Highly Fluorescent Liquid Crystals from Excited‐State Intramolecular Proton Transfer Molecules

Abstract

solid-state emitters, electroluminescent devices, organic lasers, and so on. [1][2][3][4][5] Particularly, emission through excitedstate intramolecular proton transfer (ESIPT) mechanism with an extremely large Stokes shift [6][7][8][9] (anti-Kasha rule) suppresses excitation energy dissipation via self-absorption, providing attractive material platforms for optical device fabrication. [10,11] ESIPT is a photochemical process that produces a tautomer with a different electronic structure from the original excited form (Figure 1a). The requisite of chemical structures for ESIPT is the presence of an intramolecular hydrogen bond (H-bond) between the proton donor (OH and NH 2 ) and the proton acceptor (N and CO) groups close to one another in a single molecule. Recent notable reports on the ESIPT fluorophores include the polymorph-dependent emissive crystals, [12,13] double proton transfer process with an extra-large Stokes shift, [14] amplified spontaneous emission, [15][16][17] lasing system, [18][19][20] environment-sensitive multicolored materials, [21][22][23][24] and the use as emitters with electrically generated intramolecular proton transfers. [25] Most of ESIPT molecules show significant fluorescence in the solid states, not suffering from "concentration quenching" and thus serving as aggregation-induced emission enhancement (AIEE) materials. [26][27][28] The ESIPT process requires structural relaxations upon proton transfer, and thus a certain conformational freedom is allowed for the molecular structures. [29] The conformational freedom results in allowing the possible nonradiative pathways especially in solution (Figure 1b), giving AIEE characters to the ESIPT molecules. [30,31] The AIEE character is advantageous because light outputs from the systems are extremely high embedded into actual devices to avoid the concentration quenching. [32] However, most of the previous ESIPT as well as AIEE systems have been mostly reported in the rigid solid phase such as single crystals, polycrystals, or powders [11,12] -not in liquid crystals (LCs). Although columnar LCs with ESIPT-active cores were reported, [33,34] simple rod-shaped LC molecules embedded with ESIPT-active group have been limited. [35,36] In contrast to columnar and cubic liquid crystal phases with highly ordered structures, nematic as well as several smectic subphases can easily align macroscopically by rubbed surfaces, mechanical stimuli, and electric and magnetic fields. [37][38][39] Fluorescence via excited-state intramolecular proton transfer (ESIPT) provides strong light emission with a large Stokes shift and environment-sensitive unique spectral patterns. Particular systems including 2-(2-hydroxyphenyl) benzothiazole (HBT) serve as efficient solid-state emitters with the ESIPT mechanism and aggregation-induced emission enhancement (AIEE) property, but have not been used for liquid crystalline (LC) materials. Here, rod-shaped fluorescent LCs with ESIPT characters are newly developed based on the HBT motif. The design of the targeted mole...

Near-Infrared Organic Single-Crystal Nanolaser Arrays Activated by Excited-State Intramolecular Proton Transfer

“…For instance, Seki et al reported that ESIPT-active molecule doped liquid crystal materials with controlled molecular alignment allow high fluorescent quantum yield, exhibiting promising prospects as a laser gain medium. 33,34 To better understand the excited-state dynamics of DMHC samples, we conducted time-resolved femtosecond fluorescence upconversion transients and the transient absorption. The time-resolved femtosecond fluorescence upconversion transients of DMHC single crystal monitored at 620 and 750 nm are shown in Figure 1C (the instrument response function is shown by the black squares), and the lifetimes are summarized in Table 1.…”

Molecular spectroscopy and adverse optical properties of N-(p-hexyloxy-benzylidene)–p-toluidine (HBT) liquid crystal molecule studied by DFT methodology

“…The C-O stretching expresses the maximum absorbance because oxygen atom transfers the charge to the benzene ring during the intermolecular interaction shown in figures 5 and 6, also given in table 1. Zhang et al reported [24] oxygen atom transfer the proton to the nitrogen atom during an intermolecular interaction, which is responsible for the maximum transmittance of the molecule. The HBT liquid crystal molecule suitable for sensing and filtering applications because it expresses a broad range of frequencies given in table 1 and shown in…”

Role of Conical Intersections on the Efficiency of Fluorescent Organic Molecular Crystals

“…HBT 2-(2'-hydroxyphenyl)-benzothiazole (HBT) displays SSLE upon its aggregation to herringbone single crystal 37 and liquid crystal phases. 38 While HBT has negligible QEF in organic solvents, it significantly increases in aggregate phases, amounting to 0.77 in powder samples. 29,39 For this reason, HBT-derivatives are proposed as materials for organic light-emitting diodes (OLEDs) and fluorescent probes.…”