Instrumentation time-correlated single-photon counting

The time-resolved measurements were made using standard time-correlated single photon counting techniques [9]. The instrument response function had a typical full width at half-maximum of 50 ps. Time-resolved spectra were reconstructed by standard methods and corrected to susceptibilities on a frequency scale. Stokes shifts were calculated as first moments of cubic-spline interpolations of these spectra. 

Figure 8.9 Time-resolved fluorescent lifetime analysis of Cy3 attached to double-stranded DNA (Iqbal et al., 2008b). Fluorescent decay curve for Cy3 attached to a 16 bp DNA duplex, showing the experimental data and the instrument response function (IRF), and the fit to three exponential functions (line). The decay curve was generated using time-correlated single-photon counting, after excitation by 200 fs pulses from a titanium sapphire laser at 4.7 MHz.

Figure 7.5 Time-correlated single-photon counting instrument principle. Courtesy of Jobin Yvon.

Figure 8. A schematic presentation of a time-correlated single-photon-counting instrument (see text).

Figure 9. The time profile for the decay of fluorescence from a solution of anthracene in degassed hexane. The data were obtained with a time-correlated single-photon-counting instrument such as that shown in Figure 8. The upper panel shows the raw data with a superimposed linear fit and the instrument-response function. The extracted lifetime was 5.14 ns. The lower panel shows the residuals. (Courtesy of Dr. F. N. Castellano).

Fluorescence lifetimes were measured by time-correlated single photon counting using a mode-locked, synchronously pumped, cavity-dumped pyridine I dye laser (343 nm) or Rhodamine 6G dye laser (290 nm). Emissive photons were collected at 90° with respect to the excitation beam and passed through a monochromator to a Hamamatsu Model R2809U microchannel plate. Data analysis was made after deconvolution (18) of the instrument response function (FWHM 80 ps). 

Time-resolved PL measurements were also performed using time-correlated single-photon counting (TCSPC) and photoluminescence upconversion (PLUC) spectroscopies. Descriptions of the setups can be found in refs. [14, 65], respectively. All measurements were taken in continuous-flow He cryostats (Oxford Instruments OptistatCF) under inert conditions. Finally, PL efficiency measurements were performed on simple polymer thin films spin coated on Spectrosil substrates using an integrating sphere coupled to an Oriel InstaSpec IV spectrograph and excitation with the same Ar+ laser as above. 

Figure C1.5.12.(A) Fluorescence decay of a single molecule of cresyl violet on an indium tin oxide (ITO) surface measured by time-correlated single photon counting. The solid line is the fitted decay, a single exponential of 480 + 5 ps convolved with the instrument response function of 160 ps fwhm. The decay, which is considerably faster than the natural fluorescence lifetime of cresyl violet, is due to electron transfer from the excited cresyl violet (D ) to the conduction band or energetically accessible surface electronic states of ITO. (B) Distribution of lifetimes for 40 different single molecules showing a broad distribution of electron transfer rates. Reprinted with permission from Lu and Xie [138]. Copyright 1997 American Chemical Society.

The time decay of fluorescence intensity measurements of a i-acid glycoprotein was performed with an Edinburgh Analytical Instruments CD 900 fluorometer. The technique used was time correlated single photon counting. The sample was excited with series of pulses at a frequency of 20 kHz. The time decay of ai-acid glycoprotein was measured by tlie phase method. 

While the above-described instrumentation is an excellent choice for long-lived excited states such as the Ru " and Os polypyridyl complexes, organic-based chromophores and fluorescence labels frequently used by supramolecular chemists require higher time resolution. Commercially available time-correlated single photon counting (TCSPC) instruments can readily access... 

Time-Correlated Single Photon Counting Instrumentation... 

For fluorescence measurements, by far the most versatile and widely used time-resolved emission technique involves time-correlated single-photon counting [8] in conjunction with mode-locked lasers, a typical mo m apparatus being shown in Figure 15.8. The instrument response time of such an apparatus with microchannel plate detectors is of the order of 70 ps, giving an ultimate capability of measurement of decay times in the region of 7 ps. However, it is the phenomenal sensitivity and accuracy which are the main attractive features of the technique, which is widely used for time-resolved fluorescence decay, time-resolved emission spectra, and time-resolved anisotropy measurements. Below ate described three applkations of such time-resolved measurements on synthetic polymers, derived from recent work by the author s group. 

As more and more research groups gained access to the time-correlated single photon counting instrument for measuring fluorescence decay profiles, it became clear that these models presented a serious oversimplification of the phenomena. Even now with picosecond sources and even more complex models, data interpretation remains controversial. This subject will be examined in more detail in chapters by Soutar, Phillips, and Frank. It may be that a new phase in this area of research will be opened when one has access to a series of polymer samples of controlled end groups, known stereochemistry, and low molecular weight polydispersity. 

The fluorescence decay measurements were obtained from room-temperature, degassed solutions of the polymers in benzene by the technique of time-correlated, single photon counting using a laser-excited time-resolved fluorescence spectrometer described previously [8]. The fluorescence decay analyses required iterative reconvolution with the finite instrumental response function [8]. 

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