A clear and unambiguous rationalization of chirality induction, amplification, and subsequent inversion processes has been demonstrated using an achiral Mg(II)porphyrin dimer (host) and a series of chiral diols (guests) upon stepwise formation of a 1:1 host−guest polymer and 1:2 host−guest monomer via intermolecular assembling and disassembling processes. Crystallographic characterizations are reported here for both the polymer and the monomeric complexes, which enable us to completely scrutinize the structural and geometrical changes systematically in rationalizing their optical properties. The sign of the CD couplets for both the polymer and monomer are just opposite between R and S guests, which suggests that the chirality is dictated solely by the stereogenic projection of the chiral centers. Stronger intra-and intermolecular coupling in the polymeric complexes is responsible for the highly enhanced CD couplets as compared to the monomer and have only intramolecular coupling as also observed in their X-ray structures. DFT studies clearly support the experimental observations. ■ INTRODUCTIONControlling supramolecular and macromolecular chirality has become a fascinating research area in the past two decades, not only for understanding the asymmetry of various biological systems but also due to its potential applications to materials for chiral sensing, chiral separation, asymmetric catalysis, etc. 1−3 Of particular interest is the precise regulation of chirality, which has been so far achieved by employing host− guest stoichiometry, metal coordination, solvents, anions, temperature, etc. 4,5 Porphyrins are of special interest for detailed investigations of the supramolecular chirality induction process owing to their interesting photophysical properties, versatile modification, great biological importance, and wide applicability. 5−7 Stoichiometry mediated chirality induction and inversion have been known to occur via an intramolecular 1:1 and 1:2 host−guest complexation mechanism. 6,7 There are only a few reports available in which the chirality induction and inversion processes have been rationalized with X-ray structures using the same host−guest system. 6c,d In the present work, we report stepwise chirality induction, amplification, and inversion processes occurring via intermolecular assembling and disassembling between the Mg(II)porphyrin dimer host and chiral diol guest by varying their stoichiometry. At a lower guest concentration, the 1D polymer of the 1:1 host−guest complex was formed, which upon great addition of the guest converted into the monomeric 1:2 host−guest complex. A clear rationalization of chirality induction, amplification, and inversion phenomena has been reported here, which are fully supported by single crystal X-ray structure determination and DFT calculations. ■ RESULTS AND DISCUSSIONA urea-bridged Ni(II)porphyrin dimer has been synthesized using the reported procedure. 8 After demetalation using concentrated sulfuric acid, magnesium was inserted into the free base porphyrin di...
A new class of pyridine-based spiropyran (SP) shows photoinduced reversible switching between the closed SP and ring-opened merocyanine (MC). We show that a condensed crystalline monolayer of SP on graphite can be quantitatively converted to MC upon UV irradiation. In solution only ∼10% of SP can be transformed to MC because of the establishment of a photostationary state. Using an electrical field applied by a scanning tunneling microscopy (STM) tip, single molecules are reversibly switched between SP and MC forms in their condensed phases without any threshold voltage at ambient conditions. The microscopic structure of submonolayer films of SP and MC are investigated using atomic force microscopy (AFM) and STM.
The formation of a monolayer film of bis-naphthyridyl ferrocene on highly oriented pyrolytic graphite (HOPG) at ambient conditions is demonstrated. The films are prepared by drop casting from different solvents. The microscopic structure of the films is understood using atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). The analysis reveals two different types of Phases (I and II) in the films and the relative percentage of these phases depends on the nature of the solvents used for the preparation and the thermodynamical condition. Solvents like methanol, acetonitrile and DMF exclusively select Phase-I, whereas acetone and ethanol show a mix of both phases at room temperature. The different phases are formed by different conformers of the molecule. We also show that the selectivity of one of the phases over the other is related to the difference in the energetics for the formation of these phases.
Background: The nonsteroidal contraceptive centchroman is manufactured in India. It was included in family welfare programme in April 2016 by the Ministry of Health and Family Welfare, Government of India. However, national statistics indicated its low utilization at all levels of healthcare. Objective was to identify the reasons for underutilization of centchroman in the family planning clinic, and in the postpartum period, and to implement corrective measures to improve its utilization at a tertiary hospital.Methods: The study was conducted in the department of obstetrics and gynecology at a tertiary care hospital from September 2019 to January 2022. A quality improvement action team was constituted to undertake root cause analysis. Important corrective factors were insufficient focused counselling for centchroman as contraceptive, insufficient sensitization of doctors and nurses, inconsistent supply of tablets, lack of clarity about side effects of centchroman among healthcare workers. Tablets were not readily available to postpartum women at home, which was also a contributing factor. There were the following changes implemented: sensitization of the healthcare workers, daily focused FP counselling of women in outpatient clinics and postnatal wards, and measures to ensure a continuous supply of tablets. At discharge, eligible postpartum women were given a three-month supply of tablets for ready access. ASHA (accredited social health activist) workers who are engaged in other governmental health initiatives were included in the loop to maintain contact and supply tablets to women initiated on centchroman.Results: Contraceptive utilization increased from 0.5-0.7% at beginning of the study to 29% by incorporating the solutions. Failure of contraception occurred in one user. Pearl index for centchroman in our study was 0.22%.Conclusions: Simple, practical solutions identified through quality improvement initiatives resulted in improved coverage and utilization of contraceptive. This study can be an important example for problem solving of challenges in low resource settings.
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