W.; Wong, H. N. C. A soluble

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tetrakis(trimethylsilyl)pentacene. Chem. Commun. (Cambridge) 2005, 1, 66–68. (12) Coppo, P.; Yeates, S. G. Shining light on a pentacene derivative: the role of photoinduced cycloadditions. Adv. Mater. 2005, 17, 3001–3005. 16 ACS Paragon Plus Environment

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Electric Field Promotes Pentacene Dimerization in Thin Film

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