Photoactive centre

One of the favourite generic arrangement is the molecular triad, consisting of a photoactive centre (PC), an electron donor (D) and an electron acceptor (A). In systems such as D-PC-A, the charge separated state D+-PC-A is obtained in two consecutive-electron transfer processes after excitation of PC. Of course, several variants exist, depending on the electron transfer properties of PC and its excited state, PC, as well as on the precise arrangement of the various components (PC-Ai-A2 or D2-Di-PC, in particular, if PC is an electron donor or an electron acceptor, respectively). 

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. 

Having verified that the photophysical and electrochemical properties of the Ir(terpy)2+ derivatives were satisfying the characteristics we looked for, in that the excited state energy was well above 2 eV and that reduction was easier than for the related Ru(terpy)2+ derivatives, we decided to use Ir(terpy)2+ as assembling unit and as electron relay in the following evolution of the project. We then synthesized two triads based on the Ir(III) complex, using zinc or free base tetra-aryl-porphyrin as the donor and photoactive centre and a gold tetraarylporphyrin as the acceptor. 

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