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Ask Not How Many, But Where They Are: Substituents Control Energetic Ordering of Frontier Orbitals/Electronic Structures in Isomeric Methoxy- Substituted Dibenzochrysenes Maxim V. Ivanov, Marat R. Talipov, Tushar Shivram Navale, and Rajendra Rathore J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11232 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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The Journal of Physical Chemistry

Ask Not How Many, But Where They Are: Substituents Control Energetic Ordering of Frontier Orbitals/Electronic Structures in Isomeric MethoxySubstituted Dibenzochrysenes Maxim V. Ivanov, Marat R. Talipov, Tushar S. Navale and Rajendra Rathore* Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, WI 53201-1881.

ABSTRACT. Redox properties of polycyclic aromatic hydrocarbons (PAHs) can be modulated by substitution with electron-rich groups. Here we show, using the example of dibenzo[g,p]chrysene (DBC), that substitution position (i.e., meta vs para) alters the energetic ordering of frontier molecular orbitals (FMOs), leading to cation radicals with altered electronic structures and thereby redox/optical properties. We demonstrate that a straightforward analysis of FMOs provides a valuable insight towards the rational design of novel PAHs with tailored redox properties.

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Introduction Extensive efforts continue towards the design and synthesis of new and improved (derivatized) polycyclic aromatic hydrocarbons (PAHs) as building blocks for the preparation of functional electronic and optoelectronic materials for modern photovoltaic applications.1-3 As an example, molecules based on dibenzo[g,p]chrysene4 (DBC), a twisted PAH, have been extensively explored for potential applications in the areas of sensors, non-linear optical and liquid-crystalline materials.5-8 para

O

meta

O

O

O

O

O

O

O

m-DBC

O

O

O O

DBC

O

O

m/p-DBC

O

O

p-DBC

Chart 1. Structure and naming scheme of DBCs. It is known that oxidation potentials of PAHs can be modulated by appropriate substitutions with electron rich alkyl, alkoxy, or dialkylamino groups.8-10 In larger PAHs such as DBC, it is not immediately obvious whether the substitution of para (i.e. p-DBC) or meta (i.e. m-DBC) or both meta/para (m/p-DBC) positions (see Chart 1) would achieve maximum modulation of its redox properties. In this report, we show experimentally (via generation of cation radicals and electrochemical and spectroscopic measurements) and computationally (DFT) for parent and methoxy-substituted DBCs (Chart 1) that the position of substitution can have dramatic impact on redox and optical properties and alter the electronic structures of the cation radicals. For example, the oxidation potentials of isomeric m-DBC vs p-DBC, with four methoxy substituents, surprisingly differ by some 300 mV, while the m/p-DBC, with eight methoxy groups, was found to have a higher redox potential (by 60 mV) than m-DBC.


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An analysis of frontier molecular orbitals (FMOs) can help explain these surprising trends. In particular, the position of substituents impacts the energetic ordering of the filled FMOs (i.e. HOMO/HOMO-1) and can also lead to cation radicals with altered electronic structures, and, in turn, dramatically altered redox/optical properties.11,12 In general, the spatial hole distribution in PAH cation radicals can be ascertained by inspecting the nodal structure and distribution of HOMO densities in corresponding neutral PAH.11 Accordingly, the example of DBCs presented herein will serve to demonstrate that understanding the substitutional positiondependent redox potential modulation of PAHs, based on the analysis of FMOs, can serve as a valuable tool for informed design and synthesis of novel PAH derivatives with desired redox properties. Results and discussion Synthesis and electrochemistry of DBCs and electronic spectroscopy of DBC cation radicals. Synthesis of various methoxy-substituted DBCs was carried out by adaptation of literature procedures.4-8

For

example,

an

oxidative

cyclodehydrogenation

of

the

tetrakis(3-

methoxyphenyl)ethylene and tetrakis(3,4-dimethoxyphenyl)ethylene, using either FeCl3 or DDQ/CH3SO3H as oxidants,13 afforded p-DBC and m/p-DBC, respectively, in excellent yields. Unfortunately, a similar oxidative cyclodehydrogenation of tetrakis(4-methoxyphenyl)ethylene did not produce any m-DBC;8,9 and therefore, it was accessed via an oxidative cyclization of bis(bianisyl)acetylene,

which,

in

turn,

was

prepared

by

Sonogashira

coupling

of

trimethylsilyacetylene with 2-iodobianisyl (Scheme 1 and the Supporting Information for complete experimental details).


3


X

X

y

y

DDQ/acid

DDQ/acid

X = OMe Y = OMe

X = OMe Y=H

m/p-DBC

y

y X

X

X y

I y

y

X X

SiMe3

X

PdCl2(PPh3)2

FeCl3

X=H Y = OMe

X=H Y = OMe

y

p-DBC

DDQ/acid X = H, Y = OMe

y

X

X

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y

m-DBC

X

Scheme 1. Synthesis of methoxy-substituted DBCs All DBCs were characterized by 1H/13C NMR spectroscopy and their molecular structures were established by X-ray crystallography. The X-ray structures of various DBCs (Scheme 1), including parent DBC, showed similar twist angles of 35°-37° around the central double bond (Figure S2 in the Supporting Information) irrespective of the number or position of the methoxy substituents. The cyclic voltammograms of all DBCs met the criteria of reversibility over scan rates of 50 to 1000 mV/s, when the potential window was limited in the range of their first oxidation potentials (solid curves in Figure 1).


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0.15

DBC O

0.48

O

O

O

p-DBC O

O

O

O

O

m-DBC

0.42 O

O

O

O

O O

O

m/p-DBC

1.55

1.15 0.75 + 0.35 V vs Fc/Fc

Figure 1. Cyclic voltammograms of 2 mM DBCs in CH2Cl2 (0.2 M n-Bu4NPF6) recorded at a platinum electrode at a scan rate of 200 mV/s and 22 oC. The oxidation potentials were referenced to ferrocene. The CV data show that the first oxidation potential of parent (unsubstituted) dibenzochrysene [Eox1(DBC) = 0.88 V vs Fc/Fc+] decreases by 0.42 V upon incorporation of the 8 methoxy groups in all of its meta- and para-positions [i.e. Eox1(m/p-DBC) = 0.46 V]. Surprisingly, however, a removal of 4 methoxy groups from the para-positions of m/p-DBC led to a further decrease in its oxidation potential by 60 mV [i.e. Eox1(m-DBC) = 0.40 V]. This would clearly suggest that the four para-methoxy groups in m/p-DBC do not play any role in stabilizing its cation radical. Based on this observation, one may expect that the oxidation potentials of parent DBC and tetramethoxy p-DBC should be similar. However, the oxidation potential of tetramethoxy p-DBC [Eox1(p-DBC) = 0.73 V] was found to be 0.15 V lower than the parent DBC.


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This surprisingly divergent effect of the para-methoxy groups in m/p-DBC, where they appear to destabilize the cation radical by 60 mV, and in p-DBC, where they stabilize the cation radical by 150 mV, suggests that these cation radicals must possess different electronic structures. To investigate this effect, we next examine the electronic absorption spectra of the DBC cation radicals, generated by quantitative redox titrations14 using either a hindered naphthalene cation radical (NAP+•; Ered = 0.94 V) or tetrasubstituted p-hydroquinone ether (THEO+•; Ered = 0.67 V) as oxidants15 in CH2Cl2 at 22 oC; see Supporting Information for details. A comparison of the reproducible absorption spectra of DBC+• showed remarkable similarity between the spectra of m/p-DBC+• and m-DBC+• (Figure 2), as both spectra have (i) intense bands at ~800-900 nm (λmax = 818 and 886 nm, respectively) with comparable extinction coefficients (log εmax = 4.36 and 4.28, respectively) and (ii) a small broad band in the nearinfrared region of the spectrum at ~1300 nm. Moreover, the absorption spectrum of DBC+•, although somewhat blue-shifted and less intense, bears similar spectral features as m/p-DBC+• and m-DBC+• (Figure 2). However, the absorption spectrum of p-DBC+• is rather dissimilar from the absorption spectra of other DBC cation radicals as it has a much less intense band at 808 nm (log ε808 = 3.97) and an additional broad near-IR band that extends beyond ~3000 nm, thus suggesting a different electronic structure of p-DBC+• as compared to m-DBC+• or m/p-DBC+•.


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DBC+•

m/p-DBC+•

m-DBC+•

p-DBC+•

1000

2000 Wavelength (nm)

3000

Figure 2. A comparison of the absorption spectra of various DBC cation radicals in CH2Cl2 at 22 oC. DFT modeling of the redox/optical properties of DBCs. Accurate description of the electronic structure of PAH cation radicals is challenging for many DFT functionals due to the inherent self-interaction error that causes artificial hole delocalization and leads to underestimated oxidation potentials and inaccurate identification of the nature of the excited states.16-18 These artifacts can be minimized using a hybrid density functional that contains a portion of the exact Hartree-Fock (HF) exchange.19,20 However, in many standard hybrid functionals amount of the Hartree-Fock exchange is either insufficient or excessive for a correct description of the extent of hole delocalization/stabilization.21,22 Recent benchmarking studies on a set of mixed-valence compounds20,21,23 and poly-p-phenylene wires24 have demonstrated that customization of a standard density functional by fine-tuning the amount of the exact exchange to reproduce experimental data can provide a reliable description of the electronic structures of the πconjugated cation radicals. For example, in our previous study24 we have used a one-parameter


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B1LYP25 functional where HF exchange was varied in 20-50% range to accurately reproduce oxidation potentials and the cation radical excitation energies of the alkyl-capped poly-pphenylenes with increasing number of p-phenylenes, i.e., iAPPn, n = 2-7. It was shown that 40% of the HF exchange provides a balanced description of the electronic structure of the poly-pphenylene cation radicals similar to that shown by Kaupp and coworkers23 for a reliable description of the another class of organic mixed-valence systems. Indeed, usage of modified B1LYP-40/6-31G(d) functional performed exceptionally well in reproducing the experimental redox/optoelectronic properties of a variety of poly-p-phenylene-10,26 and polyfluorene-based27 wires and other PAHs.28,29 Accordingly, in this manuscript, we performed electronic structure calculations of neutral DBCs and their cation radicals using B1LYP-40/6-31G(d) level of theory. Due to the important role of the solvation in stabilization of charged molecules and the affect on the nature of the excited states,21,30,31 we also performed a comparative analysis of the electronic structure of DBC cation radicals in solvents with varied polarity using polarizable continuum model (PCM) in linear response formalism (vide infra). Excited state calculations of the cation radicals of the DBC derivatives were performed using time-dependent DFT with B1LYP-40 functional, which has proven its reliability in reproducing correct nature of the first excited state and experimental excitation energies of a large variety of PAH cation radicals.10,27 To further validate the performance of TD-DFT in description of the excited state of π-conjugated cation radicals,32,33 we performed a benchmarking study on the performance of TD-B1LYP-40 to reproduce ab initio excitation energies of a set of several relevant to this study PAHs (Figure 3). As a reference method, we employed EOM-IPCCSD method due to its reported reliability in accurate description of the charge-(de)localized


8

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open-shell systems.34-36 Excitation energies of the PAH cation radicals were taken as the difference between first and second ionization energies (i.e., IE2-IE1) from EOM-IP-CCSD/631G(d) calculations performed at their equilibrium geometries optimized using B1LYP-40/631G(d) level of theory. Thus obtained excitation energies were then compared with the excitation energy (ν) calculated directly using TD-B1LYP-40/6-31G(d). This analysis showed that the TD-B1LYP-40/6-31G(d) level of theory correlates well with the results from EOM-IP-CCSD/6-31G(d) (R2 = 0.97, Figure 3) and reproduces ab initio energies with RMSD of 0.12 eV. Importantly, the orbitals involved in the excitation of the cation radicals obtained from TD-DFT calculations correspond to the orbitals that are involved in the first and second ionizations obtained using EOM-IP-CCSD method (see Supporting Information for additional details on the nature of transition in each molecule in Figure 3). 2.0 O O

O

O

O

O

O

O

O

O

O

O

ν (EOM-IP-CCSD), eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.1

O

O

O

O

O

O

O

0.2

1.1 ν (TD-DFT), eV

0.2 2.0

Figure 3. Left: Parent and methoxy-substituted PAH structures used in the benchmarking study. Right: Correlation plot between excitation energies of the cation radicals (structures on the left) calculated using EOM-IP-CCSD/6-31G(d) and TD-B1LYP-40/6-31G(d). Linear regression: y = 1.09x – 0.18, R2 = 0.97. Molecular orbital theory description of DBCs. It has been shown on a number of examples,12,37,38 that the electronic structures of the π-conjugated cation radicals are governed by


9


their HOMO energies and nodal arrangements. Therefore, in order to investigate the origin of the remarkable differences in the redox/optical properties of p-DBC vs m-DBC vs m/p-DBC, below we examine their FMOs. As highlighted in Figure 4 below, the relative energies of FMOs in neutral DBCs display a strong dependence on the position of the methoxy substituents. DBC

m-DBC

0.17

-5.5

m/p-DBC

0.77

-5.7 -5.9 -6.1 -6.3

0.62

Orbital Energy (eV)

p-DBC

0.09

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

-6.5

Figure 4. Comparison of the HOMO/HOMO-1 energies of neutral DBCs calculated using B1LYP-40/6-31G(d)+PCM(CH2Cl2). The two filled FMOs of DBC in Figure 3 are hereafter referred to as phenanthrene-like and biphenyl-like orbitals, based on the resemblance of their electron density distributions to those of phenanthrene and biphenyl (Figure 5).


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Figure 5. Schematic representations of phenanthrene-like (A) and biphenyl-like (B) FMOs of DBC and per-atom HOMO densities, obtained from the MO expansion coefficients of a single orbital. The orbital coefficients add up to 1.00 in phenanthrene-like orbital, while to 1.02 in biphenyl-like orbital due to the rounding error. A closer look at these FMOs suggests that the phenanthrene-like orbital bears significant electron density (or molecular orbital expansion coefficient squared, represented by yellow and blue circles in Figure 5A) at the meta carbons (0.05) and none at the para carbons. In contrast the biphenyl-like (Figure 5B) orbital bears significant electron density at both para (0.07) and meta (0.01) carbons, albeit in different amounts. It is emphasized that the energies of the FMOs in Figure 5 will respond to the substitution at meta and para carbons based on the amount of electron density coefficient, i.e., larger the coefficient at the substituent-bearing carbon, larger will be the increase in FMO energy. Based on this analysis, incorporation of the methoxy groups at meta position (i.e., mDBC) raises the energies of both FMOs (Figure 4). Addition of four more methoxy in m-DBC at the para positions (i.e., m/p-DBC) does not affect the energy of the phenanthrene-like HOMO, due to a complete lack of HOMO density at para carbons. The energy of HOMO-1 in m/p-DBC increases significantly as compared to m-DBC because the biphenyl-like orbital bears electron density at both meta and para carbons. Importantly, incorporation of the methoxy groups only at the para positions of DBC (i.e., p-DBC) does not affect the energy of its phenanthrene-like


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orbital, but raises the energy of biphenyl-like orbital to a sufficient extent that it becomes HOMO (Figure 4).39 Relatively small HOMO/HOMO-1 energy gap of 0.09 eV in p-DBC may suggest that the solvent polarity could swap the order of these orbitals and thereby result in the cation radical of p-DBC with the alternate electronic structure similar to that of m-DBC. However, changing the solvent from toluene (ε = 2.4) to dichloromethane (8.9) to dichloroethane (10.1) to acetone (20.5) to acetonitrile (35.7) did not change the ordering of the FMOs in all DBCs examined, as their HOMO/HOMO-1 energy gap remains largely constant with changing solvent polarity (Figure 6).

Figure 6. Energies [B1LYP-40/6-31G(d)] of the HOMO/HOMO-1 of DBCs (biphenyl-like/red circles and phenanthrene-like/blue circles) against dielectric constant of the solvents of different polarity, i.e., toluene (ε = 2.4), dichloromethane (8.9), dichloroethane (10.1), acetone (20.5), and acetonitrile (35.7). Electronic structure of DBC cation radicals. The FMO analysis clearly demonstrates that the energetic ordering of HOMO/HOMO-1 can be controlled by positioning of the substituents and,


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in turn, can lead to different electronic structures of the resulting cation radicals. The importance of this finding is recognized in light of prior works that have shown, through X-ray structural analysis of PAH cation radicals, that contraction/elongation of the C-C bonds upon 1-e- oxidation track in accordance with the nodal structure of HOMO (i.e., bonds with bonding HOMO lobes undergo

elongations

whereas

the

bonds

with

antibonding

HOMO

lobes

undergo

contractions).28,29,38 Moreover, the experimental bond length changes in various PAH cation radicals can be fully reproduced by DFT calculations and the calculated spin/charge distributions in these cation radicals closely resemble the HOMO density distributions.28,29,38 Indeed, the calculated spin-density distributions (Figure 7A) correspond to phenanthrene-like orbital in DBC+•, m-DBC+• and m/p-DBC+• and biphenyl-like orbital in p-DBC+•, in accordance with the nodal arrangement of HOMOs displayed in Figure 5.


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A

DBC+•

m/p-DBC+•

m-DBC+•

Page 14 of 24

p-DBC+•

B DBC+•

m/p-DBC+•

m-DBC+•

p-DBC+•

1000

2000 Wavelength (nm)

3000

Figure 7. A. Isovalue spin-density distribution plots of parent and methoxy-substituted DBC cation radicals calculated using B1LYP-40/6-31G(d)+PCM(CH2Cl2). B. Experimental and TDDFT calculated stick spectra of parent and methoxy-substituted DBC cation radicals. Orbitals involved in the lowest energy transitions are shown as insets. The spectrum of 4,4’dimethoxybiphenyl cation radical is shown as dashed curve. While spin-density distributions in DBC+•, m-DBC+• and m/p-DBC+• are visually identical to the corresponding distribution of HOMO densities in neutral DBCs, it is curious that the


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observed spin-density distribution in p-DBC+• occupies only one of the two biphenyl moieties linked via an ethylenic linkage and does not fully correspond to the biphenyl-like HOMO distribution (compare Figures 7 and 4). Such a discrepancy between spin/charge and HOMO distributions is known to arise in bichromophoric systems, in which interchromophoric electronic coupling (i.e., Hab) is far less than the reorganization energy (i.e., λ), i.e. 2Hab < λ.40-42 In such systems, the hole is known to be stabilized via dynamic hopping amongst the equivalent chromophores.43,44 Consistent with the drastically different spin-density distribution of p-DBC+• as compared to those of DBC+•, m-DBC+• and m/p-DBC+•, its electronic absorption spectrum is quite different (Figure 7B). Absorption band of p-DBC+• in the ~800-900 nm region closely resembles the absorption spectrum of 4,4’-dimethoxybiphenyl cation radical24 and therefore corresponds to the local excitation of a single biphenyl moiety of p-DBC+•. Additional (weak) broad absorption band in the near-IR region extending beyond 3000 nm then is readily assigned to as a charge transfer excitation where the hole hops between two biphenyl-like moieties. This analysis of the spectrum of p-DBC+• is further supported by the TD-DFT calculations and the charge-transfer nature of the transition displayed in Figure 7B. In contrast, (weak) broad band in the 1000-1500 nm region in the spectra of DBC+•, m-DBC+• and m/p-DBC+• involve relatively higher energy excitation of the cationic charge that is delocalized over the entire DBC core (Figure 7B). We next probed whether the nature of the transitions in DBC cation radicals changes upon changing the solvent polarity with the aid of TD-DFT calculations. As shown in Figure 8, changing the solvent from least polar toluene (ε = 2.4) to highly polar acetonitrile (35.7) did not affect the nature of the transitions in DBC cation radicals. In particular, the lowest-energy transitions in DBC+•, m-DBC+• and m/p-DBC+• remain invariant to the polarity of the solvent,


15


while the charge-transfer transition observed in p-DBC+• experiences expected change from 0.6 to 0.8 eV (Figure 8).

CH2Cl2

1.20

.

m-DBC+ .

DBC+ . m/p-DBC+ ν , eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

0.85 .

p -DBC+

0.50

0

20

40

Figure 8. Excitation energy of parent and methoxy-substituted DBC cation radicals calculated using B1LYP-40/6-31G(d) in PCM of solvents with varying polarity. Conclusions In summary, we show that the positioning of the substituents controls the energetic ordering of the FMOs in DBCs and thereby results in cation radicals with different electronic structures and in turn different redox/optical properties. The variations in electronic structures of PAH cation radicals with positioning of the substituents can be readily ascertained by an inspection of the nodal arrangements of the FMOs and their energetic ordering and remains unchanged with varied solvent. The FMO analysis presented herein will serve as a valuable tool for the informed design and synthesis of novel PAH derivatives with desired (enhanced) redox properties.


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ASSOCIATED CONTENT Supporting Information. Experimental and computational details are included in the Supporting Information. The following files are available free of charge. Supporting Information (PDF) Optimized equilibrium geometries of DBCs at neutral and cation radical states (TXT) AUTHOR INFORMATION Corresponding Author *[email protected] Notes Any additional relevant notes should be placed here. The authors declare no competing financial interests. ACKNOWLEDGMENT We thank the NSF (CHE-1508677) and NIH (R01-HL112639-04) for financial support, Professor Scott A. Reid for helpful discussions. The calculations were performed on the highperformance computing cluster Père at Marquette University and the Extreme Science and Engineering Discovery Environment (XSEDE). REFERENCES (1) Segawa, Y.; Ito, H.; Itami, K. Structurally Uniform and Atomically Precise Carbon Nanostructures. Nat. Rev. Mat. 2016, 1, 15002.


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(2) Avouris, P.; Chen, Z.; Perebeinos, V. Carbon-based Electronics. Nat. Nanotechnol. 2007, 2, 605-615. (3) Heath, J. R. Molecular Electronics. Annu. Rev. Mater. Res. 2009, 39, 1-23. (4) Buckles, R. E.; Serianz, A.; Naffziger, D. Dibenzo (gp) Chrysene: A Challenging Experiment in Organic Synthesis. Proc.. Iowa. Acad.. Sci. 1973, 80, 45-49. (5) Yamaguchi, S.; Swager, T. M. Oxidative Cyclization of Bis (biaryl) Acetylenes: Synthesis and Photophysics of Dibenzo [g, P] Chrysene-based Fluorescent Polymers. J. Am. Chem. Soc. 2001, 123, 12087-12088. (6) Toal, S. J.; Trogler, W. C. Polymer Sensors for Nitroaromatic Explosives Detection. J. Mater. Chem. 2006, 16, 2871-2883. (7) Li, C. -W.; Wang, C. -I.; Liao, H. -Y.; Chaudhuri, R.; Liu, R. -S. Synthesis of Dibenzo[g,p]chrysenes From Bis(biaryl)acetylenes Via Sequential ICl-induced Cyclization and Mizoroki-Heck Coupling. J. Org. Chem. 2007, 72, 9203-9207. (8) Navale, T. S.; Thakur, K.; Rathore, R. Sequential Oxidative Transformation of Tetraarylethylenes to 9,10-diarylphenanthrenes and Dibenzo[g,p]chrysenes Using DDQ As An Oxidant. Org. Lett. 2011, 13, 1634-1637. (9) Navale, T. S.; Zhai, L.; Lindeman, S. V.; Rathore, R. Octamethoxydibenzochrysene: Isolation and X-ray Crystallographic Characterization of a Twisted Polyaromatic Cation Radical. Chem. Commun. (Camb). 2009, 2857-2859.


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