## 3 Results and Discussion  
### Solvent Dependence of Absorption Change Spectra  
We determined the absorbance change spectra of the prepared samples via laser irradiation. Figure 3a-d shows the absorbance change spectra when 532-nm laser light was irradiated to the F-monomer-dominant water, ethanol, methanol, and butanol solution samples. Figure 3: \(Cont.\)  
Figure 2: Experimental set-up (L: lens, APD: avalanche photodiode). The absorption decreased by \(\Delta A\sim 10^{-4}\) at the monomer absorption peak and increased by \(\Delta A\sim 10^{-5}\) in the broad spectral region of the aggregates. Table 2 lists the decreased and increased peak values of the absorbance change for each sample. We observed an absorption increase of \(\Delta A\sim 10^{-3}\) for the 16-\(\upmu\)mol/L TPPS aqueous sample in our former report [48]; the difference between reference [48] and the present paper was due to differences in the NaOH concentration and laser intensity (2 times larger and 10 times smaller than in reference [48], respectively). The absorbance change in the organic solvent was smaller by one order of magnitude compared with the water solvent case (Figure 3a). There have been reports that the viscosity of solvents affects molecular aggregation [23] and that the viscosity depends on the solvents' hydrogen bonds [56]. Thus, it is conceivable that solvent hydrogen bonds could contribute to molecular aggregation and light-induced aggregation. In Table 3, the fractional volumes of the hydroxyl group in each solvent molecule are compared at room temperature. The fractional volume of the hydroxyl group in water was the largest; it was 4.5 times, 6.5 times, and 10.2 times larger than in methanol, ethanol, and butanol, respectively. The aggregation efficiency (absorbance increase/the ratio of light-gradient force potential to molecular thermal kinetic energy) was over 10 times larger for the water sample than for the alcohol samples. The fractional volume of the OH group can be correlated with the difference of absorbance change between the water solution and the alcohol solutions, as shown in Figure 3b-d and Tables 2 and 3. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Water** & **Methanol** & **Ethanol** & **Butanol** \\ \hline Absorbance Increase & \(1.77\times 10^{-4}\) & \(1.68\times 10^{-5}\) & \(1.58\times 10^{-5}\) & \(2.90\times 10^{-5}\) \\ Absorbance Decrease & \(-1.13\times 10^{-3}\) & \(-8.39\times 10^{-5}\) & \(-1.08\times 10^{-4}\) & \(-1.62\times 10^{-4}\) \\ \hline \end{tabular}
\end{table} 
Table 2: The peak values of absorbance increase and decrease in B band for each sample. Figure 3: (**a**) The absorbance change for water solution (solid line) and absorption spectra (dotted line); (**b–d**) The absorbance change for methanol, ethanol, and butanol solution (solid line) and absorption spectra (dotted line). For Figures 3–6, absorption spectrum measured in 1-mm-thick quartz cell at room temperature and absorbance change measured in 0.3-mm-thick glass cell. Throughout the TPPS molecular aggregation process, protonation of the porphyrin ring contributed to the formation of J-aggregates. Each solvent had different values for the TPPS protonation constant [57]. The constant in water is larger by one to two orders of magnitude than that in methanol and ethanol according to reference [57], which could have caused the light-induced aggregation change to differ between different solvents. Comparing the absorbance change of the aqueous sample (\(\Delta A\sim 1.77\times 10^{-4}\) for 59 \(\upmu\)mol/L) with that of the methanol sample (\(\Delta A\sim 1.68\times 10^{-5}\), for 62 \(\upmu\)mol/L), the absorbance change in the former normalized by the TPPS concentration was approximately 11 times larger than in the latter. The protonation constant in water was approximately 100 times larger than in organic solutions. Therefore, the difference in absorbance change for each solvent could be explained by a difference in the protonation constant.