•The blue PDAPMO films turn purple, red and yellow successively upon heated.•Temperature ranges for thermochromism are rather wide.•The purple-to-red transition is rapidly and completely reversible.•A possible mechanism of thermochromism is proposed.•The films can also respond to mechanical stress and organic solvents.Polydiacetylene-based periodic mesoporous organosilica (PDAPMO) films with different molar fractions of diacetylene-bridged silsesquioxane (DASi) were synthesized through the evaporation-induced self-assembly followed by the topochemical polymerization via 254 nm ultraviolet radiation. Specifically, the as-polymerized blue PDAPMO films could undergo a multi-step thermochromic process, turning purple, red and yellow successively over a wide range from room temperature to above 150 °C. Importantly, the high temperature triggered multi-step color transition process could be divided into two stages: the first reversible blue-to-purple-to-red transition and the second irreversible red-to-yellow transition. In particular, the purple-to-red transition in the first stage was rapidly and completely reversible, and the reversibility could be repeated for many heating-cooling cycles. To study the mechanism of the thermochromic response, Raman and temperature-dependent FT-IR spectra, the results of which successfully highlighted the close relationship between chromatic transitions and the conformational changes of PDA backbone, were recorded. Furthermore, the PDAPMO films could also respond chromatically to mechanical stress and organic solvents, expanding their application in portable sensors.Download high-res image (80KB)Download full-size image

A novel “Off–On” solid chemosensor based on ordered mesoporous organosilica material was constructed through the co-condensation of rhodamine B organosilanes (RBSi) and tetraethoxysilane (TEOS). The resulting rhodamine B-based hybrid mesoporous organosilicas (RBMSiO2) show good recognition ability for Al3+ on the basis of the “Off–On” mechanism modulated by the Al3+-promoted ring-opening process of the spirolactam structure in the rhodamine B diethylenetriamine (RBD). The photo-stability of RBD could be significantly improved after introducing rhodamine B diethylenetriamine into the framework of RBMSiO2. Upon addition of Al3+, fluorescence emission intensity could increase by 110-fold with the color change from colorless to rose-red, and the detection limit of this hybrid chemosensor towards Al3+ can be as low as 1.3 × 10−7 M. Accordingly, the RBMSiO2 materials have great potential application in environmental and biological fields for the detection of Al3+.

A silver nanocube (Ag NC) nanoparticle-enhanced fluorescent nanocomposite was developed. The designed nanocomposite contained an Ag NC core of 50 nm in size as enhancer, the 10 nm silica interlayer, and outer shell-periodic mesoporous organosilicas (PMOs) which incorporated the bis(rhodamine Schiff-base derivative) siloxane groups. Little fluorescence emission of nanocomposites was observed due to the existence of spirolactam in the rhodamine derivative, however, upon the addition of Cu2+, the fluorescence emission intensity increased dramatically, which resulted from ring-opening of the spirolactam in the rhodamine moieties. We showed the effect of using an Ag NC core on the fluorescence emission intensity of the novel system. The emission intensity was approximately 3-fold higher for Ag-nanocube@SiO2@PMOs than the control sample prepared by etching the silver core away with iodide. Moreover, the metal-enhanced fluorescence (MEF) mechanism in this system was investigated through measuring the fluorescence lifetime. In addition, the designed core–shell Ag-nanocube@SiO2@PMOs nanocomposite could act as a sensitive and selective sensor to detect the Cu2+, and we have been expecting that the detection limit would be reduced by reaping benefits from the plasmon resonance of Ag NC core in this nanocomposite system.

As a strategy for achieving integration of two-state rotaxane based molecular switches and ordered solid-state frameworks, a pH-driven molecular shuttle was immobilized into the framework of the periodic mesoporous organosilicas (PMOs) that possessed enough free space to accommodate the mechanical motion of β-cyclodextrins (β-CDs). In this molecular shuttle, β-CDs threaded a symmetrical molecular thread composed of a biphenyl unit, two ureido and propyl groups, and were end-trapped mechanically by two siloxane stoppers. The β-CDs, as the shuttles, could be reversibly translocated along the thread by the pH stimuli in the rigid framework, accompanied with the change of fluorescence emission of the biphenyl units. Particularly, the PMOs could be employed as a pH-controllable smart-release platform via the reciprocating movement properties of the molecular shuttle, and accelerated cargo release was achieved after acidification. Furthermore, the PMO materials show very low cytotoxicity and fine biocompatibility, which ensure their potential in biomedical applications.

Highly fluorescent and copper(II)-responsive periodic mesoporous organosilicas (TH-PMOs) were successfully obtained by using a (tetraylnitrilomethylidyne–hexaphenyl (TH))-derived tetrasiloxane (TH-Si4) as the organosilica precursor. The TH unit was embedded within the framework of PMOs by four silyl groups without forming associated species, and high fluorescence quantum yields were achieved even for the PMOs prepared from 100% organosilane precursor. The optical studies indicated that the intramolecular rotation of TH was restricted by the framework of TH-PMOs, resulting in the decline of the nonradiative decay process and enhanced monomeric fluorescence emission. The unique structure of the TH groups not only assured their aggregation-induced enhanced emission (AIEE) characteristics but also provided potential coordinating sites for metal ions. Therefore, the enhanced fluorescence of PMOs showed a highly selective response to copper ions in aqueous solution with the detection sensitivity up to the 10–8 M level. Moreover, the diffusion process of Cu2+ and the competitive effect between Cu2+ and Fe2+ on TH-PMOs were measured by STXM; the results reveal that the specific binding between TH and Cu2+ brings about a relatively high adsorption capacity of the hybrid material toward Cu2+.

Using the novel bis(rhodamine Schiff-base derivative) bridged precursor with four silyl groups (BRh–Si4) and tetraethoxysilane as the organosilica source, bis(rhodamine Schiff-base derivative) bridged periodic mesoporous organosilicas (BRhPMOs) with mesoscopic and molecular scale periodicities are prepared through the supramolecular self-assembly of ionic liquid 1-hexadecyl-3-methylimidazolium bromide (C16mimBr). BRhPMOs with BRh units distributed uniformly in the framework, which show ultra-high selectivity towards copper ions, could be used as a Cu2+ fluorescence-ON chemosensor. The recognition mechanism of BRhPMOs towards Cu2+ has been investigated by Cu K-edge X-ray absorption fine structure spectroscopy (XAFS). The high selectivity of BRhPMOs towards Cu2+ can be attributed to the strong chelation of “N” atoms of the Schiff base in the BRh units to copper ions, leading to effective charge transfer between the BRh fluorophores and targeted metal ions. Owing to the encapsulation effect of the silica network, BRh chromophores in the form of BRhPMOs after chelating with copper ions exhibit higher photostability against long-term irradiation than rhodamine 6G (R6G) or Cu2+-chelated BRh–Si4, indicating their potential applications in biosensing and bioimaging. The perfect combination of functional fluorophores with periodic mesoporous organosilicas (PMOs) will open new avenues for applications in biotechnology and information technology.

•A series of biphenyl-bridged PMOs (BpUPMOs) was successfully synthesized.•BpUPMOs exhibited unique optical properties due to the hydrogen-bonded interactions.•Charge-transfer (CT) complex between pore walls and pore channels was built.Using a novel cationic oligomer surfactant as the structure-directing agent, the fluorescent biphenyl-bridged periodic mesoporous organosilicas (BpUPMOs) were successfully synthesized through the cohydrolysis and cocondensation of 4,4′-bis(3-triethoxysilyl propylureido) biphenyl (BpUSi) and tetraethoxysilane (TEOS). The structure characterization of the samples was obtained by FT-IR spectra, small-angle X-ray scattering (SAXS), high resolution transmission electron microscopy (HRTEM), and nitrogen adsorption–desorption analyses. The results showed that ordered mesoporous structure was obtained below 40 mol% of the content of BpUSi. With the increase of the amount of BpUSi, the ordering degree of mesoporous structure of BpUPMOs decreased. Fluorescence emission spectra of BpUPMOX (X is a mole fraction of BpUSi, 0%, 10%, 20%, 30%, 40%, 50%, respectively) showed gradually enhanced and red-shifted bands from 357 to 372 nm as X was less than 30 mol%. However, with further increase of X value, the bands at 359 nm for BpUPMO40 and at 354 nm for BpUPMO50 exhibited blue-shift compared with that of the sample BpUPMO30 (372 nm). This fluorescence phenomenon induced by the amount of organosilicone in the PMOs has not been reported. Furthermore, Charge-transfer (CT) complex composed of BpUPMO20 and decylviologen was obtained, in which the biphenyl groups were as electron donors in the pore walls and the decylviologen molecules were as electron acceptors in the mesochannels of BpUPMO. UV–vis diffuse reflectance spectra and fluorescence spectra showed the existence of CT complex. Besides, the color of CT complex was over a wide range from the salmon pink to red brown.Graphical abstractA series of biphenyl-bridged periodic mesoporous organosilicas (BpUPMOs) with unique fluorescent properties is successfully synthesized. Charge-transfer (CT) complex composed of biphenyl groups as electron donors in the pore walls and the decylviologen molecules as electron acceptors in the mesochannels of PMO is obtained, which the color of CT complex is over a wide range from the salmon pink to red brown.

Rhodamine-6G Schiff-base bridged periodic mesoporous organosilicas (RSPMOs) were successfully synthesized by co-condensation of rhodamine Schiff-base derivative-bridged organosilanes and tetraethoxysilane (TEOS), using the ionic liquid 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) as a structure-directing agent. The formation mechanism of ordered mesoporous hybrid materials with crystal-like pore walls was mediated by the self-assembly of both the long-chain ionic liquid and the new organosilane precursors. As the molar fraction of the new organosilane precursors was above 1%, hybrid materials with crystal-like pore walls were obtained, which was due to the face-to-face π–π interaction between rhodamine moieties of the new organosilane precursors within the framework. Little or no fluorescence emission of the hybrid material with rhodamine Schiff-base units was observed because of the existence of spirolactam in rhodamine moieties before chelating with metal ions. However, a dramatic enhancement in fluorescence emission was shown due to opening of the spiro structure in rhodamine moieties, which was induced by intramolecular charge transfer from metal ions to rhodamine units when the hybrid materials chelated with corresponding metal ions. RSPMOs showed high sensitivity and selectivity towards Cu2+ in aqueous solutions with good tolerance for other metal ions, showing great superiority in practical application.

A family of designed gemini surfactants C14H29(CH3)2N+-(CH2)S–N+(CH3)2C14H29·2Br− (designated as C14-S-14, S = 4, 6, 8, 10) was utilized as structure-directing agent to prepare ordered porous MCM-41 silica. The samples were characterized by small angle X-ray diffraction, high-resolution transmission electron microscopy and N2 adsorption/desorption analysis. The results showed that the obtained materials possessed 2D-hexagonal periodic structure (space group 2D-p6mm), but the pore diameter obviously decreased as the length of the spacer group of the gemini surfactants increased. Two types mesoporosity, that is, framework-confined mesoporosity (primary pores) and voids between particles (textural mesoporosity) existed in the samples. In addition, the self-assembly ability of C14-S-14 was stronger than that of tetradecyltrimethylammonium bromide (TTAB, which was regarded as the corresponding monomer of gemini surfactant C14-S-14) in controlling the orderly pore structure and pore size of the porous materials.

A novel strategy involving the combination of soft-templating and solid–liquid method (CSSL) is presented to synthesize mesoporous nanocrystalline zirconia with high specific surface area, that is, the mesostructured zirconia hybrid is firstly synthesized via cooperative assembly between zirconium sulphate as inorganic precursor and 1-hexadecyl-3-methylimidazolium bromide (C16mim+Br−) as the structure-directing agent, and subsequently ground with solid magnesium nitrate salt followed by heat-treatment in air. The resulting zirconia material after calcination at 600 °C possesses a wormlike arrangement of mesopores surrounded by tetragonal ZrO2 nanocrystallites of ca. 2.3 nm. The BET surface area is 255 m2/g and the pore size is ca. 4.3 nm. However, no mesoporous structure exists in the obtained zirconia material via the simple soft-templating method at the same calcination temperature. Photoluminescence (PL) spectra of the obtained mesoporous nanocrystalline ZrO2 show a strong emission peak at ca. 394 nm under UV excitation of 250 nm wavelength.

Highly ordered supermicroporous silica was synthesized by short chains cationic trimeric surfactant [C10H21N+(CH3)2(CH2)2N+(CH3)(C10H21) (CH2)2N+(CH3)2C10H21] · 3Br− (denoted C10-2-10-2-10) with a short spacer group (s = 2) as the structure-directing agent and tetraethyl orthosilicate as the precursor. The obtained samples were characterized by small-angle X-ray diffraction, high resolution transmission electron microscopy, and N2 adsorption–desorption. The results showed that the pore structure of the resulting samples belonged to the two-dimensional hexagonal structure (space group 2D-p6mm) with a pore size from 1.92 to 2.16 nm, which was within the supermicroporous range. The high-quality supermicroporous silica was formed at a low molar ratio of C10-2-10-2-10 to tetraethyl orthosilicate (0.08:1), which indicated that the self-assembly ability of C10-2-10-2-10 was stronger than that of corresponding monovalent surfactants. We strictly compared the methods of calculating surface area and pore size of supermicroporous materials, and the surface area was found to be in the range of 910–1,135 m2 g−1 by the αs plot method. With the increase of hydrothermal temperature, the ordering of the supermicroporous structure increased first then decreased, at the same time the pore size was enlarged.

A series of well-ordered lamellar mesoporous molybdenum oxides were prepared using gemini surfactant [CnH2n+1N+(CH3)2–(CH2)2–N+(CH3)2CnH2n+1] · 2Br−(denoted as Cn-2-n, n = 12, 14 and 16) as the structure-directing agent and ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24 · 4H2O as the precursor. The obtained samples were characterized by X-ray powder diffraction, thermal analysis, transmission electron microscopy and nitrogen adsorption–desorption. Results showed that contrary to complete structure collapse after removing tetradecyltrimethylammonium bromide (TTAB) from molybdenum oxide/TTAB composite, the lamellar mesostructure was retained after removal of Cn-2-n from corresponding composite. The effects of alkyl chain length and concentration of gemini surfactants on the structure of the mesoporous molybdenum oxide were also investigated. The specific surface area of extracted sample was as high as 116 m2 g−1. The maintenance of the lamellar mesostruture was due to the strong self-assembly ability of gemini surfactants and the strong electrical interaction between gemini surfactants and molybdenum oxide.

A fluorescent material with methylene viologen units bonded into the pore walls of the mesoporous MCM-48 silica is synthesized using the method of periodic mesoporous organosilicas with bridging groups (PMOs), in which the methylene viologen units are located within the channel walls through the cohydrolysis and cocondensation of dichloride of N,N′-bis(triethoxysilylmethyl)-4,4′-bipyridinium (VP) and tetraethoxysilane (TEOS). It is found that the suspension of the hybrid emits fluorescence at ca. 380 and 420 nm, which is attributed to the S1 state (π* → π) of the viologen and the charge-transfer complex between the bipyridinium units as electron acceptor and accompanying halide (Br−, Cl−) as donor components, respectively. The fluorescent emission intensity increases with increasing the amount of the VP covalently bonded to MCM-48 framework. The fluorescent intensity of VP adsorbed on the surface of the pore channel of MCM-48 was greatly weaker than that of the hybrid MCM-48−VP at the same molar ratio of TEOS to VP. No fluorescence was observed for pure VP. The different fluorescent intensity is ascribed to the fact that restricted degree of the rotation between two pyridine rings is different. It could be prospected that this material is potentially applied in drug delivery and fluorescence probing for medical diagnosis and synchronous therapy.

Well-ordered lamellar mesoporous molybdenum(V/VI) oxides were prepared using gemini surfactant [C14H29N+(CH3)2-(CH2)2-N+(CH3)2-C14H29]·2Br− (denoted C14-2-14) as the structure-directing agent and ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24·4H2O as the precursor. The samples were characterized by X-ray powder diffraction, high-resolution transmission electron microscopy and N2 adsorption–desorption. A possible arrangement of gemini surfactant in the interlayer space of molybdenum oxide was proposed. Gemini surfactant possessed better self-assembly ability than monovalent surfactant to form well-ordered mesostructures in mild conditions. The lamellar mesopore structure is retained after removal of the gemini surfactant by soxhlet extraction. The cheap and air-stable precursor provided an advantageous access to synthesize mesoporous transition metal oxides.

Using sodium silicate as precursors and mixed surfactants of cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) as templates, rod-like mesoporous silica with hexagonal appearance were synthesized by controlling the pH value of solution during hydrolysis of ethylacetate. Results showed that at a molar ratio of 9:1 (CTAB to CTAC), regular rod-like shapes were obtained, and that with the decrease of the molar ratio of CTAB to CTAC (from 9:1 to 5.5:4.5), the amount and length of rod-like mesoporous silica decreased, but the amount of irregular particles increased. The diffraction peaks obtained with XRD were assigned to hexagonal symmetry. Type IV adsorption isotherm and an H1 hysteresis loop were observed in N2 adsorption–desorption curves.

Rhodamine-6G Schiff-base bridged periodic mesoporous organosilicas (RSPMOs) were successfully synthesized by co-condensation of rhodamine Schiff-base derivative-bridged organosilanes and tetraethoxysilane (TEOS), using the ionic liquid 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) as a structure-directing agent. The formation mechanism of ordered mesoporous hybrid materials with crystal-like pore walls was mediated by the self-assembly of both the long-chain ionic liquid and the new organosilane precursors. As the molar fraction of the new organosilane precursors was above 1%, hybrid materials with crystal-like pore walls were obtained, which was due to the face-to-face π–π interaction between rhodamine moieties of the new organosilane precursors within the framework. Little or no fluorescence emission of the hybrid material with rhodamine Schiff-base units was observed because of the existence of spirolactam in rhodamine moieties before chelating with metal ions. However, a dramatic enhancement in fluorescence emission was shown due to opening of the spiro structure in rhodamine moieties, which was induced by intramolecular charge transfer from metal ions to rhodamine units when the hybrid materials chelated with corresponding metal ions. RSPMOs showed high sensitivity and selectivity towards Cu2+ in aqueous solutions with good tolerance for other metal ions, showing great superiority in practical application.

Using the novel bis(rhodamine Schiff-base derivative) bridged precursor with four silyl groups (BRh–Si4) and tetraethoxysilane as the organosilica source, bis(rhodamine Schiff-base derivative) bridged periodic mesoporous organosilicas (BRhPMOs) with mesoscopic and molecular scale periodicities are prepared through the supramolecular self-assembly of ionic liquid 1-hexadecyl-3-methylimidazolium bromide (C16mimBr). BRhPMOs with BRh units distributed uniformly in the framework, which show ultra-high selectivity towards copper ions, could be used as a Cu2+ fluorescence-ON chemosensor. The recognition mechanism of BRhPMOs towards Cu2+ has been investigated by Cu K-edge X-ray absorption fine structure spectroscopy (XAFS). The high selectivity of BRhPMOs towards Cu2+ can be attributed to the strong chelation of “N” atoms of the Schiff base in the BRh units to copper ions, leading to effective charge transfer between the BRh fluorophores and targeted metal ions. Owing to the encapsulation effect of the silica network, BRh chromophores in the form of BRhPMOs after chelating with copper ions exhibit higher photostability against long-term irradiation than rhodamine 6G (R6G) or Cu2+-chelated BRh–Si4, indicating their potential applications in biosensing and bioimaging. The perfect combination of functional fluorophores with periodic mesoporous organosilicas (PMOs) will open new avenues for applications in biotechnology and information technology.