Fluorescence excitation , optically active

In the case of fluorescence spectra, it is the emission of the radiation from the excited state that is measured, rather than its absorption. This also provides valuable information. As an example, tetraethylorthosilicate (TEOS)-based gels were doped with two optically active organic indicators, thionin and nile blue A. Before trapping in a solgel host, thionin and nile blue A were both evaluated for solvent and protonation effects on their spectral properties. Only extreme pH values provided by HCl, NaOH, and NH4OH produced new absorption and/or fluorescence bands. The absorption and fluorescence spectra revealed a decrease in a pH 11 solution of NH4OH compared to neutral conditions (Krihak et al., 1997). 

The optically active 1,2-dioxetane of 2,4-adamantanedione (89) was synthesized. Thermal activation of 89 yielded chemiluminescence (Xmax = 420 nm characteristic of ketone fluorescence), pointing to intermediate 90 which is chiral only in its excited state due to the out-of-plane geometry of one of the two carbonyl groups. However, circular polarization of chemiluminescence measurement of 90 has not detected optical activity at the moment of emission. The authors have concluded that fast, relative to the lifetime of ketone singlet excited state, intramolecular n, it energy transfer caused racemization of 90196. 

In the experimental studies of state specific NO2 unimolecular dissociation (Miy-awaki et al., 1993 Hunter et al., 1993 Reid et al., 1994, 1993), NO2 is first vibra-tionally/rotationally cooled to 1 K by supersonic jet expansion. Ultraviolet excitation is then used to excite a NO2 resonance state which is an admixture of the optically active and the ground electronic states. [It should be noted that in the subpicosecond experiments by Ionov et al. (1993a) discussed in section 6.2.3.1, a superposition of resonance states is prepared instead of a single resonance state.] The NO product states are detected by laser-induced fluorescence. Both lifetime and product energy distributions for individual resonances are measured in these experiments. A stepwise increase in the unimolecular rate constant is observed when a new product channel opens. Fluctuations in the product state distributions, depending on the resonance state excited, are observed. The origin of the dynamical results is not clearly understood, but it apparently does not arise from mode specificity, since analyses of... 

The transient absorption (double-resonance) technique allows us to extend this study to excited vibrational levels of nonradiant states and especially of the ground electronic state. The s> level of the lower electronic state (e.g., of the So state) involving v" quanta of an optically active mode o>" is populated by the pumping pulse. Its population is monitored by the delayed probe pulse inducing a transition S> -> 0 > to the vibrationless level of the higher (Sj) state followed by the Si Sq fluorescence. The frequency of the probe pulse (equal to cu = cuoo v /o)") is too low for excitation of molecules from the vibrationless level 0>. On the other hand, the /> 0 > transitions are forbidden. The redistribution rate may thus be deduced from the dependence of the fluorescence signal on the delay between the pump and probe pulses (Maier et ai, 1977). 

Both (/f)-18 and rac-18 have a strong fluorescence at = 468 nm when excited at 390 nm. These polymers emit intense blue light under a UV lamp. It is interesting to observe that the optically active polymer exhibits significantly higher fluorescence quantum efficiency than the polymer made of racemic binaphthyls. The quantum yield of (/ )-18 is 80% but of rac-18 is only about 50% [39]. 

Abstract In the first part of this chapter we will illustrate circular dichroism and we will discuss the optical activity of chemical compounds with respect to light absorption which is at the basis of this technique. Moreover, we will introduce the phenomena that lie behind the technique of optical rotatory dispersion. We thought appropriate to include a brief description of linear dichroism spectroscopy, although this technique has nothing to do with optical activity. In the final part of the chapter we will introduce the basic principles of the luminescence teehniques based on polarized (either circularly or linearly) excitation. The experimental approach to the determination of steady-state and time resolved fluorescence anisotropy will be illustrated. For all the teehniques examined in this chapter the required instrumentation will be schematieally deseribed. A few examples of application of these techniques to molecular and supramolecular systems will also be presented. 

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