Molecular Conformation‐ and Packing‐Controlled Excited State Intramolecular Proton Transfer Induced Solid‐State Fluorescence and Reversible Mechanofluorochromism

Abstract

Excited state intramolecular proton transfer (ESIPT) materials are highly desirable because of their unique solid state fluorescence. Schiff base molecules with appropriate hydrogen donor functional group forms intramolecular H‐bonding, a basis for ESIPT, however, only few molecules show solid state fluorescence. Herein, carbazole based Schiff base molecules 1 (2‐((E)‐(4‐(9H‐carbazol‐9‐yl)phenyl‐mino)methyl)phenol), 2 (2‐((E)‐(4‐(9H‐carbazol‐9‐yl)phenylimino) methyl)‐5‐methoxyphenol), 3 (2‐((E)‐(4‐(9H‐carbazol‐9‐yl)phenyl‐imino)methyl)‐5‐(diethylamino)phenol) and 4 (1‐((E)‐(4‐(9H‐carbazol‐9‐yl)phenylimino)methyl)naphthalen‐2‐ol)) were synthesized and ESIPT induced solid state fluorescence were correlated with solid state structural conformation and molecular packing. 2 and 4 exhibited solid state fluorescence whereas 1 and 3 did not show any fluorescence even though both 1 and 3 showed strong intramolecular H‐bonding. Further structural comparison revealed that not only the intramolecular H‐bonding facilitated ESIPT, the molecular conformation and resulting molecular packing also plays important role on the solid state fluorescence. Computational studies have also been performed to get the insight on the role of molecular conformation and organization. The non‐planar twisted molecular conformation of 2 and 4 showed reversible mechanofluorochromism (MFC). Powder X‐ray diffraction indicates that hard crushing converted crystalline to amorphous/partial amorphous phase whereas heating converts amorphous to crystalline. Thus the present study provides a structural insight for developing ESIPT based new Schiff base materials.

Hexagon and network structured organic geometrical isomers with distinct intramolecular H-bonding and stimuli-induced self-reversible fluorescence switching

“…[1][2][3][4][5] Unlike the solution properties, supramolecular interactions, molecular conformation and packing exert a significant influence on the fluorescence properties in the solid state. [6][7][8][9] Extensive structure-property studies revealed that a donor (D)-acceptor (A) molecular structure with a twisted nonplanar conformation is an ideal platform for generating solid state fluorescence materials. [10][11][12][13][14] The non-planar conformation hinders aromatic π-stacking and weak intermolecular interactions restrict free intramolecular rotation and facilitate radiative processes.…”

Easily Accessible Schiff Base ESIPT Molecules with Tunable Solid State Fluorescence: Mechanofluorochromism and Highly Selective Co2+ Fluorescence Sensing

“…The strong intramolecular H‐bonding in Schiff bases produce aggregation dependent tunable as well as white fluorescence . For Schiff bases, along with intramolecular H‐bonding the molecular conformation also plays important role for making them to solid state fluorescence . The metal chelating functionality of Schiff base ESIPT molecules can also be utilized for fluorescence sensing applications .…”

Flexible control of excited state transition under pressure/temperature: distinct stimuli-responsive behaviours of two ESIPT polymorphs

“…[10][11][12] The phototautomerisation process between the enol (E*)-keto (K*) is realized through a four-level photo-cycle (E-E*-K*-K-E). In order to achieve multi-colour regulation of solid state emissions for ESIPT molecules, different strategies have been exploited, such as (i) controlling the mode of molecular packing, 13 (ii) aggregation-induced emission (AIE), (iii) restriction of intramolecular rotation (RIR), (iv) torsion/planarity of enol conformers, etc. 5,[14][15][16] Accordingly, requirements become much stricter for the molecular design, organic synthesis of ESIPT molecules and cultivation of a polycrystalline phase.…”