The low embodied energy and high power-conversion efficiency (h) over disparate light intensities renders the dyesensitized solar cell (DSSC) [1,2] a promising alternative to conventional photovoltaic technologies.[3] Significant penetration of the DSSC into the photovoltaic market, however, is hindered predominantly by the long-term stability of dyes and electrolytes under practical conditions. [4][5][6] The instability of champion (i.e., h > 10 %) dyes (which, until recently, [7] all were derivatives of [Ru(dcbpy) 2 (NCS) 2 ] (N3; dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) [2] ) in the DSSC is caused primarily by desorption of the dyes from the surface and/or liberation of the NCS À ligands from the metal centre. [5,6] While the rate of dye desorption from TiO 2 can be manipulated by replacing the À CO 2 H moiety with other anchoring groups, this strategy typically compromises electron injection into the TiO 2 .[8] An alternative approach is to replace the dcbpy ligands that comprise N3 with bidentate ligands bearing aliphatic substituents (e.g., Scheme 1 a), which serve to hinder water from reaching the surface to hydrolytically cleave the TiO 2 -dye ester linkage.[9] These groups provide the additional benefit of suppressing recombination between the electrolyte and the electrons in TiO 2 , thus leading to higher efficiencies (Scheme 1 a). [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. (The HOMO level of the sensitizer must lie lower in energy than the I À /I 3 À redox couple that resides at approximately + 0.5 V vs. normal hydrogen electrode (NHE).[21] Although the HOMO of 1 lies at + 0.70 V vs. NHE and therefore meets this criterion, [20] champion Ru-based sensitizers all have oxidation potentials higher than ca. + 0.9 V.[2] ) We therefore set out to overcome this potential shortcoming by introducing strongly electronwithdrawing ÀCF 3 substituents to the cyclometalating ligand to accommodate efficient dye regeneration. These design elements led to the preparation of 3-a Ru II complex devoid Sch...

Examination of the aqueous electrochemistry of a Co(II) complex bearing a pentadentate ligand suggests that the catalytic current corresponding to water oxidation is molecular in origin, and does not emanate exclusively from Co-oxide phases formed in situ.

On the basis of a time-dependent self-consistent density functional tight-binding (TD-DFTB) approach, we present a novel method able to capture the differences between direct and indirect photoinjection mechanisms in a fully atomistic picture. A model anatase TiO2 nanoparticle (NP) functionalized with different dyes has been chosen as the object of study. We show that a linear dependence of the rate of electron injection with respect to the square of the applied field intensity can be viewed as a signature of a direct electron injection mechanism. In addition, we show that the nature of the photoabsorption process can be understood in terms of orbital population dynamics occurring during photoabsorption. Dyes involved in both direct (type-I) and indirect (type-II) mechanisms were studied to test the predictive power of this method.

Characterization of the redox properties of TiO2 interfaces sensitized to visible light by a series of cyclometalated ruthenium polypyridyl compounds containing both a terpyridyl ligand with three carboxylic acid/carboxylate or methyl ester groups for surface binding and a tridentate cyclometalated ligand with a conjugated triarylamine (NAr3) donor group is described. Spectroelectrochemical studies revealed non-Nernstian behavior with nonideality factors of 1.37 ± 0.08 for the Ru(III/II) couple and 1.15 ± 0.09 for the NAr3(•+/0) couple. Pulsed light excitation of the sensitized thin films resulted in rapid excited-state injection (k(inj) > 10(8) s(-1)) and in some cases hole transfer to NAr3 [TiO2(e(-))/Ru(III)-NAr3 → TiO2(e(-))/Ru(II)-NAr3(•+)]. The rate constants for charge recombination [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3 or TiO2(e(-))/Ru(II)-NAr3(•+) → TiO2/Ru(II)-NAr3] were insensitive to the identity of the cyclometalated compound, while the open-circuit photovoltage was significantly larger for the compound with the highest quantum yield for hole transfer, behavior attributed to a larger dipole moment change (Δμ = 7.7 D). Visible-light excitation under conditions where the Ru(III) centers were oxidized resulted in injection into TiO2 [TiO2/Ru(III)-NAr3 + hν → TiO2(e(-))/Ru(III)-NAr3(•+)] followed by rapid back interfacial electron transfer to another oxidized compound that had not undergone excited-state injection [TiO2(e(-))/Ru(III)-NAr3 → TiO2/Ru(II)-NAr3]. The net effect was the photogeneration of equal numbers of fully reduced and fully oxidized compounds. Lateral intermolecular hole hopping (TiO2/Ru(II)-NAr3 + TiO2/Ru(III)-NAr3(•+) → 2TiO2/Ru(III)-NAr3) was observed spectroscopically and was modeled by Monte Carlo simulations that revealed an effective hole hopping rate of (130 ns)(-1).

The syntheses and physicochemical properties of nine bis-tridentate ruthenium(II) complexes containing one cyclometalating ligand furnished with terminal triphenylamine (TPA) substituents are reported. The structure of each complex conforms to a molecular scaffold formulated as [Ru(II)(TPA-2,5-thiophene-pbpy)(Me(3)tctpy)] (pbpy = 6-phenyl-2,2'-bipyridine; Me(3)tctpy = trimethyl-4,4',4''-tricarboxylate-2,2':6',2''-terpyridine), where various electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) are installed about the TPA unit and the anionic ring of the pbpy ligand. It is found that the redox chemistry of the Ru center and the TPA unit can be independently modulated by (i) placing EWGs (e.g., -CF(3)) or EDGs (e.g., -OMe) on the anionic ring of the pbpy ligand (substituted sites denoted as R(2) or R(3)) and/or (ii) installing electron-donating substituents (e.g., -H, -Me, -OMe) para to the amine of the TPA group (i.e., R(1)). The first oxidation potential is localized to the TPA unit when, for example, EDGs are placed at R(1) with EWGs at R(2) (e.g., the TPA(•+)/TPA(0) and Ru(III)/Ru(II) redox couples appear at +0.98 and +1.27 V vs NHE, respectively, when R(1) = -OMe and R(2) = -CF(3)). This situation is reversed when R(3) = EDG and R(1) = -H: TPA-based and metal-centered oxidation waves occur at +1.20 and +1.11 V vs NHE, respectively. The UV-vis spectrum for each complex is broad (e.g., absorption bands are extended from the UV region to beyond 800 nm in all cases) and intense (e.g., ε ∼ 10(4) M(-1)·cm(-1)) because of the overlapping intraligand charge-transfer and metal-to-ligand charge-transfer transitions. The information derived from this study offers guiding principles for modulating the physicochemical properties of bichromic cyclometalated ruthenium(II) complexes.