Masashi Ikegami

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.

Bis-N-nitroso-caged nitric oxides: photochemistry and biological performance test by rat aorta vasorelaxation

Three new caged nitric oxides (NOs)-BNN3, BNN5Na, and BNN5M were tested for biological use. BNNs have a strong ultraviolet (UV) absorption band (lambda(max): 300 nm, epsilon: 13.5 mM(-1) cm (-1)) extended to 420 nm and produce NO upon irradiation with 300-360 nm light in quantum yields about 2. A photoexcited BNN molecule yields two NOs with time constants of less than 10 ns for phase 1 and less than 20 micros for phase 2 at 37 degrees C, suggesting usefulness of BNNs for measuring in vivo and in vitro fast NO reactions. Upon irradiating with UV light, caged nitric oxides-loaded rat aortic strips maintained in a state of active tonic contraction effectively relaxed ( < 3 microM BNN5M loading solution concentration). BNN3 is incorporated in the lipid membrane. BNN5Na, insoluble in organic solvents but water soluble, localizes in the water phase. BNN5M, is muscle-cell-permeable and hydrolysed to BNN5Na to remain in cytosol. BNNs were thermally stable and demonstrated no observable toxicity.

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.

Photoinduced intramolecular hydrogen atom transfer in 2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxyphenyl)benzothiazole studied by laser flash photolysis

2-(2-Hydroxyphenyl)benzoxazole (HBO) and 2-(2-hydroxyphenyl)benzothiazole (HBT) underwent hydrogen atom transfer to give the tautomer in both the singlet excited state and the triplet excited state. In the singlet excited state, the keto form produced by excited state hydrogen atom transfer underwent isomerization around the quasi-double bond to give the trans-keto form. In the triplet excited state, HBO and HBT were equilibrated between the enol form and the cis-keto form and the equilibrium constant was determined by triplet sensitization and quenching experiment by using laser flash photolysis in benzene at room temperature. From these results, we have revealed the energy diagram of the hydrogen atom transfer reaction of HBO and HBT both in the singlet excited state and the triplet excited state.

Photochromism of Hydrogen Bonded Compounds

Abstract The effect of hydrogen bonding on the photochemical cis-trans isomerization is discussed on the basis of the results of absorption and fluorescence spectra as well as of the determination for the efficiencies of photochemical and photophysical processes.

Effect of the Intramolecular Hydrogen Bonding on the Photochromic Properties of the Hemiindigo Dye having a Pyrrole Ring

ABSTRACT Photochromism and fluorescence properties of 2-(2-pyrrolylidene)indolin-3-one (1) have been studied. The quantum yields of fluorescence emissions of both Z- and E-isomer were very small in protic solvents such as ethanol and water. However, the fluorescence intensity of E-1 increased by addition of bovine serum albumin (BSA) in water, indicating that E-1 should bind to the hydrophobic site of BSA.

Exciplex formation between pyrene and guanine in highly polar solvents

Guanine derivatives connected to pyrene with methylene spacers exhibited exciplex emission in highly polar solvents such as N,N-dimethylformamide, acetonitrile, and water. The result will open up a novel approach to synthesis of fluorescent nucleic bases.

Photoisomerization of 2-(2-pyrrol-2-ylethenyl)benzoxazole and 2-(2-pyrrol-2-ylethenyl)benzothiazole

Hydrogen bonded compounds 2-(2-pyrrol-2-ylethenyl)benzoxazole (4) and 2-(2-pyrrol-2-ylethenyl)benzothiazole (5) were synthesized and their photochemical behavior was studied. The (Z) isomers of 4 and 5 form intramolecular hydrogen bonds as revealed by the 1H NMR studies. The trans isomers are pale yellow in solution, but on photoirradiation, the color changes slightly to a different yellow due to the change of the absorption spectra. The quantum yield of isomerization of 4 and 5 is quite high in the range 0.4–0.6 and is somewhat dependent on the solvent properties. On triplet sensitization 4 and 5 gave T–T nabsorption spectra with a lifetime of ca. 1 μs in benzene. Since a transient absorption spectrum was not observed on direct irradiation of 4 and 5 with a 308 nm laser pulse, the cis–trans isomerization on direct irradiation should take place in the excited singlet state. On triplet sensitization 4 and 5 underwent cis–trans isomerization. These results indicate that the intramolecular hydrogen bonding is broken in the singlet excited state as well as in the triplet excited state of (Z)-4 and (Z)-5.

Triplet state behavior of pyrrole-2-carboxyaldehyde and related compounds

The triplet state behavior of pyrrole-2-carboxyaldehyde (1) and related compounds was investigated by means of nanosecond laser flash photolysis. Compound 1 did not exhibit fluorescence emission, but underwent intersystem crossing to the triplet state with an efficiency of 0.80 in benzene. The triplet lifetime of 1 depended on its concentration. The self-quenching rate constant was determined to be 1.6 × 108 M−1 s−1 in benzene. Intermolecular hydrogen bonding between the triplet state and the ground state plays an important role in the self-quenching process of 1.

Hydrogen bonded molecular assembly by reversible cyclization reaction of anthracene

A hydrogen bonded molecular assembly of 2-ureido-4(1H)-pyrimidinone was constructed by utilizing an anthracene photodimerization reaction.