Mechanical Modulation of the Solid‐State Luminescence of Tricarbonyl Rhenium(I) Complexes through the Interplay between Two Triplet Excited States

Abstract

Mechanoresponsive luminescence (MRL)m aterials promises mart devicesf or sensing, optoelectronics and security.Wepresenthere the first report on the MRL activity of two Re I complexes,o pening up new opportunities for applicationsi nt hese fields. Both complexese xhibitm arked solidstate luminescence enhancement (SLE). Furthermore, the pristinem icrocrystalline powders emit in the yellow-green region,a nd grinding led to an amorphous phase with concomitant emission redshift and shrinking of the photoluminescence (PL) quantum yields and lifetimes. Quantum chemical calculations revealed the existenceo ftwo low-lying triplet excited states with very similar energy levels, that is, 3 IL and 3 MLCT,h aving, respectively,a lmost pure intraligand (IL) and metal-to-ligand charge-transfer (MLCT) character.T ransi-tion between these states could be promoted by rotation aroundt he pyridyltriazoleÀphenylbenzoxazole bond. In the microcrystals, in which rotations are hindered, the 3 IL state induces the prominent PL emission at short wavelengths. Upon grinding, rotation is facilitated and the transitiont o the 3 MLCT state resultsi nal arger proportiono fl ong-wave-lengthP L. FTIR andv ariable-temperature PL spectroscopy showed that the openingo ft he vibrationalm odes favours non-radiative deactivation of the triplet states in the amorphousp hase. In solution, PL only arises from the 3 MLCT state.T he same mechanism accounts fort he spectroscopic differences observed when passing from crystalst oa morphousp owders, and then to solutions, thereby clarifying the link between SLE and MRL for these complexes.

Mechanofluorochromic Material toward a Recoverable Microscale Force Sensor

“…Figure 2a shows microcrystals (blue arrows) with darker inner parts and emissive edges typical of waveguiding effects previously observed in microcrystals of transition metal complexes [ 37 ] and organic fluorophores. [ 49 ] (See Figure S3, Supporting Information, for other sample images.)…”

“…Light, [ 7–11 ] heat, [ 10,13–15 ] solvent (and its vapor), [ 16–18 ] and mechanical forces, [ 2,6,19–22 ] modify the structure of materials at the (supra)molecular level to effect changes in both absorption and emission properties. Among these stimuli‐responsive processes, mechanofluorochromism, [ 20–50 ] the change in optical properties upon application of mechanical stimuli such as scratching or grinding, promises applications for force sensors. [ 20–36 ] Emission changes of coordination complex crystals, [ 24 ] polymeric microcrystals, [ 30 ] and organic fluorophores, [ 31–36 ] have been correlated to an applied force or pressure for new anticounterfeiting technologies [ 23–28 ] and biological stress probes.…”

“…Emission changes, correlated with applied force, are induced by morphological changes in the material. [ 20–50 ] This means that subsequent sensing events, after the initial application of force, necessitate recovering the original morphology—a process not so straightforward as this usually requires thermal annealing, [ 38 ] solvent fuming, [ 25,27 ] or recrystallization. [ 20–22,37 ] Self‐recovery, the spontaneous return to the initial state (of absorption, emission and morphology) of the scratched/ground material under ambient conditions, has been observed in derivatives based on Au(I) complexes, [ 28 ] pyrene, [ 39,40 ] anthracene, [ 41,42 ] tetraphenylethene, [ 43,44 ] indolylbenzothiadiazole, [ 45 ] triphenylamine, [ 46 ] boron‐coordinated β‐diketonate complexes, [ 47 ] and hexathiobenzene.…”

“…[ 20–50 ] This means that subsequent sensing events, after the initial application of force, necessitate recovering the original morphology—a process not so straightforward as this usually requires thermal annealing, [ 38 ] solvent fuming, [ 25,27 ] or recrystallization. [ 20–22,37 ] Self‐recovery, the spontaneous return to the initial state (of absorption, emission and morphology) of the scratched/ground material under ambient conditions, has been observed in derivatives based on Au(I) complexes, [ 28 ] pyrene, [ 39,40 ] anthracene, [ 41,42 ] tetraphenylethene, [ 43,44 ] indolylbenzothiadiazole, [ 45 ] triphenylamine, [ 46 ] boron‐coordinated β‐diketonate complexes, [ 47 ] and hexathiobenzene. [ 48 ] However, many MFC‐active materials are left unexplored for multiple‐use force sensing applications, not only due to the complexity of instrumentation required for such studies, [ 24,30,31,33–36 ] but also due to the lack of molecular design leading to reversibility [ 40 ] and a clear understanding of mechanism of self‐recovery.…”

Recent Trends in the Design, Synthesis, Spectroscopic Behavior, and Applications of Benzazole-Based Molecules with Solid-State Luminescence Enhancement Properties

“…Furthermore, BZ may be included into the organic ligand, without direct participation to the coordination sphere, and so they contribute to enhancing the SLE properties. This is the case for example for the SLE-active and mechanoresponsive rhenium(I) complexes developed in our group [111,112]. A new avenue is opening up in the field of coordination chemistry.…”