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

Breivogel, Aaron,Park, Myeongjin,Lee, Donggu,Klassen, Stefanie,Kü,hnle, Angelika,Lee, Changhee,Char, Kookheon,Heinze, Katja Wiley-VCH 2014 European journal of inorganic chemistry Vol. No.
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AbstractLight‐emitting electrochemical cells (LECs) with a simple device structure were prepared by using heteroleptic bis(tridentate) ruthenium(II) complexes [1](PF6)2–[3](PF6)2 as emitters. The push‐pull substitution shifts the emission energy to low energy, into the NIR region. The devices emit deep red light up to a maximum emission wavelength of 755 nm [CIE (International Commission on Illumination) coordinates: x = 0.731, y = 0.269 for [3](PF6)2], which, to the best of our knowledge, is the lowest emission energy for LECs containing bis(tridentate) ruthenium(II) complexes. A device structure of ITO/PEDOT:PSS/ruthenium(II) complex/Ag was used, and the thickness of the emitting layer was measured by AFM [ITO: indium tin oxide, PEDOT: poly(3,4‐ethylenedioxythiophene), PSS: poly(styrenesulfonate), AFM: atomic force microscopy]. To enhance the external quantum efficiency (EQE), cells were fabricated with and without poly(methyl methacrylate) (PMMA) as additive in the emitting layer.
2
Anchor‐Functionalized Push‐Pull‐Substituted Bis(tridentate) Ruthenium(II) Polypyridine Chromophores: Photostability and Evaluation as Photosensitizers

Breivogel, Aaron,Wooh, Sanghyuk,Dietrich, Jan,Kim, Tea Yon,Kang, Yong Soo,Char, Kookheon,Heinze, Katja Wiley-VCH 2014 European journal of inorganic chemistry Vol. No.
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AbstractStable push‐pull substituted heteroleptic bis(tridentate) ruthenium(II) polypyridine complexes with COOH or 2,2′‐bipyridine anchor groups have been prepared and characterized by 1H, 13C and 15N NMR 1D and 2D spectroscopy, infrared spectroscopy, elemental analysis, high‐resolution ESI mass spectrometry, electrochemistry, UV/Vis absorption spectroscopy, luminescence spectroscopy, and density functional calculations. The complexes feature a pronounced electronic directionality and high absorption wavelengths up to λmax = 544 nm extending to 720 nm as a result of favorable push‐pull substitutions. A remarkable photostability in the presence of water and coordinating ions (I–) was discovered for the tridentate complexes when compared with the standard ruthenium sensitizer N719 and tris(bidentate) [Ru(bpy)3](PF6)2, which are highly photolabile under the same conditions (photodissociation/photosubstitution). The complexes were studied as photosensitizers in dye‐sensitized solar cells. The incident photon‐to‐current conversion efficiency follows the absorption spectra into the NIR region. However, the high positive charge of the complexes (2+) favors the recombination of the injected electrons with I3– of the redox electrolyte, which is evidenced by high dark currents and short electron recombination lifetimes, leading to low cell performances compared with cells with the negatively charged N719 dye.
3
A Bis(tridentate)cobalt Polypyridine Complex as Mediator in Dye‐Sensitized Solar Cells

Mengel, Andreas K. C.,Cho, Woohyung,Breivogel, Aaron,Char, Kookheon,Soo Kang, Yong,Heinze, Katja WILEY‐VCH Verlag 2015 European journal of inorganic chemistry Vol.2015 No.20
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AbstractDye‐sensitized solar cells equipped with cationic and neutral RuII‐based sensitizers [Ru(ddpd){tpy(COOH)3}]2+ [12+; ddpd = N,N′‐dimethyl‐N,N′‐di(pyridin‐2‐yl)pyridin‐2,6‐diamine, tpy(COOH)3 = 2,2″6′,2″‐terpyridine‐4,4′,4″‐tricarboxylic acid] and [Ru(ddpd){tpy(COOH)(COO)2}] (2) with and without the coadsorbent chenodeoxycholic acid were constructed with I3–/I– or the CoIII/II‐based redox mediators [Co(bpy)3]3+/2+ (33+/2+; bpy = 2,2′‐bipyridine) and [Co(ddpd)2]3+/2+ (43+/2+) in the presence of LiClO4 and 4‐tert‐butylpyridine. The best photovoltaic performance was achieved by using the 43+/2+ shuttle and the neutral sensitizer 2 without coadsorbent. The higher short‐circuit photocurrent density and higher electron recombination lifetimes obtained with this combination suggest slow electron recombination kinetics at the TiO2 surface with the CoIII complex 43+. The slow electron transfer to 43+ is tentatively ascribed to the high‐lying π* orbitals of the electron‐rich ddpd ligands, which result in a weak electronic coupling. This contrasts with the faster recombination with 33+, which features the low‐energy π* orbitals of the bpy ligands.

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