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Non‐Kekulé quinoidal azaacences m‐A (1 a,b) were synthesized and compared to their para‐ and ortho‐quinodimethane analogues. m‐A display high diradical characters (1 b: y0 = 0.88) due to their meta‐quinodimethane (m‐QDM) topology. Electron paramagnetic, nuclear magnetic resonance spectroscopies and supraquantum interference device measurements in combination with quantum‐chemical calculations revealed singlet ground states for m‐A with singlet‐triplet gaps ΔEST (0.13–0.25 kcal mol−1) and thermally populated triplet states. These non‐Kekulé structures are over all void of zwitterionic character and possess record high two‐photon absorption cross sections over a broad spectral range in the near‐infrared. [ABSTRACT FROM AUTHOR]

Quasi‐2D perovskites are widely used in solar cells, photodetectors, and light‐emitting diodes (LEDs) due to their excellent optoelectronic properties and stability. However, much fewer all‐optical devices are realized based on quasi‐2D perovskites. Although perovskites have large nonlinear optical coefficients, the relaxation progress of excitons and carriers will slow down the response time and reduce the signal contrast of the all‐optical devices. In this paper, the exciton dynamics are investigated under off‐resonance excitation in quasi‐2D perovskite PEA2(FAPbBr3)2PbBr4. The spin‐selective optical Stark effect (OSE) is observed through circularly polarized transient absorption measurements, and a Stark shift of 3.3 meV is achieved under 1.11 eV excitation. The hot‐biexciton cooling process induced by two‐photon absorption is also observed. A spin‐based XNOR gate is demonstrated with a response time of ≈180 fs, and the logical signal contrast is enhanced through the competition of the OSE and the hot‐biexciton effect. [ABSTRACT FROM AUTHOR]

We report the results of a study of laser-induced transparency under the conditions of the optical Stark effect in CdSe nanoplatelets. The induced absorption spectra are measured by the pump–probe technique. Laser-induced transparency is observed due to the transfer of the population of the electron states of the valence band to the conduction band as a result of two-photon optical transitions under the action of pump radiation. The transparency of the medium is maintained for the lifetime of the first excited exciton level, which corresponds to 180 ps. The presence of a hypsochromic shift of the exciton absorption band by 3.1 meV for a pump power density of 39 GW/cm2 with the coincidence of pump and probe pulses due to the optical Stark effect is demonstrated. The results of fundamental studies presented in this paper can find application in the development of new photonic devices for controlling light with light. [ABSTRACT FROM AUTHOR]

"Tandem" uncaging systems, in which a photolabile protecting group (PPG) is sensitized by an energy‐harvesting antenna, may increase the photosensitivity of PPGs by several orders of magnitude for two‐photon (2P) photorelease. Yet, they remain poorly accessible because of arduous multi‐step synthesis. In this work, we design efficient tandem uncaging systems by (i) using a convenient assembly of the building blocks relying on click chemistry, (ii) introducing H‐bonding induced proximity thus facilitating (iii) photoinduced electron transfer (PeT) as a cooperative mechanism. A strong two‐photon absorber electron‐donating quadrupolar antenna and various electron‐accepting PPGs (mDEAC, MNI or MDNI) were clicked stepwise onto a "tweezer‐shaped" pyrido‐2,6‐dicarboxylate platform whose H‐bonding and π‐stacking abilities were exploited to keep the antenna and the PPGs in close proximity. The different electron‐accepting ability of the PPGs led to dyads with wildly different behaviors. Whilst the MDNI and MNI dyads showed poor dark stability or no photo‐uncaging ability due to their too high electron‐accepting character, the mDEAC dyad benefited from optimum redox potentials to promote PeT and slow down charge recombination, resulting in enhanced uncaging quantum yield (Φu=0.38) compared to mDEAC (Φu=0.014). This unique combination resulted in large 2P photo‐sensitivity in the near‐infrared window (240 GM at 710 nm). [ABSTRACT FROM AUTHOR]

Two‐photon excited fluorescence is a process in which the excited fluorophore relaxes and emits fluorescence after two‐photon absorption. Combined with appropriate two‐photon fluorescent probes, two‐photon microscopy enables observation deep inside tissue without damage. Former reports showed that the two‐photon absorption cross sections of donor–acceptor (D–A) dipoles, as well as D‐π‐D and D–A–D quadrupoles can be strengthened by using strong D–A groups. In this study, a novel water‐soluble fluorescent probe with D‐π‐A structure containing pyridine‐2,6‐dicarboxylic acid unit was synthesized. The maximum excitation wavelength and emission wavelength of the probe in aqueous solution were 328 nm and 471 nm, respectively. The fluorescence emitted by the probe can be quenched by various metal cations. The pyridyl N and carboxyl O in the probe formed chelates with Cu2+, which resulted in a fluorescence quenching response, and showed a linear relationship between fluorescent intensity and Cu2+ concentration with wide linear range and low limit of detection. CCK‐8 tests showed the probe had no obvious toxicity to 4T1 cells in the range of 0.4–50 μmol L−1, and the cell survival rate was above 86 %. The probe had a large two‐photon absorption cross section ranging from 680 nm to 820 nm. The two‐photon absorption cross section reached 100 GM at 760 nm of excitation wavelength. By means of microscopic imaging of 4T1 cells stained with the probe, one‐photon fluorescence represented blue emission, while two‐photon fluorescence represented bright green emission. The probe was proved to have good cell permeability, and preferred to target nucleus, showing application potential in two‐photon fluorescence imaging technology. [ABSTRACT FROM AUTHOR]

Conventional organic photocatalysis typically relies on ultraviolet and short‐wavelength visible photons as the energy source. However, this approach often suffers from competing light absorption by reactants, products, intermediates, and co‐catalysts, leading to reduced quantum efficiency and side reactions. To address this issue, we developed novel organic two‐photon‐absorbing (TPA) photosensitizers capable of functioning under deep red and near‐infrared light irradiation. Three model reactions including cyclization, Sonogashira Csp2−Csp cross‐coupling, and Csp2−N cross‐coupling reactions were selected to compare the performance of the new photosensitizers under both blue (427 nm) and deep red (660 nm) light irradiation. The obtained results unambiguously prove that for reactions involving blue light‐absorbing reactants, products, and/or co‐catalysts, deep red light source resulted in better performance than blue light when utilizing our TPA photosensitizers. This work highlights the potential of our metal‐free TPA photosensitizers as a sustainable and effective solution to mitigate the competing light absorption issue in photocatalysis, not only expanding the scope of organic photocatalysts but also reducing reliance on expensive Ru/Ir/Os‐based photosensitizers. [ABSTRACT FROM AUTHOR]

The Psoralen (Pso) molecule finds extensive applications in photo‐chemotherapy, courtesy of its triplet state forming ability. Sulfur and selenium replacement of exocyclic carbonyl oxygen of organic chromophores foster efficient triplet harvesting with near unity triplet quantum yield. These triplet‐forming photosensitizers are useful in Photodynamic Therapy (PDT) applications for selective apoptosis of cancer cells. In this work, we have critically assessed the effect of the sulfur and selenium substitution at the exocyclic carbonyl (TPso and SePso, respectively) and endocyclic oxygen positions of Psoralen. It resulted in a significant redshifted absorption spectrum to access the PDT therapeutic window with increased oscillator strength. The reduction in singlet‐triplet energy gap and enhancement in the spin‐orbit coupling values increase the number of intersystem crossing (ISC) pathways to the triplet manifold, which shortens the ISC lifetime from 10−5 s for Pso to 10−8 s for TPso and 10−9 s for SePso. The intramolecular photo‐induced electron transfer process, a competitive pathway to ISC, is also considerably curbed by exocyclic functionalizations. In addition, a maximum of 115 GM of two‐photon absorption (2PA) with IR absorption (660–1050 nm) confirms that the Psoralen skeleton can be effectively tweaked via single chalcogen atom replacement to design a suitable PDT photosensitizer. [ABSTRACT FROM AUTHOR]

Fluorescence imaging can be employed in fields of medical treatment, astronomical exploration, and national defense security. Traditional fluorescence imaging often takes the single-photon techniques, which is vulnerable to background interference and photobleaching. Remedially, two-photon fluorescence imaging can achieve much higher-resolution fluorescence imaging for reducing scattering and deeper depth. Hence, by assembling the tetraphenylethylene backbones with nontoxic and non-noble K+ ions, compound 1 ([(Hdma)K(H2ettc)]n, H4ettc = 4′,4′″,4′″″,4′″″″(ethene-1,1,2,2-tetrayl)tetrakis(([1,1′-biphenyl]-4-carboxylic acid))) with the crystallization-induced emissions exhibited charming fluorescence imaging under two-photon excitation microscopy (TPEM). Besides, luminescent powders based on compound 1 can achieve high-resolution fingerprint recognition, providing secure access control and identification for a novel authentication method. Compared with the commercial fluorescent dyes coumarin-6, the as-synthesized compound 1 showed great solvent stability, indicating its durability against harsh environment. Moreover, compound 1 shows mechanoluminescent properties for the perturbation of weak supramolecular interactions within ordered arrangements of the H2ettc2− ligands. This novel compound has provided an important insight to the development of two-photon fluorescence imaging and advanced external-stimuli responsive materials. [ABSTRACT FROM AUTHOR]

It is shown that the two-photon absorption (TPA)-induced losses in a non-centrosymmetric crystal reduce considerably when the angle θ between the polarization vector of incident radiation and the optical c-axis is 45°. In such a geometry of interaction in a Ca3(VO4)2 crystal, the effective TPA coefficient for 300 fs laser pulses at θ = 45° is 3.5 ± 0.5 times lower than its maximum at θ = 0°, which is more than two times higher than reported earlier in the literature. [ABSTRACT FROM AUTHOR]

A 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. [ABSTRACT FROM AUTHOR]

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene (fDAE) derivative were investigated by time-resolved absorption and fluorescence spectroscopies. Prescreening with quantum chemical calculation predicted that a derivative with methylthienyl groups (mt-fDAE) in the closed-ring isomer has a two-photon absorption cross-section larger than 1000 GM, which was experimentally verified by Z-scan measurements and excitation power dependence in transient absorption. Comparison of transient absorption spectra under one-photon and simultaneous two-photon excitation conditions revealed that the closed-ring isomer of mt-fDAE populated into higher excited states deactivates following three pathways on a timescale of ca. 200 fs: (i) the cycloreversion reaction more efficient than that by the one-photon process, (ii) internal conversion into the S1 state, and (iii) relaxation into a lower state (S1' state) different from the S1 state. Time-resolved fluorescence measurements demonstrated that this S1' state is relaxed to the S1 state with the large emission probability. These findings obtained in the present work contribute to extension of the ON–OFF switching capability of fDAE to the biological window and application to super-resolution fluorescence imaging in a two-photon manner. [ABSTRACT FROM AUTHOR]

Using fluorescence spectroscopy, the two-photon absorption cross sections of aqueous solutions of the styryl dye trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (DASPI) and its inclusion complexes with cucurbit[n]urils (CB[n] n = 6–8) have been measured. A nonmonotonic dependence of the cross section on the excitation wavelength and on the cavitand cavity size has been revealed. Compared to the free dye, a sevenfold increase in the two-photon absorption cross section has been observed in DASPI inclusion complexes with CB[8] at an excitation wavelength of 980 nm. [ABSTRACT FROM AUTHOR]

Gliomas of the brain are characterised by high aggressiveness, high postoperative recurrence rate, high morbidity and mortality, posing a great challenge to clinical treatment. Traditional treatments include surgery, radiotherapy and chemotherapy; they also have significant associated side effects, leading to difficulties in tumour resection and recurrence. Photodynamic therapy has been shown to be a promising new strategy to help treat malignant tumours of the brain. It irradiates the tumour site at a specific wavelength to activate a photosensitiser, which selectively accumulates at the tumour site, triggering a photochemical reaction that destroys the tumour cells. It has the advantages of being minimally invasive, highly targeted and with few adverse reactions, and is expected to be well used in anti-tumour therapy. However, the therapeutic effect of traditional PDT is limited by the weak tissue penetration ability of photosensitiser, hypoxia and immunosuppression in the tumour microenvironment. This paper reviews the current research status on the therapeutic principle of photodynamic therapy in glioma and the mechanism of tumour cell injury, and also analyses the advantages and disadvantages of the current application in glioma treatment, and clarifies the analysis of ideas to improve the tissue penetration ability of photosensitizers. It aims to provide a feasible direction for the improvement of photodynamic therapy for glioma and a reference for the clinical treatment of deep brain tumours.

The eye is an ideal organ for imaging by a multi-photon excitation approach, because ocular tissues such as the sclera, cornea, lens and neurosensory retina, are highly transparent to infrared (IR) light. The interface between the retina and the retinal pigment epithelium (RPE) is especially informative, because it reflects the health of the visual (retinoid) cycle and its changes in response to external stress, genetic manipulations, and drug treatments. Vitamin A-derived retinoids, like retinyl esters, are natural fluorophores that respond to multi-photon excitation with near IR light, bypassing the filter-like properties of the cornea, lens, and macular pigments. Also, during natural aging some retinoids form bisretinoids, like diretinoid-pyridiniumethanolamine (A2E), that are highly fluorescent. These bisretinoids appear to be elevated concurrently with aging. Vitamin A-derived retinoids and bisretinoidss are detected by two-photon ophthalmoscopy (2PO), using a new class of light sources with adjustable spatial, temporal, and spectral properties. Furthermore, the two-photon (2P) absorption of IR light by the visual pigments in rod and cone photoreceptors can initiate visual transduction by cis-trans isomerization of retinal, enabling parallel functional studies. Recently we overcame concerns about safety, data interpretation and complexity of the 2P-based instrumentation, the major roadblocks toward advancing this modality to the clinic. These imaging and retina-function assessment advancements have enabled us to conduct the first 2P studies with humans.

The utilization of two-dimensional (2D) materials in optoelectronic devices has witnessed a surge. Production of 2D ultrathin molybdenum disulfide (MoS2) nanosheets without the use of toxic precursors and without altering the phase of the material is still a task. We developed a cost-effective liquid-phase exfoliation method that is free of additives, utilizing ultrasound to produce a significant amount of nearly single-layered MoS2 nanosheets by employing isopropyl alcohol and deionized water as a cosolvent. This work addresses the deficiencies in the current state-of-the-art studies by exploring the mechanism governing ultrasonic cavitation, resulting in MoS2 nanosheets. Various sonication durations were altered to investigate the relationship between the duration of sonication and the dimensionality of the resulting nanosheets. The findings revealed that prolonged sonication durations yielded bilayered and trilayered nanosheets. The reduction in layer thickness was confirmed by Raman modes as the sonication time increased. Size-dependent optical studies indicated that the optical properties have enhanced as the sonication time increased. An open-aperture Z-scan investigation was conducted to explore the size-dependent nonlinear optical features of MoS2 nanosheets, employing a 532 nm neodymium-doped yttrium aluminum garnet pulsed laser in a nanosecond pulsed regime. The optical limiting performance of the nanosheets sonicated at various time intervals was accessed and it was observed that as the nanosheets' thickness decreases, the optical limiting behavior improves. Due to two-photon absorption, MoS2 nanosheets display optical limiting behaviors through reverse saturable absorption. Optical limiting thresholds as low as 2.98 × 1012 W/m2 are attained. This significant enhancement suggests that these ultrathin MoS2 nanosheets are suitable for applications in laser shielding. [ABSTRACT FROM AUTHOR]

Optical non-linearities play a crucial role in enabling efficient and ultrafast switching applications that are essential for next-generation photonic devices. ZnS phosphor material produces the best results in terms of increased luminescence quantum yield when doped with certain impurities. Nevertheless, the investigation of the third-order non-linear optical susceptibility of the phosphor materials can be exploited for various switching applications. In this regard, we review the recent advancements in the investigation of non-linear optical properties of ZnS phosphors, where the knowledge of absorption and refraction is utilized in various optical and detector applications. Furthermore, the review highlights strategies employed to enhance the non-linear optical response of phosphor materials as well as a general discussion of an attosecond optical switching scheme which can be used to fabricate devices with petahertz speeds. Consequently, we provide a solution to the unsolved problem of the significant extension of optical limiting applications to switching applications by developing design strategies to manipulate conventional ZnS phosphor material. The potential challenges and future prospects of utilizing phosphor materials for switching applications are also addressed. The strategies for manipulating ZnS phosphor can be generalized for a broad range of other materials by minimizing linear and non-linear losses, while enhancing the values of the non-linear refractive index coefficient. We propose that the figure-of-merit of ZnS material can be enhanced by using a suitable combination of pump and probe wavelength values, which can be useful for optical switching applications. [ABSTRACT FROM AUTHOR]

Detailed photophysical studies of luminescent lanthanide complexes are presented and elaborated using a newly developed thioanisolyl‐picolinate antenna and the related tacn macrocyclic ligand. The new ligand proved to sensitise Nd(III), Sm(III), Eu(III) and Yb(III) emission. Eu(III) complex showed complete energy transfer, yielding high quantum yield (44 %) and brightness, while the Tb(III) analogue underwent a thermally activated back‐energy transfer, resulting in a strong oxygen quenching of the triplet excited state. Transient absorption spectroscopy measurements of Gd(III), Tb(III) and Eu(III) compounds confirmed the sensitization processes upon the charge‐transfer antenna excitation. The triplet excited state lifetime of the Tb(III) complex was 5‐times longer than that of the Gd(III) analogue. In contrast, the triplet state was totally quenched by the energy transfer to the 4f‐metal ion in the Eu(III) species. Nonlinear two‐photon absorption highlighted efficient biphotonic sensitization in Eu(III) and Sm(III) complexes. In case of the Nd(III) compound, one‐photon absorption in 4f–4f transitions was predominant, despite the excitation at the antenna two‐photon band. This phenomenon was due to the Nd(III) 4f–4f transitions overlapping with the wavelength‐doubled absorption of the complex. [ABSTRACT FROM AUTHOR]

Simultaneous interrogation of pump and probe beams interacting in ZnO nanostructures of a two-wave mixing is proposed for dual-path data processing of optical signals by nonlinear optical effects. An enhancement in third-order nonlinear optical properties was exhibited by Al-doped ZnO thin films. Multiphoton absorption and nonlinear refraction were explored by the z-scan technique at 532 nm with nanosecond pulses. The evolution of the optical Kerr effect in the ZnO thin films was analyzed as a function of the incorporation of Al in the sample by a vectorial two-wave mixing method. Electrical and photoconductive effects were evaluated to further characterize the influence of Al in the ZnO solid samples. Potential applications of nonlinear optical parameters for encoding and encrypting information in light can be envisioned. [ABSTRACT FROM AUTHOR]

This review describes the two-photon absorption properties of dendrimers, which are arborescent three-dimensional macromolecules differing from polymers by their perfectly defined structure. The two-photon absorption process is a third order non-linear optical property that is attractive because it can be used in a wide range of applications. In this review, dendrimers that were studied for their two-photon absorption properties are first described. Then, the use of dendritic TPA chromophores for light harvesting, photopolymerization, optical power limitation, cell imaging, singlet oxygen generation, and photodynamic therapy is described. This review thus proposes an overview of the properties and possible applications of two-photon absorbing dendrimers. [ABSTRACT FROM AUTHOR]

Nanothermometers are emerging probes as biomedical diagnostic tools. Especially appealing are nanoprobes using NIR light in the range of biological transparency window (BTW) since they have the advantages of a deeper penetration into biological tissues, better contrast, reduced phototoxicity and photobleaching. This article reports the preparation and characterization of organic nanoparticles (ONPs) doped with two polychlorinated trityl radicals (TTM and PTM), as well as studies of their electronic and optical properties. Such ONPs having inside isolated radical molecules and dimeric excimers, can be two‐photon excited showing optimal properties for temperature sensing. Remarkably, in TTM‐based ONPs the emission intensity of the isolated radical species is unaltered increasing temperature, while the excimer emission intensity decreases strongly being thereby able to monitor temperature changes with an excellent thermal absolute sensitivity of 0.6–3.7% K−1 in the temperature range of 278–328 K. The temperature dependence of the excimeric bands of ONPs are theoretically simulated by using electronic structure calculations and a vibronic Hamiltonian model. Finally, TTM‐doped ONPs as ratiometric NIR‐nanothermometers are tested with two‐photon excitationwith enucleated pig eye sclera, as a real tissue model, obtaining a similar temperature sensitivity as in aqueous suspensions, demonstrating their potential as NIR nanothermometers for bio applications. [ABSTRACT FROM AUTHOR]

In the continuation of previous studies on carbon‐rich meso‐tetraarylporphyrins featuring 2,7‐fluorene units at their periphery, the effect of changing the peripheral dendritic arms for linear arms on their oxygen‐photosensitizing ability, their fluorescence and their two‐photon absorption (2PA) properties is now analyzed. Thus, starburst porphyrins possessing up to twenty conjugated fluorenyl units were isolated and studied. More precisely, a series of five new free‐base porphyrins featuring fully conjugated arms incorporating an increasing number of fluorenyl groups connected via 1,2‐alkenyl spacers were synthesized, along with their Zn(II) complexes. Upon excitation in the arm‐centred π‐π* absorption band, an efficient energy transfer takes place from the peripheral fluorenyl units to the central porphyrin core, leading to intense red‐light emission and oxygen photosensitization by the latter. More interestingly, while the linear optical properties of these porphyrins were only slightly improved compared to those of their dendrimer analogues for photodynamic therapy (PDT) or fluorescence imaging, their 2PA cross‐sections were much more significantly boosted, evidencing the key role played by different structures on nonlinear optical properties. Finally, by comparison with other porphyrin‐based two‐photon photosensitizers reported in the literature, we show that these new "semi‐disconnected" starburst systems exhibit a remarkable trade‐off between intrinsic 2PA, fluorescence and oxygen photosensitization. [ABSTRACT FROM AUTHOR]

This article reports three new two-photon absorption (TPA) materials that are quinolinium-carbazole derivates. They are 3-(N-methyl-4-ethylquinolinium iodide)-9-ethylcarbazole (M4), 3-(N-methyl-4-ethylquinolinium iodide)-9-ethylcarbazole (H2), and 3-(N-methyl-4-ethylquinolinium iodide)-9-ethylcarbazole (H4). Their TPA cross-sections are 491, 515, and 512 GM, respectively. Under the excitation of near-infrared light, their fluorescence emission is about 650 nm. The compounds can stain nucleic acid DNA with the same level of nuclear localization as Hoechst 33342. Under continuous irradiation with a near-infrared laser, the three new compounds showed less fluorescence decay than DAPI, and the average fluorescence decay rates were 0.016%/s, 0.020%/s, and 0.023%/s. They are expected to become new two-photon fluorescent probes of nucleic acid DNA because of their excellent performance. [ABSTRACT FROM AUTHOR]

We investigate the nonlinear optical response of Pb2+-doped ZnMoO4 nanostructures synthesized by chemical precipitation technique. X-ray diffraction (XRD) and Raman spectroscopy results confirm the formation of ZnMoO4 nanostructures with a triclinic structure. High-Resolution Transmission electron microscope (HRTEM) reveals the deformed spherical shape of nanostructures. Further, the successful doping and molecular bonding of Pb2+ with ZnMoO4 was evidenced by FTIR and XPS. UV–Vis absorption spectra reveals the alteration in the electronic band structure of ZnMoO4 nanostructures upon Pb doping, leading to a considerable increase in bandgap and enhanced charge transfer within ZnMoO4: Pb nanostructure. The nonlinear optical absorption and optical limiting characteristics of the nanoparticles is measured via the open aperture Z-scan technique with an Nd: YAG pulsed laser at 532 nm, exhibiting a reverse saturable absorption and a rapid decrease in the commencement of the optical limiting threshold with the increased in Pb concentration. The observed nonlinear absorption can be attributed to the crystal structure, particle size, creation of oxygen vacancies, and defects on the host lattice. These results suggest that Pb2+-doped ZnMoO4 nanoparticles have excellent prospects for optical limiting (OL) applications. This could be useful for applications such as laser protection and optoelectronic device applications. [ABSTRACT FROM AUTHOR]

A novel water-soluble substituted pyridine dicarboxylate containing a quaternary ammonium ion with potential applications in fluorescence cell imaging was synthesized and characterized. Remarkably, this compound exhibited water solubility in the absence of additional solubilizers, enabling direct dissolution in phosphate-buffered saline for cell studies. No obvious cytotoxicity was observed in 4T1 cells (mouse breast cancer cells) within the concentration range of 0.2–25 μmol L−1, with a cell survival rate above 89%. Furthermore, the compound exhibited a maximum two-photon absorption cross-section of 86 GM at an excitation wavelength of 780 nm, demonstrating high cell permeability, effective distribution in the cytoplasm, and preferential targeting of organelles. Single- and two-photon fluorescence imaging of cells revealed a distinctive blue emission and strong bright green emission, respectively. The newly synthesized substituted pyridine dicarboxylate exhibited low cytotoxicity and strong fluorescence imaging properties, demonstrating its potential in cell imaging applications.

As fast-emerging two-dimensional (2D) materials, MXenes have attracted great attention in the fields of optics and electricity. In this paper, the nonlinear optical (NLO) properties of the 2D MXene material (Nb4C3) and its MAX predecessor (Nb4AlC3) are presented. Also, the broadband NLO responses of Nb4AlC3 and Nb4C3 nanosheets had been investigated using the open aperture (OA) Z-scan technique. The results showed that 2D Nb4AlC3 exhibited significant two-photon absorption (TPA) properties in the short-wave infrared (SWIR) band. Different from Nb4AlC3, its etching residue, Nb4C3, exhibited totally opposite NLO behavior, which showed superior TPA properties in the ultraviolet (UV) band. This implied that the optical limiting (OL) properties of these two materials were exactly opposite. In the future, 2D Nb4AlC3 and Nb4C3 will be expected to be used as OL materials in the UV and SWIR bands, respectively, for such applications as laser protection, human eye protection, making optical filters, etc. Our work expanded a great potential application of 2D MXenes materials in the broadband nonlinear optical photonics. [ABSTRACT FROM AUTHOR]

Achieving high‐quality, frequency‐upconversion single‐mode lasing output is an important requirement for developing new nonlinear optoelectronic devices, such as on‐chip optical communication, nonlinear optical switches, and optical parametric amplifiers. Here, an individual CsPbBr3 microwire prepared by the anti‐solvent method is served as both gain media and microresonator to achieve two‐photon pumped frequency upconversion single‐mode lasing with the side‐mode suppression ratio of 18 dB. Meanwhile, the refractive index of orthorhombic perovskite is theoretically calculated based on first principles, and determining its square whispering‐gallery oscillation type by combining with the plane wave model and the steady‐state oscillation conditions of laser. The extracted exciton binding energy of 33.15 meV higher than the thermal ionization energy of room temperature (≈26 meV) suggests that the as‐grown CsPbBr3 microwires possess the capacity to achieve two‐photon pumped lasing output, as well further gaining deeper insights into the role of exciton‐phonon coupling in light emission. [ABSTRACT FROM AUTHOR]

Micro/nano structures with morphological gradients possess unique physical properties and significant applications in various research domains. This study proposes a straightforward and precise method for fabricating micro/nano structures with morphological gradients utilizing single-voxel synchronous control and a nano-piezoelectric translation stage in a two-photon laser direct writing technique. To address the defocusing issue in large-scale fabrication, a methodology for laser focus dynamic proactive compensation was developed based on fluorescence image analysis, which can achieve high-precision compensation of laser focus within the entire range of the nano-piezoelectric translation stage. Subsequently, the fabrication of micro/nano dual needle structures with morphological gradients were implemented by employing different writing speeds and voxel positions. The minimum height of the tip in the dual needle structure is 80 nm, with a linewidth of 171 nm, and a dual needle total length reaching 200 μm. Based on SEM (scanning electron microscope) and AFM (atomic force microscope) characterization, the dual needle structures fabricated by the method proposed in this study exhibit high symmetry and nanoscale gradient accuracy. Additionally, the fabrication of hexagonal lattice periodic structures assembled from morphological gradient needle structures and the size gradient Archimedean spiral structures validate the capability of the single voxel-based fabrication and proactive focus compensation method for complex gradient structure fabrication. [ABSTRACT FROM AUTHOR]

In the last decades, inorganic nanoparticles have attracted growing attention in the field of nanophotonics, especially for bioimaging purposes. Among them luminescent metal-, semiconductor- or oxide-based "hard" nanoparticles have been the most widely used. Yet, they raise concern with respect to toxicity and degradability issues. In that context, we have developed innovative bottom-up approaches towards ultrabright dye-based fluorescent organic nanoparticles (dFONs). Our strategy is based on the design and synthesis of custom-designed (multi)polar and polarizable dyes (PPDs) as building blocks of dFONs. These nanoparticles are readily prepared using expeditious and green protocols involving nanoprecipitation of the hydrophobic dyes in water. Their luminescence can be tuned in the whole visible region down to the Near Infra-Red I (NIR-I) region while their nonlinear optical responses can be enhanced thanks to cooperative effects. Intriguingly, the implemented strategy also enables modulating and improving the dFONs colloidal and structural stability as well as their photostability. As a result, dFONs made from PPDs that combine unprecedented brightness (up to 108M-1 ·cm-1 and 106 GM), remarkable colloidal stability and absence of toxicity, have been elaborated, providing superior substitutes to Quantum Dots. Such dFONs can be used as ultrasensitive contrast agents for in vivo two-photon angiography in small animals, while hyper-bright NIR-emitting FONs, that show remarkable photostability and excellent biocompatibility, can be successfully imaged and tracked at the single particle level in water. Furthermore, ultrabright dFONs of different colors that internalize into cells can be tracked within living cells allowing real-time multi-color single particles tracking. In contrast, stealth emitters are required for tracking cell-surface receptors or exploring the extracellular space. In this direction, spontaneously stealth, size-tunable, ultrabright and red to NIR emitting dFONs were developed. Thanks to these unique properties, these dFONs could be imaged and tracked up to 150 µmdeep in brain tissue. [ABSTRACT FROM AUTHOR]

This review presents the theoretical background concerning simplified quantum chemistry (sQC) methods to compute non‐linear optical (NLO) properties and their applications to large systems. To evaluate any NLO responses such as hyperpolarizabilities or two‐photon absorption (2PA), one should evidently perform first a ground state calculation and compute its response. Because of this, methods used to compute ground states of large systems are outlined, especially the xTB (extended tight‐binding) scheme. An overview on approaches to compute excited state and response properties is given, emphasizing the simplified time‐dependent density functional theory (sTD‐DFT). The formalism of the eXact integral sTD‐DFT (XsTD‐DFT) method is also introduced. For the first hyperpolarizability, 2PA, excited state absorption, and second hyperpolarizability, a brief historical review is given on early‐stage semi‐empirical method applications to systems that were considered large at the time. Then, we showcase recent applications with sQC methods, especially the sTD‐DFT scheme to large challenging systems such as fluorescent proteins or fluorescent organic nanoparticles as well as dynamic structural effects on flexible tryptophan‐rich peptides and gramicidin A. Thanks to the sTD‐DFT‐xTB scheme, all‐atom quantum chemistry methodologies are now possible for the computation of the first hyperpolarizability and 2PA of systems up to 5000 atoms. This review concludes by summing‐up current and future method developments in the sQC framework as well as forthcoming applications on large systems. This article is categorized under:Electronic Structure Theory > Ab Initio Electronic Structure MethodsStructure and Mechanism > Molecular StructuresElectronic Structure Theory > Density Functional TheoryElectronic Structure Theory > Semiempirical Electronic Structure Methods [ABSTRACT FROM AUTHOR]

A series of four donor–acceptor–donor derivatives bearing 4H‐pyranylidene and 4‐methylcyclohexan‐1‐one units as donor and acceptor groups respectively is designed, synthesized, and photophysically characterized. Both experimental and theoretical studies reveal good two‐photon absorption (2PA) properties for these systems. Decoration of the exocyclic position of the 4H‐pyranylidene moiety with a thiophene ring results in high 2PA cross‐section values (σ2PA) ≈700 nm, and remarkably, in the region between 900 and 1000 nm. Furthermore, all chromophores are evaluated as photoinitiators (PIs) for two‐photon‐laser printing at 780 nm, showing superior performance compared to the commonly used commercially available PI, phenylbis(2,4,6‐trimethylbenzoyl)phosphine oxide. Among the studied PIs, the ones equipped with a thiophene unit at the exocyclic position of the 4H‐pyranylidene moiety exhibit the highest efficiency, enabling fast printing using low laser powers with even lower concentrations of PI. Overall, this study shows the great potential of this new class of PIs for application in the field of 3D nanoprinting. [ABSTRACT FROM AUTHOR]

It is shown that the two-photon absorption (TPA)-induced losses in a non-centrosymmetric crystal reduce considerably when the angle θ between the polarization vector of incident radiation and the optical c-axis is 45°. In such a geometry of interaction in a Ca3(VO4)2 crystal, the effective TPA coefficient for 300 fs laser pulses at θ = 45° is 3.5 ± 0.5 times lower than its maximum at θ = 0°, which is more than two times higher than reported earlier in the literature.

We demonstrate how two-photon excitation with quantum light can influence elementary photochemical events. The azobenzene trans → cis isomerization following entangled two-photon excitation is simulated using quantum nuclear wave packet dynamics. Photon entanglement modulates the nuclear wave packets by coherently controlling the transition pathways. The photochemical transition state during passage of the reactive conical intersection in azobenzene photoisomerization is strongly affected with a noticeable alteration of the product yield. Quantum entanglement thus provides a novel control knob for photochemical reactions. The distribution of the vibronic coherences during the conical intersection passage strongly depends on the shape of the initial wave packet created upon quantum light excitation. X-ray signals that can experimentally monitor this coherence are simulated.

In the continuation of our sustained interest in porphyrin-based dendrimers and their use as luminescent photosensitizers for two-photon photodynamic therapy (2P-PDT), we wondered about the effect of changing the connectors in our macromolecular structures. We also wanted to initiate preliminary studies on meso-tetraarylporphyrins decorated with more electron-releasing arms. Thus, various meso-tetrafluorenylporphyrin-cored star-shaped and dendrimeric derivatives have been synthesized and characterized, as well as their zinc(II) complexes. In the new dendrimeric derivatives, the peripheral fluorenyl units of the dendrons are linked to the inner core either by N-phenylcarbazole (CCbz) or triphenylamine (CTpa) connectors instead of the more classic 1,3,5-phenylene (CPh) linkers previously used by us. Selected linear and non-linear optical (LO and NLO) properties were then determined for these compounds via absorption or emission studies and by two-photon excited fluorescence (TPEF) measurements. It was found that the CCbz-containing dendrimer, which has the most rigid structure, exhibits a significantly lower two-photon absorption (2PA) cross-section than its CTpa analog, presenting a more flexible structure while rather similar luminescence and singlet oxygen activation quantum yields are found for both. The origin of this unexpected discrepancy is briefly discussed based on our photophysical data. It is then demonstrated that the latter dendrimer also outperforms several closely related dendrimers in terms of 2PA action cross-section and 2PA-oxygen sensitization, making its molecular architecture quite appealing for developing new 2PA photosensitizers suited to theranostic uses.

We developed multi‐branched π‐conjugated systems using squaraine dyes with triphenylamine cores connected by ethynylene linkers. We investigated the influence of interbranch coupling between squaraine branches on their one‐photon (OPA) and two‐photon absorption (TPA) properties. These dyes with triphenylamine components showed a red‐shifted one‐photon absorption (OPA) compared to the precursor squaraine dyes. Among branched dyes, the lack of apparent splitting or shift in the absorption maxima, even as the absorption intensity increased with the number of chromophores, implies the presence of limited exciton coupling between the squaraine branches. The present squaraine dyes with triphenylamine cores exhibited a moderate TPA compared to the precursor squaraine without the triphenylamine core, due to the extended π‐conjugation. Notably, both the 3‐branched and 2‐branched dyes demonstrated additional enhancement in the TPA response, surpassing that of the monochromophoric counterpart. This resulted in achieving a substantial TPA cross section of up to 3905 GM at 830 nm. [ABSTRACT FROM AUTHOR]

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Photon‐upconversion in organic molecular systems is one of the promising technologies for future energy harvesting systems because these systems can generate excitons that possess higher energy than excitation energy. The photon‐upconversion caused by absorbing ambient heat as additional energy is particularly interesting because it could ideally provide a light‐driving cooling system. However, only a few organic molecular systems have been reported. Here, we report the anti‐Stokes photoluminescence (ASPL) derived from hot‐band absorption in a series of multi‐resonance‐type thermally‐activated delayed fluorescence (MR‐TADF) molecules. The MR‐TADF molecules exhibited an anti‐Stokes shift of approximately 0.1 eV with a high PL quantum yield in the solution state. The anti‐Stokes shift corresponded well to the 1–0 vibration transition from the ground state to the excited singlet state, and we further evaluated a correlation between the activation energy for the ASPL intensity and the TADF process. Our demonstration underlines that MR‐TADF molecules have become a novel class of ASPL materials for various future applications, such as light‐driving cooling systems. [ABSTRACT FROM AUTHOR]

In this work, we study the nonlinear absorption of ultrashort laser pulses of variable duration (0.3‒10 ps) in a bulk polycrystalline ZnSe dielectric. The transmission of the sample was measured for two wavelengths, 515 and 1030 nm, in a wide range of intensities. A nonlinear behavior of the transmission is found when the sample is irradiated with laser radiation at a wavelength of 515 nm with absorption saturation in the region of 3–5 TW/cm2. Surface damage thresholds were obtained for a wavelength of 1030 nm for pulse durations of 0.3–10 ps. [ABSTRACT FROM AUTHOR]

Partial genetically encoded 4-hydroxybenzylidene-imidazolinone (HBI)-type chromophores are new promising fluorescent probes, which are suitable for imaging and detection of living cells. However, the lack of infrared chromophores hinders the development seriously. Here more than 30 HBI-type chromophores with regular structure modifications were employed and typical spectral redshift change laws and mechanisms were investigated by quantum methods. Results show that both one-photon spectrum (OPS, absorption/emission) and two-photon absorption (TPA) can achieve large redshift via either extending conjugated lengths of frag-3 or enlarging conjugated areas of frag-1 of HBI skeleton. Spectral redshifts of all chromophores are highly related to intramolecular charge transfer (ICT), but neutral ones are closely related to the total ICT or electron-accepting-numbers of frag-3, and the high correlative factor of anions is the aromaticity of frag-2 bridge. The frag-2 bridge with high aromaticity can open a reverse charge transfer channel in anion relative to neutral, obtaining significant redshift. Based on analysis, a new 6-hydroxyl-naphthalene-imidazolinone (HNI) series, which have larger conjugated area in frag-1, are predicted. The OPS and TPA of anionic HNI ones acquire about 76–96 nm and 119–146 nm red-shift relative to traditional HBI series respectively as a whole. The longest emission of anionic HNI-4 realizes more 244 nm redshift relative to HBI-1. Our work clarifies worthy spectral regularities and redshift mechanisms of HBI-type chromophores and provides valuable design strategy for infrared chromophores synthesis in experiment. [ABSTRACT FROM AUTHOR]

Recent studies have opened the door to a new generation of photoinitiators for 3D laser nanoprinting. Therein, the simultaneous absorption of two photons, commonly referred to as two‐photon absorption, is replaced by the sequential absorption of two photons in two consecutive one‐photon absorption processes. This process has been termed two‐step absorption. Importantly, two‐step absorption can be accomplished by inexpensive compact low‐power continuous‐wave blue laser diodes instead of femtosecond laser systems in the red spectral region. Red‐shifting the second absorption step with respect to the first one results in an and‐type optical nonlinearity based on two‐color two‐step absorption. Herein, alternatives are systematically explored to the few already reported one‐ and two‐color two‐step‐absorption photoinitiators, including the search for photoinitiators that can be excited by one‐color two‐step absorption and be de‐excited by a disparate laser color. [ABSTRACT FROM AUTHOR]

The development of fluorescent probe for hydrazine (N2H4) detection has attracted much attention due to the important role of N2H4 plays in the fields of medicine, agriculture, biology and environments. In this paper, the optical properties and water solubility of two novel two-photon fluorescent molecular probes (Probe1 and Probe2) before and after the reaction with N2H4 are studied by using the density function theory. The results show that electronic distribution and transition dipole moment of the probes are obviously changed after the reaction with N2H4, thus the optical properties of the molecules are influenced and the detection of N2H4 are realized. In addition, photoinduced electron transfer processes for Probe1 and Probe2 in the presence of N2H4 are theoretically characterized, which explains the experimental observations from the microscopic mechanism. Special attention has been paid on the analysis of the two-photon absorption for the probes with the absence and presence of N2H4 by the response theory method. Both probes with good water solubility show large variation on the two-photon absorption cross section when reacts with N2H4. In particular, the two-photon absorption response of Probe2 is more obvious, so it possesses preferable two-photon fluorescence microscopic imaging ability. More importantly, the receptor effect on the sensing performances of the probes are demonstrated, providing a theoretical reference for the design and synthesis on more efficient two-photon fluorescence N2H4 probes. Our study provides necessary information on the response mechanism of the studied chemosensors and helps to establish the relationship between the structure and optical properties of two-photon fluorescence N2H4 probes. [ABSTRACT FROM AUTHOR]

In the continuation of our sustained interest in porphyrin-based dendrimers and their use as luminescent photosensitizers for two-photon photodynamic therapy (2P-PDT), we wondered about the effect of changing the connectors in our macromolecular structures. We also wanted to initiate preliminary studies on meso-tetraarylporphyrins decorated with more electron-releasing arms. Thus, various meso-tetrafluorenylporphyrin-cored star-shaped and dendrimeric derivatives have been synthesized and characterized, as well as their zinc(II) complexes. In the new dendrimeric derivatives, the peripheral fluorenyl units of the dendrons are linked to the inner core either by N-phenylcarbazole (CCbz) or triphenylamine (CTpa) connectors instead of the more classic 1,3,5-phenylene (CPh) linkers previously used by us. Selected linear and non-linear optical (LO and NLO) properties were then determined for these compounds via absorption or emission studies and by two-photon excited fluorescence (TPEF) measurements. It was found that the CCbz-containing dendrimer, which has the most rigid structure, exhibits a significantly lower two-photon absorption (2PA) cross-section than its CTpa analog, presenting a more flexible structure while rather similar luminescence and singlet oxygen activation quantum yields are found for both. The origin of this unexpected discrepancy is briefly discussed based on our photophysical data. It is then demonstrated that the latter dendrimer also outperforms several closely related dendrimers in terms of 2PA action cross-section and 2PA-oxygen sensitization, making its molecular architecture quite appealing for developing new 2PA photosensitizers suited to theranostic uses. [ABSTRACT FROM AUTHOR]

The crystal structure of the title compound, C13H9Br2N [systematic name: (E)-N,1-bis(4-bromophenyl)methanimine], is a second polymorph (Form II) crystallizing in the orthorhombic space group Pccn. The first polymorph (Form I) crystallizes in the monoclinic space group P21/c [Bernstein & Izak (1975). J. Cryst. Mol. Struct. 5, 257–266; Marin et al. (2013). J. Mol. Struct. 1049, 377–385]. The molecule is disordered about an inversion center situated in the middle of the C=N bond, similar to the situation in the monoclinic polymorph: the C=N bond length is 1.243 (7) Å. In the crystal, molecules stack along the b-axis direction and are linked by C—H...π interactions. The interatomic contacts in the crystal for both polymorphs were studied by Hirshfeld surface analysis and have notable differences. The solid-state fluorescence spectrum of Form II shows an emission peak at ca 469 nm. The two-photon absorption coefficient measured from the open aperture Z-scan technique is 1.3 × 10 −11 m W−1, hence, Form II shows optical limiting behaviour.

Genetically encoded probes with red-shifted absorption and fluorescence are highly desirable for imaging applications because they can report from deeper tissue layers with lower background and because they provide additional colors for multicolor imaging. Unfortunately, red and especially far-red fluorescent proteins have very low quantum yields, which undermines their other advantages. Elucidating the mechanism of nonradiative relaxation in red fluorescent proteins (RFPs) could help developing ones with higher quantum yields. Here we consider two possible mechanisms of fast nonradiative relaxation of electronic excitation in RFPs. The first, known as the energy gap law, predicts a steep exponential drop of fluorescence quantum yield with a systematic red shift of fluorescence frequency. In this case the relaxation of excitation occurs in the chromophore without any significant changes of its geometry. The second mechanism is related to a twisted intramolecular charge transfer in the excited state, followed by an ultrafast internal conversion. The chromophore twisting can strongly depend on the local electric field because the field can affect the activation energy. We present a spectroscopic method of evaluating local electric fields experienced by the chromophore in the protein environment. The method is based on linear and two-photon absorption spectroscopy, as well as on quantum-mechanically calculated parameters of the isolated chromophore. Using this method, which is substantiated by our molecular dynamics simulations, we obtain the components of electric field in the chromophore plane for seven different RFPs with the same chromophore structure. We find that in five of these RFPs, the nonradiative relaxation rate increases with the strength of the field along the chromophore axis directed from the center of imidazolinone ring to the center of phenolate ring. Furthermore, this rate depends on the corresponding electrostatic energy change (calculated from the known fields and charge displacements), in quantitative agreement with the Marcus theory of charge transfer. This result supports the dominant role of the twisted intramolecular charge transfer mechanism over the energy gap law for most of the studied RFPs. It provides important guidelines of how to shift the absorption wavelength of an RFP to the red, while keeping its brightness reasonably high.

Simultaneous interrogation of pump and probe beams interacting in ZnO nanostructures of a two-wave mixing is proposed for dual-path data processing of optical signals by nonlinear optical effects. An enhancement in third-order nonlinear optical properties was exhibited by Al-doped ZnO thin films. Multiphoton absorption and nonlinear refraction were explored by the z-scan technique at 532 nm with nanosecond pulses. The evolution of the optical Kerr effect in the ZnO thin films was analyzed as a function of the incorporation of Al in the sample by a vectorial two-wave mixing method. Electrical and photoconductive effects were evaluated to further characterize the influence of Al in the ZnO solid samples. Potential applications of nonlinear optical parameters for encoding and encrypting information in light can be envisioned.

The donor-acceptor interaction of a charge-coupled heterostructure encompassing a metal and an amorphous semiconductor subjected to a laser field has many potential applications in the realm of nonlinear optics. In this work, we fabricate an electron donor gold nanoparticle (AuNP) and acceptor amorphous Ge24Se76 heterostructure on a quartz substrate using a sequential thermal evaporation technique. In this charge coupled heterostructure, we demonstrate the ultrafast third-order nonlinear absorptive and refractive response, and their sign reversal compared to pristine Ge24Se76. Enhanced optical nonlinearity in these heterostructures of varying plasmonic wavelengths is due to charge transfer, verified by the Raman spectroscopy. Further, the ultrafast transient absorption measurements support the thesis of charge transfer in the AuNP/Ge24Se76 heterostructure. These findings open up exciting opportunities for developing novel device technologies with far-reaching applications in nonlinear optics.

The first-, second-, and third-order molecular nonlinear optical properties, including two-photon absorption of a series of derivatives, involving two dithienylethene (DTE) groups connected by several molecular linkers (bis(ethylene-1,2-dithiolato)Ni- (NiBDT), naphthalene, quasilinear oligothiophene chains), are investigated by employing density functional theory (DFT). These properties can be efficiently controlled by DTE switches, in connection with light of appropriate frequency. NiBDT, as a linker, is associated with a greater contrast, in comparison to naphthalene, between the first and second hyperpolarizabilities of the "open–open" and the "closed–closed" isomers. This is explained by invoking the low-lying excited states of NiBDT. It is shown that the second hyperpolarizability can be used as an index, which follows the structural changes induced by photochromism. Assuming a Förster type transfer mechanism, the intramolecular excited-state energy transfer (EET) mechanism is studied. Two important parameters related to this are computed: the electronic coupling (VDA) between the donor and acceptor fragments as well as the overlap between the absorption and emission spectra of the donor and acceptor groups. NiBDT as a linker is associated with a low electronic coupling, VDA, value. We found that VDA is affected by molecular geometry. Our results predict that the linker strongly influences the communication between the open–closed DTE groups. The sensitivity of the molecular nonlinear optical properties could assist with identification of molecular isomers. [ABSTRACT FROM AUTHOR]

Effects of hydrogen bonds on two-photon absorption (TPA) of a new donor-acceptor type green fluorescent protein chromophore analogue are investigated by employing a combined molecular dynamics and quantum chemistry method. The probable configurations of the chromophore in water are extracted from molecular dynamics simulation and the TP A properties of more than twenty hydrogen bond complexes are computed by quadratic response theory. Thereby, the structure and property relations are established. Three types of hydrogen bonds including O⋯H–O, N–H⋯O and N⋯H–O can be formed between the chromophore and water molecules. The O⋯H–O induces a little decrease of TPA cross section with a red-shifted wavelength. The N–H⋯O gives rise to a great enhancement of TP A at a longer wavelength, while the N⋯H–O decreases TP A significantly and makes the wavelength blue-shifted. The reasons for these effects are rationalized well by using a two-state model analysis. The related molecular orbitals are also plotted to visualize the charge transfer characters. In addition, the averaged TP A spectrum is obtained by calculating the probabilities of various hydrogen bond complexes. Our research could provide a good insight into the design of two-photon materials by making use of hydrogen bond networks. [ABSTRACT FROM AUTHOR]