Aromatically π-Extended Porphyrins with Large Two-Photon Absorption Cross-Sections and Bright Phosphorescence

The ability to form triplet excited states upon two-photon excitation is important for several applications of metalloporphyrins. Here we present two-photon absorption (2PA) properties of several platinum (II) porphyrins, focusing on the effect of aromatic π-extension and peripheral substitution on 2PA cross-sections. p-Extension of the tetrapyrrolic macrocycle by way of syn-fusion with two external aromatic fragments, such as in syn-dibenzo- (DBP) and syn-dinaphthoporphyrins (DNP), removes the center-of-inversion symmetry in the porphyrins macrocycle. As a result, DBPs and DNPs exhibit stronger 2PA into their one-photon-allowed states (e.g. Soret state) than fully-symmetrical non-extended Pt(II) porphyrins (D_4h symmetry). However, much stronger absorbing 2PA-allowed states still lie above the Soret state and cannot be accessed directly due to the interfering linear absorption. 2PA near the Soret state increases dramatically (up to 50 fold) upon introduction of alkoxycarbonyl groups (CO₂R) into the benzo rings in DBP. TDDFT/sum-over-states calculations reveal that in these porphyrins low-lying molecular orbitals spanning over CO₂R groups via configuration form new 2PA-allowed states located just above the Soret state. Furthermore, computations predicted major stabilization of gerade-states in fully-symmetrical D_4h Pt alkoxycarbonyltetrabenzoporphyrins. Measurements confirmed that e.g. in PtTBP(CO₂Bu)₈ 2PA cross-section near 810 nm is ~900 GM. Combined with ultrafast intersystem crossing and exceptionally bright phosphorescence 2PA makes alkoxycarbonyl-substituted p-extended porphyrins very promising candidates for several 2P applications.