FSrster energy transfer

A schematic of the FSrster energy transfer mechanism. The colored arrows are meant to represent the coupled transitions that iKcur simultaneously. 

Frequency-domain measurements of fluorescence energy transfer are used to determine the end-to-end distance distribution of donor-acceptor D-A) pairs linked by flexible alkyl chains. The length of the linker is varied from 11 to 2B atoms, and two different D-A pairs are used. In each case the D-A distributions are recovered from global analysis of measurements with different values for the FSrster distance, which are obtained by collislonal quenching of the donors. In all cases essentially the same distance distribution Is recovered from the frequency-domain data for each value of tha Ffirster distance. The experimentally recovered distance distributions are compared with those calculated from the RIS model. The experimentally recovered distance distributions for the largest chain molecules are In agreement with the predictions of the RIS model. However, the experimental and RIS distributions are distinct for the shorter D-A pairs. 

The steady-state absorption spectra of the compounds are characterized by a strongly absorbing Soret band at 429 nm (So — S2) and two minor Q-bands at 558 and 598 nm (So —> Si) (Fig. 2). Increasing size of the dendrimers does not produce any significant changes in the absorption spectra. This indicates weak interactions between the porphyrin units, i.e. the energy transfer can be described by FSrster theory. Excitation of the dendrimers in the Soret band results in two fluorescence bands (not shown here) with maxima at 600 and 655 nm (Si —> So for both transitions). Again, only minor differences can be observed dependent on size. 

The most important mechanism for singlet energy transfer is the FSrster resonance transfer ... 

Direct intermolecular electronic-eneigy transfer was first observed in this way in the gas phase, in a mixture of mercury and thallium vapours (G. Cario and J. Franck, Z Physik 17 (1923) 202). An instance of the phenomenon in solution was reported by Th. FSrster in Aim. Phys. 1 (1948) 55 Z Elektrochem. 53 (1949) 93. It is often called sensitised fluorescence . It is a case of non-radiative energy transfer, by contrast with radiative transfer in which a photon is emitted by one molecule and absorbed by another (A - A + /iv )n> + B -> B). The latter type of transfer gives little information on molecular interactions indeed the A and B molecules need not even be in the same vessel. It is of great practical importance, however, notably in photosynthesis, which makes use of photons emitted from the sun (Section 4.3.6.1). 

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