Star shaped ferrocenyl substituted triphenylamines

Abstract

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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Introduction

There has been continuously growing research interest in triphenylamine derivatives due to their low oxidation potential, and hole transport properties.¹ Triphenylamine derivatives have been widely explored in organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), nonlinear optics (NLO) and dye-sensitized 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.^4–7 Lin et al. have synthesized polyferrocene based thiophene and tripheylamine.⁹ 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 C₃-symmetric ferrocenyl-triphenylamines with systematic variation of the conjugation length.

Results and discussion

The ferrocenyl substituted triphenylamine conjugates (Fc-TPA) 3a–3c were synthesized by the Pd-catalyzed Sonogshira cross-coupling reaction (Scheme 1).⁹ The intermediate tris-(4-iodo-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 cross-coupling reaction of the tris-(4-iodo-phenyl)-amine (2) with the corresponding alkynyl-ferrocenes (a, b and c) in the presence of Pd(PPh₃)₄ and CuI as a co-catalyst, and tetrahydrofuran (THF) as a solvent resulted in the homo-coupling product. Therefore the reaction was performed in the absence of copper-iodide (CuI). The Sonogashira cross-coupling reaction of tris-(4-iodo-phenyl)-amine 2 with ethynylferrocene (a), 1-ferrocene-4-ethynyl-benzene (b), and 1-ferrocenyl-1-ynyl-4-ethynyl-benzene (c), were performed using Pd(PPh₃)₄ as catalyst, and triethylamine as a base, which resulted Fc-TPA 3a–3c in ∼70% yield (Scheme 1).

Scheme 1 Synthesis of Fc-TPA 3a–3c.

The Fc-TPA 3a–3c were purified by column chromatography and well characterized by ¹H, ¹³C NMR, and HRMS techniques. The Fc-TPA 3a was also characterized by single crystal X-ray diffraction technique. The ¹H NMR spectrum of the Fc-TPA 3a–3c shows a characteristic doublet in the region 7.56–7.40 ppm corresponding to phenyl rings of the triphenylamine core. In Fc-TPA 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.

The thermal properties of the Fc-TPA 3a–3c were explored using thermogravimetric analysis (TGA) at a heating rate of 10 °C min⁻¹ under nitrogen atmosphere (Fig. S18†). The decomposition temperature for 5% weight loss in the Fc-TPA 3a–3c were found to be 180 °C, 248 °C, 334 °C respectively. This reflects the thermal stability of Fc-TPA 3a–3c is directly proportional to the conjugation length.

X-ray analysis

The single crystal of Fc-TPA 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 skew-eclipsed conformation (Fig. 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.†

Fig. 1 Single crystal X-ray structure of the Fc-TPA 3a (a) through a-axis (dichloromethane solvent molecule) and (b) through b-axis (skew-eclipsed conformation).

The crystal packing diagram of the Fc-TPA 3a reveals, intermolecular C_cp–H⋯π interactions between the two adjacent molecules, which are interconnected via two different C_cp–H⋯π interactions. The C–H⋯π interaction between hydrogen (H21) and ferrocenyl cyclopentenyl ring (C50–C55, 3.29 Ε) results in the formation of a dimer. The C–H_cp⋯π interactions between hydrogen (H42) of the ferrocenyl cyclopentenyl ring and the phenyl ring (C9–C14, 3.04 Å) leads to the formation of 2D network (Fig. 2).

Fig. 2 Packing diagram of Fc-TPA 3a, forming 2-D network through b-axis. The secondary interactions are shown by the dashed lines.

Photophysical properties

The electronic absorption spectra of Fc-TPA 3a–3c were recorded in dichloromethane at room temperature, and the photophysical data are listed in Table 1. The Fc-TPA 3a–3c show strong absorption band between 368–386 nm corresponding to π → π* transition (Fig. 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 Fc-TPA 3a–3c in dichloromethane is shown in Fig. S10.†

Table 1 Photophysical, and electrochemical properties of compounds 3a–3c

Compounds	λmax [nm] (ε [Lmol^−1 cm^−1]) ^a	Eoxid (V)	Optical HOMO–LUMO gap (eV)^d
a Measured in dichloromethane.b The oxidation value of ferrocene unit, and.c The oxidation value of triphenylamine unit.d Optical HOMO–LUMO gap estimated from the absorption edge.
3a	368 (31 000)	0.06^b	3.02
		0.72^c
3b	381 (13 466)	0.07^b	2.85
		0.74^c
3c	386 (48 000)	0.07^b	2.78
		0.78^c
Ferrocene	—	0.00	—

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Fig. 3 Normalized electronic absorption spectra of the Fc-TPA 3a–3c in CH₂Cl₂ (1.0 × 10⁻⁴ M).

Electrochemistry

The electrochemical behavior of the Fc-TPA 3a–3c were investigated by the cyclic voltammetric (CV) analysis in dry dichloromethane (DCM) solution at room temperature using tetrabutylammonium hexafluorophosphate (TBAPF₆) as a supporting electrolyte. The electrochemical data of Fc-TPA 3a–3c are listed in Table 1 and the representative cyclic voltammogram is shown in Fig. S11.† The cyclic voltammograms of the Fc-TPA 3a–3c show two oxidation waves. The first reversible oxidation wave corresponding to ferrocene moiety at E_1/2 = 0.06 − 0.07 V, and second reversible oxidation wave corresponding to triphenylamine unit E_1/2 = 0.72 − 0.78 V.¹² The oxidation potential of the ferrocene unit in Fc-TPA 3a–3c follows the order 3c > 3b > 3a.

Computational details

In order to understand the photophysical and electrochemical properties of the Fc-TPA 3a–3c, the time dependent density functional calculations (TD-DFT) were performed.¹³ All the quantum chemical calculations were performed using the Gaussian 09 program.¹⁴ The structures (3a–3c) were optimized using CAM-B3LYP to understand their photophysical properties.^15,16 The solvent calculations were carried out in the dichloromethane (DCM) using the polarized continuum model (PCM)¹⁷ as implemented into Gaussian 09 software. The 6–31 G** basis set for C, N, H and LANL2DZ for Fe was used for all the calculations. The excitation energies were calculated by the TD-DFT approach with the lowest 600 singlet excited states to cover the 300–750 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 Fc-TPA 3a agree well with the experimental data (Fig. 4). The optimized structure of Fc-TPA 3a show eclipsed conformation of cyclopentadienyl rings of the ferrocenyl groups, whereas the crystal structure shows skew-eclipsed conformation (Fig. S12†).

Fig. 4 Comparison of selected bond lengths of (a) crystal structure and (b) CAM-B3LYP optimized structure of Fc-TPA 3a.

Time dependent density functional (TD-DFT) studies

In order to determine the nature of the excited states of Fc-TPA 3a–3c, the time dependent-density functional theory (TD-DFT) calculations were carried out. The experimental (UV-vis) and computed (TD-DFT: CAM-B3LYP) absorption spectra are shown in Fig. 5. The Fc-TPA 3a, 3b and 3c show strong absorption band calculated at 327 nm, 344 nm, and 358 nm with oscillator strength 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 (Fig. S13 and S14†).

Fig. 5 The comparison of experimental and calculated (TD-DFT at CAM-B3LYP level) absorption spectrum of Fc-TPA 3a–3c in DCM solution.

The Table 2 shows the HOMO to LUMO transitions in Fc-TPA 3a–3c contributes to the lowest excited state by 73%, 38%, and 43% respectively. Thus, the lowest excited states of Fc-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 Fc-TPA 3a–3c

Fc-TPA	TD-DFT/CAM-B3LYP (DCM)
	λmax	f	Major contribution (%)
3a	327 nm	1.4958	HOMO → LUMO (73%)
3b	344 nm	1.7963	HOMO → LUMO (38%)
3c	358 nm	2.9417	HOMO → LUMO (43%)

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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 were explained from the TD-DFT 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.

Experimental section

General experimental

All reagents were obtained from commercial sources, and used as received unless otherwise stated. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) spectra were recorded on a Bruker Avance (III) 400 MHz instrument by using CDCl₃ as solvent. ¹H 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, 77.36 ppm). Thermogravimetric analyses were performed on the Mettler Toledo Thermal Analysis system. UV-visible absorption spectra were recorded on a Cary-100 Bio UV-visible spectrophotometer. Cyclic voltammograms (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 Bruker-Daltonics micrOTOF-Q II mass spectrometer. The single crystal X-ray structure of the Fc-TPA 3a.†

X-ray crystallography

Single crystal X-ray structural studies of 3a were performed on a CCD Agilent Technologies (Oxford Diffraction) SUPER NOVA diffractometer. Data were collected at 293(2) K using graphite-monochromated Cu Kα radiation (λ_α = 1.54184 Å). The strategy for the data collection was evaluated by using the CrysAlisPro CCD software. The data were collected by the standard 'phi–omega scan techniques, and were scaled and reduced using CrysAlisPro RED software. The structures were solved by direct methods using SHELXS-97 and refined by full matrix least-squares with SHELXL-97, refining on F².¹

The positions of all the atoms were obtained by direct methods. All non-hydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2U_eq of their 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 Fc-TPA 3a–3c

In the presence of argon atmosphere a solution of tris-(4-iodo-phenyl)-amine 2 (0.2 g, 0.38 mmol) and the corresponding ethynyl ferrocene (4 equivalent) in dry THF (20 ml), added triethylamine (5 ml), Pd(PPh₃)₄ (0.100 g, 0.08 mmol), stirred for 16 h at 65 °C, 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 3a–3c around 70% yield.

Fc-TPA 1. Fc-TPA 3a. Orange solid (0.231 g, 69%) Mp > 250 °C. ¹H 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), 118.5 (Ph), 88.0 (C≡C), 71.3 (Cp), 69.9 (Cp′), 68.7 (Cp), 65.4 (Cp). HRMS (ESI-TOF): m/z calculated for C₅₄H₃₉Fe₃N 869.1067 [M]⁺, measured 869.1141 [M]⁺.

Fc-TPA 3b. Orange solid (0.297 g, 70%) Mp > 280 °C. ¹H 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 (Ph), 127.3 (Ph), 124.8 (Ph), 123.0 (Ph), 119.4 (Ph), 117.1 (Ph), 88.6 (Ph), 88.3 (Ph), 83.2 (C≡C), 68.7 (Cp′), 68.3 (Cp′), 65.4 (Cp). HRMS (ESI-TOF): m/z calculated for C₇₂H₅₁Fe₃N 1097.2071 [M]⁺, measured 1097.2078 [M]⁺.

Fc-TPA 3c. Orange solid (0.31 g, 75%) Mp > 300 °C. ¹H NMR (CDCl₃, 400 MHz, ppm): δ 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). ¹³C NMR (CDCl₃, 100 MHz, ppm): 146.7 (Ph), 132.8 (Ph), 131.4 (Ph), 131.3 (Ph), 124.0 (Ph), 123.7 (Ph), 122.5 (Ph), 117.8 (Ph), 90.8 (Ph), 90.5 (Ph), 89.3 (Ph), 85.6 (C≡C), 71.4 (C≡C), 70.0 (Cp′), 69.0 (Cp′), 64.9 (Cp). HRMS (ESI-TOF): m/z calculated for C₇₈H₅₁Fe₃N 1169.2072 [M]⁺, measured 1169.2079 [M]⁺.

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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Footnote

† Electronic supplementary information (ESI) available: The ¹H NMR, ¹³C NMR spectra, mass spectroscopy data, cyclic voltammograms, UV-vis graphs, and DFT calculations of Fc-TPA 3a–3c are provided. CCDC 995106. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13400e

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