Drastic enhancement of two-photon absorption in porphyrins associated with symmetrical electron-accepting substitution

We observe a dramatic enhancement of simultaneous two-photon absorption (TPA) in a series of substituted tetraazaporphyrins with multiple electron-accepting groups and show for the first time that the TPA cross-section increases in linear proportion with the substituentʹs Hammett constant. The measured cross-section amounts to σ2=1.6×10−47 cm4 s/photon for octakis(4-nitrophenyl)-tetraazaporphine at 770 nm. This large value is explained by both the resonance enhancement by a strong Qx(0–0) linear transition and the presence in the same spectral region of a strong two-photon allowed gerade–gerade transition.