Covalently linked systems

Paddon-Row MN (2001) Covalently linked systems based on organic components. In Balzani V (ed) Electron transfer in chemistry, vol 3. Wiley-VCH, Weinheim, pp 179-271... 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

The feasibility of intramolecular electron- and energy-transfer depends on distance and is usually studied in covalently linked systems. However, donor-acceptor dyads can be also arranged by self-assembly what resembles the situation of electron transfer in biological systems. Artificial dyads tethered by a small number of hydrogen bonds immediately dissociate in methanol or water. To improve the binding while keeping the reversibility, a photoinducible electron donor-acceptor dyad linked by a kinetically labile bond was designed. [19]... 

Steady-state fluorescence spectra recorded after the addition of the NDI or PM I acceptor to the bisporphyrin tweezer ( rrl = 660 nm), demonstrated substantial quenching (75%) with increasing quantities of the NDI or PM I acceptors. Time-resolved emission spectra recorded in toluene for the complex 26 were biexponential containing a dominant short-lived CS components (80 ps, -95%) attributed to photoinduced ET from donor porphyrin to NDI, and a minor long-lived component (Ins, 5%). The lifetime of the dominant short-lived CS state is increased two- to threefold relative to covalently linked systems under similar conditions of solvent, donor-acceptor distance and thermodynamics [37]. Charge recombination rates from 1.4 to 3.8 x 1()9s 1 were observed, depending on whether the NDI or PM I acceptor was bound within the cavity. 

Covalently Linked Systems Based on Organic Components... 

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