Azobenzene‐bridged diradical janus nucleobases with photo‐converted magnetic properties between antiferromagnetic and ferromagnetic couplings

We computationally design a series of azobenzene (AB)‐bridged double radicalized nucleobases, a novel kind of diradical Janus‐type nucleobases, and explore their spin coupling characteristics. Calculations prove that such diradical Janus‐bases not only normally match with their complementary bases, but also exhibit well‐defined diradical character with photo‐convertible intramolecular magnetic couplings (antiferromagnetic vs. ferromagnetic). Combination of four radical nucleobases (rG, rA, rC, rT) and photoswitch AB can yield 10 diradical Janus‐bases with different magnetic characteristics in which AB functions a bridge to mediate the spin coupling between two radical bases. The trans‐form supports mild antiferromagnetic couplings with the spin coupling constants (J) ranging from −153.6 cm⁻¹ to −50.91 cm⁻¹ while the cis‐form has weak magnetic couplings with ferromagnetic (0.22–8.50 cm⁻¹) for most of them or antiferromagnetic (−0.77, −1.73, −3.30 cm⁻¹) properties for only three. Further structural examination and frontier molecular orbital analyses indicate that the extended π conjugation for better spin polarization provides an effective through‐π‐bond pathway to mediate the spin coupling in the trans conformation while nonplanarity of the cis conformation weakens the through‐bond coupling and causes a competitive through‐space pathway and as an overall result inhibits the spin coupling between two spin moieties. Meanwhile, we also find that the J values of the cis conformation vary with their angle between the radical base and its linked phenylene. Furthermore, the magnetic properties of the diradical Janus‐bases can be significantly increased by interacting with metal ions. They also maintain a good UV absorption characteristics and there is a clear redshift compared with AB. This work provides a promising strategy for the rational design of photo‐convertible Janus‐base magnets as the magnetism‐tunable DNA building blocks. © 2018 Wiley Periodicals, Inc.     

The Design of a Highly Selective Fluorescent Chemosensor for Cu(II) within Wide pH Region and a Molecular Switch Controlled by pH

A new calix[4]arene-based fluorescent compound with two thio-ether groups as ionophore and one 3-alkoxy-2-naphthoic acid moiety as fluorophore has been designed, which exhibited highly selective binding of Cu ²⁺ over alkali, alkaline earth and some transition metal ions, including Co ²⁺, Ni ²⁺, Zn ²⁺, Mn ²⁺, Cd ²⁺, Pb ²⁺, Hg ²⁺, Ag ⁺, Cu ⁺, in CH ₃OH–H ₂O (2:1) within wide pH region. Moreover, the change of pH induces the consecutive quenching/revival of the fluorescence, with a concomitant distinct difference of the fluorescence quantum yield. In consequence, this system could be considered as a molecular switch.     

Synthesis of Hymecromone Derivatives Containing Chiral 1,1''''-Bi-2-naphthyl Moiety for Dual-mode Molecular Switch

Some hymecromone derivatives containing chiral 1,1'-bi-2-naphthyl moiety were synthesized and their photodimerizations were investigated. It was found that fluorescence intensity and optical rotation of the new chiral hymecromone derivatives could be regulated by light. This property has potential significance for developing a new type of dual-mode molecular switch.     

Properties of molecular switch triad compounds for photoinduced intramolecular energy transfer

This paper reports the synthesis and transient absorption decay kinetics of several triad compounds, in which a sulfur aryl section is used as the spacer for intramolecular energy transfer (IET). After flash photolysis the producing sulfur radicals will provide an ‘energy trap’ to stop the IET process, after stopping flash photolysis the sulfur radicals reversibly recombine and the IET process recovers, and then a rapid photoinduced IET switchable function is realized.     

[Ru(bpy)2tpphz]2+与Zn2+形成的双核配合物的荧光性质研究

The formation of heterobimetallic ruthenium(Ⅱ) complex was investigated by absorption and emission spectra. As an intercalator of DNA， the luminescent monometallic ruthenium(Ⅱ) complex [Ru(bpy)₂tpphz]²⁺ could coor-dinate with Zn²⁺ to form the nonluminescent heterobimetallic complex [Ru(bpy)₂(tpphz)Zn]⁴⁺. The emission in-tensity of complex decreased as increasing the amounts of Zn²⁺ and the luminescence was almost lost at the ratio of [Zn]/[Ru] of 1. After binding to DNA， the peripheral coordination site on the tpphz ligand remained accessible for Zn²⁺， the coordination occurred from the oppsite side of helix with respect to intercalated [Ru(bpy)₂tpphz]²⁺ and the nonluminescent heterobimetallic complex was formed. On the other hand， the [Ru(bpy)₂(tpphz)Zn]⁴⁺ also bound to DNA by intercalation and situated the region of the intercalated [Ru(bpy)₂tpphz]²⁺ between the base pairs of DNA. The complex looked like a molecular nut (the Zn²⁺) and bolt (the [Ru(bpy)₂tpphz]²⁺).     

Fluorescence Switch Based on a Porphyrin-perylene Dyad

Upon excitation of porphyrin or perylene subunit, the porphyrin-perylene dyad exhibits reversible fluorescence emitting and quenching by formation and neutralization of the protonated porphyrin subunit, thereby providing a prototype of molecular fluorescence switch working in an expanded spectral range.     

Selection of fluorescent aptamer beacons that light up in the presence of zinc

In order to generate nucleic acid biosensors that could undergo a reversible conformation change in the presence of the metal zinc, a random sequence pool of single-stranded DNA was immobilized on an oligonucleotide affinity column. In the presence of zinc, those species that underwent a conformational change were released from the column, collected, and amplified. A series of negative and positive selections refined the metal specificity of the selected aptamer beacons. Since the aptamer beacons contained a fluorophore, while the bound oligonucleotide contained a quencher, zinc binding also resulted in an increase in fluorescence. One of the selected beacons, Zn-6m2, bound zinc in the low micromolar range, gave a dose-dependent fluorescence signal, and showed an approximately sixfold increase in fluorescence on zinc binding. While some cross-reactivity with cadmium was observed, it should nonetheless prove possible to use the novel selection method to generate and tune the specificity of a variety of reversible metal biosensors. Such biosensors could potentially be used for continuous monitoring of metals in environmental samples.     

Graph classes characterized both by forbidden subgraphs and degree sequences

Given a set of graphs, a graph G is ‐free if G does not contain any member of as an induced subgraph. We say that is a degree‐sequence‐forcing set if, for each graph G in the class of ‐free graphs, every realization of the degree sequence of G is also in . We give a complete characterization of the degree‐sequence‐forcing sets when has cardinality at most two. © 2007 Wiley Periodicals, Inc. J Graph Theory 57: 131–148, 2008     

Entropy-Induced Selectivity Switch in Gold Catalysis: Fast Access to Indolo[1,2-a]quinolines

New N-heterocyclic compounds for organic functional materials and their efficient syntheses are highly demanded. A surprising entropy-induced selectivity switch in the gold-catalyzed intramolecular hydroarylation of 2-ethynyl N-aryl indoles was found and its exploitation led to straightforward syntheses of indolo[1,2-a]quinolines. Experimental and computational mechanistic investigations gave insight into this uncommon selectivity phenomenon and into the special reactivity of the indolo[1,2-a]quinolines. The high functional group tolerance of this methodology enabled access to a diverse scope with high yields. In addition, bidirectional approaches, post-functionalization reactions, and π-extension of the core structure were feasible. An in-depth study of the photophysical properties explored the structure-effect relationship for different derivatives and revealed a high potential of these compounds for future applications as functional materials.     

Synthesis and characterization of 2H-pyrano[3,2-c]coumarin derivatives and their photochromic and redox properties

A series of 2H-pyrano[3,2-c]chromen-5-one derivatives were synthesized and characterized. Their photochromic and redox properties were investigated by the UV-vis absorption spectroscopy. While compounds with one or two phenyl groups incorporated at the 2-position were present in both ring-opened (5a and 10a) and ring-closed (6a and 11a) forms, the incorporation of an N,N-dimethylamino group on either side of the aromatic ring resulted in formation of the ring-opened (5b and 10b) forms only. The ring-closed forms 13 and 18 with a methyl substituent at the 3-position of the pyran moiety failed to exhibit photochromic behavior. Compound 23 with an N,N-dimethylamino group on the aromatic ring displayed increasing shoulder absorption in the visible region and a distinct change of color upon UV irradiation. The non-fluorescent 10b instantly changed from dark red to colorless, when treated with sodium borohydride. The reduced 28 was blue fluorescent with a quantum yield of 0.46 and could be returned to its original color via DDQ oxidation.