Mingdi Yang

Aggregation-Induced Fluorescence Behavior of Triphenylamine-Based Schiff Bases: The Combined Effect of Multiple Forces

Eight triphenylamine (TPA)-based Schiff bases that exhibit different aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) behavior in tetrahydrofuran (THF)/water mixtures have been synthesized and characterized. The photophysical properties in solution, aqueous suspension, film, and the crystalline state along with their relationships were comparatively investigated. The single-crystal structures of 1-8 indicate that compact π···π stacking or excimers induce fluorescence quenching of 1, 2, 5, and 7. However, the existence of J aggregates or multiple intra- and intermolecular interactions restrict the intramolecular vibration and rotation, enabling compounds 3, 4, 6, and 8 to exhibit good AIE character. The size and growth process of particles with different water fractions were studied using scanning electron microscopy, which demonstrated that smaller uniformly dispersed nanoparticles in the THF/water mixtures favor fluorescence emission. The above results suggest that the combined effects of multiple forces caused by structural variation have a great influence on their molecular packing, electronic structure, and aggregation-induced fluorescence properties. In addition, piezofluorochromic experiments verified the potential applications of 4 and 6.

Substituent group variations directing the molecular packing, electronic structure, and aggregation-induced emission property of isophorone derivatives.

A series of new isophorone derivatives (1-5), incorporating the heterocyclic ring or aza-crown-ether group, with large Stokes shifts (>140 nm), have been synthesized and characterized. 1-4 display aggregation-induced emission behaviors, while dye 5 is highly emissive in solution but quenched in the solid state. It was found that the tuning of emission color of the isophorone-based compounds in the solid state could be conveniently accomplished by changing the terminal substituent group. The photophysical properties in solution, aqueous suspension, and crystalline state, along with their relationships, are comparatively investigated. Crystallographic data of 1-4 indicate that the existence of multiple intermolecular hydrogen bonding interactions between the adjacent molecules restricts the intramolecular vibration and rotation and enables compounds 1-4 to emit intensely in the solid state. The size and growth processes of particles with different water fractions were studied using a scanning electron microscope, indicating that smaller globular nanoparticles in aqueous suspension are in favor of fluorescence emissions. The above results suggest that substituent groups have a great influence on their molecular packing, electronic structure, and aggregation-induced emission properties. In addition, fluorescence cell imaging experiment proved the potential application of 5.

Four new two-photon absorbing imidazo[4,5-f]1,10-phenanthroline dye derivatives with different dipole moment orientation based on different groups: synthesis, optical characterization and bioimaging

A new series of D–π–A type two-photon absorption (2PA) dyes (1–4) based on imidazo[4,5-f]1,10-phenanthroline have been synthesized and their photophysical properties have been systematically investigated. These dyes showed obvious positive solvatochromic effects mainly due to intramolecular charge transfer (ICT). The two-photon absorption (2PA) cross-section values measured by two-photon excited fluorescence (TPEF) were determined to be 1232 GM and 4752 GM for dye 3 and dye 4, while dyes 1 and 2 do not display 2PA activity. These results demonstrate that their 2PA cross section values (σ) increase with increasing donor/acceptor (D/A) strength of the corresponding molecule. The results also imply that the replacement of the tetrazol group with the aza-crown-ether group results in an opposite charge transfer direction, which has been verified by density functional theory (DFT) calculations. In addition, a two-photon fluorescence cell imaging experiment proved the suitability of dye 3 for this potential application .

Difunctional chemosensor for Cu(II) and Zn(II) based on Schiff base modified anthryl derivative with aggregation-induced emission enhancement and piezochromic characteristics

Three new anthryl Schiff base derivatives containing a similar molecular structure were synthesized through a simple method and their fluorescent properties were investigated in detail. Among these, compound 1 displayed an aggregation-induced emission (AIE) feature, 2 exhibited an aggregation-induced emission enhancement (AIEE) property, while 3 showed aggregation-caused quenching (ACQ) behavior. Single-crystal structure and theoretical calculation analysis show that the larger conjugation and the existence of multiple intra- and intermolecular interactions restricts the intramolecular vibration and rotation, which benefit the emission in the condensed state, while the tight dimer structure and intramolecular torsional motion induce fluorescence quenching. Moreover, compound 2 can be utilized as fluorescence on–off type sensor for Cu2+ in methanol–H2O (4/1, v/v, pH 7.2) HEPES buffer solution, as well as fluorescence off–on type sensor for Zn2+ in pure methanol solution. The 2 : 1 ligand-to-metal coordination pattern of the 2-Cu2+ and 2-Zn2+ were calculated through a Jobs plot, and were further confirmed by X-ray crystal structures of complexes 2-CuBr2 and 2-ZnCl2. In addition, 2 also exhibits a piezofluorochromic characteristic.

Schiff base derivatives containing heterocycles with aggregation-induced emission and recognition ability

Three new Schiff base derivatives possessing vagarious blue aggregation-induced emission (AIE) characteristics in tetrahydrofuran (THF)–water were synthesized. Their photophysical properties in solution, aqueous suspension, film and crystalline states, along with their relationships, were comparatively investigated. The crystallographic data for dye 1 and dye 3 indicated that the existence of multiple intermolecular hydrogen bonding interactions restricted intramolecular vibration and rotation. Moreover, the sizes and growth processes of particles with different water fractions were studied using a scanning electron microscope (SEM) and dynamic light scattering (DLS). The results showed that the small homogeneous particles, assembled in an ordered fashion with appropriate water contents, exhibited distinct AIE behavior. Moreover, in a THF–H2O (4 : 1, v/v) solution of HEPES (20 mM) buffer, dye 2 showed fluorescence turn-on sensing towards Cr3+ and Al3+via chelation enhanced fluorescence (CHEF). The 2 : 1 stoichiometries of the sensor complexes (dye 2–Cr3+ and dye 2–Al3+) were calculated from Job plots based on fluorescence titrations.

Aggregation-induced and crystallization-enhanced emissions with time-dependence of a new Schiff-base family based on benzimidazole

A new Schiff-base family containing [4-(1H-benzimidazole-2-yl)-phenyl]-bis-(4-ethoxy-phenyl)-amine has been synthesized through a condensation reaction. All the derivatives possess properties of aggregation-induced and crystallization-enhanced emission (AIE and CEE), which show time-dependent characteristics at a concentration of 10 μM and are studied in detail by scanning electron microscope (SEM) and transmission electron microscope (TEM). Different aggregation forms and the growth of crystals of the compounds, could be responsible for the notably different degrees of fluorescence enhancement.

Photon-induced intramolecular charge transfer with the influence of D/A group and mode: optical physical properties and bio-imaging

Five triphenylamine containing benzimidazole derivatives (a1–a5) with different D/A groups (D and A stand for donor and acceptor, respectively), conjugation length or configuration were designed and synthesized. The one- and two-photon spectroscopic properties of these molecules were studied. All of these compounds have amazing fluorescence quantum yields in the range of 0.39–0.61. Besides, for a1–a5, the λem (emission maximum wavelength of one- and two-photons) are only dominated by the D/A groups, and the conjugation length is irrelevant, implying that the molecular rotation is in the excited-state. To better explain this phenomenon, the structure–property relationships of a1–a5, especially the molecular rotation in the excited-state, have been discussed in detail, and were identified by density functional theory calculations. Furthermore, four azo-heterocycle containing benzimidazole derivatives, which have weak electron-donating ability, were synthesized and discussed as a comparison. Finally, considering their optical action, cytotoxicity and solubility, a1, a2 and a4 were chosen for bio-imaging. The results show that the small molecule a2 is the best candidate for one-photon excited fluorescence and two-photon excited fluorescence imaging.

New dyes with enhanced two-photon absorption cross-sections based on the Cd(II) and 4′-(4-[4-(imidazole)styryl]phenyl)-2,2′ : 6′,2″-terpyridine

A series of new cadmium complexes (dyes 1–5) were obtained by assembly of the donor–bridge–acceptor (D–π–A)-type ligand (L: 4′-(4-[4-(imidazole)styryl]phenyl) 2,2′ : 6′,2″-terpyridine) with CdX2 (X=Cl, Br, I, NCS, ClO4), in which dye 3 was characterized by single crystal X-ray diffraction. Various weak interactions, including hydrogen bonds (C–H⋯N, C–H⋯X) and π–π interactions, played significant roles in the final topological structures. Linear and non-linear optical properties of ligand and the complexes were investigated. The two-photon absorption cross-section of these complexes are larger than that of ligand with maximum values of 577 GM (dye 1), 242 GM (dye 2), 171 GM (dye 3), 326 GM (dye 4), and 215 GM (dye 5) in N,N-dimethylformamide.

Syntheses, structures, and luminescent properties of zinc(II) complexes based on imidazole derivative

Two new Zn(II) coordination complexes [ZnL2(SCN)2] (I) and [ZnL2Cl2] (II) have been pre-pared by self-assembly of corresponding metal salts with (E)-2-(3-(4-(1H-imidazole-1-yl)styryl)-5,5-dimethylcyclohexo-2-enylidene) malononitrile (L) and characterized by IR spectrum, elemental analysis and single-crystal X-ray diffraction. Complexes I and II are two-dimensional (2D) networks with different topology structures. The luminescence properties were investigated.

Assembly, crystal structures, and luminescent properties of three new thiocyanate-bridging mercury(II) coordination polymers

Two ligands, 2-{5,5-dimethyl-3-[2-(pyridin-3-yl)-ethenyl]cyclohex-2-enylidene}propanedinitrile (L1) and 2-{5,5-dimethyl-3-[2-(pyridin-2-yl)-ethenyl]cyclohex-2-enylidene}propanedinitrile (L2), were synthesized. By reaction of mercury thiocyanate with L1 and L2, respectively, coordination polymers [Hg(L1)(μ1,3-SCN)2]n (1), [Hg(L1)2(μ1,3-SCN)2]n (2), and [Hg(L2)(μ1,3-SCN)(SCN)]n (3) with different structures and topologies were obtained. In 1, the thiocyanate shows μ1,3-SCN bridging coordination, and adjacent Hg(II) ions are bridged by two μ1,3-SCN ions to form an infinite chain with the remaining position of five-coordinate Hg(II) occupied by L1. In 2, the thiocyanate has the same coordination as 1. However, Hg(II) has octahedral coordination with two L1 involved in coordination. An unusual feature of 3 is the presence of two types of thiocyanates, one has a S-terminal ligand and the other has a μ1,3-SCN bridge. The mercury(II) in 3 is four-coordinated by L2 and three thiocyanates. Luminescent properties and thermal stabilities of 1–3 were studied.