Fluorescence quenching 288 viscosity dependence

The pioneering work Forster and Hoffmann [28] on the viscosity dependence of the fluorescence quantum yield of triphenylmethane dyes (TPM) has set the foundation for several reports in these dyes (Fig. 12). It was found that both an ability to twist around the carbocationic center and the donor-acceptor properties are important [66], Specifically, a strong intramolecular quenching is observed for 34 that is virtually absent (two orders of magnitude slower quenching rate) in the bridged... 

Dynamic quenching of fluorescence is described in Section 4.2.2. This translational diffusion process is viscosity-dependent and is thus expected to provide information on the fluidity of a microenvironment, but it must occur in a time-scale comparable to the excited-state lifetime of the fluorophore (experimental time window). When transient effects are negligible, the rate constant kq for quenching can be easily determined by measuring the fluorescence intensity or lifetime as a function of the quencher concentration the results can be analyzed using the Stern-Volmer relation ... 

The motions of chromophore groups and of their environment that lead to temperature-dependent fluorescence quenching are those on the nanosecond time scale. Slower motions cannot manifest themselves in effects on the excited-state lifetime (this corresponds to the limit of high viscosity). On the other hand, if the motions are considerably faster (on the picosecond time scale), then they should give rise to static quenching. 

Then, they depend also on the viscosity of the system. Specific diffusion control is characteristic of fast reactions like fluorescence quenching. In polymer formation, specific diffusion control is responsible for the acceleration of chain polymerization due to the retardation of the termination by recombination of two macroradicals (Trommsdorff effect). Step reactions are usually too slow to exhibit a dependence on translational diffusion also, the temperature dependence of their rate constants is of the Arrhenius type. 

The data for viscosities of solutions are usually given in the literature in terms of poise or centipoise (cp). For practical reasons cgs units are generally used and poise has the units dyne s cm (the dimensions of viscosity are ML" t ). Comprehensive tables of viscosities are found in the Handbook ofPhysics and Chemistry. In terms of cp the viscosity of water is 0.890,1.002 and 1.787 at 25,20 and 0 C respectively. An example of the large temperature dependence of the viscosity of water is presented by the decrease in collisional fluorescence quenching on cooling of a solution (see section 8.2). 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

In dynamic quenching (or diffusional quenching) the quenching species and the potentially fluorescent molecule react during the lifetime of the excited state of the latter. The efficiency of dynamic quenching depends upon the viscosity of the solution, the lifetime of the excited state (x ) of the luminescent species, and the concentration of the quencher [Q], This is summarized in the Stern-Volmer equation ... 

In systems where quenching is much smaller than that predicted by diffusion-controlled encounter frequencies, the reason for inefficiency may be that either a heat of activation or an entropy of activation is necessary. The dependence of Ksv on solvent viscosity then disappears. For example, bromobenzene is a weak quencher for fluorescence of aromatic hydrocarbons, the quenching constant being nearly the same in hexane as in viscous paraffins. 

Therefore, the 530-nm excited HT cannot relax via the AT form, but it must nece sarily remain within the HC manifold. As in the case of stilbene, we believe that for 1PA2N there is an intermediate I that might have a pyramidal form different from the perpendicular form of the stilbene intermediate. Relaxation from the HT form takes place via the I intermediate, with the rate being dependent upon the viscosity of the solvent. In methylcyclohexane, the time constant is found to be about 20 ps, and therefore fast enough to quench the fluorescence. In more viscous solvents, such as mixed methyT cyclohexane-cyclohexanol, the decay process becomes slower and the probability of fluorescence increases. 

---