The thoughful construction of molecular switches has led to a gamut of supramolecular systems that can be used in molecular electronics. These include molecules based on thienylethenes, spiropyrans, fulgides, dithienylphenanthrolines, and diazafluorenes. This article reviews the recent developments made in the synthesis and characterization of all these systems, thereby allowing a comparative study to validate the viability of these switchable molecules on a nanoscale. Also, the drawbacks of each class are demonstrated and, at the same time, the remedies for further improvisation are prescribed. We have made an honest attempt to present an exhaustive account of all the different photochromic switches developed by us hitherto.

A dithienylethene derivative containing a cyclobutene-1,2-dione skeleton does not exhibit photochromic properties. However, when both ketone functions are protected with cyclic acetal groups, photochromic behavior is observed.The properties of organic switches have been widely detailed in the literature. 1 One of the main challenges in employing these photochromic compounds for applications in optical memory media is to establish nondestructive readout capability. One of the ways to induce this property is to introduce gated photochromic reactivity into the system.A compound is said to possess the property of gated reactivity if it converts from a photoinert state at a particular wavelength to a photoactive state by the application of external stimuli such as reactive chemicals, 2-6 heat, 7,8 or multiphoton absorption. 9 Several gated diarylethene photochromic systems have been reported. Strategies such as esterification reactions, 2 [4 + 2] cycloaddition reactions, 3 acid/base reactions, 4 redox processes, 6 and hydrogen bondings 6,8 have been employed to manipulate the photochromic reactivity. Previously, a derivative similar to 1a, wherein methyl groups were present in lieu of phenyl substituents, was found to be photoinert. 10 The absence of any photochromic reaction was attributed to the apparent rigidity of the structure. 10 In contrast, cyclobutene-1,2-dione derivatives can perform a photochemical(1) (a) Bouas-Laurent, H.; Durr, H.

Photoisomerization of spiropyrans has been the subject of many interesting studies in a wide number of fields, such as sensors for cations, [1] sensors of pH, [2] switchable surfaces, [3] lasing effect, [4] among many others. This molecule can be photoisomerized upon UV irradiation, giving rise to the zwitterionic merocyanine (MC) form, which dramatically changes its hydrophilicity and photophysical properties (see Scheme 1). Irradiation in the visible region or heating brings back the spiropyran (SP) to the MC form. Solvent polarity and pH have been already shown to strongly influence the properties of spiropyrans.[5] SP does not absorb in the visible range, while the merocyanine, which absorbs at ca 570 nm (in DMSO), exhibits red fluorescence and its lifetime is typically about 260 ps. [6] Recently the use of spiropyrans in the field of dual emission for bio-imaging has been reported, where silica nanoparticles containing spiropyran and a green-emitting perylene derivative were shown to exhibit green or red emission depending on the UV/Vis irradiation. [7] Such systems may become even more efficient if the dyes embedded inside the porous silica nanoparticles aggregate as little as possible, as well as if the dyes are completely isolated from the biosystem being labeled. This problem may be addressed by combining spiropyrans with versatile porous materials, such as zeolite L, which are known to encapsulate organic dyes in the monomeric form and to protect them against the outer chemical environment.Zeolite L is a porous aluminosilicate material consisting of thousands of one-dimensional nanochannels parallel to each other, which run throughout the long axis of the crystal.[8] Zeolite L can be used not only as luminescent devices, [9] in bioimaging, [10] or in photodynamic treatments, [11] but also in many other applications.[12] This class of crystals can still be functionalized on glass, indium-tin oxide or other substrates and can be synthesized from 50 nm up to 20 mm, [13] which may be strategic, depending on the application. SP-functionalized zeolite L crystals with encapsulated dyes may be an interesting option to make new dual emitters, which can be used in bio-imaging experiments, even without taking advantage of energy transfer between encapsulated dyes and SP. Because cells may exhibit auto-fluorescence, a system having two different emissions at different times allows for distinguishing between the auto-fluorescence and the luminescence of the dual-emitters, simply by direct visualization of the sample in the microscope.The advantages of using zeolite L intead of, for example, silica nanoparticles, are: 1) the size of the zeolite L crystals can be synthetically tuned from 50 nm to 20 mm, in order to be used in different experiments; 2) the luminescence of the encapsulated dyes, which can be highly anisotropic, is also strong because the inserted dyes are in the monomeric form and are completely isolated from the outer bio-environment; 3) zeolite L has been already shown to be easily functionalized...

Dithienylethene-phenanthroline ligands as new photochromic systems are described. The photochemical and photophysical properties are strongly influenced by the substituents on the thiophene moiety. The photochromic properties are lost, if one or two methyl groups in position 2 and/or 2' of the target molecules (2o, 3o, 4o) are replaced by isopropyl groups. Replacement of a methyl group by a phenyl group in position 5 and/or 5' shifts the absorption maxima from 514 nm to 575 nm for the free ligand (1c, 2c) and from 530 nm to 613 nm for the corresponding ruthenium complex (Ru(1c), Ru(2c)) in its closed form. Unfortunately, the photochromic unit in its closed form can be reopened by a back reaction in the dark at room temperature. Complex Ru(2o) shows an emission with a maximum at 608 nm. The emission is quenched if the metal complex is in its closed form (Ru(2c)). Fatigue resistance is better for complex Ru(2o) than for the free ligand (2o).

Fulgimides monosubstituted with [M(bpy)(3)](2+) (M = Ru, Os; bpy = 2,2'-bipyridine) chromophore units and with a single bpy group were synthesized and investigated as components of conceivable dinuclear photochromic switches of luminescence. The E-, Z- and closed-ring (C) photoisomer forms of the bpy-bound fulgimide were successfully separated by semi-preparative HPLC. The same procedure failed, however, in the case of the [M(bpy)(3)](2+)-substituted fulgimides. Energy transfer from the excited photochromic unit to the metal-bpy centre competes with the fulgimide cyclization, reducing the photocyclization quantum yields by approximately one order of magnitude compared to the non-complexed fulgimide-bpy ligand (phi(EC) = 0.17, phi(EZ) = 0.071, phi(ZE) = 0.15 at lambda(exc) = 334 nm). The cycloreversion of the fulgimide-bpy ligand is less efficient (phi(CE) = 0.047 at lambda(exc) = 520 nm). The intensity of the (3)MLCT-based luminescence of the metal-bpy chromophore (in MeCN, phi(deaer) = 6.6 x 10(-2) and tau(deaer) = 1.09 micros for Ru; phi(deaer) = 6.7 x 10(-3) and tau(deaer) = 62 ns for Os) is not affected by the fulgimide photoconversion. These results and supporting spectro-electrochemical data reveal that the lowest triplet excited states of the photochromic fulgimide moiety in all its E-, Z- and closed-ring forms lie above the lowest (3)MLCT levels of the attached ruthenium and osmium chromophores. The actual components are therefore unlikely to form a triad acting as functional switch of energy transfer from [Ru(bpy)(3)](2+) to [Os(bpy)(3)](2+) through the photochromic fulgimide bridge.