We propose molecular-scale photochemical-reaction control using triplet-triplet (T-T) energy transfer from a donor molecule attached on a probe to an acceptor on an insulator surface. In this work, we studied the feasibility of photochemical reactions on a substrate surface using a triplet sensitizer probe. We observed an efficient T-T energy transfer from Michler's ketone on the substrate to an acceptor molecule, cinnamoyl group, on the other substrate facing it. Approximately a quarter of the cinnamoyl groups were dimerized by triplet sensitization. We used a cone-shaped dendron molecule to avoid sensitizer self-quenching caused by the triplet energy migration within the probe surface. We then confirmed efficient site separation of the cone's focal point by measuring the absorption and fluorescent properties of the rhodamine B attached to the focal point. The generation-three dendrons provide enough distance between the functional sites on the probe to reduce singlet energy transfer and it should control triplet energy migration.

Hydrogen-passivated silicon nanochains were synthesized by pulsed laser ablation of silicon target in hydrogen gas atmosphere at room temperature. When the hydrogen gas pressure was higher than 670Pa, nanochains, which have crystalline silicon core, were formed. Transmission electron microscopy showed the silicon nanochains consist of connections of silicon nanocrystals with mean diameter of 4.6 nm. It is of great interest that the silicon nanochain has crystalline structure without any intentional annealing process. Optical properties of the hydrogen-passivated silicon nanochains were presented.

This report describes the synthesis of photocrosslinkable dendrimers with peripheral cinnamamide residues, which exhibit both photoisomerization and photodimerization, and their photochemical and photophysical properties in dilute solutions and polymer matrices. Photoirradiation with 313 nm gave rise to monotonous decrease in the absorbance of trans-cinnamamide at 270 nm as a result of the photochemical reactions of the cinnamamide residues. Spectral analysis revealed the changes in the photoproduct distribution of trans- and cis-photoisomerized and photodimerized cinnamamide groups to be a function of the exposure energy. In dilute solutions, the first-generation dendrimer displayed preferential formation of cis-isomer of the cinnamamide, whereas the photodimerization took place more favorably for the third- and fifth-generation dendrimers. The photochemical behavior was strongly dependent on the dendrimer generation rather than the concentration, probably due to the extent of steric crowding among the cinnamamide residues at terminal positions. Furthermore, the third- and fifth-generation dendrimers showed capturability of a benzophenone derivative into the macromolecules and triplet-triplet energy transfer in the photocrosslinkable dendrimers. This novel phenomenon of the triplet-triplet energy transfer in the dendritic cavities suggests potential applicability to design and fabricate novel optical and electrical molecular devices.

The stabilizing effect of dendritic boxes (DBoxes) as a means to protect against photo- degradation of encapsulated dye molecules was experimentally investigated. We focused on photoinduced oxidation and studied the ability of DBoxes to protect dyes inside them from reactive singlet oxygen. Rubrene was captured in a DBox, and then the box was closed at the surface by other stable molecules. A singlet oxygen generator was added to a Rubrene solution to exaggerate photo-oxidation in the complicated total-photodegradation process, and then the solution was exposed to a laser beam to generate singlet oxygen. Bleaching of the encapsulated Rubrene in a DBox was 50 times slower than that of Rubrene alone. We also compared the dependence of surface molecules of a DBox with other kinds of molecules that have different efficiencies of quenching singlet oxygen. Further improved stability was observed by attaching singlet-oxygen-quencher molecules on the DBox surface.

A novel rigid and planar chromophore, p-phenylene benzobisthiazole unit with donor and acceptor groups, has been synthesized for second-order nonlinear optical applications. The first hyperpolarizability, β, of the nonlinear optical (NLO) chromophore has been measured by a Hyper-Rayleigh Scattering technique at 1064 nm in 1,1,1,3,3,3-hexafluoro-2-propanol solution. The β value of the NLO chromophore is 405E-30 esu, which is about seventeen times that of the p-NA (nitroaniline) value. The chromophore showed good thermal stability up to 330 °C in nitrogen atmosphere.

Two types of core-shell structured dendrimer, 2,4-bis-{3,5-bis-[3,5-bis-(3,5-di-tert- butylbenzylthio)benzylthio]benzyloxy}-6,8-dinitroquinazoline (7a) and 2,4-bis-{3,5-bis-[3,5-bis- (3,5-di-tertbutylbenzyloxy)benzyloxy]benzyloxy}-6,8-dinitroquinazoline (7b) as a referential compound were synthesized. These compounds consist of a 6,8-dinitroquinazoline ring as the functional unit and a 3,5-dimercaptobenzyl or a 3,5-dihydroxybenzyl group as the dendritic unit. In order to protect the sulfur atom in dendrimer (7a) from the oxidation reaction, a 3,5-di-tert- butylbenzyl group was designed as the outer-most unit. The synthesized dendrimer containing 3,5-dimercaptobenzyl blocks (7a) is completely novel, and is interesting as a photoelectronic material to examine what kind of influence the d-orbital electron properties give to the whole molecule.

Nonlinear optical dendritic macromolecules, called “azobenzene dendrons”, having a branching structure modified with a second-order nonlinear optical chromophore has been synthesized. An electron donor and acceptor azobenzene chromophore having a large molecular hyperpolarizability of β0=150E-30 esu, was chosen. The electronic structure of synthesized dendrons, which were expected to become dipolar due to their intermolecular attractive interaction, was proven by second-order nonlienar optical properties. The molecular hyperpolarizabilities of azobenzene dendrons were measured by the hypr-Rayleigh scattering technique. The highest molecular hyperpoalrizability was found to be 3,010E-30 esu for an azobenzene dendron having 15 chromophoric unit, where each chromophore coherently contribute to the second harmonic generation. This level of the molecular hyperpolarizability was much larger than that for a monomeric azobenzene chromophore.