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The linear and nonlinear optical properties of the heteroaromatic push-pull-push two-photon absorbing dye N-methyl-2,5-bis[1-(N-methylpyrid-4-yl) ethen-2-yl]-pyrrole ditriflate (PEPEP) are reported. The determination of the two-photon absorption (TPA) cross-section spectrum has been performed with different techniques: femtosecond TPA-white light continuum probe experiments, two-photon-induced fluorescence, and open aperture Z-scan measurements using both nanosecond and femtosecond laser pulses. The measured TPA cross sections and their wavelength dispersion show a marked dependence on the parameters of the laser pulses and on the measurement technique employed. These properties are discussed in terms of the different microscopic mechanisms that can contribute to the multiphoton absorption processes, with different weight depending on the measurement conditions and on the photophysical parameters of the dye. © 2008 American Chemical Society.

The present invention relates to new heteroaromatic compounds with high two-photon absorption activity, useful in particular as optical power limiting agents via two-photon absorption or as imaging agents in confocal laser scanning fluorescence microsopy via two-photon absorption or excitation. [on SciFinder (R)] Patent Number: 2004018456 PCT Designated States: Designated States W: AE, AG, AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, BZ, CA, CH, CN, CO, CR, CU, CZ, DE, DK, DM, DZ, EC, EE, ES, FI, GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, MN, MW, MX, MZ, NI, NO, NZ, OM, PG, PH, PL, PT, RO, RU, SC, SD, SE, SG, SK, SL, SY, TJ, TM, TN, TR, TT, TZ, UA, UG, US, UZ, VC, VN, YU, ZA, ZM, ZW, AM, AZ, BY, KG, KZ, MD, RU. Main IPC: C07D401-14. Secondary IPC: C07D497-04; A61K041-00; A61K049-00; C07D213-00; C07D207-00; C07D333-00; C07D319-00. Patent: Application: WO; Date: 20030708.; Application Number: 2003-EP7300 Patent Assignee: (Universita' degli Studi di Milano - Bicocca, Italy). Patent Country: WO; Link \#1: E:/!sandro/miei\bpdf/2004\b68\bPCT\bTPA2.pdf

We have investigated the photoinduced optical properties of a new class of chiral methacrylic polymers characterised by the presence in the side chain of an optically active pyrrolidinyl ring linked to a trans-azoaromatic system. The homopolymers are enantiomerically pure and their strong optical activity indicates that the macromolecules assume, both in solution and in solid thin films, highly homogeneous conformations with a prevailing chirality. As expected, the studied polymers exhibit reversible linear dichroism and birefringence when irradiated with linearly polarised light. By irradiating with circularly polarised light, we have discovered that it is possible to photomodulate the chiroptical properties of the polymer films. After irradiation with L-polarised light, the CD spectra of the films show a net inversion of their relative sign. The effect is reversible and the original shape of the CD spectra can be restored by pumping with R-polarised radiation. This unexpected new phenomenon can be explained in terms of the ability of the L-polarised radiation to invert the prevailing helicity of the polymeric chains. The observed effect seems to open new possibilities for the use of azobenzene-containing materials as chiroptical switches.

We report the first example of a heterocycle-based multi-branched two-photon absorption (TPA) dye. We show that the new pyridinium chromophore presents a very large TPA cross-section in the femtosecond regime. Comparison of the TPA cross-sections between the branched system and the corresponding \"monomeric unit\" shows a strong cooperative enhancement. A value of 22.6:1, corresponding to a 750% enhancement, was obsd.

The two-photon absorption (TPA) properties of two extended symmetric squaraine dyes are thoroughly characterized from the experimental and quantum-chemical point of view. The two molecules are specially engineered to have a multipolar structure with increasing complexity, D-π-A-π-D and A′-π-D-π-A-π-D-π-A′, respectively. The experimental TPA spectra, measured by means of the Z-scan technique in the femtoseconds regime, display considerably high values of TPA cross sections (σTPA) for both molecules. In particular, the squaraine with the more extended structure shows the highest value of σTPA ever reported for this class of molecules. CIS and TDDFT calculations of the one and two-photon absorption properties are carried out to clarify the origin of the observed TPA properties and fully characterize the electronic properties of these compounds. The calculations, in good agreement with the experimental data, suggest that the origin of this exceptionally high σTPA can be ascribed to the presence of a peripheral A′ group, that increases the density of excited states involved in the TPA process. © the Owner Societies 2011.

Exciton interactions and dynamics are the most important factors determining the exceptional photophysical properties of semiconductor quantum dots (QDs). In particular, best performances have been obtained for ingeniously engineered core/shell QDs. We have studied two factors entering in the exciton decay dynamics with adverse effects for the luminescence efficiency: exciton trapping at surface and interface traps, and non-radiative Auger recombination in QDs carrying either net charges or multiple excitons. In this work, we present a detailed study into the optical absorption, fluorescence dynamics and quantum yield, as well as ultrafast transient absorption properties of CdSe/CdS, CdSe/Cd0.5Zn0.5S, and CdSe/ZnS QDs as a function of shell thickness. It turns out that de-trapping processes play a pivotal role in determining steady state emission properties. By studying the excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/CdxZn1-xS (x = 0, 0.5, 1) QDs, we demonstrate the different role played by hot and cold carrier trapping rates in determining fluorescence quantum yields. Finally, the use of global analysis allows us untangling the complex ultrafast transient absorption signals. Smoothing of interface potential, together with effective surface passivation, appear to be crucial factors in slowing down both Auger-based and exciton trapping recombination processes.

The widespread application of quantum dots greatly profits from their broad absorption band. However, the variable nature of excitations within these bands is expected to result in undesired excitation energy dependence of steady state emission properties. We demonstrate the different role played by hot and cold carrier trapping in determining fluorescence quantum yields. Our analysis relates the energetic parameters with the available knowledge on the dynamics of charge trapping. It turns out that detrapping processes play a pivotal role in determining steady state emission properties. We studied excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/CdxZn1-xS (x = 0, 0.5, and 1) quantum dots to identify best performing heterostructures in terms of shell thickness and composition. Our rationalization of the observed behavior is focused on the modulation of trapping and detrapping rates. The combination of experimental results and PLQY kinetics modeling reveals the need to consider hot-carrier trapping, supporting recent dynamics observations. This work provides a deeper insight into the trapping process in quantum dots, relating its energetics and dynamics.

Semiconductor nanocrystals are rapidly spreading into the display and lighting markets. Compared with liquid crystal and organic LED displays, nanocrystalline quantum dots (QDs) provide highly saturated colors, wide color gamut, resolution, rapid response time, optical efficiency, durability and low cost. This remarkable progress has been made possible by the rapid advances in the synthesis of colloidal QDs and by the progress in understanding the intriguing new physics exhibited by these nanoparticles. In this review, we provide support to the idea that suitably engineered core/graded-shell QDs exhibit exceptionally favorable optical properties, photoluminescence and optical gain, while keeping the synthesis facile and producing QDs well suited for light emitting applications. Solid-state laser emitters can greatly profit from QDs as efficient gain materials. Progress towards fabricating low threshold, solution processed DFB lasers that are optically pumped using one- and two-photon absorption is reviewed. In the field of display technologies, the exploitation of the exceptional photoluminescence properties of QDs for LCD backlighting has already advanced to commercial levels. The next big challenge is to develop the electroluminescence properties of QD to a similar state. We present an overview of QLED devices and of the great perspectives for next generation display and lighting technologies.

The outstanding optical properties of Semiconductor Quantum Dots (QDs) have attracted much interest for over two decades. The development of synthetic methods for the production of core-shell QDs has opened the way to attaining almost ideal emitting properties. Their implementation in opto-electronic devices, such as light emitting diodes (LEDs) and lasers, requires a full understanding of the fine details of their photophysics. The exciton dynamics of core and coreshell QDs was extensively studied by means of pump and probe (P and P) and transient photoluminescence (TRPL) spectroscopies. Nevertheless, the wealth of possible exciton and multi-exciton decay mechanisms, operating on comparable time-scales, results in complex signals. In this work, the exciton dynamics of a complete CdSe/Cd1-xZnxS series is investigated, with a focus on exciton trapping processes. Insights into the energy distribution of exciton traps are unveiled by wavelength resolve QY measurements. Multicolor P and P measurements give a deeper insight into the dynamics of exciton trapping and Auger recombinations. An inversion method is proposed as a powerful tool for separating different contribution in complex P and P transients. The outcomes of this work clarify the role of core/shell interfaces and surfaces in modulating the optical properties and suggest possible routes for their improvement.

The two-photon absorption (TPA) properties of 4,4′-bis(para-di-n-butylaminostyryl)-2,2′-bipyridine (NBu2bipy) and related dipolar ([ZnY2(NBu2bipy)] (Y = Cl, CF3CO2), [Zn(2,2′-bipyridine)2(NBu2bipy)] [PF6]2) and octupolar ([Zn(NBu2bipy)3][PF6]2) complexes were investigated by the two-photon emission (TPE) technique in a femtosecond regime, working in the 730-930 nm spectral range. We found, in contrast with previous literature data, that the TPA enhancement upon coordination of a TPA active ligand to a metal center may be larger in the dipolar rather than the corresponding octupolar complex, the response being easily modulated by the choice of the ancillary ligands. © 2009 Elsevier B.V. All rights reserved.

An original and convenient three steps (swelling-poling-deswelling) process of poling a cross-linked polymer/NLO-phore composite to produce a stable second harmonic generation is described, using as a model system a cross-linked poly-amido amine network carrying covalently attached a modified Disperse Red One, a well known NLO-phore. This procedure is expected to be of wide applicability to produce composite polymeric materials with a rather stable second harmonic generation and of particular interest for NLO-phores sensitive to temperature. (c) 2007 Elsevier B.V. All rights reserved.

The amplification of Raman signals of the heteroaromatic cation 1-(N-methylpyrid-4-yl)-2-(N-methylpyrrol-2-yl)ethylene (PEP+)) bound to Au nanorods (NRs) was investigated at different excitation wavelengths to study the effect of the laser resonance with the absorption band of the PEP+ moiety and with the two plasmon oscillation modes of the NR. Two different PEP+ derivatives, differing in the length of the alkyl chain bearing the anchoring group, were used as target molecules. Raman spectra obtained exciting at 514 or at 785 nm (i.e., exciting the transverse or the longitudinal plasmon band) present a higher intensity than that at 488 nm suggesting a higher Raman amplification when the laser excitation wavelength is resonant with one of the two plasmon modes. Moreover, considering results of Discrete Dipole Approximation (DDA) calculations of the local field generated at the NR surface when either the transverse or the longitudinal plasmon modes are excited, we deduced that the resonance condition of the 514-nm laser excitation with the absorption band of the dye strongly contributes to the amplification of the Raman signal. © 2014 The Author(s).

The shell thickness and composition of CdSe–CdxZn1–xS core–shell quantum dots (QDs) are defining parameters for the efficiency of such materials as light emitters. In this work we present a detailed study into the optical absorption and fluorescence properties of CdSe–CdS, CdSe–Cd0.5Zn0.5S, and CdSe–ZnS QDs as a function of shell thickness. Moreover, the single-exciton recombination dynamics of these systems are analyzed by means of a time-correlated single-photon counting technique and directly related to the specific core–shell interfaces of the various QDs studied using a phenomenological kinetic model. The findings from this model highlight the strong role of the core–shell interface on both steady state photoluminescence and exciton recombination dynamics in these systems.

X-ray structure, electrochemical, and nonlinear optical characterization of indolic squaraines as Two-Photon absorbing (TPA) dyes in the visible region was investigated. Squaraines are the condensation product of electron rich molecules, usually activated arenes,π-excessive heterocycles and heteromorphic anhydrobases, with squaric acid. Squaraines were synthesized by refluxing the appropriate indole precursor and squaric acid in a BuOH/toluene mixture under a Dean-Stark trap for a required period of time. Cyclic voltammetry experiments were performed to probe molecular energy levels and the extent of intramolecular donor-acceptor interactions. It was reported that the design and synthesis of π-extended squaraines exhibiting very large TPA cross section efficiencies localized in the NIR region, with the peak performances above 1μm.

The structural nature of heterointerfaces in core-shell semiconductor quantum dots (QDs) plays a crucial role in tailoring their optical properties. In this work we have focused on using surface-enhanced Raman spectroscopy as a nondestructive tool to investigate the structural evolution of such interfaces in CdSe/CdS and CdSe/Cd0.5Zn0.5S colloidal QDs. A comparison between the two systems shows significant structural variation across the core-shell interfaces for the two different materials: a smooth interface for the former and an abrupt interface for the latter. This structural difference modifies the electronic structure within the QDs, which directly dictates the confinement behavior of the electrons and holes. The implications of this translate to a better understanding of why graded CdSe/CdS/Cd 0.5Zn0.5S/ZnS QDs are so lucrative for linear and nonlinear fluorescence-based applications. © 2013 American Chemical Society.

Two dimers of a heteroaromatic quadrupolar (acceptor-donor-acceptor) chromophore have been synthesized with different interchromophoric distances. Optical spectra of dimers in solution show a red shift of the linear absorption band upon decreasing the interchromophore distance, while fluorescence and two-photon absorption spectra are only marginally affected by the interactions. A bottom up approach is adopted to describe the spectra: via a detailed spectroscopic analysis of the monomeric species in solution, we define an essential-state model for the isolated chromophore and use this information to set up a model for the dimers also accounting for interchromophore electrostatic interactions. To discriminate between static screening governed by the static dielectric constant and dynamical screening at optical frequencies, we first solve the problem in the mean-field approximation and then define the excitonic Hamiltonian on the resulting best excitonic basis. Along this line, the evolution of spectral properties with the interchromophore distance is properly rationalized.

Singlet oxygen sensitization by organic molecules is a topic of major interest in the development of both efficient photodynamic therapy (PDT) and aerobic oxidations under complete green chemistry conditions. We report on the design, synthesis, biology, and complete spectroscopic characterization (vis-NIR linear and two-photon absorption spectroscopy, singlet oxygen generation efficiencies for both one-and two-photon excitation, electrochemistry, intrinsic dark toxicity, cellular uptake, and subcellular localization) of three classes of innovative singlet oxygen sensitizers pertaining to the family of symmetric squaraine derivatives originating from π-excessive heterocycles. The main advantage of π-extended squaraine photosensitizers over the large number of other known photosensitizers is their exceedingly strong two-photon absorption enabling, together with sizable singlet oxygen sensitization capabilities, for their use at the clinical application relevant wavelength of 806 nm. We finally show encouraging results about the dark toxicity and cellular uptake capabilities of water-soluble squaraine photosensitizers, opening the way for clinical small animal PDT trials. © 2008 American Chemical Society.

In this work we present an investigation of the non-linear optical (NLO) properties of two octupolar chromophores: [Zn(4,4'-bis(dibutylaminostyryl)-[2,2']-bipyridine)(3)](2+) and [Zn(4,4'-bis((E)-2-(N-(TEG)pyrrol-2-yl)vinyl)-[2,2']-bipyridine)(3)](2+) with Zn(ii) as the coordination center, using two-photon emission technique (TPE) in fs-pulse temporal regime. Compared to the free ligands, our results do not show a net increase in the two-photon absorption (TPA) cross-section for the octupolar complexes, once normalized to the ligand unit. This is in partial disagreement with a previous theoretical study investigating the first molecule where a significant increase of the TPA cross-section was predicted (X. J. Liu, et al., J. Chem. Phys., 2004, 120, 11 493).

The microwave-enhanced synthesis, comparative singlet oxygen sensitization efficiency, and nonlinear optical characterization of a new β- functionalized porphyrin and its copper complex are described. We show that the introduction of a donor-acceptor push-pull conjugated fragment in the âposition strongly perturbs the porphyrin electronic structure leading to a remarkable one- and two-photon NIR absorption enhancement. © 2006 American Chemical Society.

A novel scheme for implementing the joint exploitation of different, somehow complementary mechanisms of nonlinear transmission in an optical limiting device is proposed. As active materials we have chosen the fullerene derivative FULP, as a reverse saturable absorber, and a new heterocyclic quadrupolar dye, PEPEP, with highly efficient multiphoton absorption for nanosecond pulses. The nonlinear absorption properties of PEPEP in solution are extensively investigated for both femtosecond and nanosecond pulses. When Z-scan experiments are performed with nanosecond pulses, much larger effective cross sections are measured than with femtosecond pulses and with remarkably different wavelength dispersion. This is interpreted as due to two-photon absorption followed by one-photon absorption from the excited state. Chemically modified nonlinear molecules are incorporated in a hybrid organic-inorganic sol-gel matrix. Sufficiently high concentrations are achieved to allow the assembling of thin sol-gel disks into a "tandem" limiter with a total thickness smaller than the Rayleigh range of the focused laser beam. Preliminary testing of our limiter is reported and shows encouraging results. The resistance of the FULP-doped sol-gel glass to laser damage is substantially improved and the nonlinear attenuation at high pulse energies is enhanced.

The first examples of heterocycle-based multi-branched dyes with efficient two-photon absorption (TPA) activity are reported; the novel chromophores exhibit large TPA cross sections ( as high as 1600 x 10(-50) cm(4) s photon(-1) molecule(-1), measured with 150 fs laser pulses at 800 nm); a strong cooperative enhancement in the branched systems with respect to the one-dimensional sub-units is found

The multiphoton absorption properties of an organic push-molecule are investigate with nano- (ns) and femto-second (fs) laser sources. The active molecule is a new quadrupolar dye, named PEPEP, where heterocyclic units act as strong donor-acceptor moieties. Z-scan and pump-probe experiments (both single-color and with white light pulses) allow us to measure the value and the wavelength dispersion of the cross section for instantaneous two photon absorption (TPA). When Z-scan experiments are performed with nanosecond pulses, much larger effective cross sections are measured than with femtosecond pulses and with remarkably different wavelength dispersion. This is interpreted as due to two-photon absorption followed by one-photon absorption from the excited state (TPA-ESA). The TPA (or TPA-ESA) spectrum is measured, through pump-probe and z-scan measurements in a large visible range, in order to find the peak wavelength for multiphoton absorption.

We report the synthesis of sol-gel materials based on highly efficient heterocycle-based push-pull chromophores showing second- and third-order nonlinear optical activity. We show the proper functionalization of the best performing chromophores and their incorporation into a hybrid organic- inorganic sol-gel matrix. Different types of functionalization of the active molecule have been considered, including hydroxyl and alkoxysilyl end-groups. The functionalization strategy responded to different criteria such as stability and synthetic availability of the final molecular precursors, their solubility, and the used synthetic approach to the sol-gel material. The synthesis of the sol-gel materials has been tuned in order to preserve molecular properties and control important factors such as final concentration of the active dye in the matrix. Both acid- and base-catalyzed sol-gel synthesis has been taken into account. 3-Glycidoxypropyltrimethoxysilane and 3- aminopropyltriethoxysilane have been used as the organically modified alkoxides to prepare the hybrid organic-inorganic matrix. Characterization of the spectroscopic properties of the sol-gel materials is presented.

We report on the synthesis and characterization of a novel series of heterocycle-based chromophores for third order non-linear optics (NLO), showing enhanced two-photon absorption activity and frequency-upconverted lasing efficiency. The parent compound is a push-pull system where a pyridinium ion acts as acceptor group and N-methylpyrrole as a donor. Its efficiency in the two-photon-pumped lasing conversion from 800 to 543 nm is one of the highest ever reported. We show here the role of the position and nature of substituents in the tuning and optimization of the lasing wavelength and conversion efficiency.