Effect of free rotation in polypyridinic ligands of Ru(ii) complexes applied in light-emitting electrochemical cells

Abstract

In the present work we report the synthesis and the electrochemical, photoluminescent and electroluminescent properties of two new Ru(II) complexes described by the general formula [Ru(phen)2X](2+), where phen is 1,10-phenanthroline. The X ligand consists of a 2,2'-bipyridine (bpy) unit substituted with two phenyl rings connected to the bpy core through a saturated (Lhydro = 4,4'-diphenylethyl-2,2'-bipyridine) or a conjugated (LH = 4,4'-bis(α-styrene)-2,2'-bipyridine) carbon-carbon bridge. The photoluminescent spectra indicate that, both in solution and solid state, the complex bearing the aliphatic substitution bridges exhibits a higher quantum yield and a longer excited state lifetime than the fully conjugated complex. The new complexes were used in light-emitting electrochemical cells (LECs) showing red emission for the complex with the Lhydro ligand and no light emission for the complex incorporating the LH ligand. This and the photophysical properties make it plausible that for these complexes the degree of freedom increases with aliphatic substitution. As a consequence, the negative effect of the auto-quenching processes taking place in solid LEC devices due to the close molecular packing is limited. When compared with the archetype [Ru(phen)3](2+) complex, the complex with aliphatic substitution shows better performance in the device supporting the beneficial effect of the bulky substitution.

Push‐Pull Design of Bis(tridentate) Ruthenium(II) Polypyridine Chromophores as Deep Red Light Emitters in Light‐Emitting Electrochemical Cells

“…[78][79][80] Self-quenching can be avoided by using sterically demanding substituents. [81,82] As a result of the high local concentration (neat or 80 w/w %), high current densities (up to 518 mA cm -2 ), and high solution luminescence lifetimes (up to 841 ns), saturation and quenching effects might also be present in our LECs. Indeed, dilution with PMMA has a positive effect on the EQE.…”

Redox and Photochemistry of Bis(terpyridine)ruthenium(II) Amino Acids and Their Amide Conjugates – from Understanding to Applications

“…Thus, the deprotonated amino group is considerably engaged in the oxidation process, which renders the oxidation irreversible. [56,59] The EPR spectrum of the N-acetylated analogue [6] + ( Figure 2) prepared by reduction of [6] 2+ with decamethylcobaltocene shows a rhombic signal with g 1,2,3 = 2.005, 1.989, 1.955 and small g anisotropy in frozen solution. In aprotic acetonitrile the fully reversible simple one-electron Ru II /Ru III oxidation of amino acid ester [4] 2+ is observed at E 1/2 = 0.68 V vs. ferrocene/ferrocenium.…”

“…[1] There is a plethora of applications of polypyridine complexes of ruthenium(II) such as dye-sensitized solar cells (DSSCs), [2][3][4] light-emitting devices, [5][6][7][8] anticancer and photodynamic therapy, [9][10][11] sensing of ions [12][13][14] and small neutral molecules, [15][16][17] energy transfer, [18,19] electron transfer and mixed valency, [20][21][22][23][24][25] triplet-triplet annihilation upconversion, [26][27][28][29] and molecular data storage. [1] There is a plethora of applications of polypyridine complexes of ruthenium(II) such as dye-sensitized solar cells (DSSCs), [2][3][4] light-emitting devices, [5][6][7][8] anticancer and photodynamic therapy, [9][10][11] sensing of ions [12][13][14] and small neutral molecules, [15][16][17] energy transfer, …”

“…[53] However, despite the similar absorption characteristics and redox potentials of [Ru(bpy) 3 ] 2+ and [1] 2+ , the excited state properties differ significantly (Table 1). [55] Long-lived and highly emissive [Ru(bpy) 3 ] 2+ [46,57] 452 (13000) 615 9.4 1100 +0.89 -1.73 [1] 2+ [50,54b] 474 (10400) 629 Յ 0.0007 [b] 0.1-0.2 [c] +0.92 -1.67 [2] 2+ [8] 485 ( [3] 2+ [58] 479 (19500) 637 [f] 0.02 [f] -+0.91 -1.69 [4] 2+ [59,60] 502 (19100) 734 0.27 [d] 34 +0.68 -1.54 [5] 2+ [59,60] 501 (20700) 739 0.18 [d] 26 (92 %), 4 (8 %) +0.66 -1.60 [6] 2+ [61,62] 492 (22100) 677 0.059 22 +0.90 -1.46 [7] 2+ [50] 490 (16800) 706 0.07 [b] 50 +0.92 -1.53 [8] 2+ [63] 490 (24000) 660 --+0.87 -1.58 [9] 2+ [64] 493 (29300) [g] 660 0.029 [h] 1.1 +0.63 -1.43 [10] 2+ [50] 487 (26200) 715 0.006 [b] 1.0 +0.90 -1.66 [11] 2+ [60] 502 (35600) 707 0.053 [d] 23 -- [12] 2+ [60] 501 (24700) 659 0.053 …”

ANOMALOUS BEHAVIOR OF Ir(III) CYCLOMETALATED COMPLEXES: APPLICATION IN LIGHT-EMITTING ELECTROCHEMICAL CELLS

“…This behavior has already been observed in other LEC devices and should be ascribed to a polarization effect due to electrical excitation. 32,35 The CIE (International Commission on Illumination) coordinates determined from the electroluminescent spectrum for complex 1 are (0.51, 0.48).…”