Here we present a structural design aimed at the control of phosphorescence emission as the result of changes in molecular rotation in a crystalline material. The proposed strategy includes the use of aurophilic interactions, both as a crystal engineering tool and as a sensitive emission probe, and the use of a dumbbell-shaped architecture intended to create a low packing density region that permits the rotation of a central phenylene. Molecular rotor 1, with a central 1,4-diethynylphenylene rotator linked to two gold(I) triphenylphosphane complexes, was prepared and its structure confirmed by single-crystal X-ray diffraction, which revealed chains mediated by dimeric aurophilic interactions. We showed that green-emitting crystals exhibit reversible luminescent color changes between 298 and 193 K, which correlate with changes in rotational motion determined by variable-temperature solid-state H NMR spin-echo experiments. Fast two-fold rotation with a frequency of ca. 4.00 MHz (τ = 0.25 μs) at 298 K becomes essentially static below 193 K as emission steadily changes from green to yellow in this temperature interval. A correlation between phosphorescence lifetimes and rotational frequencies is interpreted in terms of conformational changes arising from rotation of the central phenylene, which causes a change in electronic communication between the gold-linked rotors, as suggested by DFT studies. These results and control experiments with analogue 2, possessing a hindered tetramethylphenylene that is unable to rotate in the crystal, suggest that the molecular rotation can be a useful tool for controlling luminescence in the crystalline state.
Luminescence alterations in solid-state materials upon external stimulations have attracted much attention due to their potential for the development of highly functional devices or sensors. We have previously reported the first examples of mechano-induced single-crystal-to-single-crystal (SCSC) phase transitions of gold(I) isocyanide complexes under concomitant emission-color changes. However, the reverse phase transitions of the crystals obtained after mechanical stimulation have not yet been achieved. Herein, a reversible change of the luminescence based on two SCSC phase transitions via mechanical cutting and solvent-vapor adsorption is described. Crystallization of a gold(I) complex that bears CF and biaryl moieties from CHCl/MeOH afforded a green-emitting single crystal packed in a polar space group (Pna2). The green-emitting single crystals included MeOH molecules. Upon cutting the crystal under MeOH vapor at 22 °C, the green-emitting single crystal spontaneously changed into a centrosymmetric orange-emitting single crystal (P1̅) under concomitant release of MeOH. Remarkably, the initial green-emitting crystal could be recovered from the orange-emitting crystal by a solvent-induced SCSC transition under saturated MeOH vapor. The combination of two different types of SCSC phase transitions enables the reversible structural and photoluminescent alternations.
Herein, a novel mechano-responsive luminescent (MRL) material based on crystal-to-crystal phase transitions between crystals of a chiral and those of a centrosymmetric space group, accompanied by a change of emission properties, is described. Initially, a gold complex containing a biphenyl moiety, which exhibits an achiral structure in solution, afforded an orange-emitting amorphous phase together with a viscous isotropic oil after evaporation of the solvent. Upon pricking, the orange-emitting oil spontaneously crystallized either in a centrosymmetric or in a chiral space group while simultaneously changing the emission properties. Remarkably, grinding the chiral crystals induced a solid-state phase transition to the achiral crystals under concomitant changes of the emission properties.
Herein we report ac rystalline molecular rotor with rotationally modulated triplet emission that displays macroscopic dynamics in the form of crystal moving and/or jumping, also knownassalient effects.Molecular rotor 2 with ac entral 1,4-diethynyl-2,3-difluorophenylene rotator linked to two gold(I) nodes,c rystalizes as infinite 1D chains through intermolecular gold(I)-gold(I) interactions.T he rotational motion changes the orientation of the central phenylene, changing the electronic communication between adjacent chromophores,a nd thus the emission intensities.C rystals of 2 showed the large and reversible thermal expansion/compression anisotropy, whicha ccounts for 1) an onlinear Arrhenius behavior in molecular-level rotational dynamics,w hichc orrelates with 2) changes in emission, and determines 3) the macroscopic crystal motion. Am olecular rotor analogue 3 has properties similar to those of 2,s uggesting ag eneralized way to control mechanical properties at molecular and macroscopic scales.
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