Porphyrin dimers: A theoretical understanding of the impact of electronic coupling strength on the two-photon absorption properties

Many porphyrin oligomers exhibit large two-photon absorptions (TPA) at fundamental photon energies around 1.5 eV, while the corresponding monomers have negligible TPA cross-sections in that energy range. In general, our understanding of nonlinear absorption in these compounds is rather limited compared to that of linear absorption. Here, we seek to provide insight into this issue by examining various structural aspects of porphyrin dimers and analyzing how they lead to either "pure" or double resonance-enhanced TPA cross-sections. To do so, we have carried out highly correlated quantum-chemical calculations on model chromophores which differ by their central π-conjugated bridges or acceptor moieties. In a number of such dimers, the calculated energies of the lowest two-photon active states are stabilized and display significant cross-sections as the electronic coupling strength between the two porphyrin moieties becomes significant; in these instances, the lowest electronic TPA-active state is located in an energy range preventing contributions from double resonance effects. On the other hand, in dimers in which the porphyrin moieties are either very strongly or very weakly electronically coupled, the TPA cross-sections are mainly due to double resonance effects.