Self-Assembled Pd(II) Barrels as Containers for Transient Merocyanine Form and Reverse Thermochromism of Spiropyran

Abstract

Self-assembly of a cis-blocked Pd(II) 90° ditopic acceptor [ cis-(tmeda)Pd(NO)] (M) with a tetradentate donor L [benzene-1,4-di(4-terpyridine)] in 2:1 molar ratio yielded two isometric molecular barrels MB1 and MB3 in DMSO [tmeda = N, N, N' N'-tetramethylethane-1,2-diamine]. Exclusive formation of the symmetrical tetrafacial barrel (MB1) was achieved when the self-assembly was performed in aqueous medium. The presence of a large confined cavity makes MB1 a potential molecular container. Spiropyran (SP) compounds exist in stable closed spiro form in visible light and convert to transient open merocyanine (MC) form upon irradiation with UV-light or upon strong heating. The transient MC form readily converts to the stable closed SP form in visible light. MB1 has been employed as a safe container to store the planar and unstable merocyanine isomers (MC1/2) of different spiropyran molecules (SP1/2) [SP1/2 = 6-bromo-spiropyran and 6-nitrospiropyran] for several days. The transient MC forms (MC1 and MC2) were found to be stable inside the molecular container MB1 under visible light and even in the presence of different stimuli such as heat and UV light for a long time. Such stabilization of MC forms inside the confined cavity of MB1 is noteworthy. This phenomenon was generalized by utilizing a carbazole-based molecular barrel (MB2) as a host, which also showed a similar stabilization of transient MC form in visible light at room temperature. Moreover, reverse thermochromism was observed as a result of heating of the MC1 ⊂ MB2 complex, which de-encapsulates the guest in the form of SP1 to give a colorless solution. Moreover, both the host molecules (MB1, MB2) were capable of stabilizing transient MC2 even in the solid state. Such stabilization of transient MC forms in the solid state and transformation of SP forms to MC forms in the solid state in the presence of molecular barrel are remarkable, and these properties have been employed in developing a magic ink.

Photobreathing Zwitterionic Micelles

“…In the presence of the micelles, signals from both the spiropyran and the merocyanine can be observed in the 1 H‐NMR spectrum (Figure S5). During the photoconversion, the signal from the methyl group on the nitrogen is observed to move from 2.8 to 0.9 ppm . Integration of these signals shows that 55 % of the spiropyran is stabilized as merocyanine in the micelles (as seen in the sample with 1 : 2 molar ratio of zwitterionic repeat unit to spiropyran (Figure S5)), which correlates well with the UV‐Vis spectra showing that, 10 min after turning off the illumination, 58 % of the merocyanine still remains in the micelles.…”

“…During the photoconversion, the signal from the methyl group on the nitrogen is observed to move from 2.8 to 0.9 ppm. [61] Integration of these signals shows that 55 % of the spiropyran is stabilized as merocyanine in the micelles (as seen in the sample with 1 : 2 molar ratio of zwitterionic repeat unit to spiropyran ( Figure S5)), which correlates well with the UV-Vis spectra showing that, 10 min after turning off the illumination, 58 % of the merocyanine still remains in the micelles. We note that each zwitterionic repeat unit probably interacts with multiple merocyanine units, potentially using ionic, dipolar, and π-π supramolecular interactions, since merocyanine-merocyanine aggregates are not observed in the micellar cores.…”

A Visible–Near‐Infrared Light‐Responsive Host–Guest Pair with Nanomolar Affinity in Water

“…Confining photochromic molecules in nanometric cavities may profoundly impact their photochemical properties . As previously observed, 1 o can be converted into 1 c by light irradiation at 365 nm ( φ =0.04) and reverted with wavelengths >500 nm ( φ =0.0003, λ irr =550 nm) (see the Supporting Information, Figure S9) .…”

“…Confining photochromic molecules in nanometric cavities may profoundly impact their photochemical properties. [46][47][48][49] As previously observed, 1o can be converted into 1c by light irradiationa t3 65 nm (f= 0.04) and reverted withw avelengths > 500 nm (f= 0.0003, l irr = 550 nm) (see the Supporting Information, Figure S9). [50] 1 HNMR experimentss how that both species are quantitatively interconverted (see the Supporting Information,F igure S10).…”

Reversible Photochromic Nanofiber Membrane Containing Comb‐Like Poly(octadecyl acrylate) Nanoparticles Used for Ultraviolet Intensity Indicator

“…It has been conclusively shown that polystyrene matrix as a supporter for the photochromic compounds contribute to improving photochromic efficiency, sensitivity, isomeric merocyanine stability, and photochromic fatigue resistance, meanwhile, avoiding photochromic compounds aggregation . When it comes to the micron or nanometer level, the polymeric matrix containing the photochromic compounds could be assembled well on a suitable carrier in various manners . Based on the information we have, especially, the electrospun nanofibers could be credibly employed as core components of high performance sensing devices containing the photo‐responsive compounds .…”