Light-Harvesting Two-Photon-Absorbing Polymers

A series of atactic polystyrene-based polymers was synthesized that contains grafted pi-conjugated organic and organo-metallic chromophores to investigate two-photon light-harvesting properties. The polymers feature 4-(diphenylamino)fluorene (DPAF) and Pt-DPAF as pi-conjugated units which are known to be moderately efficient two-photon absorption (2PA) chromophores. The polymers were synthesized by reversible addition-fragmentation transfer (RAFT) polymerization of 4-chloromethylstyrene, and following substitution of Cl by N-3, the DPAF and Pt-DPAF chromophores were grafted onto the polymer via the copper-catalyzed azide-alkyne click reaction. The loading of Pt-DPAF units in the polymers was varied from 0 to 20% by varying the feed ratio in the click reactions. With an increase in the Pt-DPAF content, the fluorescence quantum yield from the DPAF singlet excited state decreases and fluorescence is replaced by phosphorescence characteristic of the Pt-DPAF units at 530 nm. The emission lifetime and ultrafast transient absorption spectroscopy confirm that rapid and efficient singlet energy transfer occurs from DPAF to DPAF-Pt. Excitation of the polymers with 100 fs, near-infrared pulses gives rise to upconverted emission, and the observed emission spectra are similar to those under one-photon excitation. The results indicate that the DPAF units effectively function as 2-photon absorption light-harvesting units, transferring the excitation to the Pt-DPAF units where intersystem crossing occurs efficiently. Taken together, the results point the way to development of novel polymer-based optical power-limiting materials for ultrashort and long optical pulses.