Fluorescent responsive molecular probes

Fluorescence is also a powerful tool for investigating the structure and dynamics of matter or living systems at a molecular or supramolecular level. Polymers, solutions of surfactants, solid surfaces, biological membranes, proteins, nucleic acids and living cells are well-known examples of systems in which estimates of local parameters such as polarity, fluidity, order, molecular mobility and electrical potential is possible by means of fluorescent molecules playing the role of probes. The latter can be intrinsic or introduced on purpose. The high sensitivity of fluo-rimetric methods in conjunction with the specificity of the response of probes to their microenvironment contribute towards the success of this approach. Another factor is the ability of probes to provide information on dynamics of fast phenomena and/or the structural parameters of the system under study. 

The increasing interest of researchers for fluorescent probes can be explained by the great improvement of the sensitivity and the spatial or temporal resolution of instruments, and by the development of a wide choice of commercially available probes for particular applications (Molecular Probes, Inc., United States Lambda Fluoreszenztechnologie Ges.m.b.H., Austria). However, there is still a need for probes with improved specific response and minimum perturbation of the microenvironment, in particular in the field of ion recognition which is the object of this chapter. 

In order to prepare successful NIR molecular probe dyes, NIR dyes must meet the following criteria adequate response to analytes, high lipophilicity and/or reactive functional groups, absorbance maxima compatible with available laser diodes, high fluorescence quantum yield, molar absorptivity, and high photostability. 

Advances in fluorescence and electron spin resonance (ESR) spectroscopies have enabled the use of organic fluorescent probes for in situ characterizations of molecular environments. The utility of fluorescence methods in such studies arises from the fact that the fluorescence and ESR responses of numerous probes are highly dependent on the environment, so that specific information can be obtained by appropriate choice of probes. Fluorescence responses that have been shown to depend on a micellar environment include excitation and emission, fluorescence polarization, and quenching (or sensitization). These responses have been, in turn, related to molecular properties such as polarity, viscosity, diffusion, solute partitioning, and aggregation numbers. 

Experiments on transport, injection, electroluminescence, and fluorescence probe the spatial correlation within the film, therefore we expect that their response will be sensitive to the self-affinity of the film. This approach, which we proved useful in the analysis of AFM data of conjugated molecular thin films grown in high vacuum, has never been applied to optical and electrical techniques on these systems and might be an interesting route to explore. We have started to assess the influence of different spatial correlations in thin films on the optical and the electro-optical properties, as it will be described in the next section. 

As a result, the site-dependent nonradiative process of individual C molecules is responsible to biexponential fluorescence decay curves of CV observed in the ensemble-averaged measurement. Our single-molecule study presented in this section will open new possibilities in the experimental study of dynamic response of condensed matter, such as polymers and liquid. We further expect that dye molecules with flexible molecular structures like CV are useful to sensitive local probes for microscopic dynamics of various host mediums. 

Relative data evaluation often uses fluorescence to determine the TPA cross section because this method sensitively relates to the number of excited singlet state Si populated by TP excitation. The observed TPA cross section by fluorescence is the TPA cross section action. It is therefore necessary to divide this quantity by the quantum yield being responsible for this event that is the fluorescence quantum yield in this example. Fluorescence is not the only prerequisite to quantify relatively the TPA cross section action. In general, almost all photo-chemical/photophysical events can be applied to probe the TPA cross section action. Again, dividing the TPA cross section action by the quantum yield related to this event results in 8, an intrinsic value of the TP absorbing chromophore, which is related to molecular parameters as shown in Eq. (18). 

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