Fluorescence transient

Figure 14. Confocal fluorescence images of single molecules of (a) the model compound 69 and (c) the multichromophoric dendri-mer 70 in a polyvinylbutyral (PVB) film. The fluorescence intensity transients (fluorescence intensity versus time) exhibit typical on-off behavior for 69 (b) and jumps between different emissive levels for 70 (d). Pictures taken from ref. [48],...

FRET interactions are typically characterized by either steady-state or transient fluorescence emission signals from the donor or acceptor species. Efficient nonradiative energy transfer results in donor PL loss associated with acceptor gain in photoluminescence intensity (if the acceptor is an emitter). The rate of this energy transfer is related to the intrinsic lifetime of the isolated donor and depends strongly on the donor-acceptor separation distance ... 

Table 2 Si Lifetimes (nsec) of Selected Liquid Alkanes at Room Temperature Determined in Steady-State Excited State Quenching or in Transient Fluorescence and Absorption Studies...

Figure 20 Time-resolved transient absorption spectra observed at 0 nsec (solid line), 100 nsec (dashed line), and 1 psec (dotted line) after e and transient fluorescence spectrum of TMB observed at 300 nsec after e during PR-LFP of TMB (1.0 x 10 M) in DCE. Energy of AV532, 130 mJ.

Values of kD which appear in Table 9 were obtained by the usual biexponential decay analysis37) adapted from the intermolecular version due to Birks 7l). Despite observations made for meJo-bis( 1 -(2-naphthyl)-ethyl) ether13) and l,3-bis(2-naphthyl) propane 159) of one rise time and two decay times in the transient fluorescence of the excimer, there have been no reports confirming this complication for the compounds... 

Fig. 3. Left Transient fluorescence signal of nascent CH2 after dissociation of CH2N2 with unchirped MIR laser pulses. A biexponential fit yields time constants of 480 fs (insert) and 36 ps. Right Proposed pathway of sub-statistical dissociation on the potential energy surface with respect to the reaction coordinates Ron and CH2 out-of-plane bend. See text for details.

The analysis of the transient fluorescence spectra of polar molecules in polar solvents that was outlined in Section I.A assumes that the specific probe molecule has certain ideal properties. The probe should not be strongly polarizable. Probe/solvent interactions involving specific effects, such as hydrogen-bonding should be avoided because specific solute/solvent effects may lead to photophysically discrete probe/solvent complexes. Discrete probe/solvent interactions are inconsistent with the continuum picture inherent in the theoretical formalism. Probes should not possess low lying, upper excited states which could interact with the first-excited state during the solvation processes. In addition, the probe should not possess more than one thermally accessible isomer of the excited state. 

At very low water content (wo=0-2) the porphyrins (ZnTPP and ZnTPPS4 ) are located at the interface and are dynamically quenched by duroquinone (DQ), which is solubilized in the organic phase, with observed rate constants of k=9.1><109 dm3 mol 1 s 1, ZnTPP/DQ, wo=0 and kq= =1.9><109 dm3 mol s ZnTPPS4-/ DQ, w0=2. However, a quencher bound at the interface like anthraquinone-2-sulphonate (AQS) interacts statically with ZnTPP at the interface as shown by steady-state and transient fluorescence techniques (Table 2)... 

The initial charge separation in PS I and PS II can be followed by what are known as ultrafast optical spectroscopy techniques. Several variations on this method exist, but they can be grouped into pump-probe absorbance difference and transient fluorescence methods (25, 26). In the first instance, the sample is irradiated with a pump pulse to initiate the electron transfer and the absorbance is measured using a probe pulse at a... 

Similar to bacterial RC there is spectral and ESR evidence that a pheophytin a molecule operates as an intermediary electron acceptor in PSII-RC. Optical absorbance changes, with a spectrum similar to that of a pheophytin a anion radical could be detected in PSII-enriched particles illuminated at low redox potentials (— 0.65 V) [57,77]. The appearance of the Ph signal could be correlated to a decrease in the extent of the rise in fluorescence of PSII of chlorophyll a observed upon illumination [78]. This apparent discrepancy (reduction of an electron acceptor is expected to cause an increase of fluorescence) is now explained by the fact that the fluorescence increase is in reality a delayed fluorescence emitted by the return to the ground state of P -682 regenerated by electron transfer from the pheophytin anion [79]. The lifetime, of this transient fluorescence rise is 2-4 ns, and that of electron transfer from Ph to P -6%2 = 4 ns, when PSII particles are poised at —0.45 V [73]. This transient fluorescence increase is, however, almost totally suppressed when A,j,(Ph) is prereduced chemically before illumination. Using this experimental criterium the midpoint potential of the Ph /Ph couple has been estimated to be -0.61 V [73,80]. 

Figure 2.7. Transient absorption spectra observed at Ons, 100 ns, and 1 ps after the 8-ns electron pulse, and transient fluorescence spectrum of TMB + observed at 300 ns after the electron pulse during pulse radiolysis-laser flash photolysis of TMB (lx 10-2M) in Ar-saturated 1,2-dichloroethane. Excitation wavelength, 532 nm. Laser pulse energy, 140 mJ pulse-1.

The optical layout for the measurement of biological samples (cells) is shown in Figure 29.3b. The sample was irradiated with co-linear IR and visible light beams. The transient fluorescence from the sample was collected from the opposite side by an objective lens. In this optical layout, the spatial resolution was determined by the objective numerical aperture (NA) and the visible fluorescence wavelength IR superresolution smaller than the diffraction limit of IR light was achieved. Here, Arabidopsis thaliana roots stained with Rhodamine-6G were used as a sample. We applied this super-resolution infrared microscope to the Arabidopsis thaliana root cells, and also report the results of time-resolved measurements. 

Transient Fluorescence Image with IR Super-Resolution in Solution... 

To confirm that the transient fluorescence is indeed an action signal due to resonant infrared absorption, the fluorescence intensity in the image was measured as a function of IR wavelength (TFD-IR spectroscopy, [27]). Figure 29.4e shows the TFD-IR spectrum of Rhodamine-6G. When the IR wavelength was scanned over the... 

Figure 29.5 Picosecond time-resolved transient fluorescence images at several delay times, when the IR (diameter, 2.5 mm0) and visible (diameter, 7 mm()) lights are fixed to 3400nm and 621 nm, respectively.

Sakai, M. and Fujii, M. (2004) Vibrational energy relaxation of the 7-azaindole dimer in CCI4 solution studied by picosecond time-resolved transient fluorescence detected IR spectroscopy. Chem. Phys. Lett., 396, 298-302. 

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