Single crystals of optoelectronic materials that respond to external stimuli, such as mechanical, light, or heat, are immensely attractive for next generation smart materials. Here we report single crystals of a green fluorescent protein (GFP) chromophore analogue with irreversible mechanical bending and associated unusual enhancement of the fluorescence, which is attributed to the strained molecular packing in the perturbed region. Soft crystalline materials with such fluorescence intensity modulations occurring in response to mechanical stimuli under ambient pressure conditions will have potential implications for the design of technologically relevant tunable fluorescent materials.

Three concomitant polymorphs of 3-((4-chlorophenyl)­imino)­indolin-2-one, a Schiff’s base, are identified and sorted based on morphology and mechanical response of their crystals. Form I grows as blocks and shows brittle fracture, Form II has long needles and shows plastic bending, and Form III also has long needles and shows elastic bending under similar qualitative mechanical deformation tests. Furthermore, the brittle Form I was found to exhibit thermosalient behavior (jumping) when heated on a hot plate. The distinct mechanical behavior of the three forms is rationalized by analyzing intermolecular interaction energies from energy frameworks analysis, slip layer topology, Hirshfeld surface analysis, and nanoindentation. The quantitative nanoindentation studies unveiled that Form III has higher elastic modulus and stiffness than Forms I and II, while the hardness was lowest for the plastic Form II. Despite high structural similarity in Forms II (plastic) and III (elastic), the E of elastic Form III was found to be 3 orders of magnitude higher than that of plastic Form II crystals, which is attributed to the subtle differences in interaction energies and slip layer topology in the two cases. Consideration of slip layer topology and interaction energies from the structures are very useful for rationalizing mechanical properties, but may not be always sufficient, and one may also need to know the topology of the potential energy surface of the slip layers for understanding the distinct mechanical behavior.

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The dark-orange monomer single crystals of 1,1′-dioxo-1H-2,2′-biindene-3,3′-diyldidodecanoate (BIT-dodeca2) convert to a transparent single-crystalline polymer (PBIT-dodeca2) material via a single-crystal-to-single-crystal (SCSC) polymerization reaction under sunlight, which then undergoes reverse thermal transformation into BIT-dodeca2 single crystals, leading to reversible photo-/thermochromism, coupled with mechanical actuation. We exploit the properties of this unique material to demonstrate the formation of monomer–polymer heterostructures in selected regions of single crystals with micrometer-scale precision using a laser. This is the first example of heterostructure patterning involving monomer–polymer domains in single crystals. We reveal that the speed of photomechanical bending induced by the polymerization reaction in this example is comparable to those of the well-known diarylethene derivatives, in which electrocyclic ring-closing–ring-opening reactions operate. Furthermore, we characterize the distinct mechanical properties of the monomer and polymer using a quantitative nanoindentation technique as well as demonstrate photopatterning on a monomer-coated paper for potential use in security devices. These crystals with several advantages, such as photomechanical bending (weight lifting) even when the crystal size is large, responsiveness to both UV and visible light, distinct solubilities (the polymer is insoluble, whereas the monomer is soluble in most organic solvents) and colors, provide unique opportunities for their use at different length scales of the sample (μm to mm) for various purposes.

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