TPA Cross-section Research Articles

Impact of Classical and Quantum Light on Donor-Acceptor-Donor Molecules.

Investigations of entangled and classical two-photon absorption have been carried out for six donor (D)-acceptor (A)-donor (D) compounds containing the dithieno pyrrole (DTP) unit as donor and acceptors with systematically varied electronic properties. Comparing ETPA (quantum) and TPA (classical) results reveals that the ETPA cross section decreases with increasing TPA cross section for molecules with highly off-resonant excited states for single-photon excitation. Theory (TDDFT) results are in semiquantitative agreement with this anticorrelated behavior due to the dependence of the ETPA cross section but not TPA on the two-photon excited state lifetime. The largest cross section is found for a DTP derivative that has a single photon excitation energy closest to resonance with half the two-photon excitation energy. These results are important for the possible use of quantum light for low-intensity energy-conversion applications.

Methyl‐ and Methoxy‐substituted 2‐(Pyridin‐2‐yl)‐4‐(4‐aminophenyl)quinazolines: Synthesis and Photophysical Properties

AbstractA series of novel 2‐(2‐pyridyl)quinazoline luminophores containing a donor aryl fragment at position 4 and methyl or methoxy groups at benzene ring has been synthesized by efficient three‐step route. The linear optical properties have been studied by UV/Vis absorption and photoluminescence spectra in two solvents. We revealed that introduction of additional methyl group or the replacement of methyl substituents with methoxy ones lead to sequentially shift of absorption and emission maximum to blue region. 9H‐Carbazol‐9‐yl‐containing derivative is characterized by hypsochromically shifted absorption band compared to its NEt2 and NPh2 counterparts. Unlike 9H‐carbazol‐9‐yl‐derivative, Et2N‐ and Ph2N‐bearing quinazolines demonstrate considerable decrease in quantum yield values when going from toluene to MeCN. Solvatochromic properties have been explored, the pronounced bathochromic shift was observed in fluorescence spectra with the increase of solvents polarity. Moreover, the compounds show significant shift of emission band with the increase of water fraction in DMSO/H2O mixture. Two‐photon optical properties have been also studied, the two‐photon absorption cross‐sections in MeCN and toluene reached 120 GM and 210 GM, respectively. The presence of additional methyl group has little impact on δTPA value, while introduction of methoxy substituents dramatically decreases TPA cross sections. Additionally, a quantum‐chemical calculations of synthesized compounds were performed to support the experimental data.

Photophysical properties of several multi-branched oligomers: An ultrafast excited state dynamics and quantum chemical calculation study

Multi-branched oligomers F1, F2 and F3 were investigated by spectroscopic experiments as well as quantum chemical calculations to elucidate the structure–property relationships and intramolecular charge transfer (ICT) characteristics of typical nonlinear optical materials. A better ICT property of F2 is obtained by replacing terminal electron donors with stronger electron-donating ability. The degree of ICT is effectively increased, which can be clearly seen from the frontier molecular orbitals contribution of central anthracene between HOMO and LUMO. The central group plays a major role in the direction as well as effective path of ICT process. In the X-shaped oligomers with anthracene as central group, ICT process takes place from terminal groups of [Formula: see text] position branches, to the central anthracene group. However, the electron distribution as well as the ICT of tri-branched oligomer F3 with triazine as central group is not symmetric. ICT process of F3 takes place from terminal triphenylamine groups to the central triazine and two fluorene groups at [Formula: see text] position branches. Y-shaped oligomer F3 has a longer lifetime of ICT state as well as a higher fluorescence quantum yield, which is consistent with the results of time-resolved fluorescence spectroscopy. Z-scan results show that the TPA cross-section value of F3 is 10.6 and 2.9 times higher than that of X-shaped F1 and F2. The dynamics curves of transient absorption spectroscopy also indicate that oligomer F3 structure has relatively stronger ICT properties. To verify the substituent effect, two linear molecules F4 and F5 were also calculated. The quantum chemical calculations result shows that the branches at positions 4 and 11 of the central anthracene group have little effect on the electron cloud distribution, and there are indeed significant differences in the contributions of branches at different positions. Our results may provide some reference for molecular design.

Synthesis and evaluation of four novel nitrogen-heterocyclic ruthenium polypyridyl complexes as photosensitizers for one and two-photon photodynamic therapy.

Four novel PSs (photosensitizers) of nitrogen-heterocyclic ruthenium polypyridyl complexes Ru(dip)2(o-pipppz)(PF6)2 (Ru1) (dip = 4,7-diphenyl-1,10-phenanthroline; o-pipppz = 1-(4-aldehydephenyl)-3-(pyridazyl-2-yl)-1H-pyrazole), Ru(dip)2(o-pipp) (PF6)2 (Ru2) (o-pipp = 1-(4-aldehydephenyl)-3-(pyrid-2-yl)-1H-pyrazole), Ru(dip)2(m-pipp)(PF6)2 (Ru3) (m-pipp = 1-(4-aldehydephenyl)-3-(pyrid-3-yl)-1H-pyrazole) and Ru(dip)2(p-pipp)(PF6)2 (Ru4) (p-pipp = 1-(4-aldehydephenyl)-3-(pyrid-4-yl)-1H-pyrazole) were reported, and the photodynamic activities of these complexes were studied on 2D and 3D HeLa cancer models. The longest visible absorption wavelength of these complexes was approximately 622 nm. The four Ru(II) complexes show preferable photodynamic activity and low dark toxicity (0.2-0.4 μM) in vitro against 2D HeLa tumor cells. These complexes exhibit very high singlet oxygen quantum yields in methanol (0.70-0.95), TPA cross-sections (7-31 GM), and high penetration depth. Thus, Ru1-Ru4 were utilized as one-photon and two-photon absorbing photosensitizers in both monolayer cells and 3D multicellular spheroids (MCSs). Among them, Ru2 revealed a higher singlet oxygen yield (0.95), a larger TPA cross-section (31 GM), and the strongest phototoxicity (EC50 = 0.20 μM). Moreover, flow cytometry shows that the four Ru(II) complexes can induced cell death mainly through apoptosis upon singlet oxygen-dependent reaction.

Flexible optical limiters based on Cu3VSe4 nanocrystals.

Optical limiters are greatly needed to protect eyes and sensitive optoelectronic devices such as photodetectors and sensors from laser damage, but they are currently plagued by low efficiency. In this work, we utilized Cu3VSe4 nanocrystals (NCs) to enhance laser protection performance, and they exhibit higher saturation intensity and broader nonlinear spectral response extending into the near IR region than the C60 benchmark. A flexible optical limiter goggle prototype based on the NCs significantly attenuated the incident laser beam, with Z scan and I scan measurements demonstrating a giant nonlinear absorption coefficient β value of 1.0 × 10-7 m W-1, a large optical damage threshold of 3.5 J cm-2, and a small starting threshold of 0.22 J cm-2. Transient absorption spectroscopy disclosed that the origin of the excellent nonlinearity was associated with quasi-static dielectric resonance behavior and a large TPA cross-section of 3.3 × 106 GM was measured for Cu3VSe4 NCs, suggesting the potential of intermediate bandgap (IB) semiconductors as alternatives to plasmonic noble metals for ultrafast photonics. Hence, optical limiters based on such semiconductors offer new avenues for laser protection in optoelectronic and defense fields.

Tuning the Photophysics of Two-Arm Bis[(dimethylamino)styryl]benzene Derivatives by Heterocyclic Substitution.

The identification of novel molecular systems with high fluorescence and significant non-linear optical (NLO) properties is a hot topic in the continuous search for new emissive probes. Here, the photobehavior of three two-arm bis[(dimethylamino)styryl]benzene derivatives, where the central benzene was replaced by pyridine, furan, or thiophene, was studied by stationary and time-resolved spectroscopic techniques with ns and fs resolution. The three molecules under investigation all showed positive fluorosolvatochromism, due to intramolecular charge-transfer (ICT) dynamics from the electron-donor dimethylamino groups, and significant fluorescence quantum yields, because of the population of a planar and emissive ICT state stabilized by intramolecular hydrogen-bond-like interactions. The NLO properties (hyperpolarizability coefficient and TPA cross-section) were also measured. The obtained results allowed the role of the central heteroaromatic ring to be disclosed. In particular, the introduction of the thiophene ring guarantees high fluorescent quantum yields irrespective of the polarity of the medium, and the largest hyperpolarizability coefficient because of the increased conjugation. An important and structure-dependent involvement of the triplet state was also highlighted, with the intersystem crossing being competitive with fluorescence, especially in the thiophene derivative, where the triplet was found to significantly sensitize molecular oxygen even in polar environment, leading to possible applications in photodynamic therapy.

Computational Study on the Effect of Thienyl π-Donor on the Optical Response of Nonclassical Oligo-Pyrazinothienothiadiazole Biradicaloids.

Computational analyses were performed on nitrogen-rich oligothiadizolothiophenes TTn (n = 0-3) and their four π-donor-substituted derivatives Th-TTn (n = 0-3) to examine the optical response due to geometrical and electronic structural attributes in the longitudinal and transverse axes, respectively. Our results are understood in the context of greater conjugation in the longitudinal axis (via additional fused rings) and substitution of a thienyl π-donor in the transverse axis of the geometry of each derivative. On inspection of the frontier molecular orbitals, we found that the better electron-accepting ability with minimal sacrifice in the ionization potentials results from geometrical aspects in both longitudinal and transverse axes. Due to the narrowed highest occupied molecular orbital-lowest unoccupied molecular orbital gaps, all of the derivatives exhibit a biradicaloid character (BRC) and one-photon panchromatic absorption; however, the open-shell nature weakened the charge transfer characteristics of excitation. In both the series, the odd electron density distributions and electron localization plots amply demonstrate the weakening of ylide character in fused thiophene rings and clearly indicate to the emergence of a long-bond/single BRC in the sulfurdimithide moiety in both series. In addition, the estimated tensor components of the second hyperpolarizability as well as overall responses confirm the shift from the longitudinal to transverse axis following the substitution with the π-donor. Interestingly, the TPA cross sections show comparable behavior, but contrary to γ, π-donor thienyl substitution appears to be discouraging in getting higher TPA responses for higher homologous series. Therefore, this study opens a new conjecture on tuning better nonlinear optical properties of organic functional materials.

One and Two‐Photon Excited Fluorescence Optimization of Metal–Organic Frameworks with Symmetry‐Reduced AIEgen‐Ligand

By designing a tetraphenylethylene (TPE)-based AIEgen-ligand with reduced symmetry, we obtained two alkaline-earth metal-based MOFs (LIFM-102 and LIFM-103) with dense packing structures and low porosity as proved by single-crystal X-ray diffraction and CO2 sorption data. Excitingly, the desolvated MOFs with rigid environment and reduced lattice free solvent exhibit high quantum yields (QY, 64.9 % and 79.4 %) and excellent two-photon excited photoluminescence (TPA cross-sections, 2946.6 GM and 2899.0 GM), while maintaining the external-stimuli-responsive properties suitable for anticounterfeit fields. The effect of ligand conformation was validated by comparing the structure and fluorescence properties of the samples before and after desolvation and further verified by theoretical calculations. This work expands the study on TPE-cored materials to symmetry-reduced ligand and might bring forward novel structures and excellent photoluminescent properties in the future.

In vivo Realization of Dual Photodynamic and Photothermal Therapy for Melanoma by Mitochondria Targeting Dinuclear Ruthenium Complexes under Civil Infrared Low-power Laser.

A series of dinuclear RuII complexes with extremely high TPA cross sections in the range of 800-900 nm have been designed. The amphiphilic complex Ru3 containing tert-butyl groups has balanced performance in singlet oxygen generation and photothermal conversion and becomes the ideal drug candidate of the series. Ru3 targets mitochondria without penetrating the nucleus, which substantially increases its photodynamic therapy activity and reduces its dark cytotoxicity. Ru3 successfully suppresses melanoma tumor growth in vitro and in vivo with combined photodynamic and photothermal therapy under low light dose irradiation of an 808 nm low-power laser, avoiding the known PDT resistance in melanoma. The excellent therapeutic effect of Ru3 facilitates its applications in further human trials for larger or deeper buried tumors, thereby becoming a prospective candidate for a new generation of low-power IR-driven dual PDT/PTT drugs.

In vivo Realization of Dual Photodynamic and Photothermal Therapy for Melanoma by Mitochondria Targeting Dinuclear Ruthenium Complexes under Civil Infrared Low‐power Laser

AbstractA series of dinuclear RuII complexes with extremely high TPA cross sections in the range of 800–900 nm have been designed. The amphiphilic complex Ru3 containing tert‐butyl groups has balanced performance in singlet oxygen generation and photothermal conversion and becomes the ideal drug candidate of the series. Ru3 targets mitochondria without penetrating the nucleus, which substantially increases its photodynamic therapy activity and reduces its dark cytotoxicity. Ru3 successfully suppresses melanoma tumor growth in vitro and in vivo with combined photodynamic and photothermal therapy under low light dose irradiation of an 808 nm low‐power laser, avoiding the known PDT resistance in melanoma. The excellent therapeutic effect of Ru3 facilitates its applications in further human trials for larger or deeper buried tumors, thereby becoming a prospective candidate for a new generation of low‐power IR‐driven dual PDT/PTT drugs.

Nonlinear optical properties of the tartrazine

Tartrazine is one of the food dyes with [Formula: see text] conjugated bond. The optical nonlinear properties of tartrazine in aqueous solution were investigated by Z-scan technique at the different input intensities of picosecond laser pulse. It was found that the tartrazine exhibits strong nonlinear optical behavior at the low input intensity. The experimental results show the nonlinear refractive indices reach the order of 10[Formula: see text] esu and their TPA cross sections are up to several hundreds of GM in the experimental condition. The strong optical nonlinearity perhaps allows them to be widely applied in the photoelectric and biological field.

Synthesis, photophysical properties and two-photon absorption of benzothiazole/benzoxazole π-expanded carbazole dyes

We report the synthesis and spectroscopic characterization (NMR, HRMS) of a series of benzazole derivatives with different carbazole substituents. The photophysical properties of these molecules were investigated in several solvents of different polarity by UV–vis absorption and fluorescence spectroscopy. These dyes show strong absorption bands in the near ultraviolet region, fluorescence emission with high quantum yields in the blue-green as well as large Stokes' shifts. Two-photon absorption properties were recorded and all of the dyes show moderate TPA cross sections in the 700–800 nm zone. Molecular modeling was also used to explain spectroscopic behavior of these dyes. Interestingly, they possess high thermal stability.

Morphology-Dependent One- and Two-Photon Absorption Properties in Blue Emitting CsPbBr3 Nanocrystals.

The linear and nonlinear optical parameters and morphologic dependence of CsPbBr3 nanocrystals (NCs) are crucial for device engineering. In particular, such information in asymmetric nanocrystals is still insufficient. We characterized the OPLA (σ1) and TPA cross sections (σ2) of a series CsPbBr3 nanocrystals with various aspect ratios (AR) using femtosecond transient absorption spectroscopy (TAS). The σ1 presents a linear volume dependence of all the samples, which agrees with the previous behavior in CsPbBr3 QDs. However, the σ2 values do not exhibit conventional power dependency of the crystal volume but are also modulated by the shape-dependent local field factors. In addition, the local field effect in CsPbBr3 NCs is contributed by their asymmetric morphologies and polar ionic lattices, which is more pronounced than in conventional semiconductor NCs. Finally, we revealed that the lifetimes of photogenerated multiexcitonic species of those nanocrystals feature identical morphology independence in both OPLA and TPA.

From 2,5-Diformyl-1,4-dihydropyrrolo[3,2-b]pyrroles to Quadrupolar, Centrosymmetric Two-Photon-Absorbing A-D-A Dyes.

An original approach has been developed for the insertion of formyl substituents at positions 2 and 5 of 1,4-dihydropyrrolo[3,2-b]pyrroles by conversion of thiazol-2-yl substituents. The synthetic utility of these formyl groups was investigated, and a series of centrosymmetric A−π–D−π–A frameworks were constructed. The two-photon absorption of the quadrupolar pyrrolo[3,2-b]pyrrole possessing two dicyanovinylidene flanking groups is attributed to an S0 → (S1) → S4 transition which has a large TPA cross-section (1300 GM) for a molecule of this size.

The Role of Hydrogen Bonds and Electrostatic Interactions in Enhancing Two-Photon Absorption in Green and Yellow Fluorescent Proteins.

The spectral properties of fluorescent proteins (FPs) depend on the protein environment of the chromophore (CRO). A deeper understanding of the CRO - environment interactions in terms of FPs' spectral characteristics will allow for a rational design of novel markers with desired properties. Here, we are taking a step towards achieving this important goal. With the time-dependent density functional theory (TDDFT), we calculate one- and two-photon absorption (OPA and TPA) spectra for 5 green FPs (GFPs) and 3 yellow FPs (YFPs) differing in amino acid sequence. The goal is to reveal the roles of: (i) electrostatic interactions, (ii) hydrogen-bonds (h-bonds) and (iii) h-bonds together with distant electrostatic field in absorption spectra tuning. Our results point to design hypothesis towards FPs optimised for TPA-based applications. Both h-bonds and electrostatic interactions co-operate in enhancing TPA cross-section ( ) for the transition in GFPs. Furthermore, it seems that details of h-bonds network in the CRO's vicinity influences response to CRO - environment electrostatic interactions in YFPs. We postulate that engineering FPs with more hydrophilic CRO's environment can lead to greater . We also find that removing h-bonds formed with the CRO's phenolate leads to TPA enhancement for transition to higher excited states than S1 . Particularly Y145 and T203 residues are important in this regard.

Supramolecular Approach to Tuning the Photophysical Properties of Quadrupolar Squaraines.

In the present study, the influence of the hydrogen bonding for the one- and two-photon absorption of the prototypical squaraine dye is investigated with quantum chemistry tools. The central squaraine unit is bound by strong hydrogen bonds with 4-substituted N,N′-diphenylurea and, alternatively, N,N′-diphenylthiourea molecules, which affects to a high extend the properties of the squaraine electron accepting moiety, thus shifting its maximum absorption wavelength and enhancing the TPA cross section. The replacement of oxygen by sulfur atoms in the squaraine central ring, known to affect its photophysical behavior, is considered here as the way of modifying the strength and nature of the intermolecular contacts. Additionally, the influence of the oxygen-by-sulfur replacement is also considered in the N,N′-diphenylurea moiety, as the factor affecting the acidity of the N–H protons. The introduction of the sequence of the substituents of varying electron-donating or electron-withdrawing characters in the position 4 of N,N′-diphenyl(thio)urea subsystems allows to finely tune the hydrogen bonding with the central squaraine unit by further modification of the N–H bond characteristics. All of these structural modifications lead to the controlled adjustment of the electron density distribution, and thus, the properties affected such as transition moments and absorption intensity. Ab initio calculations provide strong support for this way of tailoring of one- or two-photon absorption due to the obtained strong hypsochromic shift of the maximum one-photon absorption wavelength observed particularly for thiosquaraine complexes and an increase in the TPA wavelength together with the increase in the TPA cross section. Moreover, the source of the strong modification of the thiosquaraine OPA in contrast to the pristine oxosquaraine upon N,N′-diphenyl(thio)urea substitution is determined. Furthermore, for the first time, the linear dependence of the non-additivity in the interaction energy on the Hammett substituent constant is reported. The stronger the electron-donating character of the substituent, the larger the three-body non-additive components and the larger their percentage to the total interaction energy.

Effect of Polymerization on the Charge-Transfer Mechanism in the One (Two)-Photon Absorption Process of D-A-Type Triphenylamine Derivatives.

To investigate the effect of polymerization (n = 1, 2, 3, and 4) on the charge-transfer (CT) mechanisms in the one (two)-photon absorption (OPA and TPA) process of D-A-type triphenylamine derivatives, charge density difference is used to graphically represent the CT characteristics. A transition density matrix is utilized to reveal the direction of CT on different groups quantitatively. With the n increasing, electrons are mainly transferred between the groups in the middle position of the molecular chain during OPA and TPA processes. Simulated results show that the energy gap and excitation energy have a good linear relationship with the reciprocal of the polymerization degree. Importantly, the polymerization effect can effectively increase the electronic transmission capability, TPA performance, and second hyperpolarizability. Besides, the simplified sum over state model reveals the variation factor of the TPA cross-section and the second static hyperpolarizability. The McRae formula and Bakhshiev formula are used to estimate the difference of dipole moments, which is an important parameter of the second hyperpolarizability. The comprehensive analysis of the nonlinear optical (NLO) parameters of triphenylamine derivatives can provide some significant guidance for molecular design and improve the NLO performance of D-A molecular materials. Also, the thermodynamic parameters can provide some theoretical supports for solving practical problems.

Fused polycyclic compounds: Synthesize, enhancing TPA cross-section by modifying torsional multiple ring

Abstract Four fused polycyclic compounds (DNBT, DNTB, DNPY, and DNNM) with different substituents were designed and synthesized to achieve excellent nonlinear optical materials. Broadband reverse saturation absorption performance based on TPA was carried out by Z-scan. Transient absorption spectrum showed that the different substituents can significantly regulate the ultrafast kinetic processes of their excited states. The relationship between excited states charge structure and nonlinear optical properties were studied by density functional theory (DFT) and sum over states (SOS) model. TPA of these compounds is modulated by local excitation in DBPy and TA fragments. Intra-molecular charge transfer weakens the TPA cross-section. These compounds are potential optical limiting (OL) materials in visible light range. The clear mechanism show that the nonlinear optical properties of organic materials can be improved purposefully through theoretical and experimental methods.

Theoretical insights on type I/II photoreactions of potential Zn(II) polypyridyl photosensitizers for two-photon photodynamic therapy

Large two-photon absorption cross-sections are vital to photosensitizers (PSs) in TP-PDT, which can be used to develop in-depth treatment for diseased cells and minimize the harm to surrounding cells. Here, we conduct a study about photophysical properties of one Ru(II) polypyridyl complex and two designed Zn(II) polypyridyl complexes by means of DFT and TD-DFT methods. The main results are as follows: firstly, the two-photon absorption spectrum of two designed complexes Zn-OMe and Zn-OCOOCH3 are all within the phototherapeutic window (550-900nm). Secondly, large SOC values and small energy gaps ΔES-T of these complexes guarantee the efficiency of ISC process. Thirdly, their T1 energy is greater than that required for generating 1O2 (0.98eV) via Type II photoreaction. In addition, the calculated results of vertical electron affinities (VEA) and vertical ionization potentials (VIP) show that these complexes are able to form superoxide ions O2(-) via Type I photoreaction. Specifically, both of two designed Zn-centric complexes have larger TPA cross-sections than that of Ru-centric complex. In a word, we are pleased to report two potential photosensitizers with excellent performance and reasonable price for Type I/II photoreactions. We expect our study will offer some theoretical guidance and help in TP-PDT.

Multibranched Octupolar Module Embedded Covalent Organic Frameworks Enable Efficient Two‐Photon Fluorescence

AbstractCovalent organic frameworks (COFs) have emerged as potential light emitting polymers for optoelectronic and optical devices, but their nonlinear optical properties, particularly two‐photon absorption and fluorescence (TPA/TPF), have seldom been explored. Herein, to construct octupolar three‐branched modules (e.g., acceptor 3‐(donor‐core), triphenylbenzene core) within a 2D cyano‐sp2c‐conjugated framework is proposed that results in two‐photon luminescent COFs, combining a large TPA cross section and high quantum yield (QY). Such octupolar module‐embedded sp2c‐conjugated COFs emit not only intense one‐photon fluorescence with QY of 27.2% in the solid state and 38.1% in tetrahydrofuran—superior to almost all reported COFs, but also efficient two‐photon fluorescence with large TPA cross section of 1225 GM—remarkably surpassing the corresponding cyano‐sp2c‐linked model compounds (104 GM). The finding highlights the synergy between sp2c‐conjugated framework and octupolar modules that leads to markedly improved TPA response owing to extended conjugated length, enhanced planarity and multidimensional intramolecular interaction. In view of the versatility of the branched chromophore, the proposed design idea is expected to be used to exploit more two‐photon active COF materials for a range of applications. Multiple uses of the COF in information encryption and warm white light‐emitting diodes are also exemplified.