Lifetime of emission

Quantum yields and lifetimes of emission (fluorescence) as well as other principal rates of deactivation have been measured on 2-hydroxy benzophenone and 2-hydroxyphenyl benzotriazole derivatives. Polymerizable UV screening agents have been prepared and copolymerized with acrylics in order to obtain transparent films containing nonfugitive UV screening agents. Preliminary results of studies of photodegradation on these copolymers are also reported here. 

Since the natural lifetime of emission, r°, is equal to l/key Equation 13.19 may... 

Different situations can be found direct determinations, where the lu-minophore is the analyte itself, or indirect determinations, where the analyte interacts with a luminophore, changing its optical properties, the quenching effect. And different measurement possibilities can be followed intensity or lifetime of emission. 

TABLE 7. Quantum yields and experimental lifetimes of emissions from ruthenium(II) and osmium(II) complexes. ... 

It was noted above that measurements of the lifetime of emission provide an alternative way of accessing the photophysical parameters of a polymer. The analogous approach in CL is to subject the oxidizing polymer to an external perturbation and then observe the change in CL as the system returns to the steady state (George, 1981). This will genemlly take the form of a short period of UV irradiation after which there is a burst of CL followed by decay, which may be analysed according to an assumed kinetic model, such as second-order... 

The radiative lifetime ir is equal to 1 / k,-. It is the real lifetime of emission of a photon that it should be measured independently of the other processes that deactivate the molecule. However, since these processes occur in parallel to the radiative one, it appears that it is impossible to eliminate them during the measurement of the radiative lifetime. Therefore we are going to measure a time characteristic of all the deexcitation processes. This time is called the fluorescence lifetime and is lower than the radiative lifetime. 

Exciplexes are complexes of two different molecules usually of 1 1 stoichiometry. Their fluorescence phenomena are similar to those described for excimers, but their formation is not restricted to aromatic systems. If the sum of effective rate constants of the non radiative processes is so high such that the lifetime of emission is undetectable, these molecules do not necessarily luminesce. In contrast to the excimers, which are non polar, the exciplexes are polar entities. It was shown by Beens and co-workers [13] that exciplexes from aromatic hydrocarbons and aromatic tertiary amines demonstrate the charge-transfer character of the complexes as reported by Knibbe and co-workers [14], and their dipole moments were greater than 3.3 x 10 C m (10 D). 

Figure 6.3 Fluorescence decay curves of the G3 glass pumped at 808 nm by monitoring emissions of 1,460 nm and 1,540 nm. The correlation coefficients for the fits by the first-order exponential decay equation are 0.9873 for Tm (1,460 nm) and 0.9878 for (1,540 nm). The inset is the lifetime of emissions as a function of Tm2S3 content.

---