To verify our hypothesis, we initially focused on the prototypical azobenzene 1 and BODIPY dye ps1 (Fig. ). Fig. (dotted brown line) shows the UV/vis absorption spectrum obtained upon mixing aqueous solutions of the respective homodimers-(E-1)2⊂C and (ps1)2⊂C-in a 1:1 molar ratio. The visible-light region of the spectrum is practically identical to that of pure (ps1)2⊂C (blue dotted line), indicating a minute fraction of the (E-1⋅ps1)⊂C heterodimer (i.e., the equilibrium shown in Fig. heavily favors the two homodimers). However, exposing this solution to a low-intensity green light (λ = 525 nm, 2.5 mW⋅cm -2 ) resulted in a substantial decrease of absorption in the near-UV region (Fig. ), indicating the E→Z isomerization of 1. This result suggests that the small amount of (E-1⋅ps1)⊂C in equilibrium with the homodimers absorbs green light, whose energy is eventually used to generate the metastable Z isomer (Fig. ). The low illumination intensities used in our studies exclude the possibility of two-photon isomerization , which we confirmed directly by power-dependence experiments (Supplementary Fig. ).