Luminescent Behavior Elucidation of a Disilane‐Bridged D–A–D Triad Composed of Phenothiazine and Thienopyrazine

Abstract

A s-p extended aryldisilane,c omprising at hienopyrazine group as an acceptor fragment and phenothiazine groups as the donor moiety,h as been prepared through the introduction of two Si À Si bridges (compound 1). X-ray diffraction analysis determined the crystal structure of 1,a nd experimental and theoretical approaches investigated its optical properties.S olvatochromic studies revealed the dual emission of 1 in all solvents tested. Compound 1 also exhibited fluorescence in the solid state upon excitation with ahand-held UV lamp,a sw ell as mechanochromic luminescent properties. The packing mode in the crystal structure,v ariation of phenothiazine conformation, morphological changes between crystalline and amorphous phases are the major factors showing reversible fluorescence under external stimuli. A theoretical conformer study found that 1 exists in distinct conformational groups differing in Gibbs free energy by less than 3kcal mol À1 .T he conformer in the crystalline state of 1 can promote the complete separation of the HOMO and LUMO between the phenothiazine donor and the thienopyrazine acceptor,l inked by the disilane linker.H OMO-LUMO energy transition in the crystalline state is forbidden due to the lacko ff rontier orbital overlap.C rystalline state emission showed LUMO ! HOMOÀ1transition (locally excited (LE) state). In the amorphous state,t he partial presence of quasiaxial conformers allows intramolecular charge-transfer type emission via energy transfer from dominant quasi-equatorial conformers.T he strategy proposed in this work provides important guidance for developing stimuli-responsive materials with controlled excited states.

Mechanofluorochromism of (D–π–)2A-type azine-based fluorescent dyes

“…7 Meanwhile, Yamanoi et al have reported that disilane-bridged D–A–D triad composed of phenothiazine and thienopyrazine exhibits b-MFC based on changes in conformational relaxation due to the flexibility of the Si–Si bond and phenothiazine groups between the crystalline and amorphous states. 74 Therefore, in order to achieve pronounced and desired MFC that can control fluorescence properties by external mechanical stimuli, it is necessary to provide a direction in the molecular design toward creating mechanofluorochromic dyes to meet the above requirements.…”