Towards More Photostable, Brighter, and Less Phototoxic Chromophores: Synthesis and Properties of Porphyrins Functionalized with Cyclooctatetraene

Free base and zinc porphyrins functionalized with cyclooctatetraene (COT), a molecule known as a good triplet‐state quencher, have been obtained and characterized in detail by structural, spectral, and photophysical techniques. Substitution with COT leads to a dramatic decrease of the intrinsic lifetime of the porphyrin triplet. As a result, photostability in oxygen‐free solution increases by two to three orders of magnitude. In non‐degassed solutions, improvement of photostability is about tenfold for zinc porphyrins, but the free bases become less photostable. Similar quantum yields of photodegradation in free base and zinc porphyrins containing the COT moiety indicate a common mechanism of photochemical decomposition. The new porphyrins are expected to be much less phototoxic, since the quantum yield of singlet oxygen formation strongly decreases because of the shorter triplet lifetime. The reduction of triplet lifetime should also enhance the brightness and reduce blinking in porphyrin chromophores emitting in single‐molecule regime, since the duration of dark OFF states will be shorter. Energy versus electron transfer: Efficient triplet‐state quenching has been achieved in porphyrins functionalized with cyclooctatetraene (COT), leading to enhanced photostability of zinc porphyrins. The energy barrier of COT ring inversion–double bond migration depends on the location of the COT moiety with respect to the porphyrin core. These novel compounds are good models for studying nonvertical energy transfer and the competition between energy‐ and electron‐transfer processes.