Star shaped ferrocenyl substituted triphenylamines

Article abstract of DOI:10.1039/c5ra13400e

This manuscript reports design and synthesis of star shaped ferrocenyl substituted triphenylamine conjugates (Fc-TPA) 3a-3c by the Pd-catalyzed Sonogshira cross-coupling reaction. Their photophysical and electrochemical properties were investigated, which are a function of the conjugation length. The time dependent density functional (TD-DFT) studies were performed to understand and support the experimental findings. The LUMO could be significantly stabilized by increasing the conjugation. The thermal stability of Fc-TPA 3a-3c can be improved by increasing the conjugation length. The single crystal X-ray structure of Fc-TPA 3a is reported, which show interesting supramolecular interactions leading to the formation of 2D-network.

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Star Shaped Ferrocenyl Substituted Triphenylamines
Rajneesh Misra*, Ramesh Maragani, Biswarup Pathak, Prabhat Gautam, Shaikh M.
Mobin
Department of Chemistry, Indian Institute of Technology Indore, Indore 452017, India
TOC
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ABSTRACT
This manuscript reports design and synthesis of Star shaped ferrocenyl substituted
triphenylamine conjugates (FcꢀTPA) 3a–3c by the Pd‐catalyzed Sonogshira crossꢀcoupling
reaction. Their photophysical, and electrochemical properties were investigated, which are a
function of the conjugation length. The time dependent density functional (TD–DFT) studies
were performed to understand and support the experimental findings. The LUMO could be
significantly stabilized by increasing the conjugation. The thermal stability of FcꢀTPA 3a–3c can
be improved by increasing the conjugation length. The single crystal Xꢀray structure of FcꢀTPA
3a is reported, which show interesting supramolecular interactions leading to the formation of
2Dꢀnetwork.
Keywords: Sonogashira crossꢀcoupling. Photophysical. Electrochemical. TDꢀDFT studies.
Introduction:
There has been continuous growing research interest on the triphenylamine derivatives due to
their low oxidation potential, and hole transport properties.1 The triphenylamine derivatives
have been widely explored in Organic lightꢀemitting diodes (OLEDs), Organic fieldꢀeffect
transistors (OFETs), Nonlinear Optics (NLO) and Dyeꢀsensitized solar cells (DSSCs).2 The
literature reveals that the photonic properties of the triphenylamine can be tuned by the
peripheral modifications.3
Our group has explored ferrocene as a strong donor, when incorporated into a variety of
molecular systems.4ꢀ6 Lin et al. have synthesized polyferrocene based thiophene and
tripheylamine.7 We were interested to incorporate the ferrocenyl moiety with varying spacer
lengths on the periphery of triphenylamine and to explore its electron donating behavior. The
literature reveals that there are limited number of reports, where the synergistic effect of donor
on its photonic properties were explored.8
In continuation of our work on ferrocenyl functionalized triphenylamine derivatives, we
were interested to design and synthesize C3ꢀsymmetric ferrocenylꢀtriphenylamines with
systematic variation of the conjugation length.
2

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Results and discussion.
The ferrocenyl substituted triphenylamine conjugates (FcꢀTPA) 3a−3c were synthesized by the
Pdꢀcatalyzed Sonogshira crossꢀcoupling reaction (Scheme 1).9 The intermediate trisꢀ(4ꢀiodoꢀ
phenyl)ꢀamine 2 was synthesized by the iodination reaction of triphenylamine (1) using KI, KIO3
as reagent, and acetic acid as a solvent for 5 h, in 80% yield.10 The Sonogshira crossꢀcoupling
reaction of the trisꢀ(4ꢀiodoꢀphenyl)ꢀamine (2) with the corresponding alkynylꢀferrocenes (a, b
and c) in the presence of Pd(PPh3)4 and CuI as a coꢀcatalyst, and tetrahydrofuran (THF) as a
solvent resulted in the homoꢀcoupling product. Therefore the reaction was performed in the
absence of copperꢀiodide (CuI). The Sonogashira crossꢀcoupling reaction of trisꢀ(4ꢀiodoꢀphenyl)ꢀ
amine 2 with ethynylferrocene (a), 1ꢀferroceneꢀ4ꢀethynylꢀbenzene (b), and 1ꢀferrocenylꢀ1ꢀynylꢀ
4ꢀethynylꢀbenzene (c), were performed using Pd(PPh3)4 as catalyst, and triethylamine as a base,
which resulted FcꢀTPA 3a–3c in ∼70% yield (Scheme 1).
Ar
I
Ar
Pd(PPh3)4
KI,KIO3
Acetic acid
THF, Et3N
N
1
5 h, 85 o
C
I
N
16 h, 65 o
C
N
I
2
Ar
Ar
3a-3c
Ar
=
Fe
Fe
Fe
b
a
c
Scheme 1. Synthesis of FcꢀTPA 3a–3c.
13
The FcꢀTPA 3a–3c were purified by column chromatography and well characterized by 1H, C
NMR, and HRMS techniques. The FcꢀTPA 3a was also characterized by single crystal Xꢀray
1
diffraction technique. The H NMR spectrum of the FcꢀTPA 3a–3c shows a characteristic
doublet in the region 7.56–7.40 ppm corresponding to phenyl rings of the triphenylamine core. In
FcꢀTPA 3a–3c monosubstituted cyclopentadienyl rings of the ferrocene exhibits a triplet
between 4.69–4.51 ppm, whereas the unsubstituted cyclopentadienyl ring of the ferrocene
exhibits a multiplet in the region 4.28–4.06 ppm.
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The thermal properties of the FcꢀTPA 3a–3c were explored using thermogravimetric analysis
(TGA) at a heating rate of 10 °C min−1 under nitrogen atmosphere (Figure S18). The
decomposition temperature for 5% weight loss in the FcꢀTPA 3a–3c were found to be 180 °C,
248 °C, 334 °C respectively. This reflects the thermal stability of FcꢀTPA 3a–3c is directly
proportional to the conjugation length.
Xꢀray analysis:
The single crystal of FcꢀTPA 3a was obtained via slow evaporation of dichloromethane solution
at room temperature, which crystallizes into centrosymmetric monoclinic space group P21/n.
The cyclopentadienyl rings of the ferrocenyl moieties show skewꢀeclipsed conformation (Figure
1). The dihedral angles between the planes containing the triphenylamine core, and the
cyclopentadienyl ring of the ferrocenyl groups (Fc 1), (Fc 2), and (Fc 3) were found to be 17.74°,
15.43° and 17.74° respectively. The important bond lengths and bond angles are listed in Table
S1.
(a)
(b)
Figure 1. Single crystal Xꢀray structure of the FcꢀTPA 3a (a) through aꢀaxis (dichloromethane
solvent molecule) and (b) through bꢀaxis (Skewꢀeclipsed conformation).
4

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The crystal packing diagram of the FcꢀTPA 3a reveals, intermolecular Ccp−H···π interactions
between the two adjacent molecules, which are interconnected via two different Ccp−H···π
interactions. The C−H···π interaction between hydrogen (H21) and ferrocenyl cyclopendenyl ring
(C50–C55, 3.29 Å) results in the formation of a dimer. The C−Hcp···π interactions between
hydrogen (H42) of the ferrocenyl cyclopendenyl ring and the phenyl ring (C9–C14, 3.04 Å) leads
to the formation of 2‐D network (Figure 2).
Figure 2. Packing diagram of FcꢀTPA 3a, forming 2ꢀD network through bꢀaxis. The secondary
interactions are shown by the dashed lines.
Photophysical properties.
The electronic absorption spectra of FcꢀTPA 3a–3c were recorded in dichloromethane at room
temperature, and the photophysical data are listed in Table 1. The FcꢀTPA 3a–3c show strong
absorption band between 368–386 nm corresponding to π→π* transition. (Figure 3).11 The red
shift in the absorption maxima for π→π* absorption band follows the order 3c > 3b > 3a. The
variation in colors of the FcꢀTPA 3a–3c in dichloromethane is shown in Figure S10.
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1.0
0.8
0.6
0.4
0.2
0.0
3a
3b
3c
300
400
500
600
700
Wavelength (nm)
Figure 3. Normalized electronic absorption spectra of the FcꢀTPA 3a–3c in CH2Cl2
(1.0 × 10−4 M).
Electrochemistry
The electrochemical behavior of the FcꢀTPA 3a–3c were investigated by the cyclic voltammetric
(CV) analysis in dry dichloromethane (DCM) solution at room temperature using
tetrabutylammoniumhexafluorophosphate (TBAPF6) as
a
supporting electrolyte. The
electrochemical data of FcꢀTPA 3a–3c are listed in Table 1 and the representative cyclic
voltammogram is shown in Figure S11. The cylic voltammograms of the FcꢀTPA 3a–3c show
two oxidation waves. The first reversible oxidation wave corresponding to ferrocene moiety at
E1/2 = 0.06–0.07 V, and second reversible oxidation wave corresponding to triphenylamine unit
E1/2 = 0.72–0.78 V.12 The oxidation potential of the ferrocene unit in FcꢀTPA 3a–3c follows the
order 3c > 3b > 3a.
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Table 1. Photophysical, and electrochemical properties of compounds 3aꢀ3c.
Compounds λmax[nm] (ε[Lmol–
1cm–1]) a
Eoxid
(V)
Optical
HOMO−LUMO
gap (eV)d
368 (31000)
0.06b
0.72c
0.07b
0.74c
0.07b
0.78c
0.00
3.02
3a
381(13466)
2.85
2.78
ꢀ
3b
386 (48000)
3c
ꢀ
Ferrocene
aMeasured in dichlomethane. b The oxidation value of ferrocene unit, and c the oxidation value of
triphenylamine unit. dOptical HOMO−LUMO gap estimated from the absorption edge.
Computational Details.
In order to understand the photophysical and electrochemical properties of the FcꢀTPA 3a–3c,
the time dependent density functional calculations (TDꢀDFT) were performed.13 All the quantum
chemical calculations were performed using the Gaussian 09 program.14 The structures (3a–3c)
15,16
were optimized using CAM–B3LYP to understand their photophysical properties.
The
solvent calculations were carried out in the dichloromethane (DCM) using the polarized
continuum model (PCM)17 as implemented into Gaussian 09 software. The 6–31G** basis set for
C, N, H and LANL2DZ for Fe was used for all the calculations. The excitation energies were
calculated by the TDꢀDFT approach with the lowest 600 singlet excited states to cover the 300ꢀ
750 nm range of UV–vis spectra. All the UV–vis spectra computed at PCM–TDDFT were
extracted from Gaussian output file using the Gaussview 5.0 program.16
The calculated (DFT) structural parameters of FcꢀTPA 3a agree well with the experimental data
(Figure 4). The optimized structure of FcꢀTPA 3a show eclipsed conformation of
cyclopentadienyl rings of the ferrocenyl groups, whereas the crystal structure shows Skewꢀ
eclipsed conformation (Figures S12).
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(a)
(b)
Figure 4. Comparison of selected bond lengths of (a) crystal structure and (b) CAMꢀB3LYP
optimized structure of FcꢀTPA 3a.
Time dependent density functional (TDꢀDFT) studies:
In order to determine the nature of the excited states of FcꢀTPA 3a–3c, the time dependentꢀ
density functional theory (TDꢀDFT) calculations were carried out. The experimental (UV‐vis)
and computed (TD‐DFT: CAMꢀB3LYP) absorption spectra are shown in Figure 5. The FcꢀTPA
3a, 3b and 3c show strong absorption band calculated at 327 nm, 344 nm, and 358 nm with good
oscillator strengths of 1.4958, 1.7963, and 2.9417 respectively. The experimental values for these
transition are 368, 381, 386 nm for 3a, 3b and 3c respectively. Therefore, the experimental and
calculated trends are similar. The molecular orbitals (MOs) associated to these transition
confirms the π→π* transition (Figures S13 and S14).
358
386
1.0
0.8
0.6
0.4
0.2
0.0
3c-CAM-B3LYP(Computational)
3c (experimental)
327
344
381
368
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
3a-CAM-B3LYP (Computational)
3a (experimental)
3b-CAM-B3LYP (Computational)
3b (experimental)
200
400
600
800
Wavelength (nm)
200
400
600
800
400
600
800
Wavelength (nm)
Wavelength (nm)
Figure 5. The comparison of experimental and calculated (TDꢀDFT at CAMꢀB3LYP level)
absorption spectrum of FcꢀTPA 3a–3c in DCM solution.
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In FcꢀTPA 3a–3c shows the HOMO→LUMO transition contributes to the lowest excited state
by 73%, 38%, and 43% respectively as shown in Table 2. Thus, the lowest excited states of Fcꢀ
TPA 3a–3c are assign to the π→π* absorption band which was further conformed by molecular
orbitals of HOMO→LUMO.
Table 2. Computed vertical transition energies and their Oscillator strengths (f) and Major
contributions for the FcꢀTPA 3a–3c.
FcꢀTPA 3a–3c
TDꢀDFT/ CAMꢀB3LYP (DCM)
Major contribution (%)
λmax
f
327 nm
1.4958
HOMO→LUMO (73%)
1
2
344 nm
358 nm
1.7963
2.9417
HOMO→LUMO (38%)
HOMO→LUMO (43%)
3
Conclusions
In summary, we have described the synthesis of ferrocenyl functionalized triphenylamines. The
photonic, electronic, and thermal properties of the triphenylamines can be tuned by varying the
spacer length. These results show the design of new materials with varying spacers for various
optoelectronic applications. The optical and electrochemical properties of the materials are
explained from the TDꢀDFT calculations. The results obtained here will be helpful in design of
molecular systems for photonic applications. The optical limiting properties of these materials
are currently ongoing in our laboratory.
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Experimental section
General experimental. All reagents were obtained from commercial sources, and used as
1
received unless otherwise stated. H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
1
recorded on a Bruker Avance (III) 400 MHz instrument by using CDCl3 as solvent. H NMR
chemical shifts are reported in parts per million (ppm) relative to the solvent residual peak
(CDCl3, 7.26 ppm). Multiplicities are given as s (singlet), d (doublet), t (triplet), q (quartet), dd
(doublet of doublets), and m (multiplet), and the coupling constants, J, are given in Hz. 13C NMR
chemical shifts are reported relative to the solvent residual peak (CDCl3, 77.36ppm).
Thermogravimetric analyses were performed on the Mettler Toledo Thermal Analysis system.
UV−visible absorption spectra were recorded on a Caryꢀ100 Bio UV−visible spectrophotometer.
Cyclic voltamograms (CVs) were recorded on a CHI620D electrochemical analyzer using glassy
carbon as the working electrode, Pt wire as the counter electrode, and the saturated calomel
electrode (SCE) as the reference electrode. The scan rate was 100 mV s−1. A solution of
tetrabutylammonium hexafluorophosphate (TBAPF6) in CH2Cl2 (0.1 M) was employed as the
supporting electrolyte. DCM was freshly distilled from CaH2 prior to use. All potentials were
experimentally referenced against the saturated calomel electrode couple but were then
manipulated to be referenced against Fc/Fc+ as recommended by IUPAC. Under our conditions,
the Fc/Fc+ couple exhibited E° = 0.38 V versus SCE. HRMS was recorded on a Brukerꢀ
Daltonics micrOTOFꢀQ II mass spectrometer. The single crystal Xꢀray structure of the FcꢀTPA
3a CCDC number is 995106.
Xꢀray crystallography. Single crystal Xꢀray structural studies of 3a were performed on a CCD
Agilent Technologies (Oxford Diffraction) SUPER NOVA diffractometer. Data were collected
at 293(2) K using graphiteꢀmonochromoated Cu Kα radiation (λα = 1.54184 Å). The strategy for
the Data collection was evaluated by using the CrysAlisPro CCD software. The data were
collected by the standard 'phiꢀomega scan techniques, and were scaled and reduced using
CrysAlisPro RED software. The structures were solved by direct methods using SHELXSꢀ97
and refined by full matrix leastꢀsquares with SHELXLꢀ97, refining on F2.1
The positions of all the atoms were obtained by direct methods. All nonꢀhydrogen atoms
were refined anisotropically. The remaining hydrogen atoms were placed in geometrically
constrained positions and refined with isotropic temperature factors, generally 1.2Ueq of their
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parent atoms. The crystal and refinement data are summarized in Table S2, and selected bond
distances and bond angles are shown in Table S3.
General Procedure for the preparation of FcꢀTPA 3aꢀ3c
In the presence of argon atmosphere a solution of trisꢀ(4ꢀiodoꢀphenyl)ꢀamine 2 (0.2 g, 0.38
mmol) and the corresponding ethynyl ferrocene (4 equivalent) in dry THF (20ml), added
triethylamine (5 ml), Pd(PPh3)4 (0.100 g, 0.08 mmol), stirred for 16 h at 65 oC, after completion
of the reaction, the reaction mixture was concentrated under reduced pressure, the crude
compound was purified by column chromatography on silica, using Hexane/ DCM (60:40), and
afforded pure compounds 3aꢀ3c around 70 % yield.
1
FcꢀTPA 1. FcꢀTPA 3a. Orange solid (0. 231 g, 69 %) Mp > 250 °C. H NMR (CDCl3, 400
MHz, ppm): δ 7.40 (d, J = 8 Hz, 6H, Ph), 7.06 (d, J = 8 Hz, 6H, Ph), 4.51 (t, 6H, Cp’), 4.26 (t,
19H, Cp’ and Cp). 13C NMR (CDCl3, 100 MHz, ppm): 146.2 (Ph), 132. 5 (Ph), 123. 9 (Ph), 118.
C
C
5 (Ph), 88.0 (
), 71.3 (Cp), 69.9 (Cp’), 68. 7 (Cp), 65. 4 (Cp). HRMS (ESIꢀTOF): m/z
calculated for C54H39Fe3N 869.1067 [M]+, measured 869.1141 [M]+.
1
FcꢀTPA 3b. Orange solid (0.297 g, 70%) Mp > 280 °C. H NMR (CDCl3, 400 MHz, ppm): δ
7.56 (s, 18H, Ph), 7.12 (d, J = 8 Hz, 6H, Ph), 4.69 (s, 5H, Cp’), 4.37 (s, 6H, Cp’ and Cp), 4.06 (s,
13H, Cp’ and Cp). 13C NMR (CDCl3, 100 MHz, ppm): 145.5 (Ph), 138.7 (Ph), 131.7 (Ph), 130.5
C
C
(Ph), 127.3 (Ph), 124.8 (Ph), 123.0 (Ph), 119.4 (Ph), 117.1 (Ph), 88.6 (Ph), 88.3 (Ph), 83.2 (
), 68.7 (Cp’), 68.3 (Cp’), 65.4 (Cp). HRMS (ESIꢀTOF): m/z calculated for C72H51Fe3N
1097.2071 [M]+, measured 1097.2078 [M]+.
1
FcꢀTPA 3c. Orange solid (0.31 g, 75%) Mp > 300 °C. H NMR (CDCl3, 400 MHz, ppm): δ
13
7.47 (d, J = 12 Hz, 18H, Ph), 7.11 (d, J = 8 Hz, 6H, Ph), 4.53 (t, 5H, Cp’), 4.28 (t, 17H, Cp). C
NMR (CDCl3, 100 MHz, ppm): 146.7 (Ph), 132. 8 (Ph), 131.4 (Ph), 131.3 (Ph), 124.0 (Ph),
C
C
C
), 71.4 (
C
123.7 (Ph), 122.5 (Ph), 117.8 (Ph), 90.8 (Ph), 90.5 (Ph), 89.3 (Ph), 85.6 (
), 70.0
(Cp’), 69.0 (Cp’), 64.9 (Cp). HRMS (ESIꢀTOF): m/z calculated for C78H51Fe3N 1169.2072
[M]+, measured 1169.2079 [M]+.
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Supporting Information
The 1H NMR, 13C NMR spectra, mass spectroscopy data, cyclic voltammograms, UVꢀvis graphs,
and DFT calculations of FcꢀTPA 3aꢀ3c are provided.
Corresponding Author
*Eꢀmail: rajneeshmisra@iiti.ac.in.
Tel.: +91 7312438710; fax: +91 7312361482.
Acknowledgements
The work was supported by DST, and CSIR Govt. of India, New Delhi. We are grateful to the
Sophisticated Instrumentation Centre (SIC), IIT Indore.
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