Yoshimitsu Sagara

Mechano- and Thermoresponsive Photoluminescent Supramolecular Polymer

Mechanoresponsive luminescent (MRL) materials change their emission color upon application of external forces. Many dyes with MRL behavior are known, but they normally do not display useful mechanical properties. Here, we introduce a new approach to overcome this problem, which relies on combining MRL compounds with the concept of supramolecular polymerization. As a first embodiment, a cyano-substituted oligo(p-phenylenevinylene), whose MRL behavior is associated with different solid-state assemblies, was derivatized with two ureido-4-pyrimidinone groups, which support the formation of a dynamic supramolecular polymer. The new material displays the thermomechanical characteristics of a supramolecular polymer glass, offers three different emission colors in the solid state, and exhibits both MRL and thermoresponsive luminescent behavior.

Rotaxanes as Mechanochromic Fluorescent Force Transducers in Polymers

The integration of mechanophores, motifs that transduce mechanical forces into chemical reactions, allows creating materials with stress-dependent properties. Typical mechanophores are activated by cleaving weak covalent bonds, but these reactions can also be triggered by other stimuli, and this renders the behavior unspecific. Here we show that this problem can be overcome by extending the molecular-shuttle function of rotaxanes to mechanical activation. A mechanically interlocked mechanophore composed of a fluorophore-carrying macrocycle and a dumbbell-shaped molecule containing a matching quencher was integrated into a polyurethane elastomer. Deformation of this polymer causes a fluorescence turn-on, due to the spatial separation of fluorophore and quencher. This process is specific, efficient, instantly reversible, and elicits an easily detectable optical signal that correlates with the applied force.

Tuning the thermo- and mechanoresponsive behavior of luminescent cyclophanes

Many cyclophanes have been investigated in dilute solution, where their internal cavities are accessible for supramolecular interactions. However, their photophysical properties in the solid state remain largely unexplored. We here report a new mechano- and thermoresponsive luminescent cyclophane that is comprised of two 9,10-bis(phenylethynyl)anthracene moieties and features two hexaethylene glycol bridges. The compound was found to exhibit a nematic liquid-crystalline phase at elevated temperature. X-ray diffraction patterns confirm that thermal and mechanical treatments induce changes in the molecular assembly, which are the basis for the observed photoluminescent color variations. The stimuli-responsive behavior of the new compound is quite different from that of a previously reported cyclophane with similar structure but shorter bridges. Thus, merely changing the ring size is an effective tool to tailor the stimuli-responsiveness and the phase behaviour of luminescent cyclophanes.

Mechanoresponsive luminescence and liquid-crystalline behaviour of a cyclophane featuring two 1,6-bis(phenylethynyl)pyrene groups

Cyclophanes have been attractive targets over recent decades because of their fascinating molecular structures and inherent ability to act as supramolecular hosts. However, cyclophanes that exhibit stimuli-responsive luminescence in their condensed states are still very rare, although luminescent cyclophanes could be suitable platforms for sophisticated photofunctional molecular assembled materials. We here report that a pyrenophane featuring two π-extended pyrene groups exhibits mechanoresponsive luminescence in the solid states and shows a nematic liquid-crystalline phase at elevated temperature. Two 1,6-bis(phenylethynyl)pyrene moieties form intramolecular excimers in a diluted chloroform solution, whereas no clear intra- and/or intermolecular excimer formation was observed for the solid state accessed through slow cooling from the nematic phase. Photoluminescence spectroscopic measurements, emission lifetime measurements, and X-ray diffraction patterns confirmed that mechanical stimuli result in conversion to much less ordered molecular assembled states in which some luminophores form excimers, leading to changes in photoluminescence colours.

Stimuli‐Responsive Dual‐Color Photon Upconversion: A Singlet‐to‐Triplet Absorption Sensitizer in a Soft Luminescent Cyclophane

Reversible emission color switching of triplet-triplet annihilation-based photon upconversion (TTA-UC) is achieved by employing an Os complex sensitizer with singlet-to-triplet (S-T) absorption and an asymmetric luminescent cyclophane with switchable emission characteristics. The cyclophane contains the 9,10-bis(phenylethynyl)anthracene unit as an emitter and can assemble into two different structures, a stable crystalline phase and a metastable supercooled nematic phase. The two structures exhibit green and yellow fluorescence, respectively, and can be accessed by distinct heating/cooling sequences. The hybridization of the cyclophane with the Os complex allows near-infrared-to-visible TTA-UC. The large anti-Stokes shift is possible by the direct S-T excitation, which dispenses with the use of a conventional sequence of singlet-singlet absorption and intersystem crossing. The TTA-UC emission color is successfully switched between green and yellow by thermal stimulation.

Linearly polarized photoluminescence from an asymmetric cyclophane showing thermo- and mechanoresponsive luminescence

The first cyclophane to exhibit linearly polarized photoluminescence in the liquid-crystalline and crystalline states is described. An asymmetric cyclophane featuring a 4,7-bis(phenylethynyl)-2,1,3-benzothiadiazole group forms a thermodynamically metastable nematic liquid-crystalline phase at room temperature. The compound sandwiched between two glass substrates coated with rubbed polyimide thin layers exhibits homogeneous alignment after shearing. The transition dipole moments of the luminophores align along the rubbing direction. As a result, linearly polarized photoluminescence is achieved at room temperature without any other host materials. Furthermore, the polarized emission is retained after the transition to the crystalline phase, which is induced by annealing at 60 °C for 1 h. The cyclophane also shows a thermal and mechanical stimuli-induced change in the photoluminescence colors.