The E and Z isomers of azobenzene have significantly different molecular geometries; therefore, we speculated that equilibrating the cage (i.e., a mixture of T and X) with one of the two isomers of azobenzene might shift the equilibrium towards either of the two cage isomers. To this end, we prepared aqueous solutions of the cage containing 12.5 mol% of X (cf. Figure ) and incubated them with E and Z isomers of three azobenzenes 1-3 (solids, used in excess) at two different temperatures (20 ºC and 40 ºC). Z-1 has a relatively short half-life, and it underwent partial back-isomerization to E-1 during the experiment; therefore, we worked with Z-stilbene (Z-4) as the thermally stable analog of Z-1 instead. In contrast, the Z isomers of tetra-o-fluoroazobenzene 52 2 and tetra-omethoxyazobenzene 53 3 are sufficiently long-lived to allow us to neglect the Z→E relaxation under the applied conditions. The experiment at 20 ºC was followed for 33 days (Figure ). During this time, the molar fraction of X in the control sample (i.e., no guest added) decreased to 8 mol%, gradually approaching the equilibrium value of 6 mol%. During the same period, the initial fraction of X (12.5 mol%) was sustained by all three Z compounds (Z-2, Z-3, and Z-4; Figure ), suggesting that X has an affinity to the Z isomer of azobenzenes and styrene. In contrast, the E isomers of 1-3 all facilitated the transformation of X into T (Figure ). These effects were amplified when the same experiment was carried out at 40 ºC (Figure ). At this temperature, the molar fraction of X in the presence of all three Z guests increased to ~20 mol% and decreased to ~5 mol% for E-1 and E-3. Remarkably, guest E-2 induced the X→T conversion almost quantitatively, with less than 1 mol% of residual X at equilibrium (Figure ). Taken together, these results indicate that the extended E isomers of azobenzenes 1-3 bind favorably to-and thus stabilize-the T form of the cage; on the other hand, the more globular Z isomers favor the X isomer of the same cage, in agreement with the more spherical shape of the putative capsule C (Figure ). Therefore, we can tentatively conclude-and will confirm below-that X = C.