Excimer fluorescence, compounds polarization

Indeed, the usual fluorescence of the isolated aromatic amines (e.g., N,N-dimethylaniline, DMA) is quenched by excimer formation in compounds I and II. In the process of prolonged irradiation of I and II solutions the emission intensity increases gradually because of the loss of the C = C double bonds in the system due to the polymerization reaction. A polar environment favors the charge transfer and, therefore, the fluorescence quenching of the monomer is drastically decreased, whereas the polymer formation increases. 

Fluorescence emission from the majority of excimers is virtually independent of solvent polarity, indicating that little of their stability is due to charge-transfer interactions. Exceptions to this rule have been found, e.g. [17] and [26]. The norm is that excimer stability is little effected by change in solvent polarity. Compound [28] is interesting in that it can form an intramolecular... 

Solute-solvent complexes of different stoichiometry have been observed between all the D-A compounds under consideration and various solvent molecules. Some of the clusters show structured excitation spectra and a narrow short-wave emission that has been assigned to the primary excited state of the vdW complex. The broad, red-shifted emission of other clusters can be explained in terms of the transformation of the vdW complexes of stoichiometry l n (n > 0) into excimers or the transition into an intramolecular CT state of the D-A chromophore which is induced by its polar partner(s) (for the complexes of stoichiometry 1 ). The main conclusion from the fluorescence behaviour of the jet-cooled vdW clusters is that dual luminescence is obviously connected with the preference of specific solute-solvent geometries. 

As a fluorescence probe, a TICT compound possesses several merits. It can probe both microviscosity and micropolarity and furthermore, the mode of molecular motion necessary to the TICT phenomenon is well defined and limited to the rotation around a particular bond. This is a difference from intramolecular exciplex or excimer formation in which multiple modes of bond rotation and bending are compounded [7]. Fluorescence polarization studies also can not be free from ambiguity of the mode of rotation [8]. 

Pyrene excimer formation still continues to be of interest and importance as a model compound for various types of study. Recent re-examinations of the kinetics have been referred to in the previous section. A non a priori analysis of experimentally determined fluorescence decay surfaces has been applied to the examination of intermolecular pyrene excimer formation O. The Kramers equation has been successfully applied to the formation of intermolecular excimer states of 1,3-di(l-pyrenyl) propane . Measured fluorescence lifetimes fit the predictions of the Kramer equation very well. The concentration dependence of transient effects in monomer-excimer kinetics of pyrene and methyl 4-(l-pyrenebutyrate) in toluene and cyclohexane have also been studied . Pyrene excimer formation in polypeptides carrying 2-pyrenyl groups in a-helices has been observed by means of circular polarized fluorescence" . Another probe study of pyrene excimer has been employed in the investigation of multicomponent recombination of germinate pairs and the effect on the form of Stern-Volmer plots ". 

Measurements of the circularly polarized fluorescence (CPF) of this compound (PPA) as a function of temperature indicate the existence of two kinds of excimers one with a negative CPF dissymmetry with an emission maximum around 460 nm and predominant at higher temperatures, the other with a positive CPF dissymmetry emitting at wavelengths longer than 500 nm and mainly formed at lower temperatures (Egusa et al., 1985). In the emission spectrum of poly-N -(9-carbazolyl)-carbonyl-L-lysine, even four distinct species are observed besides the emission of locally excited carbazole. 

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