Two-photon 3D optical data storage via aggregate switching of excimer-forming dyes

Charge transfer excitons and directional fluorescence of in-plane lateral MoSe2-WSe2 heterostructures

In this article, the linear and nonlinear optical properties of in-plane lateral MoSe2-WSe2 heterostructures are theoretically investigated. The polarization-dependent strongest optical absorption in one-photon absorption occurs in charge transfer excited states, where electrons transfer from WSe2 to MoSe2. This phenomenon is supported by the LUMO (lowest unoccupied molecular orbital) and HUMO (highest occupied molecular orbital) imaging obtained through scanning tunneling microscopy. The charge difference density and transition density matrix are used to interpret the electronic transitions, and these interpretations rely on the concept of transition density. The optical properties of two-photon absorption in its nonlinear optical process are significantly different from the excitation in one-photon absorption, where the strongest optical absorption is contributed from direct transition from the ground state to the final state without going through an intermediate excited state, due to the very large difference of permanent dipole moments between the excited and ground states. Our results also reveal directional fluorescence and physical mechanism of in-plane lateral MoSe2-WSe2 heterostructures. Our work can provide insights into the physical mechanism of the optical properties of in-plane lateral MoSe2-WSe2 heterostructures.

The enhancement of nonlinear optical properties of azulene-based nanographene by N atoms: a finishing touch

Nonlinear optical (NLO) materials play an increasingly important role in optoelectronic devices, biomedicine, micro-nano processing, and other fields. The development of organic materials with strong second or (and) third NLO properties and a high stability is still challenging due to the unknown strategies for obtaining enhanced high order NLO properties. In the present work, π-conjugated systems are constructed by doping boron or (and) nitrogen atoms in the azulene moiety of azulene-based nanographenes (formed with an azulene chain with two bridging HCCHs at the two sides of the connecting CC bonds between azulenes, A1A2A3), and the NLO properties are predicted with time-dependent density functional theory based methods and a sum-over-states model. The doping of heteroatoms induces charge redistribution, tunes the frontier molecular orbital energy gap, changes the composition of some frontier molecular orbitals, and affects the NLO properties of those nanographenes. Among the designed nanographenes, the azulene-based nanographene with two nitrogen atoms at the two ends has the largest static first hyperpolarizability (91.30 × 10-30 esu per heavy atom), and the further introduction of two N atoms at the two ends of the central azulene moiety of this nanographene results in a large static second hyperpolarizability while keeping the large static first hyperpolarizability.

Size economy and first hyperpolarizability: synthesis and nonlinear optical behavior of ferrocene-based donor-acceptor chromophores lacking π-linkers

The question of size economy in the design of chromophores for nonlinear optics is addressed in this investigation. We have synthesized directly linked donor-acceptor dyads, which lack a π-conjugated linker, the presence of which is usually considered obligatory in materials designed for nonlinear optics. Correlating linear optical data, electrochemical data, computational data and hyper Rayleigh scattering (HRS) data on ferrocene (Fc) based dyads, we demonstrate that the first hyperpolarizability of such size economical chromophores is significantly better compared to that of Fc based, traditional, larger, donor-π-acceptor chromophores. Arguably, a larger π-conjugated linker decreases the electronic communication between the donor and the acceptor and weakens the intramolecular charge transfer in such chromophores.

Control of Fluorescence of Organic Dyes in the Solid-State by Supramolecular Interactions

Fluorescent organic dyes play an essential role in the creation of new "smart" materials. Fragments and functional groups capable of free rotation around single bonds can significantly change the fluorescent organic dye's electronic structure under analyte effects, phase state transitions, or changes in temperature, pressure, and media polarity. Dependencies between steric and electronic structures become highly important in transition from a solution to a solid-state. Such transitions are accompanied by a significant increase in the dye molecular structure's rigidity due to supramolecular associates' formation such as H-bonding, π···π and dipole-dipole interactions. Among those supramolecular effects, H-bonding interactions, first of all, lead to significant molecular packing changes between loose or rigid structures, thus affecting the fluorescent dye's electronic states' energy and configuration, its fluorescent signal's position and intensity. All the functional groups and heteroatoms that are met in the organic dyes seem to be involved in the control of fluorescence via H-bonding: C-H···N, C-H···π, S = O···H-C, P = O···H, C-H···O, NH···N, C - H···C, C - H···Se, N-H···O, C - H···F, C-F···H. Effects of molecular packing of fluorescent organic dyes are successfully used in developing mechano-, piezo-, thermo- fluorochromes materials for their applications in the optical recording of information, sensors, security items, memory elements, organic light-emitting diodes (OLEDs) technologies.

Nonlinear plexcitons: excitons coupled with plasmons in two-photon absorption

The nonlinear optical properties of a D-A (donor-acceptor) conjugated organic molecule with polythiophene (PT) as the donor and indene-C₆₀ bisadduct (IC₆₀BA) as the acceptor are theoretically investigated, which exhibits a large two-photon absorption (TPA) cross-section up to 8000 GM at the wavelength of 780 nm. Combining surface plasmon resonances (SPRs) with nonlinear optics, nonlinear properties can be strongly enhanced. In this paper, an appropriate nonlinear plexciton method by the coupling of Au@Ag nanorods and an Ag film is designed, in which the TPA properties of the PT:IC₆₀BA complex can be increased by 10⁶ times. The angle dependence on polarization and incidence is investigated to obtain the maximum of plasmonic enhancement. Our results emphasize the physical mechanism of nonlinear plexcitons and provide a feasible method to improve the nonlinear properties of organic solar cell materials.

Two-Photon Absorption Cooperative Effects within Multi-Dipolar Ruthenium Complexes: The Decisive Influence of Charge Transfers

One- and two-photon characterizations of a series of hetero- and homoleptic [RuL_3-n(bpy)_n]²⁺ (n = 0, 1, 2) complexes carrying bipyridine π-extended ligands (L), have been carried out. These π-extended D−π−A−A−π−D-type ligands (L), where the electron donor units (D) are based on diphenylamine, carbazolyl, or fluorenyl units, have been designed to modulate the conjugation extension and the donating effect. Density functional theory calculations were performed in order to rationalize the observed spectra. Calculations show that the electronic structure of the π-extended ligands has a pronounced effect on the composition of HOMO and LUMO and on the metallic contribution to frontier MOs, resulting in strikingly different nonlinear properties. This work demonstrates that ILCT transitions are the keystone of one- and two-photon absorption bands in the studied systems and reveals how much MLCT and LLCT charge transfers play a decisive role on the two-photon properties of both hetero- and homoleptic ruthenium complexes through cooperative or suppressive effects.

Enhanced Multiphoton Processes in Perovskite Metasurfaces

Multiphoton absorption and luminescence are fundamentally important nonlinear processes for utilizing efficient light-matter interaction. Resonant enhancement of nonlinear processes has been demonstrated for many nanostructures; however, it is believed that all higher-order processes are always much weaker than their corresponding linear processes. Here, we study multiphoton luminescence from structured surfaces and, combining multiple advantages of perovskites with the concept of metasurfaces, we demonstrate that the efficiency of nonlinear multiphoton processes can become comparable to the efficiency of the linear process. We reveal that the perovskite metasurface can enhance substantially two-photon stimulated emission with the threshold being comparable with that of the one-photon process. Our modeling of free-carrier dynamics and exciton recombination upon nonlinear photoexcitation uncovers that this effect can be attributed to the local field enhancement in structured media, a substantial increase of the mode overlap, and the selection rules of two-photon absorption in perovskites.

The role of twisting in driving excited-state symmetry breaking and enhanced two-photon absorption in quadrupolar cationic pyridinium derivatives

Two symmetric quadrupolar cationic push-pull compounds with a central electron-acceptor (N+-methylpyrydinium, A+) and different lateral electron-donors, (N,N-dimethylamino and N,N-diphenylamino, D) in a D-π-A+-π-D arrangement, were investigated together with their dipolar counterparts (D-π-A+) for their excited-state dynamics and NLO properties. As for the quadrupolar compounds, attention was focused on excited-state symmetry breaking (ESSB), which leads to a relaxed dipolar excited state. Both electron charge displacements and structural rearrangements were recognized in the excited-state dynamics of these molecules by resorting to femtosecond-resolved broadband fluorescence up-conversion experiments and advanced data analysis, used as a valuable alternative approach for fluorescent molecules compared to time-resolved IR spectroscopy, only suitable for compounds bearing IR markers. Specifically, intramolecular charge transfer (ICT) was found to be guided by ultrafast inertial solvation, while diffusive solvation can drive the twisting of lateral groups to originate twisted-ICT (TICT) states on a picosecond time scale. Yet still, only the bis-N,N-diphenylamino-substituted compound undergoes ESSB, in both highly and sparingly polar solvents, provided that it can experience large amplitude motions to a fully symmetry-broken TICT state. Besides well-known solvation effects, this structural requirement proved to be a necessary condition for these quadrupolar cations to undergo ESSB. In fact, a more efficient uncoupling between the out-of-plane D and A+ groups in the TICT state allows a greater stabilization gained through solvation, relative to the bis-N,N-dimethylamino-substituted derivative, which instead maintains its symmetry. This different behavior parallels the two-photon absorption (TPA) ability, which is greatly enhanced in the case of the bis-N,N-diphenylamino-substituted compound, paving the way for cutting-edge bio-imaging applications.

Tunable Structural Color Patterns Based on the Visible-Light-Responsive Dynamic Diselenide Metathesis

Structural color materials with reversible stimuli-responsiveness to external environment have been widely used in sensors, encryption, display, and other fields. Compared with other stimuli, visible light is highly controllable both temporally and spatially with less damage to materials, which is more suitable for structural color patterning. Herein, a new diselenide-containing shape memory material is prepared and used for creating patterns via visible light stimulus. In this system, the structural color originates from birefringence of stretched materials, whose shapes can be fixed while maintaining the mechanical stress. The fixed stress can be released by diselenide metathesis under visible light irradiation. By regulating the wavelength or irradiation time with a commercial projector, the pattern with tunable structural colors is realized and the structural color pattern can be erased and rewritten arbitrarily. During the patterning process, the optical signal is first stored as mechanical signal and then transformed back to optical signal. It is a new method for preparing visible-light-responsive structural color material and has great potential in display devices, anticounterfeiting labels, and data storage.

Modification of side chain of conjugated molecule for enhanced charge transfer and two-photon activity

Amounts of strategies implemented to obtain improved two-photon absorption responses but remains challenging. Herein, a serials zwitterionic chromophores, TSEO1-3, with D-π-A configuration were rational designed and synthesized. Notably, by minor modification of the side chain, the obtained TSEO3 exhibited enhanced two-photon activity and considerable two-photon imaging in vitro and in vivo. It manifested that appropriate modifications of side chains that are linked to conjugated frameworks can improve the intermolecular packing order and boost charge transfer favoring two-photon activity.

Enhanced two-photon absorption of ligated silver and gold nanoclusters: theoretical and experimental assessments

Ligated silver and gold nanoclusters belonging to a non-scalable size regime with molecular-like discrete electronic states represent an emerging class of extremely interesting optical materials. Nonlinear optical (NLO) characteristics of such quantum clusters have revealed remarkable features. The two-photon absorption (TPA) cross section of ligated noble metal nanoclusters is several orders of magnitude larger than that of commercially-available dyes. Several such case studies on NLO properties of ligated silver and gold nanoclusters have been reported, making them promising candidates for various bio-imaging techniques such as multiphoton-excited fluorescence microscopy. However, the structure-property relationship is of great importance and needs to be properly addressed in order to design new nonlinear optical materials. Using small ligated silver nanoclusters as test systems, we illustrate how theoretical approaches together with experimental findings can contribute to the understanding of structure-property relationships that might ultimately guide nanocluster synthesis.

Polymer Nanosheet Containing Star-Like Copolymers: A Novel Scalable Controlled Release System

Poly(ε-caprolactone) (PCL)-based nanomaterials, such as nanoparticles and liposomes, have exhibited great potential as controlled release systems, but the difficulties in large-scale fabrication limit their practical applications. Among the various methods being developed to fabricate polymer nanosheets (PNSs) for different applications, such as Langmuir-Blodgett technique and layer-by-layer assembly, are very effort consuming, and only a few PNSs can be obtained. In this paper, poly(ε-caprolactone)-based PNSs with adjustable thickness are obtained in large quantity by simple water exposure of multilayer polymer films, which are fabricated via a layer multiplying coextrusion method. The PNS is also demonstrated as a novel controlled guest release system, in which release kinetics are adjustable by the nanosheet thickness, pH values of the media, and the presence of protecting layers. Theoretical simulations, including Korsmeyer-Peppas model and Finite-element analysis, are also employed to discern the observed guest-release mechanisms.

Self-Assembled α-Cyanostilbenes for Advanced Functional Materials

In the specific context of condensed media, the significant and increasing recent interest in the α-cyanostilbene (CS) motif [ArCHC(CN)Ar] is relevant. These compounds have shown remarkable optical features in addition to interesting electrical properties, and hence they are recognized as very suitable and versatile options for the development of functional materials. This progress report is focused on current and future use of CS structures and molecular assemblies with the aim of exploring and developing for the next generations of functional materials. A critical selection of illustrative materials that contain the CS motif, including relevant subfamilies such as the dicyanodistyrylbenzene and 2,3,3-triphenylacrylonitrile shows how, driven by the self-assembly of CS blocks, a variety of properties, effects, and possibilities for practical applications can be offered to the scientific community, through different rational routes for the elaboration of advanced materials. A survey is provided on the research efforts directed toward promoting the self-assembly of the solid state (polycrystalline solids, thin films, and single crystals), liquid crystals, nanostructures, and gels with multistimuli responsiveness, and applications for sensors, organic light-emitting diodes, organic field effect transistors, organic lasers, solar cells, or bioimaging purposes.

Spectral properties of ionic benzotristhiazole based donor-acceptor NLO-phores in polymer matrices and their one- and two-photon cellular imaging ability

A series of ionic benzotristhiazolium (BTT) push-pull chromophores, with different nitrogen donor groups and different lengths of conjugated bridges, was successfully doped in polar polymer matrices (PVC and PSS). The spectral (photophysical) properties of their low concentration thin polymeric films are compared with those in solution and are discussed in terms of matrix polarity/viscosity influence, specific polymer-chromophore interaction, structure-spectral property relationship and Twisted Intramolecular Charge-Transfer (TICT) state formation. The elimination of a non-emissive phantom and TICT state formation by restricted intramolecular rotations in the polymer matrix or viscous solvent results in a relatively high Φ_F of all the investigated NLO-phores; particularly for near-infrared NIR molecular rotors bearing diphenylamino and julolidine donor groups. Because of cationic characteristics, small molecular weight, calculated high second hyperpolarizability and significant emission efficiency dependence on surroundings' viscosity (rigidochromic effect), two dyes were chosen as candidates for potential fluorescent probes for one-photon (1P) and two photon (2P) cellular imaging. The selected BTT NLO-phore with a julolidine donor is promising as a mitochondria-specific fluorescent small molecular probe for live cell super-resolution imaging with low cytotoxicity and good photostability, and is also potentially suitable for super-resolution STED imaging.

Nanopatterning of a Stimuli-Responsive Fluorescent Supramolecular Polymer by Thermal Scanning Probe Lithography

The miniaturization of nanometer-sized multicolor fluorescent features is of continuous significance for counterfeit security features, data storage, and sensors. Recent advances in engineering of stimuli-responsive supramolecular polymeric materials that respond upon exposure to heat or mechanical force by changing their fluorescence characteristics open new opportunities as functional lithographic resists. Here, we demonstrate the patterning of a thermochromic supramolecular material by thermal scanning probe lithography (t-SPL), an emerging nanofabrication technique, which allows for ultrafast indentation with a heated probe, resulting in both fluorescent and topographic nanofeatures. t-SPL indentation reveals a linear relationship between the temperature at which material softening occurs and the indentation force in the range from 200 to 500 nN. The softening temperature decreases as the heating time increases from 4 μs to 1 ms, following time-temperature superposition behavior. Our results herein confirm that the fluorescence contrast, perceivable as a shift from red to green, was obtained by kinetic trapping of the dissociated state due to ultrarapid cooling when the probe is removed. We use t-SPL to create highly customized fluorescence patterns up to 40 × 40 μm² in size with a spatial resolution of 86 nm and change the pitch size to modify the fluorescence intensity when observed by fluorescence microscopy. As an application, multifaceted security features with nanometer resolution are explored.

Light-responsive azo-containing organogels

While azo compounds are widely employed as radical initiators, they have rarely been used as stimuli-responsive motifs in macromolecular constructs. In this study, an azo-based cross-linker was prepared and reacted with poly(vinyl alcohol) to afford a series of stimuli-responsive organogels. Irradiation of these materials with UV light causes de-cross-linking and triggers a solid-to-liquid phase transition. Model adhesives with de-bonding-on-demand capability based on this design were explored.

Synthesis, crystals of centrosymmetric triphenylamine chromophores bearing prodigious two-photon absorption cross-section and biological imaging

Two centrosymmetric D-π-D type triphenylamine chromophores with long π-conjugated bridge and strong electron-donating moiety were designed, synthesized and fully characterized. The crystal analysis revealed that multiple CH⋯π interactions existed in two chromophores, which played a crucial role in generating molecular 1D chains and 2D layers structures. Linear and nonlinear optical properties of the chromophores were systematically investigated with the aid of theoretical calculations. Two chromophores both exhibited intense and wide-dispersed one-photon/two-photon excited fluorescence, bear prodigious 2PA cross section (δ). Especially for Dye2, with ethyoxyl groups, displayed the strong 2PA activity, large cross-sections (δ_max>16,000GM) and high NLO efficiency (δ_max/MW>16GM/(g·mol)) in the range of 680-830nm in DMF. In addition, one- and two-photon fluorescence microscopy images of HepG2 cells incubated with Dye2 were obtained and found that Dye2 could effectively uptake toward living cells and display a uniformly localized in cytosolic space.

Simultaneous Two-Photon Absorption to Gerade Excited Singlet States of Diphenylacetylene and Diphenylbutadiyne Using Optical-Probing Photoacoustic Spectroscopy

Simultaneous two-photon absorption to one-photon forbidden electronically excited states of diphenylacetylene (DPA) and diphenylbutadiyne (DPB) was investigated by means of highly sensitive optical-probing photoacoustic spectroscopy. The incident laser power dependencies on photoacoustic signal intensity indicate that the signals are dominated by the two-photon absorption regime. Two-photon absorption is responsible for transitions to gerade excited states based on the selection rule. The two-photon absorption bands observed in the heat action spectra were assigned with the aid of quantum chemical calculations. The relative magnitude of the two-photon absorption cross sections of DPA and DPB was estimated, and the larger two-photon absorption cross section of DPB was related to the resonance effect with the red-shifted one-photon allowed 1(1)B1u ← 1(1)Ag transition of DPB.

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

Coumarin-Containing Polymers for High Density Non-Linear Optical Data Storage

Optical data storage was performed with various thin polymer films containing coumarin-based derivatives and by using femtosecond laser pulses as well as two-photon absorption processes. Exploring the photodimerization attribute of coumarin derivatives and using appropriate irradiation wavelengths, recording/erasing processes could be carried out in the same area. Second harmonic generation microscopy was used to read the stored information.

Design, synthesis, linear and nonlinear photophysical properties of novel pyrimidine-based imidazole derivatives

Novel pyrimidine imidazole derivatives with flexible ether chains have been synthesised and evaluated for their cell imaging performanceviaphotophysical investigations and theoretical calculations.

Tuning the thermo- and mechanoresponsive behavior of luminescent cyclophanes

The thermo- and mechanoresponsive luminescent behavior of cyclophanes is tuned just by changing the ring size.

Selective fluorescence functionalization of dye-doped polymerized structures fabricated by direct laser writing (DLW) lithography

The continuous development of the vast arsenal of fabrication techniques is a pivotal factor in the breakthrough of nanotechnology. Although the broad interest is generally focused on the reduction of the dimensions of the fabricated structures, localized functionalization of the nanomaterials emerges as a key factor closely linked to their potential applications. In particular, fabrication of spatially selective fluorescence nanostructures is highly demanded in nanophotonics, as for example in three-dimensional (3D) optical data storage (ODS), where massive storage capacity and fast writing-reading processes are promised. We have developed an innovative method to control the location and intensity of the fluorescence signal in dye-doped photopolymerized structures fabricated with Direct Laser Writing (DLW) lithography. Well-defined fluorescent pixels (area = 0.24 μm(2)) were written inside a polymer matrix with the help of a femtosecond pulsed laser (multiphoton absorption) via a thermally-induced di-aggregation of a fluorescent dye. Moreover, we have accomplished a fine control of the fluorescence intensity which can increase the storage capacity of ODS systems fabricated with this approach.

Synthesis and optical properties of novel D–π–A–π–D type cationic cyclopentadienyliron complexes of arenes

With the enlarged and annulated ICT conjugation systems, three new cationic cyclopentadienyliron complexes exhibited significantly better third-order NLO absorption properties than the commercial cyclopentadienyliron arene complexes I-261.

SCC-DFTB calculation of the static first hyperpolarizability: from gas phase molecules to functionalized surfaces

The performance of the self-consistent charge density functional tight binding (SCC-DFTB) method for calculating the first hyperpolarizability of π-conjugated compounds has been assessed with respect to results obtained with high-level ab initio methods and density functional theory (DFT). The SCC-DFTB method performs similarly or better than DFT with the PBE XC functional. Thus, if for small π-conjugated linkers SCC-DFTB can reproduce trends, for longer chains the first hyperpolarizabilities are overestimated. In the case of push-pull thiophenes, the β values are strongly overestimated, as it is also the case with the B3LYP and PBE XC functionals. On the other hand, the SCC-DFTB method closely reproduces the evolution of β in p-disubstituted benzenes as a function of the donor and acceptor groups, as estimated at the MP2 level. The reliability of SCC-DFTB to determine the bond length alternation and the dihedral angles between the aromatic rings has also been tackled, demonstrating that both are underestimated. Overall, the SCC-DFTB calculations are of the same quality as those performed with the conventional PBE XC functional on which the method was parameterized but the SCC-DFTB calculations are computationally very little demanding, and it can therefore be adopted for very large systems for screening nonlinear optical materials as well as for assessing structure-property relationships. This is illustrated with an application on the first hyperpolarizability of an indolino-oxazolidine molecular switch grafted on a SiO2 surface. This has enabled to pinpoint (i) the effect of the surface on the donor/acceptor character of the linking substituent, (ii) the impact of molecular orientation, (iii) the role of a spacer between the π-conjugated switch and the surface, (iv) the global effect of the surface on the β contrast, and also (v) the fact that the molecular switches can maintain this contrast when adsorbed.

Origin of the Surface-Induced First Hyperpolarizability in the C60/SiO2 System: SCC-DFTB Insight

Using the self-consistent charge density functional tight binding (SCC-DFTB) method, C60 molecules physisorbed on an α-quartz slab are shown to display a first hyperpolarizability, whereas, owing to their symmetry, both the α-quartz slab and C60 molecule have no first hyperpolarizabilities. A larger first hyperpolarizability is achieved when the lowest-lying (five- or six-membered) ring is situated in between two hydroxyl rows, rather than on top, because this situation favors orbital overlaps and charge transfer. Further analysis has demonstrated that (i) the first hyperpolarizability originates from the MO overlap and field-induced charge transfers from the neighboring substrate/adsorbate moieties but not to geometric relaxation of the C60 molecules at the interface and that (ii) larger first hyperpolarizabilities are associated with low surface coverage and with small distances between C60 and the surface. This contribution is a clear illustration of the emergence of second-order nonlinear optical responses (first hyperpolarizability) as a result of breaking the centrosymmetry.

Image storage in coumarin-based copolymer thin films by photoinduced dimerization

We report a technique to encode grayscale digital images in thin films composed of copolymers containing coumarins. A nonlinear microscopy setup was implemented and two nonlinear optical processes were used to store and read information. A third-order process (two-photon absorption) was used to photoinduce a controlled dimer-to-monomer ratio within a defined tiny volume in the material, which corresponds to each recorded bit of data. Moreover, a second-order process (second-harmonic generation) was used to read the stored information, which has been found to be highly dependent upon the monomer-to-dimer ratio.

Photochromic materials: more than meets the eye

Photochromic materials are a family of compounds which can undergo reversible photo-switches between two different states or isomers with remarkably different properties. Inspired by their smart photo-switchable characteristics, a variety of light-driven functional materials have been exploited, such as ultrahigh-density optical data storage, molecular switches, logic gates, molecular wires, optic/electronic devices, sensors, bio-imaging and so on. This review commences with a brief description of exciting progress in this field, from systems in solution to modified functional surfaces. Further development of these photo-switchable systems into practical applications as well as existing challenges are also discussed and put in prospect.

Giant enhancement of upconversion in ultra-small Er³⁺/Yb³⁺:NaYF₄ nanoparticles via laser annealing

Most of the synthesis routes of lanthanide-doped phosphors involve thermal processing which results in nanocrystallite growth, stabilization of the crystal structure and augmentation of luminescence intensity. It is of great interest to be able to transform the sample in a spatially localized manner, which may lead to many applications like 2D and 3D data storage, anti-counterfeiting protection, novel design bio-sensors and, potentially, to fabrication of metamaterials, 3D photonic crystals or plasmonic devices. Here we demonstrate irreversible spatially confined infrared-laser-induced annealing (LIA) achieved in a thin layer of dried colloidal solution of ultra-small ∼8 nm NaYF₄ nanocrystals (NCs) co-doped with 2% Er³⁺ and 20% Yb³⁺ ions under a localized tightly focused beam from a continuous wave 976 nm medium power laser diode excitation. The LIA results from self-heating due to non-radiative relaxation accompanying the NIR laser energy upconversion in lanthanide ions. We notice that localized LIA appears at optical power densities as low as 15.5 kW cm⁻² (∼354 ± 29 mW) threshold in spots of 54 ± 3 µm diameter obtained with a 10 × microscope objective. In the course of detailed studies, a complete recrystallization to different phases and giant 2-3 order enhancement in luminescence yield is found. Our results are highly encouraging and let us conclude that the upconverting ultra-small lanthanide-doped nanophosphors are particularly promising for direct laser writing applications.

Benzothiazoles with tunable electron-withdrawing strength and reverse polarity: a route to triphenylamine-based chromophores with enhanced two-photon absorption

A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure-property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections (δ(TPA)) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the auxiliary acceptor. In addition, a change from the commonly used polarity in push-pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are ∼500-2300 GM in the near-infrared region (740-810 nm); thereby the molecular weight normalized δ(TPA)/MW values of the best performing dyes within the series (2.0-2.4 GM·g(-1)·mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.