Photoactive arrays

FIGURE 1. Energies of the singlet and charge transfer states pertaining to the photoactive array. Reprinted with permission from reference 5. Copyright 1999 American Chemical Society... 

Herein, we wdl discuss several approaches that have led from molecular entities to supramolecular soft and hard systems. In particular, we will show how the molecular structure can be modified to induce the controlled self-assembly of transition metal complexes into sophisticated photoactive arrays with imusual properties derived from the structiu-e of the metal complexes and their intermolecular interactions in the ground and/or excited electronic states within the assemblies. We will start with a survey of the photophysical properties of selected transition metal complexes, followed by an overview of the aggregation mechanism they can undergo to. We will focus our attention on soft assemblies... 

Polymetallic complexes presenting directional energy migration are of much significance for the design of photochemical molecular devices. Large arrays of multiple photoactive and redox-active building blocks (of ruthenium- or osmium tris(bipyri-dine)-type for instance) have been constructed for such purposes [A. 10,8.25-8.27]. 

In particular, the formation of ordered arrays of photoactive units like porphyrins or of ion binding sites (e.g. macrocyclic polyethers see also [8.202]) may induce directional electron and energy transfer or ion channel features [9.171a, 9.172], Such effects are of interest for optoelectronic information storage and for ionic devices. 

The monolayer also provides an environment of variable dielectric so that intermolecular association between photoactive molecules can readily occur. For example, molecular association of pyrene within a Langmuir-Blodgett film is clearly seen through time-resolved fluorescence measurements on the picosecond timescale [92], Attenuated total reflectance studies of dyes in cast films can similarly reveal their positions and photophysical interactions [93], Photochromism in a monolayer assembly has been attributed to excitation of ion-pair charge transfer complexes formed within the array [94]. 

We will review here work wherein several types of species incorporate the Ir-bis-terpy unit and derivatives. The search for multicomponent arrays, including metal-based photoactive centres in combination with electron donor and acceptor components, was started a couple of decades ago, and one of the first systems, compound 14+, was studied by Meyer et al. (Fig. 1) [5]. In this multicomponent system, the electron donor phenothiazine (PTZ) and the bridged 2,2/-bipyridinium (DQ) units are linked to the photoactive unit by flexible methylene connections. In this early example, excitation at the Ru-based chromophore ultimately yields a PTZ+-DQ" CS state which lives 165 ns and for which the transiently stored energy is 1.29 eV. 

Perylenediimides represent another class of photoactive dyes which are characterized by their strong fluorescence emission and facile electrochemical reduction. Recently, a supramolecular bis(phthalocyanine)-perylenediimide hetero-triad (compound 42) has been assembled through axial coordination [47]. Treatment of perylenediimide 43, which has two 4-pyridyl substituents at the imido positions, with 2.5 equiv. of ruthenium(II) phthalocyanine 44 in chloroform affords 42 in 68% yield (Scheme 3). This array shows remarkable stability in solution due to the robustness of the ruthenium-pyridyl bond. Its electronic absorption spectrum is essentially the sum of the spectra of its molecular components 43 and 44 in... 

Perathoner S, Passalacqua R, Centi G, Su DS, Weinberg G. Photoactive titania nanostructured thin films. Synthesis and characteristics of ordered helical nanocoil array. Catal Today. 2007 122 3-13. 

Photoswitchable electrical communication between enzymes and electrodes has also been achieved by the application of photoisomerizable electron-transfer mediators [195, 199]. DilTusional electron mediators (viologen or ferrocene derivatives) were functionalized with photoisomerizable spiropyran/merocyanine units. These mediators can be reversibly photoisomerized from the spiropyran state to the merocyanine state (360 < A < 380 nm) and back (A > 475 nm). An enzyme multilayer array composed of glutathione reductase or glucose oxidase was electrically contacted only when the photoactive group linked to the redox relay (viologen or ferrocene derivative, respectively) was in the spiropyran state. 

Recent work has investigated the potential to control the partitioning of charge in branched arrays [230]. This has many potential applications to the development of photoactive networks and dendritic systems capable of electron transfer. Compounds 17 and 18 employ 1,3,5-triaminobenzene as the central branch point. In each molecule ANI is attached to the 1 position and serves as the electron donor. The electron acceptors again are NI and PI, and are attached to the 3 and 5 positions, respectively in compound 17. Excitation of ANI with 400 nm laser pulses results in electron transfer exclusively to the NI branch. This is due to the 0.3 V... 

Lehn [1] has also envisaged a wonderful array (literally) of supramolecular systems containing photoactive, electroactive and ionactive components. These systems can be designed and created to provide functions and properties such as light conversion by energy transfer, non-hnear optics, molecular wires. 

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