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Through–Space or Through–Bond? The Important Role of Cofaciality in Orbital Reordering and Its Implications for Hole (De)stabilization in Polychromophoric Assemblies Maxim V. Ivanov, Shriya H. Wadumethrige, Denan Wang, and Rajendra Rathore J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05804 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on July 1, 2017

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The Journal of Physical Chemistry

Through–space or through–bond? The Important Role of Cofaciality in Orbital Reordering and its Implications

for

Hole

(De)stabilization

in

Polychromophoric Assemblies Maxim V. Ivanov,a Shriya H. Wadumethrige,b Denan Wang,a and Rajendra Rathore*a a

Department of Chemistry, Marquette University, P.O Box 1881, Milwaukee, WI 53201-1881.

b

Faculty of Science, University of Ruhuna, Wellmadama, Matara 81000, Sri Lanka.

KEYWORDS. Charge transfer, frontier orbitals, electronic coupling

ABSTRACT. Developing predictive tools for the elucidation of redox and optical properties of polychromophoric assemblies is crucial for the rationale design of efficient charge-transfer materials. Here, such tools are introduced to explain the curious observation that a pair of bichromophoric electron donors based on ethanoanthracene (5) and dihydroanthracene (DHA or 11), having similar interchromophoric separation between the carbons in the region of orbital overlap, show dramatically different (>300 mV) stabilization of their cation radicals. Analysis of molecular orbital (MO) diagrams reveals the important interplay between through-space and through-bond electronic couplings, which results in HOMO/HOMO-1 swapping in 11. Unlike

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the antisymmetric SOMO that stabilizes a hole by charge resonance in 5+• (0.57 V), the symmetric SOMO in 11+• (0.88 V) does not afford hole stabilization by charge resonance, rather, the hole localizes onto a single benzenoid unit. This important finding is expected to aid in the design of polychromophoric assemblies with chromophores of graded redox potentials for optimization of long-range charge transfer.

The design of improved charge-transfer materials demands improved fundamental understanding of the structure-function relationships concerning redox/optical properties of the simplest units in polychromophoric assemblies.1-4 In a set of model cyclophane-like donors 1-5, a single cationic charge or hole is effectively stabilized (i.e. by 250-380 mV), despite the fact that the pair of (cofacial) aromatic rings are (rigidly) juxtaposed over a range of interplanar angles (i.e. 0° to 120°, see Figure 1).5 φ = 120° ΔEox = 250 mV

3

4

2.53 Å 5.09 Å

4.32 Å

30°

2.92 Å

3.92 Å

20°

3.16 Å

120°

180°

2

6.55 Å

1

2.42 Å

19°

φ = 180° ΔEox - ?

3.82 Å

3.00 Å

φ = 0° to 30° ΔEox = 320 to 380 mV 3.21 Å

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5

DHA

Figure 1. Structures and hole stabilizations, ΔEox [= Eox1(bichromophore) - Eox(model compound)], for 1-5 (330, 350, 320, 380, 250 mV, respectively) are shown.5 Considering donors 1-4, where interplanar angles vary between 0o and 30o, similar cation radical stabilization is found (i.e. 350±30 mV).5 However, increasing the interplanar angle to 120° in ethanoanthracene 5 leads to a somewhat reduced stabilization (i.e. 250 mV).5


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This, despite the fact that the separation of the carbons in the interchromophoric region of orbital overlap is smaller in 5 than in 2-4 (Figure 1). This led us to probe in detail the evolution of the redox/optical properties of such bichromophores with varying interplanar angles, especially in an extreme case where the interplanar angle is 180°, i.e. the case of 9,9,10,10tetramethyl-dihydroanthracene6 derivative DHA (Figure 1). Here, we employ a combined experimental/computational approach to systematically evaluate the redox and optical properties of a series of cyclophane-like molecules where the interplanar angle is varied in the range of 0° to 180°. Importantly, we will show that upon decreasing the cofaciality (increasing the interplanar angle) between two aromatic rings, through-space electronic coupling becomes negligible at an interplanar angle of ~160°. At larger interplanar angles, effective overlap of the orbitals of sigma framework with symmetric HOMO-1 (throughbond electronic coupling) promotes a swapping of the antisymmetric HOMO with symmetric HOMO-1. The consequence of the swapping of filled frontier molecular orbitals leads to a localization of the hole onto a single aryl unit, as demonstrated experimentally by the synthesis of DHA (Scheme 1) and comparison of its redox/optoelectronic properties with the representative cyclophane-like donors 2-4 and model compounds.

LiAlH4/reflux O O

O

Br2/CH2Cl2

MeONa/MeOH Br

DHA (X-ray structure)

EtOAc/toluene [CuBr]

Br

Br Br

Scheme 1. Synthetic scheme for DHA.


3


Synthesis of DHA was accomplished by reaction of anthracene with LiAlH4 in refluxing diglyme to afford 9,9,10,10-tetramethyldihydroanthacene,7 which was then brominated via reaction with Br2 in CH2Cl2, followed by a Cu-catalyzed substitution8 of bromides with methoxide groups to return DHA in excellent overall yield (Scheme 1). The structure of the DHA was established by 1H/13C NMR spectroscopy, mass spectrometry and by X-ray

A

B O

O

O

O

O

ε

O

M

2000 M-1cm-1

crystallography (see Scheme 1 and Supporting Information for details).

2 μA

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DHA O O

5

O O

O O

O

O

2

O O

1.5

1.0 0.5 0.0 V vs Fc/Fc+

500

1000 1500 2000 Wavelength (nm)

Figure 2. A. Cyclic voltammograms of 2 mM M, DHA, 5, and 2 in CH2Cl2 (0.2 M n-Bu4N+PF6-) at ν = 200 mV/sec and 22 °C. B. Absorption spectra of M+•, DHA+•, 5+• and 2+• in CH2Cl2 at 22 °C, see Supporting Information for the details for generation of the cation radicals.9,10 Cyclic voltammogram of DHA in CH2Cl2 showed two reversible oxidation waves, with the first oxidation potential (0.88 V vs Fc/Fc+) being 60 mV higher than the model compound M


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(0.82 V vs Fc/Fc+), indicating a destabilization of its cation radical (Figure 2A). In contrast, both 2 (0.42 V vs Fc/Fc+) and 5 (0.57 V vs Fc/Fc+), featuring identical chromophores as DHA but with reduced interplanar angles (i.e. 19° and 120°, respectively) show effective stabilization of the cationic charge (Figure 2A) in comparison to the model M. The presence of an intervalence transition in the electronic absorption spectra of the cyclophane-like cation radicals 2+• and 5+• signifies hole delocalization via through-space electronic coupling (Figure 2B).5 The similarity of absorption spectra of DHA+• and model M+• (Figure 2B) and a singular absence of the absorption in the near-infrared (NIR) region in DHA+• clearly indicates a lack of through-space electronic coupling between the pair of benzenoid rings. The observation of the destabilization of cationic charge in DHA+• was initially surprising. Indeed, the orbitals on the two benzenoid rings in DHA overlap more or less similarly to the ethanoanthracene derivative 5 (φ = 120o), as the closest aromatic carbons in 5 and DHA lie at a similar distance of 2.42 and 2.53 Å, respectively (see Figure 1). To fully address the (interplanar) angular dependence of hole stabilization/destabilization in bichromophoric donors, a series of molecules with incrementally varied interplanar angle in the whole rage of 0° - 180° was needed. While the molecules with interplanar angles in the shorter range of 0° to 30° (i.e. 1-4), medium angle of 120° (5), or at an extreme angle of 180° (DHA) have been synthesized, the molecules between the range of 120°-180° were not readily available. We

initially

approached

this

problem

using

DFT

calculations

[B1LYP-40/6-

31G(d)+PCM(CH2Cl2)],11,12 in particular, potential energy surface scans where the interplanar angle (φ) between two benzenoid rings in DHA was varied and the electronic structures and redox/optical properties evaluated computationally. Unfortunately, this approach was only


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applicable in a small angle range of 150°-180°, due to increased steric repulsion between the pair of methyl groups at angles below 150° (see structure below).

H

O

HH H H H

O

O

O

A solution to this problem was suggested by simple molecular modeling, which indicated that a series of the bichromophoric molecules with the interplanar angle ranging from 110° to ~180°, i.e. structures 5-10, could be feasible by varying the number of methylenes in the ethano bridge of 5 (Figure 3).

Figure 3. Structures of 5-11 with the interplanar dihedral angles in the range of 110° to ~180°. Superimposed 5-11 in the center show that the carbons in interchromophoric region, where orbital overlap occurs, vary only in the range 2.4-2.5 Å. The redox/optical properties and electronic structures of 5-10 and DHA (or 11) were then obtained by DFT calculations [B1LYP-40/6-31G(d)+PCM(CH2Cl2)], see Tables S3-S4 in Supporting Information. The availability of a set of experimental data points (i.e. 1-5, 11, M) allowed us to confirm that B1LYP-40 functional well reproduces the experimental redox


6

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potentials (Figure 4A). Scaling from Figure 4A then provided the oxidation potentials of 6-10, which are yet to be synthesized, revealing, e.g., that the Eox1 of 10 (0.82 V, φ = 177°) is similar to DHA (0.85 V, φ = 180°). The availability of experimental and predicted oxidation energies of 2, 5-10 reveals that increasing the interplanar angle in the range from 19° (i.e. 2) to 110° (i.e. 6) has little effect on the cation radical stabilization energy (ΔEox).13 However, for angles from 120° (i.e. 5) to 180° (i.e. 10-11) the ΔEox decreases in magnitude and reaches negligible values at interplanar angles 153°-160° (i.e. 8-9), see Figure 4B. Increasing the interplanar angle beyond 160° (i.e. in 10-11) leads to a destabilization (by 50-80 mV) of the cation radical in comparison to the model compound (M).14 The significant changes in the oxidation energies/potentials (Gox/Eox) of 5-11 are observed despite that the separation amongst the carbons in the interchromophoric region of orbital overlap does not vary significantly (2.4-2.5 Å, see Figure 3).

A

B

1.1 11

0.2

0.97 11

3

8

0.0

M

5

-0.2

5 0.3 4.7

2

2

5.2

10

0.77

7

0.7

14

9

5.7

-0.4

0°

0.78

6

90°

0.37 180°

Figure 4. Correlation plot between experimental Eox against computed oxidation energies (Gox) with the linear fit: Eox = 0.96 Gox– 4.29, R2 = 0.97. B. Plot of the scaled (from Figure 4A) Eox as well as ΔEox against interplanar angles for 2, 5-11. In order to reconcile this unusual evolution of oxidation energies with varied interplanar angles (Figure 4B), we closely scrutinize the filled frontier orbitals (i.e. HOMO and HOMO-1) in the


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context of MO theory. First, we examine the HOMO energies of 2, 5-11 (Figure 5A). The similar HOMO energies of 2 (φ = 19°) and 6 (φ = 110°) are consistent with their similar oxidation potentials (ΔEox = 350±30 mV). However, increasing the interplanar angle beyond 110° (i.e. from 120° to 160°, 5-9) leads to a decrease in HOMO energy and a corresponding increase in oxidation potential (Figures 5A and 4B). At φ ≳ 160° (9-11), the HOMO/HOMO-1 become almost degenerate, while at larger angles near co-planarity (177°/10 and 180°/11), HOMO-1 (i.e. symmetric orbital) swaps with HOMO (i.e. antisymmetric orbital).15-17

Figure 5. (A) Relative energies of HOMO and HOMO-1 for 2, 5-11 [B1LYP-40/631G(d)+PCM(CH2Cl2)]. (B/C) MO diagrams of 5 and 11. The value of

βπ

is set to zero for φ =

180°, (i.e. 11). The Δ value is approximated as the averaged difference in the ionization potentials of various cycloalkanes and 1,2-dimethoxy-4,5-dimethylbenzene.16,20


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Figure 5B shows that through-space electronic coupling (βπ) between HOMOs of a pair of benzenoid chromophores, in the absence of the through-bond electronic coupling (βσ), will produce HOMO (red bar, antisymmetric combination) and HOMO-1 (blue bar, symmetric combination) with an energy gap of 2βπ.18 However, due to symmetry consideration, only HOMO-1 and σ orbital from the alkyl framework are involved in through-bond electronic coupling (βσ) that produces a new HOMO-1, which is higher in energy by a value of λσ than the original HOMO-1 (Figure 5B).15,19 Hereafter, we refer to this orbital as σπHOMO-1 (green bars). The raising of the energy of HOMO-1 by λσ arises from the interplay between MOs of the benzenoid chromophores and alkyl framework (i.e. Δ), βπ and βσ, which in case of large ∆ − 𝛽! will approximate to a quadratic dependence (eq. 1, also see Supporting Information for additional details). 𝜆! ≈

𝛽! ! ∆ − 𝛽!

(eq. 1)

Extension of this MO analysis to the entire series in Figure 5A shows that the energy gap between HOMO and HOMO-1, arising entirely due to βπ, decreases with increasing φ and becomes zero at φ = 180° (i.e. βπ = 0). As through-bond electronic coupling impacts the energy of symmetric HOMO-1 and not of antisymmetric HOMO, in cases of their near degeneracy, the increase in the energy of

σπ

HOMO-1 leads to a swapping or energetic re-ordering. That is,

symmetric σπHOMO-1 becomes σπHOMO and antisymmetric HOMO becomes HOMO-1 (Figure 5A). The evolution of the MO energies as well as swapping of HOMO and σπHOMO-1 (in cases of 10 and 11) can be accurately reproduced with the aid of Hückel MO theory by employing the parameters βπ and βσ, extracted from the analysis of the MO diagrams of 2, 5-11, into Hückel Hamiltonian matrices (see Supporting Information for full details).


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This MO analysis raises a key question: What are the implications of the HOMO/HOMO-1 swap on the hole stabilization in cation radicals? The experiment (Figure 2), DFT calculations (Figure 4) and MO analysis (Figure 5) reveal that unlike the antisymmetric HOMO, symmetric HOMO prevents hole delocalization in bichromophoric donors despite favorable geometrical juxtapositions (Figures 3 and 6).

5 antisymmetric HOMO

11 symmetric HOMO O

O

2.5 Å

2.4 Å

OO

+.

charge resonance

OO

+. O

O

charge localization

Figure 6. Showing with the aid of spin-density plots that antisymmetric HOMO (5) delocalizes the hole by charge-resonance, while symmetric HOMO (11) results in localization. The inability of the symmetric singly occupied molecular orbital (SOMO) in 11+• toward hole stabilization is reflected in its drastically increased oxidation potential (Eox = 0.88 V vs Fc/Fc+) as compared to 5 (Eox = 0.57 V vs Fc/Fc+), where the HOMO is antisymmetric. The dramatic variation in (de)stabilization of the cation radicals of 11 and 5 occurs despite that separation amongst carbons in the interchromophoric region of orbital overlap differ only by ~0.1 Å (Figure 6). In 5+•, the charge is fully delocalized over both benzenoid units by the charge-resonance as attested by spin/charge distributions (Figure 6), and the presence of a NIR charge-resonance transition (Figure 2). In 11+• the charge is localized onto single benzenoid unit as evidenced by


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spin/charge distributions (Figure 6) and a singular absence of the charge-resonance transition (Figure 2). In accordance with molecular orbital theory and well-established rules of bonding,21,22 the symmetric HOMO (or SOMO) is expected to favor bond formation;23 however, an inter-ring bond formation in 11+• is likely to cause severe strain in the molecule due to geometrical constraints and thus is not observed. This unprecedented observation of hole localization/delocalization controlled by the symmetry of the MO will be exploited in the design of new polychromophoric assemblies by a judicious introduction of chromophores to control the symmetry of HOMOs in order to impact long-range charge transfer dynamics.

ASSOCIATED CONTENT Supporting Information. Experimental details for DHA (or 11) and computational details. The following files are available free of charge. Supporting Information (PDF) X-ray structure of DHA (CIF) Optimized equilibrium structures of 2, 5-11 and M (TXT) AUTHOR INFORMATION Corresponding Author *[email protected]


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ACKNOWLEDGMENT We thank NSF (CHE-1508677) and NIH (R01-HL112639-04) for financial support, Professors Scott A. Reid for helpful discussions and Marat R. Talipov for preliminary calculations. Calculations were performed on Père (MU) and XSEDE (NSF) computing clusters. REFERENCES (1) Fujitsuka, M.; Tojo, S.; Shibahara, M.; Watanabe, M.; Shinmyozu, T.; Majima, T. Delocalization of Positive Charge in Π-stacked Multi-benzene Rings in Multilayered Cyclophanes. J. Phys. Chem. A. 2011, 115, 741-746. (2) Morisaki, Y.; Chujo, Y. Through-space Conjugated Polymers Consisting of [2.2] Paracyclophane. Polym. Chem. 2011, 2, 1249-1257. (3) Renaud, N.; Harris, M. A.; Singh, A. P. N.; Berlin, Y. A.; Ratner, M. A.; Wasielewski, M. R.; Lewis, F. D.; Grozema, F. C. Deep-hole Transfer Leads to Ultrafast Charge Migration in DNA Hairpins. Nat. Chem. 2016, 8, 1015-1021. (4) Rathore, R.; Abdelwahed, S. H.; Guzei, I. A. Synthesis, Structure, and Evaluation of the Effect of Multiple Stacking on the Electron-donor Properties of Pi-stacked Polyfluorenes. J. Am. Chem. Soc. 2003, 125, 8712-8713. (5) Navale, T. S.; Thakur, K.; Vyas, V. S.; Wadumethrige, S. H.; Shukla, R.; Lindeman, S. V.; Rathore, R. Charge Delocalization in Self-assembled Mixed-valence Aromatic Cation Radicals. Langmuir. 2012, 28, 71-83.


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(6) DHA-based molecules are being actively explored for photovoltaic applications, see Cheng, C.-H.; Lin, J.-J.; US Patent 20090062570, 2009; Ikeda, Y.; Ito, H.; Ikeda, T.; JP Patent 2014205641, 2014. (7) Pryor, W. A.; Gleicher, G. J.; Church, D. F. Relative Reactivities of Alkylbenzenes and Related Compounds Toward Ozone. The Mechanism of Ozonation at Benzylic Positions. J. Org. Chem. 1984, 49, 2574-2578. (8) Navale, T. S.; Rathore, R. A Practical Synthesis of 1, 4, 5, 8-Tetramethoxyanthracene From Inexpensive and Readily Available 1, 8-Dihydroxyanthraquinone. Synthesis. 2012, 44, 805-809. (9) Talipov, M. R.; Boddeda, A.; Hossain, M. M.; Rathore, R. Quantitative Generation of Cation Radicals and Dications Using Aromatic Oxidants: Effect of Added Electrolyte on the Redox Potentials of Aromatic Electron Donors. J. Phys. Org. Chem. 2015, 29, 227-233. (10) Talipov, M. R.; Rathore, R. Robust Aromatic Cation Radicals as Redox Tunable Oxidants. In Organic Redox Systems: Synthesis, Properties, and Applications John Wiley & Sons: Hoboken, NJ, 2015; pp 131. (11) Talipov, M. R.; Boddeda, A.; Timerghazin, Q. K.; Rathore, R. Key Role of End-capping Groups in Optoelectronic Properties of Poly-p-phenylene Cation Radicals. J. Phys. Chem. C. 2014, 118, 21400-21408. (12) Wang, D.; Talipov, M. R.; Ivanov, M. V.; Rathore, R. Energy Gap Between the Poly-pphenylene Bridge and Donor Groups Controls the Hole Delocalization in Donor--Bridge--Donor Wires. J. Am. Chem. Soc. 2016, 138, 16337-16344.


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(13) Reilly, N.; Ivanov, M.; Uhler, B.; Talipov, M.; Rathore, R.; Reid, S. A. First Experimental Evidence for the Diverse Requirements of Excimer Vs Hole Stabilization in Π-Stacked Assemblies. J. Phys. Chem. Lett. 2016, 7, 3042-3045. (14) An increase in the interplanar angle leads to an uneven spin/charge distribution in 6+•-9+• and it should be contrasted with a complete spin/charge localization onto one ring in 10+• and 11+•, which arises due to the symmetric nature of bonding SOMO; also see Figures S4-S5 in the Supporting Information. (15) Hoffmann, R.; Heilbronner, E.; Gleiter, R. Interaction of Nonconjugated Double Bonds. J. Am. Chem. Soc. 1970, 92, 706-707. (16) Goldstein, M. J.; Natowsky, S.; Heilbronner, E.; Hornung, V. Near Cancellation of Through Space and Through Bond Interaction in Bicyclo [3.2. 2] Nona-6, 8-diene. Helv. Chim. Acta. 1973, 56, 294-301. (17) Paddon-Row, M. N. Orbital Interactions and Long-range Electron Transfer. Adv. Phys. Org. Chem. 2003, 38, 1-85. (18) Borden, W. T. Modern molecular orbital theory for organic chemists; Prentice-Hall Englewood Cliffs, New Jersey: 1975. (19) Hoffmann, R. Interaction of Orbitals Through Space and Through Bonds. Acc. Chem. Res. 1971, 4, 1-9. (20) Howell, J. O.; Goncalves, J. M.; Amatore, C.; Klasinc, L.; Wightman, R. M.; Kochi, J. K. Electron Transfer From Aromatic Hydrocarbons and Their Pi-Complexes With Metals.


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Comparison of the Standard Oxidation Potentials and Vertical Ionization Potentials. J. Am. Chem. Soc. 1984, 106, 3968-3976. (21) Anslyn, E. V.; Dougherty, D. A. Modern physical organic chemistry; University Science Books: 2006. (22) Dewar, M. J. Aromaticity and Pericyclic Reactions. Angew. Chem. Int. Ed. Engl. 1971, 10, 761-776. (23) Rathore, R.; Le Magueres, P.; Lindeman, S. V.; Kochi, J. K. A Redox-Controlled Molecular Switch Based on the Reversible C- C Bond Formation in Octamethoxytetraphenylene. Angew. Chem. Int. Ed. 2000, 39, 809-812.


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