Elucidating the Quenching Mechanism of Tris(2,2'-bipyridyl)ruthenium(II) Complex in the Water-Glycerol Binary System Based on the Microscopic Structure of the Media.

Kinetic studies on the photochemical quenching reaction of the tris(2,2'-bipyridyl) ruthenium(II) complex ([Ru(bpy)3]2+) in water-glycerol binary media were conducted based on the Einstein-Smoluchowski (E-S) theory. Dynamic and static quenching behaviors were analyzed by comparing results from time-resolved spectroscopy and emission spectroscopy. While the dynamic quenching reaction aligns well with the E-S theory, static quenching was observed, leading to a notable increase in the overall photoquenching reaction rate constant. Employing chromatography and infrared spectroscopy, we correlated the microscopic molecular structure of the binary solvent system and the solvation environment around the emitters with the reaction mechanism. This correlation was found to correspond to ion pair formation and the confinement effect of the emitter, respectively.

Reversible colour/luminescence colour changes of tetracyanoruthenium(II) complexes by sorption/desorption of water molecules in crystals.

We report colour/luminescence colour changes of M[Ru(bpy)(CN)4] crystal (M2+ = Ca2+, Sr2+, and Ba2+; bpy = 2,2'-bipyridine). The X-ray crystallographic study revealed that the crystals are constructed from linear-chains of {[Ru(bpy)(CN)4][Ca(H2O)5]}n, {[Ru(bpy)(CN)4][Sr(H2O)6]}n, and {[Ru(bpy)(CN)4]2[Ba(H2O)5]2(µ-H2O)2}n, respectively. Ru(II) complex linear chains and the hydrophilic channels composed of M2+ ion and water along them enable reversible water sorption/desorption without collapse of crystals responsible for the colour change. The emission spectra of Ca2+ and Sr2+ salts are remarkably shifted to the red side when the temperature was increased from 296 to 500 K, while Ba2+ salt shows a slight shift in the emission spectrum during the heating. The change in the interaction of M2+ ion to the equatorial CN ligand depending on the number of hydrated water molecules effectively contributes to the luminescence colour change for Ca2+ and Sr2+ salts. FT-IR spectra after heating at 473 K show the high-frequency shifts in the CN stretching mode for Sr2+ salt, while no remarkable peak shifts are observed for Ca2+ and Ba2+ salts. Thermogravimetry results indicate that heating over 470 K leads to the desorption of 5H2O from all salts, resulting in {[Ru(bpy)(CN)4][Ca]}n, {[Ru(bpy)(CN)4][Sr(H2O)]}n, and {[Ru(bpy)(CN)4]2[Ba]2(µ-H2O)2}n for linear chains. The change in the hydration structure for M2+ ions regulates the shift of CN stretching modes.

Polymorphic crystal approach to changing the emission of [AuCl(PPh3)2], analyzed by direct observation of the photoexcited structures by X-ray photocrystallography.

The photoexcited charge-transferred state of [AuCl(PPh(3))(2)] in a novel polymorphic crystal form was directly observed by X-ray photocrystallographic analysis. Its photoexcited state was completely different from the one generated in the known crystal of [AuCl(PPh(3))(2)]; the photoexcited bond-shrunk state was generated in the known crystal. This difference in the generated photoexcited state was clearly reflected by the difference in emission color. While the known crystal form showed green phosphorescence, the novel form showed blue phosphorescence under UV irradiation. The difference in the generated photoexcited state was due to the differences in steric hindrance in the crystal; bond shortening by photoexcitation was sterically allowed in the known form, while on the other hand, it was restricted in the novel form. Therefore, instead of the bond-shrunk state, the charge-transferred excited state became the lowest triplet state, and the emission color changed from green to blue (i.e., a blue shift of the emission wavelength was observed). These results mean that the photoexcited structure and the emission color of [AuCl(PPh(3))(2)] can be controlled by designing the molecular environment in the crystal.

Photocrystallographic analysis of [AuCl(PPh3)2] for elucidation of the crystal packing effect on the photophysical properties.

A metal-ligand bond shortening of [AuCl(PPh(3))(2)] by photoexcitation was analyzed by the photocrystallographic method in the unsolvated crystal. The gradual structural change of photoexcited and ground-state molecules with cooling explains the temperature dependence of the emission spectrum and the excited-state lifetime. Actually, on cooling, the ground-state molecular structure approached the excited-state structure. As a result, the HOMO-LUMO gap of [AuCl(PPh(3))(2)] became narrower and a red shift of the absorption and emission bands were observed. Below 180 K, inhibition of the bond shortening was observed due to the intermolecular interactions, which was confirmed by the temperature dependence of the photoexcited phase cell volume, the integrated emission intensity, and the excited-state lifetime measurement.

Crystal Structure and Energy Transfer in Double-Complex Salts Composed of Tris(2,2'-bipyridine)ruthenium(II) or Tris(2,2'-bipyridine)osmium(II) and Hexacyanochromate(III).

In crystals of double-complex salts [M(bpy)(3)](2)[Cr(CN)(6)]Cl.8H(2)O (M(2+) = Ru(2+), Os(2+); bpy = 2,2'-bipyridine), luminescence from (3)CT state of [M(bpy)(3)](2+) is partially quenched by [Cr(CN)(6)](3)(-) at 77 K and room temperature (RT). This quenching is attributed to intermolecular excitation energy transfer from the (3)CT state of [M(bpy)(3)](2+) to the (2)E(g) state of [Cr(CN)(6)](3)(-). Crystal structure and crystal parameters of [Os(bpy)(3)](2)[Cr(CN)(6)]Cl.8H(2)O: monoclinic, C2, a = 22.384(4) Å, b = 13.827(4) Å, c = 22.186(3) Å, beta = 90.70(2) degrees, V = 6866(2) Å(3), Z = 4, R = 0.0789, R(w) = 0.1932: are almost the same as those of [Ru(bpy)(3)](2)[Cr(CN)(6)]Cl.8H(2)O: monoclinic, C2, a = 22.414(2) Å, b = 13.7686(15) Å, c = 22.207(2) Å, beta = 90.713(8) degrees, V = 6852.9(12) Å(3), Z = 4, R = 0.0554, R(w) = 0.1679. Moreover, these double complex salts have the same distance and relative orientation between donor and acceptor. The rate of intermolecular energy transfer from [M(bpy)(3)](2+) to [Cr(CN)(6)](3)(-) was evaluated by the decay time of luminescence from (3)CT state of [M(bpy)(3)](2+) in single- and double-complex salts. The rate of energy transfer in [Os(bpy)(3)](2)[Cr(CN)(6)]Cl.8H(2)O (4.9 x 10(7) s(-)(1)) is about eight times larger than that in [Ru(bpy)(3)](2)[Cr(CN)(6)]Cl.8H(2)O (6.0 x 10(6) s(-)(1)) at 77 K. The difference of energy transfer rate is brought about by only the spectral overlap between the normalized luminescence spectrum from the (3)CT state of donor ([M(bpy)(3)](2+)) and the normalized excitation spectrum of the (2)E(g) state of acceptor ([Cr(CN)(6)](3)(-)) in the salts. Decay rates of the (3)CT state in [M(bpy)(3)](2)[Cr(CN)(6)]Cl.8H(2)O were measured as a function of temperature. A large enhancement of a decay rate from the (3)CT state was obtained for [Ru(bpy)(3)](2)[Cr(CN)(6)]Cl.8H(2)O as the temperature was increased. This result implies that an additional path from the (3)CT state of [Ru(bpy)(3)](2+) to the (2)T(2g) state of [Cr(CN)(6)](3)(-) would be opened for energy transfer with a rise in temperature in [Ru(bpy)(3)](2)[Cr(CN)(6)]Cl.8H(2)O.