Acid-base reactions, excited state

In this context, an avalanche of studies were devoted to acid-base reactions in their broadest sense (i.e., the Lewis picture), also involving complexation reactions, to the typical organic reactions of addition, substitution, and elimination types, involving nucleophilic and electrophilic reagents including the case of radicalar reactions and excited states (for a review see Ref. [11]) in which our group has... 

This section will only cover reactions in aqueous solutions. Water molecules acting as either a proton acceptor or a proton donor will thus be in close contact with an acid or a base undergoing excited-state deprotonation or protonation, respectively. Therefore, these processes will not be diffusion-controlled (Case A in Section 4.2.1). 

Dyes whose fluorescence intensity increases on binding to DNA (e.g. 3 and 4) have especially high potential as DNA marker or detector molecules. In the absence of DNA the relatively low fluorescence quantum yield of these dyes results from a radiationless deactivation of the excited state by conformational changes or acid-base reactions with the solvent. On association with DNA, however, significant suppression of the conformational flexibility and a shielding of the dye from solvent molecules within the complex occurs, leading to an increase of the emission intensity. 

Proton transfer is a particularly important transport process. Beyond acid-base reactions, proton transfer may be coupled to electron transfer in redox reactions and to excited-state chemistry. It is of enormous significance in biochemical processes where it is an essential step in hydrolytic enzyme processes and redox reactions spanning respiration, and photosynthesis where proton motion is responsible for sustaining redox gradients. In relatively recent times, proton transfer in the excited state has undergone significant study, primarily fueled by advances in ultrafast spectroscopy. 

It assumes that there is no entropy change between the ground- and excited-state acid-base reactions. In order for this assumption to be valid, there should be little geometric distortion of the excited molecule and the acid-base reactions must originate from the same sites in both the ground- and excited-state species. 

Weller s review (1961) is not confined to acid-base reactions but deals with the kinetics of excited state reactions in general. Vander Donckt (1970) covers developments of the acid-base section of the Weller field but pays more attention to physical organic aspects, such as applications of resonance theory to the interpretation of pA-shifts upon excitation and the application of linear free energy relationships. The reviews by Schulman and Winefordner (1970) and by Winefordner et al. (1971a) are directed towards possible analytical applications. 

Many aromatic amines, phenols and carboxylic acids have different pKa s in the excited state than in the ground state 83 -85). Therefore, at certain pH values a compound can undergo acid-base reactions in the excited state which do not occur in the... 

One of the interesting aspects of acid-base reactions of fluorescent molecules in fluid solutions is derived from the occurrence of protonation and dissociation during the lifetime of the excited state. This phenomenon affects the dependence of fluorescence on pH and must be considered in the development of a fluoro-metric analysis in aqueous solutions. 

Hydroxyarenes become stronger acids upon electronic excitationSuch a property of an aromatic molecule is usually described as photoacidity , and the molecules undergoing such a transition upon electronic excitation are usually named photoacids . Photoacids are Brpnsted acids, and their excited state acidity may be described in terms used for ground state acids as were defined by Brpnsted some 80 years ago - . Following Brpnsted, one usually associates acidity with a proton-transfer reaction where a proton is transferred from a proton donor (an acid) to a proton acceptor (a base) (equation 1). 

The discovery of photoacidity was made by Forster more then 50 years ago . Forster correctly explained the unusual large Stokes shift found in the fluorescence of several classes of aromatic dyes, including 1- and 2-naphthol derivatives as an indication of excited state proton-transfer reaction which results in the formation of the molecular anion still in the excited state. Thus, it become clear that excited-state proton transfer may compete with other radiative and non-radiative decay routes of the photoacid. The main modern-day importance of photoacids lies in their ability to initiate and then to follow acid-base reactions so they may be regarded as optical probes for the study of general proton-transfer reactions. 

These new data convey in addition original information on the structure of water molecules around, for instance, ions. They thus allow the distinguishing of the HjO molecules that establish H-bonds with an anion X from H2O molecules of the bulk. The lifetimes of their first excited state is much longer (2.6 psec in the vicinity of Cl ) than those of HDO molecules dissolved in heavy water (68). It allows the collection of precise information on the acid-base reaction in water, putting into evidence at least three steps with different time constants (69) and enabling measurements of their lifetimes (70). 

Such acid-base reactions take place at the carboxylic groups of suitable ligands such as L-L = 2,2 -bipyridine-4,4 -dicarboxylic acid. e.g. [Ru(bipy)2(L-L)]2+ [131]. In acidic D2O solutions of the Ru(II)-complex [Ru (NH3)5(pyridine)]2+ a light-induced H/D exchange occurs at the pyridine ligand [10,132]. This reaction is also attributed to the enhanced basicity of the coordinated pyridine in the MLCT excited state. 

If the prototropic equilibrium is established within the lifetime of the Sj state, i.e., the fluorescence lifetimes of AH and A are equal, the p/fa value can be estimated from the midpoint of fluorescence titration curve as shown in Figure 2.1, where the fluorescence quantum yields for AH and A are plotted as a function of pH values of the solution [21,47], However, such a case is rather rare for the acid-base reactions in the excited singlet state of aromatic compounds. 

Upon absorption of UV radiation from sunlight the bases can proceed through photochemical reactions that can lead to photodamage in the nucleic acids. Photochemical reactions do occur in the bases, with thymidine dimerization being a primary result, but at low rates. The bases are quite stable to photochemical damage, having efficient ways to dissipate the harmful electronic energy, as indicated by their ultrashort excited state lifetimes. It had been known for years that the excited states were short lived, and that fluorescence quantum yields are very low for all bases [4, 81, 82], Femtosecond laser spectroscopy has, in recent years, enabled a much... 

All the nucleic acid bases absorb UV radiation, as seen in Tables 11-1, 11-2, 11-3, 11-4, and 11-5, making them vulnerable to the UV radiation of sunlight, since the energy of the photons absorbed could lead to photochemical reactions. As already mentioned above, the excited state lifetimes of the natural nucleobases, and their nucleotides, and nucleosides are very short, indicating that ultrafast radiationless decay to the ground state takes place [6], The mechanism for nonradiative decay in all the nucleobases has been investigated with quantum mechanical methods. Below we summarize these studies for each base and make an effort to find common mechanisms if they exist. 

The proton transfer may occur rapidly after the excitation and form a tautomer, when either acidic or basic moieties of the same molecule become stronger acids or bases in the excited state. The majority of reactions of this type involve the proton transfer from an oxygen donor to an oxygen or nitrogen acceptor, although a few other cases are known, where a nitrogen atom can function as a donor and a carbon atom as the acceptor. Usually an intramolecular hydrogen bond between the two moieties of a molecule facilitates the proton transfer. 

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