Gen-ichi Konishi

Recent advances in twisted intramolecular charge transfer (TICT) fluorescence and related phenomena in materials chemistry

Twisted intramolecular charge transfer (TICT) is an electron transfer process that occurs upon photoexcitation in molecules that usually consist of a donor and acceptor part linked by a single bond. Following intramolecular twisting, the TICT state returns to the ground state either through red-shifted emission or by nonradiative relaxation. The emission properties are potentially environment-dependent, which makes TICT-based fluorophores ideal sensors for solvents, (micro)viscosity, and chemical species. Recently, several TICT-based materials have been discovered to become fluorescent upon aggregation. Furthermore, various recent studies in organic optoelectronics, non-linear optics and solar energy conversions utilised the concept of TICT to modulate the electronic-state mixing and coupling on charge transfer states. This review presents a compact overview of the latest developments in TICT research, from a materials chemistry point of view.

Fluorescence enhancement of pyrene chromophores induced by alkyl groups through σ-π conjugation: systematic synthesis of primary, secondary, and tertiary alkylated pyrenes at the 1, 3, 6, and 8 positions and their photophysical properties.

We have systematically synthesized 1-, 3-, 6-, and 8-alkyl-substituted pyrene derivatives using the latest synthesis methods and investigated the effects of alkyl substitution on the photophysical properties of the pyrene chromophore. Like the trimethylsilyl group, which is known to enhance the fluorescence properties of some chromophores through σ*-π* conjugation, alkyl groups (primary, secondary, and tertiary) enhanced the fluorescence quantum yield of the pyrene chromophore through σ-π conjugation in most cases. While these enhancements in the fluorescence quantum yield were beyond expectations, the results were supported by absolute measurements. These results also indicate that ubiquitous alkyl groups can be used to tune the photophysical properties of the pyrene chromophore, as well as to improve the solubility or prevent aggregation. In other words, they can be used to develop new photofunctional materials.

Solvatochromic Pyrene Analogues of Prodan Exhibiting Extremely High Fluorescence Quantum Yields in Apolar and Polar Solvents

True colors: Novel pyrene analogues of Prodan exhibit outstanding photophysical properties with remarkably high fluorescence quantum yield (QY) in solvents ranging from apolar hexane to polar methanol (see figure). This is accompanied by strong solvatochromism and large Stokes shifts. These properties have not been previously achieved in enormous solvatochromic dyes, but are quite useful for emitting materials and imaging tools.

Development of Laser Dyes to Realize Low Threshold in Dye-Doped Cholesteric Liquid Crystal Lasers

Cholesteric liquid crystals (CLCs) have attracted much attention for use in a distributed feedback (DFB) laser cavity and their application to wavelength-tunable dye lasers. [ 1–6 ] For practical applications, however, the lasing threshold must be lowered essentially to zero. Considerable attempts to achieve this have been made concerning 1) the cavity structure (e.g., use of the defect mode), [ 7–9 ] 2) the excitation condition (e.g., excitation by circularly polarized light), [ 10 , 11 ] and 3) CLC materials (e.g., use of highly birefringent CLCs). [ 12 ] Syntheses of highly ordered dye molecules along the CLC local director have also been carried out successfully. [ 13 , 14 ] However, not much effort has been directed toward fi nding ideal dye systems. [ 15 , 16 ] Almost all the LC lasers so far investigated have utilized commercial laser dyes. It is obvious that dyes with higher luminous effi ciency (high molar absorption coeffi cient and high quantum yield) have to be used to increase energy effi ciency and decrease threshold. Very recently, we reported an extremely low threshold using a highly fl uorescent pyrene dye. [ 17 ] In the study reported here, we have systematically designed and synthesized series of pyrene and anthracene derivatives and found that one of the pyrene derivatives shows an even lower lasing threshold, 23 nJ/pulse. Here we suggest a strategy to obtain ideal dyes in terms of luminous effi ciency and radiative decay rate. It is very important to design a high-performance dye in order to achieve continuous wave (cw) operation, the fi nal goal of LC lasers. Pyrene and anthracene derivatives have phenyl, biphenyl, terphenyl, and naphthyl moieties, and the π -extended pyrene with 1,3,6,8-positions and anthracene with 9,10-positions have remarkable material potentials, such as high absorption coeffi cient, high quantum yield, and high stability with respect to heat and oxidation. [ 18–22 ] Therefore we focus on the use of new pyrene and anthracene derivatives as new laser dyes. Currently, because of these important optical properties, these dyes are used in organic light-emitting diodes (OLEDs) [ 21 , 23 ]

Synthesis of diphenyl-diacetylene-based nematic liquid crystals and their high birefringence properties

We synthesized two series of diphenyl-diacetylene (DPDA)-based materials with alkoxy and alkyl tails of length m (DPDA–OCm and DPDA–Cm, respectively), and measured their nematic-phase birefringence (Δn) as a function of wavelength and temperature. We found that Δn decreases with an increase in m, possibly by a dilution effect of the low-Δn alkyl tail. Further, of the two series, Δn was found to be relatively higher in the DPDA–OCm materials, with the highest value of 0.4 obtained for DPDA–OC1 at 550 nm at 10 °C below the isotropic-to-nematic transition temperature. Further, we observed the temperature dependence for Δn, which is proportional to the order parameter (s). From extrapolation to s = 1 (the perfect orientation state), it is speculated that the DPDA–O moiety has the potential to afford a very large Δn of 0.9.

Highly Twisted N,N-Dialkylamines as a Design Strategy to Tune Simple Aromatic Hydrocarbons as Steric Environment-Sensitive Fluorophores

The steric-environment sensitivity of fluorescence of 9,10-bis(N,N-dialkylamino)anthracenes (BDAAs) was studied experimentally and theoretically. A new design strategy to tune simple aromatic hydrocarbons as efficient aggregation-induced emission (AIE) luminogens and molecular rotors is proposed. For a variety of BDAAs, prominent Stokes shifts and efficient solid-state fluorescence were observed. Calculations on BDAA-methyl suggested that in the ground state (S0) conformations, the pyramidal amine groups are highly twisted, so that their lone-pair orbitals cannot conjugate with the anthracene π orbitals. Fluorescence takes place from the S1 minima, in which one or both amine groups are planarized. The stability of the S1 excited state minima as well as destabilization of the S0 state is the origin of large Stokes shift. Experimental measurement of the nonadiabatic transition rate suggests that para disubstitution by dialkylamino (or strongly electron-donating) groups is a key for fast internal conversion. Minimum energy conical intersection (MECI) between S1 and S0 states was found to have a Dewar-benzene like structure. Although this can be reached efficiently in liquid phase for fast internal conversion, a large amplitude motion is required to reach this MECI, which is prohibited in the solid state and caused efficient AIE. This strategy is used to find experimentally that naphthalene analogues are also efficient AIE luminogens. The flexibility of alkyl chains on amino groups is also found to be important for allowed charge-transfer transition. Thus, three points [(1) highly twisted N,N-dialkylamines, (2) substitution at the para positions, (3) with flexible alkyl groups] were proposed for activation of small aromatic hydrocarbons.

Bright and photostable push-pull pyrene dye visualizes lipid order variation between plasma and intracellular membranes

Imaging lipid organization in cell membranes requires advanced fluorescent probes. Here, we show that a recently synthesized push-pull pyrene (PA), similarly to popular probe Laurdan, changes the emission maximum as a function of lipid order, but outperforms it by spectroscopic properties. In addition to red-shifted absorption compatible with common 405 nm diode laser, PA shows higher brightness and much higher photostability than Laurdan in apolar membrane environments. Moreover, PA is compatible with two-photon excitation at wavelengths >800 nm, which was successfully used for ratiometric imaging of coexisting liquid ordered and disordered phases in giant unilamellar vesicles. Fluorescence confocal microscopy in Hela cells revealed that PA efficiently stains the plasma membrane and the intracellular membranes at >20-fold lower concentrations, as compared to Laurdan. Finally, ratiometric imaging using PA reveals variation of lipid order within different cellular compartments: plasma membranes are close to liquid ordered phase of model membranes composed of sphingomyelin and cholesterol, while intracellular membranes are much less ordered, matching well membranes composed of unsaturated phospholipids without cholesterol. These differences in the lipid order were confirmed by fluorescence lifetime imaging (FLIM) at the blue edge of PA emission band. PA probe constitutes thus a new powerful tool for biomembrane research.

The effect of regioisomerism on the solid-state fluorescence of bis(piperidyl)anthracenes: structurally simple but bright AIE luminogens†

A series of regioisomers of piperidylanthracenes (PA) and bis(piperidyl)anthracenes (BPA) were synthesized and their photophysical properties examined in solution, suspension, and the solid state. Aggregation-induced emission (AIE) was observed only for 1,4-BPA and 9,10-BPA, in which two piperidyl groups are substituted at the anthracene moiety in the para-position with respect to each other. Compared to previously reported AIE luminogens, these easily obtainable para-substituted BPAs, exhibited several unique and beneficial features, such as simple structures, bright solid-state fluorescence (Φfl = 0.49 and 0.86 for 1,4-BPA and 9,10-BPA, respectively), tunable fluorescence emission, and large Stokes shifts. Results from diffuse-reflectance and fluorescence lifetime measurements demonstrated that PAs and BPAs intrinsically possess an efficient non-radiative transition pathway in the solid state, and that 1,4-BPA and 9,10-BPA may overcome this pathway. The X-ray crystallographic analysis of 1,4-BPA revealed that undesirable interchromophoric interactions can be minimised, while TD-DFT calculations suggested that the enhanced Stokes shift of 1,4-BPA arises from severe electronic repulsion between neighbouring piperidyl moieties, which presumably results in the absence of self-absorption.

Pyrene-based D–π–A dyes that exhibit solvatochromism and high fluorescence brightness in apolar solvents and water

A novel pyrene-based D–π–A dye consisting of piperidine (D) and a secondary N-alkyl carboxamide (A) was prepared. This dye showed solvatochromism and bright fluorescence in solvents with a wide range of polarities, including water. Such emission properties are derived from the role of the secondary N-alkyl carboxamide group as both a weak acceptor and a stabilizer of the n electrons on the carbonyl group.

Selective Formation of Twisted Intramolecular Charge Transfer and Excimer Emissions on 2,7-bis(4-Diethylaminophenyl)- fluorenone by Choice of Solvent

We designed and synthesized a donor-acceptor-donor dye consisting of a 2,7-disubstituted fluorenone with diethylaminophenyl moieties present as strong electron donating groups. Switching between twisted intramolecular charge transfer (TICT) emission and excimer emission was achieved, with no ground state changes, by simply changing the solvent used. In a nonpolar solvent, excimer emission was observed; with increasing polarity, the emission gradually disappeared, and the TICT emission appeared.