Efficient Upconversion by Highly Water-Soluble Cationic Sensitizer and Emitter in Aqueous Solutions with DNA

Highly water-soluble cationic palladium porphyrin as a sensitizer and 9,10-bis(4-trimethylammoniumphenyl)anthracene as an emitter were newly synthesized. They were shown to be bound and immobilized in DNA double helix assembly from absorption, fluorescence, phosphorescence, and circular dichroism spectra. Upon excitation at 532 nm in deaerated aqueous solutions, they showed weak blue upconversion fluorescence, the efficiency of which increased dramatically in the presence of DNA. The threshold power density between the second-order and first-order power dependence of upconversion fluorescence decreased to less than a half upon addition of DNA. The emitter triplet lifetime estimated from time dependences of upconversion fluorescence at low power ns pulsed laser was found to considerably increase in the presence of DNA. From these results, DNA was concluded to work effectively in concentrating both sensitizer and emitter and in migrating excited triplet states, resulting in efficient upconversion.