Reversible intramolecular cyclization reactions to form crown ethers are possible with photoactive anthracene derivatives containing two binding sites (see scheme). Different chemical inputs in the form of HgCl2 and Na+ ions result in changes in the efficiency and rate of the forward and reverse processes.
Aza‐crown ethers 2 and 3 with anthracene‐containing pendant arms have been synthesised and characterised. Both compounds bind Group 1 metal cations in solution, forming complexes of 1:1 stoichiometry. The properties of compound 2 and its complexes have been studied by a range of techniques, including NMR, UV and fluorescence spectroscopy and X‐ray crystallography. The pendant arms can adopt either a cis or a trans geometry, the cis geometry favoured with larger cations. The geometry of the complex affects the fluorescence properties of the system, with larger cations giving higher excimer/monomer ratios. Upon irradiation at λ>300 nm, coronand 2 forms the cryptand 5 through a reversible intramolecular [4π+4π] cycloaddition reaction. The rates of the forward and reverse reactions of this photochromic process are cation dependent; in particular the rate of the thermal reverse reaction is decreased by smaller cations and increased by larger cations, especially Rb+. The metal binding constants in methanol for 2 and 5 have been determined, revealing that the cryptand 5 binds Na+ and Rb+ more weakly than crown ether 2 by over two orders of magnitude.
Photochromic molecules, compounds that interconvert between one form and another upon the action of light, [1] play an important role in approaches towards the develop-ment of photo-active switches and logic devices that operate at the molecular level. [2] There are now a number of examples of multi-functional photochromic systems that respond to a separate external stimulus, be it physical or chemical, in addition to a light stimulus. [2e, f] One such example uses a spirobenzopyran unit linked to a crown ether that only exhibits thermally irreversible photochromism in the presence of metal cations. [2f] We have previously reported multifunctional photochromic systems containing bis(anthracene) crown ether molecules that complex s-block cations in addition to undergoing reversible intramolecular photocyclization reactions. [3] Other photochromic systems are based on the same photocycloaddition. [4] Here we report photochemical studies on compounds 1, 2, and 5, which are related to other ditopic ligands [5] in that they contain a 2,2'-bipyridyl (bipy) unit for binding transition metals and two sidearms containing ethereal oxygen atoms for binding alkali metals.Irradiation of acetonitrile solutions of 1 and 2 at 369 nm results in the formation of their [4p4p] photocycloadducts 3 and 4 (Scheme 1), [6] as evidenced by the disappearance of the Scheme 1. Schematic representation of the formation of crown ether photocycloadducts 3 and 4 from 1 and 2, respectively. 1 L a UV absorption band and by NMR spectroscopic studies (see Experimental Section). Data from studies carried out on 1 and 2 and their mercury(II) complexes [5a] [Hg(1)Cl 2 ] and [Hg(2)Cl 2 ], in the absence and presence of excess NaClO 4 , are displayed in Table 1. It is clear from the reaction quantum [18] Reabsorption of donor emission by neighboring acceptors instead of Förster energy transfer is considered to be nominal because of the 2-dimensional alignment of donor and acceptor chromophores as a mixed monolayer. The emission spectra from a mixed monolayer of 3 and 6 (r 4) were measured after small changes of the incidence angle a of the excitation beam (Da < 208). Only minor changes in the emission spectra were observed and the relative emission intensity from the donor and acceptor chromophores remained the same.[19] B. Arkles, CHEMTECH, 1977, 7, 766. [20] The assembly of alkoxysilanes is a two-step process that involves pretreatment of the adsorbates prior to adsorption on the substrate. Initially, siloxane dimers and trimers are formed in the adsorbate solution by an acid-catalyzed condensation. Upon immersion of the silicon wafer, these oligomers adsorb onto the silanol surface by hydrogen bonding and become covalently bound to the surface by curing the wafers. The adsorption of the chromophor oligomers from solution occurs randomly. Therefore, the chromophore composition on the monolayer may not be the same as in solution.[21] The donor (D) and acceptor (A) distribution in the siloxane dimers and trimers was calculated by reaction kinet...
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