A Piezochromic Luminescent Complex: Mechanical Force Induced Patterning with a High Contrast Ratio

Abstract

Piezochromic (mechanochromic) luminescent (PIEL) materials change their emission colors (bands) by applying appropriate pressure or other mechanical forces.[1] Their mechanical responsiveness provides fundamental basis for luminescence switches, information display/storage, and extremely significant pressure sensors in various fields. [2][3][4] The tuning of the luminescence of organic solids by mechanical forces has been realized through changing the chemical structures containing opened/closed cyclic forms [5] and double-bond E/Z configurations.[6] However, relatively high pressure is necessary to promote chemical reactions, [7] and poor reversibility has been considered as a drawback in such systems. In contrast, controlling the mode of molecular packing (aggregation states) is more attractive for both fundamental research and practical applications due to low pressure demand and good reversibility, but successful systems are quite limited. [8][9][10][11][12][13][14][15] A few reports mainly focused on transforming p-p aggregation modes of single conjugated materials under pressure, for example, (liquid) crystal to (liquid) crystal [8][9][10][11][12][13] or crystal to amorphous solid. [14][15] However, it is still difficult to predict and design materials with polymorphism. Herein, we utilize a binary complex to overcome the drawbacks from a single component system, and this strategy expands the range of PIEL materials to a large extent. In our approach, a fluorescent donor (D) and a nonemissive acceptor (A) are blent to form a binary complex to achieve piezochromic luminescence, in which the luminescence changes from quenching (black) to emitting (color) state once mechanical force is applied. We further confirmed the mechanism and realized mechanical force induced patterning. This strategy provides a new way to achieve piezochromic luminescence and special polymorphic materials are not necessary. In addition, unlike the transformation between two emission colors in traditional piezochromic luminescent materials, the contrast ratio is very high to favor observation by naked-eye due to very low fluorescence intensity in the quenching state.Many donor-acceptor systems reported have only fluorescence quenching process and no force-induced fluorescence recovery of donor. However, we propose that cocrystallized D-A systems have potential of the fluorescence recovery once force-induced phase separation occurs, which means that acceptors move away from adjacent fluorescent donors. 2,5-di(E)-distyrylfuran [16] 1 as an electron donor exhibited intensive greenish blue fluorescence both in solution and in crystal state, and planar p-conjugated structure leads to strong tendency to aggregate by p-p stacking. Meanwhile, we chose N-alkyl substituted maleimides as acceptors to quench fluorescence of donor via photo-induced electron transfer process (PET). [17][18] In fact, once the complex (1:1) of 1 and 2 a is deposited on the substrate, the fluorescence intensity of donor will be dramatically quenched. However, the fluore...

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

“…Song and Pei et al reported that an MRL mixture consisting of a luminophore and a luminescent quencher shows a high on–off contrast ratio in 2011 . They used 2,5‐di( E )‐distyrylfuran 37 as the luminophore and maleimide 38 as the quencher ( Figure ).…”

“…Molecular structures of 37 , 38 , and 39 and photographs demonstrating the reversible MRL behavior of the mixture under UV irradiation. Adapted with permission . Copyright 2011, Wiley‐VCH.…”

Solvatochromism, Reversible Chromism and Self‐Assembly Effects of Heteroatom‐Assisted Aggregation‐Induced Enhanced Emission (AIEE) Compounds

“…[4] Notably,c ontrolling the physical state of aggregationsi se asier to achieve colour changes in the materials by using external forces. [5][6][7] However,m ost chromic luminescent materials suffer from aggregation-caused quenching (ACQ) effects in the solid state, which ultimately limit the development of chromic materials. [8] The fall-known aggregation-inducede mission (AIE) or aggregation-induced enhanced emission (AIEE) phenomena reported by Tang and co-workers in 2001 effectively overcome the interference of ACQ, and have becomep opulart opics in many materials fields.…”

Enhanced Emission under Mechanical Stimuli Based on Phenanthroimidazole Derivative by Controlling ISC Process

“…Organic luminogens with enhanced emission intensity in response to mechanical stimuli become particular appealing with abroad attention in sensors, data memories, and security ink applications, for it can not only provide a dual mode switch in a multifunction system but also avoid false positive signals for quenching effect of most interference after grinding . However, while most reported works are concerned about the emission color switching, only limited mechanochromic luminogens (MLs) have been reported with intensity variation and most of them show decreasing emission efficiency, for the collapse of network constructed by intermolecular interactions would result in notorious aggregation induced quenching in condensed phase .…”

“…However, while most reported works are concerned about the emission color switching, only limited mechanochromic luminogens (MLs) have been reported with intensity variation and most of them show decreasing emission efficiency, for the collapse of network constructed by intermolecular interactions would result in notorious aggregation induced quenching in condensed phase . Till now, several strategies have been developed to synthesize “off–on” type MLs, e.g., 1) introduction of donor–acceptor (D–A) structure within binary complexes/single molecule by taking advantage of photoinduced electron transfer process/intramolecular charge transfer (ICT) or 2) cycloaddition reaction, excimer formation, defect generation, etc. However, it still remains in the preliminary stage with quite limited examples for lacking of reliable guidance.…”