Crowned spirobenzopyrans

A class of crown spirobenzopyrans 10 exhibit thermally irreversible photochromism only in the presence of alkali-metal cations (Scheme 3). The favored complexation between 10 and lithium cation was thus revealed by the effect of lithium salt on the electronic absorption spectra of 10. Lithium complexation was also corroborated on the basis of FAB mass experiments. Before addition of Lil to 10, ion peaks for MH, [M + Na] and [M + K] were detected after the addition the signals decreased, while a peak for [M + Li]+ appeared (Figure 10) . 

Crowned Spirobenzopyrans as Alkali Metal Cation Sensors.108... 

Cryptand and Bibrachial Lariat Type Crowned Spirobenzopyrans as... 

In this chapter, we will concentrate on the development of macrocycle-based fluorescent chemosensors for metal ions. Some examples from our laboratory will be discussed in detail. We have also inserted a few paragraphs dealing with the crowned spirobenzopyran ion-sensing systems since this new type of chemosensor has been developed only recently. 

In this section, various crowned spirobenzopyrans for use as alkali metal cation receptors have been presented. This new type of chromoionphore is conceptually different from the crown ether dyes because, in the case of the crown ether dyes, the absorption bands of the chromophones are shifted by complexation of cations. Crowned spirobenzopyrans might provide sophisticated ion-sensing systems such as ion sensors whose ion selectivity can be photochemically switched between cations and anions.104... 

In this section, the sensing properties of the cryptand and bibrachial lariat-type crowned spirobenzopyrans were presented. In the spirobenzopyrans, coloration was efficiently induced in the presence of the alkaline earth metal cations. The cryptand and bibrachial lariat-type crowned spirobenzopyrans represent highly sensitive and selective chemosensors for alkaline earth metal cations. 

Chemically modified crowned spirobenzopyran 112, containing a pyrenyl fluorophore attached at the nitrogen atom, can function as a fluorescence emission switch <2004T6029>. This sensor displayed a quenching of the PET fluorescence emission of the fluorophore in the absence of metal ions (the merocyanine form was not produced). When, however, the spiro form of 112 was converted into the merocyanine form by metal ion complexation of the crown ether portion of the molecule, a fluorescence resonance energy transfer (FRET) from the pyrene to the merocyanine moiety took place, producing fluorescence emission. 

In 1990, we introduced conceptually new artificial receptors, crowned spirobenzopyrans, in which recognition of alkali-metal cations induced a configurational change in the receptor framework that was accompanied by signaling... 

Soon after our publication, Kimura et al. (Osaka University) reported other versions of crowned spirobenzopyrans (13, Figure ll).54,55 They connected... 

M. Inouye, Y. Noguchi, and K. Isagawa, Sensitive and selective coloration ofcryptand-type crown spirobenzopyrans for alkaline earth metal cations, Angew. Chem. Int. Ed. Engl. 33, 1163-1166 (1994). 

K. Kimura, T. Yamashita, and M. Yokoyama, Cation-specific isomerization of crowned spirobenzopyrans, 7. Chem. Soc., Chem. Commun. 1991, 147-148. 

Crowned spirobenzopyrans as ion-responsive photochromic materials 03YGK322. 

Inouye et al. have designed a sensitive and selective cryptand-type crown spirobenzopyran sensor for alkaline-earth metal cations (Figure 92) [119]. Upon metal ion complexation, the spirocycle opens up, resulting in a large increase in the absorption in the visible range. For n = 1 and 2 the selectivity orders were found to be Ca " " > Mg " " > Sr " " and > Ba > Ca " ", respectively. 

Kimura et al. [259] reported applying organic photochromic compoimds to photochemical switching of metal-ion complexation and ionic conduction by combining photochromism with metal-ion binding property of crown ether derivatives. They synthesized vinyl polymers, incorporating a crowned spirobenzopyran moiety at the side chain ... 

The crowned spiropyran in the electrically neutral form can bind an alkali metal ion with the crown ether moiety. At the same time, the spirobenzopyran portion isomerizes to the corresponding merocyanine form photo-chemically. The zwitterionic merocyanine form of crowned spirobenzopyran moiety brings about a significant change in the metal-ion binding ability. This prompted the authors to apply the compound to photo-responsive ion-conductive materials. They observed that the ion-conductivity was increased by ultraviolet light and decreased by visible light [259]. 

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