Mechanochromic luminescence in pyrylium salts: a new class of compounds

Vishnu, V. S. (2018) Mechanochromic luminescence in pyrylium salts: a new class of compounds. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Even though several compounds are successfully reported to show prudent reversible Mechanochromic Luminescence (ML), not much systematic studies are done to develop rationale behind this process. Such serendipitously occurring compounds are shown to be widely applicable in mechano-responsive switches, rewritable storage medium, security papers, photon-harvesting materials, etc., thus it is important to explore the mechanism and principles behind such processes in more systematic and detailed fashion. Inspiring from previous reports of difluoroboron avobenzone compounds which show eminent ML in polymorphs that contain weak interaction planes, so called slip planes (showing plastic bending upon mechanical stress), a crystal engineering approach was successfully proposed to design and explain the rationale behind the ML process. In this study I am exploring the crystal engineering strategy to a new class of compounds, with 2,4,6-tris(4-bromo phenyl) pyrylium as the central cationic unit with a verities of counter anions. First attempt was made with 2,4,6-tris(4-bromo phenyl)pyrylium tetrafluoroborate (TBB) compound, which do not show any kind of emission changes in response to mechanical stimuli and is consistent with brittleness of so obtained single crystals under mechanical force. Absence of ML property in TBB is directly correlated with brittle fracture which may resulting due to interactions of same strength in all three directions and are not supposed to possess any kind of weak interaction planes. Further understandings of TBB via single crystal x-ray diffraction studies could not be accomplished due to high degree of twinning in crystal forms. To explore ML property in multi-component solids, Co-crystals of TBB were prepared with co-formers such as pyrazine, 1,4-Diazabicyclo[2.2.2]octane (DABCO), Acridine, Phenazine, 1,2-Di(4-pyridyl)ethylene (BPE) and 4,4’- bipyridine. All co-crystals synthetic trials were successful to obtain a new solid form and confirmed with a difference in melting point, but unfortunately none of them exhibit ML property. Further another derivative of pyrylium salt, 2,4,6-tris(4-bromo phenyl)pyrylium 4- methyl benzene sulfonate (TBR) was synthesized by changing counter anion by keeping cationic unit intact. TBR prepared via a simple one-pot synthetic methodology and it shows reversible ML behaviour in response to grinding stimuli in an agate mortar and pestle. Here, I succeeded to obtain slip planes formed by methyl-bromine close contacts along major face of single crystal (001), aligned nearly orthogonal to comparatively strong C-H···O hydrogen bonding networks to result an anisotropy in crystal packing. This arrangement is consistent with plastic bending of TBR single crystals under qualitative study, that is stressing the crystal with metallic needle under optical microscope. Such slip planes facilitates shearing movements of thick two dimensional sheets along (001) to enhance ML property in the system and can be brought back to initial state via another kind of external perturbations such as heat and solvent addition. Thus here I am further establishing a mechanical behaviour-ML property relationship in a new group of systems which is not discussed so far in literatures. Further the rationale behind the process is investigated with various characterization technique such as PXRD, DSC, Raman spectroscopy, which shows a stable crystalline (Form I) to metastable amorphous (Form II) state interconversion plays decisive role in emission changes. Thus in this study we introduced a new class of compounds which exhibit reversible ML property, further extended the crystal engineering approach by introducing slip planes for achieving ML system and to find rationale behind prominent emission changes. Qualitative and quantitative analyses successfully made to understand at of molecular level and establish a mechanical behaviour – ML Property correlation with the same.

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