Supramolecular polymers are arrays of low-molecular-weight building blocks held together by hydrogen bonding or other reversible noncovalent interactions. [1] They have attracted increasing attention recently because of their potential applications in the fabrication of smart functional materials whose physical properties and functions could be controlled by external stimuli. [2] Among such stimuli, light is especially attractive as it allows remote activation and control with submicrometer spatial and sub-millisecond temporal resolution without generating waste chemicals. [3] Previously reported supramolecular polymers have been shown to polymerize/ depolymerize [4] as well as switch between welldefined aggregation states [5] or gel and solution [6] upon irradiation. Azobenzene (AB) is the most commonly used chromophore in photoresponsive polymers because of the large structural difference between its E and Z isomers. However, the halflife of most (Z)-AB derivatives at room temperature is a few hours, [7] which makes polymers of pure (Z)-AB hard to prepare and study. This has so far made it difficult to exploit the (probable) differences in the physical properties and polymerization mechanisms between the two isomeric states of AB polymers.A stiff stilbene (1,1-biindane, Scheme 1) moiety offers considerable advantages over AB as a chromophore for photoresponsive supramolecular polymers. Similar to AB, stilbene exists as structurally distinct E and Z isomers. Unlike AB, the isomers are separated by an activation barrier of about 43 kcal mol À1 (i.e., a half-life of ca. 10 9 years at 300 K), thus making thermal isomerization negligibly slow at temperatures below about 420 K. [8a] The photoisomerization of either isomer of the stiff stilbene derivative proceeds with 50 % quantum yield, [8d] compared to about 20 % (l < 320 nm) or 35 % (l > 400 nm) for the photoisomerization of (E)azobenzene to the Z isomer. [8e] The stiff stilbene offers greater opportunities for peripheral substitution than AB while being generally more inert. Boulatov et al. has used the stiff stilbene moiety extensively as a molecular force probe to mimic the strain generated in diverse functional groups when they are part of a stretched polymer. [8] Herein, we report a photoresponsive supramolecular polymer comprised of quadruply hydrogen-bonded monomers of stiff stilbene with two ureidopyrimidinone moieties (Scheme 1). The selfassembly behavior and physical properties of the supramolecular polymers can be regulated by photoisomeirization of stiff stilbene. (Z)-1 and (E)-1 polymerize by ring-chain and isodesmic growth mechanisms, respectively. The polymer of Scheme 1. Photoisomerization of the UPy-linked stiff stilbene derivates (Z)-1 and (E)-1.