Porphyrins with Four Monodisperse Oligocarbazole Arms:  Facile Synthesis and Photophysical Properties

A series of novel monodisperse, well-defined, star-shaped molecules T(OCAn)Ps (n = 2-6) with a central porphyrin core and four oligocarbazole arms are synthesized from the corresponding formyl-substituted oligocarbazoles via Adler reaction. The obtained star-shaped porphyrins are intrinsically two-dimensional nanosized molecules, and the diameter of compound T(OCA6)P is 7.4 nm, representing one of the largest known star-shaped conjugated systems. Their photophysical properties have been investigated by absorption and steady-state fluorescence spectroscopy, together with the corresponding monodisperse oligocarbazole aldehyde precursors. It is found that the light-harvesting capability of T(OCAn)Ps increases with the increasing length of the arms and reaches the maximum when n = 6. A selective excitation of the oligocarbazole arms leads to the typical emission from the porphyrin cores, indicating occurrence of photoinduced intramolecular energy transfer, and the energy transfer efficiency decreases from T(OCA2)P to T(OCA6)P owing to the Förster energy-transfer process. Accordingly, the longest effective distance for Förster energy transfer is estimated to be ca. 3 nm in our system. Such star-shaped porphyrins may find applications in photonic devices, with respect to their intense emission of red light. Notably, the monodisperse oligocarbazole aldehyde precursors give twisted intramolecular charge-transfer (TICT) excited states and luminescence in polar solvents with large Stokes shifts.