Small molecule compounds having aggregation-induced emission (AIE) characteristics include donor-acceptor (D-A) structural NIR fluorophores based on 1,3-bis(dicyanomethylene)indan (BDCI), a strong and steric electron-deficient acceptor. A twisted quasi-double bond (TQDB) links each electron donor with the acceptor. The compounds can be used as NIR-II fluorescent dyes for in vivo imaging. The compounds can conjugate with bioactive molecules, such as peptides, sugars, aptamers and antibodies, to provide specific and active NIR-II fluorescent probes. The compounds can serve as active NIR-II fluorescent probes in many applications, such as, cancer cell-targeted imaging, accurate diagnosis of disease, and image-guided phototherapy.