Effective photocrosslinking reaction of dendrimers through triplet energy transfer Seiichi Furumi*, Shiyoshi Yokoyama, Akira Otomo, Shinro Mashiko ln this article, we descnbe the synthesis and photoreactions of photocrosslinkable dendrimers bearinng lrantMirnalnOyl residues at the peripheral positions. Photoirradiation of the dendrimers with 313 nm gave rise to monotonous decrease in the absorbance of trans-Cinnamates at 270 nm as a result of their photochemical reactions involving trans-to-cis photoisomerization and [2+2] photodimerization. The first-generation dendrimer displayed the preferential fomation of cis-cinnamates at the photostationary state, whereas the photodimerization took place favorably for the third- and fifth-generation dendrimers. The photodimerized rate was strongly dependent on the dendritic generation rather than the conentration of solutions, probably due to the extent of steric crowding among the cinnamates settled on the dendritic surfaces. The third and fifth-generation dendrimers enabled the capturing of a phosphorescent sensitizer into the internal dendritic cavities to generate the effective photocrosslinking of the cinnamates in an intramacromolecular manner through triplet energy transfer from the excited sensitizer. The photocrosslinking reaction of dendrimers through the triplet energy transfer might provide potential applicabilities to design and fabricate novel optical and electrical molecular devices from the bottom-up approach.