Unusual Behavior of Donor-Acceptor Stenhouse Adducts in Confined Space of a Water-Soluble Pd II 8 Molecular Vessel Rupak Saha, † Anthonisamy Devaraj, †,§ Soumalya Bhattacharyya, †,§ Soumik Das, † Ennio Zangrando, ‡ and Partha Sarathi Mukherjee* ,† † Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India ‡ Department of Chemical and Pharmaceutical Sciences, University of Trieste, Trieste 34127, Italy * S Supporting Information ABSTRACT: Donor-acceptor Stenhouse adducts (DASA) are new-generation photochromic compounds discovered recently. DASA exist normally in open form (blue/violet) and readily convert to cyclic (light yellow/colorless) zwitterionic form reversibly in the presence of green light in toluene/dioxane. In aqueous medium, the open form is not stable and converts to the cyclic zwitterionic form irreversibly. We report here a new self- assembled Pd 8 molecular vessel (MV) that can stabilize and store the open form of DASA even in aqueous medium. Reaction of the 90° acceptor cis-(tmeda)Pd(NO 3 ) 2 (M) [tmeda = N,N,N′,N′- tetramethylethane-1,2-diamine] with a symmetric tetraimidazole donor (L, 3,3′,5,5′-tetra(1H-imidazol-1-yl)-1,1′-biphenyl) in a 2:1 molar ratio yielded a water-soluble [8+4] self-assembled M 8 L 4 molecular barrel (MV). This barrel (MV) is found to be a potential molecular vessel to store and stabilize the open forms of DASA in aqueous medium over the more stable zwitterionic cyclic form, while in the absence of the barrel the same DASA exist in cyclic zwitterionic form in aqueous medium. The hydrophobic interaction between the cavity and the open form of DASA molecules benefits reaching an out-of-equilibrium or reverse equilibrium state in aqueous medium. The presence of excess MV could even drive the conversion of the stable cyclic form to the open form in aqueous medium. The host-guest complex is stable upon irradiating with green light. To the best of our knowledge, this is the first successful attempt to stabilize the open form of DASA molecules in aqueous medium and the first report on the fate of DASA in a confined space discrete molecular architecture. Furthermore, the molecular vessel has been utilized for catalytic Michael addition reactions of a series of nitrostyrene derivatives with 1,3-indandione in aqueous medium. ■ INTRODUCTION Light-driven photoswitching molecules have been of great interest in current research due to their wide applications in light-mediated catalysis, photoresponsive materials, selective drug delivery in biological systems, molecular electronics, and so on. 1 Azobenzenes, spiropyrans, dithienylethenes, and a few others 2 are the major contributors in the field of light-driven photoisomerization-related applications. The easy conversion between the two photoswitching forms of these molecules in the presence of UV light makes them easy to handle and selective in nature for various applications. However, in order to exhibit photochromism in low-energy visible light compared to potentially harmful UV light, 3 a new class of photochromic molecules was reported in 2014, called donor-acceptor Stenhouse adducts (DASA). 4 These molecules photoisomerize from colored neutral open forms to colorless zwitterionic cyclic forms with irradiation of visible light (Scheme 1) in organic solvents such as toluene and dioxane in a reversible manner. Higher molar absorptivity, reversible and effective photo- switching under visible light, a high degree of fatigue resistance, and the simple synthetic procedure make DASA more attractive to chemists than many other competitive photo- switching materials. In this short period, the DASA molecules found an intriguing place in the field of polymer science such as temperature localization for bullet impacts in explosives, targeted drug release by light-triggered micelle collapse, and controlling wettability. 5 Received: April 11, 2019 Published: May 3, 2019 Scheme 1. General Photoswitching Behaviour of DASA Molecules: DASA1 and DASA2 Are the Two Molecules That Are Investigated in This Study Article pubs.acs.org/JACS Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/jacs.9b03924 J. Am. Chem. Soc. XXXX, XXX, XXX-XXX Downloaded via INDIAN INST OF SCIENCE on May 15, 2019 at 06:08:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.