Substituent effect on two-photon absorption properties of conjugated porphyrins

Abstract

Two-photon absorption (2PA) cross section values of bisacetylene-connected bisporphyrins were measured by using a nanosecond open aperture Z-scan method. The maximum effective cross section values of alkyl and aryl meso-substituted bisporphyrins were 14,000 GM at 890 nm and 7,700 GM at 860 nm, respectively, and the value of alkyl-substituted bisporphyrin was almost twice of that of aryl-substituted one. In one-photon absorption spectra, Q-bands of alkyl-substituted bisporphyrin were red-shifted at 635 nm and 685 nm, and intensified compared with those of aryl-substituted one at 625 nm and 678 nm, respectively, indicating higher π-electron density of the former compound compared to that of the later. The large enhancement observed for alkyl-substituted bisporphyrin is attributed to the increase in π- electron density compared with aryl-substituted one. It was found that the alkyl group is better to obtain large 2PA cross section values than the aryl group as the meso-substituent of the porphyrin.