Solvent-induced Switching Research Articles

Solvent-induced switching between static and dynamic fluorescence quenching of N, S Co-doped carbon dots in sensing of Crotonaldehyde: A detailed systematic study

Blue emissive N, S co-doped carbon dots (NSCDs) (quantum yield = 12%) have been synthesized via one-pot microwave-assisted pyrolysis of optimal mole ratios of tartaric acid and thioacetamide and they are devised as a novel fluorescent probe for assessing the sensitive and selective detection of Crotonaldehyde (CH3CH=CHCHO, CT) in aqueous and ethanol solution. In the presence of CT, the fluorescence of NSCDs was observed to switch between static and dynamic quenching in the two solvents – water and ethanol, although both exhibited a positive deviation from the linearity in the Stern-Volmer (SV) plots. The positive deviation in the SV plots in the aqueous medium has been attributed to a combination of static and dynamic quenching mechanisms, while the deviation in the ethanol medium followed the static quenching model. The LOD values in water and ethanol medium are estimated to be 50 μM and 30.8 μM in the CT concentration range of 0–6 mM and 0–8 mM respectively. The fluorescence quenching mechanism in the two solvents have been differentiated and validated through fluorescence lifetime measurement using time-resolved fluorescence spectroscopy, shifting in absorption maxima using UV–Vis spectroscopy and energy level calculations. The energy level calculations of NSCDs have been carried out using cyclic voltammetry (CV) measurement and that of CT from density functional theory (DFT) calculations.

Dynamic rotaxane-branched dendrimers with precisely arranged luminogens for efficient light harvesting

Although the development of artificial light-harvesting systems (LHSs) has attracted increasing attention, the construction of novel artificial LHSs with co-existed various types of precisely arranged luminogens has been still a major challenge. Herein, starting from [2]rotaxane building blocks AN-R or TPE-R functionalized with either anthracene or tetraphenylethene (TPE) moiety, a family of novel rotaxane-branched dendrimers containing two types of energy donors on the branches and one pyrene moiety at the central core as energy acceptor have been successfully constructed through a facile, efficient, and controllable divergent approach. Notably, attributed to the solvent-induced switching feature of the rotaxane branches, the integrated rotaxane-branched dendrimers displayed an interesting dynamic feature. More importantly, as revealed by fluorescence spectra, the resultant rotaxane-branched dendrimers displayed enhanced antenna effects along with the increase of dendrimer generation. Therefore, these rotaxane-branched dendrimers described herein could serve as not only promising platforms for light harvesting but also excellent candidates for the construction of smart supramolecular materials for practical applications.

Benzoindoxazine derivatives containing carbazole for detection of CN− and its application in plant seed extracts and cell imaging

Cyanide (CN−) is a highly toxic compound that exists in many substances and is harmful to the environment and human health. Therefore, it is of great significance to develop excellent CN− ion probes, especially solvent-induced on–off fluorescent probes. Based on the condensation reaction of indolo[2,1-b][1,3]oxazine molecules with aldehydes, probes (E)-13a-(2-(9-ethyl-9H-carbazol-3-yl)vinyl)-14,14-dimethyl-10-nitro-13a,14-dihydro-8H-benzo[e]benzo[5,6][1,3]oxazino[3,2-a]indole (NCO) and (E)-13a-(2-(9-benzyl-9H-carbazol-3-yl)vinyl)-14,14-dimethyl-10-nitro-13a,14-dihydro-8H-benzo[e]benzo[5,6][1,3]oxazino[3,2-a]indole (NBO) were synthesized to detect CN−. Compared with other cyanogen ion probes, NCO and NBO have special carbazole ring structures and large conjugate systems. When CN− is added to the probe-detection solution, color changes that are visible to the naked eye can occur. The UV–vis spectrum test using differential spectroscopy shows that the probe (i) has excellent solvent-induced switching characteristics and stability (CH3OH–H2O) and (ii) high selectivity, anti-interference ability, and sensitivity for the detection of CN−. The fluorescence limit of detections (LODs) are 1.05 µM for NCO and 1.34 µM for NBO. The UV LODs are 0.83 µM for NCO and 0.87 µM for NBO. Fluorescence spectroscopy shows that the probe has remarkable fluorescence properties. Fluorescence titration experiments, liver cancer cell (Hep G2) imaging, and cytotoxicity experiments all show that the probes have high biocompatibility, low toxicity, high cell permeability, and high sensitivity for the detection of CN− in cells. In addition, NCO and NBO have been successfully used for the detection of cyanogenic glycosides in the seeds of ginkgo, crabapple, apple, and cherry. Test strips were fabricated to detect CN−. After adding CN−, the color of the test strip changed significantly—from brown to light yellow; thus, the test strips have a high application value in the fields of drug quality control, drug safety testing, and pharmacological research.

Artificial Light-Harvesting Systems Based on AIEgen-branched Rotaxane Dendrimers for Efficient Photocatalysis.

Aiming at the construction of novel platform for efficient light harvesting, the precise synthesis of a new family of AIEgen-branched rotaxane dendrimers was successful realized from an AIEgen-functionalized [2]rotaxane through a controllable divergent approach. In the resultant AIE macromolecules, up to twenty-one AIEgens located at the tails of each branches, thus making them the first successful example of AIEgen-branched dendrimers. Attributed to the solvent-induced switching feature of the rotaxane branches, the integrated rotaxane dendrimers displayed interesting dynamic feature upon the aggregation-induced emission (AIE) process. Moreover, novel artificial light-harvesting systems were further constructed based on these AIEgen-branched rotaxane dendrimers, which revealed impressive generation-dependent photocatalytic performances for both photooxidation reaction and aerobic cross-dehydrogenative coupling (CDC) reaction.

A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from Tc = 26 to 80 K.

The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) with Fe(II) affords the solid (Me2NH2)2[Fe2L3]·2H2O·6DMF. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of Fe(III) centers with mixed-valence ligands of the form (L3)(8-) that result from a spontaneous electron transfer from Fe(II) to L(2-). Upon removal of DMF and H2O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me2NH2)2[Fe2L3], and a fit to N2 adsorption data of this activated compound gives a BET surface area of 885(105) m(2)/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally characterized extended solid that contains tetraoxolene radical ligands.

Dinuclear metal complexes composed of peptide chains: Solvent-induced switching and inversion of the metal-centered chirality

The peptide-linked bisbipyridine separated by a rigid or flexible spacer formed a dinuclear FeII complex, which showed a unique solvent-induced inversion and “on–off” switching of the metal-centered chirality depending on the spacer, resulting from the supramolecular structural change between a dinuclear helicate and a mono peptide ligand-bridged dinuclear FeII complex. The rigid and flexible spacer groups connecting the peptide residues are responsible for these unique structural and chiroptical properties occurring at the metal center.

Solvent-induced switching between two supramolecular assemblies of a guanosine–terthiophene conjugate

We report here our findings on a lipophilic guanosine derivative armed with a terthiophene unit that undergoes a pronounced variation of its supramolecular organisation by changing the polarity of the solvent. In chloroform the guanosine derivative, templated by alkali metal ions, assembles via H-bonding in G-quartet based D(4)-symmetric octamers; the polar guanine bases are located into the inner part of the assembly and act as a scaffold for the terthienyl pendants. On the other hand, in the more polar (and H-bond competing) acetonitrile, different aggregates are observed in which the terthiophene chains are pi-pi stacked in a helicoidal (left-handed) arrangement in the central core, and the guanine bases (free from hydrogen bonding) are located at the periphery and exposed to the solvent. The system can be switched back and forth by subsequent addition of chloroform and acetonitrile. The solvent-induced switching can be easily followed by circular dichroism spectroscopy: the CD exciton-couplet in the guanine chromophore absorption region observed in chloroform disappears after addition of acetonitrile, indicating the disassembly of the G-quartet based octameric structure, while an intense quasi-conservative exciton splitting in the 300-450 nm spectral region becomes predominant in the CD spectrum. This latter strong bisignate optical activity can be ascribed to the helical packing of conjugated terthiophene moieties stabilised by pi-pi interactions. NMR spectra and photophysical investigations confirm the structures of the guanine-directed and thiophene-directed assemblies in chloroform and acetonitrile, respectively.

Unusual solvatochromism of the 4,4′‐bis(dimethylamino)benzophenone (Michler's ketone)–tetracyanoethene electron donor–acceptor complex

The transition energy of the charge-transfer UV/Vis absorption maxima (νmax, CT) and the structure of the Michler's ketone (MK)–tetracyanoethene (TCNE) electron donor–acceptor (EDA) complex are remarkably solvent dependent. The UV/Vis spectra of the MK–TCNE complex were measured in 20 non-protic and seven protic solvents. In non-protic solvents, νmax, CT of the EDA complex is quantitatively described by a multiple LSE relationship using the Kamlet–Taft dipolarity/polarizability (π*) and basicity (β) parameters of the solvents. The influence of the two terms β and π* on νmax, CT is opposite, indicating a qualitatively different solvent-induced stabilization of the electronic ground and excited states of the complex. As expected, increasing dipolarity/dipolarizability of the solvent causes a bathochromic band shift (positive solvatochromism), whereas the basicity of solvents is responsible for a hypsochromic band shift due to specific solvation of the TCNE site. In protic solvents, the complex formation is associated with the formation of an ionic species (νmax = 19 800 cm−1) derived from Michler's ketone due to coordination of TCNE at the carbonyl oxygen of MK (called an n-complex). The solvent-induced switching of the π-complex into the n-complex is demonstrated for mixtures of DCE with protic solvents and for silica surfaces. Copyright © 1999 John Wiley & Sons, Ltd.

Unusual solvatochromism of the 4,4′-bis(dimethylamino)benzophenone (Michler's ketone)-tetracyanoethene electron donor-acceptor complex

The transition energy of the charge-transfer UV/Vis absorption maxima (νmax, CT) and the structure of the Michler's ketone (MK)–tetracyanoethene (TCNE) electron donor–acceptor (EDA) complex are remarkably solvent dependent. The UV/Vis spectra of the MK–TCNE complex were measured in 20 non-protic and seven protic solvents. In non-protic solvents, νmax, CT of the EDA complex is quantitatively described by a multiple LSE relationship using the Kamlet–Taft dipolarity/polarizability (π*) and basicity (β) parameters of the solvents. The influence of the two terms β and π* on νmax, CT is opposite, indicating a qualitatively different solvent-induced stabilization of the electronic ground and excited states of the complex. As expected, increasing dipolarity/dipolarizability of the solvent causes a bathochromic band shift (positive solvatochromism), whereas the basicity of solvents is responsible for a hypsochromic band shift due to specific solvation of the TCNE site. In protic solvents, the complex formation is associated with the formation of an ionic species (νmax = 19 800 cm−1) derived from Michler's ketone due to coordination of TCNE at the carbonyl oxygen of MK (called an n-complex). The solvent-induced switching of the π-complex into the n-complex is demonstrated for mixtures of DCE with protic solvents and for silica surfaces. Copyright © 1999 John Wiley & Sons, Ltd.