SIMULTANEOUS 2-PHOTON EXCITATION-ENERGY TRANSFER (SEET) AND NONLINEARPHOTOCHEMISTRY

The previously demonstrated simultaneous two-photon excitation energy transfer (SEET) using a trichromophoric model compound was further inv estigated. Therefore, two identical donors (A - antenna) were covalent ly linked to an energy acceptor unit (T - target) with the different e nergy 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, popula tion of the target excited state may occur via simultaneous energy tra nsfer of two photons, one from each donor. In order to restrict revers e energy transfer from the excited target to the donor(s) it is necess ary that the excited target unit undergoes an efficient photoreaction used to monitor the SEET. In the case of E-4,4'-di (fluorescein-5 ''-y l-thioureanyl)-stilbene respectively using erythrosine as antenna the SEET was detected by photoisomerization of the stilbene unit. Concerni ng the lifetimes of the exited state of the antenna and the role of th e 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-a minoanthracene as target unit its [4+4] cyclization was investigated.