Sn(IV) Multiporphyrin Arrays as Tunable Photoactive Systems

A series of four arrays made of a central Sn­(IV) porphyrin as scaffold axially connected, via carboxylate functions, to two free-base porphyrins has been prepared and fully characterized. Three arrays in the series feature the same free-base unit and alternative tin-porphyrin macrocycles, and one consists of a second type of free-base and one chosen metallo-porphyrin. A thorough photophysical investigation has been performed on all arrays by means of time-resolved emission and absorption techniques. Specific focus has been given at identifying how structural modifications of the free-base and tin-porphyrin partners and/or variation of the solvent polarity can effectively translate into distinct photophysical behaviors. In particular, for systems SnTPP­(Fb)2 (1) and SnOEP­(Fb)2 (2), an ultrafast energy transfer process from the excited Sn­(IV) porphyrin to the free-base unit occurs with unitary efficiency. For derivative SnTPP­(FbR)2 (3), the change of solvent from dichloromethane to toluene is accompanied by a neat change in the intercomponent quenching mechanism, from photoinduced electron transfer to energy transfer, upon excitation of the Sn­(IV) porphyrin unit. Finally, for array SnTpFP­(Fb)2 (4), an ultrafast electron transfer quenching of both chromophores is detected in all solvents. This work provides a general outline, accompanied by clear experimental support, on possible ways to achieve a systematic fine-tuning of the quenching mechanism (from energy to electron transfer) of Sn­(IV) multiporphyrin arrays.