pH-Dependent Morphology and Photoresponse of Azopyridine-Terminated Poly(N-isopropylacrylamide) Nanoparticles in Water

Abstract

A series of azopyridine-terminated poly( N -isopropylacrylamide)s (PNIPAM) (C12-PN-AzPy) (∼5000 < M w < 20 000 g mol –1 , polydispersity index 1.25 or less) were prepared by reversible addition–fragmentation chain-transfer polymerization of NIPAM in the presence of a chain-transfer agent that contains an AzPy group and an n -dodecyl chain. In cold water, the polymers form nanoparticles (5.9 nm < R h < 10.9 nm) that were characterized by light scattering (LS), 1 H NMR diffusion experiments, and high-resolution transmission electron microscopy. We monitored the pH-dependent photoisomerization of C12-PN-AzPy nanoparticles by steady-state and time-resolved UV–vis absorption spectroscopy. Azopyridine is known to undergo a very fast cis-to-trans thermal relaxation when the azopyridine nitrogen is quaternized or bound to a hydrogen bond donor. The cis-to-trans thermal relaxation of the AzPy chromophore in an acidic nanoparticle suspension is very fast with a half-life τ = 2.3 ms at pH 3.0. It slows down slightly for nanoparticles in neutral water (τ = 0.96 s, pH 7.0), and it is very slow for AzPy-PNIPAM particles in alkaline medium (τ > 3600 s, pH 10). The pH-dependent dynamics of the cis-to-trans dark relaxation, supported by Fourier transform infrared spectroscopy, 1 H NMR spectroscopy, and LS analysis, suggest that in acidic medium, the nanoparticles consist of a core of assembled C12 chains surrounded by a shell of hydrated PNIPAM chains with the AzPy + end groups preferentially located near the particle/water interface. In neutral medium, the shell surrounding the core contains AzPy groups H-bonded to the amide hydrogen of the PNIPAM chain repeat units. At pH 10.0, the amide hydrogen binds preferentially to the hydroxide anions. The AzPy groups reside preferentially in the vicinity of the C12 core of the nanoparticles. The morphology of the nanoparticles results from the competition between the segregation of the hydrophobic and hydrophilic components and weak attractive interactions, such as H-bonds between the AzPy groups and the amide hydrogen of the PNIPAM repeat units.

Nanoscale Coordination Polymer of Dimanganese(II) as Infinite, Flexible Nanosheets with Photo‐Switchable Morphology

“…The absorption band at 569 cm –1 indicates the formation of metal‐oxygen bonds. In the UV/Vis spectrum (Figure S2, S3), the compound 1 in DMF solution shows two absorption bands at 308 and 454 nm assigned to π–π* and n–π* electronic transition, respectively in trans configuration . The results of the elemental analysis for C, H and N match quite well with the values calculated on the basis of the molecular formula determined by crystallography.…”

“…IR ν max (KBr), cm –1 : 538m, 569m, 625s, 793s, 841vs, 1015s, 1051vs, 1076vs, 1109vs, 1227m, 1254s, 1321m, 1418vs, 1452s, 1491w, 1601vs, 1738w, 2858w, 2926m, 2955m, 3437w. UV/Vis (DMF): λ max = 308 nm ( ε = 13609 mol –1 Lcm –1 ) (π–π* transition); λ max = 454 nm ( ε = 369 mol –1 Lcm –1 ) (n–π* transition).…”

Light, temperature, and pH control of aqueous azopyridine-terminated poly(N-isopropylacrylamide) solutions

“…It is worth mentioning that the amide group of the NIPAM repeat units and the ester group that links the AzPy end group to the main chain withhold the experimental conditions, as reported previously according to previous reports. 19,42 High sensitivity microcalorimetry (HS-DSC) thermograms recorded for aqueous solutions of C12-PN-AzPy 7 K of pH 7.0 (black squares) and pH 3.0 (red circles) are presented in Fig. 1d, where T m denotes the temperature corresponding to the maximum of the endotherm.…”

“…The C12-PN-AzPy samples were prepared by reversible additionfragmentation transfer radical polymerization using a chain transfer agent bearing an n-dodecyl group and an azopyridine moiety, as described in detail previously. 19 Their molar mass ranges from M n 5800 g mol −1 to 19 700 g mol −1 ( Table 1). Their chemical structure is presented in Scheme 1.…”

“…[16][17][18] This phenomenon is exemplified in a recent study of aqueous solutions of poly(N-isopropylacrylamides) bearing an AzPy on one chain end and an n-dodecyl group on the other end (C12-PN-AzPy) (Scheme 1a). 19 In neutral or basic water, they form "flowerlike" core-shell micelles with a core of n-dodecyl chains and AzPy groups surrounded by a PNIPAM corona. In solutions of pH 3, the micelle morphology changes as a result of the protonation of the AzPy group.…”