Crystallization-Driven Self-Assembly toward Uniform Nanofibers Containing a Donor-Acceptor Core with Near-Infrared Absorption/Emission, Photodynamic, and Photothermal Activities: A Small Variation on the Structure of Donor-Acceptor Segment Matters

Although living crystallization-driven self-assembly (CDSA) has emerged as a facile approach to generate uniform pi-conjugated-polymer-based fiber-like micelles, the extension of living CDSA to prepare uniform donor-acceptor (D-A) fiber-like micelles of controlled length with attractive near-infrared (NIR) absorption/emission, photodynamic (PD), and photothermal (PT) properties is virtually unexplored. Herein, three block copolymers composed of the same corona-forming P2VP(44) (P2VP = poly(2-vinylpyridine), the subscript is the degree of polymerization) but different D-A pi-conjugated core-forming co-oligomers of OPE4-DPP-OPE4 (OPE4 = oligo(p-phenylene ethynylene), DPP = diketopyrrolopyrrole) are designed and synthesized to probe the steric and electronic effect of co-oligomers of OPE4-DPP-OPE4 on their CDSA behaviors and photophysical properties. One co-oligomer contains a DPP with two 2-ethylhexyl side chains flanked by two thiophene-OPE4 segments (b-T-OPE4-DPP-OPE4). The variation of the second one is the replacement of 2-ethylhexyl side chains with linear octyls (l-T-OPE4-DPP-OPE4). The last one is composed of a DPP with two linear octyl side chains flanked by two phenylene-OPE4 segments (l-P-OPE4-DPP-OPE4). Uniform fiber-like micelles can be generated by a self-seeding approach for b-T-OPE4-DPP-OPE4-b-P2VP(44) and l-P-OPE4-DPP-OPE4-b-P2VP(44), whereas polydisperse fiber-like micelles only can be obtained for l-T-OPE4-DPP-OPE4-b-P2VP(44) likely due to the multiple packing modes of l-T-OPE4-DPP-OPE4. Intriguingly, the aging of micelles of l-T-OPE4-DPP-OPE4-b-P2VP(44) can promote the conversion of the packing mode of l-T-OPE4-DPP-OPE4 units to their most thermodynamically stable packing mode, and thus, self-seeding of aged seed micelles of l-T-OPE4-DPP-OPE4-b-P2VP(44) gave uniform fiber-like micelles of controlled length. More interestingly, micellar solution of b-T-OPE4-DPP-OPE4-b-P2VP(44) exhibits NIR absorption/emission, PT, and PD activities upon the light irradiation at 660 nm. Although micellar solution of l-T-OPE4-DPP-OPE4-b-P2VP(44) also shows NIR absorption/emission and PT activity, its PD activity is much lower relative to that of b-T-OPE4-DPPa-OPE4-b-P2VP(44). The micellar solution of l-P-OPE4-DPP-OPE4-b-P2VP(44) exhibits neither NIR absorption/emission nor PT/PD activities upon the light irradiation at 660 nm. All these results showed that a small variation of structure of OPE4-DPP-OPE4 can lead to distinct CDSA behaviors and absorption/emission/PD/PT properties.