Multiphoton Upconversion Fluorescence Properties of Heteroaromatic Push-Pull Polymer

Abstract

Two novel heteroaromatic push-pull fluorescence polymers poly [(2,5-diphenylene-1,3,4-oxadiazole) -4,4′-vinylene-alt-N,N′-bis(4-phenylenevinylene) -phenylamine](P1) and poly [(2,5-diphenylene-1,3,4-oxadiazole) -4,4′-vinylene-alt-N-ethyl-3,6-carbazolevinylene](P2) were synthesized by the Wittig reaction and the Heck reaction. Polymers P1 and P2 have decomposition temperatures of 373 and 412 ℃,respectively,and exhibit good thermal stabilities. Their electrochemical properties were explored by cyclic voltammetry. The highest occupied molecular orbital(HOMO) energy levels of P1 and P2 were-5.39 and-5.81 eV,respectively,and the lowest unoccupied molecular orbital(LUMO ) energy levels of P1and P2were-2.81 and-3.09 eV,respectively. Three-photon and two-photon upconversion fluorescence spectra of both polymers were measured using a femtosecond Ti:sapphire that was pumped by a femtosecond laser at 1250 and 800 nm. Strong three-photon upconversion fluorescence with maximum emission wavelengths of 510 nm for P1 and 491 nm for P2 in THF solution were observed. By pumping at 800 nm,strong two-photon upconversion fluorescences at 511 nm for P1 and 495 nm for P2 in THF were measured. Maximum peaks of single-photon fluorescence in THF were at 503 nm for P1 and at 475 nm for P2 with fluorescence quantum yields of 0.80 for P1 and 0.31 for P2. The effect of solvent polarity on multi-photon fluorescence was studied. Increased solvent polarity were found to induce a marked red-shift of the emission band in polymers P1 and P2.