Fluorescence switches behavior

TTF-based D-A systems have been extensively used in recent years to play around photoinduced electron transfer processes. Typically, when an electron acceptor moiety that emits fluorescence intrinsically is linked to TTF (D), the fluorescence due to the A moiety may be quenched because of a photoinduced electron transfer process (Scheme 15.1). Accordingly, these molecular systems are potentially interesting for photovoltaic studies. For instance, efficient photoinduced electron transfer and charge separation were reported for TTF-fullerene dyads.6,7 An important added value provided by TTF relies on the redox behavior of this unit that can be reversibly oxidized according to two successive redox steps. Therefore, such TTF-A assemblies allow an efficient entry to redox fluorescence switches, for which the fluorescent state of the fluorophore A can be reversibly switched on upon oxidation of the TTF unit. 

Fig. 20.2. Photo-switchable probes constructed from activator-reporter pairs, (a) Spectrally distinct reporters exhibit photo-switching behavior. The lower panel shows the fluorescence time traces of Cy5, Cy5.5, and Cy7 when paired with a Cy3 dye as the activator. The upper panel shows the green laser pulses used to activate the reporters. The red laser was continuously on, serving to excite fluorescence from the reporters and to switch them off to the dark state, (b) The same reporter can be activated by spectrally distinct activators. The lower panel shows the fluorescence time traces of Cy5 paired with different activators, Alexa Fluor 405 (A405), Cy2, and Cy3. The upper panel shows the violet (405nm, magenta line), blue (457nm, blue line), and green (532 nm, green line) activation pulses...

This purely organic knot synthesis is appealing because in its simplest form it is a one-pot procedure that affords reasonable yields and unique possibilities for the preparation of derivatives in the context of the knots prepared to date. This advantage is at the same time the principal defect of the molecules, as they have yet to show any recognition behavior, and the knot is essentially a closed system. However, the functionalization of the knots makes it possible to use them as scaffolds for pendant groups which have some function, for example, as fluorescent switches. The many other possibilities for these knots have yet to be explored, although the synthetic foundation is laid. 

Dickson, R.M. Cubitt, A.B. Tsien, R.Y Meemer, W.E., On/Off Blinking and Switching Behavior of Single Molecules of Green Fluorescent Protein, Nature (1997), 388,355-358. 

Switching systems based on photochromic behavior,I29 43,45 77-100 optical control of chirality,175 76 1011 fluorescence,[102-108] intersystem crossing,[109-113] electro-chemically and photochemical induced changes in liquid crystals,l114-119 thin films,170,120-1291 and membranes,[130,131] and photoinduced electron and energy transfer1132-1501 have been synthesized and studied. The fastest of these processes are intramolecular and intermolecular electron and energy transfer. This chapter details research in the development and applications of molecular switches based on these processes. 

Besides fluorescence spectroscopy, time-resolved spectroscopy can rely on the measurement of excited (singlet or triplet) state absorption. Similarly to ground-state absorption, the spectral and absorbance properties may be altered by CyD complexation and yield information about the behavior of the complex in the excited state in addition, the time dependence (formation and decay) of the excited state absorption yields information about the kinetics and dynamics of the system. This is illustrated by the behavior of the lowest triplet state of naphthalene as measured by nanosecond spectroscopy using a Q-switched Nd YAG laser at 266 nm for excitation [21]. The triplet-triplet absorption spectra were measured in neat solvents (water and ethanol) and in the presence of a- and -CyD (Fig. 10.3.3). The spectra in ethanol and H2O had the same absorption maximum, but the transition was considerably weaker and broadened in H2O. Both CyDs induced a red shift, and a-CyD additionally narrowed the main band considerably. Fig. 10.3.4 shows the effect of a-CD concentration on the time evolution of the triplet-triplet absorption at 416 nm in the microsecond range. Triplet decay was caused by O2 quenching a detailed kinetic analysis of the time dependence yielded two main components which could be assigned to the free guest and the 1 2 complex, in full... 

Pyrene has a nonallowed 0 0 transition (ground state to excited state) that is markedly medium-dependent. Media effects are readily monitored through pyrene s fluorescence vibrational structure [5]. The compounds 1--aminopyrene (1-AP) and 1-pyrenecarboxylic acid (PCA) each possess two chromophores that interact in the excited state the extent of the interaction depends on the media. The lone pair of the amine chromophore of 1-AP acts as an internal switch for photophysics. Previous work [6] showed that the photophysical behavior of 1-AP is dominated by protonation, or blocking, of the lone pair (Figure 30.1). Protonation of the amino group leads to a complex, 1-APH" , that exhibits photophysical properties similar to those of pyrene the free 1-AP system... 

As a specific example, we can consider H+-driven YES and PASS 1 logic gates 5 and 6, respectively, both of which carry the same fluorophore, that is, the same excitation and emission colors. However, tag 5 switches on its fluorescence as pH is reduced to low values while 6 shows pH-independent fluorescence. This is the distinction between the two tags. Many other distinctions, that is, other Boolean logic actions, can be built up in similar ways. The different pH-dependent fluorescence behavior of 5 and 6 arises from the presence of the tertiary amine receptor in 5 and its absence in 6. As seen in the previous sections, the amine is the donor component in the PET process whereas the anthracene fluorophore serves as the acceptor. Thus, fluorescence is minimal in neutral solutions, but under... 

Surprisingly, each spot showed discrete jumps of fluorescence intensity in the transient measurement the intensity was intermittent and switched rapidly on and off states as shown in Fig. 5a. This behavior is often found in single molecule spectroscopy due to photobleaching of the chromophore... 

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