•A novel pyridinyl-organoboron complex POB has been designed and synthesized.•Complex POB exhibited high fluorescence quantum yields in both monomer and aggregation state.•POB exhibited a ratiometric chemosensor to detect Hg2+ with high sensitivity, selectivity and rapid dynamic, which can be visualized by naked eyes from cyan to green.•POB film can be used for the visual detection of gaseous acid in atmosphere.Recently, most of fluorescent sensors have been focused on in diluted solutions because π-π stacking of traditional fluorescent materials would quench their emissions in solid state, and meanwhile some fluorescent materials in aggregation were reported as chemosensors to detect target analytes although they always were involved in photonic and electronic devices. Fluorescent probes in both the diluted solutions and aggregation have scarcely been developed in reported works. In this study, we report a novel pyridinyl-organoboron complex as a probe to detect mercury ions (Hg2+) in diluted solution and gaseous acid/base in aggregation. The as-synthesized probe with strong fluorescence contained a pyridine unit, as the reactive site of both Hg2+ and gaseous acid in the environmental monitoring. Upon the addition of Hg2+, the fluorescence ratio of I480/I515 was changed immediately accompanying with the color variation from cyan to green under a UV lamp. On the other hand, the complex probe was prepared as a film on a glass pallet for the detection of gaseous acid with the emissive color evolution from yellow to orange-red. Interestingly, the film sensor for acid/base gas had a reversible property with 10 cycles at least due to its much higher photostability.A pyridinyl-organoboron complex is an excellent fluorescent sensor of Hg2+ with high sensitivity, selectivity and rapid dynamics. Meanwhile, the film of complex can be used for the visual detection of gaseous acid in atmosphere.

Three tetraphenyl imidazole derivatives functionalized by an α-cyanostilbene unit (3a–3c) have been designed and synthesized. A combination of the representative aggregation-induced emission enhancement fluorophore and propeller sharp tetraphenyl imidazole unit in the same molecule achieved the integration of their function. 3a–3c emitted weakly in dilute organic solvents and showed an evident solvatochromic effect caused by strong intramolecular charge transfer (ICT), which has been confirmed by density functional theory (DFT) calculations. Meanwhile, in the solid state, they exhibited obvious fluorescence and stimuli-responsive emission. The main emission peak of compound 3a was red-shifted from 519 nm to 550 nm after grinding. The reason could be explained that the destruction of the crystalline structure leads to the planarization of molecular conformation or the increase of conjugation.

A ‘Λ’ structural A–π–D–π–A type triphenylamine derivative (2E,2′E)-3,3′-(4,4′-(phenylazanediyl)bis(4,1-phenylene))diacrylic acid (abbreviated as L) with intramolecular charge transfer (ICT) character has been designed, synthesized and characterized. The morphology and linear optical properties of L nanostructures can be tuned by changing the solvent and/or the acidity of the solution. The possible growth mechanism is discussed through the single crystal structure and the corresponding molecular stacking style of L molecules. Further, the strong coordination effect between the carboxylic acid group of L and the metal ions is discussed, which also brings about changeable morphology, linear optical properties and enhanced nonlinear optical properties, two photon absorbance (TPA) included, which makes them suitable for applications in two-photon biological imaging, such as in HepG2 cells.

•α-Cyanostilbene-functionalized linear bis-Schiff base 4 exhibits AIE character.•4 exhibits sensitive and selective fluorescence turn-on response towards Hg2+.•4 can be used for discrimination Hg2+ by “naked eye” or using UV lamp.•4 can detect trace Hg2+ in organic solvent, water environment and living cells.A novel aggregation-induced emission (AIE) active linear conjugated bis-Schiff base compound, (E)-N,N′-bis(4-(Z)-2-(4-butoxyphenyl)-1-cyanovinylphenyl)-1,4-benzenebismethaneimine (4) containing α-cyanostilbene unit was designed, synthesized and characterized. The distinct change of color and/or fluorescence emission of 4 + Hg2+ in tetrahydrofuran (THF) and THF/water solution provided a simple and easy discrimination way for Hg2+ by “naked eye” or using a UV lamp. The aggregating patterns of 4 were applied to explain the mechanism of sensing. The as-synthesized sample showed comparatively high selectivity and sensitivity to Hg2+ through fluorescence “turn-on” response with a low detection limit (3.4 × 10−9 M in THF, 2.4 × 10−7 M in THF/water). The results implied that the as-made sample could effectively act as Hg2+ sensor not only in THF and THF/water but in living cells.

A D–π–A type triphenylamine derivative (E)-3-(4-(4-(diphenylamino)styryl)-phenyl)acrylonitrile (abbreviated as L) is designed, synthesized and characterized by single crystal X-ray diffraction analysis, which is demonstrated to exhibit superior AIEE (aggregation-induced enhanced emission) properties. Self-aggregates of L obtained with and without the guidance of metal ions (Cd2+ or Cu2+) showed morphology-dependent luminescent properties. The coupling and synergy effects between L and metal ions bring about an energy transfer process between the components, which further results in red-shifted absorption and fluorescence, an enhanced fluorescent quantum yield and tunable FL lifetime. The interactions between L and metal ions also change nonlinear optical properties, including two-photon excited fluorescence (2PEF), two-photon absorption (2PA) cross-section (δ), two-photon action cross-section (ηδ) and two-photon absorption coefficient (β), which further leads to successful application of the samples as a two-photon fluorescent probe for labelling the intercellular section. The study indicates that the preparation of nanohybrids from metal ions and an AIEE organic compound is a promising method to prepare 2PA probes possessing a high ηδ for biological imaging.

•Five triphenylamine derivatives containing cyclohexyl have been designed and synthesized.•The compounds possess high quantum yields both in solution and solid state.•The high emission characters are utilized for picric acid (PA) detection in dual system.Five triphenylamine derivatives containing cyclohexyl have been designed and synthesized with a solvent-free green procedure. The compounds possess high quantum yields both in solution and solid state. Our investigation shows that the steric effects of the chair conformation cyclohexyl and strong intramolecular hydrogen bond play a key role for the high emission. Moreover, a conjugated triphenylamine core and strong polar groups (pyridinyl and hydroxy) and their location in the molecule formed a typical non-planar push–pull electronic structure. This special chemical structure endows the molecules with solvatochromic effect and exhibit interesting fluorescent behavior in binary solvent systems of THF-H2O due to the strong intramolecular charge transfer (ICT), which has been confirmed by DFT calculation, absorption and fluorescence spectra of the compounds in different polar solvents. In addition, their high emission characters, both in solution and solid state are utilized for picric acid (PA) detection.Triphenylamine derivatives containing cyclohexyl can emit highly efficient luminescence both in solution and solid state and have been utilized for picric acid detection.

A type of water-soluble Au based nanohybrid is constructed through a D–π–A (D = donor, A = acceptor) fluorophore-carbazole derivative (abbreviated as L) coupled with gold nanoparticles (NPs) under the guidance of calf thymus DNA (ct-DNA). The coupling interactions between components at the interface increase the strength of donor–acceptor pair within the nanohybrid, which results in an energy transfer process and further brings about a dramatic blue-shifted single-photon absorption (about 53 nm from 450 nm for L to 397 nm for the hybrid) and fluorescence (∼77 nm blue-shifted), decreased FL lifetime (1.65 ns for L, 0.53 ns for the hybrid), and blue-shifted two-photon excited fluorescence (2PEF, 638 nm for L, 574 nm for the hybrid). The coupling effect amongst the nanohybrid also leads to enhancement of nonlinear optical properties, including the two-photon absorption (TPA) cross section (δ, 137.60 GM for L, 651.07 GM for the hybrid) and two-photon absorption coefficient (β, from 0.003669 cm GW−1 to 0.01401 cm GW−1). The two-photon fluorescence and good water solubility of the hybrid have proven to be potentially useful for two-photon microscopy imaging in living cells such as HepG2.

A pyridine-based small-molecule receptor (L) for the specific recognition of TNP was synthesised and characterised by 1H NMR, 13C NMR and FT-IR spectra, and the optical properties were studied by UV-Vis absorption and PL spectra. The results showed that the fluorescence of L was strongly quenched by picric acid accompanied by an obvious colour change from transparent to yellow, which indicated that the receptor L can serve as a small-molecule sensor for TNP. Crystal structure determination and theoretical calculation of the corresponding host–guest complex L·TNP were also performed.

•Hg2+ probe (1) was designed by combining AIEE-active fluorophore and chelating group.•The AIEE property of 1 was investigated in detail.•The excellent AIEE property was utilized for Hg2+ detection in a “turn-on” mode.•1 has been successfully applied to detect Hg2+ in aqueous and living cells.By combination of the AIEE-active fluorophore and coordination binding group, a new selective and sensitive probe (1) for Hg2+ detection has been synthesized. 1 exhibits weak fluorescence in dilute solution, but exhibits dramatically enhanced emission in aggregation. In the solid state, a remarkable red-shifted emission and longer PL lifetime of 1 are present, which is caused by the formation of excimers due to the π–π stacking interactions of the coumarin groups. This AIEE effect of 1 is utilized to design probe for the detection of Hg2+ in a luminescence “turn-on” mode. The functionalization of the AIEE-active molecule with chelating group provides the potential binding sites for Hg2+. The selective coordination-induced aggregation bursts the emission. 1 has been successfully applied to detect Hg2+ in aqueous and living cells.By combination of coumarin and α-cyanostilbene derivative, a new Hg2+ fluorescent probe (1) with AIEE is synthesized. The “turn-on” response is triggered by Hg2+ ion coordination inducing fluorogen aggregation. 1 has been successfully applied to detect Hg2+ in aqueous and living cells.

A novel Schiff base derivative (Z)-3-(4-(hexyloxy)phenyl)-2-(4-((E)-2-hydroxybenzylidene amino)phenyl)acrylonitrile (L) was designed, synthesized and characterized. L was used as a Zn2+ selective, turn-on, fluorescent chemosensor with a detection limit of 0.1 μM in DMF. 1:1 stoichiometric complex formation of L wih Zn2+ was confirmed through fluorescent titration experiments and Job’s plot. The enhancement of fluorescence intensity of L with addition of Zn2+ is the consequence of the inhibited isomerization of the C=N bond, namely chelation-enhanced fluorescence (CHEF) effect.

Two easily-prepared pyridine-based derivatives of (Z)-2-(4-amino-phenyl)-3-(pyridine-4-yl)acrylonitrile (I)and (Z)-2-phenyl-3-(pyridin-4-yl)acrylonitrile (II) were designed, synthesised and characterised. Due to the formation of a complex with Hg2+, hence leading to an enhanced ICT effect, I exhibits a visible colour change from light yellow to orange, rendering it suitable for use as a naked-eye sensor for rapid detection of Hg2+ in an aqueous ethanol solution. When mixed with Hg2+, I interacts with Hg2+ in a2:1 (Y1-Hg2+) stoichiometry via a coordination bond with an association constant of 7.7 × 108 M−2 (R2 = 0.96). The present probe I exhibits excellent reproducibility, reversibility, sensitivity and selectivity with the presence of low concentration of Hg2+ (1.74 × 10−10 M).

A novel α-cyanostilbenic derivative, (Z)-2-(4-aminophenyl)-3-(4-(dodecyloxy)phenyl) acrylonitrile, is reported, wherein a Z–E isomerization process is accompanied by a remarkable decrease in compatibility with liquid crystals. Accordingly, fluorescent-molecule-dispersed liquid crystals have been prepared through a photoisomerization-induced phase separation method. The photoluminescence can be repeatedly switched by an electric field. Modulating the excitation light offers a new technique to explore electrically controllable photoluminescence.

A novel cyanostilbene derivative (I) is synthesized and sufficiently characterized, which is also utilized as the initiator to prepare poly(ε-caprolactone) (I-PCL) and poly(D,L-lactide) (I-PLA) via ring-opening polymerization (ROP). Notably, molecule I exhibits superior aggregation-induced enhanced emission (AIEE) behavior in the mixed solvent of THF and water. The bulky cyano groups are linked to vinylene moieties in molecule (I) generating intramolecular coplanarity and preventing π–π stacking, which leads to enhanced emission in the aggregated state. Crystallographic data of I confirm that the existence of multiple intermolecular interactions among adjacent molecules restricts intramolecular vibration and rotation. In addition, molecule I can self-organize into ordered microtubes in appropriate THF/water mixtures exhibiting significant optical waveguide properties. Furthermore, the polymers, I-PCL and I-PLA, also show AIEE behaviors since the single chromophore I is anchored on the polymer chains. As compared with the corresponding small molecule I, the target polymers possess not only preferable thermostabilities but also the property of easy fabrication. The luminogen aggregation of the initiator is well-realized and even enlarged after polymerization, due to the linkage of polymer chains. In the end, the polymer I-PLA has a great prospect in white light-emitting diode (LED) application, which emits daylight white light.

A novel luminescent liquid crystal, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(4-(dodecyloxy)phenyl)acrylonitrile) (PDPA), is reported, which shows green, yellow and orange colors depending on the self-assembled structure. The luminescence of PDPA can vary between those three different colors invoked by mechanical shearing and thermal annealing. Moreover, an excellent liquid crystal phase transition of PDPA is observed using polarized optical microscopy (POM) and differential scanning calorimetry (DSC) analysis. Finally, a prototype device is fabricated that demonstrates rewritable fluorescence optical recording with multistimuli luminescence tuning.

The PEI@Mg2SiO4 hybrid composite is readily synthesized for adsorbing carbon dioxide (CO2) and nitrophenol compounds (NACs), in which more than 11% PEI is embedded. By the removal rate of 4-nitrophenol (10 mg L−1) and the capture capacity of CO2, the Mg2+:PEI:SiO32− optimal mole ratio of the PEI@Mg2SiO4 is 1:0.6:1.5. The composite shows the highest adsorption amounts of 0.94 mmol g−1 CO2 at 50 °C. The heat treatment regeneration exhibits a good stability, e.g. the CO2 capture capacity decreases by only 20% after four cycles, and the desorption rate of CO2 remains more than 91%. It also exhibites a fast adsorption and high capacity for 4-nitrophenol (NP), 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenyl group (TNP). The adsorption capacity of NP, DNP and TNP is 2.40 mmol g−1, 2.66 mmol g−1 and 4.85 mmol g−1 respectively, which is more than that of the conventional sorbents. This work presents the higher capacity and eco–friendly sorbent for CO2 and nitrophenol compounds.

A D–π–A type triphenylamine derivative 3-(4-(diphenylamino)phenyl)acrylic acid (abbreviated as L) is designed, synthesized and characterized by single crystal X-ray diffraction analysis. The self-aggregation of L nanostructures in different acidity aqueous solutions and/or under the guidance of rare earth (RE) ions is studied, along with the corresponding optical properties. In strong acidic conditions, L molecules self-assemble to form two dimensional nanostructures. While in neutral aqueous solutions, L tends to form nanorods. In the strong alkali environment, nanofibers were obtained. Moreover, in acidic conditions, the existence of Tm3+ induces the L molecules to self-aggregate into nanoplates. Flower-like six branched structures are observed when Yb3+ is used. The changed morphology leads to tunable linear optical properties.

Luminogenic materials with AIE (aggregation-induced emission) and AIEE (aggregation-induced enhanced emission) characteristics have attracted substantial research interest due to their potential applications in optics, electronics environmental and the biological sciences. Here, we successfully synthesized two novel indolo[3,2-b]carbazole derivatives (compounds 3 and 4) with strong symmetrical groups, which were demonstrated to exhibit superior AIEE properties. Both 3 and 4 contain the same π-conjugated core structure, but slightly differ in the cyano-substituted stilbene group, giving rise to a high charge mobility, similar optical properties and energy band-gaps. The bulky cyano group attached to a vinylene moiety generates intramolecular co-planarity and prevents π–π stacking, whereby the emission in the aggregates can be enhanced. In addition, it is found that the cyano group also plays an important role in the formation of J-type aggregation, which is advantageous to the appearance of AIE and/or AIEE properties. A slight modification of the terminal substituent group renders compounds 3 and 4 AIEE properties entirely. As revealed by crystallographic data, the multiple intermolecular hydrogen bonding in 4 restricts intramolecular vibration and rotation, which enables 4 to emit intensely in the solid state. Moreover, cyano groups endow 3 with an improved self-assembly ability under the proper conditions. The molecules of 3 self-organize into one-dimensional ordered microfibers and/or nanorods that show a significant optical waveguide with different water fractions, indicating that the aqueous suspension of microfibers favors fluorescence emission. Our results demonstrate that the AIEE properties of the novel indolo[3,2-b]carbazole derivatives are highly related to the molecular structure and aggregate packing.

A facile and efficient green protocol for the synthesis of carbazole derivatives containing a six-membered ring, via an aldol condensation and solvent-free Michael addition reaction, has been developed. The compounds were characterized by 1H NMR, 13C NMR, IR, ESI-MS and HR-MS spectroscopic techniques and also, for one example, by X-ray crystallography. The cyclohexyl unit was introduced into the carbazole derivatives and utilized as a sterically demanding group for the purpose of increasing the non-planarity of the carbazole derivatives and reducing self-quenching, leading to improved optical and electronic properties. The influence of solvent polarity on the ultraviolet absorption and fluorescence characteristics was observed and investigated. The majority of the carbazole derivatives have high fluorescence quantum yields in dichloromethane, ethanol, acetonitrile and DMF.Highlights► A facile protocol for the synthesis of carbazole derivatives has been developed. ► The cyclohexyl group was introduced into carbazole derivatives. ► The solvent effect on the UV absorption and fluorescence was studied.

Two heterocycle-based derivatives that can be used as two-photon absorption chromophore, 9-butyl-3-(2, 6-diphenylpyridin-4-yl)-9H-carbazole (BDPYC) and 9-butyl-3-(4-(2, 6-diphenylpyridin-4-yl)styryl)-9H-carbazole (BDPSC) have been successfully synthesized and fully characterized by elemental analysis, IR, 1H NMR, 13C NMR and MS. The molecules possess D-π-A structures, but have different π bridge. The 9-butylcarbazole is used as a donor (D), and the pyridine ring is used as an acceptor (A). One- and two-photon absorption and excited fluorescence properties in various solvents were experimentally investigated. Two-photon initiated optical data recording experiments have been carried out under 740 nm laser radiation, and the possible mechanism of optical data storage is discussed based on theoretical calculations.

A novel ferrocene derivative which has four stereo centers has been prepared by an efficient synthetic method based on a solvent-free reaction. The compound was characterized by IR, 1H-NMR, 13C-NMR, ESI-MS, single crystal X-ray diffraction and TG analysis. The crystal structure shows that the cyclohexyl group is in a chair conformation and the two ferrocenyl moieties are nonequivalent. There are two intramolecular hydrogen bonds and two intermolecular hydrogen bonds in the compound, which involve two hydroxyls and generate a dimer, respectively. The electrochemical properties of the compound are discussed.

Two donor-π-acceptor (D-π-A) type 2,2′: 6′,2″-terpyridyl-based organic heterocyclic molecules have been efficiently synthesized via solvent-free Wittig reactions in good yields. One crystal structure of them was determined by single crystal X-ray diffraction determination. The two compounds exhibit sensitive single-photon-excited fluorescence (SPEF) emission with high fluorescence quantum yields and long lifetimes. The two-photon-excited fluorescence (TPEF), two-photon absorption (TPA) cross-sections and two-photon initiation polymerization (TPIP) microfabrication experiments have been investigated. Experimental results confirm that they are effective organic two-photon photopolymerization initiators.

Lead sulfide (PbS) is a major ore mineral of lead and an important semiconductor. The small band gap and large exciton Bohr radius make PbS an interesting system for studying the effect of size confinement. The precise control of their size and size distribution is an important goal of modern materials chemistry. In this work, lead sulfide (PbS) nanocubes have been successfully synthesized in an ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6) (ILs). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed the cubic morphology of the synthesized nanocrystals with the length about 100 nm. X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) showed the nanocubes were cubic PbS with a lattice constant of 5.93 Å.

A D–π–A type triphenylamine derivative (E)-3-(4-(4-(diphenylamino)styryl)-phenyl)acrylonitrile (abbreviated as L) is designed, synthesized and characterized by single crystal X-ray diffraction analysis, which is demonstrated to exhibit superior AIEE (aggregation-induced enhanced emission) properties. Self-aggregates of L obtained with and without the guidance of metal ions (Cd2+ or Cu2+) showed morphology-dependent luminescent properties. The coupling and synergy effects between L and metal ions bring about an energy transfer process between the components, which further results in red-shifted absorption and fluorescence, an enhanced fluorescent quantum yield and tunable FL lifetime. The interactions between L and metal ions also change nonlinear optical properties, including two-photon excited fluorescence (2PEF), two-photon absorption (2PA) cross-section (δ), two-photon action cross-section (ηδ) and two-photon absorption coefficient (β), which further leads to successful application of the samples as a two-photon fluorescent probe for labelling the intercellular section. The study indicates that the preparation of nanohybrids from metal ions and an AIEE organic compound is a promising method to prepare 2PA probes possessing a high ηδ for biological imaging.

Luminogenic materials with AIE (aggregation-induced emission) and AIEE (aggregation-induced enhanced emission) characteristics have attracted substantial research interest due to their potential applications in optics, electronics environmental and the biological sciences. Here, we successfully synthesized two novel indolo[3,2-b]carbazole derivatives (compounds 3 and 4) with strong symmetrical groups, which were demonstrated to exhibit superior AIEE properties. Both 3 and 4 contain the same π-conjugated core structure, but slightly differ in the cyano-substituted stilbene group, giving rise to a high charge mobility, similar optical properties and energy band-gaps. The bulky cyano group attached to a vinylene moiety generates intramolecular co-planarity and prevents π–π stacking, whereby the emission in the aggregates can be enhanced. In addition, it is found that the cyano group also plays an important role in the formation of J-type aggregation, which is advantageous to the appearance of AIE and/or AIEE properties. A slight modification of the terminal substituent group renders compounds 3 and 4 AIEE properties entirely. As revealed by crystallographic data, the multiple intermolecular hydrogen bonding in 4 restricts intramolecular vibration and rotation, which enables 4 to emit intensely in the solid state. Moreover, cyano groups endow 3 with an improved self-assembly ability under the proper conditions. The molecules of 3 self-organize into one-dimensional ordered microfibers and/or nanorods that show a significant optical waveguide with different water fractions, indicating that the aqueous suspension of microfibers favors fluorescence emission. Our results demonstrate that the AIEE properties of the novel indolo[3,2-b]carbazole derivatives are highly related to the molecular structure and aggregate packing.

A novel α-cyanostilbenic derivative, (Z)-2-(4-aminophenyl)-3-(4-(dodecyloxy)phenyl) acrylonitrile, is reported, wherein a Z–E isomerization process is accompanied by a remarkable decrease in compatibility with liquid crystals. Accordingly, fluorescent-molecule-dispersed liquid crystals have been prepared through a photoisomerization-induced phase separation method. The photoluminescence can be repeatedly switched by an electric field. Modulating the excitation light offers a new technique to explore electrically controllable photoluminescence.

A ‘Λ’ structural A–π–D–π–A type triphenylamine derivative (2E,2′E)-3,3′-(4,4′-(phenylazanediyl)bis(4,1-phenylene))diacrylic acid (abbreviated as L) with intramolecular charge transfer (ICT) character has been designed, synthesized and characterized. The morphology and linear optical properties of L nanostructures can be tuned by changing the solvent and/or the acidity of the solution. The possible growth mechanism is discussed through the single crystal structure and the corresponding molecular stacking style of L molecules. Further, the strong coordination effect between the carboxylic acid group of L and the metal ions is discussed, which also brings about changeable morphology, linear optical properties and enhanced nonlinear optical properties, two photon absorbance (TPA) included, which makes them suitable for applications in two-photon biological imaging, such as in HepG2 cells.

Three tetraphenyl imidazole derivatives functionalized by an α-cyanostilbene unit (3a–3c) have been designed and synthesized. A combination of the representative aggregation-induced emission enhancement fluorophore and propeller sharp tetraphenyl imidazole unit in the same molecule achieved the integration of their function. 3a–3c emitted weakly in dilute organic solvents and showed an evident solvatochromic effect caused by strong intramolecular charge transfer (ICT), which has been confirmed by density functional theory (DFT) calculations. Meanwhile, in the solid state, they exhibited obvious fluorescence and stimuli-responsive emission. The main emission peak of compound 3a was red-shifted from 519 nm to 550 nm after grinding. The reason could be explained that the destruction of the crystalline structure leads to the planarization of molecular conformation or the increase of conjugation.