Triplet-state probes phosphorescence

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. 

The quenching of benzophenone phosphorescence has been used by Mar and Winnik (1981) as a photochemical probe of hydrocarbon chains in solution. The bimolecular reaction for quenching the triplet state of 4-methoxy-carbonylbenzophenone [24] by 1-pentene occurs at rates which are below the diffusion limit by two to three orders of magnitude. Consequently, the intramolecular quenching reactions of to-alkenyl esters of benzophenone-4-carbo-xylic acid [25] occurs under conformational control. In [25] the point of... 

Fig. 14. Schematic representation of energy levels and transitions for fluorescence and related processes kic, rate constant for interval conversion fcF, rate constant for fluorescence fcISC, rate constant for intersystems crossing fc[cp> rate constant for internal conversion from triplet state kp, rate constant for phosphorescence S, energy level for the first excited singlet state after solvent rearrangement for a polarity probe in a polar solvent.

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

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. 

The electronic excited state is inherently unstable and can decay back to the ground state in various ways, some of which involve (re-)emission of a photon, which leads to luminescence phenomena (fluorescence, phosphorescence, and chemiluminescence) (22). Some biologic molecules are naturally fluorescent, and phosphorescence is a common property of many marine and other organisms. (Fluorescence is photon emission caused by an electronic transition to ground state from an excited singlet state and is usually quite rapid. Phosphorescence is a much longer-lived process that involves formally forbidden transitions from electronic triplet states of a molecule.) Fluorescence measurement techniques can be extremely sensitive, and the use of fluorescent probes or dyes is now widespread in biomolecular analysis. For example, the large increase in fluorescence... 

The environmental sensitivity of the fluorescence and phosphorescence of phenylalanine, tryptophan and tyrosine, and their side chains, is often examined when considering the macromolecular luminescence of natural peptides and proteins. Therefore, lower-lying singlet and triplet states of toluene, aniline and phenol have been extensively studied as the simplest models of the proteins mentioned above, respectively. Knowledge of the various aspects of electronic spectra of the corresponding aromatic amino acids is often exploited to probe those of the proteins137. In other words, accurate information on both... 

A variation of the quenching method leading to Eq. (25) is to follow the decay of the triplet state from the emission of a luminescent quencher, which is much more intense than the phosphorescence of the probe [192]. In this case, the observed rate constant corresponds to the growth and subsequent decay in the emission profile of the quencher. However, an additional rate constant, corresponding to the emission lifetime of the quencher, has to be included in Eq. (25). The exit rate constant was determined to be 2.5 x 10 s for 1-bromonaphthalene when quenched by either Eu or Tb. This value is the same (Table 16) as that determined using the anionic quencher, NOf [62], showing that this method is useful. However, care should be taken to eliminate the possibility of reverse energy transfer. 

From magnetic resonance spectroscopy [49] it is well-known that IB effects are adequately circumvented by the tricks of a spin echo experiment. For instance, in a two-pulse echo experiment, IB effects are averaged out and one probes spin dephasing determined by time-dependent fluctuations characteristic of HB only (and not IB). More specifically, a nll-r-n microwave pulse sequence is applied, where the first nil pulse creates a coherent superposition state for which a la = 1 and the n pulse, applied at time r after the first pulse, generates a spin coherence (the echo) at time 2r after the initial pulse. The echo amplitude is traced with r. The echo amplitude decay time is characteristic of the pure dephasing dynamics. For phosphorescent triplet states it is possible to make the echo optically detectable by means of a final nil probe pulse applied at time f after the second pulse [44]. In Fig. 3b, the optically detected echo amplitude decay for the zero-field transition at 2320 MHz of... 

The lifetimes of the sublevels of Pd (qol)2 in the emissive triplet state were determined by means of microwave recovery experiments. At 1.4 K, the triplet state sublevels are thermally isolated from each other, and the individual lifetimes of the microwave-pumped spin levels are reflected in the biexponential kinetics of the recovery signal [81]. Figure 25 shows a typical transient,probed for the D-E transitions for site 1 . Labeling the triplet substates as l), Il), and IIl), such that eni>ei> n>0 (i.e., 2 = in- i, D- = i-en), the sublevel lifetimes as determined for site 1 from the fits are Ti = 90 ms, Th= 180 ps, and ein = 80 ps. These results are in very good agreement with the triplet sublevel lifetimes as determined from phosphorescence decay measurements [851. Likewise, for site... 

In the frame of CSTPM, the following dynamics parameters of the cascade system components can be experimentally measured the spin label rotation correlation time and spin relaxation parameters, the fluorescence and phosphorescence polarization correlation times, the singlet and triplet state quenching rate constants, the rate constant of photoisomerization, and the rate constant of the triplet-triplet energy transfer. This set of parameters is a cumulative characteristic of the dynamic state of biomembranes in the wide range of the probes amplitude and characteristic time. 

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. 

Even when the intramolecular vibrations do not mix with the phonons, they are coupled by the intermolecular potential, through its dependence on the internal molecular coordinates. This coupling determines the band structure of the vibrational excitons or vibrons in molecular crystals, which can be studied by inelastic neutron scattering (see Fig. 12). In the case of TCB crystals the vibron band structure has been observed recently by laser phosphorescence [59, 104]. This is a rather special achievement since, normally, optical techniques probe only the g = 0 excitations. It is related to the long lifetime of triplet electronic excitons in TCB, which are scattered into different q states. By phosphorescent de-excitation q 0 triplet excitons to the electronic ground state it is possible to detect the g 0 vibrons, with intensities controlled by the (very low) temperature and by the phosphorescence delay time. 

Figure 7.14 Possible radiationless processes following creation of 6 benzene, one of the vibronic levels which is El-accessible from ground-state benzene (cf. Fig. 7.13). This level may undergo internal conversion (IC) to an isoenergetic, vibration-ally hot So molecule, or it may undergo intersystem crossing (ISC) to an isoenergetic level in triplet state T.,. The T - So phosphorescence transition can be monitored for experimental evidence of ISC. Time-dependent S — Sq fluorescence decay furnishes a probe for depopulation of S., through radiative (fluorescence) and nonradiative (IC, ISC) decay.

Electron transfer rates between adrenaline and related benzene diols and complexes of iron(III) with some substituted 1,10-phenanthrolines have been reported [67] in surfactant systems. In cationic systems the reactions take place in the aqueous phase and reaction rates are lower than they are in simple aqueous systems, but in anionic surfactant systems the reaction rates are enhanced, reactions probably taking place at the micellar interface. The rates of exit and entrance of aromatic compounds from and into micelles have recently been studied using phosphorescence decay measurements [68] exit rate constants of aromatic hydrocarbons are of the order of 10 to 10 s " S whereas values of 10 to 10 (moll ) s have been reported for intramicellar energy transfer processes. Release of aromatic phosphorescence probes from micelles followed by their deactivation in the aqueous phase is hence expected to be an important mode of deactivation of the triplet state [69]. Kinetic schemes for triplets that are partitioned between aqueous and micellar phases are considered for the cases of single occupancy and double occupancy of the micellar units. 

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