Intramolecular Energy Transfer in a Series of Star-Shaped Molecules with a Central Porphyrin Core and Four Oligocarbazole Arms

A series of star-shaped molecules T(OCAn)Ps (n = 2–6) based on porphyrin as a core and oligocarbazole as monodisperse arms has been investigated by steady-state spectroscopy, femtosecond transient absorption (TA) spectroscopy, and quantum chemical calculations. UV–visible absorption spectra and quantum chemical calculation reveal that there is conjugation between the first carbazole units (the nearest to porphyrin) and porphyrin core but not between the neighboring carbazole units. The photo-excitation of T(OCAn)Ps results in efficient intramolecular energy transfer from the oligocarbazole (donor) to the porphyrin core (acceptor). The intramolecular energy-transfer time constants in T(OCAn)Ps (n = 2–6) obtained by global analysis of the femtosecond TA spectra are in the range of 0.60–22.27 ps. It is confirmed that the light-harvesting capability of porphyrin core could be improved by covalently linked oligocarbazole arms, while the excitation energy-transfer efficiency remains above 99%. This work provides a combined experimental-theoretical elucidation of ultrafast intramolecular energy-transfer processes, which could help develop novel star-shaped materials to achieve higher efficiency and better light-harvesting capability.