Excited-state Proton Transfer ESPT

Excited State Proton Transfer (ESPT) from the Neutral Chromophore... 

The crucial requirement of excited-state proton transfer (ESPT) is suggested by the failure of 1-naphthyl methyl ether to undergo self-nitrosation under similar photolysis conditions. The ESPT is further established by quenching of the photonitrosation as well as 1-naphthol fluorescence by general bases, such as water and triethylamine, with comparable quenching rate constants and quantum yield. ESPT shows the significance in relation to the requirement of acid in photolysis of nitrosamines and acid association is a photolabile species. 

In a review of the theories of excited-state proton-transfer (ESPT) reactions, Ar-naut and Formosinho [1] observed that excited-state prototropism as a subject of research has been much less popular, even in the realm of photochemistry, despite its unquestionable importance in fundamental and applied photochemistry. The pace of research in this area has more or less remained the same during all the years since then. 

Upon excitation of the A state an excited-state proton transfer (ESPT) occurs in which the proton is transferred from the chromophore... 

Figure 5.6 shows a model for the photophysical behavior of GFP [41]. According to the model the excited A state of the chromophore can undergo an excited-state proton transfer (ESPT) to form the excited intermediate form, I (pathway A2), which emits radiation at 505 nm (pathway II), or the excited A state can undergo fluorescence quenching NAC by means of a r-OBF or a HT (pathway A3). Excitation of the anionic B state leads to excited B which fluoresces (Bl). The model also contains a zwitterionic form that has not gained much acceptance in the literature. 

For many molecules, due to extensive redistribution of electron densities, acid-base property in the excited state differs considerably from that in the ground state [33 For instance, aromatic amines are weakly basic in the ground state. But many of them become acidic in the excited state and readily donate a proton to a proton acceptor to produce the anion in the excited state. Such a molecule, which behaves as an acid in the excited state, is called a photoacid similarly, photobases are those that display basic properties in the excited state. In many cases, excited state proton transfer (ESPT) results in dual emission bands. One of these emission bands arises om the neutral excited state and bears mirror image relation with the absorption spectrum. The other emission band is due to the excited deprotonated (anion) or protonated species and exhibits a large Stokes shift. 

Zhao et al. created an approach to red-shift the emission wavelength based on the excited state proton transfer (ESPT) process (Figure 5.8). In PCys-B, two Cys sensing sites (acrylate ester and aldehyde) were installed ortho to each... 

The excited state properties of hydroxyaromatic compounds (phenols, naphthols, etc) are of interest to a wide audience in chemistry, including those interested in the environmental decomposition of phenols, chemical physicists interested in the very fast dynamics of excited-state proton transfer (ESPT) and excited-state intramolecular proton transfer (ESIPT), physical chemists interested in photoionization and the photochemical pathways for phenoxyl radical formation, and organic photochemists interested in the mechanisms of phenol and hydroxyarene photochemistry. Due to space limitations, this review is restricted to molecular photochemistry of hydroxyaromatic compounds reported during the last three decades that are of primary interest to organic photochemists. It also includes a brief section on the phenomenon of enhanced acidity of phenols and other hydroxyaromatics because this is central to hydroxyarene photochemistry and forms the basis of much of the mechanistic photochemistry to be discussed later on. Several reviews that offer related coverage to this work have also appeared recently. This review does not cover aspects of electron photoejection from phenols or phenolate ions (and related compounds such as tyrosine) or phenol OH homolysis induced photochemically, as shown in Eq. (39.1), as these are adequately covered elsewhere ... 

Jiminez, M.C., Miranda, M.A. and Tormos, R., Photocychzation of 2-cinnamylphenols via excited state proton transfer (ESPT) involving the lowest-lying styrenic singlet. Tetrahedron, 53, 14729, 1997. 

Pyranine (PY) is a large aromatic molecule (Scheme 4) which shows excited-state intermolecular proton-transfer reaction with water molecules [12]. Several studies have been reported on the intermolecular proton transfer from PY to water. The interaction at the excited-state results in an excited-state proton-transfer (ESPT) reaction producing an anionic form, which emits a greenish-yellow fluorescence band. Here, we will consider only those of CD complexes using ultrafast spectroscopy [13]. In presence of CD, a 1 1 complex is formed and the normal emission of PY in water due to the enol form (E, 440 nm) increases, while that due to the anionic (A, 550 nm) structure decreases (Scheme 4). The change clearly shows the effect of CD on the emission behaviour. Gating the emission of E (440 nm) and of A (550 nm), the fs study reported a 0.8 ps component assigned to solvation of locally excited (LE) enol prior to proton transfer, and a 2-3 ps component attributed to solvent-assisted interconversion of LE to... 

---