Simultaneous two-photon excitation energy transfer (SEET) and non-linear photochemistry

The previously demonstrated simultaneous two-photon excitation energy transfer (SEET) using a trichromophoric model compound was further investigated. Therefore, two identical donors (A - antenna) were covalently linked to an energy acceptor unit (T - target) with the different energy levels preventing an energy transfer of a single photon. At high intensity illumination (laser exposure) of a trichromophoric system A-T-A, sufficient to excite both of the appended donor subunits, population of the target excited state may occur via simultaneous energy transfer of two photons, one from each donor. In order to restrict reverse energy transfer from the excited target to the donor(s) it is necessary that the excited target unit undergoes an efficient photoreaction used to monitor the SEET. In the case of E-4,4'-di (fluorescein-5 ''-yl-thioureanyl)-stilbene respectively using erythrosine as antenna the SEET was detected by photoisomerization of the stilbene unit. Concerning the lifetimes of the exited state of the antenna and the role of the sulphur (heavy atom effect) in the A and T linking thiourea group we propose a triplet mechanism (similar to triplet-triplet annihilation) for both of the above mentioned model compounds. Intending to use 2-aminoanthracene as target unit its [4+4] cyclization was investigated.