New Iridium Complex Coordinated with Tetrathiafulvalene Substituted Triazole-pyridine Ligand: Synthesis, Photophysical and Electrochemical Properties

A new iridium(III) complex based on the triazole-pyridine ligand with tetrathiafulvalene unit, [Ir(ppy)2(L)]PF6 (1), has been synthesized and structurally characterized. The absorption spectra, luminescent spectra and electrochemical behaviors of L and 1 have been investigated. Complex 1 is found to be emissive at room temperature with maxima at 481 and 510 nm. The broad and structured emission bands are suggested a mixing of LC (π–π*) and CT (MLCT) excited states. The influence of iridium ion coordination on the redox properties of the TTF has also been investigated by cyclic voltammetry.


Introduction
For several decades, tetrathiafulvalene (TTF) and its derivatives were extensively developed by scientists in photofunctional materials [1][2][3][4][5][6][7][8] because of their strongly electron-donating and attractive reversible redox properties. As a consequence, a large synthetic effort has also been devoted to the preparation of materials that exhibit synergy or coexistence between conductivity and luminescence. Coordination of TTF-containing ligands to transition metal centers is typically achieved by functionalizing TTF with nitrogen atom. 9 Very recently, we have reported a new nitrogen-containing TTF-based ligand, 2-(1-(2-((4',5'-bis(methylthio)-  10 The results hence led us to further design other metal complexes based on the TTF-containing thioethylbridged triazole-pyridine ligand.
Iridium(III) complexes have widely been employed in organic light-emitting devices (OLEDs), as they have high phosphorescence quantum efficiency, long excitedstate lifetime and excellent color tenability. [11][12][13][14] Therefore, the association of the redox-active TTF unit with cyclometalated iridium(III) complex is intriguing in coordination chemistry and material chemistry. In this work, we report the synthesis of a new bis-cyclometallated TTF-based iridium(III) complex with ppy as C ∧ N ligand, [Ir(ppy) 2 (L)]PF 6 (1) (Scheme 1). Their electrochemical and photophysical properties are also investigated.

1. Materials and Measurements
was synthesized in our previous work, 8 and an improved preparation method was used to synthesize the cyclometalated iridium chlorobridged dimer [Ir(ppy) 2 Cl] 2 in good yield. 15 All solvents were dried using standard procedures. Solvents used for electrochemistry and spectroscopy were spectroscopic grade. 1 H NMR and 13 C NMR spectra were recorded on a Bruker AM 400 MHz instrument. Chemical shifts were reported in ppm relative to Me 4 Si as internal standard. FT-IR spectra were taken on a Nicolet 6700 FTIR spectrometer (400-4000 cm -1 ) with KBr pellets. ESI-MS spectra were recorded on an Esquire HCT-Agilent 1200 LC/MS spectrometer. The elemental analyses were performed on a Vario EL Cube Analyzer system. UV-vis spectra were recorded on a Hitachi U3900/3900H spectrophotometer. Fluorescence spectra were carried out on a Hitachi F-7000 spectrophotometer.

3. Cyclic Voltammetry
Cyclic voltammetry (CV) was performed on a CHI 1210B electrochemical workstation, with a glassy carbon electrode as the working electrode, a platinum wire as the counter electrode, an aqueous saturated calomel electrode (SCE) as the reference electrode, and 0.1 M n-Bu 4 NClO 4 as the supporting electrolyte.

1. 2. Emission Properties
The relative emission spectra of ligand L and complex 1 in degassed CH 2 Cl 2 solution at room temperature are also given in Fig. 1. Upon excitation at 438 nm, complex 1 displays two intense emission maxima at ca. 481 and 510 nm. As for L, the emission band occurs at about 462 nm (λ ex = 363 nm). Therefore the vibronically structured emission of 1 is probably derived from a mixing of 3 LC ( 3 π-π*) and 3 CT ( 3 MLCT) excited states. 19,20

2. Electrochemical Properties
The electrochemical behaviors of the ligand L and iridium complex 1 were investigated by cyclic voltamme-try in CH 2 Cl 2 solution ( Fig. 2 and Table 1). Both compounds (L and 1) exhibit two reversible one-electron oxidation processes, which are associated with the successive oxidation of the TTF unit to TTF + and TTF 2+ . Additionally, complex 1 show a irreversible oxidation peak (E p ox ) at 1.88 V, which is attributed to the metal-centered Ir 3+ /Ir 4+ oxidation couple. 21,22 In comparison with the ligand L, the two oxidation waves for complex 1 are shifted to more negative potentials. The observed results are different from the previous reported work, 23 it is possible that the triazole-pyridine unit is grafted on the TTF core through a non-conjugated spacer group, which is disadvantageous to intramolecular electron transfer and communications. 10

Conclusions
In conclusion, a new iridium(III) complex 1 based on tetrathiafulvalene-substituted triazole-pyridine ligand, has been synthesized and fully characterized by 1 H NMR, 13 C NMR, mass spectrometry, FTIR and elemental analyses. The photophysical and electrochemical properties have been measured and analyzed. The luminescent spectra show that the emissive state originates from mixed intraligand and metal-to-ligand charge transfer 3 (π→π * + MLCT) transitions. The electrochemical studies reveal that 1 undergo reversible TTF/TTF +· /TTF 2+ redox processes and one irreversible Ir 3+ →Ir 4+ oxidation process. The research plays a role in designing new photoelectric functional materials, and more work is going on in our laboratory.