Amine-Amine Electronic Coupling through a Dibenzo[a,e]pentalene Bridge

Article abstract of DOI:10.1021/acs.orglett.5b03408

Three dibenzo[a,e]pentalene derivatives containing two redox-active amine substituents have been prepared. The degree of amine-amine electronic coupling through the dibenzo[a,e]pentalene bridge greatly depends on the substitution positions. Three monoamin

Full text of DOI:10.1021/acs.orglett.5b03408

Letter
pubs.acs.org/OrgLett
Amine−Amine Electronic Coupling through a Dibenzo[a,e]pentalene
Bridge
Jun-Jian Shen, Jiang-Yang Shao, Xiaozhang Zhu,* and Yu-Wu Zhong*
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, 2 Bei Yi Jie, Zhong Guan
Cun, Beijing 100190, China
S
* Supporting Information
ABSTRACT: Three dibenzo[a,e]pentalene derivatives contain-
ing two redox-active amine substituents have been prepared. The
degree of amine−amine electronic coupling through the
dibenzo[a,e]pentalene bridge greatly depends on the substitu-
tion positions. Three monoamine compounds have been
prepared for comparison studies. The experimental data and
analysis were corroborated by time-dependent density functional
theory results of mixed-valent compounds.
ibenzo[a,e]pentalene (DBP) is a ladder-type ring-fused
Dhydrocarbon with unique electronic and antiaromatic
properties.1 Thanks to recent successes in the development of
synthetic methodology,2 DBP derivatives have now become
readily available from ordinary starting materials. This has
greatly stimulated the studies of DBP derivatives in various
fields of organic electronics and optoelectronics, including
organic field-effect transistors,3 organic photovoltaics,4 and
photoluminescence.5 We report herein the use of DBP as a
conjugated bridge to mediate the electronic coupling between
two mixed-valent redox sites. Interestingly, the placement of
redox sites on different positions of the DBP core gives rise to
significantly different degrees of coupling.
Electronic communications between mixed-valent redox-
active components have received continuous attention since
the pioneering work of Creutz and Taube.6 Apart from
inorganic or organometallic complexes,7 purely organic redox-
active components have been used as redox termini in mixed-
valent compounds.8,9 Three C2-symmetric compounds with
two amine substituents on the 2,7- (1), 3,8- (2), and 5,10- (3)
positions of the DBP core have been prepared for the purpose
of electronic coupling studies (Figure 1). In addition, three
DBP derivatives 4−6 with one amine substituent have been
Figure 1. Diamine compounds 1−3 with a dibenzo[a,e]pentalene
bridge and monoamine model compounds 4−6.
prepared for comparison. Triarylamines are useful redox sites
for constructing organic mixed-valent systems due to the well-
defined N•+/0 process in a relatively low potential region and a
high extinction coefficient of intervalence charge-transfer
(IVCT) transitions.9 The attachment of long alkyl chains
(C8H17) is beneficial for improving the solubility of these
compounds. These compounds were prepared via a Pd(dba)2-
catalyzed (dba = dibenzylideneacetone) C−N coupling of di(p-
octylphenyl)amine with 2,7-dichloro-5,10-diphenylDBP, 3,8-
dichloro-5,10-diphenylDBP, or 5,10-di(p-chlorophenyl)DBP,
which were in turn synthesized via a Pd(OAc)2/n-Bu4NOAc-
catalyzed homoannulation10 of a chloro-substituted o-alkyny-
laryl iodide substrate. The details of synthesis and character-
ization are provided in the Supporting Information (SI).
Compounds 1 and 2 show two well-defined N•+/0 redox
couples in the potential window between +0.4 and +1.2 V vs
Ag/AgCl, as displayed by the cyclic voltammograms (CVs) in
Figure 2a. The potential splitting (ΔE) between two waves is
Received: November 28, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03408
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 2. (a) CVs of 1 and 2. (b) CV and DPV of 3. (c) CVs of 4−6.
Measurement conditions: glassy carbon working electrode; Pt wire
counter electrode; Ag/AgCl in aq NaCl as reference electrode; 0.1 M
Bu4NClO4/CH2Cl2. Potentials are summarized in Table 1.
240 and 310 mV for 1 and 2, respectively. Both compounds
have a large comproportionation constant Kc for the
equilibrium [N0−N0] + [N•+−N•+] → 2[N0−N•+] (Table 1),
Figure 3. Absorption spectral changes of 1 (a), 2 (b, c), and 3 (d, e) in
0.1 M Bu4NClO4/CH2Cl2 by stepwise electrolysis at an ITO glass
electrode. The applied potentials shown in the insets are referenced vs
Ag/AgCl.
a
Table 1. Electrochemical Data
b
c
d
compd
E1 (V)
E2 (V)
Ecath (V)
ΔE (mV)
Kc
1
2
3
4
5
6
+0.72
+0.66
+0.88
+0.80
+0.79
+0.90
+0.96
+0.97
+0.96
−1.19
−1.19
−1.28
−1.17
−1.18
−1.23
240
310
80
1.2 × 104
1.8 × 105
23
oxidation step (double oxidation; potential was further
increased to +1.20 V), these new bands continued to increase.
The sharp absorption band around at 750 nm of 12+ is
characteristic of the N•+-localized transition.9,11 The main
absorption bands at 1100 nm of 1•+ and 12+ are attributable to
the bridge to N•+ charge-transfer (BNCT) transitions, as
supported by the time-dependent density functional theory
(TDDFT) results below. It is surprising that no lower energy
absorptions that could be assigned to potential IVCT
transitions were observed for 1•+.
In stark contrast, an intense and broad absorption band at
2200 nm (εmax = 8900 M−1 cm−1) appeared in the single
oxidation of 2, which decreased in the double-oxidation step
(Figure 3b,c). This band is attributed to the IVCT transitions
of 2•+, and this assignment is further supported by the TDDFT
results discussed below. For compound 3 with a smaller
potential splitting, shallow and broad IVCT absorptions were
observed in the single oxidation step (Figure 3d, εmax < 3000
M−1 cm−1).
When the three monoamine compounds 4−6 were subjected
to similar oxidative electrolysis, only the appearance of the
BNCT transitions (between 800 and 1500 nm) and the N•+-
localized transition (around 700 nm) were observed (Figure S1,
SI). No other lower energy absorptions appeared. This is in
agreement with the assignment of the near-infrared absorptions
of 1•+−3•+; namely, IVCT transitions are only observed for 2•+
and 3•+.
a
Potentials are reported vs Ag/AgCl. Potentials vs ferrocene0/+ can be
estimated by subtracting 0.45 V. Potentials of the cathodic scan. ΔE
b
c
d
= E2 − E1. Kc = 10(ΔE/59)
.
indicating that the one-electron-oxidized state has good
thermodynamic stability. In comparison, the potential splitting
between two N•+/0 processes of 3 is much smaller (around 80
mV, Figure 2b), and the splitting is more discernible from the
differential pulse voltammogram (DPV). The monoamine
compounds 4−6 show one N•+/0 redox wave at similar
potential region (Figure 2c). In addition, one cathodic redox
wave around −1.2 V attributed to the reduction of the DBP
core3 is observed for all of the six compounds prepared.
In order to examine the electronic coupling of the above
compounds, they are subjected to oxidative electrolysis at a
transparent indium−tin oxide (ITO) glass electrode. Figure 3
shows the absorption spectral changes of three diamine
compounds recorded during the oxidative spectroelectrochem-
ical measurements. In the first one-electron oxidation step of 1
(single oxidation; potential was gradually increased from +0.70
to +0.90 V vs Ag/AgCl), some new absorption bands between
700 and 1200 nm appeared. In the second one-electron
B
DOI: 10.1021/acs.orglett.5b03408
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 4. TDDFT results Me-1+ (a), Me-2+ (b), and Me-3+ (c).
The IVCT band of 2•+ is well separated from the higher
TDDFT calculations have been performed on the DFT-
optimized structures of the open-shell compounds Me-1+, Me-
2+, and Me-3+ by replacing the octyl chains of 1+−3+ with
methyl groups (see details in Table S1, Figure 4, and Figures
S4−S6). The predicted D1 excitation of Me-1+ have potential
IVCT character, which is dominated by the spin transition from
the β-highest occupied spin orbital (HOSO) to the β-lowest
unoccupied spin orbital (LUSO). However, this excitation has
very weak oscillator strength (f = 0.01) and very low energy (λ
= 7442 nm), which means that the IVCT transition of this
compound is essentially forbidden. The predicted D3 excitation
of Me-1+ (f = 0.3783, λ = 1060 nm) is responsible for the
observed absorptions at 1100 nm of 1+. This excitation has the
dominant spin transition from β-HOSO-1 to β-LUSO,
attributable to the BNCT transitions. In comparison, IVCT
transitions with large oscillator strengths have been reproduced
for Me-2+ and Me-3+ at 2144 and 2629 nm, respectively. Both
transitions are dominated by the β-HOSO → β-LUSO
excitations, where the p orbitals of the amine atom are in
out-of-phase and in-phase combination, respectively, and both
orbitals have large contributions from the bridge segment. This
kind of excitations and orbital compositions are observed in
most TDDFT results of mixed-valent bistriarylamines.9d,h,15 In
the higher energy region, BNCT, amine-to-bridge charge
transfer, and bridge-localized transitions are predicted (Table
S1). It should be kept in mind that DFT calculations often tend
to overestimate delocalization and give symmetrical delocalized
structures. The TDDFT results based on these structures
should be taken with care since 3•+ is clearly a class II
compound from the experimental side.
It is strange that the no IVCT band has been observed and
predicted for 1•+ or Me-1+. The β-HOSO and β-LUSO orbitals
of Me-1+ are localized on the triarylamine segment; however,
those of Me-2+ and Me-3+ are delocalized. A similar situation is
present for the spin density distribution (Figure S7, again
keeping in mind the apparent delocalization of DFT
calculations). The DFT-calculated energy splitting between
the highest occupied molecular orbital (HOMO) and
HOMO−1 of neutral Me-1, Me-2, and Me-3 is 0.022, 0.31,
and 0.22 eV, respectively (Figure S8). This trend does
reproduce that of the experimentally determined degree of
electronic coupling of 1•+−3•+. The attachment of two amine
energy absorptions (Figure S2). The full bandwidth at half-
height (Δν1/2
theoretical value for a class II system, as determined by
Δν1/2,theo
= (2310Eop)1/2 = 3200 cm−1,12 where Eop is the
̃
, 1700 cm−1) is much narrower with respect to the
̃
energy of the IVCT band (4500 cm−1). In addition, the IVCT
band of 2•+ is distinctly asymmetric with some shallow shoulder
bands on the high energy side (probably from vibronic
structures). This indicates that 2•+ is a strongly coupled system
and likely belongs to a delocalized class III system. The
electronic coupling Vab can either be estimated by 2Vab = Eop
(for a class III system, Vab,III = 2250 cm−1) or Vab = (μgeEop)/
eRab (see details in the SI for a strongly coupled class II system,
Vab,II = 480 cm−1), where μge is the transition dipole moment
(6.0 D), e is the elementary charge, and Rab is the diabatic
electron transfer (ET) distance and taken to be the geometrical
N−N distance (11.6 Å). The large deviation between the Vab,III
and Vab,II values suggests a large error in estimating the ET
distance.13
The IVCT band of 3•+ is very broad, and the center of the
band is located around 2700−2800 nm, where big noises from
the solvent are present. It is difficult to distinguish the whole
shape of the IVCT band. The near-infrared absorptions of 3•+
were thus fitted to multiple symmetric curves using Gaussian
functions (Figure S3). The deconvoluted IVCT band has a
reasonable Δν1/2
Vab value is estimated to be 250 cm−1 by analyzing the higher
energy side of the deconvoluted IVCT band using Vab
0.0206(εmaxEopΔν1/2
1/2/(Rab),12 where Rab is again taken to be
̃
value of 3200 cm−1 (see details in the SI). The
=
̃
)
geometrical N−N distance (14.6 Å). Note that for a symmetric
IVCT band, this equation gives exactly the same value as Vab =
(μgeEop)/eRab. Due to the error in deconvoluting the IVCT
band and estimating the ET distance, the calculated Vab value
should be taken with care.
The Vab values of 1•+ (Vab,II) and 3•+ are smaller than that of
4,4′-bis(N,N-di-p-anisylamino)tolane with a conjugated bridge
but comparable N−N distance (Rab = 12.48 Å; Vab = 3.1 kcal/
mol =1080 cm−1).14 However, this comparison should be taken
with great care because the estimated Vab values are greatly
dependent on the measurement conditions and calculation
methods.
C
DOI: 10.1021/acs.orglett.5b03408
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Chem. Commun. 2015, 51, 693. (c) Dai, G.; Chang, J.; Zhang, W.; Bai,
S.; Huang, K.-W.; Xu, J.; Chi, C. Chem. Commun. 2015, 51, 503.
(d) Liu, C.; Xu, S.; Zhu, W.; Zhu, X.; Hu, W.; Li, Z.; Wang, Z. Chem. -
Eur. J. 2015, 21, 17016.
(4) Kawase, T.; Fujiwara, T.; Kitamura, C.; Konishi, A.; Hirao, Y.;
Matsumoto, K.; Hurata, H.; Kubo, T.; Shinamura, S.; Mori, H.;
Miyazaki, E.; Takimiya, K. Angew. Chem., Int. Ed. 2010, 49, 7728.
(5) Xu, F.; Peng, L.; Shinohara, K.; Nishida, T.; Wakamatsu, K.;
Uejima, M.; Sato, T.; Tanaka, K.; Machida, N.; Akashi, H.; Orita, A.;
Otera, J. Org. Lett. 2015, 17, 3014.
(6) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1969, 91, 3988.
(7) (a) Aguirre-Etcheverry, P.; O’Hare, D. Chem. Rev. 2010, 110,
4839. (b) Chisholm, M. H.; Lear, B. J. Chem. Soc. Rev. 2011, 40, 5254.
(c) Hildebrandt, A.; Lang, H. Organometallics 2013, 32, 5640.
(d) Kubiak, C. P. Inorg. Chem. 2013, 52, 5663. (e) Mandal, A.;
Agarwala, H.; Ray, R.; Plebst, S.; Mobin, S. M.; Priego, J. L.; Jimenez-
Aparicio, R.; Kaim, W.; Lahiri, G. K. Inorg. Chem. 2014, 53, 6082.
(f) Pieslinger, G. E.; Albores, P.; Slep, L. D.; Baraldo, L. M. Angew.
Chem., Int. Ed. 2014, 53, 1293. (g) Canzi, G.; Goeltz, J. C.; Henderson,
J. S.; Park, R. E.; Maruggi, C.; Kubiak, C. P. J. Am. Chem. Soc. 2014,
136, 1710. (h) Cao, Z.; Xi, B.; Jodoin, D. S.; Zhang, L.; Cummings, S.
P.; Gao, Y.; Tyler, S. F.; Fanwick, P. E.; Crutchley, R. J.; Ren, T. J. Am.
Chem. Soc. 2014, 136, 12174. (i) Shao, J.-Y.; Zhong, Y.-W. Chem. - Eur.
J. 2014, 20, 8702.
substituents on the 2,7-positions of the DBP core results in
near-degenerate HOMO and HOMO−1 orbitals, which is in
agreement with the absence of efficient amine−amine coupling
in 1•+.
In summary, the degree of amine−amine electronic coupling
through the DBP bridge was found to be greatly dependent on
the substitution pattern of the redox sites. Efficient coupling is
present between two amine substituents on the 3,8- and 5,10-
positions of DBP. However, little coupling is present between
the two distal amine sites through the 2,7-positions of the
bridge. The latter system is dominated by the bridge to
aminium charge transfer in the one-electron oxidized state. This
information is of great significance for the design of new mixed-
valent systems and DBP derivatives for optoelectronic
applications. In addition, the amine-substituted DBP derivatives
possess appealing electrochemical properties and rich electronic
absorptions at different redox states, which may make them
useful in near-infrared electrochromism and redox-driven
molecular switches.16
ASSOCIATED CONTENT
* Supporting Information
■
S
(8) (a) Nelsen, S. F. Chem. - Eur. J. 2000, 6, 581. (b) Hankache, J.;
Wenger, O. S. Chem. Rev. 2011, 111, 5138. (c) Heckmann, S.;
Lambert, C. Angew. Chem., Int. Ed. 2012, 51, 326.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03408.
(9) (a) Lambert, C.; Noll, G. J. Am. Chem. Soc. 1999, 121, 8434.
̈
(b) Lambert, C.; Noll, G.; Schelter, J. Nat. Mater. 2002, 1, 69.
̈
Complete synthesis and experimental details, TDDFT
results, and NMR and mass spectra of new compounds
(PDF)
(c) Jones, S. C.; Coropceanu, V.; Barlow, S.; Kinnibrugh, T.;
Timofeeva, T.; Bred
126, 11782. (d) Barlow, S.; Risko, C.; Chung, S.-J.; Tucker, N. M.;
Coropceanu, V.; Jones, S. C.; Levi, Z.; Bredas, J.-L.; Marder, S. R. J.
́
as, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2004,
́
AUTHOR INFORMATION
Corresponding Authors
Am. Chem. Soc. 2005, 127, 16900. (e) Lacroix, J. C.; Chane-Ching, K.
I.; Maquere, F.; Maurel, F. J. Am. Chem. Soc. 2006, 128, 7264.
(f) Sakamoto, R.; Sasaki, T.; Honda, N.; Yamamura, T. Chem.
■
*E-mail: xzzhu@iccas.ac.cn.
*E-mail: zhongyuwu@iccas.ac.cn.
Notes
Commun. 2009, 5156. (g) Volker, S. F.; Renz, M.; Kaupp, M.;
̈
Lambert, C. Chem. - Eur. J. 2011, 17, 14147. (h) Barlow, S.; Risko, C.;
Odom, S. A.; Zheng, S.; Coropceanu, V.; Beverina, L.; Bredas, J.-L.;
Marder, S. R. J. Am. Chem. Soc. 2012, 134, 10146. (i) Nie, H.-J.; Chen,
X.; Yao, C.-J.; Zhong, Y.-W.; Hutchison, G. R.; Yao, J. Chem. - Eur. J.
2012, 18, 14497. (j) Parthey, M.; Vincent, K. B.; Renz, M.; Schauer, P.
A.; Yuft, D. S.; Howard, J. A. K.; Kaupp, M.; Low, P. J. Inorg. Chem.
2014, 53, 1544.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Grant Nos. 21472196, 21271176, 91333113, and 21221002)
and the Strategic Priority Research Program of the Chinese
Academy of Sciences (Grant No. XDB 12010400) for funding
support.
(10) Shen, J.; Yuan, D.; Qiao, Y.; Shen, X.; Zhang, Z.; Zhong, Y.; Yi,
Y.; Zhu, X. Org. Lett. 2014, 16, 4924.
(11) Sreenath, K.; Thomas, T. G.; Gopidas, K. R. Org. Lett. 2011, 13,
1134.
(12) Hush, N. S. Coord. Chem. Rev. 1985, 64, 135.
(13) (a) Oh, D. H.; Boxer, S. G. J. Am. Chem. Soc. 1990, 112, 8161.
(b) Nelsen, S. F.; Konradsson, A. E.; Weaver, M. N.; Telo, J. P. J. Am.
Chem. Soc. 2003, 125, 12493. (c) Heckmann, A.; Lambert, C.; Goebel,
M.; Wortmann, R. Angew. Chem., Int. Ed. 2004, 43, 5851.
(14) Lambert, C.; Noll, G. Angew. Chem., Int. Ed. 1998, 37, 2107.
(15) (a) Renz, M.; Theilacker, K.; Lambert, C.; Kaupp, M. J. Am.
Chem. Soc. 2009, 131, 16292. (b) Sutton, C.; Korzdorfer, T.;
Coropceanu, V.; Bredas, J.-L. J. Phys. Chem. C 2014, 118, 3925.
(16) (a) Bill, N. L.; Lim, J. M.; Davis, C. M.; Bahring, S.; Jeppesen, J.
O.; Kim, D.; Sessler, J. L. Chem. Commun. 2014, 50, 6758. (b) Chang,
C.-C.; Yueh, H.; Chen, C.-T. Org. Lett. 2011, 13, 2702. (c) Yao, B.; Ye,
X.; Zhang, J.; Wan, X. Org. Lett. 2014, 16, 5378.
REFERENCES
■
(1) (a) Saito, M. Symmetry 2010, 2, 950. (b) Hopf, H. Angew. Chem.,
Int. Ed. 2013, 52, 12224. (c) Kawase, T.; Nishida, J.-i. Chem. Rec. 2015,
15, 1045.
(2) (a) Zhang, H.; Karasawa, T.; Yamada, H.; Wakamiya, A.;
Yamaguchi, S. Org. Lett. 2009, 11, 3076. (b) Kawase, T.; Konishi, A.;
Hiro, Y.; Matsumoto, K.; Kurata, H.; Kubo, T. Chem. - Eur. J. 2009, 15,
2653. (c) Levi, Z. U.; Tilley, T. D. J. Am. Chem. Soc. 2009, 131, 2796.
(d) Xu, F.; Peng, L.; Orita, A.; Otera, J. Org. Lett. 2012, 14, 3970.
(e) Maekawa, T.; Segawa, Y.; Itami, K. Chem. Sci. 2013, 4, 2369.
(f) Zhao, J.; Oniwa, K.; Asao, N.; Yamamoto, Y.; Jin, T. J. Am. Chem.
Soc. 2013, 135, 10222. (g) Chen, C.; Harhausen, M.; Liedtke, R.;
Bussmann, K.; Fukazawa, A.; Yamaguchi, S.; Petersen, J. L.; Daniliuc,
C. G.; Frohlich, R.; Kehr, G.; Erker, G. Angew. Chem., Int. Ed. 2013, 52,
̈
5992. (h) London, G.; von Wantoch Rekowski, M.; Dumele, O.;
Schweizer, W. B.; Gisselbrecht, J.-P.; Boudon, C.; Diederich, F. Chem.
Sci. 2014, 5, 965. (i) Li, H.; Wang, X.-Y.; Wei, B.; Xu, L.; Zhang, W.-
X.; Pei, J.; Xi, Z. Nat. Commun. 2014, 5, 4508.
(3) (a) Nakano, M.; Osaka, I.; Takimiya, K.; Koganezawa, T. J. Mater.
Chem. C 2014, 2, 64. (b) Li, C.; Liu, C.; Li, Y.; Zhu, X.; Wang, Z.
D
DOI: 10.1021/acs.orglett.5b03408
Org. Lett. XXXX, XXX, XXX−XXX