Proton photoinduced

New IR techniques introduced for the study of prototropic tautomerism include IR dicroism for the photoinduced double proton transfer in por-phine 71 (Scheme 24) [89CPH(136)165], and IR spectroscopy in a supersonic jet (less than 50 K) for demonstrating the presence, in these conditions, of 2//-benzotriazole (57b) [96CPL(262)689]. 

Photoinduced oxidation of 1,4-dimethoxybenzene (DMB) and tetrahydrofuran (THF) by [Au(C N N-dpp)Cl]+ in acetonitrile upon UV/Vis irradiation have been observed. The time-resolved absorption spectrum recorded 12 (xs after excitation of [Au(C N N-dpp)Cl] with a laser pulse at 35 5 nm showed the absorption band of the DMB radical cation at 460nm, whereas upon excitation at 406 nm in the presence of THF, a broad emission characteristic of the protonated salt of 2,9-diphenyl-l,10-phenanthroline (Hdpp ) developed at 500 nm. 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

Photoinduced proton transfer reactions, undoubtedly, belong to the most important transformations in chemistry [20], Proton transfers can take place both in the ground and excited states of organic compounds, and the most important for... 

Keywords Excited-state intramolecular proton transfer Fluorescence dye Photoinduced electron transfer Proton coupled electron transfer Relaxation dynamics... 

When this probability is equal to 1 (uniform concentration), the reaction is of pseudo-first order. This is the case, for example, in photoinduced proton transfer in aqueous solutions from an excited acid M (=AH ) (see Section 4.5) M is always within the encounter distance with a water molecule acting as a proton acceptor, and thus proton transfer occurs effectively according to a unimolecular process. This is also the case of photoinduced electron transfer in aniline or its derivatives as solvents an excited acceptor is always in the vicinity of an aniline molecule as an electron donor. In both cases, the excited-state reaction occurs under non-diffusive conditions and is of pseudo-first order. 

In the presence of photoinduced proton transfer, the steady-state fluorescence intensities are given by Eqs (4.55) and (4.56). In the absence of deprotoration (i.e. in a very acidic solution such that k i [H30+] 1/tq), when the experimental conditions (concentrations, excitation and observation wavelengths, sensitivity of the instrument) are kept strictly identical, the fluorescence intensities is (Iah )o = C o- Rewriting Eqs (4.55) as fAH = C , the following ratio is obtained... 

In almost all applications, fluorescent pH indicators are employed in a pH range around the ground state pKa (even if the excited state pK is different). Therefore, the absorption (and excitation) spectrum depends on pH in the investigated range. These indicators can be divided into three classes (see formulae in Figure 10.2) on the basis of the elementary processes (photoinduced proton transfer or electron transfer) that are involved. 

Class C Fluorophores that undergo no photoinduced proton transfer but only photoinduced electron transfer. The fluorescence quantum yield of these fluorophores is very low when they are in the non-protonated form because of internal quenching by electron transfer. Protonation (which suppresses electron transfer) induces a very large enhancement of fluorescence (see Section 10.2.2.5). The bandshapes of the excitation and fluorescence spectra are independent of pH. 

The mechanism of p-lactam formation is thought to involve photoinduced electron transfer from nitrogen to the benzoyl group followed by transfer of a tertiary y-proton from one of the isopropyl groups to the carbonyl oxygen... 

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