Structural characterization of quaterphenyl cation radical: X-ray crystallographic evidence of quinoidal charge derealization in poly-p-phenylene cation radicals

Article abstract of DOI:10.1021/ja0720356

Quaterphenyl derivative (QP) containing tert-butyl solubilizing groups at the terminal positions yields a stable cation radical salt that was isolated, and its structure was established by X-ray crystallography. The crystal structure of neutral QP and its

Full text of DOI:10.1021/ja0720356

Published on Web 06/09/2007
Structural Characterization of Quaterphenyl Cation Radical: X-ray
Crystallographic Evidence of Quinoidal Charge Delocalization in
Poly-p-phenylene Cation Radicals
Moloy Banerjee, Sergey V. Lindeman, and Rajendra Rathore*
Department of Chemistry, Marquette UniVersity, P.O. Box 1881, Milwaukee, Wisconsin 53201-1881
Received March 22, 2007; E-mail: rajendra.rathore@marquette.edu
The study of π-conjugated oligomers and polymers has attracted
considerable attention since these materials hold promise for
applications in the emerging area of molecular electronics and
nanotechnology.1 Of these, poly-p-phenylenes (PPP) are especially
important due to their optoelectronic properties,2 high conductivity
when doped with either n- or p-type dopants,3 potential usage as
active constituents of blue light emitting diodes,4 and high thermal
and oxidative stability.5 Unfortunately, their lack of processability,
especially when they are unsubstituted, has hampered their study
as well as exploitation of their extensive potential for practical
applications.6
Several studies have been conducted where PPP oligomers have
Figure 1. (A) Cyclic voltammograms of 2 × 10-3 M QP in CH2Cl2
containing 0.1 M n-Bu4NPF6 at scan rates between 25 and 400 mV s-1
,
been utilized as spacers or bridges in molecular assemblies con-
sisting of electron donor-bridge-electron acceptor (D-B-A) to
explore their potential as functional molecular wires for long-range
electron transport.7 The mechanism of charge transport (CT) in these
assemblies is proposed to occur by a combination of highly distance-
dependent coherent (i.e., superexchange) and weakly distance-
dependent noncoherent (i.e., hopping) mechanisms.7 In order to gain
a better understanding of various CT mechanisms operating in these
PPP-based molecular wires, a quantitative understanding as to how
the charge is stabilized by multiple aryl moieties that constitute
the spacer is highly desirable. Therefore, attempts to isolate an
oxidized polyphenyl cation radical to gain X-ray crystallographic
evidence as to how the creation of a hole leads to structural changes
in various PPPs are pertinent. We have recently undertaken studies
to this effect, and herein we report the isolation of both the neutral
as well as the cation radical salt of a quaterphenyl derivative (QP)
having tert-butyl solubilizing groups at the terminal positions,
enabling us to unequivocally establish from the crystallographic
parameters that a single charge is delocalized by a quinoidal
distortion of the phenylene rings. It is also noted that the tert-butyl
substitutions at terminal positions in larger PPP oligomers merely
aid in improving their solubility without affecting the electronic
properties to any significant extent as the HOMO resides largely
on the central phenylene rings.8 The details of these preliminary
finding are discussed herein.
and (B) UV-vis absorption spectrum of cation radical QP+• obtained by
-
NO+SbCl6 oxidation in dichloromethane at 22 °C.
reversible oxidation potential was calibrated with ferrocene as
internal standard (Eox ) 0.45 V vs SCE) and was found to be 1.47
V versus SCE corresponding to the formation of a monocation.
The high electrochemical reversibility of oxidation of QP
prompted us to isolate its cation radical in solution by chemical
oxidation using nitrosonium hexachloroantimonate as a 1-e- oxidant
according to the stoichiometry in eq 1.
QP + NO+SbCl6 DCM 8 QP+•SbCl6- + NOv
(1)
-
-10 °C
Thus, a solution of QP in anhydrous dichloromethane was added
-
to crystalline NO+SbCl6 under an argon atmosphere at -10 °C.
The gaseous nitric oxide produced was entrained by bubbling argon
through the solution to yield a deep red solution, which upon
-
spectrophotometric analysis indicated the formation of QP+•SbCl6
as judged by the appearance of characteristic twin absorption bands
at λmax ) 490 and 1302 nm (Figure 1B).9 Twin absorption bands
in QP+• are reminiscent of the absorption bands observed in the
spectrum of the biphenyl cation radical generated by laser flash
photolysis or pulse radiolysis.10 The molar extinction coefficient
of pink-red QP+• was estimated to be 54 800 ( 3000 M-1 cm-1
(see Supporting Information).
The 4,4′′′′-di-tert-butylquaterphenyl was prepared from 4,4′′-
diiodobiphenyl and 4-tert-butylphenylboronic acid via a Pd(0)-
mediated Suzuki reaction in 67% yield and was purified by
crystallizations from a CHCl3-MeOH mixture (see Supporting
Information for the experimental details and spectral data).
Next, the quaterphenyl derivative was subjected to electrochemi-
cal oxidation at a platinum electrode as a 2 × 10-3 M solution in
dichloromethane containing 0.2 M n-Bu4NPF6 as the supporting
electrolyte. The cyclic voltammograms (Figure 1A) consistently
met the reversibility criteria at various scan rates of 25-400
mV/s, as they all showed cathodic/anodic peak current ratios of
ia/ic ) 1.0 (theoretical) as well as the differences between anodic
and cathodic peak potentials of Epa - Epc ∼ 70 mV at 22 °C. The
-
The stability of the QP+•SbCl6 prompted us to attempt the
isolation of its crystalline salt as follows. For example, an excellent
crop of dark red crystals, suitable for X-ray crystallographic studies,
were obtained by a slow diffusion of toluene in a dichloromethane
-
solution of QP+•SbCl6 during a period of 2 days at -30 °C.
-
The crystal structure of the QP+•SbCl6 revealed that two
symmetrically independent molecules of QP+• having essentially
the same structure are stacked as dimeric pairs which are separated
by intercalated toluene molecules, as shown in Figure 2. The close
interplanar distance between the two QP+• in the dimeric pair of
∼3.1 Å, which is less than the equilibrium van der Waals separation
of 3.4 Å, is indicative of electronic coupling between them.11 Such
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10.1021/ja0720356 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
ring, we can estimate that the rings A-D bear the following positive
charges: 0.15 e (A), 0.46 e (B), 0.33 e (C), and 0.06 e (D). As
such, the asymmetric charge distribution further attests to the strong
electronic coupling within the “shifted” dimeric associates.11
These salient features of the QP cation radical can only be
rationalized, when one considers that the single charge is stabilized
by a quinoidal valence bond resonance structure. Such a resonance
form having a quinoidal structure leads to unequal bond distortions
in the aryl rings as well as planarization of the aryl rings for
effective π-conjugation with the magnitude of changes being the
highest in the central rings. It is worthwhile to note that a previously
reported ab initio SCF-LCAO-MO calculation of a lithium-doped
quaterphenyl anion radical showed a similar quinoidal distortion
as observed herein for the QP+•.14
In summary, the isolation and X-ray crystal structure determi-
-
Figure 2. The crystal structure of QP+•SbCl6 cation radical, with the
-
nation of QP+•SbCl6 provides unequivocal evidence for the
packing diagram showing that the toluene molecules are embedded between
the stacked dimeric pairs.
quinoidal stabilization of the cationic charge or polaron, which is
responsible for the high conductivities in PPP oligomers in their
doped state. The defect state induces regions of quinoidal confor-
mation and thus serves as a physical model for the insulator-metal
transition in PPP.3 Studies are underway for a more comprehensive
investigation of the optoelectronic properties of PPP oligomers and
will be reported in due course.
a close packing of the QP+• in dimeric pairs leads to a slight
longitudinal displacement with respect to each other to compensate
for the steric crowding caused by the terminal tert-butyl groups.
A closer look at the structural parameters of the cation radical
QP+•, along with a comparison with its neutral form, points to four
important observations:
Acknowledgment. We thank the National Science Foundation
(CAREER Award) for financial support.
Supporting Information Available: Preparation and spectral data
for QP and the X-ray structural data for the neutral QP, QP+•SbCl6
.
-
This material is available free of charge via the Internet at http://
pubs.acs.org.
(i) The central bond distance of 1.440 Å between the two inner
benzene rings B and C in QP+• is considerably shorter than the
two peripheral C-C bonds between the outer rings and the
penultimate ones (i.e., 1.454 Å between rings A and B and 1.463
Å between rings C and D). This represents a contraction of the
central C-C bond (between ring B and C) by 5 pm in the oxidized
form in comparison to 1.1 pm in neutral QP.12 (ii) The longitudinal
bonds inside the rings are contracted, whereas the transverse bonds
are lengthened in comparison to neutral QP, indicative of a quinoidal
bond alteration.13 In numerical terms, the longitudinal bond
contraction results in bond lengths of 1.377 Å (ring A), 1.362 Å
(ring B), 1.368 Å (ring C), and 1.381 Å (ring D) from an average
of 1.38 Å in the neutral QP and a consequent lengthening of the
transverse bonds to 1.397/1.409 Å (ring A), 1.419/1.424 Å (ring
B), 1.421/1.413 Å (ring C), and 1.407/1.399 Å (ring D). The
magnitude of the bond contractions and expansions reaches a
maximum in the inner rings with the peripheral rings being less
affected. (iii) The torsional angles between the rings A/B/C are
∼0.1-15°, resulting in a nearly coplanar conformation. However,
for the peripheral ring D, this angle slightly increases primarily
due to the steric hindrance of the tert-butyl group of the adjacent
stacked molecule. Thus, the formation of the cation radical smooths
out the torsional motion of the interconnected aryl rings from their
typical values of ∼30° between the outer rings and ∼10° between
the central rings as in the neutral form, in order to allow effective
stabilization of the cationic charge by quinoidal distortions.13 (iv)
Considering the magnitude of quinoidal distortion to be a linear
function of the amount of charge allocated within the corresponding
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