Triplet-Triplet Energy Transfer in Photocrosslinkable Dendrimers
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Triplet-Triplet Energy Transfer in Photocrosslinkable Dendrimers Seiichi Furumi,1 Akira Otomo,1 Shiyoshi Yokoyama1,2 and Shinro Mashiko1 1 Nanotechnology group, Kansai Advanced Research Center, Communications Research Laboratory, 2 PRESTO, Japan Science and Technology Corporation (JST), 588-2 Iwaoka, Nishi-ku, Kobe 651-2492, JAPAN.
ABSTRACT 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.
INTRODUCTION Molecular interactions between ground and excited states of organic molecules have been the long-standing subject of considerable interest. Among them, electronic energy transfers and migration from excited donor to acceptor species are crucial photophysical events in the molecular assemblage systems [1-3], because of their playing in requisite roles in not only natural phenomena of photosynthesis [4], but also technological advancement in performance of organic light-emitting diodes (OLEDs) [5,6]. The energy transfers occur by a radiative process through the absorption of emitted exposure (trivial) as well as by non-radiative processes, divided into two mechanisms of the dipole-dipole interaction (Förster transfer) [7] and the electron exchanges (Dexter transfer) [8]. The Förster energy transfer is resonantly generated by the dipole-dipole interaction, resulting in the long-range energy transfer of 10 nm. The Dexter mechanism by electron exchanges, on the other hand, is a very important type of energy transfer observed in diffusion-controlled conditions such as rigid solutions and glassy solid states [1-3,8]. This is because the process requires electrical collisions between donors and acceptors during the relatively long excitation
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