## 4 Discussion
From the present experiment, it is clarified that the absorption change of the F-monomer aqueous solution with [NaOH] = 1 mM was induced by the triplet excited-state absorption and SA effects due to the triplet state formation as explained below. Immediately after N\({}_{2}\) bubbling, \(|\Delta A|\) at 412 nm of the F-monomer aqueous solution becomes 4.6 times larger (Figure 2b). N\({}_{2}\) bubbling decreased the dissolved oxygen concentration and increased the triplet excited-state absorption and SA effects as a result of the extension of the triplet state lifetime. This conclusion is consistent with the previous reports of the transient absorption change of F-monomer [3; 4; 5]. We tried to measure DAS by avoiding the real excitation using an 808 nm laser, but no DAS was observed (Figure 3). This result suggests that the DAS induced by 532 nm laser irradiation was caused only by the real excitation. To summarize, a phenomenon such as \"highly efficient\" light-induced molecular aggregation [1; 2] in bulk solution does not exist. One of the causes of the misinterpretation is that the triplet-triplet absorption spectrum of the TPPS monomer nearly coincides accidentally with that of the red-shifted energy of TPPS J-aggregates as observed in [42]. From the present experiment, we confirmed the reproducibility of the previously reported results in ref. [2] that \(|\Delta A|/A\) of the water-MeOH mixed solution of F-monomer with the high proportion of MeOH to water (approximately 0.80 mole fraction of MeOH) was approximately 0.1 times that of the F-monomer aqueous solution (Figure 5). This result is consistent with the fact that the oxygen solubility in MeOH is approximately 20 times larger than that in water [47]. It is pointed out in ref. [47], that the reason why the oxygen solubility in MeOH is higher than that in water is that non-polar oxygen molecules are easily dissolved in less polar solvents. Let us discuss that the dependence of \(|\Delta A|/A\) of the water-MeOH mixed solution of the F-monomer on the mole fraction of MeOH. When the NaOH concentration is 1 mM, the DAS is presumed to be caused by the triplet state formation because the shape of the DAS is similar to that induced by the triplet state formation. The reason why \(|\Delta A|/A\) decreases with the mole fraction of MeOH is explained as follows. Since the oxygen solubility in MeOH is approximately 20 times larger than that of water ref. [47], it is expected that the dissolved oxygen concentration increases as the proportion of MeOH to water increases. This is also supported by the ref. [48], where the dissolved oxygen concentration increases with the ratio [ethanol]/[water] in a mixed solvent of the two. In contrast, it is difficult to explain the dependence of \(|\Delta A|/A\) of the mixed solution of F-monomer on the mole fraction of MeOH when the NaOH concentration is 10 mM. Here, we introduce the hypothesis that the increase in absorption change is caused by the decrease in dissolved oxygen concentration. Because the shape of the DAS is similar to that induced by the triplet state formation (Figure 6b), the origin of the absorption change is considered to be that. Because \(|\Delta A|/A\) increased by mixing MeOH, the lifetime of the triplet state probably extends by mixing MeOH. The result of \(F/A\) decrease with \(|\Delta A|/A\) increase (Figures 5b and 8b) is consistent with the interpretation of the extension of the triplet state lifetime: Longer triplet state lifetime decreases the ratio of the singlet states to the triplet states during the pump light irradiation. However, the cause of the extension of the triplet state lifetime is unclear. Because the shape of the absorption spectra of the aqueous solution and the mixed solution of the F-monomer is similar (Figure 6), the form of F-monomer unlikely changed. A possible cause is a change in solvent conditions, especially a decrease in dissolved oxygen concentration.