Tetracyanoethylene substituted triphenylamine analogues

Article abstract of DOI:10.1016/j.tetlet.2014.10.148

A set of tetracyanoethylene (TCNE) substituted triphenylamine analogues (4-6) exhibiting strong intramolecular charge transfer (ICT) were designed and synthesized by the [2+2] cycloaddition-retroelectrocyclization reaction of 3 (tris-(4-phenylethynyl-phenyl)-amine) with TCNE. The reaction was found to be temperature dependent. The blue shift in the π → π^? transition and intramolecular charge transfer (ICT) in amines 4-6 were found to be directly proportional to the number of TCNE units. The computational study shows good agreement with the experimental results and reveals that as the number of TCNE units in amine increases, HOMO-LUMO gap increases.

Full text of DOI:10.1016/j.tetlet.2014.10.148

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Tetracyanoethylene substituted triphenylamine analogues
⇑
Rajneesh Misra , Ramesh Maragani, Prabhat Gautam, Shaikh M. Mobin
Department of Chemistry, Indian Institute of Technology Indore, MP 452017, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A set of tetracyanoethylene (TCNE) substituted triphenylamine analogues (4–6) exhibiting strong
Received 18 September 2014
Revised 29 October 2014
Accepted 30 October 2014
Available online xxxx
intramolecular charge transfer (ICT) were designed and synthesized by the [2+2] cycloaddition–retro-
electrocyclization reaction of 3 (tris-(4-phenylethynyl-phenyl)-amine) with TCNE. The reaction was
⁄
found to be temperature dependent. The blue shift in the
p
?
p
transition and intramolecular charge
transfer (ICT) in amines 4–6 were found to be directly proportional to the number of TCNE units. The
computational study shows good agreement with the experimental results and reveals that as the num-
ber of TCNE units in amine increases, HOMO–LUMO gap increases.
Keywords:
Donor–acceptor
Cycloaddition
Charge transfer
Photophysical
Computational
HOMO–LUMO
Ó 2014 Elsevier Ltd. All rights reserved.
Triphenylamine is a strong donor and its functionalized deriva-
tives have been explored for various optoelectronic applications.
Pd-catalyzed Sonogashira cross-coupling, and [2+2] cycloaddi-
tion–retroelectrocyclization reactions, respectively (Scheme 1).
The intermediate 2 was synthesized by the iodination reaction of
1
Tripheylamine derivatives have received substantial interest in
photoconductors due to their excellent hole transporting proper-
1
7
triphenylamine 1. The Pd-catalyzed Sonogashira cross-coupling
reaction of 2 {(tris-(4-iodo-phenyl)-amine} with ethynyl-benzene
2
ties. The photonic properties of the triphenylamines can be tuned
3
18
by the peripheral modification. Incorporation of strong acceptors
resulted in 3 in ꢀ70% yield. The [2+2] cycloaddition–retroelectro-
at the periphery of the triphenylamine results in strong intramo-
lecular charge transfer (ICT).4 Diederich and co-workers have
explored tetracyanoethylene (TCNE) as a strong electron acceptor,
which undergoes [2+2] cycloaddition–retroelectrocyclization reac-
tion with electron rich alkynes.8–13 Wang and co-workers have
attached cyano acceptors on the periphery of one of the phenyl
units of triphenylamine and studied its intramolecular charge
transfer (ICT) properties.14 Michinobu et al. have explored tetracy-
anoethylene (TCNE) and tetracyanoquinodimethane (TCNQ)
substituted triphenylamine as a photosensitizer in dye-sensitized
cyclization reaction of tris-(4-phenylethynyl-phenyl)-amine 3 with
TCNE was found to be temperature dependent.
–7
The reaction of TCNE with 3 at room temperature resulted in a
mono substituted product, whereas the microwave assisted reac-
tion with varying temperatures resulted in di and tri TCNE substi-
tuted products.
The [2+2] cycloaddition–retroelectrocyclization reaction of
amine 3 with excess TCNE at room temperature for 24 h resulted
in an exclusive mono TCNE substituted amine 4 in 54 % yield.
The reaction of tris-(4-phenylethynyl-phenyl)-amine 3 with TCNE,
under microwave condition at 120 °C for 6 h resulted in amine 5,
whereas increasing the reaction temperature to 150 °C under
microwave for 6 h resulted in amine 6 (Scheme 1).
1
5
solar cells (DSSCs). Our group is interested in the design and syn-
thesis of donor–acceptor materials for optoelectronic applica-
1
6
tions. We were interested to incorporate the TCNE acceptor at
the periphery of the phenyl units of the triphenylamine and to
explore its photophysical properties. Therefore we designed and
synthesized tris-(4-phenylethynyl-phenyl)-amine 3, and their
TCNE derivatives 4–6 and studied their photophysical, electro-
chemical, and computational properties.
The amines 3–6 were purified by column chromatography and
1
13
characterized by H NMR and C NMR, and HRMS techniques.
The single crystal of amine 3 was obtained via slow evaporation
of dichloromethane solution at room temperature, which shows a
monoclinic P21/n space group. The crystal structure of 3 is shown
in Figure 1. The dihedral angles between the triphenylamine core,
and the substituted phenyl rings were found to be 28.82°, 24.61°,
and 52.55°. Important bond lengths and bond angles are listed in
the Table S2 (see ESI for details).
The tris-(4-phenylethynyl-phenyl)-amine 3, and their tetra
cyano ethylene (TCNE) derivatives 4–6 were synthesized by the
⇑
Corresponding author. Tel.: +91 7312438710; fax: +91 7312361482.
E-mail address: rajneeshmisra@iiti.ac.in (R. Misra).
http://dx.doi.org/10.1016/j.tetlet.2014.10.148
040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
0
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2
R. Misra et al. / Tetrahedron Letters xxx (2014) xxx–xxx
NC
NC
CN
CN
Dichloroethane,rt
2
4 h
N
CN
CN
NC
NC
4
I
NC
NC
A
NC
NC
CN
CN
CN
CN
Pd(PPh3)4
KI,KIO3
Acetic acid
N
N
THF, Et3N
Dichloroethane,
o
o
8
5
C
o
16 h, 65
C
1
20 C,
N
N
3
I
I
microwave,
6h
CN
CN
1
2
NC
NC
5
NC
NC
CN
CN
NC
NC
CN
CN
Dichloroethane,
o
N
1
50 C,
CN
microwave, NC
CN
CN
6h
NC
NC
CN
NC
6
Scheme 1. Synthesis of amines 3–6.
The electronic absorption, and fluorescence spectra of the tris-
(
4-phenylethynyl-phenyl)-amine 3, and their TCNE derivatives 4–
6
were recorded in dichloromethane at room temperature
(
Fig. 2), and their photophysical data are listed in Table 1. Tris-
⁄
(
4-phenylethynyl-phenyl)-amine 3 shows
p
?
p
p
transition at
⁄
3
67 nm, whereas amines 4–6 show blue shifted
?
p
transition
at 342 nm, 313 nm and 299 nm, respectively. The donor–acceptor
amines 4, 5, and 6 exhibit low energy charge transfer bands at
1
4,19,20
5
10 nm, 507 nm, and 494 nm, respectively.
These results
show good agreement with the DFT calculation, which shows that
increasing the number of TCNE groups in 4–6 results in a higher
HOMO–LUMO gap. The HOMO–LUMO gap in amine 4–6 is directly
proportional to number of TCNE groups. Tris-(4-phenylethynyl-
phenyl)-amine 3 shows high fluorescence at 434 nm, whereas their
TCNE derivatives 4–6 show quenching of fluorescence, reflecting
strong donor–acceptor interaction. Figure S15 displays the colors
of the amines 3–6 in dichloromethane.
The electrochemical behavior of the amines 3–6 was investigated
by the cyclic voltammetric (CV) analysis in dry dichloromethane
Figure 1. Single crystal X-ray structure of amine 3.
(
DCM) solution at room temperature using tetrabutylammonium
6
hexafluorophosphate (TBAPF ) as a supporting electrolyte. The elec-
a
b
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1000000
800000
600000
400000
200000
0
3
4
3
4
5
6
5
6
300
400
500
600
700
800
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 2. (a) Normalized electronic absorption spectra of the amines 3–6 in CH
2
Cl
2
(1.0 Â 10À4 M) and (b) fluorescence spectra of the amines 3–6.
Please cite this article in press as: Misra, R.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.10.148

R. Misra et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 1
Photophysical, and electrochemical properties of amines 3–6
(L mol cm–1))a
k
em (nm)
Stokes shift (nm)
U
b
E
oxid (V)
c
E
red (V)
HOMO–LUMO gap (eV)
f
HOMO–LUMO gapg (eV)
Amines
k
max (nm) (
e
f
3
4
367 (107,000)
342 (65,333)
434
433
461
414
437
454
480
68
91
0.74
0.17
0.92
1.20
—
3.04
2.12
3.45
2.32
c
e
À0.50
d
5
10 (31,334)
313 (64,666)
07 (45,354)
299 (51,600)
94 (81,600)
À1.12
À0.55
À1.14
À0.56
À1.18
1.33c
1.35c
e
d
e
d
2.02
2.45
2.42
2.79
5
6
101
155
0.04
0.01
5
4
a
Measured in dichloromethane.
b
The fluorescence quantum yields using quinine sulfate for the amines 3–6 as a standard in dichloromethane solution were performed. The oxidation potential of
triphenylamine unit (Calomel electrode (SCE) as the reference electrode).
c
The oxidation potential of triphenylamine unit (Calomel electrode (SCE) as the reference electrode).
The reduction potential of TCNE group.
Optical band gap estimated from the absorption edge.
Theoretical values from DFT calculations.
d,e
f
g
Amines
HOMO
LUMO
trochemical data are listed in Table 1 and the representative cyclic
voltammograms are shown in Figure S14. Tris-(4-phenylethynyl-
phenyl)-amine 3, shows one reversible oxidation wave correspond-
ing to the triphenylamine unit at E1/2 = 0.92 V. The incorporation of
TCNE groups results in harder oxidation potential of triphenylamine
(
irreversible oxidationwave) in amines 4–6, E1/2 = 1.20–1.33 V, indi-
4
cating a strong donor–acceptor interaction. Amines 4–6 show two
irreversible reduction waves between À1.12 to À1.18 V and À0.50
3
21
to À0.56 V corresponding to TCBD unit.
In order to understand the geometry and electronic properties
of the amines 3–6, density functional theory (DFT) calculations
⁄
⁄
using Gaussian 09 at the B3LYP/6-31G for C, H, and N were per-
formed.
-
4.91
-1.46
2
2–24
The optimized structures of the amines 3–6 show dif-
ferent dihedral angles between the peripheral phenyl groups and
triphenylamine core. The peripheral phenyl groups show planar
conformation (2.08°) with respect to the triphenylamine core in
3. The incorporation of the TCNE groups leads to the loss of planar-
ity between the substituted phenyl groups and triphenylamine
core in 4–6 (see Supporting information Table S1).
4
5
The frontier molecular orbitals of amines 3–6 are shown in Fig-
ure 3. The HOMO orbital in amine 3 is localized on the entire tri-
phenylamine core, whereas LUMO is delocalized on 2/3 of the
triphenylamine core. In amines 4–6, LUMO is localized on the
peripheral electron withdrawing TCNE groups, whereas the HOMO
remains confined to the triphenylamine unit.
The HOMO–LUMO gaps in amines 4–6 are directly proportional
to the number of TCNE groups. The incorporation of TCNE groups
in amine 3 results in greater stabilization of the HOMO as well
LUMO energy level. The HOMO–LUMO gap in amines 3–6 follows
the order 4 > 5 > 6 > 3.
-3.19
-
5.51
TCNE derivatives of triphenylamine analogues have been syn-
thesized by varying the reaction temperature and conditions. The
⁄
blue shift in the
p
?
p
transition and intramolecular charge trans-
fer (ICT) and HOMO–LUMO gap in amines 4–6 were found to be
directly proportional to the number of TCNE units. The results
obtained here well help in the design of materials with varying
ICT and HOMO–LUMO gap. Detailed photophysical studies of these
materials are currently ongoing in our laboratory.
-6.01
-3.59
Acknowledgements
6
The work was supported by DST and CSIR Govt. of India, New
Delhi.
Supplementary data
-
6.63
-3.83
Supplementary data (The optimized structures dihedral angles,
experimental procedure, cyclic voltammograms, H NMR, C NMR
1
13
Figure 3. Frontier molecular orbitals of amines 3–6.
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R. Misra et al. / Tetrahedron Letters xxx (2014) xxx–xxx
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