π-Bridged Dimer Strategy of Aggregation-Induced Emission Molecules to Achieve Very Strong Absorption Ability

Abstract

Aggregation-induced emission (AIE) molecules have attracted widespread attention due to their remarkable fluorescence properties in the aggregated state. However, the highly twisted structures of AIE molecules significantly disrupt the π-conjugations, thus resulting in weak absorption abilities (i.e., small molar absorption coefficients ε). To overcome this problem, herein we have proposed an efficient molecular design strategy: π-bridged dimer of AIE molecules. Accordingly, two series of AIE dimer molecules, TPE-BTO-Dimer 1‒6 and DTPE-BTO-Dimer 1‒6 with various π-bridged moieties, have been newly synthesized. In comparison to the corresponding AIE monomer molecules TPE-BTO and DTPE-BTO, the dimer molecules retain the AIE character while exhibit largely improved absorption abilities (the ε values are increased by 2.3‒3.7 times to 6.01‒9.54 × 10⁴ M⁻¹ cm⁻¹) as well as significantly redshifted absorption maxima. The theoretical calculations have revealed that the π-bridged dimer strategy dramatically increases the oscillator strength of electron transition from the ground state to an excited state and thus results in a large ε. In the transient absorption studies, the local excited state components of dimer molecules are obviously higher than those of monomer molecules, which further confirms the effectiveness of π-bridged dimer strategy. Moreover, one of the AIE dimer molecules DTPE-BTO-Dimer 6 with near-infrared (NIR) emission has been applied in NIR fluorescence imaging-guided photothermal therapy. The very strong absorption ability has enabled its nanoparticles to exhibit a high photothermal conversion efficiency of 73% under the 655 nm laser irradiation and thus display a desired photothermal therapy performance.