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 ) energylevels of P1 andP2were -2.81and-3.09eV, 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 maximumemission 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 nmfor P1 and 495 nmfor P2 in THF were measured.Maximumpeaks of single-photon fluorescence in THF were at 503 nmfor P1 and at 475 nmfor P2 with fluorescence quantumyields 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.

Key words: Heteroaromatic polymer, Upconversion fluorescence, Three-photon fluorescence, Two-photon fluorescence, Solvatochromic effect