Interestingly, the vastly different heterodimer populations in 1+ps1 vs. 9+ps1 mixtures do not translate into major differences in the reaction kinetics: the former comprises only ~2% heterodimer but requires only twice as much time as the latter (with a heterodimer fraction of ~80%) to reach a photostationary state (PSS). This finding suggests that the rate-limiting step of DESC is the photoisomerization reaction rather than guest shuttling between the cages, which led us to hypothesize that the process should work efficiently also with catalytic amounts of the PS. Indeed, as shown in Fig. , decreasing the amount of ps1 to only 0.05 eq with respect to 9 extended the time required to reach the PSS fourfold but did not markedly affect its composition. homodimeric inclusion complexes of a PS and an E-azoarene (PS2⊂C and E2⊂C, respectively) and (bottom) the heterodimeric complex (E⋅PS)⊂C. Only E residing within the heterodimer-but not the homodimer-can be switched with visible light. The resulting Z is encapsulated as a sole guest and cannot be sensitized either. b, Absorption spectrum of a 1:1 mixture of (ps1)2⊂C and (E-1)2⊂C (dotted brown line) and changes in the spectra accompanying irradiation with green light (λ ≈ 525 nm, denoted with green shading). Dotted blue line: absorption spectrum of (ps1)2⊂C. c, Absorption spectrum of a 1:1 mixture of (ps1)2⊂C and (E-9)2⊂C (dotted brown line; predominantly (E-9⋅ps1)⊂C) and changes in the spectra accompanying irradiation with green light. Dotted blue line: absorption spectrum of (ps1)2⊂C. d, The X-ray crystal structure of the heterodimeric complex (E-9⋅ps1)⊂C (from the left: front view, side view, and top view; light-gray = cage C; dark-gray = E-9; green = ps1; water molecules, counterions, and cage protons omitted for clarity). e, Following DESC of 9 in the presence of different equivalents of ps1 (the data were normalized to the 0-1 range, except the experiment with no PS; for raw data, see Supplementary Fig. ). f, Evolution of the emission intensity of ps1 under 515 nm light (used both to induce DESC and excite fluorescence) as a function of the amount of 9. g, More than 100 cycles of reversible photoisomerization of 9 induced solely by visible light (E→Z: DESC with 525 nm light for 2 min; Z→E: direct photoexcitation using 435 nm light for 30 s). The amount of the E isomer is proportional to absorbance at 353 nm; the absorption at 480 nm originates from the (ps1)2⊂C homodimer.