Unveiling the fluorescence lifetime changes of a solvatochromic aurone probe dye upon wetting in an emissive polymer brush matrix by deconvolution of TCSPC data

The wetting processes of polymer brushes, embedded with a fluorescent probe, were investigated with time-correlated single photon counting. A suitable aurone dye was synthesized, which exhibits moderate fluorescence in aprotic solvents, poor fluorescence in protic solvents and practically no fluorescence in aqueous solution. Poly(diethylene glycol methyl ether methacrylate) (PDEGMA) brushes of ca. 200 nm thickness were obtained by polymerization from a Ti surface by surface-initiated atom transfer radical polymerization (SI-ATRP). The dye-loaded brushes show fluorescence of the dye comparable with the autofluorescence of the PDEGMA brushes and the subjacent α-bromoisobutyryl bromide (BiBB) functionalized polydopamine (PDA) layer. With a global analysis of fluorescence decays, the lifetimes of the complex multicomponent materials were deconvoluted. The dry polymer loaded with the dye possesses a five-exponential fluorescence decay with the characteristic decay times of ~ 3.0 ns for the dye, and ~ 6.0, 1.8 and 0.45 ns for PDEGMA brushes as well as < 0.05 ns for the PDA-BiBB layer. Because of the strong decrease of the fluorescence lifetime of the dye down to ~ 2.4 ns when submersed in water and a concomitant decrease of the lifetimes of the matrix to 4.5, 1.3 and 0.40 ns, both dye and matrix fluorescence are feasible reporter probes for the analysis of wetting processes of polymers with fluorescence lifetime imaging. The contrast of the intensity images of the aurone, which is low because of background fluorescence, increases by a factor of 3.1 in the time-resolved imaging at certain time delay after excitation. Hence, the incorporation of the aurone in the polymer brushes is a promising and versatile system for the investigation of (de)wetting dynamics employing fluorescence lifetime imaging and nanosecond time-resolved fluorescence imaging microscopy.