Characterization of Dimethylsulfoxide / Glycerol Mixtures: A Binary Solvent System for the Study of "Friction-Dependent" Chemical Reactivity

The properties of binary mixtures of dimethylsulfoxide and glycerol, measured by several techniques, are reported. Special attention is given to those properties contributing or affecting chemical reactions. In this respect the investigated mixture behaves as a relatively simple solvent and it is especially well suited for studies on the influence of viscosity in chemical reactivity. This is due to the relative invariance of the dielectric properties of the mixture. However, special caution must be taken with specific solvation, as the hydrogen-bonding properties of the solvent changes with the molar fraction of glycerol.Comment: 49 pages including appendix, 20 figures and 89 reference

Photophysical and photocatalytic behavior of cobalt(III) 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin

Although kinetically inert cationic Co(III)TMPyP5+ (H2TMPyP4+ = 5,10,15,20-tetrakis(methylpyridinium-4-yl)porphyrin) was considered earlier to be very weakly emissive, both the spectrum and the lifetime of its fluorescence could be determined. Besides, this complex proved to be favorable for outer-sphere photoinduced reduction of the metal center in the presence of triethanolamine (TEOA) as electron donor quenching the triplet excited state of this metalloporphyrin. The corresponding cobalt(II) porphyrin formed in this way was also photoactive; it forwarded an electron to a suitable acceptor (e.g., methylviologen) upon irradiation, regenerating the starting complex. Hence, this system may be a candidate for hydrogen generation from water by utilization of visible light

Photophysical and photochemical properties of manganese complexes with cationic porphyrin ligands: effects of alkyl substituents and micellar environment

Although Mn(III) porphyrins were considered earlier to be very weakly emissive, the fluorescence displayed by Mn(III) complexes with the cationic TMPyP2+ ligand (H2TMPyP4+ = 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin) as well as with its other alkyl (such as hexyl and dodecyl) derivatives proved to be strong enough for a comparative study. Elongation of the alkyl substituent increased both the quantum yield and the lifetime of the emission for the Mn(III) porphyrins, while resulted in an opposite effect for the corresponding free bases in homogeneous solutions. The presence of cationic micelles, however, reversed this tendency regarding the emission lifetime of the complexes. These cationic metalloporphyrins were applied in a photocatalytic system involving triethanolamine (TEOA) as a sacrificial electron donor and methylviologen (MV2+) as an acceptor. In the first step of the catalytic process outer-sphere photoinduced reduction of the metal center took place via quenching of the triplet excited state of these metalloporphyrins by TEOA. The corresponding manganese(II) porphyrins formed in this way were also photoactive; they forwarded an electron to MV2+ upon irradiation, regenerating the starting complex. Elongation of the alkyl substituents increased the quantum yield of the formation of methylviologen radical (MV•+) in this system, but considerably decreased the durability of the photocatalyst. Anionic micelles totally hindered the photoinduced generation of Mn(II) porphyrins, while cationic micellar environment appreciably increased the efficiency of the accumulation of MV•+

Photophysical and photocatalytic behavior of nickel(II) 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin

Kinetically inert cationic Ni(II)TMPyP4+ (H2TMPyP4+ = 5,10,15,20-tetrakis(methylpyridinium-4-yl)porphyrin) displayed a characteristic fluorescence (τ = 1.2-1.4 ns, Φ = 2.0×10-3), which was quenched with triethanolamine (TEOA) in a static way. This complex proved to be an efficient photocatalyst in the system containing TEOA and methylviologen (MV2+) as electron donor and acceptor, respectively. Interestingly, however, deviating from the behavior of the analogous Co(III) and Mn(III) complexes in such a system, TEOA did not dinamically quench the triplet excited state of Ni(II)TMPyP4+ (τ = 6.31 μs), hence no reduction of the metal center occured upon irradition. Instead, in the presence of this electron donor (at 1×10-3M) the excited-state lifetime dramatically increased (to τ = 36.6 μs), indicating the formation of a Ni(II)TMPyP4+-TEOA associate. This longer-lived triplet was efficiently quenched by MV2+ (kq = 9.9×106 M-1s-1), leading to the formation of MV●+. The overall quantum yield of this one-step photoassisted electron transfer is considerably high (Φ = 0.011-0.013 at Soret-band irradiation). Hence, this system, combined with a suitable co-catalyst, may be applicable for visible light-driven hydrogen generation from water

Visible light-driven photophysics and photochemistry of water-soluble metalloporphyrins

Metal ions can form normal (in-plane) metalloporphyrins, fitting into the central hole of the porphyrin ring, or several of them are located out of the ligand plane, resulting in sitting-atop (SAT) complexes. Kinetically inert water-soluble complexes of Mn(III), Co(III), and Ni(II) with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin display a weak, short-lived fluorescence. This can be affected by elongation of the alkyl substituent and using micellar environment in the case of Mn(III) porphyrins. In the presence of a suitable electron donor (triethanolamine, TEOA) and acceptor (methylviologen, MV2+), these metalloporphyrins proved to be efficient photocatalysts transferring electrons between the ground-state donor and acceptor via outer-sphere mechanism. In these systems triplet excited-state Mn(III) and Co(III) porphyrins are dynamically quenched with TEOA. The Mn(II) and Co(II) complexes formed in this way need also photoexcitation for the transfer of electron to the ground-state acceptor. However, the triplet excited state of Ni(II)TMPyP4+ cannot be dynamically quenched with TEOA. Instead, this electron donor forms an associate with Ni(II)TMPyP4+ in a ground state equilibrium. The lifetime of the triplet excited state of this species is much longer than that of the nickel(II) porphyrin alone, and it can undergo an efficient dynamic oxidative quenching with MV2+. Thus, a one-step electron transfer can be realized between the electron donor and acceptor, generating MV•+, which can be utilized for hydrogen generation from water. Lanthanide(III) porphyrins are of typical SAT complexes, the photophysical and –chemical features of which can be tuned by the size of the metal center. Anionic, early lanthanide(III) mono- and bisporphyrin complexes exhibit very similar photoinduced properties as a consequence of a special type of aggregation, through the peripheral substituents. The rather slow formation of complexes and transformation between the mono- and bisporphyrins can be accelerated by the irradiation of the system. These by-processes play considerable roles beside the photoredox degradation and demetalation. Depending on the wavelength of irradiation, two types of photoproducts can appear: during the photolysis at the Soret-band, a radical type intermediate can be observed, which disappears in dark. However, irradiation at the Q-bands, generates the formation of a new, stable photoproduct

Magnetic field modulation of the delayed fluorescence yield in the photoionization reaction of N, N, N', N'-tetramethyl- p

External magnetic field effects on the recombination fluorescence (MARY effect) in the photoionization reaction of N,N,N′,N′-Tetramethyl-p-phenylenediamine (TMPPD) in water and DMSO/water mixture are studied. Relatively large magnetic field effects (MFE), ∼ 2–4%, on the fluorescence yield are observed in the extremely polar water solvent under magnetic fields as small as 3 mT. Such MFE is hardly expected in water due to instability and very fast escape of the solvated electron from the solvent cage. Enhancement in the signal-to-noise ratio and superior time resolution characterizing the technique of field modulation allowed the detection of a very short lived radical ion pair (about 1 ns). The observed MARY spectra illustrate that the singlet radical ion pair is more reactive than the triplet one