Two-photon Absorption Cross-section Research Articles

Efficiently emissive, strongly solvatochromic and lipid droplet-specific, fluorescent probes for mapping polarity in vitro

The abnormal change of lipid droplets (LDs) polarity has been believed to be closely associated with many diseases such as fatty liver. Thus, the detection of LDs polarity plays a particular role in the diagnosis of LDs polarity-related diseases. Herein, two isoindole-based fluorescent probes have been derived and they show evident solvatochromic effects, large two-photon absorption cross-section, LDs-specificity, and high photostability and excellent performance in monitoring LDs-polarity changes in living cells. Owing to the existence of intramolecular charge transfer process, these isoindole-based probes exhibited high sensitivity between the emission wavelength maxima and solvent polarity. In spite of that, they maintained high fluorescence quantum yields up to 0.87. Moreover, the detection of LDs-polarity was demonstrated in living cells via confocal laser scanning fluorescence imaging. Importantly, the isoindole-based probe maintained strong fluorescence with high contrast and clarity after seven days of degradation in a zebrafish model, indicating that it is photostable enough and suitable for long-term tracking and observation of biological dynamic changes. All these features render the isoindole chromophore an effective implement for the diagnosis of LDs polarity-related diseases.

Theoretical Study of a Two-Photon Fluorescent Probe Based on Nile Red Derivatives with Controllable Fluorescence Wavelength and Water Solubility.

Hypochloric acid (HOCl) plays a vital role in the natural defense system, but abnormal levels of it can cause cell damage, accelerated human aging, and various diseases. It is of great significance to develop new probes for detecting HOCl in biosystems nondestructively and noninvasively. The purpose of this work is to explore new chemical modification strategies of two-photon excitation fluorescence (TPEF) probes to improve the poor water solubility and low efficiency in imaging applications. Nil-OH-6 has a two-photon absorption cross-section value as high as 243 GM and attains a good quantum yield of 0.49. In addition, the modification of terminal groups with different azetidine-heterospirocycles or N,N-dialkyl fused amino groups to Nile Red can effectively improve the fluorescence efficiency as well as increase the solubility to some extent. This study provides some strategies to simultaneously improve the fluorescence performance and solubility of these two-photon probes and, hence, reliable guidance and a foundation for the subsequent synthesis of TPEF probes based on Nile Red.

Anthracene Cu(I) compounds with fluorescence and two-photon absorption properties

A new ligand (L), 2-(2,3,6,7-tetramethoxy-9-(pyridin-2-yl)-9,10-dihydroanthracen-10-yl) pyridine, has been designed and synthesized. The anthracene ring in the ligand is curved and cannot form a large π bond conjugated system. Ligands have no fluorescence or other optical properties. When the ligand reacts with cuprous nitrate (CuNO3) to form complex (1), [LCuNO3], the anthracene ring magically straightens, and the maximum deviation of all the atoms on the anthracene ring from the anthracene ring plane is only 0.05873 Å. Complex (1) has a large two-photon absorption cross-sectional area and strong fluorescence properties. The nonlinear absorption of complex (1) (in DMF) was measured by the open-aperture Z-scan technique at a 532 nm wavelength. The results of Z-scan experiments show that complex (1) has remarkable nonlinear optical absorption effects with the two-photon absorption cross-section σ = 5969.78 GM. The ligand (L) and complex (1) all have certain thermal stability and their melting points are 169 °C for (L), and 297 °C for (1), respectively. Their properties indicate that they have potential applications.

Quantum metrology of two-photon absorption

Two-photon absorption (TPA) is of fundamental importance in super-resolution imaging and spectroscopy. Its nonlinear character allows for the prospect of using quantum resources, such as entanglement, to improve measurement precision or to gain new information on, e.g., ultrafast molecular dynamics. Here, we establish the metrological properties of nonclassical squeezed light sources for precision measurements of TPA cross sections. We find that there is no fundamental limit for the precision achievable with squeezed states in the limit of very small cross sections. Considering the most relevant measurement strategies -- namely photon counting and quadrature measurements -- we determine the quantum advantage provided by squeezed states as compared to coherent states. We find that squeezed states outperform the precision achievable by coherent states when performing quadrature measurements, which provide improved scaling of the Fisher information with respect to the mean photon number $\sim n^4$. Due to the interplay of the incoherent nature and the nonlinearity of the TPA process, unusual scaling can also be obtained with coherent states, which feature a $\sim n^3$ scaling in both quadrature and photon-counting measurements.

Optical, Electrochemical, Third-Order Nonlinear Optical Investigations of 3,4,5-Trimethoxy Phenyl Substituted Non-Aqueous Phthalocyanines.

A new series of non-aqueous phthalocyanines having 3,4,5-trimethoxy phenyl group at peripheral positions in which the central cavity possessing Cu(II), Zn(II), and without metals has been synthesized, and its absorption, fluorescence (steady-state and excited state lifetimes), electrochemical, and third-order nonlinear optical (NLO) properties were evaluated. Absorption studies data suggest that all three phthalocyanines obey Beer–Lambert’s law, and the redox properties indicate that both oxidation and reduction reactions are macrocyclic centered. The singlet quantum yields were measured in different solvents and were found to be in the range of 0.2–0.5 in the case of free-base, whereas it was in the range of 0.1–0.5 in zinc derivative, while the time-resolved fluorescence data revealed lifetimes of typically a few ns. The third-order NLO properties were investigated using the Z-scan technique with kilohertz (for retrieving true electronic nonlinearities) and megahertz repetition rate femtosecond pulses at 800 nm. Intensity-dependent Z-scan studies revealed robust NLO coefficients for solutions and thin films (two-photon absorption cross-sections of 9,300–57,000 GM) of these molecules suggesting a strong potential for optical switching, imaging, and optical limiting applications.

Subcellular Targeted Nanohoop for One- and Two-Photon Live Cell Imaging.

Fluorophores are powerful tools for interrogating biological systems. Carbon nanotubes (CNTs) have long been attractive materials for biological imaging due to their near-infrared excitation and bright, tunable optical properties. The difficulty in synthesizing and functionalizing these materials with precision, however, has hampered progress in this area. Carbon nanohoops, which are macrocyclic CNT substructures, are carbon nanostructures that possess ideal photophysical characteristics of nanomaterials, while maintaining the precise synthesis of small molecules. However, much work remains to advance the nanohoop class of fluorophores as biological imaging agents. Herein, we report an intracellular targeted nanohoop. This fluorescent nanostructure is noncytotoxic at concentrations up to 50 μM, and cellular uptake investigations indicate internalization through endocytic pathways. Additionally, we employ this nanohoop for two-photon fluorescence imaging, demonstrating a high two-photon absorption cross-section (65 GM) and photostability comparable to a commercial probe. This work further motivates continued investigations into carbon nanohoop photophysics and their biological imaging applications.

Substituent Modulation for Highly Bright 9-Borafluorene Derivatives with Carbazole Pendant.

A series of 3,6-di-tert-butyl carbazole-functionalized 9-borafluorene derivatives have been prepared with outstandingly strong photoluminescence with quantum yields up to ca. 100 and 94% for Mes*BF-pCz in solution and film, respectively. 1,3,5-Tris(trifluoromethyl)benzene (FMes)-substituted compounds exhibit enhanced Lewis acidity with coordination to weak nucleophiles like tetrahydrofuran, resulting in a long afterglow at low temperature. The large two-photon absorption cross-section of ca. 1103 GM for Mes*BF-pCz at 800 nm in CH2Cl2 indicated its potential application in bioimaging.

Synthesis and characterization of the novel 4-(1-(pyridin-4-yl) ethoxyl) substituted bis(phthalocyaninato) rare earth complexes and investigation of their two-photon absorption-based third-order non-linear optical properties

Novel 4-(1-(pyridin-4-yl) ethoxyl) substituted double-decker rare earth phthalocyanine complexes were synthesized starting from (R)-4-(1-(pyridin-4-yl) ethoxy) phthalonitrile with proper rare earth metal acetates in octanol catalyzed by 1,8-diazabicyclo[5.4.0]undec‑7-ene at 300 °C. The new starting phthalonitrile compound was obtained from (R)-1-(pyridin-4-yl) ethan-1-ol and 4-nitrophthalonitrile in acetonitrile at reflux temperature in the presence of potassium carbonate as a catalyst. The characterization of the synthesized and isolated compounds was performed by elemental analysis, infrared, ultraviolet-visible, proton and correlation spectroscopy nuclear magnetic resonance, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopic methods. Nonlinear optical responses of all complexes are investigated using the Z-scan method. The nonlinear absorption coefficient, two-photon absorption cross-section, and the imaginary part of the nonlinear susceptibility of complexes are calculated for each discrete concentration. All these nonlinear parameters of our rare-earth coordinated double-decker phthalocyanines present a decrement due to the aggregation under high optic powers, which result in sequential two-photon absorption for smaller ionic radii.

Functionalizing Collagen with Vessel-Penetrating Two-Photon Phosphorescence Probes: A New In Vivo Strategy to Map Oxygen Concentration in Tumor Microenvironment and Tissue Ischemia.

The encapsulation and/or surface modification can stabilize and protect the phosphorescence bio‐probes but impede their intravenous delivery across biological barriers. Here, a new class of biocompatible rhenium (ReI) diimine carbonyl complexes is developed, which can efficaciously permeate normal vessel walls and then functionalize the extravascular collagen matrixes as in situ oxygen sensor. Without protective agents, ReI‐diimine complex already exhibits excellent emission yield (34%, λ em = 583 nm) and large two‐photon absorption cross‐sections (σ 2 = 300 GM @ 800 nm) in water (pH 7.4). After extravasation, remarkably, the collagen‐bound probes further enhanced their excitation efficiency by increasing the deoxygenated lifetime from 4.0 to 7.5 µs, paving a way to visualize tumor hypoxia and tissue ischemia in vivo. The post‐extravasation functionalization of extracellular matrixes demonstrates a new methodology for biomaterial‐empowered phosphorescence sensing and imaging.

Strong near-infrared and ultrafast femtosecond nonlinearities of a covalently-linked triply-fused porphyrin dimer-SWCNT nanohybrid

Functional materials displaying large ultrafast third-order optical nonlinearities across a wide spectral region and broad temporal domain are required for all-optical signal processing. Particularly desirable is nonlinear optical (NLO) activity at near-infrared (NIR) wavelengths with femtosecond pulses. Herein the first triply-fused porphyrin dimer (TFP)-functionalized single-walled carbon nanotube (SWCNT) nanohybrid was successfully constructed by covalently grafting TFPs onto SWCNT. The results of Z-scan techniques demonstrate that the newly obtained TFP-SWCNT nanohybrid was found with a strong NLO performance under both nanosecond and femtosecond irradiation. In the nanosecond regime, an enhancement in optical limiting (OL) of the TFP-SWCNT nanohybrid is seen at 532 nm when compared with the performance of porphyrin monomer-functionalized SWCNT nanohybrid Por-SWCNT. Under femtosecond irradiation, the TFP-SWCNT nanohybrid exhibits a particularly strong OL effect with a giant two-photon absorption (TPA) cross section value (ca. 15,500 GM) at 800 nm pulses that mainly stems from intense TPA of TFP, in sharp contrast to the Por-SWCNT nanohybrid which exhibits only saturable absorption under identical irradiation. These results demonstrate that the newly-developed TFP-SWCNT nanohybrid is a very promising OL candidate for practical applications across wide spectral and temporal domains, and that covalently functionalizing carbon-based materials with triply-fused chromophores may be a useful approach to engineering adaptable photonic devices with broad-ranging NLO activity.

Preparation and linear/nonlinear optical properties of a gold-terpyridine nanohybrid constructed through thiocyanate coordinating bridge

In this study, a gold-based nanohybrid was designed and prepared, which was constructed through interfacial coordination interaction between gold nanoparticals (Au NPs) and terpyridine-Zn (II) complexes (L-Zn(NCS)2, L = N,N-diphenyl-4-(2,2':6′,2″-tepyridin-4-yl)aniline) with thiocyanate unit as the coordination bridge. The optimized molar ratio of L-Zn(NCS)2 to chloroauric acid was selected as 1:15 through UV-vis absorption spectra. Compared with that of pure Au NPs or free L-Zn(NCS)2, the electron absorption of the nanohybrid was enhanced and the single photon excited fluorescence was weakened. Enhanced nonlinear optical properties and enlarged two-photon absorption cross section (δTPA = 21870.56 GM) were also captured. Furthermore, the nanohybrid exhibited excellent photothermal conversion performance, which was beneficial to be applied in phototherapy against living cancer cells. HeLa cells co-cultivated with the nanohybrid were gradually apoptotic within 10 min irradiation at 860 nm.

Two-photon absorption characterization and comparison of Au(I) fluorenyl benzothiazole complexes in the visible wavelength regime

We use the two-photon excited fluorescence method to determine the two-photon absorption (2PA) cross sections of three series of (fluorenyl benzothiazole) gold(I) complexes in the visible wavelength range from 570 to 700 nm. We compare the effect of ancillary ligand substitutions on the 2PA magnitudes and find that the ancillary ligand does not drastically affect either the magnitude or the shape of 2PA. Even so, moderate 2PA cross sections were measured that ranged from 10 to 1000 s of GM (Göppert-Mayer, =10-50cm4s/photon), making these types of complexes nonlinear optical materials for two-photon absorbing applications.

Octupolar Acrylonitrile-Bridged 2D-Conjugated Polymers Enable Bright Far-Red Emission with Intense Two-Photon Absorption via Alkoxylation Chemistry.

Herein, alkoxylation chemistry is introduced as a "one-stone-three-birds" solution for exploring a new family of highly-fluorescent octupolar 2D-conjugated organic polymers/frameworks (OCOPs/OCOFs) combining far-red emission, high fluorescence quantum yield (QY), and strong two-photon absorption (TPA). Both alkoxy-substituted OCOP and OCOF comprising acrylonitrile-bridged strongly-coupled donor3-(acceptor core) chromophores densely packed in either disordered or ordered forms, exhibit significantly redshifted emission. They produce high QY of 22.2% and 27.8% in tetrahydrofuran, large TPA cross section of 600 and 1124 GM, and 2-3 folds and 15-30 folds that of non-alkoxylate amorphous counterpart respectively. Combined theoretical and experimental studies reveal unique "one-stone-three-birds" role of the alkoxylation in realizing red-shifted-emission, improved QY and TPA enabled by inducing steric hindrance effect for weakened π-π stacking, and triggering p-π conjugation effect for electronically engineering octupolar chromophores, while the crystalline engineering enables enforced coplanarity conformation and improved π-electron delocalization for further improved QY and TPA. The robust and biocompatible pentoxy-substituted polymer can be used not only as metal-free red-emissive phosphor for efficient warm white light-emitting diodes, but also as efficient two-photon fluorescence probes for bio-imaging.

Impact of Five-membered Heterocyclic Rings on Photophysical Properties Including Two-photon Absorption Character

Abstract A series of dipolar-type terphenyl derivatives 1–3 featuring an electron donor (p-NMe2C6H4 group) and acceptor (p-NO2C6H4 group) unit was synthesized and their photophysical properties were investigated. By using aromatic heterocycles (thiophene, furan, pyrrole) as the π-linkers between the donor and acceptor units, significant effects on the bathochromic shift and enhancement of the two-photon absorption wavelength and the two-photon absorption cross-sections were observed, respectively.

Rational design, facile synthesis, and linear/nonlinear optical properties of novel two-photon absorption stilbene derivatives with different configurations

Four novel two-photon absorption compounds (1MOBI, 2MOHA, 3BrBI, and 4BrHA), which have assembled stilbene π-bridge, different electron acceptors and donors, were synthesized conveniently via the one-step Horner-Wadsworth-Emmons reaction. Their configurations are A-π-D-π-A, D′-π-D-π-D′, A′-π-A-π-A′, and D-π-A-π-D, respectively. Their linear and nonlinear optical properties including linear absorption, one-photon excited fluorescence, two-photon absorption, and two-photon excited fluorescence, were systematically investigated in various solvents. The results demonstrate that the compounds with dihexylamino donors exhibit good optical properties. 2MOHA and 4BrHA exhibit large two-photon absorption cross-sections (2183 and 1737 GM) in DMF, which are larger than those of many known stilbene derivatives. The relationships between the structures and the optical properties were studied aided with the time-dependent density functional theory calculations.

Impact of Changing the Core in Tetrapyrrolic Dendrimers Designed for Oxygen Sensitization: New Fluorescent Phthalocyanine-Based Dendrimers with High Two-Photon Absorption Cross-sections

In the continuation of our sustained interest for porphyrin-based dendrimers and their use as luminescent photo-sensitizers for two-photon photodynamic therapy (2P-PDT), we wondered about the effect of changing the central porphyrin core for a phthalocyanine core in these macromolecular structures. Thus, related phthalocyanine-based dendrimers possessing up to 16 conjugated 9,9-dibutyl-2-fluorenyl endgroups at their periphery were now prepared and studied. As for porphyrin analogues, it was found that an efficient energy transfer occurs from the peripheral fluorenyl units toward the central phthalocyanine unit of these compounds, leading to intense red light emission and photosensitization of oxygen. While the linear optical properties of these new dendrimers are only slightly improved compared to those of their porphyrin-cored analogues for PDT and fluorescence imaging, their two-photon absorption (2PA) cross-sections are much more significantly boosted, evidencing the key role played by the central tetrapyrrolic unit in this respect. The impact of this structural change in relation with the impact resulting from the change in dendrimer generations is then discussed with the help of DFT and molecular dynamic calculations.

Physical Mechanism of Photoinduced Charge Transfer in One- and Two-Photon Absorption in D-D-π-A Systems.

The photoinduced charge transfer process of a D-π-A molecule (W1) and three D-D-π-A molecules (WS5–WS7) with triphenylamine as a donor was studied theoretically. D-D-π-A molecules are formed by inserting donors between the triphenylamine and π-linker (π-bridge) on the base of the W1 molecule. The results showed that donor insertion resulted in a red shift in the absorption spectrum, and the absorption intensity increased to a certain extent. A visualization method was used to observe the charge transfer of the four molecules in the process of one- and two-photon absorption (TPA). The local excitation enhanced charge transfer excitation in the TPA process was analyzed and discussed, and the insertion of the thiazolo[5,4-d]thiazole donor showed the largest TPA cross-section. This work contributed to the profound understanding of D-D-π-A molecules and the design of large cross-section TPA molecules.

Nonlinear optical properties of a new panchromatic series of water-soluble bicationic push-pull fluorophores

Six water-soluble bications, characterized by a symmetric A+-π-D-π-A+ structure, were synthesized and comprehensively studied for their absorption and fluorescence properties, and their nonlinear optical response. Generally, the photobehavior exhibited by the six systems was found to be dependent on the molecular dimensionality and conjugation, unveiling the role of different electron-donor (dimethoxy-phenylene vs bithiophene) and electron-acceptor (methyl-pyridinium, methyl-quinolinum, and pyrimidin-pyridinium) substituents bound in different positions (ortho/para). Steady-state measurements revealed this series as a panchromatic family of dyes with noticeable absorption and emission abilities, covering the entire visible spectrum - from the near-ultraviolet to the near-infrared spectral region. Femtosecond-resolved spectroscopy disclosed the presence of ultrafast intramolecular charge transfer processes, as also predicted by TD-DFT calculations, from the central electron-rich moiety of the molecule towards the two lateral positively charged units. Further and for the first time, a solvatochromic method was successfully applied to this kind of quadrupolar double-charged compounds in order to estimate their static and dynamic second-order hyperpolarizability coefficients. Thanks to their appreciable fluorescence capability, significant Two-Photon Absorption cross-sections were also measured for all six dyes by means of the Two-Photon Excited Fluorescence technique. These findings, together with their good water solubility, designate these eye-catching fluorophores as potential candidates for bioimaging applications.

Oxidative C-H Homocoupling of Push-Pull Benzothiazoles: An Atom-Economical Route to Highly Emissive Quadrupolar Arylamine-Functionalized 2,2'-Bibenzothiazoles with Enhanced Two-Photon Absorption.

Copper(II)-catalyzed C-H/C-H coupling of dipolar 2-H-benzothiazoles end-capped with triphenylamine moieties affords highly fluorescent 2,2'-bibenzothiazoles with quadrupolar (D-π-A-π-D) architecture displaying large two-photon absorption (TPA) cross sections (543-1252 GM) in the near-infrared region. The notably higher TPA performance as compared to quadrupolar π-systems with a widely used 2,2'-bipyridine core, along with the ease of the synthesis and chelating N^N ability, makes the title biheteroaryl platform an attractive building block for a large scope of functional dyes exploiting nonlinear optical phenomena.

Enhanced optical nonlinearities under collective strong light-matter coupling

Optical microcavities and metallic nanostructures have been shown to significantly modulate the dynamics and spectroscopic response of molecular systems. We present a study of the nonlinear optics of a model consisting of $N$ anharmonic multilevel systems (e.g., Morse oscillators) undergoing collective strong coupling with a resonant infrared microcavity. We find that, under experimentally accessible conditions, molecular systems in microcavities may have nonlinear phenomena significantly intensified due to the high quality of polariton resonances and the enhanced microcavity electromagnetic energy density relative to free space. Particularly large enhancement of multiphoton absorption happens when multipolariton states are resonant with bare molecule multiphoton transitions. In particular, our model predicts two-photon absorption cross section enhancements by several orders of magnitude relative to free space when the Rabi splitting $\Omega_R$ is approximately equal to the molecular anharmonic shift $2\Delta$. Our results provide rough upper bounds to resonant nonlinear response enhancement factors as relaxation to dark states is treated phenomenologically. Notably, ensembles of two-level systems undergoing strong coupling with a cavity (described by the Tavis-Cummings model) show no such optical nonlinearity enhancements, highlighting the rich phenomenology afforded by multilevel anharmonic systems. Similar conclusions are expected to hold for excitonic systems that share features with our model (e.g., molecular dyes with accessible S_0 -> S_1 -> S_2 transitions) and strongly interact with a UV-visible cavity.