Effect of substitution on the optical properties and HOMO-LUMO gap of oligomeric paraphenylenes

Article abstract of DOI:10.1021/jp108619n

A series of dialkyl amino benzophenone dimers with various alkyl chain lengths is presented. Gaussian B3LYP/6-31G(d) calculations show that the band gap decreases within the dimer series as a function of the donor group efficiency. Theoretical calculation

Full text of DOI:10.1021/jp108619n

13228
J. Phys. Chem. A 2010, 114, 13228–13233
Effect of Substitution on the Optical Properties and HOMO-LUMO Gap of Oligomeric
Paraphenylenes
Brian D. Koepnick,‡,§ Jeremy S. Lipscomb,† and Darlene K. Taylor*,†
Department of Chemistry, North Carolina Central UniVersity, 3105 M. Townes Science Building, 1801 Concord
Street, Durham, North Carolina 27707, United States, North Carolina School of Science and Math, 1219 Broad
Street, Durham, North Carolina 27705, United States
ReceiVed: September 9, 2010; ReVised Manuscript ReceiVed: October 16, 2010
A series of dialkyl amino benzophenone dimers with various alkyl chain lengths is presented. Gaussian B3LYP/
6-31G(d) calculations show that the band gap decreases within the dimer series as a function of the donor
group efficiency. Theoretical calculations show that the interaction between phenyl-phenyl rings is more
important than simple donor-acceptor effects. We report the experimental and electro-optical properties of
one of these dimers, N,N-(dibutyl)-4-amino-benzophenone. The experimental and theoretical results enabled
us to design a new dimer. Altogether, side chain substituents reported herein tune the theoretical band gap of
paraphenylene based dimers by over 8.86 eV.
Introduction
unit has a greater effect on charge transfer in the resulting
material.22 Other studies on thiophenes have shown that large
substituent groups tend to increase steric hindrance between
monomer units. This potentially distorts the planarity of the
dihedral angle and thus the π-orbital overlap between neighbor-
ing monomer units.23
The potential for semiconducting polymers to outperform their
silicon-based counterparts has inspired the development of
organic-based solar cells,1,2 light-emitting diodes (LEDs),3-5 and
photodetectors.6 Modest progress in organic versions of these
technologies has occurred through several strategies7 that
primarily utilize three small band gap materials platforms:
aniline,8 phenylene vinylene,9-11 and thiophene.12-16 Funda-
mental studies are needed to design better organic electronic
materials. One approach widely overlooked in the literature is
fundamental studies of high band gap polymers. Invaluable
lessons can be learned about designing better semiconducting
organic polymers by tuning high band gap polymers to push
the boundaries of their wavelength absorption.
To this end, we have approached the field of organic
electronic materials by focusing on paraphenylene oligomers
and polymers. Oligomeric paraphenylenes (OPP) are a class
of organic semiconductors known for their excellent thermal
and mechanical stability. Biphenyl is the simplest version of
these molecules, but it is the para hexaphenyl version that has
been widely reported for its optical properties and tested as an
emitting layer in LEDs.17-19 Fundamental studies on the
electronic properties of hexa paraphenyls have emerged;20
however, there are still unresolved structural questions pertaining
to these highly luminescent materials and the class of paraphe-
nylenes in general. Specific questions remain regarding the
contributions of substitution on the phenyl ring and planarity
between phenyl rings.
On the basis of these facts and our ongoing interest in diethyl
amino substituted poly(p-benzophenone),24 we have chosen to
alter systematically the side-chain substituent group of a series
of paraphenlyene dimers as shown in Figure 1 to tune the
resulting electrooptical properties. In all but one case, the
substitution pattern involves either variations or replacement
of the alkyl chains of the (dialkylamino) side-chain substituent.
Theoretical studies show that the length of the alkyl chain group
on the side-chain substituent group has very little effect on the
electro-optical properties of the dimer series. However, a critical
alkyl chain length of four carbons was apparent as dimer A4
showed deviations in trends observed for the other dimers in
series A. We prepared and characterized A4 to confirm our
theoretical observations. Experimental measurements of absorp-
tion, fluorescence emission, and redox potentials for dimer A4
are reported. The results provide guidance for improving the
design of paraphenylenes to produce more conductive materials.
The possibilities are demonstrated in the case of B2, in which,
in addition to dialkylamino substituents, the ketone functional
group in the benzophenone unit has been replaced by a divinyl
cyano substituent. Dimers A0, A4, B0, B1, and B2 show
theoretical optical absorption maxima that span over a modest
140.3 nm.
It is well established that side-chain substituent groups readily
alter the conductivity of organic semiconductors;21 however,
little effort has been devoted to systematic structure-property
relationships uniquely characterized by side chain substituted
paraphenylenes. The positions of substituent groups along a
polymer backbone as well as the number of substituent groups
can affect conductivity. Studies have shown that adding two
acceptor or donor groups to each fluorene-phenylene monomer
Experimental Methods
Theoretical Calculations. The geometrical and electronic
properties of the substituted oligoparaphenylenes were per-
formed with the Gaussian 03 program package.25 The calcula-
tions were optimized by means of the Becke three-parameter
hybrid functional with Lee-Young-Parr26 correlation functional
(B3LYP) using the 6-31G(d) basis set unless otherwise indi-
cated. Molecular orbitals were visualized using WebMO. The
theoretical absorption spectra of the dimers were calculated on
* Towhomcorrespondenceshouldbeaddressed.E-mail:dtaylor@nccu.edu.
‡ North Carolina School of Science and Math.
† North Carolina Central University.
§ Current address: Wake Forest University, Winston-Salem, NC 27106.
10.1021/jp108619n  2010 American Chemical Society
Published on Web 11/19/2010

Effect of Substitution on Oligomeric Paraphenylenes
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13229
SCHEME 1: Synthesis of Dimer A4a
a Reagents and conditions: (i) 180 °C (neat); (ii) POCl3, N,
N-dibutylaniline, 100 °C; (iii) (Ph3P)2-NiCl2, Zn, PPh3, Bipy, THF,
60 °C.
3-Chloro-N-phenylbenzamide (1). The condensation of
3-chlorobenzoic acid (20 g, 130 mmol) with excess aniline
(14.37 g, 154.3 mmol) yields the crude product. The excess
aniline is removed by atmospheric distillation at 182 °C. The
resulting solid is washed in acidic and basic aqueous solutions,
and dried in vacuum after TLC showed complete absence of
aniline. This reaction has shown product yields of up to 98%.
The material is utilized without further purification.
Figure 1. Structures of paraphenylene side-chain substituted dimers.
(3-Chlorophenyl)(4-(dibutylamino)phenyl)methanone (2).
The chlorinated monomer was prepared by a condensation
reaction between 1 (16 mmol, 3.7 g) and N,N-dibutylaniline
(53.5 mmol, 12.1 mL) in the presence of phosphorus oxychloride
(22 mmol, 2.0 mL). An exothermic reaction was noted at around
80 °C before the reaction was cooled in an ice bath. The reaction
was allowed to continue at 100 °C for 3 h. The crude resulting
oil was stirred in 5% hydrochloric acid before it was washed
with water. The organic product was taken up in ethyl acetate,
dried over magnesium sulfate, and concentrated by rotovap to
yield ∼4 g of crude, brown, oily product. A crude product/
silica mixture was prepared by dissolving the oil in ethyl acetate,
mixing with silica (∼100 mL), and evaporating the solvent.
Meanwhile, a large column of silica (∼180 g) slurried in
cyclohexane was made up. The crude product/silica mixture was
placed on top and eluted with ethyl acetate/cyclohexane (1:4),
collecting 50 mL fractions. The first five fractions, identified
by TLC to be purified product, were combined, and the solvent
was removed in a vacuum to give a yellow oil. Yield: 2.47 g,
45%. 1H NMR (CDCl3, TMS, 300 MHz): δ 0.98 (t, 6H, J ) 6
Hz), 1.36 (sextuplet, 4H, J ) 6 Hz), 1.6 (quintuplet, 4H, J ) 6
Hz), 3.55 (t, 4H, J ) 6 Hz), 6.62 (d, 2H, J ) 9 Hz), 7.36 (t,
2H, J ) 6 Hz), 7.47 (dd, 1H, J1 ) 2 Hz, J2 ) 7 Hz), 7.58 (dd,
1H, J1 ) 2 Hz, J2 ) 7 Hz), 6.68 (d,1H, J ) 2 Hz), 7.94 (d, 2H,
J ) 9 Hz). APCI/ESI-MS: m/z 344.2 [M - H]-, tR ) 9.36
min.
the B3LYP 6-31G optimized structures using the semiempirical
method of Zerner’s intermediate neglect of differential overlap
(ZINDO).27
1
Instruments and Measurements. H and 13C NMR (300
MHz) spectra were measured on a Varian Inova spectrometer
in deuterated chloroform. Chemical shifts (δ) for proton are
reported in parts per million downfield from tetramethylsilane
and are referenced to it (TMS 0.0 ppm). Coupling constants
(J) are reported in Hertz. Chemical shifts for carbon are reported
in parts per million downfield from TMS and are referenced to
residual solvent peaks: carbon (CDCl3, 77.0 ppm). Mass spectra
were recorded on an Agilent Technologies 1200 series equipped
with a XTerra MS (C-18, 5.0 µm) 3.0 × 100 mm column.
Retention times (tR) were obtained on the same instrument using
HPLC/LC. Analytical thin-layer chromatography was performed
on precoated 250 µm layer thickness silica gel 60 F 254 plates
(EMD Chemicals Inc.). Visualization was performed with
ultraviolet light. Elemental analysis was performed on Perkin-
Elmer 2400 series II CHN analyzer.
Materials and Reagents. All materials were purchased from
either Aldrich or Fisher and used without further purification
unless otherwise noted. Tetrahydrofuran (THF) was freshly
distilled prior to use. Bipyridine (Bipy) was purified by
recrystallization from ethanol. Triphenylphosphine (TPP) was
recrystallized from cyclohexane. Both reagents were dried under
vacuum for 24 h at room temperature and stored in the glovebox
under nitrogen. The catalyst, (Ph3P)2NiCl2, and zinc (100 mesh,
99.999%) were also stored under nitrogen. Dimer A4 was
synthesized in a three-step process as shown in Scheme 1.
Biphenyl-2,3′-diylbis((4-(dibutylamino)phenyl)metha-
none) (A4). A previously flame-dried, three-neck, 100 mL,
round-bottom flask equipped with a magnetic stir bar was
charged with 0.589 g (9.01 mmol, 3.1 equiv) of zinc, 0.380 g
(0.581 mmol, 0.2 equiv) of (Ph3P)2-NiCl2 catalyst, 0.152 g
(0.581 mmol, 0.2 equiv) of triphenylphosphine (TPP), and

13230 J. Phys. Chem. A, Vol. 114, No. 50, 2010
Koepnick et al.
0.0454 g (0.291 mmol, 0.1 equiv) of bipyridine (Bipy). The
reagents were combined in the glovebox under a nitrogen
atmosphere. The flask was sealed with rubber septa, purged with
nitrogen, and charged with 6 mL of THF via syringe. The
catalyst mixture was stirred and heated in an oil bath at 50 °C
under the nitrogen purge. Once the color of the catalyst mixture
changed from deep green to dark red, 1.0 g (9.0 mmol) of
monomer dissolved in 3 mL of THF was added via syringe.
The reaction continued at 50 °C overnight. The reaction mixture
was then washed with 5% HCl solution to remove excess zinc
and stirred overnight. The product was taken up in ethyl acetate,
dried over magnesium sulfate, and concentrated by rotovap. The
crude product was separated from TPP by flash column using
cyclohexane as an elutant. Once TLC confirmed the absence
of TPP from the elutant, a brown band containing dimer and
unreacted monomer was flash-eluted using ethyl acetate. Silica
was added, and a crude product/silica mixture was obtained by
rotovaporation to remove the solvent. Meanwhile, a column of
silica (∼90 g) slurried in cyclohexane was made up. The crude
product/silica mixture was placed on top and eluted with ethyl
acetate/cyclohexane (1:4), collecting 50 mL fractions. The
column was followed by TLC, the first several fractions were
combined, and the solvent was removed in vacuo to give yellow
crystals. Yield: 45%. Anal. Calcd for (C42H52N2O2): C, 81.78;
H, 8.50; N, 4.54; O, 5.19. Found: C, 81.68; H, 8.47; N, 4.61.
1H NMR (CDCl3, TMS, 300 MHz): δ ) 0.962 (t, 12H, J ) 7
Hz), 1.36 (sextet, 8H, J ) 7 Hz), 1.605 (quintet, 8H, J ) 8
Hz), 3.34 (t, 8H, J ) 7 Hz), 6.64 (d, 4H, J ) 8 Hz), 7.528 (t,
2H, J ) 8 Hz), 7.841 (m, 8H), 7.9 (s, 2H). 13C NMR (CDCl3,
300 MHz): δ ) 14.2, 20.5, 29.5, 51.1, 110.5, 123.9, 128.2, 128.8
(d, J ) 17.4 Hz, Caryl), 129.97, 133.3, 140.5 (d, J ) 51.9 Hz,
C-C), 151.8, 194.8 (CO). UV-vis [methanol, λmax (ε)]: 245
(1.83), 369 (2.26). APCI/ESI-MS: m/z 617.4 [M - H]-, tR )
8.53 min.
Figure 2. Influence of alkyl chain length (n) on band gap of the six
dimers in series An (where n ) 0, 1, 2, 4, 6, or 8).
TABLE 1: The Band Gaps (Eg) by B3LYP/6-31G of
Various Substituted Paraphenylene Dimers
dimer
couplinga
Eg (eV)
A0
TT
HT
TT
TT
HT
TT
HT
TT
HT
TT
HT
TT
HT
TT
TT
4.13
3.92
3.82
3.77
3.74
3.76
3.82
3.77
3.86
3.78
3.79
4.52
4.59
3.54
1.17
A1
A2
A4
A6
A8
B0
B1
B2
a TT ) tail-to-tail; HT ) head-to-tail.
be at 4.7 eV vs vacuum with an additional +0.2 V potential
difference between Ag/AgCl and NHE. Before each experiment,
the cell was purged with N2 for 5-10 min, and a blanket of N2
was maintained during the experiment.
Optical Spectroscopy. Absorption spectra of dimer A4 in
solution were measured with a Hewlett-Packard model 8453
UV-vis spectrophotometer with HP Vectra Workstation. The
fluorescence spectra of dimer A4 were observed on a Perkin-
Elmer LS 55 fluorescence spectrometer at ambient temperature.
In both absorbance and emission spectra, the samples were
contained in 1-cm path-length quartz cells.
Results and Discussion
Band Gap Calculations of Series A Dimers. The theoretical
band gap (Eg) is the transition energy required to excite electrons
from the ground state to the first dipole-allowed excited state.
A crude and widely accepted28-30 estimate of this quantity is
the energy difference between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO). The energy of the HOMO or LUMO level can be
altered by substituents. Figure 2 shows that in going from an
amino side group (A0) to a (diethylamino) side group (A2),
the band gap is lowered by 0.36 eV. This behavior is a result
of the electron-releasing effect created by the introduction of
the alkyl side chains. Further increase in the number of carbon
atoms (n) has no effect on the resulting Eg.
It should be noted that this study has focused on the tail-to-
tail (TT) coupling of dimers shown in Figure 1. However,
geometry optimization calculations were also performed for
select head-to-tail (HT) dimer molecules (structures not shown),
and the computed band gaps were very similar to those reported
for the TT verison (see Table 1). This suggests that HT and TT
coupling have very little effect on the resulting bandgap, and
others have reported similar findings for poly(benzo-2,1,3-
thiadiazol-4,7-diyl)-(dihexyl[2,2′]dithiophene-5,5′-diyl).31 Ap-
parently, the critical number of carbon atoms in the An dimer
series is n ) 4. Butyl and higher alkyl chain side groups induce
Electrochemistry. Cyclic voltammograms were recorded
using a BAS potentiostat (CV-50W with C3 cell stand)
interfaced to a personal computer and software supplied by the
manufacturer. Acetonitrile (anhydrous, 99.8%, e0.001% water
content) was obtained from Aldrich and used as received.
Tetrabutylammonium hexafluorophosphate (Bu4NPF6) and fer-
rocene were recrystallized several times from freshly distilled
THF using Schlenck line techniques. All experiments were
conducted in 0.1 M Bu4NPF6 in CH3CN under ambient
laboratory conditions. A working volume of 5 mL was
transferred to the cell by syringe immediately prior to the start
of the experiment.
A normal one-compartment cell was utilized with a Pt disk
working electrode (MF-2013, BAS), a Pt wire auxiliary electrode
(MW-1032), and a Ag/AgCl reference electrode (MF-2052,
BAS). The silver reference electrode was calibrated against the
redox potential of ferrocene/ferrocenium ion (evaluated as the
average of Epa ) 0.609 V and Epc ) 0.514 V) as the internal
standard in an identical cell without sample. No electroactive
species were observed in the potential range of interest (0.0-1.5
V vs Ag/AgCl) in the absence of ferrocene. All potentials are
reported vs Ag/AgCl. Electrochemical potentials were converted
to vacuum assuming the normal hydrogen electrode (NHE) to

Effect of Substitution on Oligomeric Paraphenylenes
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13231
Figure 3. Influence of alkyl chain length (n) on HOMO and LUMO
in series An (where n ) 0, 1, 2, 4, 6, or 8).
Figure 5. Band gaps for infinite chain unsubstituted paraphenylene
(PPP) and amino, diethylamino, and dibutyl amino substituted versions
of parabenzophenone were determined by plotting band gaps in
respective oligomers against the inverse of the number (n) of monomer
units and extrapolating the number of units to infinity.
SCHEME 2: Dihedral Angle, ꢀ, between Monomer Units
trend was also observed in the octamer analogs, where the six
dihedral angles of A04 spanned fewer degrees than the six
dihedral angles of A24 (i.e., ∆ꢀA0 < ∆ꢀA2). We conclude that
the more dihedral angles present in the chain, the more likely
the range of dihedral angles will increase. It would be beneficial
to understand how ꢀn is effected by interchain packing in A44.
Others have observed that increasing the oligomer length in
unsubstitued paraphenylenes (PPP) resulted in an increased
interchain packing and thus a decreased torsion angle as one
progressed from diphenyl and terphenyl (45-50°)17to hexaphe-
nyl (30-40°).33
The band gap of monomer, dimer (A4), and tetramer
oligomers of dibutyl amino benzophenone were extrapolated
to determine theoretical estimates for the band gap of the
corresponding polymer. Figure 5 shows that the electrical
properties of poly(dibutyl amino benzophonone) (Eg ) 3.4 eV)
are similar to that of unsubstituted poly(paraphenylene) (Eg )
3.5 taken from the literature)34 and higher than poly(diethyl
amino benzophenone) (Eg ) 2.5 eV) or poly(aminobenzophe-
none) (Eg ) 2.4 eV). These values have been calculated by
plotting band gaps of energy-minimized oligomers against the
inverse of the number (n) of monomer units and extrapolating
the number of units to infinity. For the oligomers reported here,
we conclude that the steric interaction of the side groups on
the phenyl rings dominate the distorted backbone geometry more
than the stabilizing effects gained by the electrons being
delocalized over a planar backbone. The net effect is that the
π-overlap between monomer units decreases, leading to a
decreased bandwidth and thus an increased band gap.
a competing steric hindrance effect that negates any further
increase in the energy of the HOMO (see Figure 3).32 The side
chain dialkylamino substituents have no effect on the energetic
position of the LUMO. The combined effect is an unaltered Eg
for A4, A6, and A8.
The band gap is also expected to be affected by the torsion
between adjacent monomer units. This is denoted as ꢀn in
Scheme 2. An ideal spatial alignment is obtained when ꢀn ) 0,
but this is rarely observed, since the side chain groups introduce
repulsive or attractive forces that twist the phenyl-phenyl ring
alignment away from planarity. For the An dimers under
investigation, the average dihederal angle is 38°. It is surprising
that the addition of carbon atoms in the side chain dialkyl amino
group (i.e., the bulking up of the side chain appendage with n
) 0-8) has very little effect on the respective ꢀn. There is,
however, a small but noticeable peak in the dihedral angle when
going from dimer A2 to A4 (see Figure 4). To evaluate the
significance of this increase in ꢀ, select versions of the dimers
in the An series were extended to longer oligomer lengths;
energy minimization calculations were performed on A0x, A2x,
and A4x at the B3LYP/3-21G level of theory where x ) 2 and
4 (i.e., tetramer and ocatomer oligomer analogues). The calcula-
tions showed that the range of dihedral angles within a given
tetramer increased as the alkyl chain length increased in going
from A02 (52-55°) to A22 (54-58°) to A42 (37-56°). This
Although not a large difference, the 3° increase in optimal
dihedral angle configuration for the A4 dimer over all other
dimers in the A series led to a closer examination of this
molecule. The dihedral angle of A4 was fixed at various values;
the optimized geometry was calculated, followed by a calcula-
tion of the MO energies to calculate the band gap as a function
of torsional angle. Figure 6 shows that the minimal band gap
for the essentially planar A4 molecule is 3.74 eV and increases
to a maximum value of 3.92 eV as the dihedral angles increase
to 180°. The optimal dihedral angle for A4 is 40°, resulting in
a band gap of 3.76 eV. This is very similar to the band gap
reported by Zade et al. for the unsubstituted poly(paraphenylene)
(3.78 eV).35 This again leads to the conclusion that A4 exhibits
little improvement over PPP. It can be concluded that the
Figure 4. Torsional angle between monomer units of dimers in series
A as a function of alkyl side chain group.

13232 J. Phys. Chem. A, Vol. 114, No. 50, 2010
Koepnick et al.
Figure 8. Cyclic voltammogram of 0.8 mM A4 in 0.1 M Bu4NPF6/
CH3CN.
Figure 6. Band gap energy calculated for A4 as a function of fixed
dihedral angle.
TABLE 2: Summary of Absorption and Emission Data for
Dimer A4
λmax abs (nm) λmax emis (nm) Eogpt (eV)a
E
(eV)b Etgh (eV)c
El
HOMO
368 460 3.37
5.20
3.74
a Optical band gap derived from the absorption (λmax ) 368 nm)
of the solution spectrum. b Electrochemically derived energy
position of the HOMO band. c Theoretical calculation of band gap.
The data compare very well with the values of optical and
theoretical band gap discussed previously. A summary of the
electrooptical properties for dimer A4 is presented in Table 2.
Enhancing the Paraphenylene Platform. From the com-
bined theoretical and experimental studies, we set out to explore
substituent groups that more effectively lowered the band gap
of oligoparaphenylenes. Table 1 shows that 2,2′-(biphenyl-3,3′-
diylbis((4-(dibutylamino)phenyl)methan-1-yl-1-ylidene))dima-
lononitrile (B2) has the lowest calculated band gap of the dimers
under investigation. Although the electron-donating substituent
group is unchanged between A4 and B2, the calculated HOMO
level is raised by 0.62 eV in the latter. The calculated LUMO
level is lowered by 1.97 eV by changing the ketone in A4 to a
divinyl cyano group. The combined effect is a significantly lower
calculated band gap for B2 that deviates from expectations based
on a simple donor-acceptor effect. With no change in the donor
part of the dimer, altering the acceptor group in B2 should
mainly affect the LUMO. The reduction of the band gap in dimer
B2 consisting of divinyl cyano groups must be attributed to the
combined effect of a better electron acceptor [CO versus
CdC-(C′N)2] as well as a further extension of the π-conjuga-
tion. Furthermore, calculated optical absorption of B2 provides
a significant red shift from A4. The dimers A0, A4, B0, B1,
and B2 together span a modest 140.3 nm window, from 238.9
to 379.2 nm. The successful tuning of the theoretical absorption
maxima (see Figure 9) is promising and suggests that substit-
uents can be designed to enhance the electrooptical properties
of paraphenylenes.
Conclusions. We have prepared a substituted polyparaphe-
nylene dimer that looked promising from initial design and
theoretical calculations. On closer investigation, this dimer and
the others within this series that we chose to investigate showed
only modest improvements over the unsubstituted poly(par-
aphenylenes). The alkyl chain length on the side chain sub-
stituent group was found to have very little effect on the
calculated HOMO-LUMO band gaps or torsional angles
between monomer units of the dimers. We were, however,
successful at showing theoretically that a divinyl cyano version
of our dimers has a lower band gap and a red shift in the optical
absorption. This is a successful outcome for this class of
materials that will be explored experimentally in future studies.
Figure 7. Optical absorption spectrum (s) and fluorescence emission
spectrum (- ·) of oligomer A4 dissolved in methanol.
electron-releasing effect on the energy of the HOMO of A4
has a smaller influence on band gap than steric hindrance of
the side chain group.
Experimental Calculations of Band Gap for A4. We report
the first experimental determination of the optical bandgap in
semiconducting A4 dimers. The target dimer was synthesized
as presented in Scheme 1. The absorption and emission spectra
are overlaid in Figure 7. The UV-vis absorption spectrum of
A4 has no structural features and shows a maximum absorption
at 369 nm. This value was used to determine the optical
HOMO-LUMO gap (Eogpt ) 3.37 eV).
Unlike the absorption spectrum, the fluorescence emission
spectrum is structured. A broad peak centered at 460 nm is
observed to have a weak shoulder at long wavelength. The single
site substitution on the phenyl backbone is the most likely reason
for the assymmetric emission peak structure. Beljonne et al.36
have observed similar effects in their substituted oligothiophenes
and concluded that the structured emission spectra were due to
a more planar conformation in the excited state dimer.
The energy level of the electronic states of the HOMO and
LUMO can be studied using electrochemistry. The electro-
chemical properties of dimer A4 were investigated by cyclic
voltammetry, and the results are shown in Figure 8. More than
one peak is observed in the voltammogram. This is quite
common, and others have attributed this to the formation of
polarons followed by bipolarons.37 The potential for the oxida-
tion peak was extracted from the first sweep and found to be
0.48 V versus Ag/AgCl. Electrochemical potentials were
converted to vacuum assuming the normal hydrogen electrode
(NHE) to be at 4.7 eV vs vacuum with an additional +0.2 V
potential difference between Ag/AgCl and NHE. Thus, the
energetic position of the HOMO level is 5.2 V. We were unable
to obtain any reduction processes for dimer A4 and, therefore,
cannot report on the electrochemically determined LUMO level.

Effect of Substitution on Oligomeric Paraphenylenes
J. Phys. Chem. A, Vol. 114, No. 50, 2010 13233
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Figure 9. Calculated absorption spectra of dimers A0, A4, B0, B1,
and B2. The geometry optimizations were carried out with DFT
(B3LYP 6-31G) methods, and the theoretical spectra of the dimers under
study were obtained with ZINDO methods.
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Acknowledgment. This work was supported by the Petro-
leum Research Fund of the American Chemical Society (No.
45702-GB10). The authors thank the Cluster Computing Group
at Earlham College and the Shodor Education Foundation for
computational server time.
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