Proton transfer photoinduced

When the solvent polarity increases, the exciplex band is red-shifted. The intensity of this band decreases as a result of the competition between de-excitation and dissociation of the exciplex. 

It should be noted that de-excitation of exciplexes can lead not only to fluorescence emission but also to ion pairs and subsequently free solvated ions. The latter process is favored in polar media. Exciplexes can be considered in some cases to be intermediate species in electron transfer from a donor to an acceptor (see Section 4.3). 

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). 

The acidic or basic properties of a molecule that absorbs light are not the same in the ground state and in the excited state. The redistribution of the electronic density upon excitation may be one of the possible causes of this observation. The most interesting cases are those where acids and bases are stronger in the excited state 

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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... 

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. 

Photoinduced proton transfer may be generated through the large variation in acidity or basicity of functional groups in the excited states of specific structures [8.226] and lead to photoinduced pH jumps [8.227,8.228]. Changes of optical properties by tautomerisation in the excited state [8.229] occur, for instance, in the fluorescent states of bipyridyl diols [8.230a] and form the basis of a proton transfer laser process [8.230b]. 

Of much significance is the realization of long-lived photo generated tautomeric states and long-range proton transfer (LRpT) processes. The latter could lead to proton transfer charge separated states to be put in parallel with the extensively studied charge separation by photoinduced electron transfer (see Section 8.2.3). A number of systems present photochromism on the basis of photoinduced proton transfer [8.229]. 

The signature of proton transfer in solution is the red-shifted fluorescence of the deprotonated chromophore [4-7], From the transition frequencies of absorption and emission and the ground-state dissociation constant, the dissociation constant in the excited state can be calculated [4-7], The time scales of proton transfer processes generally are very short, of the order of picoseconds or below [7], Only recently has it become possible to detect the photoinduced proton transfer dynamics in solution in real time [9,10],... 

Election transfer remains one of the most important processes explored when using interfacial supramolecular assemblies and given the emerging area of molecular electronics, this trend is set to continue. Therefore, Chapter 2 outlines the fundamental theoretical principles behind the electiochemically and photochemi-cally induced processes that are important for interfacial supramolecular assemblies. In that chapter, homogeneous and heterogeneous electron transfer, photoinduced proton transfer and photoisomerizations are considered. 

Figure 2.16 Intramolecular photoinduced proton transfer in 2,5-bis-(2-benzoxazolyl)hydroqui-none, as reported by Grabowska et al. [24]...

However, photoinduced proton transfer is still not well understood at the molecular level. In terms of its study, it has significant experimental advantages over electron transfer. It can be detected through vibrational spectroscopic techniques, can diffract X-rays and may undergo isotope exchange, thus permitting studies of kinetic isotope effects. 

Yasuda, M., Sone, T., Tanabe, K., and Shima, K. (1994) Animation of o-alkenylphenols via photoinduced proton transfer. Chemistry Letters, 453-456. 

The two-color technique has been used to obtain direct confirmation of Tn —> Sx reverse intersystem crossing (RISC) for a variety of systems including 54 and 55 [32-37], a variety of cyanine dyes (56-58) [38], substituted isoalloxazines (59-62) [39], and the phototautomers of several systems produced by photoinduced proton transfer (63-67) [40-46]. In these systems, irradiation into the T-T... 

RISC has been observed by two-color irradiation for a series of photo-tautomers generated by photoinduced proton transfer [40-46]. [The parent molecules, 3-hydroxyflavone (63), 2, 3, 4, 5, 6 -pentamethylphenyl-3-hydroxyflavone (64), 7-hydroxy indanone (65), 2-(2 -hydroxyphenyl)benzothiazole (66), and 2,2 -bipyridine-3,3 -diol (67), are shown in Chart 3.] Following proton transfer in the... 

Depending on the photophysical properties the pH fluorescent sensors can be divided into three classes (1) chemosensors that undergo photoinduced proton transfer, (2) chemosensors that undergo photoinduced electron transfer, and (3) those sensors that undergo neither proton nor electron transfer. 

The last group of fluorescent sensors is based on neither photoinduced proton transfer nor photoinduced electron transfer. The best-known example of this kind of molecular device is fluorescein (Figure 16.2e). The evolution of the fluorescence spectrum versus pH should be similar to that of the absorption spectrum. In other words, when increasing the pH, the absorption and emission bands of the acidic form should decrease with a concomitant increase in the absorption and emission bands of the basic form [1],... 

The amination of 2-alkenylphenols occurred efficiently compared to 2-allylphenols and -naph-thols69. The mechanism involves a proton exchange equilibrium between the phenolic and amino functions and the photoinduced proton transfer (PPT) from the ammonium ion to the alkenyl group, followed by attack of the amine on the intermediate benzylic carbocation. No photoamination of O-methylated and O-acetylated phenols occurred at all. As a single example of diastereoselective amination, the amine 6 was produced from 5 with good yield and diastereoselectivity, although the configuration was not determined. 

Figure 7.2 (A) Schematic representation of photoinduced proton-transfer reaction in the enol form (E ) and twisting motion in the keto-type (K ) structure to generate the keto-rotamer (KR ) ofexcited l -hydroxy,2 -acenaphthone (HAN). (B) Illustration of HAN molecule in water and complexed with one or two CD nanocavities.

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