Cyclometalated Ru Complexes of Type [Ru^II(N^∧N)₂(C^∧N)]^z: Physicochemical Response to Substituents Installed on the Anionic Ligand

Abstract

The electrochemical and photophysical properties of a series of Ru(II) complexes related to [Ru(dcbpyH(2))(2)(ppy)](1+) (1; dcbpyH(2) = 4,4'-dicarboxy-2,2'-bipyridine; ppy = 2-phenylpyridine) were examined to elucidate the effect of modifying the anionic fragment of the C--N ligand with conjugated substituents (R). Included in this study is a family of compounds (2-5) consisting of one or two -NO(2) groups installed meta, ortho, and para to the organometallic bond. A suite of compounds with electron-donating and withdrawing groups (e.g., R = -F (6), -phenyl (7), -4-pyridine (8), -thiophene-2-carbaldehyde (9)) were also evaluated. Deprotonated forms of select compounds were isolated as tetrabutylammonium salts to benefit solution studies. All complexes were structurally characterized by a combination of mass spectrometry, (1)H and (13)C NMR spectroscopy, and/or elemental analysis. The electronic absorption spectra for all of the compounds reveal three broad bands over the 350-700 nm range. The maximum wavelength of the lowest energy absorbance bands for complexes modified with electron-withdrawing groups are hypsochromically shifted up to 45 nm relative to 1; the weakly emitting compounds (i.e., 1, 3, 6-9) display a hypsochromic shift of up to 63 nm compared to 1. Emission was not observed in cases where the -NO(2) group was positioned meta to the Ru-C bond. The sensitivity of the oxidation potentials to the nature, number, and position of the electron-withdrawing/-donating substituents for the entire set of compounds reflect a highest occupied molecular orbital (HOMO) character extended over the metal, the anionic portion of the C--N ligand, and, in the case of 7-9, the conjugated R group. The reduction potentials indicate that the lowest unoccupied molecular orbital (LUMO) is localized to the C--N ligand where R = -NO(2), and on the dcbpyH(2) ligands for all other compounds. This assessment was corroborated by time-dependent density functional theory (TD-DFT) studies.

Strategies for Optimizing the Performance of Cyclometalated Ruthenium Sensitizers for Dye‐Sensitized Solar Cells

“…[40,42] The orbital character of the low-lying excited states resides on the π* framework of the N ∧ N ligands (e.g., anchoring dcbpy ligands); the π*-system of the C ∧ N ligand often lies higher in energy by ca. 1.2 eV.…”

“…Unlike N3, however, C ∧ N ligands provide the opportunity for further chemistry because the HOMO level can be easily modulated through the judicious installation of terminal substituents. [42] This feature provides a significant amount of leverage for optimizing the numerous factors that must be in alignment for efficient sensitization (e.g., appropriate ground-and excited-state redox properties, high molar extinction coefficients, appropriate electron-transfer kinetics). [13,43] A strategy that has proven to be effective for conventional dyes is the use of a tris-heteroleptic ligand environment; indeed, the significant majority of champion dyes are variations of N3, where one dcbpy ligand has been replaced with a bpy ligand bearing bulky, conjugated substituents.…”

Luminescent Cyclometalated Dialkynylgold(III) Complexes of 2‐Phenylpyridine‐Type Derivatives with Readily Tunable Emission Properties

“…Despite this, the use of ppy-type phosphors containing alternative metal centers remains relatively less extensively explored and rather underdeveloped, with only some reports on ppy derivatives of platinum(II) [3] and ruthenium(II). [4] In contrast to the isoelectronic platinum(II) compounds that are known to show rich luminescence properties, [5] very few examples of luminescent goldA C H T U N G T R E N N U N G (III) complexes have been reported, [6] which probably stems from the presence of lowenergy d-d ligand field (LF) states and the electrophilicity observed for the goldA C H T U N G T R E N N U N G (III) metal center.…”

A Trisheteroleptic Cyclometalated RuII Sensitizer that Enables High Power Output in a Dye‐Sensitized Solar Cell

“…[2] Chemical strategies for avoiding the labile RuÀNCS bond have been realized recently; [10,11] indeed, we [12] and others [13,14] have 1+ (ppy = 2-phenylpyridine) provide a versatile platform in this respect because: 1) the highest occupied molecular orbital (HOMO) is extended over the metal and anionic ring thus enabling its modulation through judicious installation of substituents at the À R 2 site in Scheme 1 b; [15] and 2) the low-lying excited states, which contain orbital character that resides on the p* framework of the dcbpy ligand(s), are poised for electron injection into the TiO 2 . [10,11,[15][16][17][18] This scenario leaves open the opportunity to replace one dcbpy with a bidentate ligand capable of suppressing recombination and enhancing the optical properties as per the aforementioned protocol (Scheme 1). [2,19] While we recently demonstrated synthetic access to trisheteroleptic Ru sensitizers (e.g., 1 and 2; Scheme 2), [20] we learned that removing the acid linkers raises the HOMO level of the sensitizer to potentially compromise dye regeneration.…”