Low-bandgap conjugated donor-acceptor copolymers based on porphyrin with strong two-photon absorption

Two new low-bandgap, conjugated donor (D)-acceptor (A) copolymers of porphyrin with 2, 3-bis(4-trifluoromethylphenyl)pyrido[3, 4-b]pyrazine (P1) and perylene diimide (P2) were synthesized by Sonogashira coupling polymerization and compared with porphyrin-dithienothiophene D-D copolymer (P3). All these polymers possess good thermal stability with decomposition temperatures over 300 °C. Polymers P1 and P2 in films exhibit strong absorption in near-IR (820-950 nm) with optical bandgaps as low as 1.15 eV; their Q-bands red shift 60-190 nm compared to that of P3, while the Soret bands are similar. The HOMO (-5.3 to -5.4 eV) and LUMO (-3.6 to -4.0 eV) of the D-A polymers are lower than that of the D- D polymer. Two-photon absorption (2PA) properties of the polymers were investigated by the femtosecond Z-scan method. The D-A polymer P2 exhibits 2PA cross sections over 7000 GM/repeat unit at telecommunication wavelengths (1320 and 1520 nm), larger than that of P1 and P3, due to the very strong, rigid, and coplanar perylene diimide acceptor and strong D-A intramolecular charge transfer.