Tunable Rh2(II,II) Light Absorbers as Excited-State Electron Donors and Acceptors Accessible with Red/Near-Infrared Irradiation.

A series of dirhodium(II,II) paddlewheeel complexes of the type cis-[Rh2(µ-DTolF)2(µ-L)2][BF4]2, where DTolF = N,N'-di( p-tolyl)formamidinate and L = 1,8-naphthyridine (np), 2-(pyridin-2-yl)-1,8-naphthyridine (pynp), 2-(quinolin-2-yl)-1,8-naphthyridine (qnnp), and 2-(1,8-naphthyridin-2-yl)quinoxaline (qxnp), were synthesized and characterized. These molecules feature new tridentate ligands that concomitantly bridge the dirhodium core and cap the axial positions. The complexes absorb light strongly throughout the ultraviolet/visible range and into the near-infrared region and exhibit relatively long-lived triplet excited-state lifetimes. Both the singlet and triplet excited states exhibit metal/ligand-to-ligand charge transfer (ML-LCT) in nature as determined by transient absorption spectroscopy and spectroelectrochemistry measurements. When irradiated with low-energy light, these black dyes are capable of undergoing reversible bimolecular electron transfer both to the electron acceptor methyl viologen and from the electron donor p-phenylenediamine. Photoinduced charge transfer in the latter was inaccessible with previous Rh2(II,II) complexes. These results underscore the fact that the excited state of this class of molecules can be readily tuned for electron-transfer reactions upon simple synthetic modification and highlight their potential as excellent candidates for p- and n-type semiconductor applications and for improved harvesting of low-energy light to drive useful photochemical reactions.

New Rh2(II,II) Complexes for Solar Energy Applications: Panchromatic Absorption and Excited-State Reactivity.

The new heteroleptic paddlewheel complexes cis-[Rh2(µ-form)2(µ-np)2][BF4]2, where form = p-ditolylformamidinate (DTolF) or p-difluorobenzylformamidinate (F-form) and np = 1,8-napthyridyine, and cis-Rh2(µ-form)2(µ-npCOO)2 (npCOO- = 1,8-naphthyridine-2-carboxylate), were synthesized and characterized. The complexes absorb strongly throughout the ultraviolet (λmax = 300 nm, ε = 20 300 M-1 cm-1) and visible regions (λmax = 640 nm ε = 3500 M-1 cm-1), making them potentially useful new dyes with panchromatic light absorption for solar energy conversion applications. Ultrafast and nanosecond transient absorption and time-resolved infrared spectroscopies were used to characterize the identity and dynamics of the excited states, where singlet and triplet Rh2/form-to-naphthyridine, metal/ligand-to-ligand charge-transfer (ML-LCT) excited states were observed in all four complexes. The npCOO- complexes exhibit red-shifted absorption profiles extending into the near-IR and undergo photoinitiated electron transfer to generate reduced methyl viologen, a species that persists in the presence of a sacrificial donor. The energy of the triplet excited state of each complex was estimated from energy-transfer quenching experiments using a series of organic triplet donors (E(3ππ*) from 1.83 to 0.78 eV). The singlet reduction (+0.6 V vs Ag/AgCl) potentials, and singlet and triplet oxidation potentials (-1.1 and -0.5 V vs Ag/AgCl, respectively) were determined. Based on the excited-state lifetimes and redox properties, these complexes represent a new class of light absorbers with potential application as dyes for charge injection into semiconductor solar cells and in sensitizer-catalyst assemblies for photocatalysis that operate with irradiation from the ultraviolet to ∼800 nm.

Electron injection into titanium dioxide by panchromatic dirhodium photosensitizers with low energy red light.

Two new Rh2(ii,ii) dyes were synthesized and anchored to TiO2 for charge injection upon irradiation. The 1ML-LCT (metal/ligand-to-ligand charge transfer) excited state is populated upon excitation, which decays to the corresponding 3ML-LCT state. Ultrafast electron injection into TiO2 from the Rh2(ii,ii) dyes was achieved with low energy, red light excitation.

Photocatalytic H2 production by dirhodium(ii,ii) photosensitizers with red light.

Photocatalytic proton reduction to generate H2 was achieved with the photosensitizers Rh2(DTolF)2(npCOO)2 (DTolF = p-ditolylformamidinate; npCOO- = 2-carboxylate-1,8-naphthyridine; 1) and [Rh2(DTolF)2(qnnp)2][BF4]2 (qnnp = 2-(quinolin-2-yl)-1,8-naphthyridine; 2) using a relay system containing the sacrificial donor BNAH (1-benzyl-1,4-dihydronicotinamide), electron acceptor MV2+ (methylviologen), and Pt nanoparticles as the catalyst with 655 nm irradiation. Comparison of the H2 evolution under similar experimental conditions show comparable activity of the Rh2(ii,ii) complexes (λirr = 655 nm) to that of the prototypical [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine; 3) with λirr = 447 nm. This work demonstrates the ability of the new panchromatic Rh2(ii,ii) complexes to achieve photocatalysis with red light.