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The covalent photoadduct (PA) between [Ru(TAP)3]2+ (TAP = 1,4,5,8-tetraazaphenanthrene) and guanosine monophosphate (GMP) opened the way to interesting photobiological applications. In this context, the PA's capability upon illumination to give rise to the addition of a second guanine base is especially interesting. The origins of these intriguing properties are for the first time thoroughly investigated by an experimental and theoretical approach. The PA's spectroscopic and redox data combined with TDDFT results corroborated with resonance Raman data show that the properties of this PA (pK a around 7) depend on the solution pH. Theoretical results indicate that the acid form PA.H + when excited should relax to MLCT (metal-to-ligand charge transfer) excited states, in contrast to the basic form PA whose excited state should have LLCT/ILCT (ligand-to-ligand charge transfer/intra ligand charge transfer) characteristics. Ultrafast excitation of PA.H + at pH 5.9 produces continuous dynamic processes in a few hundred picoseconds involving coupled proton-electron transfers responsible for luminescence quenching. Long-lived species of a few microseconds capable of reacting with GMP are produced at that pH, in agreement with the formation of covalent addition of a second GMP to PA, as shown by mass spectrometry results. In contrast, at pH 8 (mainly nonprotonated PA), other ultrafast transient species are detected and no GMP biadduct is formed in the presence of GMP. This pH dependence of photoreaction can be rationalized with the different nature of the excited states, thus at pH 8, unreactive LLCT/ILCT states and at pH 5.9 reactive MLCT states. [ABSTRACT FROM AUTHOR]

Thefirst systematic steady-state and time-resolved emission studyof firefly oxyluciferin (emitter in firefly bioluminescence) and itsanalogues in aqueous buffers provided the individual emission spectraof all chemical forms of the emitter and the excited-state equilibriumconstants in strongly polar environment with strong hydrogen bondingpotential. The results confirmed the earlier hypothesis that excited-stateproton transfer from the enol group is favored over proton transferfrom the phenol group. In water, the phenol-keto form is the strongestphotoacid among the isomers and its conjugate base (phenolate-keto)has the lowest emission energy (634 nm). Furthermore, for the firsttime we observed green emission (525 nm) from a neutral phenol-ketoisomer constrained to the keto form by cyclopropyl substitution. Theorder of emission energies indicates that in aqueous solution a seconddeprotonation at the phenol group after the enol group had dissociated(that is, deprotonation of the phenol-enolate) does not occur in thefirst excited state. The pH-dependent emission spectra and the time-resolvedfluorescence parameters revealed that the keto-enol tautomerism reaction,which can occur in a nonpolar environment (toluene) in the presenceof a base, is not favored in water. [ABSTRACT FROM AUTHOR]

The effects of the nonprotonated and protonated calix[6]crypturea 1/1•H+ on the PF6- and Cl- salts of a luminescent Ru-TAP complex (TAP = 1,4,5,8-tetraazaphenanthrene) were investigated. Thus, the phototriggered basic properties of this complex were examined with 1•H+ in acetonitrile (MeCN) and butyronitrile (BuCN). The Ru excited complex was shown to be able to extract a proton from the protonated calixarene, accompanied by a luminescence quenching in both solvents. However, in BuCN, the Cl- salt of the complex exhibited a surprising behavior in the presence of 1/1•H+. Although an emission decrease was observed with the protonated calixarene, an emission increase was evidenced in the presence of nonprotonated 1. As the Cl- ions were shown to inhibit the luminescence of the complex in BuCN, this luminescence increase by nonprotonated 1 was attributed to the protection effect of 1 by encapsulation of the Cl- anions into the tris-urea binding site. The study of the luminescence lifetimes of the Ru-TAP complex in BuCN as a function of temperature for the PF6- and Cl- salts in the absence and presence of 1 led to the following conclusions. In BuCN, in contrast to MeCN, in addition to ion pairing, because of the poor solvation of the ions, the luminescent metal-to-ligand charge transfer (³MLCT) state could reach two metal-centered (³MC) states, one of which is in equilibrium with the ³MLCT state during the emission lifetime. The reaction of Cl- with this latter ³MC state would be responsible for the luminescence quenching, in agreement with the formation of photosubstitution products. [ABSTRACT FROM AUTHOR]

The photoreaction mechanism of [Ru(TAP)2(phen)]2+ and [Ru(TAP)3]2+ (TAP = 1,4,5,8-tetraazaphenanthrene) with tryptophan (Trp), N-acetyl-Trp, and Lys-Trp-Lys is examined. The existence of a photoelectron-transfer process from the amino acid unit is demonstrated by laser flash photolysis experiments. The back electron transfer (BET) from the reduced complex to the oxidized amino acid, occurring at the microsecond time scale, corresponds approximately to an equimolecular-bimolecular process; however, it is disturbed by another reaction, originating from the oxidized Trp. Moreover, in competition with the BET, the reduced and oxidized intermediates give rise to an adduct. The latter is clearly detected by gel electrophoresis experiments in denaturing conditions, with a system composed of an oligonucleotide derivatized at the 3′ end by the RuIITAP complex and hybridized with the complementary sequence functionalized at the 5′ end by the tripeptide Lys-Trp-Lys. Thus, upon illumination, a cross-linking between the two strands is observed, which originates from the presence of a Trp residue. [ABSTRACT FROM AUTHOR]

Time-resolved femtosecond stimulated Raman spectra (FSRS) of a prototypical organometallic photosensitizer/photocatalyst ReCl(CO) 3 (2,2'-bipyridine) were measured in a broad spectral range ∼40-2000 (4000) cm -1 at time delays from 40 fs to 4 ns after 400 nm excitation of the lowest allowed electronic transition. Theoretical ground- and excited-state Raman spectra were obtained by anharmonic vibrational analysis using second-order vibrational perturbation theory on vibrations calculated by harmonic approximation at density functional theory-optimized structures. A good match with anharmonically calculated vibrational frequencies allowed for assigning experimental Raman features to particular vibrations. Observed frequency shifts upon excitation (ν(ReCl) and ν(CC inter-ring) vibrations upward; ν(CC, CN) and ν(Re-C) downward) are consistent with the bonding/antibonding characters of the highest occupied molecular orbital and the lowest unoccupied molecular orbital involved in excitation and support the delocalized formulation of the lowest triplet state as ReCl(CO) 3 → bpy charge transfer. FSRS spectra show a mode-specific temporal evolution, providing insights into the intersystem crossing (ISC) mechanism and subsequent relaxation. Most of the Raman features are present at ∼40 fs and exhibit small shifts and intensity changes with time. The 1450-1600 cm -1 group of bands due to CC, CN, and CC(inter-ring) stretching vibrations undergoes extensive restructuring between 40 and ∼150 fs, followed by frequency upshifts and a biexponential (0.38, 21 ps) area growth, indicating progressing charge separation in the course of the formation and relaxation of the lowest triplet state. Early (40-150 fs) restructuring was also observed in the low-frequency range for ν(Re-Cl) and δ(Re-C-O) vibrations that are presumably activated by ISC. FSRS experimental innovations employed to measure low- and high-energy Raman features simultaneously are described and discussed in detail.

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.