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A 1,2-bis(2-methylbenzothiophene-3-yl)maleimide model ( DAE) and two dyads in which this photochromic unit is coupled, via a direct nitrogen-carbon bond ( Ru-DAE) or through an intervening methylene group ( Ru-CH 2-DAE ), to a ruthenium polypyridine chromophore have been synthesized. The photochemistry and photophysics of these systems have been thoroughly characterized in acetonitrile by a combination of stationary and time-resolved (nano- and femtosecond) spectroscopic methods. The diarylethene model DAE undergoes photocyclization by excitation at 448 nm, with 35% photoconversion at stationary state. The quantum yield increases from 0.22 to 0.33 upon deaeration. Photochemical cycloreversion (quantum yield, 0.51) can be carried out to completion upon excitation at lambda > 500 nm. Photocyclization takes place both from the excited singlet state (S 1), as an ultrafast (ca. 0.5 ps) process, and from the triplet state (T 1) in the microsecond time scale. In Ru-DAE and Ru-CH 2-DAE dyads, efficient photocyclization following light absorption by the ruthenium chromophore occurs with oxygen-sensitive quantum yield (0.44 and 0.22, in deaerated and aerated solution, respectively). The photoconversion efficiency is almost unitary (90%), much higher than for the photochromic DAE alone. Efficient quenching of both Ru-based MLCT phosphorescence and DAE fluorescence is observed. A complete kinetic characterization has been obtained by ps-ns time-resolved spectroscopy. Besides prompt photocyclization (0.5 ps), fast singlet energy transfer takes place from the excited diarylethene to the Ru(II) chromophore (30 ps in Ru-DAE, 150 ps in Ru-CH 2-DAE ). In the Ru(II) chromophore, prompt intersystem crossing to the MLCT triplet state is followed by triplet energy transfer to the diarylethene (1.5 ns in Ru-DAE, 40 ns in Ru-CH 2-DAE ). The triplet state of the diarylethene moiety undergoes cyclization in a microsecond time scale. The experimental results are complemented with a combined ab initio and DFT computational study whereby the potential energy surfaces (PES) for ground state (S 0) and lowest triplet state (T 1) of the diarylethene are investigated along the reaction coordinate for photocyclization/cycloreversion. At the DFT level of theory, the transition-state structures on S 0 and T 1 are similar and lean, along the reaction coordinate, toward the closed-ring form. At the transition-state geometry, the S 0 and T 1 PES are almost degenerate. Whereas on S 0 a large barrier (ca. 45 kcal mol (-1)) separates the open- and closed-ring minima, on T 1 the barriers to isomerization are modest, cyclization barrier (ca. 8 kcal mol (-1)) being smaller than cycloreversion barrier (ca. 14 kcal mol (-1)). These features account for the efficient sensitized photocyclization and inefficient sensitized cycloreversion observed with Ru-DAE. Triplet cyclization is viewed as a nonadiabatic process originating on T 1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S 0 at closed-ring geomet...

For the structurally rigid homometallic dinuclear complexes (ttp)Ru(tpy-tpy)Ru(ttp) 4+ and (ttp)Ru(tpy-phtpy)Ru(ttp) 4+ , we have obtained ground-state absorption spectra and transient-absorption difference spectra at room temperature and luminescence spectra and lifetimes in the temperature interval from room temperature to the rigid matrix (90 K); the solvent was acetonitrile or butyronitrile (tpy is 2,2′:6′,2′′-terpyridine, ttp is 4′-p-tolyl-2,2′:6′,2′′-tpy, and ph is 1,4-phenylene). The gathered spectroscopic data indicate that after absorption of visible light, formation of the luminescent metal-to-ligand charge transfer (MLCT) excited states takes place, which involves the bridging ligand (BL). Since we found that (ttp)Ru(tpy-tpy)Ru(ttp) 4+ is a good luminophore (λ max ) 720 nm, Φ ) 4.7 × 10 -3 , and τ ) 570 ns) while both (ttp)Ru(tpy-ph-tpy)Ru(ttp) 4+ (λ max ) 656 nm, Φ ) 1.1 × 10 -4 , and τ ) 4 ns) and the reference mononuclear complex Ru(ttp) 2 2+ (λ max ) 640 nm, Φ ) 3.2 × 10 -5 , and τ ) 0.9 ns) are not, we have explored the effects brought about by the delocalization and energy content of the luminescent state. The study of the temperature dependence of the luminescence lifetimes indicates that two main nonradiative paths, i and ii, are responsible for deactivation of the luminescent state. Path i directly connects the luminescent and ground states; within the frame of the "energy-gap law", vibronic analysis of low-temperature luminescence profiles enables one to correlate the delocalization of the M f BL CT state and the extent of structural distortions occurring at the accepting ligand. Thermally activated decay via a metal-centered (MC, of dd orbital origin) excited state characterizes path ii, with a MLCT-MC energy separation ∆E ) 3800, 2300, and 1600 cm -1 for (ttp)Ru(tpy-tpy)Ru-(ttp) 4+ , (ttp)Ru(tpy-ph-tpy)Ru(ttp) 4+ , and Ru(ttp) 2 2+ , respectively. At room temperature, for this limited series of complexes it is found that nonradiative processes governed by the "energy-gap law" play a minor role as compared to thermally activated processes, the ratios of the rate constants being k nr act /k nr dir ≈ 16, 1900, and 7000 for (ttp)Ru(tpy-tpy)Ru(ttp) 4+ , (ttp)Ru(tpy-ph-tpy)Ru(ttp) 4+ , and Ru(ttp) 2 2+ , respectively.

The rate constants for the quenching of excited Ru(bpy)32+ by methylviologen (MV2+) and Ru(NH3)5py3+ have been studied in aqueous solution as a function of the concentration (0.01-1 M) and type (NaCl, NaC104, CaCl2) of added electrolyte. With MV2+ as quencher, the yield of products escaping cage recombination and the rate constant of their back electron transfer reaction have also been studied as a function of the concentration of added NaCl. The results have been compared with predictions based on expressions available in the literature for the ionic strength dependence of diffusional parameters kd and k-¿. With uni-univalent electrolytes, the Debye and Eigen equations appear to be adequate for the calculation of kd and k-¿, respectively, provided that the appropriate numerical integration over the interreactant distance is performed. Approximations leading to more tractable expressions (such as, e.g., those leading to a Bronsted-Bjerrum ionic strength dependence of kd) give rise to serious disagreement with experiments. Specific counterion effects (C104" faster than Cl-) are observed that can be best interpreted in terms of changes in the rate of the unimolecular electron-transfer step within an encounter complex including the counterion. Also, counterion concentration rather than ionic strength better represents (Olson-Simonson effect) the salt effects obtained with the CaCl2 electrolyte.