Off the

Article pubs.acs.org/JPCC

Nodal Arrangement of HOMO Controls the Turning On/Off the Electronic Coupling in Isomeric Polypyrene Wires Maxim V. Ivanov, Khushabu Thakur, Anitha Boddeda, Denan Wang, and Rajendra Rathore* Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin 53201-1881, United States S Supporting Information *

ABSTRACT: The charge transfer along π-conjugated wires is largely governed by the interchromophoric electronic coupling that depends on the geometry (e.g., interchromophoric dihedral angle) and electronic structure of the chromophores. Herein, we demonstrate that stabilization of the cationic charge (hole) in polypyrene cation radicals and the extent of hole delocalization can be easily controlled by modulating the nodal arrangement of the HOMO. For example, 2,2′-linked para-polypyrenes show nonexistent electronic coupling owing to a nodal arrangement of HOMO that is unfavorable for orbital overlap, despite a favorable interchromophoric dihedral angle. A repositioning of the linkage between two pyrenes from para to meta positions produces a far less favorable interchromophoric dihedral angle, yet the electronic coupling turns on due to a favorable nodal arrangement of HOMO, which allows interchromophoric orbital overlap. This surprising finding has been demonstrated through the synthesis and systematic examination of the redox and optical properties of meta-polypyrenes (m-Pyn), which reveals a sizable delocalization of the hole in m-Pyn+• that extends to three pyrene units, only two benzenoid units less than in typical poly-pphenylene wires. These findings of widespread interest, supported by density functional theory (DFT) and the Marcus-based multistate model, will impact the rational design of new charge-transfer materials for photovoltaic and molecular electronics applications.

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INTRODUCTION Development of new π-conjugated molecular wires with enhanced redox and optical properties holds significant promise for applications in photovoltaic devices and molecular electronics.1−4 The rational design of new wirelike materials using polycyclic aromatic hydrocarbons as repeating units rather than single p-phenylenes may extend the effective length of charge/exciton delocalization;5,6 however, one must consider other factors, such as interplanar dihedral angles and/or the electronic structure of the chromophore, in order to maximize the interchromophoric electronic coupling.5−11 For example, linking a pair of fluorenes at the 2,2′ position (i.e., bifluorene) allows an effective stabilization of the cationic charge (hole), as judged by a pronounced lowering of its oxidation potential by 370 mV compared to fluorene (Figure 1).12 Surprisingly, linking a pair of pyrenesa chromophore with rich optoelectronic properties13−16at (para-)2,2′-positions, leads to little or no stabilization of its cation radical, yet the interchromophoric dihedral angle is identical to that in bifluorene (Figure 1). Even more surprising, the isomeric (meta-)3,3′-linked bipyrene affords a stabilization of ∼70 mV, despite possessing an interchromophoric dihedral angle that is much larger than para-bipyrene or bifluorene (Figure 1).17 Reflecting on the origin of these surprising observations, a visual inspection of the nodal arrangements of the corresponding HOMOs suggests that absence of overlap between monomeric HOMOs in para-bipyrene as opposed to bifluorene © XXXX American Chemical Society

Figure 1. Structures and HOMOs of bifluorene (R = n-hexyl), (para− )2,2′-linked and (meta-)3,3′-linked bipyrenes. The ΔEox (= Edimer ox ) reflects stabilization of the cationic charge (see Figure S5 in Emonomer ox the Supporting Information).

accounts for the very different interchromophoric electronic coupling in these two structurally similar molecules (see red circles in Figure 1). In contrast, a simple alteration of the linkage from para- to meta-bipyrene turns on the electronic coupling due to the appreciable orbital overlap despite a dramatically increased interplanar dihedral angle (Figure 1). This simple analysis of the nodal structure of HOMOs suggests that the meta-linked polypyrenes could potentially afford longrange hole delocalization; however, it is not clear whether the Received: March 9, 2017 Published: April 11, 2017 A

DOI: 10.1021/acs.jpcc.7b02264 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Preparation of m-Pyn (n = 1-5)1

hole delocalization in higher homologues (n > 2) will extend beyond two units. To resolve this question, we undertook the synthesis of a series of long (up to 3.7 nm) meta-linked polypyrenes (m-Pyn, n = 1−5), decorated with long alkyl chains for improved solubility, and determined redox and optical properties of the neutral m-Pyn and their cation radicals (m-Pyn+•). The availability of experimental data together with DFT calculations and theoretical modeling reveals that, despite large interplanar dihedral angles in m-Pyn, the effective length of hole delocalization in m-Pyn+• extends to at least three pyrene units, which is only two benzenoid units less than the hole delocalization in strongly coupled poly-p-fluorene (Fn+•) wires.12,18,19 Details of this comparative study of hole distribution/stabilization in m-Pyn versus Fn wires is described herein.

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RESULTS AND DISCUSSION Synthesis. The synthesis of soluble oligomers of polypyrenes linked at 3,3′ positions (m-Pyn, n = 1−5) was accomplished using 2,7-dioctylpyrene (Py1, see Scheme 1), which was prepared from the parent pyrene by adapting standard literature procedures. An oxidative coupling of Py1 using DDQ/CH3SO3H as an oxidant20 afforded m-Py2 in excellent yield. Synthesis of m-Py3 was accomplished by Suzuki coupling21,22 between mono and bis-boronic esters of dioctylpyrene obtained from the corresponding mono and dibromo-dioctylpyrene (Scheme 1). The syntheses of higher homologues (n = 4−5) was similarly accomplished by Suzuki coupling of dibromo derivatives of m-Py2 and m-Py3 with monoboronic ester of Py1 (see Scheme 1). The structures of various m-Pyn were established by 1H/13C NMR spectroscopy and MALDI mass spectrometry and were further confirmed by X-ray crystallography of m-Py2 as a representative molecule (see the Supporting Information for details). Electronic Absorption and Emission Spectroscopy of m-Pyn. The structured electronic absorption band of m-Pyn shifts red with increasing n, as measured by the onset (Figure 2A, Table 1), and follows an expected linear trend with cos[π/ (n + 1)] (Figure 2B).12,23−25 Moreover, the slope of the trendline for m-Pyn is roughly half of the slope for polyfluorene wires (Figure 2B), indicating a smaller interchromophoric coupling in m-Pyn as compared to Fn.12 The emission spectra of m-Pyn, Figure 2C, evolves from structured emission of Py1 into a structureless band that shifts red with increasing n. The plots of νem against cos[π/(n + 1)] for m-Pyn and Fn are compared in Figure 2D. The slope for mPyn is roughly one-fifth of the slope for Fn, suggesting that the varied interchromophoric coupling in m-Pyn and Fn has a pronounced impact on the evolution of the emission energies, which saturate at n = 3−4 in m-Pyn as compared to n = 6−7 in Fn.12,26 Electrochemistry and Electronic Spectroscopy of mPyn+•. Electrochemical oxidation of m-Pyn in CH2Cl2 showed reversible cyclic voltammograms with multiple waves that correspond to the number of pyrenes in a given m-Pyn (Figure 3A). A list of oxidation potentials referenced to ferrocene is compiled in Table 1. A plot of first oxidation potentials (Eox1) against cos[π/(n + 1)] showed a rapid saturation beyond mPy3, suggesting that the hole delocalization does not extend beyond three pyrene units (Figure 3B). Moreover, the slope of the trendline in Eox1 vs cos[π/(n + 1)] plot for m-Pyn is roughly

1a

Aqueous HBr (48%), H2O2 (30%), ether/methanol (1:1). b1Octyne, (PPh3)2PdCl2, (iPr2)NH/THF, 50 °C. cRepeated crystallization from n-hexanes. dPd−C/H2, EtOAc/EtOH (2:1). eDDQ, CH2Cl2/MeSO3H (9:1). fNBS (2 equiv) NH4NO3/MeCN. g2,7Dioctylpyrene-3-boranate ester, Pd(PPh3)4, aqeous Na2CO3, DME, reflux. hNBS (1 equiv), NH4NO3, MeCN. iPinacolotodiborane, Pd(dppf)Cl2, KOAc, dioxane, reflux. jPd(PPh3)4, aq Na2CO3, DME, reflux.

half of the slope for Fn (Figure 3B), suggesting a relatively smaller interchromophoric coupling in m-Pyn+• as compared to Fn+•.12 Electronic absorption spectra of cation radicals of m-Pyn (Figure 3C) were obtained by quantitative redox titrations using three different aromatic oxidants, i.e. THEO+•, NAP+•, B


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The Journal of Physical Chemistry C

Figure 2. (A) Electronic absorption spectra of m-Pyn (n = 1−5) in CH2Cl2. (B) Plot of the onset energies (inset, part A) of m-Pyn (red) and the energies of absorption maxima of Fn (blue) against cos[π/(n + 1)]. (C) Emission spectra of m-Pyn (n = 1−5) in CH2Cl2. (D) Plot of the energies of emission maxima of m-Pyn (red) and Fn (blue) against cos[π/(n + 1)].

Table 1. Experimental Onset λonset (nm) of the Absorption Spectra, λem (nm) of the Emission Maxima, Oxidation Potentials Eox1 (V vs Fc/Fc+) of m-Pyn, and λabs (nm)/εabs (104 M−1 cm−1) of the Absorption Maxima of m-Pyn+• n

1

2

3

4

5

λonset λem Eox1 λabs εabs

366 382 0.67 694 5.1

412 450 0.61 2366 14.3

427 458 0.56 2580 20.8

436 464 0.55 2739 20.8

442 464 0.55 2743 19.1

Figure 3. (A) Cyclic/square-wave voltammograms of 1 mM m-Pyn in CH2Cl2 (0.1 M n-Bu4NPF6) at 200 mV s−1 and 22 °C. (B) Plot of Eox1 of m-Pyn (red) and Fn (blue) against cos[π/(n + 1)]. (C) Absorption spectra of m-Pyn+• in CH2Cl2 at 22 °C. (D) Plot of energies of the absorption maxima of Pyn+• (red) and Fn+• (blue) against cos[π/(n + 1)]. (E) Spectral changes observed upon the reduction of 0.031 mM THEO+• in CH2Cl2 (3 mL) by addition of 15-μL increments of 0.86 mM stock solution of m-Py2 in CH2Cl2. (F) Deconvolution of UV−vis absorption spectrum from each titration point in part E into its component spectra, i.e. THEO+• and m-Py2+•. (C) Plot of the mole fractions of THEO+• (red) and m-Py2+• (blue) against the added equivalents of m-Py2. Symbols represent experimental points, while the solid lines show the best fit to experimental points using ΔG1 = Eox(mPy2) − Ered(THEO+•) = −116 mV.

and ANT+•,26−28 see Figures S9−S22 in the Supporting Information. As an example, absorption spectra of m-Py2+• were obtained by an incremental addition of a concentrated solution of m-Py2 to a solution of [THEO+•SbCl6−] (Figure 3E), followed by numerical deconvolution29 performed at each titration point (Figure 3F and G), which confirmed a 1:1 stoichiometry of the redox reaction, i.e., eq 1:

the vertically excited state does not extend beyond 3−4 pyrene units (Figure 3D, Table 1). Electronic Structure Calculations. (TD-)DFT calculations of absorption energies (νabs) and oxidation energies (Eox1) of m-Pyn and absorption energies of D0 → D1 transitions in mPyn+•, at the B1LYP-40/6-31G(d)+PCM(CH2Cl2) level of theory,19 showed an excellent agreement with the experiment (see Tables 1, S6 and Figures S23−S25 in the Supporting Information). For example, the computed νabs of m-Pyn (n = 1− 7) follow a linear cos[π/(n + 1)] trend, while Eox1 and the energies of D0 → D1 transitions in m-Pyn+• show a breakdown from cos[π/(n + 1)] trend (compare Figures 2 and 3 with S23−S25).

Reliable energies of the absorption maxima of m-Pyn+•, obtained from reproducible electronic absorption spectra, showed a linear cos[π/(n + 1)] dependence only up to 3−4 pyrene units followed by saturation, suggesting that the hole in C


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The Journal of Physical Chemistry C In neutral m-Pyn, the HOMOs are delocalized over the entire chain with a bell-shaped distribution (Figure 4). However,

Figure 4. (left) Isovalue (0.001 au) plots of HOMOs of m-Pyn and plots of spin densities (0.01 au) of m-Pyn+• in D0 (middle) and D1 (right) states, superimposed with corresponding per-unit barplot representations. Per-unit barplot quantification of HOMO-densities in m-Pyn and spin/charge densities in m-Pyn+• was obtained from the HOMO expansion coefficients and natural population analysis (NPA),30,31 respectively.

spin/charge (hole) distribution in ground (D0) state in m-Py2+• and m-Py3+• is similar to HOMO distribution, while in higher homologues, the hole distribution does not extend beyond three pyrene units (Figure 4). In the vertically excited (D1) state, the hole distribution extends away from the middle unit and shows a slight spillover beyond three units (e.g., 0.07 e− in the outer units in m-Py5+•, Figure 4). Availability of the X-ray structures32 of tetraisopropylpyrene (TIP) and its cation radical TIP+• allowed us to benchmark the oxidation-induced structural reorganization computed for mPyn+•. In particular, oxidation-induced bond length changes in the TIP → TIP+• transformation obtained from X-ray crystallography follow a linear correlation with the results from DFT calculations (Figure 5A) and are identical to the bond length changes computed for Py1 → Py1+• (Figure 5B). Furthermore, structural reorganization in smaller homologues of m-Pyn+• (n = 1−3) follows the distribution of HOMO and its nodal arrangementi.e., bonds with bonding HOMO lobes undergo elongation, while bonds with antibonding lobes undergo contraction (Figure 5A). Additionally, we find a reduction in the interchromophoric dihedral angles, e.g., the single dihedral angle in m-Py2+• is reduced by 22°, i.e. from 71° to 49°, while, in m-Py3+•, the two dihedral angles are reduced by 16°, i.e. from 71° to 55°. The distribution of bond length contractions/ elongations parallels spin/charge distribution and is limited to three units for larger m-Pyn+• (i.e., n ≥ 3). The accompanying reductions in the dihedral angles in these Pyn+• (i.e., n ≥ 3) are associated only to spin/charge-bearing pyrenes (Figure 5B). Notably, such direct correspondence between distributions of the charge and structural reorganization (compare Figures 4 and 5B) is often referred to as polaron formation, i.e. the coupling between charge and nuclear motions.33−35 A comparison of the spin/charge distribution and accompanying structural changes in m-Pyn, p-Pyn, and Fn shows that the interchromophoric dihedral angles in m-Pyn+• are more than twice of those in Fn+• (e.g., 49° vs 20° for n = 2), yet the hole distribution in m-Pyn+•, is extended over three pyrene units as compared with four fluorene units in Fn+•, a difference

Figure 5. (A) Correlation of the oxidation-induced bond length changes in TIP → TIP+• transformation obtained by X-ray crystallography32 and DFT calculations (y = 1.05x + 0.06, R2 = 0.97). (B) Distribution of the oxidation-induced changes of the representative C−C bonds and dihedral angles between adjacent pyrenes in m-Pyn+• obtained from DFT calculations. Also see Figure S27 in the Supporting Information.

of only two benzenoid units. At the same time, in poly pyrene cation radicals p-Pyn+•, the hole distribution is limited to one pyrene unit for all n, despite a favorable dihedral angle (Figure S29 in the Supporting Information). In order to understand such a dramatic influence of the linkage between pyrenes on the extent of hole delocalization, below we compare m-Pyn, p-Pyn, and Fn in the context of the coarse-grained representation of the Hückel molecular orbital theory12,36 and Marcus two- and multistate models. Molecular Orbital Theory. A simple visual inspection of the HOMOs in F2, m-Py2, and p-Py2 indicates that, due to their different nodal arrangements, there is a strong overlap in F2 and no overlap in p-Py2 due to a complete absence of HOMO density at the carbons linking two pyrene monomers (red circles, Figure 6). As a result, the splitting between symmetric and antisymmetric linear combinations of monomeric HOMOs (i.e., 2β) and thus the electronic coupling (β) is drastically different in F2 and p-Py2 (Figure 6), although the dihedral angle is similar (37° vs 39°). A simple alternation of the linkage from para to meta leads to appreciable orbital overlap because of the presence of substantial HOMO density at the carbons linking two pyrene monomers and hence electronic coupling (β = 0.14 eV), despite an unfavorable dihedral angle (39° vs 71°). D


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hole stabilization lies at the borderline between the two regimes, i.e. static delocalization and dynamic hopping (Figure 7, middle). Note that the electronic coupling of F2+• and mPy2+• can be directly read from the excitation energies at the equilibrium angle, i.e., Hab(F2+•) = 0.60 eV and Hab(m-Py2+•) = 0.33 eV, while the reorganization energy (λ) can be obtained by extrapolation of the nonlinear curve to the equilibrium angle, i.e., λ(F2+•) = 0.33 eV and λ(m-Py2+•) = 0.56 eV. Also note that there is no switchover in the delocalization mechanism for pPy2+• because the hole is always localized onto one pyrene unit and the dihedral angle scan can only provide the reorganization energy λ(p-Py2+•) = 0.56 eV (Figure 7, right). Marcus-Based Multistate Model. We incorporated values of Hab and λ obtained from Figure 7 into our recently developed multistate model (MSM)an extension of the Marcus two-state model to multiple unitsto simulate the hole distribution in m-Pyn+• and Fn+•.19,45 In MSM, n × n Hamiltonian matrices H(x) are constructed and subsequently diagonalized for a range of values x along the reaction coordinate. This provides the adiabatic ground state potential energy surface G0(x) with the minimum defining the position of the hole (i.e., xmin). The lowest-energy eigenvalue at this point defines the oxidation energy (i.e., G0(xmin)) and the corresponding eigenvector defines the hole distribution. A plot of the oxidation energies G0(xmin) thus derived by MSM against cos[π/(n + 1)] reproduces the experimentally observed evolution of the oxidation potentials of m-Pyn (compare Figures 8A and 3B). In particular, the MSM captures precisely even the modest reduction of the oxidation energies in m-Pyn as compared with Fn (−60 mV vs −170 mV, Figure 8A). Moreover, the hole distributions in m-Pyn+• from MSM matches closely to that obtained from DFT calculations (compare Figures 8B and 4). A closer look at the potential energy surfaces of various mPyn+•, obtained from the MSM, suggests that starting from n = 4, multiple isoenergetic structures with small interconversion barriers (10 mV or 0.23 kcal/mol) are possible (Figure 8B). Indeed, DFT calculations of m-Py4+• identified a transition state structure where the hole is distributed over two units in the middle of the chain, which possessed an electronic energy 8 mV (or 0.19 kcal/mol) higher than two equilibrium structures where the hole resides over one side of the chain (Figure 8C). Notably, the single imaginary frequency (i550 cm−1) of the transition state structure corresponds to a mode that involves vibrations of the C−C bonds in two middle pyrene units (see Table S8 in the Supporting Information). The dynamic interconversion between m-Pyn+• isomers with such a minimal barrier may provide a pathway for the charge transfer along the long polypyrene wires.46,47

Figure 6. Assessment of electronic coupling (β) from the bonding and antibonding HOMOs (i.e., symmetric and antisymmetric linear combinations of monomeric HOMOs) of F2, m-Py2, and p-Py2.

Marcus Two-State Model. Analysis of the frontier molecular orbitals quickly provides an estimate of the electronic coupling; however, the extent of hole delocalization in corresponding cation radicals is expressed by the interplay between the electronic coupling (Hab ≈ β) and structural/ solvent reorganization energy (λ).37−40 For example, in the strong coupling regime (i.e., 2Hab > λ, class III), the hole is delocalized over both units and the absorption energy of the cation radical gives the electronic coupling, i.e. νabs = 2Hab (Figure S30A in the Supporting Information). In the weak coupling regime (i.e., 2Hab < λ, class II), the hole may dynamically hop, and the absorption energy provides the reorganization energy, i.e. νabs = λ. However, it is often difficult to distinguish between these two cases, especially for systems with weak electronic coupling, which may lie in between (i.e., 2Hab ≈ λ).41−43 The mechanism of hole delocalization can be readily discerned by computing the excitation energies of biaryl cation radicals at varying dihedral angles (φ).44 As shown in Figure 7,

Figure 7. Dihedral angle scans of F2+•, m-Py2+•, and p-Py2+• obtained as series of constrained optimizations with fixed interchromophoric dihedral angle (φ = 0°−90°) and subsequent calculation of D0 → D1 excitation energies using single-point TD-DFT. Equilibrium and switchover angles are indicated by arrows. Two-state Marcus models of F2+•, m-Py2+•, and p-Py2+• based on the calculated parameters (Hab and λ) for equilibrium geometries are also shown.

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CONCLUSIONS In conclusion, we have carefully examined the evolution of the redox and optical properties of the polypyrenes wires, linked at 2,2′ (meta) positions, i.e. m-Pyn. Both experimental data and DFT calculations showed that oxidation potentials (Eox) of mPyn and excitation energies (νabs) of m-Pyn+• quickly saturate with increasing n, i.e. no change was observed after n = 3 for Eox and after n = 3−4 for νabs, suggesting that the hole is confined to 3−4 pyrene units. Quantification of the electronic couplings β (via molecular orbital theory, Figure 6) and Hab (via dihedral angle scan, Figure 7) showed that despite a roughly 3-fold decrease in the electronic couplings in m-Pyn as compared to typical poly-p-

starting from a completely planar geometry, an increase in φ leads to a switchover of the mechanism of hole stabilization from static delocalization (linear dependence) to a dynamic hopping (nonlinear dependence), where the switchover angle corresponds to the case where 2Hab = λ. In F2+•, the mechanism of hole stabilization is static delocalization because the equilibrium angle of 20° is much smaller than the switchover angle of 63° (Figure 7, left). However, in m-Py2+•, the equilibrium angle of 49° is close to the switchover angle of 52°, and thus, the mechanism of the E


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Rotatable images of compound C64 H82 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Maxim V. Ivanov: 0000-0002-6171-9524 Rajendra Rathore: 0000-0001-7387-7936 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSF (CHE-1508677) and NIH (R01HL112639-04) for financial support and Professors Scott A. Reid and Marat R. Talipov for helpful discussions. The calculations were performed on the high-performance computing cluster Père at Marquette University and the Extreme Science and Engineering Discovery Environment (XSEDE) funded by NSF (TG-CHE130101).

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(1) Lin, Y.; Zhan, X. Oligomer Molecules for Efficient Organic Photovoltaics. Acc. Chem. Res. 2016, 49, 175−183. (2) Li, C.; Liu, M.; Pschirer, N. G.; Baumgarten, M.; Müllen, K. Polyphenylene-based Materials for Organic Photovoltaics. Chem. Rev. 2010, 110, 6817−6855. (3) Facchetti, A. Π-conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications. Chem. Mater. 2011, 23, 733−758. (4) Carroll, R. L.; Gorman, C. B. The Genesis of Molecular Electronics. Angew. Chem., Int. Ed. 2002, 41, 4378−4400. (5) Seth, J.; Palaniappan, V.; Johnson, T. E.; Prathapan, S.; Lindsey, J. S.; Bocian, D. F. Investigation of Electronic Communication in Multiporphyrin Light-harvesting Arrays. J. Am. Chem. Soc. 1994, 116, 10578−10592. (6) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. Soluble Synthetic Multiporphyrin Arrays. 1. Modular Design and Synthesis. J. Am. Chem. Soc. 1996, 118, 11166−11180. (7) Branzea, D. G.; Pop, F.; Auban-Senzier, P.; Clérac, R.; Alemany, P.; Canadell, E.; Avarvari, N. Localization Versus Delocalization in Chiral Single Component Conductors of Gold Bis(dithiolene) Complexes. J. Am. Chem. Soc. 2016, 138, 6838−6851. (8) Elbing, M.; Bazan, G. C. A New Design Strategy for Organic Optoelectronic Materials by Lateral Boryl Substitution. Angew. Chem., Int. Ed. 2008, 47, 834−838. (9) Chukharev, V.; Tkachenko, N. V.; Efimov, A.; Guldi, D. M.; Hirsch, A.; Scheloske, M.; Lemmetyinen, H. Tuning the Ground-state and Excited-state Interchromophore Interactions in Porphyrin-fullerene Π-stacks. J. Phys. Chem. B 2004, 108, 16377−16385. (10) Kong, D.-D.; Xue, L.-S.; Jang, R.; Liu, B.; Meng, X.-G.; Jin, S.; Ou, Y.-P.; Hao, X.; Liu, S.-H. Conformational Tuning of the Intramolecular Electronic Coupling in Molecular-Wire Biruthenium Complexes Bridged by Biphenyl Derivatives. Chem. - Eur. J. 2015, 21, 9895−9904. (11) Talipov, M. R.; Navale, T. S.; Rathore, R. Nodal Arrangement of HOMOs in Polychromophoric Molecules and Assemblies Controls Interchromophoric Electronic Coupling. Angew. Chem., Int. Ed. 2015, 54, 14468−14472. (12) Ivanov, M. V.; Talipov, M. R.; Boddeda, A.; Abdelwahed, S. H.; Rathore, R. Hückel Theory+ Reorganization Energy = Marcus-Hush TheoryBreakdown of the 1/n Trend in Π-Conjugated Poly-pphenylene Cation Radicals Is Explained. J. Phys. Chem. C 2017, 121, 1552−1561. (13) Kim, S.; Kim, B.; Lee, J.; Shin, H.; Park, Y.-I.; Park, J. Design of Fluorescent Blue Light-emitting Materials Based on Analyses of Chemical Structures and Their Effects. Mater. Sci. Eng., R 2016, 99, 1− 22.

Figure 8. (A) Plot of oxidation energies G0(xmin) of m-Pyn and Fn against cos[π/(n + 1)] scaled to experimental oxidation potentials. (B) Potential energy surfaces and barplot representations of the hole distributions in m-Pyn+• obtained from MSM. (C) Potential energy surface of m-Py4+• obtained from MSM and spin-densities of two isomers and the transition state of m-Py4+• obtained from DFT.

phenylenes such as poly fluorenes (Fn), the effective length of hole delocalization is only two benzenoid units less in m-Pyn+• than in Fn+•. Furthermore, a slightly more confined hole distribution in m-Pyn+• leads to existence of several isoenergetic electronic structures, which can easily interconvert due to small interconversion barriers between them, providing a pathway for a long-range charge transfer. Finally, we have demonstrated that a simple alternation in the linkage from (meta) 2,2′-positions to (para) 3,3′-positions in the isomeric polypyrenes turns off the electronic coupling and limits the hole delocalization to a single unit. This on/off switching of the electronic coupling is ultimately controlled by the nodal structure of the HOMO and can be easily predicted by a simple visual inspection of the filled frontier molecular orbitals (FMOs), akin to the prediction of chemical reactivity and (regio-)stereoselectivity of pericyclic reactions based on the FMOs,48 which provides a valuable tool for the design of nextgeneration materials for photovoltaics and molecular electronics applications.

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02264. Experimental and computational details (PDF) Coordinates of the optimized structures (TXT) Rotatable images of compound C32 H42 (CIF) F


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