Furan-modified thiadiazolo quinoxaline as an electron acceptor for constructing second near-infrared aggregation-induced emission fluorophores for beyond 1300 nm fluorescence/photoacoustic imaging and photothermal therapy

Creation of new fluorophores is important for understanding the structure-property relationship, by which the required optical properties are likely to be attained. Herein, through theory calculation, it is found that furan-modified thiadiazolo quinoxaline acting as an electron acceptor can endow donor-acceptor-donor (D-A-D) type second near-infrared (NIR-II) fluorophores with longer emission wavelength than the other thiadiazolo quinoxaline-based acceptors containing pyridine, pyrrole, thiophene, and phenyl groups, respectively. On the basis of this theoretical prediction, a D-A-D type NIR-II fluorophore with 6,7-di(furan-2-yl)-[1,2,5]thiadiazolo[3,4-g] quinoxaline (DFTQ) as the acceptor and dithieno[3,2-b:2 ',3 '-d]pyrrole (DTP) as the donor is designed and synthesized, and the aggregation-induced emission (AIE) function is further achieved by introducing the AIE units of tetraphenylethylene (TPE) and triphenylamine (TPA), respectively, totally forming three NIR-II fluorophores DFTQ-DTP, DFTQ-DTPE, and DFTQ-DTPA. For biological applications, the fluorophores are encapsulated by amphiphilic DSPE-PEG2000 to generate water-dispersible nanoparticles (NPs). Almost the whole emission of each of the NPs falls into the NIR-II spectral range, with part emission beyond 1300 nm. By using DFTQ-DTPA NPs as the contrast and photothermal therapy (PTT) agent, high-resolution in vivo fluorescence imaging is achieved in the greater than 1300 nm window, and their good performance in photoacoustic imaging and high tumor PTT efficacy in tumor-bearing mice are also demonstrated. Taken together, this work mainly provides a strong electron acceptor for constructing long-emitting fluorophores, and by using the electron acceptor, a AIE fluorophore with desirable quantum yield (QY) and photothermal conversion efficienciy (PCE) is synthesized and demonstrated to be promising in fluorescence/photoacoustic imaging and PTT. By using a furan-modified thiadiazoline quinoline as an electron acceptor, we synthesize a NIR-II AIE fluorophore (DFTQ-DTPA) with long emission wavelength, and desired fluorescence quantum yield (QY) and photothermal conversion efficiency (PCE). After encapsulation with DSPE-PEG2000, the properties of DFTQ-DTPA in beyond 1300 nm NIR-II fluorescence imaging, photoacoustic imaging, and photothermal therapy are studied.image