Conformational changes of meso-Aryl substituted expanded porphyrins upon protonation: Effects on photophysical properties and aromaticity

meso-Hexakis(pentafluorophenyl) [30]heptaphyrin(1.1.1.1.1.1.0) and mew-hexakis(pentafluorophenyl) [38]non-aphyrin( 1.1.0.1.1.0.1.1.0) have been investigatwith a particular focus on their photophysical properties affected by protonation with acids using steady-state and time-resolved spectroscopic measurements along with femtosecond Z-scan method. It was found that the smaller Stokes shift and longer excsinglet/triplet state lifetimes of protonated [30]heptaphyrin and [38]nonaphyrin compared to their distorted neutral counterparts are strongly associated with the rigid and planar molecular structures. Much larger two photon absorption cross-section vof protonated [30]heptaphyrin and [38]nonaphyrin (6300 and 6040 GM) than those of their neutral forms (1350 and 1300 GM) also reflect the enhanced rigidity and planarity as well as aromaticity. In parallel with this, the nucleus-independent chemical (NICS) values of protonated forms exhibit large negative values, -14.3 and -11.5 ppm for [30]heptaphyrin and [38]nonaphyrin, respectively, at central positions. Thus we have demonstrated the structure-property relationships between molecular planarit, photophysical properties, and aromaticity of expanded porphyrins upon protonation based on our experimental and theoretical investigations. This study also promises a possibility of structural control of expanded porphyrins through protonation in which molecular flexibilities of expanded porphyrins lead to distorted structures especially as the number of pyrrole rings increases.