Ken-ichi Sakai

Using proton-transfer laser dyes for organic laser diodes

Photopumping measurements for the dilute thin films of the representative proton-transfer laser dye, 2-(2-hydroxyphenyl)benzothiazole (HBT) revealed that it exhibited better stimulated-emission performance, especially at the high doping level of 26wt%. This suggests that HBT has an ability to form high gain media, in addition to an inherent potential for widely tuning gain wavelength. Both are advantageous for overcoming the polaron-absorption problem that is a major obstacle to making organic laser diodes.

A blue-white-yellow color-tunable excited state intramolecular proton transfer (ESIPT) fluorophore: sensitivity to polar–nonpolar solvent ratios

We report the emission wavelength tunability of an excited state intramolecular proton transfer (ESIPT) fluorophore, 2,4-dibenzothiazolylphenol (2,4-DBTP), and its application to white light generation. In a nonpolar solvent, yellow fluorescent 2,4-DBTP had a maximum emission around 560 nm, but did not absorb light in the blue region (400–500 nm) because of its large Stokes shift. In a polar solvent, however, a phenol proton of 2,4-DBTP involving in ESIPT is dissociated by solvent molecules, giving rise to blue emission. By dissolving the fluorophore in a polar–nonpolar solvent mixture, we observed white emission with a broad band ranging from 400 to 650 nm, a consequence of the small overlap between the absorption spectrum of the yellow-emitting form and the fluorescence spectrum of the blue-emitting form. We also successfully fabricated a white-emitting polymer film using yellow-emitting 2,4-DBTP and a representative blue fluorescent dye (perylene) as dopants. Our fluorophore will be useful for environmentally sensitive fluorescent probes and white organic light-emitting diodes.

An ESIPT fluorophore with a switchable intramolecular hydrogen bond for applications in solid-state fluorochromism and white light generation

A novel excited state intramolecular proton transfer (ESIPT) fluorophore of BTImP, in which benzothiazole (BT) and imidazole (Im) rings are respectively linked to the 2,6 positions of a phenol, was designed and synthesized. The switching of two intramolecular hydrogen bonds from the phenol proton to either the BT or Im nitrogen was reversibly induced by external acid/base stimuli due to protonation/deprotonation of the Im site. The fluorescence color of BTImP was thus changed with high contrast between green and orange, which could be achieved even in a BTImP-doped Nafion film. Writing a letter on the film using acidic or basic water as ink was demonstrated. Furthermore, the property that the protonated state of BTImP is sensitive to the surrounding anions provides other possibilities not only for blue/orange fluorochromism in Nafion film by dry–wet treatments, but also for white light generation in solution by tuning of the excitation wavelength.

Highly efficient solid-state red fluorophores using ESIPT: crystal packing and fluorescence properties of alkoxy-substituted dibenzothiazolylphenols

An excited-state intramolecular proton transfer (ESIPT) fluorophore, 2,6-bis(benzothiazol-2-yl)phenol, was modified with alkoxy groups at the 4-position to obtain the methoxy (OMe), ethoxy (OEt), propoxy (OPr), and butoxy (OBt) derivatives. The derivatives exhibit bright red fluorescence in chloroform, giving the same fluorescence spectra with a maximum (λmax) at 619 nm. However, in the crystalline state, the λmax values of OMe and OEt are bathochromically shifted, producing a deeper red color, whereas those of OPr and OBt are hypsochromically shifted producing an orange color. X-ray analysis of the OMe and OPr crystals shows that OMe molecules interact strongly with each other through sulfur–sulfur contacts, whereas the OPr molecules are stacked in an eclipsed arrangement. Assuming that the OMe and OPr crystals are J- and H-aggregates, respectively, the difference in solid-state fluorescence could be explained by the Davydov exciton coupling theory. The OEt derivative was the best solid-state red fluorophore (λmax = 633 nm) with a fluorescence quantum yield of 0.32. Therefore, ESIPT fluorophores are promising for developing a highly efficient solid-state red-emitting material with relatively small π-conjugation and no bulky groups.

Highly emissive excited-state intramolecular proton transfer (ESIPT) inspired 2-(2′-hydroxy)benzothiazole–fluorene motifs: spectroscopic and photophysical properties investigation

Tuning or switching of the solid state luminescence of organic materials is an attractive target for both basic research and practical applications. In the present study, solid state emissive compounds with very high quantum efficiencies (ΦF up to 68%) were achieved by chemical alteration of the excited state intramolecular proton transfer (ESIPT) 2-2′-hydroxy benzothiazole (HBT) unit. Five ESIPT inspired compounds based on fluorene were synthesized via Suzuki coupling reaction. Their photophysical properties were studied by means of steady state absorption, emission spectra and a time resolved emission method in solid as well as in solution of different polarities. The fluorophores showed absorption in the UV region and emission in the visible region with large Stokes shift (∼232 nm). Efficient yellow emissive compounds showed very high quantum yields (ΦF = 55–68%) in the solid state, which are the highest quantum yields in the solid state to the best of our knowledge, for fluorene based ESIPT molecules. The fluorescence lifetime in the solid state is between 3.48–5.21 ns, while it is 5–10 fold less in chloroform (0.52–0.75 ns) solution. The optical properties of these compounds are sensitive towards the polarity of the medium. The structural properties, such as X-ray single crystal analyses, DSC and TGA were studied, and the lack of stacking and/or hydrogen bonding interactions around HBT motifs reveals enough room for ESIPT in the series of molecules even in their solid state. The DFT computations were performed to support experimental results and the calculations are well in line with the experimental results. These suggest high quantum efficiency ascribed to the large orbital energy difference between HOMOs and LUMOs of enol and keto forms transformed via ESIPT, and hence, singlet energy localization onto the keto form. The intra-molecular charge transfer nature between fluorene and HBT units plays a key role for the localization of energy on HBT motifs in their excited states.

Cation–Anion Dual Sensing of a Fluorescent Quinoxalinone Derivative Using Lactam–Lactim Tautomerism

A quinoxalinone derivative capable of lactam-lactim tautomerization was designed as a new fluorescence probe for sensing of cation (M(+) = Li(+) and Na(+)) and anion (X(-) = F(-), Cl(-), Br(-), and CH3COO(-)) in organic solvents. In THF, the minor lactam tautomer exhibited a weak fluorescence band at 425 nm with a typical Stokes shift of ∼4400 cm(-1), whereas the major lactim tautomer exhibited an intense fluorescence band at 520 nm with large Stokes shift of ∼8900 cm(-1) due to excited-state intramolecular proton transfer (ESIPT). The presence of either cations or anions was found to promote lactim-to-lactam conversion, resulting in the lowering of the ESIPT fluorescence. The lone pairs on the alkylamide oxygen and the quinoxalinone ring nitrogen of the lactam were found to bind Li(+) to form a 1:2 coordination complex, which was confirmed by single crystal X-ray structural analysis and fluorescent titrations. In addition, the N-H bond of the lactam was able to recognize anions via N-H···X hydrogen bonding interactions. Where X = F(-) or CH3COO(-), further addition of these anions caused deprotonation of the lactam to generate an anionic state, consistent with the crystal structure of the anion prepared by mixing tetrabutylammonium fluoride and the quinoxalinone derivative in THF. Dual cation-anion-sensing responses were found to depend on the ion-recognition procedure. The anionic quinoxalinone derivative and its Li(+) complex, which are formed by the addition of CH3COO(-) and Li(+), respectively, displayed different fluorescence enhancement behavior due to the two anions exchanging with each other.

AIE active triphenylamine–benzothiazole based motifs: ESIPT or ICT emission

Two novel donor–π bridge–acceptor compounds containing excited state intramolecular proton transfer (ESIPT) and non-ESIPT units based on triphenylamine–benzothiazole were synthesized via Suzuki coupling reaction. Their photophysical properties were studied in the solid state as well as in solutions with solvents of different polarities. The fluorophores showed absorption in the UV region and emission in the visible region in both solutions and solid state. The optical properties of these compounds are highly dependent on solvent polarity. Significant positive solvatochromism (∼30 nm absorption and ∼80 nm emission red shift in polar solvents) were observed for both the compounds. Large Stokes shift (∼15 000 cm−1), polarity sensitive optical properties and very high quantum efficiencies (∼90%) in solvents and the solid state are the striking features of the synthesized compounds. Intramolecular charge transfer (ICT) characteristics of the compounds were supported experimentally and computationally. Halochromism and the intermolecular charge transfer phenomenon were used for investigation of ESIPT emission for compound 9 over ICT emission.

A photoluminescent six-coordinated zinc(II) complex with hydroxides as axial ligands, [Zn(Hhpa)2(OH)2] (Hhpa = 3-hydroxypicolinamide)

A highly luminescent zinc(II) complex has been prepared using 3-hydroxypicolinamide; it has a six-coordinated octahedral structure with hydroxides as axial ligands in solution.

Partially-oxidized Ni(dmit)2 salt with copper ions

The d-π hybrid charge-transfer system, Cu 0.39 [Ni(dmit) 2 ] was prepared, and the crystal and electronic band structures were investigated. In the crystal, a unique layered structure is constructed, which results from the stacks of dimerized Ni(dmit) 2 being largely slipped along the long molecular axis. The copper ions are located at the interlayer spaces with large thermal factors. From the tight-binding band calculation, a quasi-one-dimensional Fermi surface was obtained.

Crystal-to-crystal structural transformation of hydrogen-bonding molecular crystals of (imidazolium)(3-hydroxy-2-quinoxalinecarboxylate) through H2O adsorption–desorption

The Bronsted acid–base reaction between 3-hydroxy-2-quinoxalinecarboxylic acid (Hhqxc) and imidazole (Im) in acetone–H2O yielded two 1 : 1 salts: anhydrous (HIm+)(hqxc−) (1) and hydrated (HIm+)(hqxc−)·(H2O) (2), where HIm+ and hqxc− are imidazolium and 3-hydroxy-2-quinoxalinecarboxylate, respectively. Single-crystal X-ray structural analyses and vibrational spectra were consistent with the lactam tautomer of the hqxc− anion, which formed π-dimers in both 1 and 2. Each π-dimer in crystal 1 was connected by intermolecular N–H⋯O hydrogen-bonding interactions to form a linear hqxc− chain, whereas the π-dimers in crystal 2 formed an intermolecular N–H⋯O hydrogen-bonding zigzag chain. The HIm+ cations existed in the crystalline space between the hydrogen-bonding hqxc− anionic chains. A reversible crystal-to-crystal structural transformation between crystals 1 and 2 was observed following H2O adsorption–desorption, which was confirmed by powder X-ray diffraction measurements, single-crystal X-ray structural analyses, and H2O adsorption–desorption isotherms at 323 K. The structural rearrangement of the hqxc− anions was achieved through changes in the intermolecular hydrogen-bonding interactions. The temperature- and frequency-dependent dielectric constants of crystal 2 revealed a dielectric peak at ~330 K owing to thermal fluctuations of H2O molecules within the crystal.