Unveiling the mechanism of reverse saturable absorption in zinc tetrapyridyl porphyrins

Reverse saturable absorption (RSA) is one of the important mechanism for a molecule showing optical-limiting property. The experiment found that zinc tetrapyridyl porphyrins (ZnTPyP) exhibit distinct RSA characteristics at both 470 nm and 532 nm wavelengths. To understand the origin of the RSA in ZnTPyP, a theoretical analysis based on the linear-response time-dependent density functional theory(LR-TDDFT) for the ground state absorption (GSA) and the pseudo-wavefunction formulation of LR-TDDFT(PWF-TDDFT) for excited states absorption (ESA) were conducted. Experimentally observed RSA in ZnTPyP are well predicted. It is shown that the transitions of $S_1\to S_{13,14,15,18}$ are the origin of the observed RSA feature in ZnTPyP. We found that these excited states transitions exhibit intramolecular charge transfer character and the pyridyl groups play an important role in the charge transfer process. It is also noticed that the pyridyl groups results in a red shift for both the GSA and ESA of porphyrin. In addition, the central zinc atom causes the energy gap between the positions with prominent absorption in GSA and ESA increased. These observations indicate that pyridyl groups and central zinc atom play an important role in tuning the spectral window and strength of RSA in the porphyrin complexes. These observations provide guidance for designing novel optical-limiting materials based on the porphyrin derivatives.