Optical and Structural Properties of ESIPT Inspired HBT–Fluorene Molecular Aggregates and Liquid Crystals

Abstract

In bulk materials, positional isomers not only help in understanding how slight difference in molecular structure alters the crystal packing and optical properties, but also play a key role in developing new type of materials for functional applications. A detailed study on the photophysical properties of fluorene-HBT positional isomers in solution and in the solid state providing a molecular level understanding of the factors which influence fluorescence behavior is reported. Two molecules Ia and IIa were synthesized by Suzuki coupling reaction and their photophysical properties were compared to positional isomers Ib and IIb. Crystal structure analyses and density functional theory (DFT) computation studies were performed to understand structure-properties relation and the results reveal that changing substitution pattern has a marked influence on their packing modes and luminescence properties. Strong noncovalent interactions (π-π) in the solid state hamper the excited state intramolecular proton transfer (ESIPT) process which causes fluorescence quenching in the solid state (Ia and IIa = Φ, 28-40%; Ib and IIb = Φ, 55-67%). Compounds show solvent-responsive and aggregation induced emission (AIE) properties. Bent structures of Ia with double and symmetric substitution of ESIPT motifs exhibit particularly unique condensed phase upon heating, confirmed as a nematic liquid crystalline phase, and this is the first report on the ESIPT and AIE active liquid crystalline materials with a banana-shaped molecule.

A TD‐DFT investigation of the photo‐induced excited state intramolecular proton transfer dynamics for the novel 5,5′‐(9,9‐dihexyl‐9H‐fluorene‐2,7‐diyl)bis(2‐benzo[d]thiazol‐2‐yl)phenol) system

“…For convenience, the simulated electronic transition energies, relative oscillator strengths, and compositions of first three transitions have been listed in Table . It can be found clearly that the absorption peak of the first transition (S 0 → S 1 ) is calculated to be 402 nm, which is in good agreement with the experimental result (382 nm) . At least, it can be confirmed that the TDDFT/B3LYP/6‐31G(d) theoretical level is enough to describe the Ia‐enol system.…”

“…Herein, it is clear that the first excited state mainly owns the ππ*‐type‐character with the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). From Table , it should be noticed that the HOMO‐LUMO transition owns a very large oscillator strength (2.5471), which means the evident absorption in previous experiment . Herein, we just show the HOMO and LUMO orbitals in this present work, because this transition almost includes the entire percentage (ie, 95.46%).…”

“…Thus, the ESIPT process along with double intramolecular hydrogen bonds should be the fundamental model in this connection. Recently, Padalkar and coworkers designed and synthesized the novel 5,5′‐(9,9‐dihexyl‐9H‐fluorene‐2,7‐diyl)bis(2‐benzo[d]thiazol‐2‐yl)phenol) (abbreviated as Ia) . They explored the molecular packing and optical properties for the Ia system experimentally and presented that the strong noncovalent interactions in solid state play roles in hampering the ESIPT process that leads to fluorescence quenching in solid state.…”

“…In fact, Ia owns two intramolecular hydrogen bonds, which means it may refer to single or double proton transfer behaviors in the excited state. However, it was not considered in Padalkar et al In addition, experiments might just provide the indirect information about photochemical and photophysical properties. Even though a few simple theoretical simulations were implemented in previous work, the solvent effect was not touch on in their calculations.…”

“…However, it was not considered in Padalkar et al In addition, experiments might just provide the indirect information about photochemical and photophysical properties. Even though a few simple theoretical simulations were implemented in previous work, the solvent effect was not touch on in their calculations. Therefore, in this work, in‐depth theoretical investigations about ESIPT behavior and mechanism of Ia are necessary.…”