Covalent Dimers of 1,3-Diphenylisobenzofuran for Singlet Fission: Synthesis and Electrochemistry

Article abstract of DOI:10.1021/jo502004r

Abstract The synthesis of covalent dimers in which two 1,3-diphenylisobenzofuran units are connected through one phenyl substituent on each is reported. In three of the dimers, the subunits are linked directly, and in three others, they are linked via an

Full text of DOI:10.1021/jo502004r

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Article
Covalent Dimers of 1,3-Diphenylisobenzofuran for
Singlet Fission: Synthesis and Electrochemistry
Akin Akdag, Abdul Wahab, Pavel Beran, Lubomir Rulisek, Paul Ionut Dron, Jiri Ludvik, and Josef Michl
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo502004r • Publication Date (Web): 10 Nov 2014
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Covalent Dimers of 1,3-Diphenylisobenzofuran for Singlet Fission: Synthesis and
Electrochemistry
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‡* and
†
§
Akin Akdag,§†¶ Abdul Wahab,‡,Z Pavel Beran,†
Josef Michl§†*
Lubomír Rulíšek, Paul I. Dron, Jiøí Ludvík,
§ Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215,
†
USA; Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech
Republic, 16610 Prague6, CzechRepublic; ‡ J. HeyrovskýInstitute of PhysicalChemistry, Academy
of Sciences of the Czech Republic, Dolejškova 3, 18223 Prague 8, Czech Republic
michl@eefus.colorado.edu, jiri.ludvik@jh-inst.cas.cz
Table of Contents graphic:
______________________________________________________________________________
Present addresses: ¶ Department of Chemistry, Middle East Technical University, 06800 Ankara,
Turkey;Z Departmentof Chemistry, Universityof ScienceandTechnologyMeghalaya, KillingRoad,
Baridua, 9th Mile, Ri-Bhoi District, Mehalaya - 793101, India
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Abstract
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Thesynthesisofcovalentdimersin whichtwo1,3-diphenylisobenzofuranunitsareconnected
through one phenyl substituent on each is reported. In three of the dimers, the subunits are linked
directly, and in three others, via an alkane chain. A seventh new compound in which two 1,3-
diphenylisobenzofuranunits shareaphenyl substituent isalsodescribed. Thesematerialsareneeded
for investigations of the singlet fission process, which promises to increase the efficiency of solar
cells. Electrochemical oxidation and reduction of the monomer, two previously known dimers, and
the seven new compounds have been examined and reversible redox potentials have been compared
with results obtained from densityfunctional theory. Although the overall agreementis satisfactory,
some discrepancies are noted and discussed.
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Introduction
1,3-Diphenylisobenzofuran (1) is a long-known and commercially available compound that
has seen considerable use for semiquantitative detection of singlet oxygen.1 It originallyelicited our
interest becausetheoreticalconsiderations led to the suggestion2 that it should be efficientas a model
chromophore for singlet fission,3 a photophysical process in which a singlet excited chromophore
shares its energy with a ground-state neighbor and both end up in their excited triplet states.
Materials that undergo efficient singlet fission are veryrare4 but are of considerable interest for solar
cells, since in principle they permit an increase in maximum theoretical cell efficiency by a factor
of ~1.4.5 An examination of the photophysics of a thin layer of polycrystalline 1 at a series of
temperatures indeed revealed efficient singlet fission.6,7,8 Irradiation produced up to two triplet
excitations per absorbed photon, implying that both the chromophore itself and the inter-
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chromophore coupling in one of the two known modifications of the solid are favorable.9,10
Although the high transparency of 1 in most of the visible region of the spectrum11 and its
sensitivity to the combination of air and light preclude any practical use in a solar cell, it is of
considerableinterest as a model compoundin fundamentalstudies ofthestill onlypoorlyunderstood
nature of the singlet fission process. One of the issues that are currentlymost intriguing is the design
of efficient inter-chromophore coupling. This has two aspects, (i) the mutual orientation of the
chromophores, and (ii) the nature of their covalent attachment: directly through a single bond, or
indirectlythrough a coupling group such as methylene. The approachthat others12,13 and we14,15 have
chosen is to prepare various covalent dimers of chromophores known to undergo singlet fission
efficiently as neat solids, and to examine their dilute solutions for evidence of singlet fission.
None of the dimers examined so far were efficient. This could be due to a varietyof reasons,
for instance to the loss of the favorable entropic contribution associated with the ability of the two
triplets to diffuse apart in acrystal. One likelyreason is apparent upon inspection of the approximate
theoreticalexpressions3,4 fortheinter-chromophorecouplingstrengths. Accordingtotheseformulas,
both contributing components, the minor direct path and the major path mediated by virtual charge-
transferstates, should be inefficientwhen the coupling is linear, as it has been in all dimers examined
so far. The theory suggests that slip-stacking the chromophores will be much more favorable, and
indeed will be the best arrangement among those considered up to now.
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The first two covalent dimers of 1 that were synthesized and tested, 2 and 3, were linear.
They exhibited no detectable singlet fission in solutions in non-polar solvents, but triplet formation
was observed in small yields up to ~10% in polar solvents, whereit proceededthrough an observable
intramolecular charge-transfer state.10,14,15 It is not clear whether each charge-transfer excited
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molecule produced two triplets in a second phase of two-phase singlet fission, or just one triplet by
ordinary intersystem crossing. In any event, the observation has drawn attention to the possible
importance of two-step singlet fission. The investigation of 2 and 3 clearly needs to be
complemented by an examination of a larger selection of covalent dimers of 1. Presently, we report
the synthesis of compounds 4 - 10, designed to test both the effects of the choice of the covalent link
(4 - 6) and the effects of conformational mobility that would permit stacking (7 - 9). The related
double isobenzofuran 10, which is not a true dimer of 1, was added to the series for comparison.
Since the redox properties of potential singlet fission molecules would be critical for their
use in solar cells, we have examined the electrochemistry of the monomer and the dimers, both
experimentally and computationally. The electrochemical behavior of 10 is quite distinct and this
compound requires a more detailed examination, the results of which will be reported separately.
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Chart 1. Structures 1 - 10.
Results
Synthesis. The preparation of dimers 2 and 3 was described elsewhere.15 The dimers 4 - 9
were prepared by the addition of a solution of a dilithiated linker compound, prepared from a
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dibromo precursor, to a solution of 3-phenylphthalide (11, Scheme 1). Inverse addition did not yield
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the desired products.
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Scheme 1. Preparation of covalent dimers.
Some of the required linker compounds were known,16,17 and the routes used to prepare the
new ones are shown in Scheme 2.
Scheme 2. The synthesis of linker compounds.
The precursors for dimers 8 and 9 were synthesized in the same fashion (Scheme 2).18,19
A
condensation of 4-bromoacetophenone with 4-bromobenzaldehyde or 3-bromobenzaldehyde in the
presence of sodium ethoxide yielded excellent yields of 12 and 14. Their catalytic hydrogenation
(H2, Pd/C, 1 atm) produced 1,3-diphenylpropane, but 13 and 15 were obtained by reduction with
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triethylsilane in trifluoroacetic acid.20 Reduction of the nitro group in 4'-bromo-3-nitro-1,1'-
biphenyl21 to the amine followed by the Sandmayer reaction afforded 3,4'-dibromo-1,1'-biphenyl.
The general method gave only traces of 10 and a two-step procedure was used instead,
where 1-(4-bromophenyl)-3-phenylisobenzofuran22 was lithiated and treated with 11 to yield 10.
Electrochemistry. DC polarographyand cyclic voltammetry(CV) of 1 - 10 wereperformed
on mercury (reduction) and platinum (oxidation) electrodes in N,N-dimethylformamide (DMF).
Oxidation on a platinum electrode was also examined in acetonitrile at room temperature and in
liquid SO2 at several temperatures from -70 to 22 EC (4 was very slightly soluble in liquid SO2, and
only at the higher temperatures).
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For reduction processes on a hanging mercurydrop electrode (HMDE) and in polarographic
measurements, the number of electrons involved was determined from the relative height of
normalizedcurrentsandcomparisonwith p-nitrotoluene,whichgivesacleanreversibleone-electron
reduction. The number of electrons involved in anodic processes was determined by comparison of
normalized peak currents with that of ferrocene, which undergoes a reversible one-electron
oxidation. Judgingbyconcentrationandscanratedependence,alloxidation andreductionprocesses
were diffusion controlled, except that 3 showed an adsorption pre-wave in CV and polarography, as
discussed below.
Oxidation. Cyclic voltammograms were measured in liquid SO2 (at 0.08 to 5 V/s and at !70
to 22 EC), in acetonitrile (5.0 V/s), and in DMF (0.2 V/s). In liquid SO2 containing Oct4NClO4, the
monomer 1 and two of the dimers (2, 4) undergo oxidation that is completely reversible above 0 EC
(Figure 1, Table 1). Their peak current ratios are close to unity and the peak separations are
comparable to that of Fc+/Fc couple under the same experimental conditions (66 mV for 1). Clean
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reversibility was not observed in the absence of a perchlorate salt. The oxidation behavior of the
other dimers is quasi-reversible: their cathodic|/anodic peak currentratio is much lower than one and
the peak separation is 100 - 150 mV. For 4 two reversible one-electron oxidations are observed at
0.27 V and 0.38 V vs. Fc+/Fc (shown in more detail in the Supporting Information, Figure 1S).
Similarly, 10 is oxidized in two reversible one-electron processes with peak separations of 46 and
58 mV. Figure 2 shows the noteworthy "inverted" temperature dependence of reversibility for 1 at
0.1 V/s.
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In DMF (Figure 3) and in acetonitrile all oxidations areirreversibleat the available scan rates
(0.05 – 5.0 V/s) at room temperature, presumably due to a reaction of the primary products with the
solvent. The parent 1 exhibits a single irreversible oxidation wave in both solvents. However, at -70
EC and a high scan rate (5.0 V/s), it appears to show a vague cathodic return peak, indicating some
reversibility. This is especially true in DMF, although at this temperature the solution is a semi-
transparent semi-solid (melt). Except for 4 the dimers all exhibit a single oxidation wave in DMF
and acetonitrile at a potential close to that of 1. This appears to be a two-electron process, although
the normalized peak currents of the dimers are less than double those of 1. In DMF, 4 exhibits two
very weak, probably one-electron processes at 0.82 and 0.88 V (vs. SCE), and 10 displays three
irreversible anodic oxidation peaks. In acetonitrile, these processes occur at similar potentials, but
are not well defined.
The oxidation potentials obtained at the same scan rate in liquid SO2, acetonitrile, and DMF
using different reference electrodes are mutually consistent within ±0.04 V – cf. the three sets of
measured oxidation potentials in Tables 1 and 2.
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Figure 1: Cyclic voltammograms of 1 - 10 in liquid SO2 (Pt
electrode, 0.1 V/s, 0.1 M Oct4NClO4, vs. Fc+/Fc, at 15 EC).
Sample concentrations were 4.5 (±1.0) × 10-4 M except for 4,
which was lower because it is very poorly soluble.
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Figure 2: Temperature dependence of cyclic voltammograms of 1 in
+
4, vs. Ag /Ag).
liquid SO (Pt electrode, 0.1 V/s, 0.1 M Oct NClO
2
4
Figure 3: Cyclic voltammograms of 1 - 10 in DMF (0.2 V/s, 0.1 M Bu4NPF6, vs. SCE, at room
temperature). Sample concentrations were 3.5 (±1.0) × 10-4 M. Reductions at hanging mercury
drop electrode (HMDE) and oxidations at Pt disk electrode (therefore the current cannot be on the
same scale).
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Table 1. CV data for oxidation of 1 - 10 in liquid SO2 measured vs. Fc+/Fc (at 15 EC, 0.1 V/s) and
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in acetonitrile measured vs. SCE (room temperature, 5.0 V/s) at Pt stationary disk electrode.
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Compd
Liquid SO2
E1/2 (vs. Fc+/Fc)a
Acetonitrile
E1/2 vs. SCEb Epa vs. SCE
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0.35 (67)
0.28 (112)
0.34 (106)
0.855
0.785
0.845
0.88
0.78
0.82
0.91
0.87
0.88
0.81
0.78
0.75
0.49c
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10
0.27 (60); 0.38 (92)
0.33 (113)
0.37 (130)
0.32 (119)
0.27 (101)
0.29 (143)
0.16 (46); 0.48 (58)
0.775; 0.885
0.835
0.875
0.825
0.775
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0.665; 0.985
a Eox = (Epa + Epc)/2 ; measured data vs. Fc/Fc+. Values in parentheses are anodic/cathodic peak separations
b
ÄEp in mV.
Eox recalculated vs. SCE: ESCE – EFc/Fc+ = 505 mV (average experimental value of the
difference). c Poorly defined peak.
Table 2. CV data for oxidation (Epa) and reduction (Epc) of 1 - 10 in DMF (0.2 V/s) measured vs.
SCE.a
Compd
Epa
0.85
0.81
0.82
Epc(1)b
-1.84 (65)
-1.88 (90)
-1.89 (98)
E
pc(2)b
Epc(3)
-2.26
-2.31
-2.31
-2.35
-2.29
-2.26
-2.30
-2.30
-2.29
-2.24
Epc(4)
E
pc(5)
1
2
3
4
5
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9
10
0.82, 0.88
0.84
0.87
-1.69 (57) -1.79 (61)
-1.77 (65) -1.87 (60)
-1.84 (103)c
-2.49
-2.45
-2.41
-2.65
-2.67
-2.69
0.80
0.80
-1.88 (75)
-1.87 (66)
0.83 -1.86 (80)
0.66, 0.91, 1.00 -1.62 (66) -1.83 (66)
-2.61
-2.81
a
b
Oxidation at Pt stationary disk electrode and reduction at HMDE.
Values in parentheses are
c
anodic/cathodic peak separations ÄEpin mV of reversible couples. Broad cathodic and anodic peaks most
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probably contain two very close reversible couples.
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Reduction. Typical voltammograms recorded at 0.2 V/s are shown in Figure 3 and the
results are summarized in Table 2. The parent 1 undergoes a reversible one-electron reduction at
-1.81 V followed by a quasi-reversible one-electron reduction at -2.28 V. No other reduction peak
was found up to the discharge at -2.90 V. The quasi-reversibility of the second reduction step is
manifested at a lower temperature (-20 EC) and a higher scan rate (5.0 V/s) by the appearance of a
small anodic counterpeak (Figure 4). When the switching potential is set only a little more negative
than the first reduction peak, its full reversibility is observed even at room temperature (Figure 3).
However, when it is set a little more negative than the secondreduction peak, the anodic counterpeak
of the first reversible couple becomes smaller.
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o
Figure 4: Cyclic voltammogram of 6.5 × 10-4 M 1 in DMF at -20 C
(HMDE, 5.0 V/s, 0.1 M Bu4NPF6, vs. SCE).
The reduction of the dimers 2, 3, and 7 - 9 is very similar to that of the monomer 1 (Table
2). In cyclic voltammograms a single reversible one-electron redox couple at -1.81 to -1.84 V is
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followed by another reduction peak at -2.29 to -2.31 V, which is irreversible. The normalized
current of all reduction processes in the dimers is less than twice that of the monomer 1. In
polarograms, and more clearly in cyclic voltammograms, 3 shows a weak pre-wave at -1.71 V,
attributed to adsorption of the first reduced species, the radical anion of 3.
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The cyclic voltammograms of the dimers 4 - 6 present more complicated reduction patterns:
each of these dimers undergoes two successive reversible one-electron reductions E(1) and E(2)
followed by two irreversible reduction steps E(3) and E(4) (Figure 3, Table 2). Another irreversible
reduction peak appears at more negative potentials (~ -2.67 V, E(5)). The corresponding
polarographic curve possesses an approximately two-electron height.
Like its oxidation, the reduction of 10 is easier than those of 1 - 9. Two successive reversible
one-electron reductions occur at -1.62 and -1.83 V and are followed by two irreversible reductions
at -2.24 and -2.61 V. The second irreversible reduction at -2.61 V appears to involve more than two
electrons. At even more negative potentials (-2.81 V), another reduction takes place.
Calculations. Since single bonds between aromatic rings are typically twisted by 20 - 40E,
conformational flexibility provides a considerable challenge to the calculation of the oxidation and
reduction potentials of the isobenzofurans 1 - 10. In 1 - 6 and 10, the existence of choices for the
relative sense of these twists is responsible for the presence of two conformers even in the monomer
1,11 and of a larger number of conformers in the dimers. In 7 - 9, the situation is further complicated
byadditional conformational isomerism in the saturatedchain that connects the two isobenzofurans.
In the study of electrochemical properties, it also needs to be considered that conformational
isomerism may be different in neutral, oxidized, and reduced molecules.
Fortunately, conformers differing only in the relative sense of twist about single bonds
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between aromatic rings can be expected to exhibit only minute differences in oxidation-reduction
and most other properties. This has been established in the case of 1, where the only observed
differentiating feature between the conrotatory and disrotatoryconformation is a small difference in
the shape of the vibrational envelope of the first electronic absorption band.11 We have verified this
expectation further by locating all 32 conformational minima in the ground states of each of the
dimers 4 - 6 and finding that their calculated highest occupied molecular orbital (HOMO from DFT)
and lowestunoccupied molecular orbital (LUMO from DFT) energies are identical within 0.01 eV.23
In this work, we considered 10 conformers for each compound, spanning the conformational space
corresponding to rotation about single bonds linking the two monomers. The lowest energy
conformer of the neutral, the oxidized, and the reduced forms was used for the calculation of
reduction potentials. The molecular geometries of the lowest energy conformers can be found in
Supporting Information.
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Accordingto PBE+D3/def2-TZVP energiescalculatedfor7, 8, and9, thestackedconformers
are favored by 3, 9, and 10 kcal.mol-1, respectively. However, we expect the extended conformers
to be favored by the entropic term in the expression for free energy by 4 - 6 kcal.mol-1, as estimated
from the Ä(EZPVE - RTlnQ) term (Supporting Information), and we find that they are also favored by
COSMO-RS solvation energies, byamounts rangingfrom ~4 to ~7 kcal.mol-1. Therefore, we expect
only the extended conformer to be present in the solvents used. This expectation is in line with the
finding that the computed reduction potentials agree better with the observed ones when extended
conformations are used for 7 - 9.
The calculated oxidation and reduction potentials are collected in Table 3 together with the
individual terms (solvation energies, ionization energies, zero-point energy, and the in-vacuum
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entropic and thermal contributions).
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Table 3. Calculated Oxidation and Reduction Potentials of 1 - 10.a
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SO2
SO2
DMF
DMFg
Cmpd. IEb /eV ÄÄGgpc /eV ÄÄGsolvd /eV
E0calcd /Ve E01/2exp /V E0calcd /Vf
E0pa exp /V
1e- ox
6.38
5.72
5.71
5.64
5.81
5.93
5.68
5.65
5.70
5.68
0.01
0.01
-0.02
0.00
0.02
0.00
0.01
0.02
0.07
0.00
0.85
0.78
0.84
0.77
0.83
0.87
0.82
0.77
0.79
0.66
0.85
0.81
0.82
0.82, 0.88
0.84
0.87
0.80
0.80
1
2
3
4
5
6
7
8
-1.28
-0.82
-0.78
-0.84
-0.85
-0.92
-0.78
-0.76
-0.76
-0.95
0.57 (0.95)
0.36 (0.74)
0.37 (0.75)
0.27 (0.65)
0.43 (0.81)
0.47 (0.85)
0.37 (0.75)
0.36 (0.74)
0.47 (0.85)
0.19 (0.57)
0.58
0.38
0.39
0.28
0.45
0.49
0.39
0.38
0.49
0.20
0.83
0.66
9
10
1e- red
0.99
1.53
1.50
1.72
1.55
1.46
1.58
1.56
1.54
1.62
0.15
0.13
0.17
0.11
0.13
0.17
0.14
0.14
0.08
0.13
1.73
1.33
1.32
1.32
1.36
1.42
1.26
1.19
1.28
1.43
1
2
3
4
5
6
7
8
9
10
-1.66 (-1.96) -1.84
-1.53 (-1.83) -1.88
-1.53 (-1.83) -1.89
-1.35 (-1.65) -1.69
-1.49 (-1.79) -1.77
-1.48 (-1.78) -1.84
-1.54 (-1.84) -1.88
-1.64 (-1.94) -1.87
-1.62 (-1.92) -1.86
-1.34 (-1.64) -1.62
a All values are in eV or V, potentials are against the SCE (+0.2444 V vs. NHE) in DMF and acetone (mimic
b
of SO2 solvent). See computational details, Eqs. 1 and 2. Adiabatic ionization energies or electron
affinities calculated in vacuum at the PBE+D3/def2-TZVP//PBE-def2-SVP level. c The difference in gas-
phase chemical potential (including ZPVE) calculated at the PBE+D3/def2-SVP level. d The difference in
e
solvation energies between the oxidized and reduced forms, calculated using the COSMO-RS method. In
f
parentheses, values increased by a constant shift of +0.38 V. In parentheses, values reduced by a constant
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shift of -0.30 V. e Oxidations in DMF are irreversible.
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Discussion
The synthesis of the dimeric isobenzofurans 2 - 10 by a single-step double coupling
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proceededsmoothlywith the soleexception of10, which had to be preparedin two separatecoupling
steps. It provided access to a series of compounds suitable for investigations of singlet fission, some
of which have already yielded results.23 The other aspect of the present study, gaining knowledge
of the redox potentials of 1 and its dimers, is also of immediate interest in our studies of the
photophysics of compounds containing the 1,3-diphenylisobenzofuran chromophore, since
intramolecular excited state charge transfer can represent a serious competitor to the desired singlet
fission event. The redox potentials will be ultimately relevant for the choice of materials into which
an electron or a hole will need to be injected if they are to be tested in solar cells.
Oxidation and Reduction of the Monomer 1. The UV-visible absorption spectra of the
radicalions of1 havebeenmeasuredusingpulseradiolysis and interpreted,11 but the redox potentials
were unknown.
We find that the oxidation of 1 is relativelyeasyand in liquid SO2 occurs by a reversible one-
electron transfer only about 0.35 V above the oxidation potential of ferrocene. The resulting radical
cation is fairly stable. The unusual temperature dependence of reversibility presented in Figure 2
invites additional study. In DMF and acetonitrile the situation is more complicated in that the
oxidation current corresponds to an irreversible two-electron process, based on comparison with the
one-electron standard, ferrocene. The primary radical cation undergoes a fast follow-up chemical
reaction, most likely with the solvent, and then a second electron transfer occurs (ECEC
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mechanism). The follow-up reaction is probably suppressed at low temperatures and therefore at
high scan rates partial reversibility is observed.
7
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9
The reduction of 1 is a relatively difficult reversible 1-electron process, 2.1 V below the
reduction of ferricenium. The electrochemical reduction mechanism of 1 is apparently of the EEC
type:
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1 + e- 6 1G •
E1 (rev)
1G • + e- 6 12G
E2 (quasirev)
12G + H+ 6 products
Quantum chemical calculations of oxidation-reduction potentials are becoming a standard
tool complementing experimental data24,25,26,27,28,29 and can be helpful for the interpretation of
reversibleredox behavior. Since reversible potentials are available for 1, both for oxidation in liquid
SO2 and for reduction in DMF, a meaningful absolute calculation of both potentials versus SCE is
possible, using the latest value27 E0abs(NHE) = 4.281 V for the absolute potential of the normal
hydrogen electrode NHE and the standard value 0.2444 V for the potential of SCE vs. NHE. The
oxidation potential of 1 is 0.35 V measured against Fc/Fc+ (Table 1) and 0.85 V after conversion to
SCE as a reference, using the measured difference of 0.505 V between the two reference electrodes
(cf. the calculated30 difference of 0.516 V in acetonitrile). The reduction potential is !1.84 V
measured against SCE (Table 2). The experimental values relative to the SCE, 0.85 and !1.84 V,
can be compared with the absolute computed values converted to SCE, 0.57 and !1.66 V,
respectively (Table 3).
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Thus, the value computed for the oxidation process is insufficiently positive by ~0.3 V and
the value computed for the reduction process is insufficiently negative by ~0.2 V. This is not a very
impressive agreement, and the most likely culprits are errors in (i) the COSMO-RS calculation of
absolute solvation energies and (ii) the calculation of ionization and electron attachment energies.
It is also likely that acetone is only an imperfect mimic for liquid SO2 in this type of calculation, but
parameters for the latter are not available. Admittedly, absolute calculations of redox potentials are
difficult and one can hope for better agreement when comparing the calculated redox potentials
within a series of related compounds.
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Oxidation and Reduction of Dimers 2 - 9. The observed oxidation as well as reduction
potentials generallydiffer only little from to those of the monomer 1, placing considerable demands
on computational interpretations. With the exception of the reduction of 4 - 6, the oxidation and the
reduction of the dimers occur in one single electrochemical step, suggesting that they proceed on
both isobenzofuranunits independentlyatthesamepotential. Theeffectofintramolecularelectronic
communication between the two ð systems through the bridge is negligible. Apparently, the
saturated alkane bridges in 2, 7, 8, and 9 prevent electron delocalization, as does the perpendicular
orientation of the two benzene rings in 3, imposed by the ortho methyl substituents. In these cases
the reduction response is analogous to that of 1, but with twice the current.
The situation is different in the reduction of the directlyconjugated compounds 4 - 6. Instead
of two 2-electron reduction peaks as in 2, 3, and 7 - 9, two pairs of 1-electron reductions are
observed, as each electron transfer that increases the negative charge impedes the next reduction
step. This separation of individual reduction steps shows that in this case the effect of electronic
communication between the two isobenzofuran units is significant.
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Not surprisingly in view of the shape of the frontier molecular orbitals of 1,11 the extent of
interaction between the two halves in the dimers increases in the order 6 < 5 < 4. The optimized
computed structures of the radical anions of 4 - 6 aremore planar than the neutral parent compounds.
This is reflected in an equal difference between E(3) and E(4) in all three dimers. The most negative
reduction process E(5), observed only in the case of 4 - 6, is probably due to the biphenylene bridges
which become non-innocent and reducible (cf. Table 2). The origin of the irreversibility of the
second reduction processes in all the dimers is probablythe high basicityof the resulting tetraanions,
which allows them to abstract a proton from the solvent.
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The location and the nature of the covalent bridging unit in the dimers 2 - 9 have a
remarkably small effect on the oxidation potentials in both DMF and liquid SO2, which differ from
that of 1 by at most 80 mV. The 40 - 60 mV easier oxidation of 7 - 9 relative to 1 is not likely to be
all due to the electron donor effect of the alkane bridge, since the redox potentials of 1 and 3 are
nearly identical. This argument is weakened by the location of the methyl groups in the latter in the
poorlyconjugatingmeta positions, but it neverthelessappearslikelythattheslightlyeasieroxidation
of 7 - 9 reflects a stacking stabilization of their radical cations. The limiting currents observed for
the oxidation of the dimers are less than twice that of 1, undoubtedly partially due to their smaller
diffusion coefficients, but probably at least in part due to an increased fouling of the electrode. The
current response observed for 6 and 7 is even smaller because of their poor solubility.
The reduction potentials within the series2 - 9 varyonlyslightlymore, within about 200 mV,
and are again all roughly equal to that of 1. As might be expected, the best conjugated dimer 4 is
the easiest to reduce.
A comparison with calculations is somewhat less reliable than was the case for 1, since even
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in liquid SO2 only the oxidation of 2, 4, and 10 is fully reversible, and the others are only
quasireversible. The reductions are all reversible. The calculations correctly predict little variation
within the series, but exaggerate it slightly, both for oxidation and for reduction potentials. An
empirical adjustment of the calculated values by a constant shift improves the agreement in both
cases and reduces the typical discrepancy to less than 0.1 V (Table 3).
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In contrast, it is probably significant that the calculations of both oxidation and reduction
potentials fail to reproduce the difference between the dimers and the monomer 1. They predict the
dimers to be 0.1 - 0.3 V easier to oxidize and to reduce than the monomer, and the observed
differences are much smaller. Although it is possible that some of the problem originates in
incorrect treatment of conformational isomerism, it seems more likely that this relatively poor
performance of theory reflects an inadequate treatment of the interplay between changes in the
ionization and electron attachment energies on the one hand and changes in the solvation energies
on the other hand, as one goes from the dimers to the monomer. Table 3 shows that the calculated
increase in the ionization potential and in the electron attachment energy upon going to the less
conjugated monomer 1 is ~0.7 eV, and this is counteracted by a 0.4-0.5 eV gain in solvation energy
of the smaller ions from 1. In reality, the two opposing quantities approximately cancel and the
redox potentials of the dimers are very similar to those of the monomer. It is not clear which of the
opposingquantities is calculatedincorrectly, or if both are. The situation contrasts with our previous
more satisfactoryexperience with the oxidation of a series of anions that were all of comparable size
and showed no conformational isomerism,26,31 and provides a cautionarynote for future calculations
of redox potentials of series of species that differ significantly in size. At the same time, it provides
a good test case for the development of better theoretical procedures.
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Oxidation and Reduction of 10. This compound is 0.2 V easier to reversibly oxidize or to
reduce than the monomer 1. The calculated differences are again too large, 0.4 V for oxidation and
0.3 V for reduction. This double isobenzofuran exhibits a peculiar behavior upon cycling and we
plan to examine it more thoroughly.
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Experimental Part and Computational Details
Electrochemistry. Materials. Tetra-n-butylammonium hexafluorophosphateobtainedfrom
Fluka AG (>98 %) was recrystallized twice from ethanol, dried in a vacuum oven and kept in a
vacuum desiccator over P2O5. Anhydrous SO2 gas (99.98 %) obtained from Linde Technoplyn,
Prague, Czech Republic, was condensed at !70 °C in a dried flask and purified by stirring
magnetically for 30 min with highly activated alumina (basic type 5016A, Fluka AG). N,N-
dimethylformamide (DMF) purchased from Fluka absolute (>99.8 %) was purified by azeotropic
distillation with 5% water and 5% benzene followed by vacuum fractionation under argon
atmosphere.32
Measurements. All voltammetric measurements were performed using an AUTOLAB
potentiostat (PGSTAT30) controlled by a PC with GPES software. DC polarography was recorded
using an analog potentiostat PA3 connected with an XY recorder (Laboratorní Pøístroje Praha), the
controlled drop time was 2 s, and a typical scan rate was 10 mV/s at the sensitivity of 50 nA/cm.
][ClO ].
The supporting electrolytes were 0.1 M [NBu4][PF6] and [NOct4
4
Cyclic voltammetric measurements in liquid SO2 were carried out in a single compartment
glass cell using a Pt disk (1.0 mm diameter) working electrode, a Pt plate counter electrode and an
Ag rod as a quasi-reference electrode. Ferrocene (Fc+/Fc) was used (0.3 – 0.5 mM) as an internal
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reference. The volume of the five-neck conical shaped electrochemical cell was ~ 10 mL. The cell
containing enough substrate, electrolyte and ferrocene, along with a magnetic stirrer, was placed on
the vacuum line (7.0 mbar) and then filled and evacuated with argon several times. Purified SO2 was
distilled into the cell at -70 °C to prepare a sub-milimolar solution (~ 4 mL) and the measurements
were performed at the same temperature and above, up to 22 EC, at scan rates typicallyranging 0.08 -
5.0 V/s under argon atmosphere.
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In DMF, cyclicvoltammetricandpolarographicreductionexperimentswerecarriedout using
a hanging mercury drop working electrode (HMDE), a Pt plate counter electrode and a saturated
calomel electrode (SCE) with a nonaqueous salt bridge as a referenceelectrode at room temperature,
while for oxidation experiments the Pt disk was used as the working electrode. Sub-mM solutions
were prepared by dissolving the dimer in DMF (8 mL) and the measurements were performed under
Ar atmosphere after several min of deaeration. The scan rates of CVs were 0.1 to 5.0 V/s. The
reproducibility of potential measurements was ± 5 mV.
Computational Details. Quantum chemical calculations were performed using the
TURBOMOLE 6.4 program. Geometry optimizations were carried out in the RI-DFT-PBE33
approximation.34 During the optimization, the def2-SVP basis set was employed on all atoms,35
whereas the def2-TZVP basis set was used for the final single-point calculations to obtain molecular
energies.36 For all open-shellsystems, theunrestrictedKohn-Sham (UKS) formalism was used. The
addition of an empirical dispersion term to the DFT calculations (DFT+D337) changed the calculated
reduction potentials by less than 0.01 V and those values are not reported.
Solvation effectsweretakeninto accountbyusingtheCOSMO-RS(conductor-likescreening
model for realistic solvation) method.38,39 The standard COSMO calculations40 required by the
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COSMO-RS protocol were carried out employing the TURBOMOLE 6.4 program. The
recommended protocol involving the Becke-Perdew (B-P) functional41,42,43 for the vacuum and the
år = 4 (ideal screening) calculations (at the år = x optimized geometries, where x is the dielectric
constant of the solvent) together with the def-TZVP basis set was used. Final COSMO-RS
calculations were carried out using the COSMOtherm44 program.
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The Gibbs free energy and reduction potentials were calculated from
G = Eel + ÄGsolv + EZPVE + pV – RT ln(qtrans qrot qvib).
E0/V = 27.21(Gox/a.u. – Gred /a.u. ) – E0abs(NHE)/V,
(1)
(2)
where Eel is the vacuum energy of the system (RI-PBE/def2-TZVP//RI-PBE/def2-SVP) and Gsolv is
the solvation free energy (from RI-BP/def-TZVP,COSMO-RS, å = 1, å = 4). EZPVE is the zero-point
energy, and pV –RT ln(qtrans qrot qvib) accounts for thermal corrections to the enthalpy and entropy
terms, obtained from a frequencycalculation with the procedure used for the geometryoptimization
(RI-PBE/def2-SVP level), 298 K, and 1 atm using the ideal gas approximation. Gox/red is the Gibbs
free energy of the oxidized (reduced) form and the E0abs(NHE) is the absolute potential of the normal
hydrogen electrode (NHE). The latest value27 E0abs(NHE) = 4.281 V was used.
Synthesis. General Information. All reagents were commercial and were used without
further purification unless otherwise noted. All reactions were monitored byTLC with GF254 silica
gel coated plates. Flash column chromatography was carried out by using 200–300 mesh silica gel
1
at increased pressure. H NMR and 13C NMR spectra were recorded with a 400 MHz spectrometer
in CDCl3 solutions using tetramethylsilane as the internal standard unless stated otherwise. ä values
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are given in ppm and coupling constants (J) in Hz. Melting points were measured on a melting point
apparatus and are uncorrected. IR spectra were recorded in KBr pellets and ESI TOF was used to
obtain HRMS.
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Dimer Synthesis: General Procedure. To a solution of a dibromo compound (1 mmol) in
THF (10 mL) at -78 EC was added 1.6 M BuLi in hexanes (3 mmol). The mixture was stirred at -78
EC for 10 min under Ar atmosphere and cannulatedinto a solution of 3-phenylphtalide (11), 3 mmol)
in THF (10 mL). The red mixture was stirred at room temperature for 10 min and then at reflux
temperature for another 30 min. It was cooled to rt, 5% aqueous HCl (20 mL) was added, and the
mixture was extracted with ether. The ether phase was dried with MgSO4, the solvent was
evaporated, and the residue purified as stated below.
Bis(p,p’-1,3-diphenylisobenzofuran) (4) was purified by recrystallization from boiling
DMSO to yield very fine red needle-like crystals (1.42 g, 82%), mp 297 - 298 oC. The compound
is not stable in the presence of light and air; it reacts with oxygen to form an oxidized form of 4. The
oxidized form is more soluble in dichloromethane than is the parent compound. Therefore, this
compound was dissolved in dichloromethane and filtered to remove the impurities from the
crystallized pure compound under inert conditions. Alternatively, we also ran continuous Soxhlet
1
extraction with dichloromethane. H NMR (600 MHz, CD2Cl2) ä 8.10 (d, J = 8.2, 4H), 8.01 (d, J
= 8.2, 4H), 7.92 (ddd, J = 2.4, 5.5, 9.4, 4H), 7.86 (d, J = 8.2, 4H), 7.52 (dd, J = 5.7, 9.6, 4H), 7.33
13
(dd, J = 4.1, 11.4, 2H), 7.16 – 7.03 (m, 4H). C NMR (201 MHz, CD2Cl2) ä 144.51, 144.02, 139.01,
132.08, 131.17, 129.55, 127.72, 127.60, 125.98, 125.91, 125.59, 125.32, 122.99, 122.82, 120.75,
120.74. IR(KBr):v 3020(w), 1589(s), 1484(s), 1442(m), 1203(s) cm-1. HRMS (ESI+) calcd for
C40H27O2+ (M+H+) 539.2011, found 539.2015. Anal. Calcd. for C40H26O2 : C 87.73, H 4.97. Found:
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C 87.83, H 4.73.
3,3'-(Biphenyl-3,4'-diyl)bis(1-phenylisobenzofuran) (5) was recrystallized from an
7
o
1
8
acetone-methanol mixture (1:10) as a yellow solid (80 % yield). Mp 198-199 C. H NMR (400
MHz, CD2Cl2) ä 8.31 (s, 1H), 8.16 (d, J = 8.5 Hz, 2H), 8.05 (d, J = 8.4 Hz, 4H), 8.04 – 7.93 (m, 5H),
7.92 (d, J = 8.4 Hz, 2H), 7.71 – 7.62 (m, 2H), 7.61 – 7.51 (m, 4H), 7.38 (td, J = 7.5, 1.0 Hz, 2H),
7.21 – 7.08 (m, 4H). 13C NMR (101 MHz, CD2Cl2) ä 144.16, 143.76, 141.39, 139.24, 132.34,
131.71, 131.01, 129.75, 129.19, 127.84, 127.25, 125.64, 125.54, 125.51, 125.22, 124.99, 124.97,
123.92, 123.23, 122.63, 122.55, 122.43, 122.34, 120.39, 120.32. IR(KBr): í (cm-1) 3050(w),
1600(s), 1470(m), 1320(m), 1200 (m). MS ESI(+) calcd for [C40H26O2Li]+ 545.2088, found
545.2101.
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3,3'-Bis(3-phenylisobenzofuran-1-yl)biphenyl (6) was recrystallized from an acetone-
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methanol mixture (1:10) as a yellow solid (92% yield). Mp 203-205 EC. H NMR (400 MHz,
CD2Cl2) ä 8.31 (m, 2H), 8.02 (m, 4.0 Hz, 1H), 7.98 (m, 2H), 7.92 (m, 2H), 7.67 (m, 4H), 7.52 (m,
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4H), 7.34(m, 2H), 7.11(m, 4H). C NMR (101 MHz, CD2Cl2) ä 144.19, 143.75, 141.94, 132.34,
131.70, 129.76, 129.20, 127.26, 126.00, 125.68, 125.54, 124.98, 124.07, 123.66, 122.62, 122.34,
120.40, 120.29. IR(KBr): í (cm-1) 3023(w), 1597(s), 1460(m), 1307(s), 1190(s). MS ESI+ calcd
for [C40H26O2Li]+ 545.2088, found 545.2075.
1,2-Bis[4-(3-phenylisobenzofuran-1-yl)phenyl]ethane(7)wasrecrystalizedfrommethanol
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(85 % yield). Mp > 230 dec. H NMR (400 MHz): ä 7.95 (d, 4H, J = 7.5 Hz), 7.88 (d, 4H, J = 7.5
Hz), 7.84 (m, 4H), 7.49 (t, 4H, J = 7.51 Hz), 7.30 (m, 6H), 7.02 (m, 4H), 3.03 (s, 4H); 13C NMR (75
MHz): ä 144.1, 143.6, 140.7, 132.0, 130.0, 129.4, 129.2, 127.0, 125.1, 125.0, 122.3, 122.0, 120.6,
37.9. IR: í 3040(w), 2900(w), 1590(s) cm-1. MS ESI+ calcd for [C42H30O2Li]+ 573.2401, found
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573.2415.
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1,3-Bis(4-(3-phenylisobenzofuran-1-yl)phenyl)propane (8) was recrystalized from
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methanol (80% yield). Mp 150 EC. H NMR (400 MHz): ä 7.95 (d, 4H, J = 7.5 Hz), 7.88 (d, 4H,
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J=7.5 Hz), 7.84 (m, 4H), 7.49 (t, 4H, J = 7.51 Hz), 7.30 (m, 6H), 7.02 (m, 4H), 2.75 (t, 4H, J = 7.51
Hz), 2.06 (m, 2H); 13C NMR (75 MHz): ä 144.3, 143.6, 141.4, 132.0, 129.6, 129.3, 129.1, 127.0,
125.4, 125.1, 124.9, 122.3, 122.0, 120.5, 120.4, 35.4, 33.0. IR(KBr): í 3050(w), 2950(w), 1590(s)
cm-1. MS ESI(+) calcd for [C43H32O2Li]+ 587.2557, found 587.2566.
1-Phenyl-3-(3-(3-(4-(3-phenylisobenzofuran-1-yl)phenyl)propyl)phenyl)isobenzofuran(9)was
recrystallized from methanol (yield 80%). Mp 130 oC. 1H NMR (400 MHz): ä 7.96(m, 4H), 7.89(d,
2H, J = 8.43 Hz), 7.83(m, 6H), 7.49(dt, 4H, J = 2.02, 8.06 Hz), 7.43 (t, 1H, J = 7.88 Hz), 7.35(d, 2H,
J = 8.24 Hz), 7.17(d, 1H, J = 7.69 Hz), 7.02(m, 4H), 2.79(m, 4H), 2.11(m, 2H); 13C NMR (75 MHz):
ä 144.2, 143.1, 141.4, 132.0, 131.9, 129.6, 129.4, 129.1, 127.4, 127.1, 127.0, 126.0, 126.4, 125.4,
125.1, 125.0, 124.9, 122.7, 122.3, 122.0, 120.5, 120.4, 120.3, 35.7, 35.4, 33.0. IR(KBr): í 3050(w),
2950 (w), 2870 (w) 1590(s) cm-1. MS ESI(+) calcd for [C43H32O2Li]+ 587.2557, found 587.2565.
1,4-Bis(3-phenylisobenzofuran-1-yl)benzene(10). Toasolution of1-(4-bromophenyl)-3-
phenylisobenzofuran22 (1.2 mmol) in dry THF (20 mL) at -78 EC was added 1.6 M n-BuLi in
hexanes (1.6 mmol). The mixture was stirred at -50 EC under Ar atmosphere. After 1.5 h, 3-
phenylphthalide (11, 1.6 mmol) was added and the reaction mixture was maintained at -50 °C under
Ar atmosphere for 2 h. The red mixture was allowed to warm to room temperature overnight and
HCl (36%, 2.5 mL) was added. After stirring one more hour diethyl ether (15 mL) was added and
the organic phase was separated. More diethyl ether (50 mL) was added and the mixture was left
resting for 1 h in the dark. The resulting solid was filtered and dried in the dark to give 105 mg of
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a bright red solid. Yield 19%. Mp 221-222°C. H-NMR (THF-d8, 400 MHz, ä ppm) 8.14 (s, 4H),
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8.05-7.95 (m, 8H), 7.52-7.48 (t, J = 7.8 Hz, 4H), 7.32-7.28 (t, J = 7.5 Hz, 2H), 7.11-7.06 (m, 4H).
13C-NMR (THF-d8, 101 MHz, ä ppm) 143.84, 143.37, 131.45, 129.61, 128.77, 126.78, 125.31,
125.24, 124.74, 124.53, 122.53, 122.36, 120.08. IR (í, cm-1) 3092 (w), 3055 (w), 1625 (s), 1597 (m),
1500 (s), 1390 (m), 1212 (m), 1115 (m), 999 (m). MS (ESI+) calcd for C34H23O2+ (MH+) 463.1698,
found 463.1720. Anal. Calcd for C34H22O2: C, 88.29; H, 4.79. Found: C, 88.42; H, 5.15.
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1 - ( 3 - B r o m o p h e n y l ) - 3 - ( 4 - b r o m o p h e n y l ) p r o p a n e
( 1 3 ) .
(E)-3-(3-bromophenyl)-1-(4-bromophenyl)prop-2-en-1-one (12) was prepared by a modified
literature procedure.18 To a mixture of 4-bromoacetophenone (2.2 g, 11 mmol) and 3-
bromobenzaldehyde (2 g, 11 mmol) in ethanol (50 mL) was added a solution of NaOH (0.66 g) in
water (15 mL). The mixture was stirred for 30 min. A light yellow solid 12 formed and was
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collected by filtration (3.9 g, 97%). H NMR (300 MHz): ä 7.89 (d, 2H, J = 8.46 Hz), 7.79 (t, 1H,
J = 1.71 Hz), 7.73 (d, 1H, J = 15.7 Hz), 7.66 (d, 2H, J = 8.34 Hz), 7.54 (m, 2H), 7.46 (d, 1H, J =
15.66 Hz), 7.30 (t, 1H, J = 7.83 Hz). A solution of 12 (3.32 g, 9 mmol) in trifluoroacetic acid (30
mL) containing triethylsilane (9 mL) was stirred for 30 min and quenched with water (100 mL). The
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product was extracted with dichloromethane to give 2.80 g (88 % yield) of 13 as viscous oil. H
NMR (300 MHz): ä 7.41 (d, 2H, J = 5.1 Hz), 7.33 (m, 2H), 7.162 (t, 1H, J = 4.5 Hz), 7.1 (d, 1H,
J = 4.5), 7.06 (d, 2H, J = 5.1 Hz); 13C NMR (75 MHz): ä 144.3, 140.8, 131.5, 131.4, 130.2, 129.9,
129.0, 127.1, 122.4, 119.6, 34.9, 34.7, 32.5; MS ESI(-) calcd for [C15H14Br2Cl]- 388.9135, found
388.9147.
1,3-Bis(4-bromophenyl)propane (15). (E)-1,3-bis(4-bromophenyl)prop-2-en-1-one (14)
was prepared by a modified literature procedure.18,19 To a mixture of 4-bromoacetophenone (2.2 g,
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0.011 mol) and 4-bromobenzaldehyde (2 g, 0.011mol) in ethanol (50 mL) was added a solution of
NaOH (0.66 g) in water (15 mL). The mixture was stirred for 30 min while a light yellow solid
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product 14 formed. Filtration yielded 3.8 g (94% yield). H NMR (300 MHz): ä 7.9 (d, 2H, J = 8.7
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Hz), 7.76 (d, 1H, J = 15.69 Hz), 7.67 (d, 2H, J = 8.7 Hz), 7.58 (d, 2H, J = 8.67 Hz), 7.52 (d, 2H, J
= 8.58 Hz), 7.48 (d, 2H, J = 15.72 Hz). A solution of 14 (2.6 g, 7 mmol) was dissolved in a mixture
of trifluoroacetic acid (~26 mL) and triethylsilane (10 mL) and the milky white mixture was stirred
for 30 min. The reaction was quenched with water (100 mL) and the product was extracted with
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dichloromethane to give 2.13 g (85 % yield) of 15 as a viscous oil. H NMR (300 MHz): ä 7.42 (d,
4H, J = 8.49 Hz), 7.06 (d, 4H, J = 8.58 Hz), 2.60 (t, 4H, J = 7.56 Hz), 1.92 (m, 2H); 13C NMR (300
MHz): ä 140.9, 131.4, 130.2, 119.6, 34.6, 32.6; MS ESI (-) Calcd for [C15H14Br2Cl]- 388.9135,
found 388.9147.
3,4'-Dibromobiphenyl45,46,47 was prepared from 3-amino-4'-bromobiphenyl, obtained from
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4'-bromo-3-nitro-1,1'-biphenyl. H NMR (400 MHz, CDCl3) ä 8.41 (s, 1H), 8.25 – 8.17 (m, 1H),
7.86 (m, 1H), 7.62 (m, 3H), 7.49 (d, J = 8.5 Hz, 2H). It was synthesized as reported20,48 except that
10% Pd/C was used instead of 5% Pd/C. To a solution of 4-bromo-3’-nitrobiphenyl (1.5 g, 5.4
mmol) in ethanol (100 mL) was added Sn (3 g) and concentratedaqueous HCl (25 mL). The mixture
was refluxed for 6 h, cooled to room temperature, and made alkaline with a KOH solution (6 M).
The mixture was extractedwith dichloromethane(3 × 50 mL) and the combinedorganicphaseswere
dried over Na2SO4. After filtration of drying agent, the solvent was evaporated to yield 3-amino-4’-
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bromobiphenyl (1.3 g, 96 %). H NMR (500 MHz, CD2Cl2) ä 7.57 (d, J = 8.5 Hz, 2H), 7.45 (d, J
= 8.5 Hz, 2H), 7.26(t, J = 7.8 Hz, 1H), 6.98 (m, 1H), 6.89 (t, J = 2Hz, 1H), 6.73 (m, 1H), 3.78 (s,
2H). The amine was used in the next step without purification. A solution of 3-amino-4’-
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bromobiphenyl (1.3 g, 5.2 mmol) in 30% H2SO4 (30 mL) was cooled below 5 C. An ice-cold
solution of NaNO2 (2.5 equiv.) in water (10 mL) was added slowly and the mixture was stirred for
20 min. This solution was slowly added to a solution of CuBr (5 g) in 48% HBr (150 mL), pre-
cooled in an ice bath. After warming to room temperature, the mixture was stirred for 4 h at 60 EC,
cooled to room temperature, and extracted with ether (3 × 50 mL). The combined ether phases were
dried over MgSO4, filtered, and ether was evaporated. The crude material was purified by column
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chromatography (9:1 hexanes:ethyl acetate) to give the product (1.20 g, 74 %) as an oil. H NMR
(400 MHz) ä 7.69 (t, J = 2.4Hz, 1H), 7.58 (d, J = 11.2 Hz, 2H), 7.48 (m, 2H), 7.42 (d, J = 11.2Hz,
2H), 7.32 (t, J = 10.4 Hz, 1H).
Acknowledgement. This material is based upon work supported by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-
SC0007004. Work in Prague was supported by the AS CR, RVO 61388963 and 61388955, and by
GACR grants 13-21704S and 14-31419S.
Supporting Information. Spectral characterization of new compounds and calculated
energies and geometries of compounds 1 - 10, and an enlarged view of the cyclic voltammogram of
4. This material is available free of charge via the Internet at http://pubs.acs.org.
References
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2.
Howard, J. A.; Mendenhall, G. D. Can. J. Chem. 1975, 53, 2199-2201.
Paci, I.; Johnson, J. C.; Chen, X.; Rana, G.; Popoviæ, D.; David, D. E.; Nozik, A. J.; Ratner,
M. A.; Michl, J. J. Am. Chem. Soc. 2006, 128, 16546-16553.
Smith, M. B.; Michl, J. Chem. Rev. 2010, 110, 6891-6936.
3.
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