Modulating the Shape of Short Metal-Mediated Heteroleptic Tapes of Porphyrins

In view of developing artificial light-responsive complex systems, the preparation of discrete and robust heteroleptic assemblies of different chromophores in precisely defined positions is of great value since they would allow to investigate directional processes unavailable in symmetrical architectures. Here we describe the preparation, through a modular stepwise approach, and characterization of four novel and robust metal-mediated heteroleptic 4+3 porphyrin tapes, labeled D₄ -T₄ -D₄ , D₃ -T₄ -D₃ , D₄ -T₃ -D₄ , and D₃ -T₃ -D₃ , where a central meso-tetrapyridylporphyrin (either 3'-TPyP=T₃ or 4'-TPyP=T₄ ) is connected to two equal cis-dipyridylporphyrins (either 3'cisDPyMP=D₃ or 4'cisDPyMP=D₄ ) through four {t,c,c-RuCl₂ (CO)₂ } fragments. Whereas D₄ -T₄ -D₄ is flat, the tapes containing at least one 3'PyP, i. e. D₃ -T₄ -D₃ , D₄ -T₃ -D₄ , and D₃ -T₃ -D₃ , have unprecedented - and well defined - 3D geometries, and each exists in solution as a pair of stereoisomers in slow conformational equilibrium. The X-ray molecular structures of two such conformers, the C-shaped (D₃ -T₄ -D₃ )_C and the z-shaped (D₄ -T₃ -D₄ )_z , were determined and are fully consistent with the solution NMR findings.