Design and Synthesis of Charge-Transfer-Based Conjugated Polyelectrolytes as Multicolor Light-Up Probes

Cationic poly[9,9-bis(6'-N,N,N-trimethylammonium)hexyl)fluorene-alt-4,7-(2,1,3-benzothiadiazole) dibromide] (PFBT) and poly[9,9-bis(6'-(N,N,N-trimethylammonium)hexyl)fluorenyldivinylene-alt-4,7-(2,1,3-benzothiadiazole) dibromide] (PFVBT) are designed and synthesized to serve as multicolor light-up probes for biomolecular quantification. Because of the charge-transfer electronic states of the polymers, they exhibit weak fluorescence in aqueous media which can be significantly enhanced by increasing the hydrophobicity of polymeric microenvironment. Molecular orbital simulations further reveal that the presence of vinyl bonds endows PFVBT with a stronger charge-transfer character relative to that of PFBT. Both PFBT and PFVBT exhibit linear fluorescence enhancement as a function of bovine serum albumin (BSA) or DNA concentration in buffer solution, allowing effective biomolecular quantification. Of particular interest is that the light-up responses of PFBT or PFVBT in the presence of BSA and DNA are different, featuring biomolecule-dependent yellow-to-golden and orange-to-red light-up signatures, respectively. With a more sensitive light-up response, PFVBT can quantify biomolecules more effectively than PFBT does, which highlights the crucial role of charge transfer in determining the microenvironment-responsive fluorescence of conjugated polyelectrolytes for chemical and biological sensing.