Fluorescence spectra temperature effects

A third possible channel of S state deexcitation is the S) —> Ti transition -nonradiative intersystem crossing isc. In principle, this process is spin forbidden, however, there are different intra- and intermolecular factors (spin-orbital coupling, heavy atom effect, and some others), which favor this process. With the rates kisc = 107-109 s"1, it can compete with other channels of S) state deactivation. At normal conditions in solutions, the nonradiative deexcitation of the triplet state T , kTm, is predominant over phosphorescence, which is the radiative deactivation of the T state. This transition is also spin-forbidden and its rate, kj, is low. Therefore, normally, phosphorescence is observed at low temperatures or in rigid (polymers, crystals) matrices, and the lifetimes of triplet state xT at such conditions may be quite long, up to a few seconds. Obviously, the phosphorescence spectrum is located at wavelengths longer than the fluorescence spectrum (see the bottom of Fig. 1). 

Let us consider in greater detail the temperature dependence of the position of the maximum in the fluorescence spectrum of melittin (Figure 2.12). Three characteristic temperature regions may be distinguished. At T< 30 °C the spectrum does not depend on the temperature with excitation at both 280 nm and 305 nm in this case the red-edge effect is maximal. Evidently, the condition xR xF holds in this region. In the range 30 to 50 °C there is a... 

The formation of a benzene dimer from association of a ground state monomer and an excited monomer is a well-established observation of this species was first noted by Dammers de Klerk (270) and Ivanova and co-workers (271) in a study of the effects of concentration on the fluorescence spectrum of benzene. Broad structureless emission on the long wavelength side of monomer emission at room temperature is clearly observed at concentrations... 

Given this relationship only one of the two states may be fluorescent. If both were fluorescent we would observe a change in the emission spectrum with temperature and a wavelength dependence for the fluorescence lifetime. These effects were not observed. This leaves only four possible kinetic relationships between 5] and S2. 

For temperature measurement by single-dye fluorescence, the temperature sensitivity of a dye, specifically its quantum efficiency, effectively defines the temperature resolution of the measurement itself. Rhodamine B is the most common temperature-dependent fluorescent dye used in both macro- and microscale liquid applications because of its relatively strong temperature sensitivity of 2.3 % in water over a temperature range of 0-120 °C. This dye is also soluble in many other organic solvents, like ethanol, making it a practical choice in a variety of microfluidic applications. Moreover, its absorption spectrum is rather broad (470-600 nm with a peak at 554 nm), meaning it can be readily excited with conventional illumination sources like mercury-arc lamps as well as argon-ion (continuous) and Nd YAG (pulsed) lasers. Further, its emission spectrum is also... 

Hu, Z. and Margulis, C. J. 2006. A study of the time-resolved fluorescence spectrum and red edge effect of ANF in a room-temperature ionic liquid. J. Phys. Chem. B 110,11025-11028. 

In 1971, adrenodoxin, an iron-sulfur protein with a single tyrosine residue and no tryptophan was shown to fluoresce at 331 nm upon 280-nm excitation at neutral pH/20 1 On cooling from room temperature to 77 K, the emission maximum shifts to 315 nm. The redox state of the iron does not have any effect on the tyrosine emission. From these results, an exciplex between the excited singlet state of tyrosine and an unidentified group was suggested as the cause of the anomalous emission energy/2031 Later studies have shown that the excitation spectrum is a red-shifted tyrosine spectrum, that removal of the iron to form the apoprotein has no effect on the emission, and that heat, low pH, guanidine hydrochloride, urea, and LiCl all cause the emission... 

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