Photophysical quenching

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

The role of disorder in the photophysics of conjugated polymers has been extensively described by the work carried out in Marburg by H. Bassler and coworkers. Based on ultrafast photoluminescence (PL) (15], field-induced luminescence quenching [16J and site-selective PL excitation [17], a model for excited state thermalizalion was proposed, which considers interchain exciton migration within the inhomogenously broadened density of states. We will base part of the interpretation of our results in m-LPPP on this model, which will be discussed in some detail in Sections 8.4 and 8.6. 

More recently, several groups have investigated electrostatic effects on the fluorescence quenching of hydrophobic chromophores covalently attached to various polyanions. The photophysics of the chromophores incorporated in the polyeletrolytes at small mole fractions is relatively simple, because no interaction is expected to occur between the incorporated chromophores. For this reason, most of the studies have focused on amphiphilic polyeletrolytes loaded with a low amount of hydrophobic chromophores. 

Keywords Fluorescence quenching, Nucleobase, Photophysics, Photochemistry, Quantum Chemistry,... 

The photophysical properties of lanthanide ions are influenced by their local environment, the nature of the quenching pathways available to the excited states of sensitizing chromophores, and the presence of any available quenchers (as we have seen when discussing bioassay). All of these factors can be exploited for the sensing of external species. 

C. Tan, M.R. Pinto, and K.S. Schanze, Photophysics, aggregation and amplified quenching of a water-soluble poly(phenylene ethynylene), Chem. Commun. 446-447, 2002. 

Electron transfer (Chapter 6), considered as a photophysical process, involves a photoexcited donor molecule interacting with a ground-state acceptor molecule. An ion pair is formed, which may undergo back electron transfer, resulting in quenching of the excited donor. 

One of the first examples describing the idea of fluorescence signal amplification in NPs was reported by Montalti et al. [197]. In an early work, silicaNPs were covered with a layer of dansyl (Dns) moieties and were evaluated for their pH response. Photophysical studies indicated that protonation had a dramatic effect on the fluorescence of the NPs, leading to the quenching of both, the protonated Dns and... 

Vogtle F, Gestermann S, Kauffmann C et al (1999) Polypropylene amine) dendrimers with peripheral dansyl units protonation, absorption spectra, photophysical properties, intraden-drimer quenching, and sensitization processes. J Am Chem Soc 121 12161-12166... 

Tab. 4.1. Main photophysical processes responsible for fluorescence quenching...

The effects of photophysical intermolecular processes on fluorescence emission are described in Chapter 4, which starts with an overview of the de-excitation processes leading to fluorescence quenching of excited molecules. The main excited-state processes are then presented electron transfer, excimer formation or exciplex formation, proton transfer and energy transfer. 

The basic principle of this method of recognition is a cation-induced conformational change bringing closer together (or moving away) two moieties able to interact and induce photophysical effects excimer or exciplex formation (or disappearance), electronic energy transfer and quenching. 

In spite of the numerous studies on practical applications of luminescence sensors, the understanding of the fundamental primary processes and underlying photophysics is still in its infancy. Only when details of the types of sites occupied by the sensor molecules, the local environment, and the quenching processes are understood and correlated with a variety of sensor molecules and supports can new materials and supports be rationally designed. 

Our results demonstrated clearly that the lifetime data are more sensitive to subtleties of the micromechanistic photophysics. In this case we were able to establish inadequacies of the two-component model that were not detected by intensity quenching measurements alone. It is also clear that resolution of the detailed mechanism in these complex polymer systems will require even better lifetime data than we are able to obtain with a conventional flash lamp-based time-correlated photon counting system. 

E. R. Carraway, J. N. Demas, and B. A. DeGraff, Photophysics and oxygen quenching oftransition-metal complexes on fumed silica, Langmuir 7, 2991-2998 (1991). 

Table 10.4. Photophysical Properties (Quantum Yield (<2>), Lifetime (to), Wavelength at Absorption (Aexc), and Emission Maximum (Aem)) of Chloride Probes and Their Quenching Constants (Ks h...

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