Pyrene-like HOMO Governs Polaron Delocalization in Model Graphitic

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C: Energy Conversion and Storage; Energy and Charge Transport

Pyrene-like HOMO Governs Polaron Delocalization in Model Graphitic Strips: A Combined Experimental and Computational Analysis Maxim V. Ivanov, Ruchi Shukla, Sergey V. Lindeman, Denan Wang, and Rajendra Rathore J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06068 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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The Journal of Physical Chemistry

Pyrene-like HOMO Governs Polaron Delocalization in Model Graphitic Strips: A Combined Experimental and Computational Analysis Maxim V. Ivanov,§* Ruchi Shukla, Sergey V. Lindeman, Denan Wang and Rajendra Rathore

†

Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201 KEYWORDS. Polycyclic aromatic hydrocarbons, charge-transfer, polaron delocalization

ABSTRACT. Polycyclic aromatic hydrocarbons (PAH) with large graphitic cores have attracted significant attention as charge-transfer materials for photovoltaic and molecular electronics applications. In this work, we probe the redox and optoelectronic properties of novel hexabenzo[a,c,fg,j,l,op]tetracene (HBT) and its methoxylated analogue

HBT to seek whether

MeO

long HBT-based graphitic strips can be viable candidates as efficient charge-transfer material. Our data reveals that, despite the presence of eight electron-rich methoxy groups in

HBT, the

MeO

redox/optoelectronic properties of these two HBTs are very similar, an unusual finding in comparison with other smaller PAHs and poly-p-phenylene wires. Precise crystal structures of neutral HBTs and their cation radicals in comparison with carefully benchmarked DFT calculations revealed that the polaron in both HBT and HBT is mainly localized at the central +•

MeO

+•

pyrene, with a minor spillover to peripheral biphenyl moieties, and mirrors the distribution of

ACS Paragon Plus Environment

1

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pyrene-like HOMO of neutral HBTs. Finally, aided by the DFT calculations, we show that redox properties of long HBT-based graphitic strips are nearly invariant to both substitution and length, with the polaron delocalization limited to only three pyrene moieties. This finding suggests that this class of wires could be promising candidate materials for long-range charge transfer studies.

Introduction The rational design of novel π-conjugated charge-transfer materials with enhanced redox and optical properties is critically important for improving the efficiency of modern photovoltaic and molecular electronics based devices. For example, in the course of a rational design of a new 1-4

charge-transfer wire, one may consider constructing a poly-p-phenylene ( PP ) wire with pR

n

phenylene as a repeat unit (Figure 1A). Additional tailoring of the wire can be accomplished by 5,6

judicious choice of the substituents at the repeat unit to modulate its HOMO energy, HOMO 7,8

nodal arrangement, and interplanar angle between the repeat units. Moreover, incorporation of 9

10

appropriate end-capping groups (i.e., alkyl, alkoxy, dialkylamino or R = A, AO, A2N),

11,12

can

skew the hole distribution towards one end of the wire (Figure 1A). This list of well-defined parameters to design novel wires has emerged from detailed analysis of the experimental redox/optical properties of various poly-p-phenylene-based wires with the aid of electronic structure calculations and theoretical modeling via molecular orbital and Marcus-Hush theories.

11-

13


2

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Figure 1. Structures, HOMO and spin-density plots of poly-p-phenylene wires

11,12

(A),

representative examples of PAHs (B), and PAH-based wire related to this work (C). Based on these design principles, the utilization of various polycyclic aromatic hydrocarbons (PAHs), possessing rich redox and optoelectronic properties,

14-17

to construct extended PAH-based

wires is highly desired (Figure 1B). In contrast to a poly-p-phenylene wire, the choice of repeat unit in such a wire is unclear, as multiple PAH-moieties could be envisioned as the repeat unit, as shown in the graphitic strip pictured in Figure 1C. Among various possible PAH-moieties, hexabenzo[a,c,fg,j,l,op]tetracene (HBT) represents the smallest unit that exhibits essential structural motifs of its longer homologue, yet contains an extended π-system that is only two carbons smaller than well-studied hexa-peri-hexabenzocoronene. In order to gain a fundamental understating of the electronic structure of such PAH-based wires, we have performed a careful analysis of the redox, optical and structural properties of


3


HBT and its methoxylated analogue

Page 4 of 27

HBT via electrochemical analysis, generation and

MeO

absorption and EPR spectroscopies of their cation radical, X-ray crystallography and DFT calculations. Our data showed that the redox/optical properties and hence the extent of polaron delocalization in HBT and HBT are quite similar, in contrast to the properties of other PAHs +•

MeO

+•

and poly-p-phenylene wires. For example, comparing redox properties of HBT and

HBT we

MeO

surprisingly found that incorporation of eight methoxy groups led to a mere 170 mV lowering of the oxidation potential—a remarkably small stabilization considering that incorporation of only two methoxy groups in benzene lowers its ionization energy by more than 1 volt. Quantitative assessment of the polaron delocalization via X-ray crystallography and benchmarked DFT calculations showed that the polaron is primarily localized at the central pyrene moiety of HBT

+•

and HBT with a minor spillover to the peripheral biphenyl moieties. Finally, DFT calculations MeO

+•

of longer HBT-based wires showed that their redox properties are weakly dependent on the wire length and substitution, suggesting that this class of graphitic strips could be a promising material for long-range charge transfer. Although synthesis of various similar PAHs with extended graphitic cores have been reported,

18-21

their promise as long-range charge-transfer materials has yet to be explored/realized.

Details of the experimental/computational approach described herein shed light on the viability of these graphitic strips as novel charge-transfer materials. Results and discussions Synthesis. Parent HBT and its octamethoxy analogue the well-established Suzuki coupling

22,23

HBT were prepared by adaptation of

MeO

and Scholl protocols

24,25

(i.e. Scheme 1). For example, the

precursors for HBT/ HBT can be accessed via Suzuki coupling with readily-available MeO

tetrabromopyrene /dibromobiphenyl with veratroleboronic acid/phenanthrene boronic acid, 26


4

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respectively, followed by Scholl reaction using FeCl as oxidant. A caveat is that the oxidative 3

cyclization using FeCl produces a significant amount of isomeric cyclized product together with 3

HBT. This issue can be obviated by replacing FeCl with DDQ/CH SO H as oxidant to

MeO

27

3

3

3

produce HBT quantitatively. See the Supporting Information for complete experimental details MeO

as well as the characterization data of all compounds from Scheme 1.

Scheme 1. Synthesis of HBT and

HBT. a. Br /[Fe powder]/CCl /22 C/4h; b. Pd(PPh ) /aq

MeO

Na CO /DME/Toluene/reflux/12 h; c. 2

3

2

4

FeCl /CH Cl /0 C/30 min. 3

2

2

o

d.

o

3 4

DDQ/CH Cl -MeSO H 2

2

3

(9:1)/~0 C/30 min. o

Electrochemistry. The redox properties of HBT/ HBT were evaluated by electrochemical MeO

oxidation at a platinum electrode as a 1 mM solution in CH Cl containing 0.1 M n-Bu N PF as +

2

2

4

6

the supporting electrolyte. Both HBT/ HBT showed highly reversible cyclic voltammograms MeO

(CVs) with multiple redox waves: E (HBT) = 0.61, 1.15 V vs Fc/Fc and E +

ox1,2

ox1,2,3,4

( HBT) = 0.44, MeO

0.79, 1.05, 1.30 V vs Fc/Fc (Figure 2). The CV of HBT was expected to be reversible, based +

MeO

on the expected high stability of its cation radical as a result of methoxy group substitution; however, it was somewhat surprising that the CV of HBT was found to be completely reversible,


5


Page 6 of 27

despite the presence of substitution-labile carbons (i.e., at the periphery of biphenyl moieties in HBT) that should have rendered its cation radical to be highly susceptible to dimerization. 0.61

HBT 0.44

HBT

MeO

O

O O

O

O

O O

1.6

1.2 0.8 0.4 V vs Fc/Fc+

O

0.0

Figure 2. Cyclic (solid) and square-wave (dashed) voltammograms of HBT and

HBT in

MeO

CH Cl (0.1 M n-Bu N PF ) at ν = 200 mV/s and 22 C. +

2

2

4

°

6

Electronic spectroscopy of HBT and +•

HBT . Electrochemical reversibility of HBT/ HBT

MeO

+•

MeO

was further confirmed by the generation of their cation radicals via quantitative

28,29

redox titrations

using robust aromatic oxidant salts, such as tetrasubstituted p-hydroquinone ether [THEO SbCl

– 6

+•

] (THEO = 1,2,3,4,5,6,7,8-octahydro-9,10-dimethoxy-1,4:5,8-dimethano-anthracene, E = 0.67 red

V vs Fc/Fc+, λ = 518 nm, ε = 7300 cm M ) and magic blue [MB SbCl ] (MB = tris-4max

-1

max

-1

+•

bromophenylamminium, E = 0.70 V vs Fc/Fc , λ +

red

max

= 728 nm, ε

max

– 6

= 28,200 cm M ); see -1

-1

30

Supporting Information for full details. For example, Figure 3A shows electronic spectra obtained upon incremental addition of yellowish solution of HBT/ HBT (blue/red lines) to a MeO

bright red solution of [THEO SbCl ]. A complete consumption of the oxidation and clean +•

– 6

formation of HBT / HBT was accompanied by the change in the solution color to deep brown +• MeO

+•

and further confirmed by numerical deconvolution at each titration point (Figure 3B). A plot of mole fractions against the added equivalents of HBT/ HBT established a 1:1 stoichiometry for MeO


6

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both redox reactions (Figure 3C). Similar spectra were obtained using [MB SbCl ] as an oxidant +•

– 6

(Figure S9 in the Supporting Information). The solutions of both HBT / HBT were completely +• MeO

+•

stable at ambient temperatures for several days, if protected from moisture, and their spectra remained unchanged at tenfold higher concentration or upon cooling to -50 °C, suggesting a lack of any discernable aggregation. Noteworthy, the incorporation of eight methoxy groups in HBT does not impact the molar absorptivity of HBT cation radicals (Figure 3B) and only shifts red the near-IR absorption band by some 500 nm, without adding any new features in the spectrum.

Figure 3. Top panel: A. Spectral changes observed upon the reduction of 0.048 mM THEO

+•

in CH Cl (3 mL) by addition of 1.1 mM solution of HBT in CH Cl . B: Deconvolution of each 2

2

2

2

spectrum panel A into its component spectra, i.e. THEO (black), and HBT (blue). C: Plot of the +•

mole fractions of THEO (black) and HBT (blue) against the added equivalents of neutral +•

+•

HBT. Symbols represent experimental points, while the solid lines show best-fit to experimental points. Bottom panel: A. Spectral changes observed upon the reduction of 0.051 mM THEO in +•

CH Cl (3 mL) by addition of 1.7 mM solution of 2

2

HBT in CH Cl . B: Deconvolution of each

MeO

2

2

spectrum panel A into its component spectra, i.e. THEO (black), and HBT (blue). C: Plot of +•

MeO

the mole fractions of THEO (black) and HBT (blue) against the added equivalents of neutral +•

MeO

+•


7


Page 8 of 27

HBT. Symbols represent experimental points, while the solid lines show best-fit to

MeO

experimental points. EPR spectroscopy of HBT and +•

and

HBT . In order to probe paramagnetic properties of HBT

MeO

+•

+•

HBT , we next resorted to EPR spectroscopy. The EPR experiment was carried out at 20

MeO

+•

°C and frequency of 9.479 GHz. First derivative EPR spectra shown in Figure 4 below display quintet signal with hyperfine splitting of a = 3.8 G (g-value = 2.002) for HBT and a = 3.3 G (g+•

H

value = 2.003) for

H

HBT . The lines are in the 1:4:6:4:1 ratio, which is characteristic of

MeO

+•

interaction of unpaired electron with four equivalent hydrogens of pyrene. Thus, EPR pattern suggests that the cationic charge is mainly localized on a central pyrene moiety (vide infra).

Figure 4. EPR spectra of 0.5 mM HBT and 0.5 mM HBT in CH Cl at 20 °C. +•

MeO

+•

2

2

X-ray crystallography. The high stability of HBT / HBT cation radicals allowed the growth +• MeO

+•

of high quality single crystals of [HBT SbCl ] and [ HBT SbCl ] by allowing a slow mixing of +•

6

MeO

+•

6

the carefully layered CH Cl solutions of the HBT / HBT salts with toluene for 48 hours at -10 +•

2

MeO

+•

2

C (see Supporting Information for additional details). ORTEP diagrams of the generated crystals

o

of HBT, HBT, [HBT SbCl ], and [ HBT SbCl ] are shown in Figure 5 below. MeO

+•

6

MeO

+•

6


8

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Figure 5. ORTEP diagrams (50% probability) of HBT, HBT, [HBT SbCl ], and [ HBT SbCl ]. MeO

MeO

Noteworthy, both HBT and

+•

+•

6

6

HBT may exist in two nearly isoenergetic conformations

MeO

differing by a twist of the HBT core, pictured below in Chart 1, where two outer biphenyl moieties lie on opposite sides of the central di-tert-butylpyrene, with a large interplanar dihedral angle of ~35-42°, either in syn or anti arrangement. DFT calculations confirmed that the free energy difference between syn and anti conformations does not exceed 1 kcal/mol (Table S7 in the Supporting Information).

Chart 1. Structures of syn and anti forms of HBT.

The crystal structures of cationic [HBT SbCl ]/[ HBT SbCl ] and neutral HBT/ HBT all +•

6

MeO

+•

6

MeO

show rather similar syn twists in the HBT core (Figure 6), suggesting that crystal packing forces must play a role in the preference of syn arrangement over anti. Furthermore, the peripheral biphenyl moieties in HBT, [ HBT SbCl ] and [HBT SbCl ] crystals lie parallel to each other, MeO

MeO

+•

6

+•

6


9


Page 10 of 27

resulting in a similar packing of their crystal structures, whereas in the parent neutral HBT crystal, there are two alternating layers of HBT molecules (Figure 6). In [ HBT SbCl ] and MeO

+•

6

[HBT SbCl ] crystals, partially overlapping biphenyl moieties form cavities on both sides of the +•

6

central pyrene framework, which are filled by SbCl counter-anions and solvent molecules, 6

ensuring that cationic HBTs are well separated from each other by SbCl counter-anions, 6

resulting in discrete layers of cations and anions along the crystallographic ab plane (Figure 6).

Figure 6. The packing diagrams of HBT, HBT, [HBT SbCl ] and [ HBT SbCl ]. MeO

+•

6

MeO

+•

6

While it is well-known that the relative arrangement of species in a crystal is defined by steric and packing forces, it is important to note that in both [ HBT SbCl ] and [HBT SbCl ] crystal MeO

+•

6

+•

6

structures counter-anions are positioned in close proximity to the pyrene moiety, signifying that most of the cationic charge is located at the central pyrene. Indeed, a close inspection of the oxidation-induced bond length changes in the HBT®HBT

+•

and

HBT ® HBT

MeO

MeO

+•

transformations from X-ray crystallography showed (Figure 7A) that the most significant bond contractions and elongations are confined at the center of the graphitic core. For example, C-C bonds of the pyrene moiety (bonds a-f) undergo changes up to 3.1 pm, while changes in the


10

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aromatic C-C bonds at the peripheral biphenyl moieties (bonds g-p) do not exceed 1.3 pm and are below the commonly applied 3σ criterion. Remarkably, C-O bonds in

HBT underwent a

MeO

+•

negligible 0.6 pm contraction, in contrast to a typical contraction of ~5 pm found in monochromophoric 1,2- or 2,5-dimethoxybenzenoids,

31,32

the cationic charge at the periphery of

further supporting that the presence of

HBT/HBT is minor and most of the charge is

MeO

concentrated in the center of the molecule.

Figure 7. A. Barplot representation of the oxidation-induced bond length changes upon HBT®HBT and +•

HBT® HBT transformations obtained from X-ray crystallography. B.

MeO

MeO

+•

Correlation plots between oxidation-induced bond length changes in HBT / HBT against those +• MeO

+•

in tetraisopropyl pyrene (Py). Error bars correspond to the uncertainties in C-C bond length 33

changes: σ[HBT] = 0.75 pm and σ[MeOHBT] = 0.67 pm. We have recently shown

34,35

that the cationic charge distribution in a polychromophoric cation

radical can be assessed from the X-ray crystallography data by analysis of its oxidation-induced


11


Page 12 of 27

bond length changes in comparison with a model monochromophoric compound used as the reference. For example, a slope from the linear correlation between bond length changes in pyrene moiety of HBT/HBT against bond length changes in tetraisopropyl pyrene provides a MeO

33

fraction of the charge localized at the pyrene moiety of HBT /HBT . This analysis showed that MeO

+•

+•

most of the cationic charge is localized at the pyrene moiety, i.e., 84% in HBT and 74% in +•

HBT , with the rest of the charge residing at the peripheral biphenyls (Figure 7B). It is

MeO

+•

noteworthy that the presence of eight methoxy groups in HBT has barely depleted the charge MeO

+•

at the central pyrene (by ~ 10%) towards the peripheral biphenyls. Although X-ray crystallography provides important information on the distribution of the oxidation-induced structural reorganization, more detailed information on the properties of neutral PAH and its cation radical can be obtained from electronic structure calculations, e.g., using density functional theory (DFT). However, it is known that DFT methods may predict unrealistic charge distributions in PAH cation radicals,

36-38

and therefore one has to establish a

correspondence between the electronic structures of HBT / HBT obtained from experiment +• MeO

+•

(i.e., electrochemistry, spectroscopy, X-ray crystallography) and those obtained from DFT calculations. Here we employ a B1LYP functional with 40% of Hartree-Fock exchange (i.e., B1LYP-40), which has been parameterized to reproduce experimental redox and optical 11

properties of π-conjugated cation radicals. Importantly, a direct assessment of the performance of this and other functionals in reproducing the electronic structure of PAHs against X-ray crystallography data remains unexplored, largely due to a limited number of precise crystal structures of neutral and cationic PAHs, especially those having large graphitic cores. Thus, availability of the precise crystal structures of neutral and cation radical in HBT / HBT allows +• MeO

+•


12

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us to perform a detailed comparison of the oxidation-induced bond length changes between DFT and X-ray crystallography data. DFT calculations of HBT/ HBT and their cation radicals. DFT calculations using B1LYPMeO

40/6-31G(d)+PCM(CH Cl ) 2

2

showed

that

both

HBT®HBT

+•

and

HBT® HBT

MeO

MeO

+•

transformations are accompanied by a structural reorganization that is mainly confined to the pyrene moieties, in agreement with results from X-ray crystallography. Indeed, a linear regression analysis of oxidation-induced bond length changes of the pyrene core obtained by Xray crystallography against those obtained by DFT calculations shows a perfect linear correspondence (Figure 8A). Remarkably, even changes of the C-C bonds of the peripheral biphenyls (which are below the 3σ criterion) fall on the overall trend-line (compare grey and orange symbols in Figure 8A).

Figure 8. A. Correlation plots of the oxidation-induced bond-length changes of the pyrene moiety in HBT and

HBT obtained by X-ray crystallography and DFT calculations using

MeO


13


Page 14 of 27

B1LYP-40/6-31G(d)+PCM(CH Cl ). Grey circles correspond to the bonds of the peripheral 2

2

biphenyls and were not included in the linear regression. B. Isovalue plots of HOMO of HBT/ HBT and spin-density plots of HBT / MeO

+•

HBT

MeO

+•

calculated using B1LYP-40/6-

31G(d)+PCM(CH Cl ). 2

2

Consistent with these observations, a visual inspection of the HOMO (i.e., orbital from which oxidation occurs) of neutral HBT and

HBT showed that both HBTs have nearly identical

MeO

electron density distributions of HOMO that are dominated by pyrene-like nodal arrangement, with most of the electron density concentrated at the pyrene moiety (Figure 8B). Importantly, oxidation-induced bond length changes from both X-ray crystallography and DFT calculations follow the nodal arrangement of the HOMO, i.e., bonds that correspond to the bonding lobes undergo elongation, while bonds the correspond the antibonding lobes undergo contraction (Figure 6). Turning to the electronic structure of the cation radicals, natural population analysis (NPA)

39

shows that the spin-density distribution in HBT and HBT is slightly more localized than the +•

MeO

+•

distribution of HOMO arising from the oxidation-induced structural reorganization that “traps” the cationic charge (Figure 8B). Remarkably, the amount of spin-density at the pyrene moiety of HBT / HBT obtained from NPA is nearly identical to the value of the slope obtained from the +• MeO

+•

analysis of the oxidation-induced bond length changes (Figure 7B), suggesting that the charge and structural reorganization distributions mirror each other. Note that the self-trapped charge together with the bond-length changes and reorganization in the local environment (e.g. solvent) are often referred to as a polaron,

40-42

although in its original formulation term polaron has been

used to refer to a strong interactions between electronic and nuclear degrees of freedom in the ionic crystals.

43


14

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We emphasize that due to the direct correspondence between charge and structural reorganization distributions, X-ray crystallography can be utilized to quantify the extent of polaron delocalization in PAH cation radicals given that precise crystal structures of polychromophoric PAH together with the corresponding monochromophoric compound at neutral and cation radical states are available. Besides providing an experimental evidence for polaron delocalization, this can serve as an important source of reference data to validate a computational method and this approach has been previously demonstrated on the examples of triptycenes and pillar[5]arene. 44

35

Calculation of the vertically excited states of HBT and +•

HBT using TD-DFT revealed that

MeO

+•

the excitation of both HBT cation radicals involve similar orbitals in their lowest-energy transitions, resulting in nearly identical intensities and thus similar shapes of the near-IR bands in their experimental spectra (Figure 9). As most of the cationic charge in both HBT and HBT +•

MeO

+•

is concentrated at the central pyrene moiety, the methoxy groups do not significantly impact the polaron delocalization/stabilization, resulting in similar electronic absorption spectra as well as similar oxidation potentials (Figure 2).

Figure 9. Simulated TD-DFT stick-spectra of HBT and +•

HBT superimposed with the

MeO

+•

corresponding experimental spectra and the orbitals involved in the lowest-energy transition.


15


Page 16 of 27

Finally, we emphasize that incorporation of eight methoxy groups into HBT results in a small (0.17 V) lowering of the oxidation potential, in striking contrast with the effect of substituents on the redox properties of other smaller PAHs. For example, incorporation of eight methoxy groups in dibenzo[g,p]chrysene (DBC) lowers its oxidation potential by 0.42 V, while incorporation of 45

only two methoxy groups in benzene lowers its ionization energy by 1.4 V.

46,47

Indeed, it has been

shown in a series poly-p-phenylene wires that the effect of end-capping methoxy groups on the oxidation potential of the wire decreases with increasing wire length.

11,12

In unsubstituted poly-p-

phenylenes (PP ), a longitudinal (biphenyl-like) nodal arrangement of HOMO leads to an n

efficient orbital overlap and thereby extensive polaron delocalization involving 8 p-phenylenes, as judged by the saturation of their oxidation potentials when plotted against cos[π/(n+1)] trend (Figure 10A).

11,12

Note that use of the alternate 1/n trend does not necessarily reflect polaron

localization.

13

Figure 10. A. Evolution of the calculated [B1LYP-40/6-31G(d)+PCM(CH Cl )] oxidation 2

2

energies of unsubstituted and methoxy-substituted PP and PAH against cos[π/(n+1)] trend. B. n

n

Spin-density plots of unsubstituted and methoxy-substituted PP and PAH cation radicals n

n

calculated using B1LYP-40/6-31G(d)+PCM(CH Cl ). 2

2


16

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Upon incorporation of the end-capping methoxy groups, the oxidation potentials of the resulting substituted wires (i.e.,

PP ) lower significantly, with the polaron distribution skewed

MeO

n

toward the end of the wire as represented by the spin-density plots in Figure 10B.

As the wire

length increases, the difference in the oxidation potentials between PP and

n

11,12

PP decreases,

MeO

n

reaching a constant value after n = 8 (Figure 10A). In contrast, for the PAH-based wire introduced in Figure 1C, i.e., PAH where PAH = HBT, calculations predict that the difference n

3

in oxidation potentials between PAH and its methoxylated analogue PAH is relatively minor MeO

n

n

and reduces further to negligible values as the size of the graphitic strips increases (Figure 10A). In addition, oxidation potentials of both PAH and PAH show a relatively minor dependence n

MeO

n

on the size of the strip that is also evident by the extent of polaron delocalization, which remains confined within three pyrene moieties even in the longer strips (Figure 10B). Presence of methoxy groups in PAH drives the polaron toward the end of the graphitic strip, however the MeO

+• n

extent of polaron delocalization remains similar to that in unsubstituted PAH . In contrast, in +• n

poly-p-phenylene wires the polaron is delocalized over 8 p-phenylenes in unsubstituted PP , +• n

while in PP it is limited to only 5-6 p-phenylenes (Figure 10B). MeO

+• n

Conclusions Inspired by the synthetic availability of various PAHs with extended graphitic cores, in this work we synthesized hexabenzo[a,c,fg,j,l,op]tetracene (HBT) and its methoxylated analogue HBT, and probed their redox, optical and structural properties via electrochemistry, electronic

MeO

spectroscopy of their cation radicals, X-ray crystallography and carefully benchmarked DFT calculations. Our data showed that the redox/optical properties of these two HBTs and the extent of the polaron delocalization in their cation radicals are quite similar, in contrast to the properties of other smaller PAHs and poly-p-phenylene wires.


17


Page 18 of 27

Quantitative assessment of the polaron delocalization via X-ray crystallography and benchmarked DFT calculations showed that most of the polaron distribution is concentrated at the central pyrene moiety, with only a minor spillover to the peripheral biphenyl moieties in both HBT and +•

HBT . We realized that the availability of the precise crystal structures of neutral

MeO

+•

and cation radicals of PAHs with large graphitic cores such as HBT opens new avenues for parameterization of the density functionals that are known to poorly describe charge delocalization in π-conjugated systems. Finally, aided by the DFT calculations of long PAH-based graphitic strips we showed that their redox properties weakly depend on the size and presence of substituents, suggesting that this class of wires could be promising candidate materials for long-range charge transfer.

7,48,49

With the

synthesis of a various similar PAHs with extended graphitic cores being already reported, the 18

data presented in this work may serve as a reference for future studies to probe the viability of these graphitic strips as novel charge-transfer materials. ASSOCIATED CONTENT Supporting Information. Supporting Information includes synthesis and characterization data, X-ray crystallography and computational details. The following files are available free of charge. Supporting Information (PDF) Crystal structures of HBT, HBT, [HBT SbCl ] and [ HBT SbCl ] (CIF) MeO

+•

6

MeO

+•

6

Equilibrium geometries of PAH and PAH optimized at neutral and cation radical states (TXT) n

MeO

n

AUTHOR INFORMATION Corresponding Author


18

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*[email protected] † Deceased February 16, 2018 §Present address: Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, USA.

ACKNOWLEDGMENT We thank the NSF (CHE-1508677) and NIH (R01-HL112639-04) for financial support and Professor Scott A. Reid (Marquette University) for helpful discussions. We acknowledge the valuable assistance of Prof. Brian Bennett (Marquette University) in the EPR measurements, which were obtained on an instrument supported by an NSF MRI award (NSF CHE-1532168). The calculations were performed on the high-performance computing cluster Père at Marquette University and the Extreme Science and Engineering Discovery Environment (XSEDE). REFERENCES (1) Liu, Z.; Lau, S. P.; Yan, F. Functionalized Graphene and Other Two-dimensional Materials for Photovoltaic Devices: Device Design and Processing. Chem. Soc. Rev. 2015, 44, 5638-5679. (2) Roncali, J.; Leriche, P.; Blanchard, P. Molecular Materials for Organic Photovoltaics: Small Is Beautiful. Adv. Mater. 2014, 26, 3821-3838. (3) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45, 723-733. (4) Chen, J.; Cao, Y. Development of Novel Conjugated Donor Polymers for High-efficiency Bulk-heterojunction Photovoltaic Devices. Acc. Chem. Res. 2009, 42, 1709-1718. (5) Wenger, O. S. Photoinduced Electron and Energy Transfer in Phenylene Oligomers. Chem. Soc. Rev. 2011, 40, 3538-3550.


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Pyrene-like HOMO Governs Polaron Delocalization in Model Graphitic

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