Triplet-state probes absorption

The use of excited triplet states as probes occurred in concert with the development of fluorescent probe methodologies but have not been employed to the same extent. Phosphorescence quantum yields in solution are generally much lower than fluorescence quantum yields, and when the emission of triplet states is measured, the signal-to-noise advantage of the fluorescent probes is lost. In most cases, triplet excited states are followed by their absorption spectra using laser flash photolysis. A second reason for the scarcer reports on the use of excited triplet states probes in supramolecular systems is the fact that the laser flash photolysis technique has not been as widespread as fluorescence techniques. This situation has changed over the past decade and we expect that the number of smdies which employ excited triplet states will increase. 

Reticulum ATPase [105,106], Owing to the long-lived nature of the triplet state, Eosin derivatives are suitable to study protein dynamics in the microsecond-millisecond range. Rotational correlation times are obtained by monitoring the time-dependent anisotropy of the probe s phosphorescence [107-112] and/or the recovery of the ground state absorption [113— 118] or fluorescence [119-122], The decay of the anisotropy allows determination of the mobility of the protein chain that cover the binding site and the rotational diffusion of the protein, the latter being a function of the size and shape of the protein, the viscosity of the medium, and the temperature. 

Fig. 10.28 a) Stationary absorption spectrum of the N3 Ru dye (dotted curve), of the oxidized N3 dye (dashed curve), both in absolute ethanol, and the difference of these curves (solid curve), b) Transient transmission signal due to N3 dye molecules anchored to the colloidal Ti02 electrode in ultrahigh vacuum. The parameter is the time delay between pump puLse and probe pulse.s. (The results were nearly the same for electrodes immersed in the solution.) The absorption spectrum of the excited triplet state of the N3 dye, measured in ethanol, is shown for comparison (solid squares). (After ref. [51])... 

The first reported application of the technique by Kessler and Wilkinson [12] in 1981, deals with various aromatic hydrocarbons chemisorbed on Y-alumina. The samples were adsorbed at less than monolayer levels, and the transient spectra observed were shown to be due to triplet-triplet absorptions. Figure 6 shows the time resolved triplet-triplet absorption spectra obtained from 3% coverage of acridine on powdered silica [14] demonstrating the sensitivity of the method and its ability to probe acid-base reactions at catalytic surfaces. Turro and his coworkers [25,26] have studied various molecules on silica surfaces. The lifetimes of the triplet states of valerophenone and diphenyl-butyrophenone adsorbed on powdered silica were determined to be 0.3 and 0.9 [is, respectively [25] which is at least two orders of magnitude greater than in homogeneous solution. Turro et al [26] also demonstrated triplet energy transfer from benzophenone to naphthalene on silica surfaces via static and dynamic pathways. 

Transient absorption spectroscopy has been a conventional technique for studying the configurational (conformational) changes that are reflected in the Tn <— T, optical transition. Picosecond to microsecond time-resolved, pump and probe measurement has been widely used to examine the triplet-state isomerizations as far as each ris-T, and the all-trans-Tj species are selectively observed. Even when their wavelengths are very similar to one another, the SVD and global-fitting analysis can successfully identify a set of triplet species appearing in different time-scales, when a correct kinetic model is built. 

The polarized triplet-triplet absorption spectra of biphenyl, carbazole, and phenanthrene have been compared, and phenanthrene has been found to differ considerably from the other two molecules.290 T-T Absorption spectra of cyano-substituted benzenes,291 several aromatic vapours,292 tetracene in THF,293 and hexachloroacetone 294 have been reported, and an interesting use of the triplet state of anthracene as a probe for measuring the rates of lateral diffusion of molecules of biological interest has been described.295 Absorption spectra of... 

The conditions for probes to be suitable for direct spectroscopic studies have been discussed in Section II.A. The mobility of the probe between two compartments (i.e., the homogenous phase and the complexation site in the supramolecular system) competes with the decay of the excited triplet states in those compartments. This competition is only possible if the probe s excited state lifetime is either very much longer or of the order of magnitude of the rates for association and dissociation from the supramolecular system. Conceptually, the mechanism is analogous to the formation of excimers and the observation of monomer and excimer emission. The solution for the kinetic equations has been described for excimers several decades ago [179] and can be directly applied to the decay for triplet states in the presence of an organized system. This method assumes that at a particular wavelength the molar absorptivity of the free (P) and complexed triplet state (HP) are different. The absorbance is thus related to the sum of the probe concentrations ... 

As well as linear spectroscopy, the nonlinear spectroscopic techniques of electroabsorption, third harmonic generation and two-photon absorption have all been deployed to investigate other excited states. In addition, phosphorescence probes and photoinduced absorption have been used to investigate the triplet states. In view of the spectral shifts arising from disorder and variations in the chemical structures, these investigations reveal a remarkably consistent picture for PPP, PFO, and PPV. 

Fluorescent BODIPY dyes are known to show valuable photophysieal properties, such as high thermal and photochemical stability, relatively high absorption coefficients, high fluorescence quantum yields, negligible triplet-state formation, and sharp absorption and fluorescence emission spectra. Therefore, BODIPY derivatives are highly attractive as building block for the construction of molecular sensors. Here, we briefly discussed three photophysical mechanisms in the design of BODIPY probes. 

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