More Than 50-Fold Enhanced Nonlinear Optical Response of Porphyrin Molecules in Aqueous Solution Induced by Mixing Base and Organic Solvent

Eiji Tokunaga,Takayoshi Kobayashi,Satoshi Imura

doi:10.3390/app11114892

Abstract

The difference absorption spectrum (DAS) of porphyrin molecules (tetraphenyl-porphyhrin sulfonic acid, TPPS) in aqueous solution induced by continuous wave laser irradiation has been reported previously. It was interpreted that the DAS was caused by the formation of TPPS aggregates induced by laser irradiation. However, transient spectra similar in their shape have already been reported and are attributed to the excited-state absorption and saturable absorption (SA) effects due to the triplet state formation in TPPS. In the present study, we investigated the triplet quenching effect by O2 on the DAS of TPPS aqueous solution and revealed that it originated from the triplet state formation. We also found that mixing the appropriate amount of MeOH and NaOH in TPPS aqueous solution increased its absorbance change by more than 50 times. This may be due to the decrease in dissolved oxygen concentration by mixing them. This result suggests the possibility of controlling the performance of NLO materials by adjusting the solvent mixture ratio and base/acid concentration.

Highlights

IntroductionPublisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations
The transient spectra of a TPPS aqueous solution similar in shape to the difference absorption spectrum (DAS) in ref. [1,2] had been already reported, and it was concluded that the absorption change was induced by the triplet excited-state absorption and saturable absorption (SA)
4πεr3 where ε is the permittivity of the medium surrounding J-aggregates, N is the number of molecules in a single aggregate, r is the distance between the molecules and θ0 is the angle between the transition dipole moment of the component TPPS molecule and the axis connecting the centers of the transition dipoles [41]

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Change in the absorption spectrum of TPPS (tetraphenylporphyhrin sulfonic acid) aqueous solution induced by continuous wave (CW) laser (532 nm) irradiation [1] was observed. The origin of the puzzling phenomenon was interpreted to be the formation of TPPS aggregates induced by light gradient force of the laser from an unknown mechanism of “highly efficient” light-induced molecular aggregation [1,2]. The transient spectra of a TPPS aqueous solution similar in shape to the difference absorption spectrum (DAS) in ref. [1,2] had been already reported, and it was concluded that the absorption change was induced by the triplet excited-state absorption and saturable absorption (SA) The transient spectra of a TPPS aqueous solution similar in shape to the difference absorption spectrum (DAS) in ref. [1,2] had been already reported, and it was concluded that the absorption change was induced by the triplet excited-state absorption and saturable absorption (SA)

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Discussion

Conclusion