## 1 Introduction  
Recently, change in the absorption spectrum of TPPS (tetraphenylporphyrin 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]. However, 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) effect due to the triplet state formation in TPPS monomers [3, 4, 5]. Here in this paper we investigated whether the absorption change previously observed was induced by the triplet state formation or the aggregation of TPPS monomers.  
Even if the phenomenon we observed is the SA effect due to the triplet state formation in TPPS, it is important to investigate its behavior. The SA effect in dye molecules is a kind of nonlinear optical (NLO) response, which can be applied to phase-conjugated wave generation and passive Q-switching. Recently, the SA effect of dye-doped polyvinyl alcohol (PVA) was reported [6]. It was found that the SA effect was able to be tuned by changing the dye concentration. It was also reported that the SA effect of the dye was increased by forming a donor-acceptor system between polyoxometalate (POM), an electron acceptor,and the dye, an electron donor [7]. The increase in the SA effect was attributed to the depletion of the ground state in the dye due to the transfer of electrons in the dye to POM. Recently, there have been many reports on reverse saturable absorption (RSA) effect of dyes [8; 9; 10; 11; 12]. Absorbers with RSA properties have higher absorption efficiency with higher incident light intensity. RSA effect is caused by the accumulation of excited states and multiphoton absorption processes [13]. RSA effect by the triplet excited-state absorption has also been reported [14; 15; 16]. An important application of RSA effect is the protection of eyes and sensitive devices from intense light. J-aggregates [17; 18] composed of dye-molecules aligned in one dimensional direction are attracting attention as nonlinear optical materials. The second- and third-order NLO response of J-aggregates is generally known to be large [19; 20; 21; 22].  
There are many studies on the triplet state in porphyrin molecules because of its possible application to cancer treatment and hydrogen photoproduction. For example, porphyrin molecules have been used as the photosensizers in a cancer treatment called photodynamic therapy treatments [23; 24; 25; 26; 27; 28; 29; 30; 31]. Porphyrin has also been investigated as the photosensizer of hydrogen photoproduction [32; 33; 34; 35; 36]. This is because the photocatalysts such as TiO\({}_{2}\) do not absorb visible light so that photosensitizers which are sensitive to visible light such as porphyrin molecules are needed for the best use of sunlight. TPPS is one of the porphyrin molecules which is attracting attention as the photosensitizers of photodynamic therapy and hydrogen photoproduction, because the quantum yield of the triplet state formation in TPPS is as high as 0.78 [5].  
In addition to the TPPS aqueous solution, we studied water-methanol mixed solutions of TPPS following ref. [2], where DAS of water-alcohol mixed solutions (dominated by alcohol) of TPPS were studied. We have found that by mixing methanol (MeOH) with the TPPS aqueous solution and adjusting NaOH concentration between 1 mM and 10 mM, the amount of the absorbance change increased up to 50 times. If the observed absorption changes were caused by the SA effect, this result suggests the possibility of controlling the performance of NLO materials by adjusting the solvent mixing ratio and base/acid concentration.