Synthesis and Photophysical Properties of a Series of Dimeric Indium Quinolinates

A novel class of quinolinol-based dimeric indium complexes (1–6) was synthesized and characterized using 1H and 13C(1H) NMR spectroscopy and elemental analysis. Compounds 1–6 exhibited typical low-energy absorption bands assignable to quinolinol-centered π–π* charge transfer (CT) transition. The emission spectra of 1–6 exhibited slight bathochromic shifts with increasing solvent polarity (p-xylene < tetrahydrofuran (THF) < dichloromethane (DCM)). The emission bands also showed a gradual redshift, with an increase in the electron-donating effect of substituents at the C5 position of the quinoline groups. The absolute emission quantum yields (ΦPL) of compounds 1 (11.2% in THF and 17.2% in film) and 4 (17.8% in THF and 36.2% in film) with methyl substituents at the C5 position of the quinoline moieties were higher than those of the indium complexes with other substituents.


Introduction
The creation of tris(8-hydroxyquinolinato)aluminum (Alq 3 ) by Tang and Van Slyke pioneered a new era of group 13-based organometallic luminescent materials that can be used in versatile optoelectronic fields [1]. Numerous efforts and approaches have been used to modulate the quinolinate ligands and expand their applications in organic light-emitting diodes (OLEDs) [2][3][4][5][6]. In this context, particular emphasis has been placed on the development of tris-incorporated metal complexes (Mq 3 ). Owing to the ease of introducing various substituents at the C2 and C5 positions of the quinolinolate moiety, studies of various tris-organometallic complexes based on quinolinate derivatives have also been conducted [7,8]. These complexes are endowed with photophysical properties that originate from the control of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Specifically, the systematic variation in the substituents at the C5 position of the quinolinolate groups led to excellent optical properties such as emission-color tuning and enhanced quantum efficiencies [9][10][11][12][13][14][15][16][17][18]. However, most of the previous studies primarily focused on tris-complexes.
Recently, our group reported a series of quinolinol-based indium complexes in which the sequential introduction of quinolinate ligands to the indium center could control both the emission color and quantum efficiency ( Figure 1) [19]. Importantly, the dimeric indium complex (InMeq 1 ) with a quinolinate ligand exhibited the highest quantum efficiency the emission color and quantum efficiency ( Figure 1) [19]. Importantly, the dimeric indium complex (InMeq1) with a quinolinate ligand exhibited the highest quantum efficiency (ΦPL = 59% in the poly(methyl methacrylate) (PMMA) film) compared to all the indium luminophores reported to date. In this study, we designed a series of dimeric indium quinolinates with different substituents (Me, Br, and Ph) at the C5 position of two types of quinolinate ligands (q and Meq) to prove the substitution effects for developing potential indium-based luminescent materials. The detailed synthetic procedures and optical properties of these complexes were investigated.

Synthesis of [5-phenyl-8-quinolinolate In(III)-Me 2 ] 2 (3)
This compound was prepared in a manner analogous to the synthesis of 1 using 3a (0.12 g, 0.55 mmol). The desired compound 3 was obtained as a yellow solid (0.11 g, 60%). 1  A toluene solution (10 mL) of InMe 3 (0.080 g, 0.50 mmol) was added to a toluene solution (20 mL) of 4a (0.095 g, 0.55 mmol) at room temperature. The reaction mixture was stirred for 12 h, and the insoluble parts were collected by filtration. The remained solid was washed with diethyl ether (3 × 20 mL) and dried in vacuo to obtain 4 as a pale-yellow solid (0.098 g, 62%). 1 (5) This compound was prepared in a manner analogous to the synthesis of 4 using 5a (0.095 g, 0.55 mmol). The desired compound 5 was obtained as a dark yellow solid (0.11 g, 58%). 1 (6) This compound was prepared in a manner analogous to the synthesis of 4 using 6a (0.13 g, 0.55 mmol). The desired compound 6 was obtained as a yellow solid (0.13 g, 67%).

Cyclic Voltammetry
The cyclic voltammetry (CV) measurements were performed in a deoxygenized MeCN (0.5 mM) solution with a three-electrode cell configuration (platinum working and counter electrodes and an Ag/AgNO 3 reference electrode (0.1 M in MeCN)) using an AUTO-LAB/PGSTAT12 system at room temperature. Tetra-n-butylammonium hexafluorophosphate (n-Bu 4 PF 6 ) in MeCN (0.1 M) was used as the supporting electrolyte. The redox potentials were investigated at a scan rate of 100 mV/s and determined with respect to the ferrocene/ferrocenium (Fc/Fc + ) redox couple.

Photophysical Properties
The samples for the UV-vis absorption and photoluminescence (PL) measurements were prepared using degassed solvents (p-xylene, THF, and DCM) in 1 cm quartz cuvettes (50 µM) at 298 K. The absolute PL quantum yields (Φ PL ) of indium complexes 1-6 in THF solution were obtained using a Horiba Fluoromax-4P spectrophotometer equipped with a 3.2 inch integrating sphere (HORIBA, Edison, NJ, USA) at 298 K. The fluorescence decay lifetimes (τ) were measured using a FLS920 fluorescence spectrophotometer (Edinburgh Instruments, Livingston, UK) in time-correlated single-photon-counting (TCSPC) mode with a picosecond pulsed diode laser (EPL 375-ps) as a light source and a microchannel plate photomultiplier tube (MCP-PMT, 200-850 nm) as a detector at room temperature.

Synthesis and Characterization
Scheme 1 shows the routes for the synthesis of dimeric quinoline-based indium complexes 1-6, which were easily produced in moderate yields (58-67%) by the reaction of 1.1 equivalent of the corresponding quinolines (1a-6a) with InMe 3 in toluene at room temperature. Based on previously reported results, all the complexes were expected to exist as dimeric species in solution [20]. All the complexes were found to possess good solubility in common organic solvents. The formation of 1-6 was confirmed by 1 H and 13 C{ 1 H} NMR spectroscopy (Figures S1-S6) and elemental analysis. In particular, specific singlet signals assignable to the In-Me bonds were clearly observed in both the 1 H (ca. 0.1 ppm) and 13   This compound was prepared in a manner analogous to the synthesis of 4 using 6a (0.13 g, 0.55 mmol). The desired compound 6 was obtained as a yellow solid (0.13 g, 67%). 1

Cyclic Voltammetry
The cyclic voltammetry (CV) measurements were performed in a deoxygenized MeCN (0.5 mM) solution with a three-electrode cell configuration (platinum working and counter electrodes and an Ag/AgNO3 reference electrode (0.1 M in MeCN)) using an AU-TOLAB/PGSTAT12 system at room temperature. Tetra-n-butylammonium hexafluorophosphate (n-Bu4PF6) in MeCN (0.1 M) was used as the supporting electrolyte. The redox potentials were investigated at a scan rate of 100 mV/s and determined with respect to the ferrocene/ferrocenium (Fc/Fc + ) redox couple.

Photophysical Properties
The samples for the UV-vis absorption and photoluminescence (PL) measurements were prepared using degassed solvents (p-xylene, THF, and DCM) in 1 cm quartz cuvettes (50 μM) at 298 K. The absolute PL quantum yields (ΦPL) of indium complexes 1-6 in THF solution were obtained using a Horiba Fluoromax-4P spectrophotometer equipped with a 3.2 inch integrating sphere (HORIBA, Edison, NJ, USA) at 298 K. The fluorescence decay lifetimes (τ) were measured using a FLS920 fluorescence spectrophotometer (Edinburgh Instruments, Livingston, UK) in time-correlated single-photon-counting (TCSPC) mode with a picosecond pulsed diode laser (EPL 375-ps) as a light source and a microchannel plate photomultiplier tube (MCP-PMT, 200-850 nm) as a detector at room temperature.

Synthesis and Characterization
Scheme 1 shows the routes for the synthesis of dimeric quinoline-based indium complexes 1-6, which were easily produced in moderate yields (58-67%) by the reaction of 1.1 equivalent of the corresponding quinolines (1a-6a) with InMe3 in toluene at room temperature. Based on previously reported results, all the complexes were expected to exist as dimeric species in solution [20]. All the complexes were found to possess good solubility in common organic solvents. The formation of 1-6 was confirmed by 1 H and 13 C{ 1 H} NMR spectroscopy (Figures S1-S6) and elemental analysis. In particular, specific singlet signals assignable to the In-Me bonds were clearly observed in both the 1 H (ca. 0.1 ppm) and 13 C( 1 H) NMR (ca. −5.0 ppm) spectra of all the indium complexes. Scheme 1. The synthetic routes for producing a series of dimeric indium quinolinate complexes. Scheme 1. The synthetic routes for producing a series of dimeric indium quinolinate complexes.

Photophysical and Electrochemical Properties
To examine the photophysical properties of the dimeric indium complexes, UV−vis absorption and PL experiments were performed ( Figure 2  (50 µM) solution at 298 K. All the complexes 1-6 exhibited typical low-energy absorption bands in the range of 380 to 406 nm. The absorption bands can be ascribed to the quinolinol-centered π−π* charge transfer (CT) transition. The absorption maximum (λ abs ) of these complexes gradually redshifted on increasing the electron-donating ability of the substituents at the C5 position of the quinolinate groups. The emission spectra of 1-6 displayed broad peaks in the range of 507 (green) to 523 (yellow) nm in THF, corresponding to a typical CT transition. The emission bands featured a gradual redshift phenomenon with an increase in the electron-donating effect of the substituents at the C5 position of the quinoline group ( Figure 2 and Table 1). These results are not well-matched with the Hammett σ constants [30]. However, the observation indicated that the introduction of substituents with a high electron-donating effect at the C5 position of the quinolinolate group caused an increase in the HOMO energy levels in all the indium complexes. Furthermore, the emission maxima (λ em ) of 1-6 exhibited slight bathochromic shifts in response to an increase in solvent polarity (p-xylene < THF < DCM) ( Table 1; Figures S7 and S8). Such emission behavior indicated that all the dimeric indium complexes possessed polarized excited states. The solvatochromic nature of the broad emission bands confirmed that the PL spectra of compounds 1-6 correspond to the quinoline-based intramolecular charge transfer (ICT) transitions. The PL spectra of the compounds in the film (10 wt% doped with PMMA) displayed trends similar to those in the THF solution ( Figure S9). The emission lifetime (τ) of 1-6 was measured to be in the range of nanoseconds in both the THF solution and the film state, indicating fluorescence (Table 1; Figures S10-S13).
Based on the electrochemical data obtained from CV measurements in MeCN, 1-6 showed totally irreversible oxidation processes ( Figure 3 and Table 1). The HOMO energy levels and bandgaps (E g ) of all the complexes were calculated using the measured onset oxidation potentials with the absorption edges (λ abs , edge ). Contrary to the expectation, the calculated HOMO levels were found to decrease when the electron-donating effect of substituents at the C5 position of the quinoline groups increased. However, the calculated E g values gradually decreased, which is consistent with the photophysical results.   3 Measured in the film state (10 wt% doped on poly(methy methacrylate), PMMA) at 298 K. 4 Absolute PL quantum yields. 5 kr = Φem/τ. 6 knr = kr(1/Φem−1). 7 Oxidation onset potentials in DMSO (c = 50 mM, scan rate 100 mV s −1 ) with reference to the ferrocene/ferrocenium (Fc/Fc + ) redox couple. 8 Highest occupied molecular orbital (HOMO) energy level calculated from Vox. 9 Calculated from λabs,edge.
Based on the electrochemical data obtained from CV measurements in MeCN, 1-6 showed totally irreversible oxidation processes ( Figure 3 and Table 1). The HOMO energy levels and bandgaps (Eg) of all the complexes were calculated using the measured onset oxidation potentials with the absorption edges (λabs,edge). Contrary to the expectation, the calculated HOMO levels were found to decrease when the electron-donating effect of substituents at the C5 position of the quinoline groups increased. However, the calculated Eg values gradually decreased, which is consistent with the photophysical results.

Conclusions
In summary, we prepared a new series of quinolinol-based dimeric indium complexes (1-6) that exhibited low-energy absorption bands assignable to quinolinol-centered π-π* CT transition. The emission spectra of 1-6 exhibited a gradual redshift as the electron-donating effect of substituents at the C5 position of the quinoline groups increased. The quantum efficiencies of 1 and 4, which had methyl substituents at the C5 position of the quinoline groups (q or Meq), were higher than those of the indium complexes with other substituents (Br and Ph). Consequently, these results provide a new perspective on the development of quinolinol-based dimeric indium complexes as potential organometallic luminophores. Further studies are underway to develop quinolinol-based indium complexes with improved quantum efficiencies for application as efficient luminescent materials.

Conclusions
In summary, we prepared a new series of quinolinol-based dimeric indium complexes (1-6) that exhibited low-energy absorption bands assignable to quinolinol-centered π-π* CT transition. The emission spectra of 1-6 exhibited a gradual redshift as the electrondonating effect of substituents at the C5 position of the quinoline groups increased. The quantum efficiencies of 1 and 4, which had methyl substituents at the C5 position of the quinoline groups (q or Meq), were higher than those of the indium complexes with other substituents (Br and Ph). Consequently, these results provide a new perspective on the development of quinolinol-based dimeric indium complexes as potential organometallic luminophores. Further studies are underway to develop quinolinol-based indium complexes with improved quantum efficiencies for application as efficient luminescent materials.