Assuming only monomeric species were present at 4 mM (for the Z form), we could calculate the Mw using the diffusion coefficients according to the following formula: Mw = (Dmonomer/D) . Conversion to molecular weight and plotting against concentration (Fig. ), provides evidence to indicate both (E)-I and (Z)-I undergo concentration dependent stepgrowth oligomerisation. We also constructed a DOSY calibration curve using 2,2'-dipyridyl and PMMA standards and determined Mw values from D values based on the calibration (see SI); this approach gave much lower Mw values which we consider to be less realistic given the dimerization affinities and concentrations in these systems. Overall, the data suggest that I forms short oligomers rather than supramolecular polymers which likely derives from the moderate strength for the triply hydrogen-bonded heterodimerization. For II, there were more pronounced differences between the D values for the E and Z photostationary states at all concentrations (Fig. ) when compared I. Blue irradiated II was found to have lower D value than green light irradiated (Z)-II (which exhibited D values in a similar range to I in the low concentration regime). This implies a more significant difference in the assembly state for the E and Z states of II. For low concentration samples of II (< 24 mM) D was approximately constant for the Z isomer. The D values for the E isomer when converted to Mw imply a species double the size of the Z isomer and exhibited a small decrease as the concentration increased, suggesting only moderate chain extension at these concentrations. Taken together, these data suggest the assembly state of II at low concentration differs between the E and Z states, which we attribute to the formation of a folded macrocycle for the Z isomer and cyclic hydrogen-bonded oligomers (likely dimers) for the E isomer. Molecular modelling suggests that intramolecular hydrogen bonding for the E isomer is sterically suppressed, providing a rationale for the cyclization to give dimers in the low concentration regime (see SI). At higher concentrations of II (> 24 mM) the D values for Z isomer solutions decreased moderately indicating a switch in the ring-chain equilibria from cyclic species to oligomers. For E isomer, the D values decreased more rapidly than those for the Z isomer indicating a switch from cyclic oligomers to supramolecular polymers. When converted to molecular weight and plotted against concentration (Fig. ), it is evident that the species formed by (E)-II are significantly larger than any observed for I or (Z)-II samples consistent with step-growth polymerization for (E)-II but oligomerization for (Z)-II at higher concentrations. Prior studies on a shorter ditopic synthon bearing UPy units linked via a tetrafluoroazobenzene photoswitch reported DOSY data only at on concentration but could be used to ascertain the E isomer preferred a polydisperse assembly and the Z isomer former shorter oligomers with the smallest oligomer a cyclic dimer (see also discussion of viscosity data for II below). For III, more complex behaviour was observed (Fig. ). At 4 mM the difference in D value between blue irradiated (E)-III and green light irradiated (Z)-III differed as was the case for II. When converted to Mw, the D values suggest the E isomer is larger than the Z isomer indicating the latter is monomeric whilst the former tends towards dimeric at low concentration. As concentration increases, the D value for the Z isomer undergoes small changes until ~ 24 mM and this then decreases at first dramatically and then from ~ 30 mM with a shallower gradient. For the E isomer D decreases significantly but in an uneven manner up to ~ 24 mM and then at a shallower gradient. Thus, the DOSY data imply three phases in the concentration dependent behaviour of III. These phases are not as pronounced in the Mw versus concentration plots (Fig. ) derived from the DOSY data and we interpret them to arise from the concentration dependent variation in the UPy•DAN and UPy•UPy speciation and associated chain stoppering effect. Taken together, the data suggest the assembly state of III at low concentration differs between the E (dimer) and Z (monomer) states, then switches to supramolecular polymers > mM. As for II, (E)-III and (Z)-III assemble to different extents or states, but both appear to form larger assemblies which we attribute to the higher UPy•DAN affinity.