Molecular fluorescence emission lifetime

The lifetimes of molecular fluorescence emissions are determined by the competition between radiative and nonradiative processes. If the radiative channel is dominant, as in the anthracene molecule, the fluorescence quantum yield is about unity-and the lifetime lies in the nanosecond range. In molecular assemblies, however, due to the cooperative emission of interacting molecules, much shorter lifetimes—in the picosecond or even in the femtosecond range—can theoretically be expected an upper limit has been calculated for 2D excitons [see (3.15) and Fig. 3.7] and for /V-multilayer systems with 100 > N > 2.78 The nonradiative molecular process is local, so unless fluorescence is in resonance by fission (Section II.C.2), its contribution to the lifetime of the molecular-assembly emission remains constant it is usually overwhelmed by the radiative process.118121 The phenomenon of collective spontaneous emission is often related to Dicke s model of superradiance,144 with the difference that only a very small density of excitation is involved. Direct measurement of such short radiative lifetimes of collective emissions, in the picosecond range, have recently been reported for two very different 2D systems ... 

FRET manifests itself through the quenching of donor fluorescence and a reduction of the fluorescence lifetime, accompanied by an increase in acceptor fluorescence emission. The efficiency of the energy-transfer process varies in proportion to the inverse sixth power of the distance separating the donor and acceptor molecules. Consequently, FRET measurements can be utilised as an effective molecular ruler for determining distances between molecules labelled with an appropriate donor and acceptor fluorophore, provided they are within lOnm of each other. 

Figure 27-1 Energy-level diagram shows some of the processes that occur during (a) absorption of incident radiation, (b) nonradiative relaxation, and (c) fluorescence emission by a molecular species. Absorption typically occurs in 10 s, while vibrational relaxation occurs in the 10 " to 10 " s time scale. Internal conversion between different electronic states is also very rapid (10 - s), while fluorescence lifetimes are typically lO- to I0 s.

The addition of -CD to an aqueous naphthalene solution caused the growth of its molecular fluorescence and the appearance of excimer emission [130]. By lowering the temperature of the solution, the excimer intensity grew at the expense of that of the monomer. The excimer emission was attributed to the association of 1 1 complexes to give 2 2 / -CD-naphthalene inclusion compounds. In air-saturated solutions, the three species have the following lifetimes 40 ns (free naphthalene), 48 ns (1 1 complex), and 68 ns (2 2 complex). The quenching rate constants derived from these lifetimes by the addition of I" were 6x 10 dm mol s , 3.9x 10 dm mol s , and 1.8 x 10 dm mol s , respectively, which confirmed the protection furnished by the cavity to the included molecules. 

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