Improving the Stability of Organic Semiconductors: Distortion Energy

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Improving the Stability of Organic Semiconductors: Activation Strain Energy vs. Aromaticity in Substituted Bistetracene Simil Thomas, Jack Ly, Lei Zhang, Alejandro L. Briseno, and Jean-Luc Bredas Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02552 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 19, 2016

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Chemistry of Materials

Improving the Stability of Organic Semiconductors: Distortion Energy vs. Aromaticity in Substituted Bistetracene

Simil Thomas†, Jack Ly‡, Lei Zhang‡, Alejandro L. Briseno‡, and Jean-Luc Bredas†,*

†

Laboratory for Computational and Theoretical Chemistry of Advanced Materials, Solar and

Photovoltaics Engineering Research Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia. ‡

Department of Polymer Science & Engineering, Conte Polymer Research Center, University of

Massachusetts, Amherst, Massachusetts 01003, United States.

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Abstract Polycyclic aromatic hydrocarbons (PAHs) have been widely explored as molecular semiconductors in organic electronic devices such as field-effect transistors or solar cells. However, their tendency to undergo photooxidation is a primary limitation to their practical applications. Bistetracene derivatives have recently been demonstrated to possess much larger photooxidation stability than the widely investigated pentacene and rubrene, while maintaining high charge-carrier mobilities. Here, using several levels of density functional theory, we identify the origin of the increased stability of bistetracene with respect to molecular oxygen by systematically investigating the [4+2] cycloaddition (Diels-Alder) photooxidation reaction mechanism. Importantly, our computational results indicate that endoperoxide formation in bis(2-(trimethylsilyl)ethynyl) bistetracene (BT) occurs not on the ring with least aromaticity, but rather on the ring with smallest distortion energy. This feature was subsequently confirmed by experimental NMR analyses. The oxidation activation barriers of bistetracene, pentacene, and rubrene are found to be 17.7, 13.6, and 14.4 kcal/mol, respectively, in agreement with the observed order of stability of these molecules with respect to oxidation reactions in solution. In the cases of BT and pentacene, the rates of electron transfer to create charged species (PAH+ and O2-) are at least two orders of magnitude lower than that of the charge recombination process (back to PAH and O2); for rubrene, both of these processes are calculated to be of the same order of magnitude, in agreement with experimental electron paramagnetic resonance spectroscopy observations.


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1. Introduction Molecular semiconductors such as pentacene and rubrene have garnered considerable interest as model systems since they serve as active materials in organic photovoltaic (OPV) devices1, organic field-effect transistors (OFET)2-5, Hall effect devices, etc.6-7 Moreover, hole mobilities greater than 5 cm2/Vs2-3 in pentacene and 15 cm2/Vs4-5,

8

in rubrene have been reported in

transistor-type configurations; these values are significantly larger than that in hydrogenated amorphous silicon transistors (1 cm2/Vs), a reference material for thin-film transistors.9-10

However, when exposed to light and oxygen in solution, pentacene and rubrene quickly degrade via a photooxidation reaction to produce transannular endoperoxides within a few minutes11-12. The increased reactivity of the central ring of polyacenes towards Diels-Alder reactions with molecular oxygen in the presence of light has been attributed to a reduction in local aromaticity of the individual rings towards the central ring.13-14 Since the highest occupied molecular orbital (HOMO) is stabilized by the oxidation of pentacene (due to reduced conjugation length), the generated endoperoxide molecule cannot easily exchange a hole with an adjacent pentacene, which represents a barrier for hole transport. Also, the endoperoxide structure is nonplanar and creates a local lattice deformation that acts as a scattering center. Hence, charge mobility in the device decreases with an increase in the number of oxidized species15-16. Similar features have been observed in rubrene.12 Understanding the origin of the factors preventing oxidative degradation can thus represent a major step towards the rational design of chemically stable organic semiconductors. 12, 17-20



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Polycyclic aromatic hydrocarbons (PAHs) react photochemically with O2 via two mechanistic pathways: 1) In the electron transfer mechanism (referred to as type I), a photoexcited PAH molecule transfers an electron to a neighboring O2 to create a PAH cation (PAH+) and an oxygen anion (O2-); these two charged species subsequently react to form an endoperoxide.21 2) In the energy transfer mechanism (referred to as type II), excited PAHs undergo intersystem crossing (ISC) from an excited singlet state (S1) to the triplet state (T1), which has a longer lifetime. If the triplet energy of the PAH is greater than that of the singlet-triplet gap (∆ST) of O2, the PAH molecule can transfer its energy to the 3O2 molecule, producing an excited 1O2 molecule.21 Singlet oxygen (1O2) is highly reactive with a lifetime of 45 min in vacuum, 4.4 µsec in water,22 and 240 µsec in chloroform.23 Generated 1O2 reacts with PAH to form an endoperoxide through a concerted, or a biradical step-wise mechanism. The type-I and type-II mechanisms are illustrated in Figure 1. Earlier theoretical studies, based on the G3(MP2) computational method, reported a concerted mechanism for 1,4-addition reactions between 1O2 and PAHs such as benzene, naphthalene, anthracene, tetracene, and pentacene.24 Subsequent studies on polyacenes, using density functional theory (DFT) calculations at the B3LYP/6-31G* level, showed that the smaller analogs, such as benzene and naphthalene, undergo a concerted reaction pathway whereas the larger analogs undergo a biradical stepwise mechanism.25 However, since B3LYP fails to account for dispersion interaction effects, the concerted transition state cannot be properly located when using such a functional. Therefore, here, we have chosen to systematically investigate the PAH degradation mechanism in more detail at several levels of DFT, including dispersion corrected functionals.


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Figure 1. Electron transfer (type I) and energy transfer (type II) pathways for the photooxidation reaction of PAHs with molecular oxygen along with the general chemical structure of the formed endoperoxide of triisopropylsilylethynyl-substituted (TIPS) bistetracene (TIPS-BT).

Triisopropylsilylethynyl (TIPS) substituted pentacene has been reported to be more stable than pentacene itself.26-27 This property has been attributed to a reduced singlet-triplet energy gap ∆ST, thereby making sensitization difficult and the energy-transfer mechanism inaccessible.11 We recently reported the synthesis of an ethynylsilyl-substituted bistetracene (BT)28 with a hole mobility as large as 6.1 cm2/Vs and more stable than pentacene in chlorinated solvents. A recent study on the Diels-Alder reactivity of pentacene and bistetracene with C60 by Cao et al. showed that these reactivities are related to distortion energies.14 Here, our goal is to investigate the origin of the increased stability of this substituted bistetracene compared to pentacene and rubrene with respect to the oxidation reaction, which is one of the major limitations for practical applications of organic semiconductors. The molecular structures are given in Figure 2.



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Figure 2. Chemical structures of: (A) triisopropylsilylethynyl-substituted (TIPS) bistetracene (TIPS-BT); (B) pentacene; and (C) rubrene, along with the ring numbering considered throughout this work.

2. Methodologies 2.1. Computational Methods The DFT calculations were performed using the Gaussian 09 Revision D.01 suite of programs29 and generally the 6-31G(d) basis.30-31 We have initially considered the ωB97X-D32, B3LYP33, CAM-B3LYP34, and M06-2X35 functionals. Restricted and unrestricted DFT methodologies were used for closed-shell and open-shell species, respectively. The B3LYP and CAM-B3LYP functionals were found to be inadequate to locate the concerted transition state during endoperoxide formation of pentacene, rubrene, and triisopropylsilylethynyl-substituted bistetracene (TIPS-BT), whereas those transition states are well located with the ωB97X-D and M06-2X functionals. Frequency calculations were carried out: (i) to confirm that the geometries correspond to local minima (no imaginary frequency) or transition states (one imaginary frequency pertaining to the reaction coordinates, which was confirmed in addition by intrinsicreaction-coordinate (IRC) calculations); and (ii) to provide the thermochemical data at standard temperature (298.15 K) and pressure (1 atm), which include entropy contributions. The energies were further refined by considering the 6-311+G(d,p) basis. The influence of solvent was 6 ACS Paragon Plus Environment

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modelled by using the IEF-PCM solvation model within the self-consistent reaction field (SCRF) framework (taking into account a dielectric constant, ε = 4.71, equivalent to chloroform, to maintain consistency with experiment).36 We have found that optimizing the geometry within the IEF-PCM model has only a marginal effect on geometrical parameters; thus, gas-phase optimized geometries are reported throughout this work. The energetics of the reactions are presented on the basis of relative Gibbs energies (∆Grel) with respect to infinitely separated reactants. The ∆ST energy difference between the triplet ground state and singlet excited state of O2, as calculated via the ωB97X-D functional with a spin projection technique,37 is 23.5 kcal/mol (1.02 eV), in good agreement with the experimental value of 22.5 kcal/mol (0.98 eV).38 When calculated using the M06-2X functional, the ∆ST of the O2 molecule is significantly overestimated (28.5 kcal/mol), which may result in an underestimation of the barrier heights of the reaction. Therefore, the properties calculated using the ωB97X-D functional with spin contamination removal are considered throughout this study unless otherwise specified. The distortion/interaction model,39-41 known as the activation strain model,42-43 has been investigated to understand the origin of regioselectivity of oxidation on various rings of TIPS-BT. In this model, the activation energy (∆Eact) is decomposed into the distortion energy (∆Edist) of each fragment and the interaction energy between the fragments (∆Eint): ∆E = ∆E + ∆E

(1)

The distortion energy is the energy required to distort the reactants from their ground-state structures to those in the transition state without allowing for interaction between the reactants. To calculate ∆Edist, each transition state structure is separated into two fragments, and singlepoint calculations are carried out on each of the fragments. The energy difference between the 7 ACS Paragon Plus Environment


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distorted structure in the transition state (or at any point in the IRC path) and the optimized ground-state structures is defined as the distortion energy. The interaction energy (∆Eint) is then the energy difference between the activation energy and distortion energy. The local aromaticity indices of TIPS-BT, pentacene, and rubrene were calculated at the B3LYP/6-31G* level of theory. We have calculated aromaticity indices such as Nuclear Independent Chemical Shift (NICS),44 harmonic oscillator model of aromaticity (HOMA),45-46 multicenter index (MCI)47-48. MCI indices were calculated within the Becke-rho atomic partition49 and computed using the APOST-3D and ESI-3D programs.50-51 The rate of electron transfer was estimated within the framework of semi-classical Marcus theory52 as: "

! 4 1 '

= # $% & 2! ℎ 4

where ∆G is the Gibbs free energy of the reaction; VRP is the electronic coupling between the final and initial states, and λ is the reorganization energy. The ∆G values for the forward -

electron-transfer reaction (∆Gf) (PAH* +O2 → PAH+ +O2 ) and the backward electron-transfer reaction (∆Gb) ( PAH + O → PAH + O ) were calculated as the energy difference between the final and initial adiabatic states.53-54 The electronic couplings between the molecules were evaluated for the lowest energy conformer using the fragment molecular orbital (FMO) approach.55 Approximately 2000 conformations of the systems of interest with O2 were generated through a scan of O2 over the molecule to determine the lowest energy conformer.


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2.2. Experimental Methods For the kinetic NMR study, a solution of n-octyldiisopropylsilyl-ethynylene substituted bistetracene (NODISPA-BT) (4 mg) in CDCl3 (1 ml) was stirred in ambient laboratory light/air at room temperature. 1H NMR measurements were performed with a Bruker Avance 400 at 3, 12, and 24 days of exposure.

3. Results and Discussions To investigate the possibility of energy transfer from TIPS-BT, pentacene, and rubrene towards O2, both the vertical and adiabatic ∆ST values of O2, TIPS-BT, pentacene, and rubrene were calculated at the ωB97X-D/6-31G* level; the results are collected in Table 1. As mentioned above, the ωB97X-D/6-31G* ∆ST value of O2 (1.02 eV) is in excellent agreement with the experimental excitation energy (0.98 eV). The long lifetime of the excited triplet state and the large range of ∆ST values (0.4 eV) of TIPS-BT, pentacene, and rubrene, see Table 1, enable them to undergo resonance energy transfer with O2 followed by photooxidation reaction.

Table 1. Calculated ∆ST values of O2, TIPS-BT, pentacene, and rubrene at the ωB97X-D/6-31G* level. All energies are given in eV.

Molecule

∆ST(vertical) ∆ST(adiabatic)

O2

1.02

1.02

TIPS-BT

1.10

0.70

Pentacene

1.22

0.85

Rubrene

1.51

1.08 9

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From a purely qualitative standpoint, the increased stability of the TIPS-BT compared to that of pentacene has been previously attributed to the fact that the former has two Clar aromatic sextets whereas the latter has only one (we recall that a larger number of Clar aromatic sextets is considered to increase the stability of aromatic molecules).56 In this context, we have chosen to calculate the local aromaticity indices of each ring in the TIPS-BT, pentacene, and rubrene molecules, see Table 2.57 The trends in aromaticity predicted by the NICS, HOMA, and MCI values, when compared to available experimental observations for pentacene and rubrene, suggest that the MCI approach is a more reliable aromaticity index (see SI for a more detailed discussion).

Table 2. NICS (in ppm), HOMA, and MCI atomicity indices of various rings in pentacene, rubrene, and TIPS-BT. Ring numbering is given in Figure 2.

system Pentacene

Rubrene

TIPS-BT

ring

NICS

HOMA

MCI

1

-7.04

0.48

0.020

2

-12.04

0.54

0.016

3

-13.49

0.56

0.016

1

-7.07

0.52

0.022

2

-9.61

0.44

0.016

3

-9.11

0.97

0.057

1

-10.60

0.63

0.024

2

-14.16

0.71

0.022

3

-10.81

0.43

0.011

4

-6.67

0.53

0.016


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We note that, in addition to the aforementioned aromaticity indices, in which both MCI and HOMA predict that ring 1 has the largest aromatic character and ring 3 the lesser, we have also calculated additional aromaticity indices proposed in the literature including the paradelocalization index (PDI)58 as well as the Iring59, INB60, and ING indexes.60 In the case of TIPS-BT, we find all these indices to be in agreement with the MCI results (Table I in the SI). Hence, based on aromaticity criteria grounded on MCI results, ring 3 is expected to be more prone to an oxidation reaction than rings 1, 2, or 4. To assess the role of aromaticity in controlling the stability of the molecules under investigation, we have evaluated the activation energy for endoperoxide formation on each ring of the aromatic molecule using transition state theory. We have investigated both the concerted and stepwise reaction mechanisms of TIPS-BT, pentacene, and rubrene for each of their individual rings. We find that the concerted Diels-Alder oxidation reaction is favorable compared to the stepwise mechanism (see Figure 7 and Figure S1 in the SI), with the exception of rubrene (Figure S2 in the SI), where the concerted and stepwise mechanisms are found to have similar activation barriers. For TIPS-BT, the oxidation of ring 2 is found to be the most favorable as it corresponds to the lowest activation energy barrier (∆G‡ = 17.7 kcal/mol via a concerted path,

BT

TSr2, as shown in

Figure 3); it occurs in a concerted fashion, with the reaction being exergonic with a free energy gain of 19.4 kcal/mol to form a stable endoperoxide product (BT4r2; Figure 3). The ∆G‡ values for the concerted pathway of the oxidation reaction at ring 1, ring 3, and ring 4 of TIPS-BT are 27.0 kcal/mol, 19.3 kcal/mol, and 31.0 kcal/mol, respectively (see Figure S1 in the SI). The stepwise and concerted mechanisms of endoperoxide formation of ring 2 of TIPS-BT, the most energetically favorable, are illustrated in Figure 3; the stepwise mechanism proceeds via a radical 11 ACS Paragon Plus Environment


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coupling reaction with a biradical nature for

BT

2r2 and

BT

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3r2, which has been verified via a spin

density analysis, see Figure 4.

Figure 3. Concerted (solid lines) and stepwise (dashed lines) reaction pathways of 1O2 with TIPS-BT leading to endoperoxide formation on ring 2. Gibbs energies (in kcal/mol) are given for each state.


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Figure 4. Illustration of the ωB97X-D spin densities in the BT2r2 and BT3r2 states of Figure 3 (the red and blue colors represent spin up and spin down electron densities, respectively).

Interestingly, according to the aromaticity analysis described above, ring 3 is the least aromatic (as predicted by MCI and HOMA values) and would be expected to undergo oxidation more readily than ring 2, while the activation free energy barriers suggest that the endoperoxide formation on ring 2 is more favorable by 1.6 kcal/mol. (We note that results for activation free energies obtained using the M06-2X functional are close to ωB97X-D values, see Table S2 in the SI). In order to understand the origin of this regioselectivity of endoperoxide formation, we have carried out distortion/interaction analyses for the concerted transition states of endoperoxide formation on rings 1, 2, 3, and 4 of TIPS-BT, see Figure 5.40-41 From Figure 5, it appears that the distortion energy is minimum on ring 2 (11.9 kcal/mol) and is 2.2 kcal/mol smaller than that on ring 3 (14.1 kcal/mol). However, the interaction energy is similar on ring 2 and ring 3. This is in agreement with the recent study on Diels-Alder reaction of bis(2-(trimethylsilyl)ethynyl) 13 ACS Paragon Plus Environment


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bistetracene with C60 where Cao et al. found that the lowest distortion energy is on ring 2.14 These findings again highlight that aromaticity is not the main factor controlling the stability; the distortion energy required to access the transition state plays a critical role. The distortion energies of the transition states for the TIPS-BT oxidation on various rings correlate well with the transition state energies, which points to the most favorable sites for TIPS-BT oxidation. Moreover, the distortion energy difference (2.2 kcal/mol) calculated between ring 2 and ring 3 is consistent with the activation free-energy difference (1.6 kcal/mol). These results underline that a balance between distortion and interaction energies, instead of aromaticity, actually determines the regioselectivity of endoperoxide formation in TIPS-BT. (We note that we also carried out similar calculations with water and find that it does not affect the photo-stability of the neutral TIPS-BT molecule).

Figure 5. Distortion/interaction (activation strain) analysis for the concerted transition state of O2 with ring 1, ring 2, ring 3, and ring 4 of TIPS-BT. ∆Edist, ∆Eint, and ∆Eact are given in kcal/mol. 1


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The proposed regioselectivity of endoperoxide formation based on the activation energy analysis was subsequently confirmed by the results of

1

H NMR experiments on the n-

octyldiisopropylsilyl-ethynylene substituted bistetracene (NODIPSA-BT), whose structural backbone is similar to that of TIPS-BT, see Figure 6 (we note that NODIPSA-BT was chosen for this experiment as it is more soluble in chloroform than TIPS-BT itself). Starting with the pristine PAH in the bottom spectrum16, this compound slowly converts into the endoperoxide after several days, which indicates that NODIPSA-BT has a large activation barrier against oxidation. The new peaks arising at 6.75 and 7.0 ppm with a relative intensity ratio of 1:1 are indicative of the protons on ring 2 being shifted further upfield, a consequence of having the endoperoxide forming on that ring. As the oxidized product becomes more concentrated in solution with time, peaks in the further downfield positions broaden as a result of the lower molecular symmetry; discrete individual peaks are no longer observed as they become wider and merge. The spectra at 12 and 24 days are a zoom-in view to clearly observe this peak broadening, which is the reason why the solvent peak (at ~7.3 ppm) as well as all other peaks appear to grow in intensity. Isolating the endoperoxide product did not prove to be possible, but the observation of two new peaks arising and the simultaneous broadening of all other aromatic peaks further downfield gave proof that our species only decomposed after an extended amount of time in solution. We note that the recent study by Cao et al. on the Diels-Alder reaction of C60 with TIPS-BT also showed that the lowest barrier for the reaction is on ring 2,14 which is in line with our experimental observation of Diels-Alder reaction of 1O2 with NODIPSA-BT.



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Figure 6. (A) Proposed endoperoxide formation upon photooxidation of n-octyldiisopropylsilylethynylene substituted bistetracene (NODISPA-BT). (B) 1H-NMR spectra monitoring the photooxidation of NODISPSA-BT with various time intervals.

To provide a baseline for comparison, we have also considered pentacene and rubrene. From earlier experimental61 and theoretical studies25 of pentacene, it has been shown that endoperoxide formation occurs at the central ring. Therefore, we have limited our investigations of the reaction 16 ACS Paragon Plus Environment

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of pentacene with oxygen and focused on the central ring; both the concerted and stepwise reaction pathways for the Diels-Alder reaction of pentacene with 1O2 are illustrated in Figure 7. The activation free energy barrier for the concerted reaction is 13.6 kcal/mol (via PTSr3), which is 16.2 kcal/mol lower than that of the stepwise mechanism (via PTS1r3, ∆G=29.8 kcal/mol) and the reaction is exergonic (∆Grel= -31.6 kcal/mol, Figure 7). This result suggests that a concerted mechanism is preferred over the step-wise mechanism that was previously reported.25 The activation energy barrier (∆G‡) for the endoperoxide formation on pentacene (13.6 kcal/mol) is smaller than that on TIPS-BT (17.7 kcal/mol), which is in agreement with the experimentally observed higher stability (half-life) of pentacene than that of TIPS-BT towards photooxidation reaction.

Figure 7. Concerted (solid lines) and stepwise (dashed lines) reaction pathways of 1O2 with pentacene. Free energies are given in kcal/mol.



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In the case of rubrene, our calculations indicate that the Diels-Alder reaction with 1O2 occurs on ring 2, in agreement with experimental observations.22 The concerted path has a ∆G‡ of 14.4 kcal/mol (via RTSr3), which is lower than that of TIPS-BT and higher than that of pentacene (Figure 8). It is interesting to note that ∆G‡ of the stepwise mechanism (via RTS1r3) for the endoperoxide formation at ring 2 of rubrene (15.3 kcal/mol) is similar to that of the concerted mechanism; hence, both of these mechanisms appear to be energetically possible in rubrene. The resulting rubrene endoperoxide is stable by about 20.7 kcal/mol. The biradical nature of the intermediates and transition states of the stepwise mechanism has also been verified via a spin density analysis, see Figure S3 in the SI. For the concerted pathways of rubrene oxidation at ring 1 and ring 3, see Figure S2 in the SI.

Figure 8. Concerted (solid lines) and stepwise (dashed lines) reaction pathways of 1O2 with rubrene. Free energies are given in kcal/mol. 18 ACS Paragon Plus Environment

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The experimentally observed half-lives of TIPS-BT, pentacene, and rubrene in solution (5 days,28 3 min,62 and 40 min,12 respectively) correlate well with our calculated ∆G‡ values of 17.7 kcal/mol, 13.6 kcal/mol, and 14.4 kcal/mol, respectively. As stated above, the activation barrier for the oxidation of TIPS-BT is 4.1 kcal/mol higher (at ring 2) than that of pentacene (at ring 3); this corresponds to an approximately 1,000 times higher stability of TIPS-BT than pentacene and is well in line with the experimental observation. Moreover, the distortion energy of the concerted oxidation on ring 2 of TIPS-BT (11.9 kcal/mol) is higher than that on ring 3 of pentacene (6.1 kcal/mol) and on ring 2 of rubrene (9.2 kcal/mol). Even though a similar photodegradation mechanism is expected in the solid-state environment, the crystal packing in the solid state has to be taken into account for a proper understanding of device degradation.63

Finally, we have also investigated the electron transfer mechanisms shown in Figure 1. The rates of electron transfer between TIPS-BT, pentacene, or rubrene with 3O2 for the forward and backward reactions are calculated using Eq. (2) and presented in Table 3, along with the parameters entering the above equation.

For pentacene and TIPS-BT, the rate of forward -

reaction to create the charged species (PAH+ +O2 ) is at least two orders of magnitude smaller than that of their recombination (backward reaction). Thus, we expect that degradation in pentacene and TIPS-BT does not involve an electron transfer mechanism (type I). However, in rubrene, both the forwards and backwards reactions are of the same order, again in agreement with previous experimental detection of the rubrene radical cation and oxygen radical anion using EPR spectroscopy, indicating that an electron transfer process is possible.19



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Table 3. Gibbs free energies of forward electron transfer reaction (∆Gf), backward electron transfer reaction (∆Gb), electronic couplings between donor and acceptor molecule (VRP), reorganization energy (λ) and rate of electron transfer (k) reactions (Parameters for the forward electron transfer reaction are denoted by f subscript and those for the backward electron recombination reaction are denoted by b subscript).

Molecule

∆Gf [∆Gb]

|VRP|f [|VRP|b]

λf [λb]

kf [kb]

(eV)

(meV)

eV

s-1

TIPS-BT

-0.46 [-1.67]

7 [17]

1.24 [1.26]

6.5x109 [1.1x1012]

Pentacene

-0.84 [-1.50]

0.004 [0.37]

1.26 [1.31]

6.1x104 [1.5x109]

Rubrene

-1.74 [-0.60]

104 [442]

1.26 [1.31]

2.8x1013 [6.6x1013]

4. Conclusion

We have described a detailed investigation, at the long-range corrected density functional theory level, of the [4+2] cycloaddition (Diels-Alder) reaction mechanism for a recently synthesized bistetracene derivative. Our aim was to elucidate the origin of the increased stability towards oxidation displayed by the substituted bistetracene compared to pentacene and rubrene. We find that the singlet-triplet gaps of pentacene, rubrene, and TIPS-BT are similar to the excitation energy of molecular oxygen and can lead to singlet oxygen generation, which opens up a pathway for degradation. The calculated activation free energy barrier of the oxidation reaction in TIPS-BT is 4.1 kcal/mol higher than that of pentacene, and 3.2 kcal/mol higher than that of rubrene.


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Local aromaticity indices calculated with electron sharing indices (MCI) are found to be more reliable than those based on nuclear independent chemical shift (NICS). Interestingly, if aromaticity were the main factor controlling the reaction, degradation of TIPS-BT with singlet oxygen would be expected to take place on ring 3 (which is one of the rings shared by the two tetracene subunits). However, our results show that the free energy barrier for oxidation on ring 2 (pertaining to a single tetracene subunit) is lower by 1.6 kcal/mol than on ring 3. We were able to rationalize these results on the basis of a distortion/interaction energy analysis, which points out that the difference in activation energy primarily originates in distortion energy. Experimental 1H NMR chemical shift analyses validate the theoretical prediction by confirming that endoperoxide formation occurs on the ring with smallest amount of distortion energy. Our work underlines that, in order to enhance the stability of polyaromatic hydrocarbons towards oxidation: (i) the singlet-triplet energy gap (∆ST) of the organic molecules should be substantially smaller than that of oxygen; and (ii) the distortion energy of the aromatic rings should be increased, e.g., via incorporation of a two-dimensional PAH framework, which is consistent with the increased stability of the 2D-like bistetracene.



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ASSOCIATED CONTENT Supporting Information Local aromaticity indices, oxidation reaction pathways of TIPS-BT and rubrene at various rings, and spin density plots. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements. This work has been supported by King Abdullah University of Science and Technology (KAUST) and by ONR-Global (Award N62909-15-1-2003). We are grateful to the KAUST Supercomputing Laboratory and the KAUST IT Research Computing Team, for providing continuous assistance as well as computational and storage resources. J.L. acknowledges funding from the National Science Foundation (DMR-1508627); L.Z. thanks the Office of Naval Research (N000147-14-1-0053); S.T. thanks Dr. Manjaly J. Ajitha for useful discussions.


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Improving the Stability of Organic Semiconductors: Distortion Energy

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