Time-resolved fluorescence and phosphorescence spectroscopy

MSC1412, National Institutes of Health, Bethesda, MD 20892-1412, USA Dept, of Applied Biology and Chemical Technology, Hong Kong Polytechnic University, Hong Kong, PRC 

1 Basic photophysics and time dependence of fluorescence and phosphorescence decays 

The intrinsic time dependence of fluorescence decays is derived in Section 3.3 of Chapter 2. There it is shown that if a population of excited singlet-state molecules is generated instantaneously, its size decreases exponentially with time, as does the intensity of the emitted photons. The fluorescence lifetime tj is typically used to describe the rate of decay, where ts = Icr + Icnr -l- kisc + kpR [equation 12, Chapter 2) and kR, krjR, kisc and kpR are the rate constants for the 

The steady-state fluorescence intensity 1(X) at emission wavelength X is related to the time-resolved decay parameters through 

Analogous to the fluorescence quantum yield (equation 4, Chapter 2), we can define a quantum yield for intersystem crossing (Section 2.2.1, Chapter 2), which is the fraction of initially excited molecules that makes it from Sj to and is given by. 

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Fig. 3 Relative temperature dependent time resolved fluorescence and phosphorescence yields for the o-diketone moiety of O44 upon excitation of the benzene cromophore at 266nm. The resulting emission is a manifestation of S-S and T-T intra-ET. The boxcar overager aperture duration A and aperture delay 6 used for the time resolved spectroscopy are shown. 

This chapter briefly reviews recent progress in the investigation of fluorescent and phosphorescent properties of stilbenes as well as such phenomena as triplet-triplet and singlet-singlet energy transfer and Raman scattering. The trends in this area include the use of a wide arsenal of stilbenes, employment of elaborated experimental methods such as nano and picosecond time-resolved absorption and fluorescent spectroscopy, and the use of modern theoretical calculations, for example, density function theory. The importance of these research endeavors for further basic and applied applications of stilbenes cannot be overestimated. 

Using time resolved laser induced luminescence spectroscopy the relative yields of T-T Intra-ET and S-S Intra-ET were determined. Typical examples are shown in Figs. 3-6 where comparison is made between the relative quantum yields of fluorescence and phosphorescence of the o-diketone chromophore upon direct excitation of the a-diketone chromophore at 430 nm to that obtained by exciting the aromatic moiety at 266nm followed by S-S and T-T Intra-ET. The results indicate competition between T-T Intra-ET and S-S Intra-ET leading to a more efficient T-T process whenever S-S Intra-ET becomes less efficient. 

Fluorescence applied to oil identification has been an active field, with 17 papers presented on the subject at the last three Pittsburgh Conferences. A number of interesting developments for fluorescence and low-temperature luminescence (LTL) are described by Eastwood et al. (58). These include synchronous scanning, difference spectrofluorometry, synchronous difference spectroscopy, derivative spectroscopy, and total luminescence (or contour) spectroscopy and combinations of these techniques. In a recent presentation, Eastwood and Hendrick (59) reported an extension of their low-temperature luminescence studies to include polarized excitation and emission spectroscopy, and time-resolved phosphorescence. Preliminary studies of polarization effects indicate that differences exist in low-temperature polarized luminescence spectra of oils, which may aid in oil identification. In the time-resolved phosphorescence spectra of oils, the most significant difference observed was enhancement of the vanadyl porphyrin signal at approximately 700 nm for short delay times (20 fxsec). 

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