•Two novel pH probes (L1 and L2) were facilely synthesized by one-step reaction.•Fluorescence intensity ratio of L1 and L2 can quantify the pH fluctuations.•L1 and L2 show high quantum yield and large two-photon absorption cross-section.•L1 and L2 show excellent selectivity response and remarkable color change to H+.•L1 and L2 were successfully utilized to mapping pH difference intracellularly.The pH value plays a significant role in lots of systems such as chemical reactions and biological processes. Herein, two novel two-photon excited fluorescent (2PEF) pH probes abbreviated as L1 and L2, based on the A-π-D-π-A system were facilely synthesized by one-step solid-state reaction at room temperature. Probes L1 and L2 exhibited linear response over the weakly acidic pH range (4.33–6.73 for L1, 2.99–5.28 for L2) and remarkable emission ratio enhancement with the pKa of 5.12 and 3.85, respectively. Interestingly, the position of the nitrogen atom had influence on the pH-dependent single-photon excited fluorescence, two-photon absorption (2 PA) across-section and linear region of pH response in probes L1 and L2. Furthermore, the probes exhibited favorable two-photon absorption, high sensitivity, low cytotoxicity, and capability of mapping intracellular pH distribution. Therefore, probes L1 and L2 could act as practical tool for the sensitively detection of pH in weakly acidic conditions and tracking pH difference in living cells via two-photon fluorescence imaging.Dynamic monitoring the pH fluctuations is critical for many industrial and biological processes and hence many researchers devoted themselves to develop novel sensors for monitoring pH variations. Herein, two novel two-photon excited fluorescent (2PEF) pH probes abbreviated as L1 and L2, based on the A-π-D-π-A system were facilely synthesized by one-step solid-state reaction at room temperature. Probes L1 and L2 exhibited linear response over the weakly acidic pH range (4.33–6.73 for L1, 2.99–5.28 for L2) and remarkable emission ratio enhancement with the pKa of 5.12 and 3.85, respectively. Interestingly, the position of the nitrogen atom had influence on the pH-dependent single-photon excited fluorescence, two-photon absorption (2 PA) across-section and linear region of pH response in probes L1 and L2. Furthermore, the probes exhibited favorable two-photon photophysics, high sensitivity, low cytotoxicity, and capability of mapping intracellular pH distribution. Therefore, probes L1 and L2 could act as practical tool for the sensitively detection of pH in weakly acidic conditions and tracking pH difference in living cells via two-photon fluorescence imaging.

•A series of novel nitro and amino molecules (Z1-Z3 and L1-L3) had been synthesized.•The photophysical properties were systematically investigated.•Z1-Z3 possessing excellent two-photon absorption cross-section were measured by Z-scan method.•L1-L3 were applied to monitor mitochondria in living cells (HepG2) and zebrafish 3D imaging.Two-photon fluorescent probes have emerged as promising molecular tools for imaging subcellular organelles. Here, the facile synthesis of three novel nitro molecules (Z1-Z3) with lager two-photon absorption cross-section and three novel amino molecules (L1-L3) based on mitochondrial probes with two-photon absorption are presented. Their photophysical properties have been investigated both experimentally and theoretically. The study of nonlinear optical property revealed that Z1-Z3 possess more excellent two-photon absorption (2PA) cross-section values than L1-L3 and the 2PA cross-section values were enhanced with increasing electron-donating strength of the end group when fixed the opposite group (nitro or amino). Furthermore, in consideration of the water solubility and fluorescent intensity, L1-L3 possessing high specificity for mitochondrial localization were applied to biology, which is advantageous in comparison with commercially available mitochondrial trackers. Due to their low cytotoxicity, these small molecules L1-L3 offer a promising platform to directly monitor mitochondria in living cells and zebra fish.

•Three stilbazolium salts with A-π-A model have been synthesized and fully characterized.•A relative strong intramolecular charge transfer (ICT) transition•The excellent third-order NLO response in the near-infrared (IR) region by Z-scan technique using femtosecond laser.•The different terminal substituent group of the pyridinium plays a vital role in third-order NLO behavior.A series of water-soluble stilbazolium salts with A-π-A (A: Acceptor) model have been synthesized and fully characterized. The results obtained from absorption spectra and TD-DFT computational studies show that there is a relative strong intramolecular charge transfer (ICT) transition from pyridine unit to pyridine cation of the stilbazolium salts. Furthermore, it is found that the three stilbazolium salts (T1, T2, T3) show the strong two-photon absorption (2PA) response in the near-infrared (IR) region by Z-scan technique using femtosecond laser. And the stilbazolium salt T3 shows the largest two-photon absorption cross-section and third-order nonlinear optical (NLO) coefficient χ(3) at 730 nm, indicating the different terminal substituent group of the pyridinium plays a vital role in third-order NLO behavior.

A novel dithiocarbazate derivative ligand based on triphenylamine with D-π-A model (HL) and its corresponding transition metal complexes were designed, synthesized and fully characterized. The crystal structures of NiL2 and CuL2 have been confirmed by single crystal X-ray diffraction analysis, indicating that the coordinates to metal-ion to form a five-membered chelated ring with an excellent square-planar conformation, which further form high electron delocalization. Crystal structure determination also confirmed that the structures of NiL2 and CuL2 are isostructural. Open and closed aperture Z-scan measurements were devoted to explore the third-order nonlinear optical (NLO) properties with the aid of femtosecond pulse laser from 680 to 1080 nm. The results revealed that complexes NiL2 and CuL2 possessed large 2PA cross-sections 11235.33 and 12018.99 GM, as well as optical power limiting properties.Two novel D-π-A Ni(II) and Cu(II) complexes have been designed, synthesized and fully characterized. They have potential applications as NLO materials in near-infrared region.

How can imaging be improved? Coordination polymers (CPs) show fascinating potential in optoelectronic optics but limited potential in bioimaging. Without doubt, it was very meaningful when CPs were first used in second-harmonic generation (SHG) imaging. Herein, through reasonable design and synthesis, a series of nonlinear optical CPs bearing very good one-photon excited fluorescence (OPEF), two-photon excited fluorescence (TPEF) and very strong SHG properties has been presented. Further study demonstrated that the nanoscale CPs show very strong SHG signals which have been applied in the three-dimensional imaging of thick block tissue with higher spatial resolution through simultaneous multichannel nonlinear optical (NLO) imaging technology. After simple encapsulation by polymeric micelles, the nanoscale CPs were successfully applied in SHG bio-imaging within the living cells. This finding throws light on the design of nanoscale NLO CPs and offers a simple avenue to develop novel effective exogenous SHG imaging agents.

Two-photon (TP) microscopy has advantages for biological imaging in that it allows deeper tissue-penetration and excellent resolution compared with one-photon (OP) microscopy. Herein, two-photon activated nonlinear optical (NLO) materials, a terpyridine derivative ligand with a donor–acceptor (D–A) model and its corresponding four zinc(II) complexes (Z1–Z4), were assembled and fully characterized. Based on their systematic photophysical investigations, it was found that the third-order NLO response in the near-infrared (NIR) region was significantly enhanced for the symmetrical complex Z4 compared with the other asymmetric complexes (Z1–Z3). To further explore its utility in biological systems, Z4 was selected as a two-photon probe for cellular lipid membranes, which was confirmed both by two-photon confocal microscopy and transmission electron microscopy (TEM). Furthermore, it was found that the fluorescence intensity was enhanced for complex Z4 with the addition of bioactive liposomes (1 μg mL−1). Its recognition mechanism was demonstrated by experimental methods, as well as molecular modeling calculations. These findings should open a new pathway for functional metal complexes as lipid membrane targeting probes.

Small, biocompatible and water-soluble molecules with moderate two-photon absorption (2PA) cross-section values (σ) are in extreme demand for specific bioimaging applications. Herein, two novel imidazole–pyrimidine hexafluorophosphate derivatives (6P and 10P) are efficiently synthesized through improved Knoevenagel condensation and Ullmann reactions with high yield. Based on systematic photophysical investigations and theoretical calculations, the structure–property relationships indicate that the different donor groups have large influences on their optical properties. The 2PA cross-section values (σ) are obtained both by Z-scan and two-photon excited fluorescence (2PEF) measurements. It is found that the two-photon absorption cross section values (σ) in the near-infrared region are significantly enhanced for 6P and 10P, which confirms that these small molecules display a suitable turn-ON fluorescence response for two-photon fluorescence microscopy (2PFM). Comprehensively considering the high solubility of the introduced hexafluorophosphate group, 6P and 10P possess high specificity for mitochondrial localization, showing a temperature-independent pathway, possibly passive infusion, which is advantageous in comparison with the commercially available mitochondrial trackers. Due to their low cytotoxicity, these small molecules offer a promising platform to directly monitor the intestinal system in live zebrafish larvae.

•A series of metal complexes based on the ferrocenyl terpyridine were synthesized and fully characterized.•The electrochemical properties of the complexes 1-6 were studied by cyclic voltammetric experiments.•The third-order nonlinear optical properties in the near-IR range were investigated by the Z-scan technique.A series of zinc (II) and cadmium (II) complexes based on the ferrocenyl terpyridine were synthesized and fully characterized. Firstly, their structures were confirmed by single crystal X-ray diffraction analysis, which revealed that the central metal atom adopted a distorted tetragonal pyramidal geometry coordination model. Secondly, the photophysical properties of the complexes were investigated with aid of experimental and theoretical calculation methods. The results indicate that there are ICT, π⋯π*, LLCT and LMCT transition in the six complexes. Thirdly, their electrochemical behaviors showed that the ferrocenyl (Fc) terpyridine complexes exhibited a quasi-reversible Fc+-Fc redox couple. The E1/2 values of them were influenced by different halides. Lastly, the third-order nonlinear optical properties in the near-IR range were systematically determined by open/close aperture Z-scan methods using tunable femtosecond laser. The results revealed that LCdBr2 possesses largest 2 PA coefficient (β = 0.416 cm/GW) and cross sections (σ = 17835 GM) compared to others. However, the LZnBr2 possess largest values of the cubic hyperpolarizability χ(3) (5.64 × 10−13 esu), it shows that the ferrocenyl-based metal bromine complexes have better NLO properties.A series of novel D-A Cd(II) and Zn(II) Complexes have been synthesized and fully characterized. They have potential applications as NLO materials in near-infrared region.Download high-res image (202KB)Download full-size image

Designing small molecules with large two-photon absorption cross sections is urgently needed. In the present work, six novel chalcone thiophene derivatives were obtained by tuning the terminal electron-withdrawing/donating groups rationally. Their structure–property relationship was investigated both experimentally and theoretically. Crystallographic studies revealed that structural diversity was mainly influenced by ethyloxy groups. Both the alternation of electron-withdrawing/donating groups and the polarity of different solvents had a considerable influence on their photophysical properties. The results of UV-vis and one-/two-photon excited emission spectra showed that the high polarity solvents increased the quantum yields (Φ) and two-photon absorption cross-sections (σ) of type C chalcones, while they decreased those of type OC chalcones. It should be highlighted that as they are small molecules, favorable Φ and σ values were obtained. Finally, a preliminary attempt has been made in the biological imaging field, with satisfactory results that C-1 and OC-1 could locate uniformly in a cytosolic membrane-like system.

As a semi-autonomous organelle, the eukaryotic mitochondrion possesses an individual DNA and protein synthesizing system. Therefore, the mitochondrial DNA-targeting probes are powerful tools to understand the functions in physiological processes. Herein, we report a novel Ru(II) complex (HLRu) based on a phenanthroline derivative to target mitochondrial DNA. The nonlinear optical property study revealed a reverse nonlinear optical refraction character from self-focusing to self-defocusing response before and after complexation with Ru(II). Furthermore, the two-photon absorption and high biocompatibility of complex HLRu highlight its potential applications in biological processes. It was found that complex HLRu specifically binds to mitochondrial DNA in living cells and enables imaging of tissues with minimal auto-fluorescence. As a result, this complex HLRu probe offers a promising platform to directly monitor mitochondrial DNA in living cells.

Imaging of RNA in living cells is a potential tool to understand intracellular RNA function. Therefore, an effective two-photon fluorescent probe with a reasonable two-photon action cross-section to label RNA is now urgently required. In this work, a series of novel two-photon absorbing terpyridine ZnX2 (X = Cl, Br, I) complexes have been designed and an effective RNA imaging probe has been obtained. The results revealed that OTP-ZnCl2 possesses large Stokes shift and two-photon action cross-section. Furthermore, live cell imaging experiments indicated that OTP-ZnCl2 could stain nucleoli in living cells by binding with nucleoli RNA. The mechanism of selective nucleoli staining of OTP-ZnCl2 was studied systematically via both experiments and molecular modeling calculations. Due to its low cytotoxicity, good membrane permeability and counterstain compatibility with the commercial fluorescent nucleic acid dye Hoechst 33342, as well as its ability to label RNA in living cells, OTP-ZnCl2 is a promising candidate for the detection of nucleic acid in living cells.

•Photophysical properties were tuned by subtle substitute changes on host framework.•Crystallography structures of novel terpyridine derivatives were determined.•Two-photon absorption properties were studied both experimentally and theoretically.•Relationships between the structures and properties were investigated.•Applications in bioimaging for the terpyridine derivatives were carried out.Five novel 2, 2′:6′, 2″-terpyridine derivatives (L1-L5) with subtle substitute changes have been designed and synthesized. Their structures were determined by the single crystal X-ray diffraction analysis. It was observed that L1 and L2 containing an acceptor (–NO2) group exhibited three strong absorption bands, while L3, L4 and L5 containing donors (–OCH3/NH2) exhibited two absorption bands similar to that of ligand L0. Two-photon absorption cross sections were measured by two-photon excited fluorescence and open aperture Z-scan methods, respectively. Further, the two-photon absorption properties were explained by the hybrid density functional theory (DFT/B3LYP). The results indicated that L5 with amino group possessed the largest two-photon absorption cross-section values. The systematic investigations revealed that the linear/nonlinear optical properties could be tuned by subtle terminal unit changes for the Donor–Acceptor framework. Due to the existed –NH2 moiety, the subcellular acidic organelle targeting in living cells finally observed using such compounds under two-photon confocal microscopy.

•Two novel ferrocene derivatives have been designed, synthesized and fully characterized.•The electrochemical properties of the two compounds were studied by cyclic voltammetric experiments.•The third-order nonlinear optical properties were investigated by Z-scan technique.Two novel ferrocene derivatives, N,N-diphenyl-4-E-((1-ferrocenyl) ethenyl) aniline (1), N,N-(4,4-diethoxyphenyl)-4-E-((1-ferrocenyl)ethenyl)aniline (2), were designed, synthesized and fully characterized by 1H NMR, 13C NMR, FT-IR and MALDI-TOF-MS spectra. Structure of (1) was confirmed by X-ray diffraction analysis. The UV–visible absorption spectra of the two compounds were investigated and verified by density functional theory (DFT) calculations. Electrochemical and third-order nonlinear optical (NLO) properties have been shown by cyclic voltammetric experiments and open/closed aperture Z-scan measurements. The results show that the two compounds both are highly π-electron delocalization system, indicating potential applications as NLO materials. It is noteworthy that (1) possesses larger two-photon absorption (2 PA) coefficient β, 2 PA cross section σ and the third-order nonlinear susceptibility (χ(3)) than (2).Two novel D1-π-D2 ferrocenyl compounds, (1) and (2), were designed, synthesized and fully characterized. They have potential applications as NLO materials and all-optical switching.

•Two new two photon chemosensors CCP and COP were synthesized and fully characterized.•They show strong fluorescence quenching and good ratiometric responses.•The two-photon fluorescence intensity could be effectively enhanced on adding EDTA.•The internalization (CCP → Fe3+, COP → Cu2+) with cells and shows the “light switch” property.Two novel NIR-region two-photon fluorescent probes CCP and COP, show strong fluorescence quenching and good ratiometric responses toward Fe3+ and Cu2+, respectively; and their two-photon fluorescence are reversible by the subsequent addition of EDTA. CCP and COP are valuable candidates for two-photon imaging in the biological transparency window.

A series of asymmetrical D–π–D type thiophene-based chromophores with small sizes and different electron-donating groups were designed and synthesized in high yield. Single-crystal X-ray diffraction analysis and theoretical calculations were carried out to further explore the electronic structural features of the chromophores. It was observed that multiple C–H⋯π interactions and C–H⋯O hydrogen bonds played an important role in crystal stacking. Systematic investigations, including fluorescence quantum yields, fluorescence lifetime, and two-photon absorption (2PA) cross sections, revealed that the photophysical properties of the thiophene-based chromophores can be tunable by the modulation of electron-donating terminal units. Furthermore, by introducing a carbonyl group in the π-bridge center, one of the chromophores with a small size exhibited significantly enhanced two-photon absorption with a 36-fold increase of the 2PA cross section compared with that of the corresponding chromophore. Finally, due to their superior 2PA character in the near-infrared region (700–900 nm), two-photon excited bioimaging applications were carried out for the chromophores. The results of live cell imaging experiments showed that the chromophores can be effectively penetrated into the cytosol of HepG2 cells and their physiological activity can be detected in vitro using two-photon fluorescence microscopy.

It is still a challenge to obtain two-photon excited fluorescent bioimaging probes with intense emission, high photo-stability and low cytotoxicity. In the present work, four Zn(II)-coordinated complexes (1–4) constructed from two novel D–A and D–π–A ligands (L1 and L2) are investigated both experimentally and theoretically, aiming to explore efficient two-photon probes for bioimaging. Molecular geometry optimization used for theoretical calculations is achieved using the crystallographic data. Notably, the results indicate that complexes 1 and 2 display enhanced two-photon absorption (2PA) cross sections compared to their corresponding D–A ligand (L1). Furthermore, it was found that complex 1 has the advantages of moderate 2PA cross section in the near-infrared region, longer fluorescence lifetime, higher quantum yield, good biocompatibility and enhanced two-photon excited fluorescence. Therefore, complex 1 is evaluated as a bioimaging probe for in vitro imaging of HepG2 cells, in which it is observed under a two-photon scanning microscope that complex 1 exhibits effective co-staining with endoplasmic reticulum (ER) and nuclear membrane; as well as for in vivo imaging of zebrafish larva, in which it is observed that complex 1 exhibits specificity in the intestinal system.

Three different pyridinium salts with I−, BPh4− or [Eu(TTA)4]− anions were synthesized and fully characterized. The crystal structures and 1H-NMR spectra of the dipyridinium cations showed that the coplanarity of the cation was controlled by the type of anion. Their one- and two-photon absorption properties were studied in combination with their structure. The two-photon absorption properties were shown to be tunable by the counter-anions. A pyridinium salt containing L[Eu(TTA)4]2 was selected as a marker for use in fluorescence microscopy to image living cells. This salt clearly displayed the cell structure and was used successfully in two-photon excited fluorescence microscopy.

Three novel organooxotin complexes (Z1, Z2 and Z3) were synthesized by reaction of L1 (2-cyano-3-(4-(diphenylamino)phenyl) acrylic acid) with n-Bu2SnO, Ph3Sn(OH) and nBu6Sn2O. The structures of the three complexes have been confirmed by single-crystal X-ray diffraction analysis. The metal complex Z1 features a ladder framework while Z2 and Z3 show discrete structures. The UV-vis absorption, single-photon excited fluorescence (SPEF) and two-photon excited fluorescence (2PEF) of the complexes have been systematically studied, suggesting that the three metal complexes have strong two-photon absorption (2PA) and large 2PA cross-sections. It is noteworthy that Z2 possesses an enhanced two-photon absorption, and Z3 exhibits larger 2PA cross-section per molecular weight compared to L1. Finally, high anti-tumor activity of these three metal complexes has also been identified.

A novel 4′-(4-(diphenylamino)thienyl)-2,2′:6′,2′′-terpyridine ligand (L) based on thiophene and its LZnX2 complexes (X = Cl, Br, I, SCN) was designed, synthesized and characterized by elemental analysis, far-IR, MALDI-TOF-MS, and single crystal X-ray diffraction analysis. Structural studies revealed that the central zinc(II) atom adopted a distorted trigonal bipyramidal coordination model. However, there were different hydrogen bonds and stacking models with different counter anions in the crystals. The absorption properties of the compounds were investigated with the aid of TD-DFT computational methods. Furthermore, the third-order nonlinear optical (NLO) properties were systematically studied via open-aperture Z-scan methods using a tunable wavelength femtosecond laser. The results from photophysical property investigations suggested that the complexation of the thiophene-based terpyridine ligand with zinc halides resulted in strong ICT/LLCT bands of about 450 nm, and the complexes exhibited strong nonlinear optical response in the near-infrared range around 850 nm. Above all, the two-photon absorption (2PA) cross-section values (σ) were enhanced by coordination with zinc and influenced by halide ions, reaching up to 2583 GM (X = Br).

•Five novel Ru (II) polypyridyl complexes were synthesized and fully characterized.•The photophysical properties of them were systematically investigated.•The complexes all display excellent third-order NLO properties.•The σ of cyclometalated complexes are larger than that of noncyclometalated ones.•The five complexes are all intercalation model with DNA.Five novel ruthenium (II) polypyridyl (N∧N∧N or N∧N∧C) complexes (1–5) containing carbazole with flexible substituents have been synthesized, and fully characterized. The crystal structure of complex 5 was determined by X-ray diffraction analysis. The photophysical properties of them were systematic studied. The complexes exhibited ligand-centered π–π∗ transition in the range of 290–350 nm and metal-to-ligand charge transition bands in the range of 450–650 nm. The third-order nonlinear optical response was measured by Z-scan technique for the complexes. The results revealed that cyclometalated (N∧N∧C) complexes 1 and 2 possess a stronger third-order nonlinear optical response than those of noncyclometalated (N∧N∧N) complexes 3, 4 and 5. The DNA binding behaviors of the complexes have been investigated by spectroscopic and viscosity measurements. It was found that the complexes can interact with DNA in different modes, including classical intercalation for complexes 1–3 and partial intercalation for complexes 4–5.Five novel Ru (II) polypyridyl complexes have been synthesized and their third-order nonlinear optical properties and DNA-binding behaviors have been systematically studied.

•A novel chromophore with D–π–A configuration was reported.•The structure was determined by single crystal X-ray diffraction analysis.•The relationships between the structures and properties were investigated.•The cis-trans isomerization of the ethylene group had large influence on its properties.•The two-photon absorption cross-sections were determined by Z-scan technique.One novel 2,2′:6′,2″-terpyridine ligand L and its complexes (Zn(S)2L, Cd(S)2L) were synthesized and systematically characterized. There were cis-trans isomerization of the ethylene group in compounds of L, Zn(S)2L and Cd(S)2L though some characterization. Finally it was found that the two-photon absorption cross section values (σ) for the complexes in the near infrared region (∼800 nm) were dramatically enhanced compared to the free D–π–A type ligand.One novel 2,2′:6′,2″-terpyridine ligand L and its complexes Zn(S)2L, Cd(S)2L were synthesized, they display interesting cis-trans isomerization and excellent NLO properties.

Here we have designed and synthesized a novel analogous cis-platinum complex (TDPt) with strong two-photon absorption properties and higher stability upon laser irradiation. Interestingly, a higher cytotoxicity against three types of cancer cells compared to that of commercial cis-platinum was observed. The initial confocal micrographs showed that lysosomes may be the biological targets of such TDPt, except the conventional presumed DNA. This hypothesis was further confirmed by the two-photon microscopy and transmission electron microscopy micrograph. These results form an important basis for future “on-site observation” of the anticancer mechanism of the Pt(II) complex.

•One novel ligand, 4′-ferrocenyl-4, 2′: 6′, 4″-terpyridine, have been designed and synthesized.•Five novel metal-organic hybrid materials have been prepared by self-assembly method.•The structural features of the ligand and the five complexes have been systematic investigated.•The optical band gaps of the ligand and the five complexes have been confirmed by the diffuse reflectance spectra.Five novel Zinc (II) metal-organic hybrid materials, [(ZnCl2L)6·6(H2O)·6(CHCl3)] (1), [(ZnBr2L)6·6(H2O)·6(CHCl3)] (2), [ZnI2L·2(CHCl3)]n(3), [Zn(SCN)2L·(CHCl3)]n(4), and [Zn(CHCOO)2L·2(H2O)]n(5), have been assembled by a novel ligand 4′-ferrocenyl-4, 2′: 6′, 4″-terpyridine and different zinc salts under ambient conditions, and their structures have been confirmed by single-crystal X-ray diffraction analysis. The results indicate that both complexes 1 and 2 adopt a metallohexacycle consisting of six ligands and six zinc ions, and complexes 3, 4 and 5 are all one-dimensional polymers. The supramolecular frameworks are constructed by complexes 1–5 through the C–H⋯X (X = Cl, Br, I, S, O or π) or π–π intermolecular interactions. In addition, the optical band gaps were confirmed by use of the optical diffuse reflectance spectra, and the results show that the ligand and complexes 1–5 should have semiconductor properties.Five novel metal-organic hybrid materials have been prepared by self-assembly method, and their structural features were systematic investigated.

•Four Zn(II) terpyridine complexes with different coordination anions were designed and synthesized.•The linear and nonlinear spectroscopic properties were carried out.•The results revealed that two Zn(II) complexes possessed larger TPA cross-sections.•The complex of ZnLCl2 showed superior optical power limiting property.Four novel Zn(II) terpyridine complexes (ZnLCl2, ZnLBr2, ZnLI2, ZnL(SCN)2) based on carbazole derivative group were designed, synthesized and fully characterized. Their photophysical properties including absorption and one-photon excited fluorescence, two-photon absorption (TPA) and optical power limiting (OPL) were further investigated systematically and interpreted on the basis of theoretical calculations (TD-DFT). The influences of different solvents on the absorption and One-Photon Excited Fluorescence (OPEF) spectral behavior, quantum yields and the lifetime of the chromophores have been investigated in detail. The third-order nonlinear optical (NLO) properties were investigated by open/closed aperture Z-scan measurements using femtosecond pulse laser in the range from 680 to 1080 nm. These results revealed that ZnLCl2 and ZnLBr2 exhibited strong two-photon absorption and ZnLCl2 showed superior optical power limiting property.Graphical abstractFour novel Zn(II) terpyridine complexes (ZnLCl2, ZnLBr2, ZnLI2, ZnL(SCN)2) based on carbazole derivative group were designed, synthesized and fully characterized. Their photophysical properties including absorption and one-photon excited fluorescence, two-photon absorption (TPA) and optical power limiting (OPL) were further investigated systematically and interpreted on the basis of theoretical calculations (TD-DFT). The results of NLO determination revealed that two Zn(II) complexes possessed larger two-photon absorption cross-sections and ZnLCl2 complex showed superior optical power limiting property which suggested a large polarizability, meeting the requirements for third-order NLO materials offering promise for applications in optical limiting in the near infrared region.

A specific series of D–π–A (1A–3A) and D–π–A (1B–3B) structural chromophores with various electron donors and π-conjugated bridges were designed, synthesized, and fully characterized. Their crystal structures were determined by single crystal X-ray diffraction analysis. The one/two-photon absorption properties of the chromophores have been successfully tuned by using different electron donors and π-bridges. Interestingly, it was found that chromophore 3B shows quite weak fluorescence in pure DMSO, while a significant AIE (aggregation-induced emission) effect is observed in water–DMSO (v/v 90%) mixtures with a sharp increase in fluorescence intensity of about 11 times. Furthermore, chromophore 3B shows strong two-photon excited fluorescence (TPEF) and has a large 2PA action cross-section (394 GM). The results of live-cell imaging experiments show that 3B can be effectively used as a bio-imaging probe in two-photon fluorescence microscopy towards HepG2 cells in vitro. The TPEF images of 3B not only exhibit cellular cytosol uptake but also exhibit actin regulatory protein uptake which are different from OPEF images.

To explore the photophysical properties of coordination compounds with enhanced two-photon absorption, two novel six-coordinated metal complexes (ML2, M = Cd(II), Zn(II)) from carbazole β-diketone ligand (HL = 4,4,4-trifluoro-1-(9-butylcarbazole-3-yl)-1,3-butanedione) were prepared and fully characterized. Their crystal structures were determined by X-ray diffraction analysis. Both variable temperature 1H NMR spectra and MALDI-TOF mass spectrometry proved that the coordination compounds exhibit good stability in solution. The results of time-dependent density functional theory (TD-DFT) calculations indicated that the complexation of the ligands with metal ion extends the electronic delocalization in the coordination compounds, leading to enhanced two-photon absorption. The photophysical properties for the coordination compounds were identified relying on both experimentally and theoretically studies. Finally, confocal microscopy and two-photon microscopy fluorescent imaging of HepG2 cells labeled with the Zn(II) complexe revealed its potential applications as a biological fluorescent probe.

•The molecular geometry optimization is based on crystal data.•The photophysical properties of them were systematically investigated.•The two-photon cross-sections were measured by two-photon excitation fluorescent and Z-scan methods.•The di- and hexa-branched pyrimidine derivatives exhibit large two-photon cross-section in 800–890 nm regions in DMF solution.•The potential application of the di-branched chromophores in two-photon biological imaging was evaluated.A series of mono-, di- and hexa-branched pyrimidine derivatives possessing two-photon absorption have been synthesized. The spectral features of the dyes showed solvatochromic with significant bathochromic shifting of the emission spectra and large Stokes shifts in more polar solvents mainly due to the intramolecular charge-transfer. The two-photon absorption cross-section values were measured by two-photon excited fluorescence and open aperture Z-scan methods. The results revealed that the two-photon absorption cross-section values were enhanced with increasing electron-donating strength of the end group and branch number. The photophysical properties of the dyes were further investigated with the aid of theoretical calculations. In addition, the di-branched dyes has the largest values of two-photon absorption cross-section per molecular weight, low cytotoxicity and can be internalized by human liver cancer cell line and accumulates in the cytoplasm, allowing for live cell two-photon fluorescence imaging, so it is promising for in vitro and vivo cellular imaging applications.

•Two novel dinuclear Cu(I) and Ag(I) complexes were designed, synthesized.•The molecular geometry optimization is based on crystal data.•The complexes display significantly enhanced two-photon absorption.•The Ag(I) complex possesses a large χ(3) in near IR region.A novel thiosemicarbazide ligand L and its dinuclear complexes M2(dppm)2L(NO3)2 [where dppm = bis(diphenylphosphino)methane, M = Cu(I) (1), Ag(I) (2)] were designed, synthesized and characterized by 1H NMR spectroscopy, elemental analysis, IR, MALDI-TOF mass spectroscopy, and single crystal X-ray diffraction analysis. The third-order nonlinear optical (NLO) properties of the metal complexes were determined by the open/close aperture Z-scan technique. The results reveal that the complexes possess much larger third-order nonlinear susceptibility and two-photon absorption cross sections than that of its free ligand L in the near IR region, especially for complex 2. The photophysical properties of them were investigated and interpreted on the basis of theoretical calculations (TD-DFT).A novel thiosemicarbazide ligand L and its dinuclear complexes M2(dppm)2L(NO3)2 [where dppm = bis(diphenylphosphino)methane, M = Cu(I) (1), Ag(I) (2)] were designed, synthesized and characterized by single crystal X-ray diffraction analysis.

•Two novel Nickel(II) complexes were designed, synthesized and fully characterized.•The Ni(II) complexes have higher electron delocalization.•The Nickel(II) complexes exhibit large two-photon absorption in narrow near-infrared range.•The Nickel(II) complexes exhibit superior optical power limiting property.•It possesses large values of the real part of the cubic hyperpolarizability χ(3).Two novel Nickel(II) complexes (NiL21 and NiL22) with remarkable two-photon absorption (TPA) and optical power limiting (OPL) properties were synthesized and fully characterized. Single crystals were obtained and solved by X-ray diffraction analysis. Their photophysical properties had been further investigated both experimentally and theoretically. The third-order nonlinear optical (NLO) properties (TPA and OPL) were investigated by open/closed aperture Z-scan measurements using femtosecond pulse laser in the range from 680 to 1080 nm. The results revealed that the two Nickel(II) complexes exhibited strong two-photon absorption and superior optical power limiting properties, which are much better than that of the free ligands.Graphical abstractTwo novel Nickel(II) complexes (NiL21 and NiL22) with remarkable two-photon absorption (TPA) and optical power limiting (OPL) properties were synthesized and fully characterized. Single crystals were obtained and solved by X-ray diffraction analysis. Their photophysical properties had been further investigated both experimentally and theoretically. The third-order nonlinear optical (NLO) properties (TPA and OPL) were investigated by open/closed aperture Z-scan measurements using femtosecond pulse laser in the range from 680 to 1080 nm. The results revealed that the two Nickel(II) complexes exhibited strong two-photon absorption and superior optical power limiting properties, which are much better than that of the free ligands.

•Synthesis and characterization of two novel 6′-phenyl-2,2′-bipyridine ligands (L1, L2) and their CdL1,2I2 complexes are presented.•Single crystal study indicated that Cd(II) ion is in a distorted tetrahedral structure.•Linear absorption and single-photon exited fluorescence properties have been investigated.•The results showed that the complex 2 has the longest fluorescence lifetime.•Linear absorption spectral features are studied by TD-DFT methods.Two novel 6′-phenyl-2,2′-bipyridine ligands (L1, L2) and their CdL1,2I2 complexes (1, 2) were synthesized and characterized by elemental analysis, 1H NMR, IR, MALDI-TOF spectroscopy, and single crystal X-ray diffraction analysis. The results reveal that the central cadmium(II) atom in the complexes was coordinated by two iodide ions and two nitrogen atoms from L1, L2, forming a distorted coordination geometry. The electronic absorption properties of them were investigated on the basis of theoretical calculations (TD-DFT).Two novel 6′-phenyl-2,2′-bipyridine ligands (L1, L2) and their CdL1,2I2 complexes (1, 2) were designed, synthesized. Spectral properties were studied based on both experimentally and theoretically.

Biological metal detecting, small molecule probes bearing nonlinear optical (NLO) response provide powerful alternatives due to their favorable photophysical properties (e.g. excitation wavelength in the near-IR region), cell permeability (due to their size), and chemical structure flexibility. Here, we present a series of pyrimidine-based NLO biological metal probes, especially a novel copper specific one which taken into account of the small volume-scaled and low cost-scaled nonlinear optical response of TPP and TPP–Cu2+ discussed in our present work. The photophysical properties of the probes were thoroughly investigated. 1H NMR and theoretical computation prove the binding interaction between the probe and the copper ion, which supports the functions of the molecule as a fluorescence signaling unit showing strong fluorescence quenching upon copper metal ion binding. On the other hand, the two-photon absorption cross-section of the novel copper probe increased from 275 to 591 GM (λex = 830 nm) after interacting with copper ion. It was further demonstrated that the NLO response for the copper(II) ion probe could be used for biological copper detection in live cells.

•A novel chromophore with A′−D−A configuration was reported.•The structure was determined by single crystal X-ray diffraction analysis.•The relationships between the structures and properties were investigated.•The two-photon absorption cross-sections were determined by Z-scan technique.•The results indicate that auxiliary moiety (A′) largely influence on its properties.A novel chromophore (Z)-ethyl-3-(4-((4-([2,2′:6′,2″-terpyridin]-4′-yl)phenyl) (phenyl) amino)phenyl)-2-cyanoacrylate(3), constructed with triphenylamine moiety as the electron donor (D), 2, 2: 6, 2-terpyridine moiety as an electron acceptor (A), and ethyl cyanocaetate group as an auxiliary electron acceptor (A′), has been designed and synthesized. The crystal structures of 3 and its mediator 2 (4″-(4′-(4-(Diphenylamino) phenyl) aldehyde)-2,2′:6′,2″-Terpyridine), have been determined by single crystal X-ray diffraction analysis. The linear and nonlinear spectra of these chromophores were investigated on the basis of experimental and calculation methods. The two-photon absorption (TPA) cross-sections of 1–3 were determined by a femtosecond open-aperture Z-scan technique. The maximum value of the TPA cross-section (σ) for 3 is 7938.3 GM in DMF solution. However, much weaker two-photon absorption responses were observed at the same condition for chromophore 1 and 2, respectively. The results of the work indicate that, the photophysical properties of the D–A configuration group are influenced largely by the auxiliary moiety (A′) attached.

A novel europium(III) β-diketonate complex exhibiting bright two-photon-sensitized luminescence is synthesized and applied as a two-photon-sensitized luminescent probe to stain DNA in live cells.

One novel bisferrocene pyrazole derivative, bis [2-(5-trifluoromethyl-3-ferrocenyl) pyrazolyl] methane (abbreviated as (3)), was synthesized and fully characterized. A single crystal of (3) was obtained and solved by X-ray diffraction analysis. The bisferrocene derivative exhibits MLCT (metal to ligand charge transfer) and π→π* transitions in the UV-visible range, which have been verified by density functional theory (DFT) calculations. Its electrochemical properties were studied with the aid of cyclic voltammetry (CV), differential pulse voltammetry (DPV) and rapid scan time-resolved Fourier transform infrared spectroscopy (RS-TRS FT-IR) analysis. Furthermore, the electrochemical mechanism was elucidated based on the results from the cyclic voltabsorptometry (CVA) determination technique. (3) apparently shows a single wave in the cyclic voltammetric experiments which indicates there is no intermediate, however, the intermediate of (3) was observed by employing the RS-TRS FT-IR spectroelectrochemistry technique. The detailed investigation brought us safely to the conclusion that the methylene can also act as a linker, leading to electronic communication in either D–π–D and A–π–A systems.

Two new metal-centered ferrocene complexes Ni(SCN)2(L)4 (1) and Cu(OAc)2(L)2 (2) (L = 1-[1-ferrocenylmethyl]imidazole) have been synthesized and characterized by elemental analysis, single crystal X-ray diffraction analysis, spectroscopic and cyclic voltammetric measurements. The geometry of Ni(II) in 1 is octahedral, with four ligands in the equatorial plan and two thiocyanate anions at the axial site, while that of Cu(II) in 2 is a distorted octahedron formed by two chealted OAc− and two ligands. Single crystal X-ray diffraction studies reveal that there is partial electron delocalization from ferrocene to imidazole in the two complexes. Electrochemical measurements exhibit that complexes 1 and 2 undergo similar reversible one electron redox processes, suggesting that the ferrocene moieties are equivalent and there are no interactions among them.

A series of carbazole-functionalized carboxylate ligands (N-carbazolylacetic acid (L1), 4-carbazol-9-yl-benzoic acid (L2), and 3-(4-carbazol-9-yl-phenyl)-acrylic acid (L3)) and their corresponding silver complexes were designed and synthesized and the structures were determined by single crystal X-ray diffractions. The silver atoms in the complexes are in tetrahedral geometry coordinated with two oxygen from carboxylic and two phosphorous atoms from triphenylphosphine. The complexes exhibit excellent electrochemical characters in solution and strong photoluminescence in the solid state. The emission wavelengths of the compounds can be tuned (from ultraviolet to visible region) by introducing of the second coordinating ligand triphenylphosphine and by elongation of the conjugation moieties.A series of carbazole-functionalized carboxylate ligands and silver complexes were obtained. The emissions of their photoluminescence can be tuned by introducing of the second coordinating ligand and/or by elongation of the conjugation moieties.

How can imaging be improved? Coordination polymers (CPs) show fascinating potential in optoelectronic optics but limited potential in bioimaging. Without doubt, it was very meaningful when CPs were first used in second-harmonic generation (SHG) imaging. Herein, through reasonable design and synthesis, a series of nonlinear optical CPs bearing very good one-photon excited fluorescence (OPEF), two-photon excited fluorescence (TPEF) and very strong SHG properties has been presented. Further study demonstrated that the nanoscale CPs show very strong SHG signals which have been applied in the three-dimensional imaging of thick block tissue with higher spatial resolution through simultaneous multichannel nonlinear optical (NLO) imaging technology. After simple encapsulation by polymeric micelles, the nanoscale CPs were successfully applied in SHG bio-imaging within the living cells. This finding throws light on the design of nanoscale NLO CPs and offers a simple avenue to develop novel effective exogenous SHG imaging agents.

A novel europium(III) β-diketonate complex exhibiting bright two-photon-sensitized luminescence is synthesized and applied as a two-photon-sensitized luminescent probe to stain DNA in live cells.

Three novel organooxotin complexes (Z1, Z2 and Z3) were synthesized by reaction of L1 (2-cyano-3-(4-(diphenylamino)phenyl) acrylic acid) with n-Bu2SnO, Ph3Sn(OH) and nBu6Sn2O. The structures of the three complexes have been confirmed by single-crystal X-ray diffraction analysis. The metal complex Z1 features a ladder framework while Z2 and Z3 show discrete structures. The UV-vis absorption, single-photon excited fluorescence (SPEF) and two-photon excited fluorescence (2PEF) of the complexes have been systematically studied, suggesting that the three metal complexes have strong two-photon absorption (2PA) and large 2PA cross-sections. It is noteworthy that Z2 possesses an enhanced two-photon absorption, and Z3 exhibits larger 2PA cross-section per molecular weight compared to L1. Finally, high anti-tumor activity of these three metal complexes has also been identified.

A novel 4′-(4-(diphenylamino)thienyl)-2,2′:6′,2′′-terpyridine ligand (L) based on thiophene and its LZnX2 complexes (X = Cl, Br, I, SCN) was designed, synthesized and characterized by elemental analysis, far-IR, MALDI-TOF-MS, and single crystal X-ray diffraction analysis. Structural studies revealed that the central zinc(II) atom adopted a distorted trigonal bipyramidal coordination model. However, there were different hydrogen bonds and stacking models with different counter anions in the crystals. The absorption properties of the compounds were investigated with the aid of TD-DFT computational methods. Furthermore, the third-order nonlinear optical (NLO) properties were systematically studied via open-aperture Z-scan methods using a tunable wavelength femtosecond laser. The results from photophysical property investigations suggested that the complexation of the thiophene-based terpyridine ligand with zinc halides resulted in strong ICT/LLCT bands of about 450 nm, and the complexes exhibited strong nonlinear optical response in the near-infrared range around 850 nm. Above all, the two-photon absorption (2PA) cross-section values (σ) were enhanced by coordination with zinc and influenced by halide ions, reaching up to 2583 GM (X = Br).

To explore the photophysical properties of coordination compounds with enhanced two-photon absorption, two novel six-coordinated metal complexes (ML2, M = Cd(II), Zn(II)) from carbazole β-diketone ligand (HL = 4,4,4-trifluoro-1-(9-butylcarbazole-3-yl)-1,3-butanedione) were prepared and fully characterized. Their crystal structures were determined by X-ray diffraction analysis. Both variable temperature 1H NMR spectra and MALDI-TOF mass spectrometry proved that the coordination compounds exhibit good stability in solution. The results of time-dependent density functional theory (TD-DFT) calculations indicated that the complexation of the ligands with metal ion extends the electronic delocalization in the coordination compounds, leading to enhanced two-photon absorption. The photophysical properties for the coordination compounds were identified relying on both experimentally and theoretically studies. Finally, confocal microscopy and two-photon microscopy fluorescent imaging of HepG2 cells labeled with the Zn(II) complexe revealed its potential applications as a biological fluorescent probe.

Biological metal detecting, small molecule probes bearing nonlinear optical (NLO) response provide powerful alternatives due to their favorable photophysical properties (e.g. excitation wavelength in the near-IR region), cell permeability (due to their size), and chemical structure flexibility. Here, we present a series of pyrimidine-based NLO biological metal probes, especially a novel copper specific one which taken into account of the small volume-scaled and low cost-scaled nonlinear optical response of TPP and TPP–Cu2+ discussed in our present work. The photophysical properties of the probes were thoroughly investigated. 1H NMR and theoretical computation prove the binding interaction between the probe and the copper ion, which supports the functions of the molecule as a fluorescence signaling unit showing strong fluorescence quenching upon copper metal ion binding. On the other hand, the two-photon absorption cross-section of the novel copper probe increased from 275 to 591 GM (λex = 830 nm) after interacting with copper ion. It was further demonstrated that the NLO response for the copper(II) ion probe could be used for biological copper detection in live cells.

One novel bisferrocene pyrazole derivative, bis [2-(5-trifluoromethyl-3-ferrocenyl) pyrazolyl] methane (abbreviated as (3)), was synthesized and fully characterized. A single crystal of (3) was obtained and solved by X-ray diffraction analysis. The bisferrocene derivative exhibits MLCT (metal to ligand charge transfer) and π→π* transitions in the UV-visible range, which have been verified by density functional theory (DFT) calculations. Its electrochemical properties were studied with the aid of cyclic voltammetry (CV), differential pulse voltammetry (DPV) and rapid scan time-resolved Fourier transform infrared spectroscopy (RS-TRS FT-IR) analysis. Furthermore, the electrochemical mechanism was elucidated based on the results from the cyclic voltabsorptometry (CVA) determination technique. (3) apparently shows a single wave in the cyclic voltammetric experiments which indicates there is no intermediate, however, the intermediate of (3) was observed by employing the RS-TRS FT-IR spectroelectrochemistry technique. The detailed investigation brought us safely to the conclusion that the methylene can also act as a linker, leading to electronic communication in either D–π–D and A–π–A systems.

Imaging of RNA in living cells is a potential tool to understand intracellular RNA function. Therefore, an effective two-photon fluorescent probe with a reasonable two-photon action cross-section to label RNA is now urgently required. In this work, a series of novel two-photon absorbing terpyridine ZnX2 (X = Cl, Br, I) complexes have been designed and an effective RNA imaging probe has been obtained. The results revealed that OTP-ZnCl2 possesses large Stokes shift and two-photon action cross-section. Furthermore, live cell imaging experiments indicated that OTP-ZnCl2 could stain nucleoli in living cells by binding with nucleoli RNA. The mechanism of selective nucleoli staining of OTP-ZnCl2 was studied systematically via both experiments and molecular modeling calculations. Due to its low cytotoxicity, good membrane permeability and counterstain compatibility with the commercial fluorescent nucleic acid dye Hoechst 33342, as well as its ability to label RNA in living cells, OTP-ZnCl2 is a promising candidate for the detection of nucleic acid in living cells.

As a semi-autonomous organelle, the eukaryotic mitochondrion possesses an individual DNA and protein synthesizing system. Therefore, the mitochondrial DNA-targeting probes are powerful tools to understand the functions in physiological processes. Herein, we report a novel Ru(II) complex (HLRu) based on a phenanthroline derivative to target mitochondrial DNA. The nonlinear optical property study revealed a reverse nonlinear optical refraction character from self-focusing to self-defocusing response before and after complexation with Ru(II). Furthermore, the two-photon absorption and high biocompatibility of complex HLRu highlight its potential applications in biological processes. It was found that complex HLRu specifically binds to mitochondrial DNA in living cells and enables imaging of tissues with minimal auto-fluorescence. As a result, this complex HLRu probe offers a promising platform to directly monitor mitochondrial DNA in living cells.

It is still a challenge to obtain two-photon excited fluorescent bioimaging probes with intense emission, high photo-stability and low cytotoxicity. In the present work, four Zn(II)-coordinated complexes (1–4) constructed from two novel D–A and D–π–A ligands (L1 and L2) are investigated both experimentally and theoretically, aiming to explore efficient two-photon probes for bioimaging. Molecular geometry optimization used for theoretical calculations is achieved using the crystallographic data. Notably, the results indicate that complexes 1 and 2 display enhanced two-photon absorption (2PA) cross sections compared to their corresponding D–A ligand (L1). Furthermore, it was found that complex 1 has the advantages of moderate 2PA cross section in the near-infrared region, longer fluorescence lifetime, higher quantum yield, good biocompatibility and enhanced two-photon excited fluorescence. Therefore, complex 1 is evaluated as a bioimaging probe for in vitro imaging of HepG2 cells, in which it is observed under a two-photon scanning microscope that complex 1 exhibits effective co-staining with endoplasmic reticulum (ER) and nuclear membrane; as well as for in vivo imaging of zebrafish larva, in which it is observed that complex 1 exhibits specificity in the intestinal system.

A specific series of D–π–A (1A–3A) and D–π–A (1B–3B) structural chromophores with various electron donors and π-conjugated bridges were designed, synthesized, and fully characterized. Their crystal structures were determined by single crystal X-ray diffraction analysis. The one/two-photon absorption properties of the chromophores have been successfully tuned by using different electron donors and π-bridges. Interestingly, it was found that chromophore 3B shows quite weak fluorescence in pure DMSO, while a significant AIE (aggregation-induced emission) effect is observed in water–DMSO (v/v 90%) mixtures with a sharp increase in fluorescence intensity of about 11 times. Furthermore, chromophore 3B shows strong two-photon excited fluorescence (TPEF) and has a large 2PA action cross-section (394 GM). The results of live-cell imaging experiments show that 3B can be effectively used as a bio-imaging probe in two-photon fluorescence microscopy towards HepG2 cells in vitro. The TPEF images of 3B not only exhibit cellular cytosol uptake but also exhibit actin regulatory protein uptake which are different from OPEF images.

Designing small molecules with large two-photon absorption cross sections is urgently needed. In the present work, six novel chalcone thiophene derivatives were obtained by tuning the terminal electron-withdrawing/donating groups rationally. Their structure–property relationship was investigated both experimentally and theoretically. Crystallographic studies revealed that structural diversity was mainly influenced by ethyloxy groups. Both the alternation of electron-withdrawing/donating groups and the polarity of different solvents had a considerable influence on their photophysical properties. The results of UV-vis and one-/two-photon excited emission spectra showed that the high polarity solvents increased the quantum yields (Φ) and two-photon absorption cross-sections (σ) of type C chalcones, while they decreased those of type OC chalcones. It should be highlighted that as they are small molecules, favorable Φ and σ values were obtained. Finally, a preliminary attempt has been made in the biological imaging field, with satisfactory results that C-1 and OC-1 could locate uniformly in a cytosolic membrane-like system.

A series of asymmetrical D–π–D type thiophene-based chromophores with small sizes and different electron-donating groups were designed and synthesized in high yield. Single-crystal X-ray diffraction analysis and theoretical calculations were carried out to further explore the electronic structural features of the chromophores. It was observed that multiple C–H⋯π interactions and C–H⋯O hydrogen bonds played an important role in crystal stacking. Systematic investigations, including fluorescence quantum yields, fluorescence lifetime, and two-photon absorption (2PA) cross sections, revealed that the photophysical properties of the thiophene-based chromophores can be tunable by the modulation of electron-donating terminal units. Furthermore, by introducing a carbonyl group in the π-bridge center, one of the chromophores with a small size exhibited significantly enhanced two-photon absorption with a 36-fold increase of the 2PA cross section compared with that of the corresponding chromophore. Finally, due to their superior 2PA character in the near-infrared region (700–900 nm), two-photon excited bioimaging applications were carried out for the chromophores. The results of live cell imaging experiments showed that the chromophores can be effectively penetrated into the cytosol of HepG2 cells and their physiological activity can be detected in vitro using two-photon fluorescence microscopy.