DCM crown

Figure 2,7. Absorption and fluorescence spectra of DCM-crown and its complexes with perchlorate salts in acetonitrile. DCM-H is the compound in which the crown is replaced by a hydrogen atom this compound cannot undergo intramolecular charge transfer and its fluorescence quantum yield is very low (3 x 1(C) (adapted from Ref, 40.)...

In calcium chelators Indo-1 (17) and Fura-3 (18b) (Figure 2.9),(18) the fluoropho-res have donor-acceptor stilbene-like structures rigidified so as to avoid photoisomerization. Based on the same principle, Fura-2 (18a)a8) is one of the most popular calcium indicator for microscopy of individual cells because, in contrast to Quin-2 (see Section 2.2.5.), the excitation spectrum is blue shifted on cation binding, thus allowing intensity-ratio measurements. On the other hand, there is almost no shift of the emission spectrum, which can be interpreted along the same line as DCM-crown (see earlier in this section). 

Finding a supramolecular system capable of fast and spatially controllable release of cations upon irradiation is of great interest for the study of intracellular process triggered by an ion concentration jump [42, 43, 45-47]. The complex of metal ions with a crown ether-linked merocyanine (DCM-crown) is an example of system (Scheme 7.9) which can temporarily release ions [46]. DCM-crown consists of DCM, a well-known laser dye, in which the dimethylamino group has been replaced by a macrocycle (monoaza-15-crown-5) that can bind metal ions [42]. 

Demonstration of the photorelease has been done in particular with Sr + [46]. This process was monitored on several time scales providing evidence for (1) the delayed formation in 9 ps of the charge transfer state of the merocyanine chromophore following ultrafast photodisruption of the nitrogen - cation interaction, (2) the cation movement away from the excited chromophore into the bulk in 400 ps, (3) recombination of the complex in the ground in about 120 ns. These three steps are respectively illustrated in Fig. 7.17a, b, c (see caption for details). Similar transient absorption studies have been carried out on a PDS-crown-Ca + complex, where PDS is an aza-crown derivative of a substituted stilbene [47]. The spectrodynamics observed on the short time scale are very similar to those found in step (1) of the above description, with in particular a delayed rise of a stimulated emission band attributed to a solvent-separated cation-probe pair. Although the full scenario of the cation photoejection from the DCM-crown-Sr, is complex [46], the spectra shown in Fig. 7.17 demonstrate that at least part of the photoexcited complexes does eject the ion into the bulk. 

Fig. 7.17. Characterization of the photo-ejection of a Sr ion from a compiex with DCM-crown and of their recombination in the ground state. (Adapted from ref. [46]).

DCM-crown (10,11 0)). It is generally admitted that the fluorescence emission of DCM and its derivatives results almost solely fiom an intramolecular charge transfer... 

Table II. Emissive properties of DCM-crown and its complexes in acetonitrile.

Figure 3. Absorption and emission spectra of DCM-crown in acetonitrile before and after full complexation by alkali or alkaline-earth metal ions. (Adapted from ref. 10),...

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