Large 2PA Cross-sections Research Articles

High-performance two-photon absorption optical limiter and stabilizer based on phase-pure thick-shell CdSe/CdS core/shell quantum dots

To achieve high-sensitivity two-photon absorption (2PA) optical limiters and stabilizers, two-photon active materials need to have large 2PA cross-sections. Phase-pure wurtzite (WZ) CdSe/CdS core/shell quantum dots (QDs) with a shell thickness of 11 CdS monolayers were prepared by a high-temperature pyrolysis method, which possess a large volume and nearly defect-free core/shell interfaces. The 2PA cross-section of QDs is measured to be as large as 1.5 × 105 GM by the nonlinear transmittance method. Due to the large 2PA cross-section of phase-pure thick-shell WZ CdSe/CdS core/shell QDs, we successfully explored their application in the field of optical limiting and stabilization. Finally, we successfully fabricated a high-sensitivity optical stabilizer made of a polymer (polymethylmethacrylate) matrix comprising phase-pure thick-shell WZ CdSe/CdS core/shell QDs, which can reduce the amplitude fluctuation by ∼67%. In addition, the device reduces the input energy by ∼40%, indicating that the device can also be applied as an optical limiter. This work promotes the application of optical limiters and stabilizers based on QDs in practical work to a certain extent.

New Two-Photon Absorbing Squaraine Derivative with Efficient Near-Infrared Fluorescence, Superluminescence, and High Photostability

The nature of linear photophysical and nonlinear optical properties of a new squaraine derivative 2,4-bis[4-(azetidyl)-2-hydroxyphenyl]squaraine (1) with efficient near-infrared (NIR) emission was comprehensively analyzed based on spectroscopic, photochemical, and two-photon absorption (2PA) measurements, along with quantum chemical analysis. The steady-state absorption, fluorescence, and excitation anisotropy spectra of 1 and its fluorescence emission lifetimes revealed the multiple aspects of the electronic structure of 1, including the relative orientations of the main transition dipoles, effective rotational volumes in solvents of different polarities, and a maximum molar extinction of 1.35 × 10-5 M-1·cm-1, which is unusually small for similar symmetric squaraines. The degenerate 2PA spectrum of 1 was obtained over a broad spectral range under femtosecond excitation, using standard open-aperture Z-scan and two-photon induced fluorescence methods, revealing maximum 2PA cross sections of ∼400 GM. Squaraine 1 exhibited efficient superluminescence emission in the polar solvent (dichloromethane) at room temperature under femtosecond pumping conditions. Quantum chemical analysis of the electronic structure of 1 was performed using the DFT/TD-DFT level of theory and found to be in good agreement with experimental data. The new squaraine derivative 1 displayed high fluorescence quantum yield, efficient NIR superluminescence, large 2PA cross sections, and high photostability with a photodecomposition quantum yield ∼4 × 10-6, suggesting its potential for applications in two-photon fluorescent bioimaging and lasing.

Going beyond the borders: pyrrolo[3,2-b]pyrroles with deep red emission.

A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2-b]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500–600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520–670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section (σ2PA) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800–900 nm with the maximum σ2PA ∼50–250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca. −5.35 eV making them relatively electron rich in spite of the presence of two B−–N+ dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2-b]pyrrole core, directly to the di(radical cation) stage.

Polymerization-Enhanced Two-Photon Photosensitization for Precise Photodynamic Therapy.

Two-photon excited photodynamic therapy (2PE-PDT) has attracted great attention in recent years due to its great potential for deep-tissue and highly spatiotemporally precise cancer therapy. Photosensitizers (PSs) with high singlet oxygen (1O2) generation efficiency and large two-photon absorption (2PA) cross-sections are highly desirable, but the availability of such PSs is limited by challenges in molecular design. In this work, we report that the polymerization of small-molecule PSs with aggregation-induced emission (AIE) could yield conjugated polymer PSs with good brightness, high 1O2 generation efficiency, and large 2PA cross-sections. A pair of conjugated polymer PSs were designed and synthesized, and the corresponding AIE PS dots were prepared by nanoprecipitation, which exhibited outstanding 2PE-PDT performance in in vitro cancer cell ablation and in vivo zebrafish liver tumor treatment. Our work highlights a strategy to design highly efficient PSs for 2PE-PDT.

Two-Photon Absorption and Two-Photon-Induced Gain in Perovskite Quantum Dots.

Perovskite quantum dots (PQDs) emerged as a promising class of material for applications in lighting devices, including light emitting diodes and lasers. In this work, we explore nonlinear absorption properties of PQDs showing the spectral signatures and the size dependence of their two-photon absorption (2PA) cross-section, which can reach values higher than 106 GM. The large 2PA cross section allows for low threshold two-photon induced amplified spontaneous emission (ASE), which can be as low as 1.6 mJ/cm2. We also show that the ASE properties are strongly dependent on the nanomaterial size, and that the ASE threshold, in terms of the average number of excitons, decreases for smaller PQDs. Investigating the PQDs biexciton binding energy, we observe strong correlation between the increasing on the biexciton binding energy and the decreasing on the ASE threshold, suggesting that ASE in PQDs is a biexciton-assisted process.

New frog-type Dibenzo[a,c][1,2,5]thiadiazolo[3,4-i]phenazine heterocyclic derivatives with aggregation-enhanced one- and two-photon excitation NIR fluorescence

Enabling organic fused heterocycles to exhibit aggregation-induced (or enhanced) near infrared (NIR) emission (AIE/AEE) and solution processability is very interesting but rarely reported since the large π-plane and the strong intramolecular charge transfer are prone to quench the fluorescence emission. In this article, a new solution-processable donor–acceptor frog-type dibenzo[a,c][1,2,5]thiadiazolo[3,4-i]- phenazine (DTP) derivative was designed and synthesized. It was characteristic of strong electron donors and solubilizing alkoxyphenyl groups linking respectively at the 3,6- and 10,14-positions of DTP core. The existence of multiple twisted and rotatable aryl moieties enabled this kind of fused heterocycles to exhibit aggregation-enhanced emission (AEE) feature. Benefiting from the extended π-plane and AEE effect, the very large 2PA cross section and the aqueous aggregation-enhanced one- and two-photon excitation NIR fluorescence were achieved, and the preliminary experiment evidenced its applicable in fluorescence bioimaging.

Solvent Effect on the Third-Order Nonlinear Optical Properties of α- and β-Tertbutyl Phenoxy-Substituted Tin(IV) Chloride Phthalocyanines.

This paper investigates the third-order nonlinear optical properties of 4α-(4-tert-butylphenoxy) phthalocyaninato dichlorotin(IV) (α-SnOtBpPc) and 4β-(4-tert-butylphenoxy) phthalocyaninato dichlorotin(IV) (β-SnOtBpPc) in different organic solvents. The third-order susceptibilities of α-SnOtBpPc and β-SnOtBpPc are reported in different solvents, using Z-scan techniques with 10 ns laser pulses at 532 nm. Their nonlinear absorption coefficient and absorption cross sections were also determined. The molecular imaginary components of the second-order hyperpolarizability Im[γ] of α-SnOtBpPc and β-SnOtBpPc were found to be 2.60 × 10-31 and 2.94 × 10-31 esu (tetrahydrofuran), 2.12 × 10-31 and 2.54 × 10-31 esu (chloroform), 3.06 × 10-31 and 2.54 × 10-31 esu (dichloromethane), and 1.27 × 10-31 and 1.50 × 10-31 esu (toluene), respectively. This study found that substitution at the α position has an effect of lowering the two-photon (2PA) cross section value for α-SnOtBpPc compared to that for β-SnOtBpPc, with values of 64.30 and 456.65 GM, respectively. The large 2PA cross section observed in β-SnOtBpPc is attributed to the decreased energy difference between the virtual state and the LUMO.

Deep-red polymer dots with bright two-photon fluorescence and high biocompatibility for in vivo mouse brain imaging

With high contrast and deep penetration, two-photon fluorescence (2PF) imaging has become one of the most promising in vivo fluorescence imaging techniques. To obtain good imaging contrast, fluorescent nanoprobes with good 2PF properties are highly needed. In this work, bright 2PF polymer dots (P dots) were applied for in vivo mouse brain imaging. Deep-red emissive P dots with PFBT as the donor and PFDBT5 as the acceptor were synthesized and used as a contrast agent. P dots were further encapsulated by poly(styrene-co-maleic anhydride) (PSMA) and grafted with poly(ethylene glycol) (PEG). The P dots-PEG exhibit large two-photon absorption (2PA) cross-sections (δ≥8500g), good water dispersibility, and high biocompatibility. P dots-PEG was further utilized first time for in vivo vascular imaging of mouse ear and brain, under 690–900nm femtosecond (fs) laser excitation. Due to the large 2PA cross-section and deep-red emission, a large imaging depth (~720µm) was achieved.

Syntheses, characterizations and third-order NLO properties of a series of Ni(II), Cu(II) and Zn(II) complexes using a novel S-benzyldithiocarbazate ligand

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 1080nm. 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.

Packing of Large Two- and Three-Photon Activity Into Smallest Possible Unsymmetrical Fluorene Chromophores.

The quantum chemical study of one-, two-, and three-photon absorption (1PA, 2PA, and 3PA) properties for a set of compact fluorene derivatives (FD) with combination of different donor and acceptor moieties on both sides of fluorene ring system is presented. The main goal of the study is to pack large two-photon (2P) and three-photon (3P) activity into smallest possible chromophore. Linear, quadratic, and cubic response time-dependent density functional theory was used to calculate 1PA, 2PA, and 3PA properties, respectively. We used CAMB3LYP/cc-pVDZ level of theory for all the property calculations. The 2P and 3P transition probabilities were recalculated using two-state model approach and found to be in good agreement with the response theory results for first excited state. To include the contributions from higher states, the three-state model was also employed to recalculate the 2P transition probabilities and found to be in excellent agreement with response theory. The 2P/3P tensor elements were also analyzed to find reasons behind large 2P/3P activities. All the orbitals involved in transition processes were studied in detail by both molecular orbital pictures (qualitatively) and overlap diagnostic Λ-values (quantitatively). The study reveals that the novel fluorene derivatives FD-12 and FD-13 have shown large 2PA cross-section values of 1100 G.M. and 1030 G.M.; and 3PA transition probabilities of 6.10 × 10(10) a.u. and 4.85 × 10(10) a.u., respectively, for transition S0 → S1. The largest 3PA transition probability of 4.04 × 10(11) a.u. was found with FD-12 for S0 → S2 excitation. The linear relationship between Λ-values and 2PA cross-section values was also studied.

Pyrrolo[3,2-b]pyrroles-From Unprecedented Solvatofluorochromism to Two-Photon Absorption.

A combined experimental and theoretical study of the two-photon absorption (2PA) properties of a series of quadrupolar molecules possessing a highly electron-rich heterocyclic core, pyrrolo[3,2-b]pyrrole, is presented. In agreement with quantum-chemical calculations, large 2PA cross-section values, σ2PA ≈10(2) -10(3) GM (1 GM=10(50) cm(4) s photon(-1) ), are observed at wavelengths of 650-700 nm, which correspond to the two-photon allowed but one-photon forbidden transitions. The calculations also predict that increased planarity of this molecule through removal of two N-substituents leads to further increase in the σ2PA values. Surprisingly, the most quadrupolar pyrrolo[3,2-b]pyrrole derivative, containing two 4-nitrophenyl substituents at positions 2 and 5, demonstrates a very strong solvatofluorochromic effect, with a fluorescence quantum yield as high as 0.96 in cyclohexane, whereas the fluorescence vanishes in DMSO.

Synthesis, crystal structures and two-photon absorption properties of triphenylamine cyanoacetic acid derivative and its organooxotin complexes.

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.

Dibutylaminophenyl- and/or Pyridinyl-Capped 2,6,9,10-Tetravinylanthracene Cruciforms: Synthesis and Aggregation-Enhanced One- and Two-Photon Excited Fluorescence

This article reports the synthesis, one- and two-photon absorption (2PA), and emission properties of 2,6,9,10-tetraarylvinylanthracene derivatives in both THF solution and aqueous media. The results show that the center-crossed integration of aggregation-induced emission-active 9,10-diarylvinylanthracenes and aggregation-quenched emission-active 2,6-diarylvinylanthracenes does not surely constitute aggregation-enhanced emission (AEE) dyes, and only the dibutylamino-containing cruciforms exhibit AEE effect. The investigation of two-photon properties reveals that the dibutylamino-containing cruciforms show large 2PA cross sections (δmax up to 1500 GM) in both THF solution and THF/water (1/9) mixture, and the two-photon fluorescence action cross sections in aqueous media are about 10 times higher than those in THF solution because of their much higher fluorescence quantum yields in aqueous media. This work has demonstrated that the nature of the building blocks could significantly affect the aggregation behaviors and fluorescence properties of anthracene-centered cruciforms. Cruciforms with excellent 2PA and AEE properties could be constructed by using anthracene as the π-center and suitable aromatic units as building blocks.

Phenylene Vinylene Platinum(II) Acetylides with Prodigious Two-Photon Absorption

The linear and nonlinear optical properties of a series of linear and cross-conjugated platinum(II) acetylide complexes that contain extended p-(phenylene vinylene) chromophores are reported. The complexes exhibit very high femtosecond two-photon absorption (2PA) cross section values (σ(2) up to 10,000 GM), as measured by nonlinear transmission (NLT) and two-photon excited fluorescence (2PEF) methods. The large 2PA cross sections span a broad range of wavelengths, 570-810 nm, and they overlap with the triplet excited state absorption. Spectral coincidence of high cross section 2PA and triplet absorption is a key feature giving rise to efficient dual-mode optical power limiting (OPL).

1, 3, 5-Triazine-cored derivatives dyes containing triphenylamine based two-photon absorption: Synthesis, optical characterization and bioimaging

A series of new one, two and three-branched two-photon absorption triazine dyes containing triphenylamine with a π-bond and a σ-electron pair as a bridge, and different electron-donating groups, have been synthesized and their photophysical properties have been systematically investigated. These dyes showed obvious solvatochromic effects, i.e., significant bathochromic shifting of the emission spectra and larger Stokes shifts were observed in more polar solvents mainly due to intramolecular charge-transfer (ICT). The two-photon absorption (2PA) cross section values were determined by two-photon excited fluorescence (TPEF) measurements in DMF. This result further proved that a σ-electron pair as a bridge is an efficient way to transfer charge as well as a π bridge to provide a large 2PA cross section, and that their 2PA cross section values (δ) increase with increasing electron-donating strength of the end group and branch number. In addition, two-photon fluorescence cell imaging of dye 7a in HeLa and MCF-7 cancer cells was demonstrated.

Theory and computations of two-photon absorbing photochromic chromophores

Exponential growth in information technology generates ever increasing amounts of data, making recording density of the storage media crucially important. Two-photon absorption was proposed as a basis for high-density multi-layer technology for optical memory and logic devices. This technology suggests to use polymers, doped with photochromic compounds that undergo a reversible photoinduced isomerization, or photoswitching. In this review we consider recent theoretical works and benchmarking studies of the DFT-based methods, capable to predict two-photon absorption (2PA) and photochemical activity, Next we review the applications of these methods to design a prototype molecule that combines the photon-mode recording property of photochromic compounds with large 2PA cross-section. We conclude that a posteriori Tamm-Dancoff approximation to the second order CEO approach in Density Functional Theory is the powerful tool for both quantitative predictions and qualitative understanding of the excited state processes in photophysics and photochemistry. We also emphasize general principles for the rational design of a two-photon operated photoswitch.

Linear and Nonlinear Spectroscopy of a Porphyrin−Squaraine−Porphyrin Conjugated System

The linear and nonlinear absorption properties of a squaraine-bridged porphyrin dimer (POR-SQU-POR) are investigated using femto-, pico-, and nanosecond pulses to understand intramolecular processes, obtain molecular optical parameters, and perform modeling of the excited-state dynamics. The optical behavior of POR-SQU-POR is compared with its separate porphyrin and squaraine constituent moieties. Linear spectroscopic studies include absorption, fluorescence, excitation and emission anisotropy, and quantum yield measurements. Nonlinear spectroscopic studies are performed across a wide range (approximately 150 fs, approximately 25 ps, and approximately 5 ns) of pulsewidths and include two-photon absorption (2PA), single and double pump-probe, and Z-scan measurements with detailed analysis of excited-state absorption induced by both one- and two-photon absorption processes. The 2PA from the constituent moieties shows relatively small 2PA cross sections; below 10 GM (1 GM = 1 x 10(-50) cm4 s/photon) for the porphyrin constituent and below 100 GM for the squaraine constituent except near their one-photon resonances. In stark contrast, the composite POR-SQU-POR molecule shows 2PA cross sections greater than 10(3) GM over most of the spectral range from 850 to 1600 nm (the minimum value being 780 GM at 1600 nm). The maximum value is approximately 11,000 GM near the Nd:YAG laser wavelength of 1064 nm. This broad spectral range of large 2PA cross sections is unprecedented in any other molecular system and can be explained by intramolecular charge transfer. A theoretical quantum-chemical analysis in combination with different experimental techniques allows insight into the energy-level structure and origin of the nonlinear absorption behavior of POR-SQU-POR.

A New Series of Quadrupolar Type Two‐Photon Absorption Chromophores Bearing 11, 12‐Dibutoxydibenzo[a,c]‐phenazine Bridged Amines; Their Applications in Two‐Photon Fluorescence Imaging and Two‐Photon Photodynamic Therapy

AbstractA new series of quadrupolar type two‐photon absorption (2PA) chromophores 3–9 bearing a core arylamine‐[a,c]phenazine‐arylamine motif are synthesized in high yields. Palladium‐catalyzed Stille coupling and CN coupling reactions are utilized to prepare target chromophores. Detailed characterization and systematic studies of these molecules, including absorption and fluorescence emission, are conducted. These compounds are found to exhibit very large 2PA cross section values, for example, ∼7000 GM at 800 nm for 8 in toluene. Two‐photon‐induced fluorescence imaging is successfully demonstrated in vitro using compound‐8‐encapsulated silica nanoparticles with excellent bio‐compatibility. In combination with the capability of both one‐ and two‐photon singlet‐oxygen sensitizations, this nanocomposite demonstrates its promising potential in dual functionality toward two‐photon fluorescence imaging and two‐photon photodynamic therapy.

Strong Two-Photon Absorption in Push−Pull Phthalocyanines: Role of Resonance Enhancement and Permanent Dipole Moment Change upon Excitation

We study near-IR two-photon absorption (2PA) spectra of nonsymmetric push−pull metal-free phthalocyanines, which are present in solution at room temperature as a mixture of two spectrally nonequivalent tautomers T1 and T2. Large 2PA cross sections, σ2 = (1−3) × 103 GM, are obtained in the excitation wavelength region λex = 815−850 nm and are explained by (1) a new charge transfer state appearing in the one-photon absorption (1PA) spectrum of these compounds at 420−430 nm and (2) a strong resonance enhancement within the quasi-three-level system when the Q-state(s) serve as the real intermediate state(s). In the Q-region, λex = 1100−1500 nm, 2PA and 1PA transitions coincide, and the 2PA strength of any pure electronic or vibronic band is quantitatively described in terms of a two-level approximation.

Very efficient two-photon induced photo-tautomerization in non-symmetrical phthalocyanines

In this work we demonstrate that non-symmetrically substituted phthalo-naphthalocyanine molecule can have extremely large 2PA cross section in near-resonance conditions of excitation. We also show the possibility to switch between its two tautomeric forms upon two-photon excitation at 77 K. By measuring 2PA cross section, quantum efficiency of transformation in these two forms, as well as quantum yield of fluorescence, we evaluate the figure of merit for fast re-writable 3D information storage and show that this figure meets the criteria of reliable and fast (one bit per one femtosecond pulse) 2PA writing or reading.