Here we report on photochemical and photophysical properties of poly(propylene imine) dendrimers tethering cinnamamide groups at the peripheral positions. Photoexcitation of the dendrimer solutions with 313 nm brought about monotonous decrease of absorption band of trans-cinnamamide around 270 nm as a result of trans-to-cis photoisomerization and [2+2] photocycloaddition. The first-generation dendrimer showed preferential formation of cis-isomer, whereas photocycloaddition was more favorable for the third- and fifth-generation dendrimers. Interestingly, the third- and fifth-generation dendrimers could encapsulate phosphorescent donors into the dendrimer nanocavities. When the dendrimers capturing the donors were excited at 365 nm, photocycloaddition proceeded efficiently through triplet-triplet energy transfer. By analyzing phosphorescence spectra with theoretical Perrin's formula, we found that this triplet-triplet energy transfer is quenched within a radius of similar to 0.5 nm. Such triplet-triplet energy transfer within dendrimer nanocavities Would provide promising strategy to design and fabricate novel molecular devices by utilizing the dendrimers. Crown Copyright (c) 2009 Published by Elsevier Ltd. All rights reserved.