A new family of donor–acceptor systems comprising tin(IV) porphyrin and anthracene subunits: Synthesis, spectroscopy and energy transfer studies

A new family of covalently linked ‘Sn(IV) porphyrin-anthracene’ diad (1), triad (2) and tetrad (3) donor–acceptor (D–A) systems have been designed and synthesized in good-to-moderate yields. While diad 1 possesses one anthracene subunit at the peripheral (meso) position of the tin(IV) porphyrin scaffold, triad 2 possesses two trans axial anthracene subunits at the tin(IV) centre. On the other hand, tetrad 3 is endowed with both the peripheral and axial anthracene subunits in its architecture. These D–A systems have been fully characterised by elemental analysis, FAB-MS, UV-Vis, 1H and 13C NMR and electrochemical methods. UV-Vis, NMR and redox data suggest the absence of intramolecular p–p interaction between the porphyrin and the anthracene/s in 1–3. Fluorescence from the anthracene subunit in 1 and 3 is found to be quenched in comparison with the fluorescence of free anthracene in four different solvents. This is not the case with compound 2. Excitation spectral data provides evidence for an intramolecular excitation energy transfer (EET) from the singlet anthracene to the porphyrin in 1 and 3. The energy transfer efficiency is in the order: 2 (almost negligible) < 3 (~30%) < 1 (nearly quantitative), with the peripheral anthracene right arrowporphyrin pathway being largely favoured. This orientation dependence of EET could be analysed using Forster’s dipole dipole mechanism.