Kinetics from fluorescence spectra

Such determinations of rotationally inelastic integral cross sections ajcm for collision-induced transitions in excited molecules obtained from measurements of satellite lines in the fluorescence spectrum have been reported for a large variety of different molecules, such as I2 [983, 984], Li2 [985, 986], Na2 [987], or NaK [988]. For illustration, the cross section a (A7) for the transition 7 7 + A7 in excited NaJ molecules induced by collisions Na + He are plotted in Fig. 8.8. They rapidly decrease from a value a(AJ = 1) 0.3 nm to a(AJ = 8) 0.02 nm. This decrease is essentially due to energy and momentum conservation, since the energy difference AE = E(J A7) — E(J) has to be transferred into the kinetic energy of the collision partners. The probability for this energy transfer is proportional to the Boltzmann factor exp[—A /(A r)] [989]. 

Figure 12. Salt effect on the relative R 0H/R 0 quantum yield in two different solvents Water (left panel) and a 50/50% (by volume) mixture of met Hanoi/water [11c]. Circles are experimental data obtained from the relative height ratio of the two peaks in the steady-state fluorescence spectrum e.g., Figure 10. Dashed and full curves correspond to the Debye-Hiickel expression (with finite ion-size correction) [21] and the Naive Approximation [17, 11c], respectively. Both models employ the zero-salt kinetic parameters.

Time-Resolved Fluorescence Spectra. Preliminary evidence indicates that the fluorescence spectrum of each of the kinetic components described above for pAT-3 chromatophores is essentially the same (the peak is about 890 nm, as it is in wild type chromatophores). This implies that all of the fluorescence components arise from the same emitter. An additional spectral component with a lifetime of about 1 ns is present in isolated reaction centers from the mutant and has a maximum emission wavelength to the blue of 870 nm. This probably represents contaminating pigments in the preparation. 

Measurements of the fluorescence spectra and fluorescence decay kinetics of the TbAl2nCl6 +3 vapor complex(es) have been performed in the 350-700 nm range (Hessler et al. 1977, Carnall et al. 1978, Caird et al. 1981a-c). Fluorescence was excited using a pulsed xenon flashlamp and the principal lines in the spectra were identified as emission from the D4, D3 and levels of the Tb ion. Figure 14 shows the fluorescence spectrum from the D4 level to the ground state Fj multiplet. Hessler et al. (1977) discussed the temperature dependence of the fluorescence lifetime, while Caird et al. (1981a-c) were the first to apply the Judd-Ofelt theory of forced electric dipole transitions to combined absorption and fluorescence data sets. 

Finally time dependent fluorescence spectra and kinetics can be obtained from the rate matrix and the spectrum of each eigenstate, fi. The time dependent fluorescence, F(t), can be written in terms of the eigenvalues and eigenvectors of the rate matrix K ... 

Chlorophyll itself shows a short-lived red fluorescence in solution. The green plants show a delayed fluorescence of approximately the same spectrum under specific conditions in which the electron transport chain is blocked. This delayed fluorescence results from the recombination of the charges (a process well known in electroluminescence), and its kinetics are complex and the decay quite long (several seconds). 

Figure 8 shows a pair of typical time-resolved fluorescence decay traces for 100 / M pyrene in supercritical CO2 (Tr = 1.02 pr = 1.17). Note that the ordinate is logarithmic. The upper and lower panels show results for selective observation in the monomer (400 +. 10 nm) and excimer (460 + 10 nm) regions of the pyrene emission spectrum. Several interesting features are apparent from these traces. First, both decay processes are not single exponential. Second, the excimer emission has a significant contribution from a species that "grows in" between 30 - 75 ns this is a result of the excimer taking time to form (i.e., k in Figure 1). Third, the fits between the experimental data and the model shown in Figure 1 are good. Detailed analysis of these decay traces (10,11,21-26) yields the entire ensemble of photophysical kinetic parameters for the pyrene excimer in supercritical C02.

The time-resolved emission spectra were reconstructed from the fluorescence decay kinetics at a series of emission wavelengths, and the steady-state emission spectrum as described in the Theory section (37). Figure 4 shows a typical set of time-resolved emission spectra for PRODAN in a binary supercritical fluid composed of CO2 and 1.57 mol% CH3OH (T = 45 °C P = 81.4 bar). Clearly, the emission spectrum red shifts following excitation indicating that the local solvent environment is becoming more polar during the excited-state lifetime. We attribute this red shift to the reorientation of cosolvent molecules about excited-state PRODAN. 

This subject was initiated by Forster (1949), who took up Weber s earlier observation (Weber, 1931) that the fluorescence of 1-naph-thylamine-4-sulphonate changes in wavelength at a markedly different pH from the absorption spectrum. Forster s work was developed by Weller and was reviewed by him in 1961, and by others later, viz., Vander Donckt (1970), Schulman and Winefordner (1970), and Schulman (1971a). A section on the subject also appears in new photochemical textbooks, among which we mention a clear exposition of its kinetic and chemical aspects by Parker (1968). 

Fig. 13. (A) Schematic representation of the interaction between neighboring residues of the His-tag (6 consecutive Histidine residues) and an NTA-complexed Ni2+ ion. (B) The performance of LHCII immobilization via chelating interaction. SPR kinetic curve of LHCII immobilization and regeneration cycles, monitored with an Nd YAG DPSS laser (X = 473 nm). (C) Surface plasmon field-enhanced fluorescence emission spectrum of surface attached LHCII compared with the fluorescence emission from free LHCII in solution, excited by an Nd YAG DPSS laser (X — 473 nm).

Miscellaneous Physical Chemistry. A kinetic study has been made of the electrochemical reduction of /8-carotene. The photoelectron quantum yield spectrum and photoelectron microscopy of /3-carotene have been described. Second-order rate constants for electron-transfer reactions of radical cations and anions of six carotenoids have been determined. Electronic energy transfer from O2 to carotenoids, e.g. canthaxanthin [/8,/3-carotene-4,4 -dione (192)], has been demonstrated. Several aspects of the physical chemistry of retinal and related compounds have been reported, including studies of electrochemical reduction, the properties of symmetric and asymmetric retinal bilayers, retinal as a source of 02, and the fluorescence lifetimes of retinal. Calculations have been made of photoisomerization quantum yields for 11-cis-retinal and analogues and of the conversion of even-7r-orbital into odd-TT-orbital systems related to retinylidene Schiff bases. ... 

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