Fluorescence Lifetime-Based Schemes

Fluorescence lifetime measurements on the aggregate have shown that the rate constant of the intermolecular energy transfer from the zinc porphyrin unit to the free-base porphyrin unit has been evaluated to be 3.0 x 109 s-1. This value is reasonable from a model in which dendritic donor 6b and acceptor 5a contact each other directly at their exterior surfaces (Scheme 2). Therefore, electrostatic assembly of positively and negatively charged dendrimers provides a promising supramolecular approach to construct photofunctional materials with nanometric precision. 

The various possible schemes for fluorescence sensing are summarized in Figure 1.1. At present, most fluorescence assays are based on the standard intensity-based methods, in which the intensity of the probe molecule changes in response to the analyte of interest. However, there has been the realization that lifetime-based methods possess intrinsic advantages for chemical sensing. (A more detailed description of... 

Sigal studied the benzene d-6 sensitized isomerization of butene-2 at pressures below 0.1 mm., where the interval between collisions is long relative to the fluorescence lifetime. The butene-2 isomer concentration was monitored by long-path infrared spectrometry. The data are consistent with a kinetic scheme based on collisions being rate determining for intersystem crossover at low pressures. 

There continues to be an enormous amount of activity in the area of PET, much of it directed towards the development of systems capable of delivering artificial photosynthesis. Many of these systems involve porphyrin units as electron-donors and thus it is appropriate to consider them in this section of the review. A number of new fullerene-porphyrin dyads have been reported. A pyrazolinofullerene (155) has been constructed which facilitates efficient PET when strong donors such as iV,Ar-diethylaniline or ferrocene are linked to the pyrazoline ring. A photosynthetic multi-step ET model (156) based on a triad consisting of a meso,meso- inked porphyrin dimer connected to ferrocene and Ceo as electron-donor and electron-acceptor, respectively, has been synthesized and its ET dynamics (Scheme 38) have been investigated using time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. ... 

First attempts to measure electron transfer reactions in these maquettes focused on the Ru -P -V system. All ruthenium-modified complexes were prepared according to standard procedures (4, 23). Time-resolved fluorescence measurements of Ru(II )-Pyj revealed lifetimes of about 490 ns, consistent with those of Ru(2,2 -bipyiidine)3 (24). Based on this lifetime, the only ET-rates that would be directly measurable by this technique are those faster than about 2 X 10 s. A possible ET reaction takes place as shown in Scheme I. 

Fig. 1 The time-resolved detection scheme is based on a sequoice of excitation pulses Te, delay period Tdeijy, and detection window Tg, repeated with a repetition period 7). The delay period allows the prompt fluorescence dashed line) to decay before detection of the long lifetime limiinescence solid lines) starts. The shaded area indicates the detection window [14]...

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