Paraphenylene dimers with diphenylamine donor groups: Synthesis and photophysics

Article abstract of DOI:10.1016/j.tetlet.2013.03.141

A novel paraphenylene dimer (D696) with electron donating diphenylamine side chain groups has been prepared. Optical absorption measurements were theoretically and experimentally determined showing a red shift in the absortivity relative to dialkylamine f

Full text of DOI:10.1016/j.tetlet.2013.03.141

Tetrahedron Letters 54 (2013) 3097–3100
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Paraphenylene dimers with diphenylamine donor groups: synthesis
and photophysics
Khoa Le a, Lokendra B. Chand a, Colette Griffin a, Alfred L. Williams b, Darlene K. Taylor a,b,
⇑
a Department of Chemistry, North Carolina Central University, Durham, NC 27707, USA
b Department of Pharmaceutical Sciences, North Carolina Central University, Durham, NC 27707, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel paraphenylene dimer (D696) with electron donating diphenylamine side chain groups has been
prepared. Optical absorption measurements were theoretically and experimentally determined showing
a red shift in the absortivity relative to dialkylamine functionalized benzophenone dimers. The fluores-
cence of D696 was quenched in a concentration dependent manner in the presence of reduced graphene
oxide (RGO).
Received 15 March 2013
Accepted 28 March 2013
Available online 12 April 2013
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
Paraphenylene
Graphene oxide
Molecular orbitals
Emission
Absorption
The large absorption coefficients, high charge mobility, excel-
lent thermal and mechanical stability, and affordable large scale
production costs of semiconducting polymers position these mate-
rials to outperform their silicon-based counterparts for application
in light emitting diodes, photovoltaic devices, and nonlinear op-
tics.1–5 However, applications have not been fully realized due to
charge transport inefficiencies in these materials.
Polyparaphenylenes and oligomeric paraphenylenes (OPPs)
have lagged behind other semiconductor materials because of
intrinsic limitations in flexibility, symmetry breaking, or distortion
properties, and hole injection difficulties as a result of high HOMO
levels. These limitations can be addressed by the selective intro-
duction of sterically demanding and electronically rich or favorable
substitution groups.
our promising theoretical results with OPP functionalized with
diphenylamine donor groups was previously unobtainable as the
dimers were not synthetically available. We describe here the syn-
thesis of the 4-(N,N-diphenylamino)benzoyl substituted paraphen-
ylene dimer (D696). The structure of D696 is illustrated and
compared with the dibutylamino substituted dimer A4 reported
earlier (Fig. 1). Important issues that we address in this work in-
clude the role of the substitution chemistry on the absorption
and emission properties of the dimer. In addition, we address the
role of coupling the dimers in a head-to-head (HH) conformation
where the dimer contains a mirror plane of symmetry upon the
union of the monomers (i.e., the two carbonyl groups face each
We and others continue to show an interest in model paraphen-
ylene oligomers derivatized with donor and acceptor groups for
the study of photoinduced charge transfer.6–11 Our previous exper-
imental and theoretical substitution strategies involved the forma-
tion of butyl amine side chain groups.6 In those studies, we
demonstrated the potential to red shift the theoretical band gap
of paraphenylene based dimers by the substitution of electron
donating groups including various alkyl amine and diphenyl amine
units. While earlier studies report that the spectral absorption
and emission of oligomeric semiconductors are influenced by
structural modification arising from the introduction of different
electron donating and withdrawing groups,12–14 confirmation of
N
O
O
N
D696
O
O
N
N
A4
⇑
Corresponding author. Tel.: +1 919 530 6463; fax: +1 919 530 5135.
E-mail address: dtaylor@nccu.edu (D.K. Taylor).
Figure 1. Structures of paraphenylene dimers derivatized with the donor group
diphenylamine (D696) and dibutylamine (A4).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.141

3098
K. Le et al. / Tetrahedron Letters 54 (2013) 3097–3100
other in a planar view of the dimer). Furthermore, we observe effi-
cient quenching of the fluorescence of D696 by reduced graphene
oxide, a photo-induced electron transfer process reported in the
literature.15
As shown in Scheme 1, the synthesis of D696 was accomplished
in six-steps (see Supplementary data). Briefly, diphenylamine was
coupled with ethyl 4-bromobenzoate by using the Buchwald ami-
nation chemistry.16 The resulting yellow viscous oil underwent
alkaline hydrolysis17 to provide the corresponding acid as a white
solid in 72% yield for two-steps. Subsequent reaction with phenol
in the presence of DCC and DMAP produced the ester 3 in good
yield. Initial attempts to convert the ester 3 into the ketone 4 using
a Fries rearrangement reaction with Lewis acids such as AlCl3,
scandium triflate, and BF3ÁOEt2 in different solvents resulted in
poor yields.18 The rearrangement was however successfully carried
out, in a yield of 76%, when the reaction was performed in neat tri-
flic acid at 0 °C followed by warming to room temperature. Finally,
the hydroxyl group of 4 was activated by triflic anhydride to pro-
duce triflate 5 which was then dimerized in the presence of a pal-
ladium catalyst to afford D696 as a yellow solid in 55% yield.
The redox behavior of D696 was investigated by cyclic voltam-
metry (CV). The studies were performed using a solution of D696
(1.64 Â 10À4 M) prepared in DMF with n-Bu4NPF6 (0.1 M) as a sup-
porting electrolyte. As shown in Figure 2, the dimer exhibits a
reversible anodic wave at ꢀ0.75 V versus AgNO3/Ag which is
attributed to the electron donating tendency of the diphenylamine
group. The energy associated with this oxidation is taken to be the
HOMO energy level. D696 exhibits no observable cathodic wave.
Thus, the LUMO energy level of D696 was calculated by subtracting
Figure 2. CV curve of D696 in acetonitrile.
extrapolated from the intersection between the absorption and
emission spectra (see Fig. S1 in Supplementary data).
The absorption spectra of D696 taken in DMF solution at a con-
centration of 5.49 Â 10À5 M is shown in Figure 3. This is very close
to our theoretically determined UV–vis spectra previously reported
for diphenylamine functioned OPP.6 The spectrum features a rela-
tively broad absorption in the ultraviolet and slight trailing in the
visible region with maxima of 290 nm and 368 nm. These two in-
tense absorption maxima suggest p–
p⁄ transition and intramolec-
ular charge transfer between the diphenylamine donor group and
the HOMO energy level and the zeroth–zeroth energy (E0–0
)
the carbonyl group which acts as a modest acceptor.
The absorption, emission, and electrochemical properties of
D696 are listed in Table 1. From our experimental data, we calcu-
lated the band gap of D696 to be 2.90 V. This band gap is lower
than unsubstituted paraphenylene and polypyrrole (3.1 eV) but
higher than other polymers such as polythiophene and poly(2-
vinylpyridine).
The geometry of the ground state molecular orbitals of D696
was fully optimized by the density functional theory (DFT) with
Becke’s three-parameter (B3) exchange functional along with the
Lee–Yang–Parr (LYP) nonlocal correlation functional implementing
the modest 6-31G(d,p) basis set. The calculations were carried out
in WebMO environment interfaced with GAUSSIAN 94/98/03 software
package. The plots of the HOMO and LUMO of the dimer are given
in the Supplementary data (Fig. S2) and the respective electron
densities images are presented in Figure 4. The absorption spectra
1. Pd(OAc)2, BINAP, Cs2CO3
O
H
N
DMF, 12 h, 116 oC
OEt
+
2. NaOH, MeOH/THF
12 h, rt
3. H3O+, rt, 72 %
(2 steps)
Br
O
O
O
O
OH
TfOH, 0 oC-rt
19 h, 76%
N
N
Phenol, DCC
DMAP, DCM
0
oC, 12 h, 82%
3
2
OH
O
OTf
Tf2O, TEA, DCM
0 oC, 12 h, 71%
N
N
4
5
N
PdCl2, dppf, B2pin2, K2CO3
DMSO, 80 oC, 55%
O
O
N
D696
Scheme 1. Synthesis of D696.
Figure 3. Absorption spectra of D696 in DMF.

K. Le et al. / Tetrahedron Letters 54 (2013) 3097–3100
3099
Table 1
on the head to tail union of the diphenylamine OPP monomer. This
suggests that the gas phase computational models really should be
altered to include solvent effects that would average the rotational
energies sampled by the phenyl rings of the donor.
The increase in the absorption wavelength of D696 relative to
A4 is due to the extension of p-electron conjugation in the mole-
Absorption, emission, and electrochemical properties of D696
Absorptiona
Kmax (nm)
Emissiona
Kem (nm)
Oxidation potential datab
c
E0–0 (V)
Eox (V)
Eox À E0–0 (V)
368
290
534
–
À2.15
0.75
–
2.90
–
–
cule with the addition of the two phenyl groups. It is important
to note that the HOMO and LUMO molecular orbitals show local-
ized electron density on the diphenylamine and carbonyl units,
respectively. It is noteworthy that the LUMO electron density of
D696 is located primarily on the carbonyl group, suggesting the ex-
cited electron can be quenched by an efficient electron trap.
To probe the interaction between the excited-state of D696 and the
reduced graphene oxide (RGO) in solution, fluorescence spectra of
D696 at different RGO concentrations were recorded. The intensity of
the D696 fluorescence bands decreases with the increase in RGO con-
centration as illustrated in Figure 5. This confirms a strong interaction
between the D696 and RGO and is presumably due to an excited state
phenomenon since we do not observe ground state charge-transfer in
the absorption spectra of the physically mixed D696/RGO solution. In
fact, the two absorbance bands of D696 increase in intensities with
the graphene oxide concentration (see Fig. S3 in Supplementary data)
and this is expected due to the increasing intensity of the graphene
oxide absorption band around 230 nm. We also do not see new
absorption bands attributable to charge-transfer.
a
Absorption and emission spectra were measured in DMF solution at room
temperature.
b
The oxidation potential was measured in acetonitrile with 0.1 M tetrabutyl-
ammonium hexfluorophosphate (Bu4NPF6) as the supporting electrolyte (working
electrode: glassy carbon; counter electrode: Pt wire; reference electrode: Ag/Ag+).
c
E0–0 was estimated from the onset point of absorption spectra.
Quenching of fluorescence of aromatic molecules by RGO has
been observed in porphyrin,19 porphyrin-fullerene/nanohorn20,21
,
and pyrene carbon nanotube systems.22 Both electron transfer
and energy transfer have been suggested to explain the fluores-
cence quenching.23 Additional studies are required to determine
the mechanism of fluorescence quenching observed in our system.
We can conclude that RGO acts as an efficient acceptor for the pho-
toinduced charge separated species produced in D696. This feature
could be exploited in the design of photovoltaic cells.
In summary, we have synthesized a novel paraphenylene dimer,
D696 and shown its absorption, emission, and electrochemical
properties. We found the band gap of D696 to be 2.90 V, which is
relativity high for the application of solar cells directly but by add-
Figure 4. Molecular orbitals of D696: (A) LUMO; and (B) HOMO.
ing a strong p-acceptor the band gap could be lowered.
Acknowledgments
C.G. acknowledges support from the UNC EFRC: Center for Solar
Fuels, an Energy Frontier Research Center, funded by the US
Department of Energy, Office of Basic Energy Sciences under Award
DE-SC0001011. K.L. and L.C. acknowledge support from the NSF
Centers of Research Excellence in Science and Technology (CREST)
Supplemental Award, HRD-0833184.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.03.
141.
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