Ultrafast time-resolved fluorescence

The time-resolved techniques that are usually used for FLIM are based on electronic-basis detection methods such as the time-correlated single photon counting or streak camera. Therefore, the time resolution of the FLIM system has been limited by several tens of picoseconds. However, fluorescence microscopy has the potential to provide much more information if we can observe the fluorescence dynamics in a microscopic region with higher time resolution. Given this background, we developed two types of ultrafast time-resolved fluorescence microscopes, i.e., the femtosecond fluorescence up-conversion microscope and the... 

Geipel, G., Acker, M., Vulpius, D., Bernhard, G., Nitsche, H., and Fanghanel, T. (2004). An ultrafast time-resolved fluorescence spectroscopy system for metal ion complexation studies with organic hgands. Spectrochim. Acta Part A—Mol. Biol. Spectrosc. 60(1-2), 417-424. 

Time-resolved fluorescence is perhaps the most direct experunent in the ultrafast spectroscopist s palette. Because only one laser pulse interacts with the sample, the mediod is essentially free of the problems with field-matter time orderings that arise in all of the subsequently discussed multipulse methods. The signal... 

Recent developments in laser technology and fast detection methods now allow the kinetic behaviour of the excited state species arising from absorption of radiation by polymers to be studied on time-scales down to the picosecond region ( ). An example of a time-resolved fluorescence spectrometer which can be used to study such ultrafast phenomena is illustrated in Figure 5 Q). 

As described in the previous section, the femtosecond fluorescence up-conversion microscope enabled us to visualize microscopic samples based on position-depen-dent ultrafast fluorescence dynamics. However, in the imaging measurements using the fluorescence up-conversion microscope, XY scanning was necessary as when using FLIM systems. To achieve non-scanning measurements of time-resolved fluorescence images, we developed another time-resolved fluorescence microscope. 

To study the excited state one may use transient absorption or time-resolved fluorescence techniques. In both cases, DNA poses many problems. Its steady-state spectra are situated in the near ultraviolet spectral region which is not easily accessible by standard spectroscopic methods. Moreover, DNA and its constituents are characterised by extremely low fluorescence quantum yields (<10 4) which renders fluorescence studies particularly difficult. Based on steady-state measurements, it was estimated that the excited state lifetimes of the monomeric constituents are very short, about a picosecond [1]. Indeed, such an ultrafast deactivation of their excited states may reduce their reactivity something which has been referred to as a "natural protection against photodamage. To what extent the situation is the same for the polymeric DNA molecule is not clear, but longer excited state lifetimes on the nanosecond time scale, possibly of excimer like origin, have been reported [2-4],... 

The ultrafast excitation-energy transfer of the J-aggregates on the octahedral AgBr is studied by the time-resolved fluorescence-anisotropy decay (r(t)) measurements [9]. They are biphasic with two time constants of -0.15 ps and 2-7 ps as shown in Fig. 6. Each phase should reflect some difference in the orientation of the dye molecules of the J-aggregates. 

Time-resolved fluorescence spectroscopy has been used to probe ultrafast interfacial electron transfer since the 1970s (Gerischer and Willig, 1976 Sakata et al, 1990 Willig et al, 1990 Miller et al, 1995 Heimer and Meyer, 1996 Rehm et al, 1996). 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

Dynamic solvent effects have been studied for other solution reactions, including proton transfer reactions, isomerizations, and time-resolved fluorescence studies. This is an important area of modern research involving ultrafast kinetic experiments in solution. 

An unusual example of delayed fluorescence exemplifying El Sayed s rules (Section 2.1.6) was recently reported for the triplet sensitizer xanthone,127 which undergoes ultrafast ISC within 1 ps. Delayed fluorescence with a lifetime of 700 ps was observed in aqueous solution. Temperature-dependent steady-state and time-resolved fluorescence experiments indicate that the T2(n,it ) state, which is primarily accessed by ISC from Si(ji,ji ), is nearly isoenergetic with the Sj state. The delayed fluorescence is attributed to reverse ISC from T2(n,it ), in competition with internal conversion to Tl(7I,7l ). 

The radiationless decay has been investigated by ultrafast polarization spectros-copy [44] and time-resolved fluorescence [45,46]. The results confirm that the radiationless decay occurs by an ultrafast internal conversion, due to intramolecular motion about the bridging bond of the chromophore in the excited state, that the isomerization is nearly barrierless, and that there is only a very weak dependence on medium viscosity, thereby implying that the isomerization occurs by a volume-conserving motion such as a hula twist [47]. 

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