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We have studied the interfacial electron‐transfer dynamics on TiO2film sensitized with synthesized ruthenium(II)–polypyridyl complexes—[RuII(bpy)2(L1)] (1) and [RuII(bpy)(L1)(L2)] (2), in which bpy=2,2′‐bipyridyl, L1=4‐[2‐(4′‐methyl‐2,2′‐bipyridinyl‐4‐yl)vinyl]benzene‐1,2‐diol, and L2=4‐(N,N‐dimethylaminophenyl)‐2,2′‐bipyridine—by using femtosecond transient absorption spectroscopy. The presence of electron‐donor L2and electron‐acceptor L1ligands in complex 2introduces lower energetic ligand‐to‐ligand charge‐transfer (LLCT) excited states in addition to metal‐to‐ligand (ML) CT manifolds of complex 2. On photoexcitation, a pulse‐width‐limited (<100 fs) electron injection from populating LLCT and MLCT states are observed on account of strong catecholate binding on the TiO2surface. The hole is transferred directly or stepwise to the electron‐donor ligand (L2) as a consequence of electron injection from LLCT and MLCT states, respectively. This results an increased spatial charge separation between the hole residing at the electron‐donor (L2) ligand and the electron injected in TiO2nanoparticles (NPs). Thus, we observed a significant slow back‐electron‐transfer (BET) process in the 2/TiO2system relative to the 1/TiO2system. Our results suggest that RuII–polypyridyl complexes comprising LLCT states can be a better photosensitizer for improved electron injection yield and slow BET processes in comparison with RuII–polypyridyl complexes comprising MLCT states only.

We have synthesized ruthenium(II)– and osmium(II)–polypyridyl complexes ([M(bpy)2L]2+, in which M=OsIIor RuII, bpy=2,2′‐bipyridyl, and L=4‐(2,2′‐bipyridinyl‐4‐yl)benzene‐1,2‐diol) and studied the interfacial electron‐transfer process on a TiO2nanoparticle surface using femtosecond transient‐absorption spectroscopy. Ruthenium(II)‐ and osmium(II)‐based dyes have a similar molecular structure; nevertheless, we have observed quite different interfacial electron‐transfer dynamics (both forward and backward). In the case of the RuII/TiO2system, single‐exponential electron injection takes place from photoexcited nonthermalized metal‐to‐ligand charge transfer (MLCT) states. However, in the case of the OsII/TiO2system, electron injection takes place biexponentially from both nonthermalized and thermalized MLCT states (mainly 3MLCT states). Larger spin–orbit coupling for the heavier transition‐metal osmium, relative to that of ruthenium, accounts for the more efficient population of the 3MLCT states in the OsII‐based dye during the electron‐injection process that yields biexponential dynamics. Our results tend to suggest that appropriately designed OsII–polypyridyl dye can be a better sensitizer molecule relative to its RuIIanalogue not only due to much broader absorption in the visible region of the solar‐emission spectrum, but also on account of slower charge recombination.

The synthesis and characterization of the first examples of core‐modified corrole dimers linked through the mesopositions are described. The dimers are obtained by a simple AgI‐ or FeIII‐catalyzed coupling reaction in near‐quantitative yields. The corroles obtained are metalated with metal ions, such as CuIIand NiII. The electronic‐absorption spectral studies reveal weak electronic interaction between the two subunits and the exciton coupling observed for the free‐base corrole dimer (1717 cm−1) is lower than that for the corresponding protonated derivative (4081 cm−1). The solution‐state structure derived from 1H and two‐dimensional NMR spectral studies reveals a noncoplanar arrangement of two corrole units. Geometry optimization at the B3LYP‐631G level also confirms the noncoplanar arrangement of corroles with a dihedral angle of 64.8° between two corrole planes. The electron paramagnetic resonance (EPR) and magnetic characterization studies on the paramagnetic copper‐metalated dimer indicate that both copper ions behave as independent spins without any noticeable interaction. Results of fluorescence studies reveal a bathochromic shift of about 60 nm upon dimerization. The first hyperpolarizability (β) measured by using the hyper Rayleigh scattering (HRS) method reveals doubling of the βvalues on progressing from monomer to dimer, attributed to enhanced π conjugation. The use of copper dimers in the photocleavage of DNA is also explored. It is shown that the bimetallic copper dimer selectively cleaves the nucleic acids without affecting the proteins, suggesting a possible application of the copper complex in the removal of nucleic acid contaminants from protein extracts through a simple photolytic pathway.