Light-Harvesting Two-Photon-Absorbing Polymers

A series of atactic polystyrene-based polymers was synthesized that contains grafted π-conjugated organic and organometallic chromophores to investigate two-photon light-harvesting properties. The polymers feature 4-(diphenylamino)­fluorene (DPAF) and Pt–DPAF as π-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 N3, 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.