Atsuya Momotake

Photoisomerization of Stilbene Dendrimers: The Need for a Volume-conserving Isomerization Mechanism¶

Highly branched stilbene dendrimers were synthesized and their photochemical behavior was studied. Even the stilbene dendrimer with molecular weight over 6500 underwent trans–cis isomerization in the excited singlet state within the lifetime of 10 ns. The photoisomerization of C=C double bond of stilbene dendrimers in the excited state may proceed by a volume‐conserving novel mechanism such as hula‐twist rather than conventional 180° rotation around the C=C double bond based on fluorescence and isomerization experiments.

Water-soluble stilbene dendrimers

The third generation of novel photo-responsive water-soluble stilbene dendrimer (trans- and cis-G3 WSD) undergoes unusual one-way trans-to-cis isomerization to give 100% of cis isomer at the photostationary state on UV irradiation in water.

A new class of azobenzene chelators for Mg2+ and Ca2+ in buffer at physiological pH

Abstract A new class of azobenzene-based chelators, trans - 3a and trans - 3b ( 3a and 3b ), were designed and synthesized in two steps. Both 3a and 3b were readily dissolved in a buffer solution at physiological pH. The values of the dissociation constant of 3a and 3b for Mg 2+ and Ca 2+ were determined by the Hills plot; K d Mg =1.12 mM and K d Ca =660 μM for 3a and K d Mg =158 μM and K d Ca =200 μM for 3b , respectively. On irradiation at 489 nm light, 3a isomerized to give cis -form, which underwent cis -to- trans thermal isomerization in darkness at room temperature. The change in the absorption spectrum of the irradiated solution of 3a in the presence of Mg 2+ , showing the cis -to- trans thermal isomerization, indicates that the affinity of cis - 3a for Mg 2+ is lower than that of 3a .

Photochemical cyclization reaction in cis-tetramethoxystilbene

Abstract trans -3,3′,5,5′-Tetramethoxystilbene (TMST) underwent cis–trans isomerization on photoirradiation. However, cis -3,3′,5,5′-tetramethoxystilbene efficiently underwent cyclization reaction to give dihydrophenanthrene (DHP) type compounds followed by oxidation to give 2,4,5,7-tetramethoxyphenanthrene (TMP), but the isomerization to the trans isomer did not practically take place.

Photoisomerisation and fluorescence behaviour of polyphenylene-based stilbene dendrimers

The photoinduced structural changes of three polyphenylene-based stilbene dendrimers, G1, G2, and G3, has been studied. They all exhibit fluorescence emission with considerably higher quantum efficiency and longer fluorescence lifetimes compared with the parent stilbene. These dendrimers undergo mutual cis --> trans isomerisation on direct irradiation with 310 nm light at room temperature. In a solvent glass at 77 K, one-way cis --> trans isomerisation was observed for G2. A volume-conserving isomerisation mechanism is presumed to be operating in the cis-to-trans isomerisation of G2 at 77 K.

Multimodal two-photon imaging using a second harmonic generation-specific dye

Second harmonic generation (SHG) imaging can be used to visualize unique biological phenomena, but currently available dyes limit its application owing to the strong fluorescent signals that they generate together with SHG. Here we report the first non-fluorescent and membrane potential-sensitive SHG-active organic dye Ap3. Ap3 is photostable and generates SH signals at the plasma membrane with virtually no fluorescent signals, in sharp contrast to the previously used fluorescent dye FM4-64. When tested in neurons, Ap3-SHG shows linear membrane potential sensitivity and fast responses to action potentials, and also shows significantly reduced photodamage compared with FM4-64. The SHG-specific nature of Ap3 allows simultaneous and completely independent imaging of SHG signals and fluorescent signals from various reporter molecules, including markers of cellular organelles and intracellular calcium. Therefore, this SHG-specific dye enables true multimodal two-photon imaging in biological samples.

1,3,5-Tristyrylbenzene dendrimers: a novel model system to explore oxygen quenching in a highly organized environment

Control of the efficiency of photoisomerization by oxygen was demonstrated for 1,3,5-tristyrylbenzene dendrimers 1-3; the large dendrons do not significantly affect the photochemistry itself due to volume-conserving isomerization mechanisms but may affect the mobility of the molecules.

Synthesis and photophysical properties of water-soluble dendrimers bearing a phthalocyanine core

Aggregation behaviors of phthalocyanine derivatives which have a dendritic structure were examined in an aqueous solution. We prepared the first- and second-generation dendrimers (WG1-PC and WG2-PC), having water-soluble substituents as a peripheral group, to be resolved in water. From spectroscopic measurement, WG2-PC acted in a monomeric manner, even in an aqueous solution, in contrast to WG1-PC, which exhibited spectra attributed to aggregation. This implies that a higher generation of water-soluble dendrimers can prevent a phthalocyanine core from forming an aggregation. The introduction of the second-generation dendrons to an aggregatable core may be effective to avoid self-aggregation and an undesirable deactivation process.

Intramolecular photoinduced processes of newly synthesized dual zinc porphyrin-fullerene triad with flexible linkers

Zinc porphyrin-fullerene-zinc porphyrin triad, in which two zinc porphyrin (ZnP) moieties and a fullerene (C60) moiety are linked by flexible bonds and which is intended to be a working model of the photosynthetic antenna-reaction centre, has been newly synthesized. Its photophysical properties have been investigated by both time-resolved emission and transient absorption techniques. Excitation of the zinc porphyrin moiety of the triad induced charge separation, generating the radical ion pair, in which the electron localizes on the C60 moiety and the hole localizes on the zinc porphyrin moiety. In polar solvents, the charge-separated states decayed with lifetimes of 300-600 ns returning to the ground state. Compared with ZnP-C60 dyad, ZnP-C60-ZnP triad showed longer lifetimes for the radical ion pair due to the conformation of the two ZnP moieties. The effects of the coordinating reagents on the zinc atom have been studied, with the expectation of conformational change of the two ZnP moieties with respect to C60.

Photochemical and complexation studies for new fluorescent and colored chelator

Abstract The photo-induced intramolecular proton (or hydrogen atom) transfer (ESIPT) and metal binding properties of Oxa-ester were studied in methanol solution. The dissociation constant between Oxa-ester and the metal ion was highly dependent with the metal ion and was determined by Hill plots or iterative least squares fitting to be 447 mM, 14.9 mM, and 290 nM for Ca2+, Zn2+, and Cu2+, respectively, in methanol. The fluorescence intensity of Oxa-ester greatly increased with increasing concentration of Zn2+, while the fluorescence intensity decreased with the addition of Ca2C and Cu2+. Transient absorption spectroscopy revealed that Oxa-ester underwent ESIPT to give Z-NH isomer in the excited state in benzene, while Oxa did not undergo ESIPT probably due to the hydrogen bonding with solvent water.