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

Abstract

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 substituent's Hammett constant. The measured cross-section amounts to σ 2=1.6×10 −47 cm 4 s/photon for octakis(4-nitrophenyl)-tetraazaporphine at 770 nm. This large value is explained by both the resonance enhancement by a strong Q x (0–0) linear transition and the presence in the same spectral region of a strong two-photon allowed gerade– gerade transition.