•Four norcyanine dyes with benzo[c,d]indolium moiety were synthesized.•Dyes' λmax and λem were in the region 488–618 nm and 583–651 nm, respectively.•Dye D1 had high fluorescent spectral sensitivity at pH 5.0–8.0, with a pKa of 5.0.•Four dyes exhibited high photostability and no or mild cytotoxicity.•Dye D1 achieved pH-dependent fluorescence imaging in living cells at pH 3.0–8.0.Fluorescence pH imaging in living cells is a rapidly expanding research direction, however, it relies on the development of pH-sensitive fluorescent imaging agents. Here four norcyanine dyes with benzo[c,d]indolium moiety, exhibiting high spectral sensitivity with pH changes, were synthesized for fluorescence pH imaging in living cells, and characterized by 1H NMR, 13C NMR, IR, UV–Vis and HRMS. The investigation of their spectral properties in methanol and water showed that the absorption and emission maxima were in the region 488–618 nm and 583–651 nm, respectively, and four dyes exhibited high photostability. The pH spectral titrations showed that selective dye D1 had pH-dependent absorption spectral changes within the pH range of 2.4 to 9.4, and high fluorescent spectral sensitivity at pH 5.0–8.0, with a pKa of 5.0. A cell association study indicated that dye D1 exhibited no or mild cytotoxicity at the application dose and duration, and could be accumulated in cells and mainly distributed in the cytoplasm, giving red fluorescence imaging. In particular, dye D1 could achieve pH-dependent fluorescence imaging in living cells with the increase of pH from 3.0 to 8.0, at excitation wavelength of 543 nm and receiving wavelength of 655–755 nm, which was valuable for studying the weak acidic, neutral and weak alkaline biological tissue compartments.

Photodynamic therapy (PDT) represents a modern and noninvasive therapeutic approach, however, it relies on the development of photosensitizers. Here five new benzo[c,d]indole rhodamine complex merocyanines (BIRCM) D1-D5, displaying low dark toxicity and significant photo toxicity, were synthesized as PDT photosensitizers, and characterized by 1H NMR, IR, UV–Vis and HRMS. The investigation of their absorption spectra in different solvents showed that the absorption maxima and molar extinction coefficient were in the region 507–679 nm and 0.21 × 104–1.27 × 105 L · mol−1cm−1, respectively. The evaluation of PDT activity showed that only irradiation could not kill SMMC-7721 cells, and the cell survival rate and inhibition rate at the application dose and duration was 92%–87% and 78%–49%, respectively. Especially, using D2, absorbed in the red zone, as photosensitizer for PDT analyzed its effect on SMMC-7721 cells survival, it could be found that the cell survival rate was 92% without irradiating and the cell inhibited rate was 78% under irradiating at concentrations of 2.5 × 10−6 mol/L, displaying low dark toxicity and high photo toxicity, which was valuable for PDT of some microvascular diseases or other superficial diseases.Download high-res image (478KB)Download full-size image

•A series of merocyanine dyes with excellent spectral properties were synthesized.•Dyes showed solvent-dependent changes on fluorescence spectra in different solvents.•Interaction of dye 7d with BSA showed ratiometric response to solvent/protein.•Dyes 7b–7h exhibited almost no cytotoxicity and stained live cell cytoplasm and mice.A series of merocyanine dyes with excellent spectral properties were synthesized through changing the heterocyclic base structure for protein labeling and imaging in biological science, such as living cells and living mice. Dyes 7c–7h showed solvent-dependent changes on emission wavelength and fluorescence intensity in water and hydrophobic solvents, which were further analyzed by theoretical calculations and absorption spectra. The interaction of dye 7d with bovine serum albumin (BSA) showed an obvious ratiometric fluorescence response to solvent/protein environment, with the potential to report protein conformational changes. Investigation of the cytotoxicity and bioimaging capability displayed that dyes 7b–7h showed low cytotoxicity, and could stain living cell cytoplasm and be used for imaging in living mice with bright fluorescence at the application dose, which was suggested as fluorescent reagents for imaging in biological science.A series of novel merocyanine dyes 7a–7h were prepared. The interaction of dye 7d with BSA showed an obvious ratiometric fluorescence response to solvent/protein environment. Cytotoxicity and bioimaging capability of the dyes 7b–7h were also investigated.

A convenient approach for the direct synthesis of meso-substituted thiazole orange (meso-TO) analogues has been unprecedentedly developed through the AlCl3-catalyzed reaction of parent TO with benzyl alcohol derivatives. Single-crystal X-ray structures show that the prepared new meso-TO analogues are nonplanar, forming a sharp contrast to planar TO. The spectral properties show that nonplanar meso-TO analogues do not aggregate, existing in monomer form (M) in PBS buffer, and have little effect of solvatochromism in different solvents. In comparison with the parents, meso-TO analogues exhibit a large Stokes shift, excellent light fastness, and inertness to singlet oxygen. A cellular association study demonstrates that incorporation of a benzyl group at the meso position methine of parent TO can decrease the cytotoxicity, change staining area in cells, and emit long-wavelength fluorescence for an extended time, which are useful for the development of smarter TOs for imaging in biological science.

Six new double N-alkylated dimethine cyanine dyes were synthesized and the crystal structures of two of the dyes were analyzed by X-ray diffraction. The investigation of the spectral properties of the dyes in different solvents indicated that the absorption maxima of the dyes decreased with the increase of the basicity of the heterocyclic nucleus, and the increase of the solvent dielectric constants of the protonic solvents and non-protonic solvents. The six dyes all emitted fluorescence, and had a larger Stokes shift in water. The interaction between the dye molecules and the six biological molecules showed that one dye exhibited a larger enhancement of fluorescent quantum yield in the presence of DNA. Investigation of the cytotoxicity and cell staining of the selected dye showed that the dye had virtually no toxicity at the application dose and duration used and that it could stain cytoplasm, suggesting its potential application as a fluorescent reagent with which to observe and analyze the characteristics of living cells.

Three kinds of new double D–π–A mono-cyanines D1, D2 and D3 were synthesized for the surface modification of monocrystalline silicon to prepare near infrared silicon-based materials. There was a strong surface photovoltage response in the range of 900–1250 nm compared with silicon materials when double D–π–A mono-cyanines were bound on the monocrystalline silicon material surface. To explore the effect of the double D–π–A mono-cyanines on the light absorption characteristics of silicon materials, the absorption properties, films on monocrystalline silicon surfaces and the surface photovoltage response of three double D–π–A mono-cyanines were investigated. The results demonstrated that the order of the intensity of the surface photovoltage response was Si/D3 > Si/D2 > Si/D1, and no absorption properties of the silicon materials in the near infrared region were improved, yielding near infrared silicon-based materials.

A kind of trinucleus dimethine cyanine dye: 1-methyl-2,6-bis[2-(furan-2-yl)vinyl]pyridinium iodide (1) was synthesized and characterized by 1H NMR, 13C NMR, IR, MS, UV–Vis spectroscopy and elemental analysis. The crystals of dye 1, obtained from slow evaporation of solvent acetone, crystallized in the triclinic space group P − 1 with a = 9.6501(16) Å, b = 10.2308(17) Å, c = 10.7341(17) Å, V = 887.2(3) Å3, and Z = 2 (at 298(2) K), and it was stabilized by the hydrogen bonds and intermolecular face-to-face π⋯π aromatic stacking interactions. Crystallographic, IR, 1H NMR and UV–Vis data of dye 1 were compared with the results of density functional theory (DFT) method, and the calculated molecular geometries, vibrational bands, 1H NMR chemical shifts and UV–Vis maximum absorption were consistent with the experimental results. The fluorescence spectra were predicted in four different solvents with CIS/PCM methods. Compared with experimental values, the absolute deviations of emission maxima were −17.4 nm in chloroform, 6.3 nm in DMSO, 4.9 nm in methanol, and 6.8 nm in water, respectively. And the experimental fluorescence spectra were nicely reproduced by the simulated fluorescence spectra for each solvent.Highlights► A kind of trinucleus dimethine cyanine dye 1 was synthesized and characterized. ► The structure of dye 1 was experimentally and theoretically studied. ► The electronic emission spectra were predicted in four different solvents with CIS/PCM. ► The simulated fluorescence spectra duplicated the experimental one for each solvent.

Two merocyanine dyes, each with an isoxazolone nucleus, were synthesized and characterized by NMR, IR, UV–Vis spectroscopy, elemental analyses and single X-ray diffraction. Crystallographic data revealed that one dye crystallized in an orthorhombic system, Cmca space group, while the other dye crystallized in a monoclinic system, P21/n space group. In the case of both dyes, the molecules adopted the most stable ketonic tautomeric form, and the crystal packing was stabilized by hydrogen bonds and π–π stacking interactions. The absorption spectra of the dyes in various solvents of different polarities were studied at room temperature. The equilibrium geometries, vibrational data and electronic spectra of the compounds were also investigated by the B3LYP method. Importantly, the predicted results fit to the experimental data, showing that this quantum chemical technology is an efficient approach to predict the properties of merocyanine dyes.Highlights► Two merocyanine dyes with an isoxazolone nucleus were synthesized and characterized. ► The molecules adopted the most stable ketonic tautomeric forms. ► Dyes crystallized in orthorhombic Cmca and monoclinic P21/n space groups. ► Crystal packing was stabilized by hydrogen bonds and π–π stacking interactions. ► The equilibrium geometries, vibrational and absorption spectra were investigated.

Two rhodanine complex merocyanine dyes 9a and 9b were synthesized and their structures were confirmed by 1H NMR, IR, MS, HRMS and UV–Vis spectra. From the spectral properties of the two dyes, it could be found that the λmax of the dyes showed hypsochromic shifts with the increase of permittivity in protonic solvents, and bathochromic shifts with the increase of refractive index in non protonic solvents. The interactions of two dyes with DNA or BSA were also studied under physiological conditions. The results showed that the quantum yield of DNA-dye 9a was up to 29.5 times compared with free dye 9a. Dyes 9a and 9b were researched in Photodynamic Therapy (PDT) as well. It was demonstrated that supplementation of dye 9a or 9b as photosensitizers for PDT in K562 cells decreases the survival rate.Highlights► The two rhodanine complex merocyanine dyes were synthesized. ► The spectral properties of two dyes were investigated in different solvents. ► The interaction of two dyes and DNA or BSA was studied. ► Using the dyes as photosensitizers for PDT could decrease K562 cells’ survival rate.

1,3-Dimethyl-2-[4-chloro-styryl]-benzimidazolium iodide (1) was synthesized and characterized by X-ray diffraction, 1H NMR, MS, IR, UV–vis spectra and elemental analysis. The crystals are monoclinic, space group P21/c, with a = 12.507(3) Å, b = 7.3259(19) Å, c = 36.705(9) Å, V = 3358.9(15) Å3, and Z = 4 (at 296(2) K). Crystal stacking scheme indicates the face-to-face π⋯π aromatic stacking interactions. Molecular geometries, frequencies, IR, 1H NMR and UV–vis were calculated at DFT/TD-DFT level using two hybrid exchange–correlation functionals, B3LYP and PBE1PBE. The stability of the molecule arising from hyperconjugative interaction and charge delocalization had been analyzed using natural bond orbital (NBO) analysis. These calculations on (1) provide deep insight into its electronic structure and properties.Highlights► A styrylcyanine dye with benzimidazole nucleus was synthesized. ► The dye crystallizes in the P21/c space group with four molecules per unit cell. ► The hydrogen bonds and π⋯π aromatic stacking interactions exist in the crystal. ► The IR, 1H NMR and UV–vis obtained by DFT/TD-DFT are in good agreement with the experimental values. ► NBO analysis reveals the charge transfer within the molecule.

A merocyanine dye with isoxazolone nucleus: 4-[(5-methoxy-1H-indole-3-yl)-methylene]-3-methyl-isoxazole-5-one (1) was synthesized and characterized by X-ray diffraction, 1H NMR, 13C NMR, IR, UV–Vis spectra and elemental analysis. The crystals are monoclinic, space group P21/n, with a = 6.4195(16), b = 7.5759(19), c = 25.181(6) Å, V = 1217.9(5) Å3, and Z = 4 (at 298(2) K). Crystal stacking scheme indicates the hydrogen bonds and intermolecular face-to-face π⋯π aromatic stacking interactions generate a unique parallelogram-void supramolecular architecture in solid state of the dye (1). By DFT calculations, molecular electrostatic potential clearly shows that the formation of hydrogen-bonding interaction [N(2)H(2)⋯O(2) and C(1)H(1A)⋯N(1)] in the crystal structure is between the positive and negative regions. In addition, molecular geometry, tautomeric forms, 1H NMR and 13C NMR were calculated by DFT/GIAO (the gauge invariant atomic orbital) methods are in good agreement with the experimental values. The correlation value between experimental 13C chemical shifts and the calculated magnetic isotropic shielding tensors is 0.9989, and the absolute deviation is 0.2–4.4 based on B3LYP/6-31G*-PCM (the polarizable continuum model) level.Highlights► A merocyanine dye with isoxazolone nucleus was synthesized. ► The dye crystallizes in the P21/n space group with four molecules per unit cell. ► The hydrogen bonds and π⋯⋯π aromatic stacking interactions exist in the crystal. ► MEP shows the hydrogen bonds are formed between the positive and negative regions. ► NMR spectra obtained by DFT/GIAO are in good agreement with the experimental values.

2-Styryl-β-naphthathiazole (1) was synthesized and characterized by X-ray diffraction, 1H NMR, MS, IR, UV–vis spectra. The crystals are orthorhombic, space group P212121, with a = 6.338(3), b = 9.242(5), c = 25.128(12) Å, V = 1471.9(12) Å3, and Z = 4 (at 298(2) K). Crystal stacking scheme indicates the edge-to-face π⋯π aromatic stacking interactions; Molecular geometries, frequencies, IR, 1H NMR and UV–vis were calculated at DFT/TD-DFT level using two hybrid exchange–correlation functionals, B3LYP and PBE1PBE. These calculations on (1) provide deep insight into its electronic structure and property.

Four asymmetric monomethine indocyanine dyes were rapidly synthesized by the condensation of indole quaternary salts with 2-methylthio quinoline quaternary salt in the presence of triethylamine under solvent-free, microwave irradiation. The effects of microwave power and irradiation time on yield were examined. The products were identified using elemental analysis, IR, MS, UV–Vis spectra, 1H and 13C NMR. The absorption of the dyes was investigated both experimentally and theoretically. Calculations performed using a combination of the time-dependent density functional theory (TD-DFT) and the polarizable continuum model (PCM) reproduced the π → π∗ type absorption bands of the dyes. Multiple linear regression, applied to the theoretical absorption maxima in different solvents, fitted well with experimental data. Resonance frequency calculations were undertaken to study the IR spectra of the dyes and the calculated results were in good accordance with experimental values.

The absorption and emission properties as well as electronic structure in the ground (S0) and excited states (S1) of seven dimethine cyanine dyes were investigated by time dependent density functional theory (TD-DFT) and configuration interaction singles (CIS) levels. The effect of water on the absorption and emission spectra of the dyes was taken into account using the polarizable continuum model (PCM). TD-DFT calculations provided a correct description of the electronic absorption spectra and showed that the dominant transitions of seven dye molecules presented a π–π* character. Scaling factor 0.72 was used on the absorption and emission wavelengths obtained by CIS method, and the scaled emission wavelengths were in good agreement with the experimental values. Compared with experimental counterparts, the average relative deviations of the absorption and emission maxima were about −2.4% and 1.7%, respectively.

A series of dimethine cyanine dyes were synthesized in a fast, efficient and high yield by the condensation of quaternary salts with 1H-indole-3-carbaldehyde in the presence of piperidine under solvent-free microwave irradiation. The products were identified by 1H NMR, IR, UV–vis spectroscopy and elemental analysis. The absorption and fluorescence properties of the dyes in both the free state and DNA or BSA were investigated. Significant enhancement of the fluorescent quantum yield was observed for all the dyes in the presence of DNA, with one compound demonstrating excellent sensitivity as a fluorescent probe.

A hemicyanine dye: 2-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-pyridinium iodide (1) was synthesized and characterized by X-ray diffraction, 1H NMR, MS, IR, UV–Vis spectra and elemental analysis. The crystals are monoclinic, space group P21/c, with a = 7.1231(8), b = 20.672(2), c = 10.9642(12) Å, V = 1605.7(3) Å3, and Z = 4 (at 298(2) K). Crystal stacking scheme indicates the hydrogen bonds and intermolecular π⋯π interactions; the molecular planes arrange in a head-to-tail fashion along the ab plane due to the strong ‘push-pull’ electron system. Molecular geometries, frequencies and absorption properties were calculated at DFT/TD-DFT level using two hybrid exchange-correlation functionals, B3LYP and PBE1PBE. These calculations on (1) provide deep insight into its electronic structure and property.

The ground-state geometries, the lowest energy transitions and the electronic spectra for a series of styryl dyes with quinoline nucleus have been studied with TD-DFT including SCRF approach. TD-DFT calculations provide a correct description of the UV–Vis spectra of these dye molecules. Moreover, the π → π∗ type absorption bands of unsubstituted or substituted styrylquinolines are all reasonably well reproduced by TD-DFT at the B3LYP/6-311G∗∗ level. Compared with our previously reported experimental data, the average relative deviation of λmax is −2.56% in gas phase and −2.48% in methanol solvent.