Luminescent nanoparticles are becoming fundamental tools to the field of bioimaging. The optimization of their size, brightness and stability is key for applications ranging from contrast agent assisted surgery to diagnosis and therapeutics. A plethora of formulations have been documented which can be split into inorganic, organic and hybrid categories. While each class has their own advantages and limitations, controlling the interactions occurring between nanoparticles and cellular membranes is of the utmost importance. In particular, a major challenge for various applications, especially molecular imaging of membrane receptors, is to prevent non-specific interactions. Towards this goal, popular strategies based on coating nanoparticles with PEG or zwitterionic moieties have been developed to yield stealth nanoparticles. In this study, we present a series of spontaneously water-soluble and stealth organic nanoparticles. These fluorescent nanoparticles, made from original articulated bis-dipolar dyes, show vanishing interactions with living cells as bare nanoparticles. Moreover, thanks to their brightness and stability, they can be tracked as isolated single emitters in aqueous environments. These stealth nanoparticles thus hold promise for molecular imaging of specific membrane receptors, such as neuronal receptors, after bioconjugation with dedicated targeting agents.
Recent studies showed that the excitation spectral window lying between 1.6 and 1.8 μm is optimal for in-depth three-photon microscopy of intact tissues due to the reduced scattering and absorption in this wavelength range. Hence, millimeter penetration depth imaging in a living mouse brain has been demonstrated, demonstrating a major potential for neurosciences.
Further improvements of this approach, towards much higher imaging frame rates (up to 15-20 s/frame in previous achievements) requires the development of advanced molecular optical probes specifically designed for three-photon excited fluorescence in the 1.6 -1.8 μm spectral range.
In order to achieve large three-photon brightness at 1700 nm, novel molecular-based fluorescent nanoparticles which combine strong absorption in the green-yellow region, remarkable stability and photostability in aqueous and biological conditions have been designed using a bottom-up route. Due to the multipolar nature of the dedicated dyes subunits, these nanoparticles show large nonlinear absorption in the NIR region.
These new dyes have been experimentally characterized through the measurement of their three-photon action cross-section, fluorescence spectra and lifetimes using a monolithically integrated high repetition rate all-fiber femtosecond laser based on soliton self-frequency shift providing 9 nJ, 75 fs pulses at 1700 nm. The main result is that their brightness could be several orders of magnitude larger than the one of Texas Red in the 1700 nm excitation window.
Ongoing experiments involving the use of these new dyes for in vivo cerebral angiography on a mouse model will be presented and the route towards three-photon endomicroscopy will be discussed.
In order to provide a green alternative to QDs for bioimaging purposes and aiming at designing bright nanoparticles combining both large one- and two-photon brightness, a bottom-up route based on the molecular engineering of dedicated red to NIR emitting dyes that spontaneously form fluorescent organic nanoparticles (FONs) has been implemented. These fully organic nanoparticles built from original quadrupolar dyes are prepared using a simple, expeditious and green protocol that yield very small molecular-based nanoparticles (radius ~ 7 nm) suspension in water showing a nice NIR emission (λem=710 nm). These FONs typically have absorption coefficient more than two orders larger than popular NIR-emitting dyes (such as Alexa Fluor 700, Cy5.5 ….) and much larger Stokes shift values (i.e. up to over 5500 cm-1). They also show very large two-photon absorption response in the 800-1050 nm region (up to about 106 GM) of major promise for two-photon excited fluorescence microscopy. Thanks to their brightness and enhanced photostability, these FONs could be imaged as isolated nanoparticles and tracked using wide-field imaging. As such, thanks to their size and composition (absence of heavy metals), they represent highly promising alternatives to NIR-emitting QDs for use in bioimaging and single particle tracking applications. Moreover, efficient FONs coating was achieved by using a polymeric additive built from a long hydrophobic (PPO) and a short hydrophilic (PEO) segment and having a cationic head group able to interact with the highly negative surface of FONs. This electrostatically-driven interaction promotes both photoluminescence and two-photon absorption enhancement leading to an increase of two-photon brightness of about one order of magnitude. This opens the way to wide-field single particle tracking under two-photon excitation
Whereas structure-properties relationships have been widely investigated at the molecular level, supramolecular
structure-property relationships have been somewhat overlooked. In many cases, interchromophoric interactions are
found to be detrimental (in particular in second-order NLO) and a lot of efforts have been devoted to circumvent and
control these effects to achieve efficient NLO materials for electrooptics. At opposite, we have implemented a countermainstream
route based on the confinement of push-pull chromophores in close proximity within organic nanodots
where both their number and relative position/distance are controlled by covalent attachment onto appropriate organic
scaffolds. In such multichromophoric organic superstructures (namely covalent nanoclusters), dipole-dipole interactions
can be tuned by playing on the internal architecture (topology, number of chromophoric subunits, length of the covalent
linkers) and on the nature and properties (polarity, polarizability) of the chromophoric subunits. Following this strategy,
we present the investigation of two series of such organic nanoclusters built from push-pull chromophores where
through-space interactions are shown to modify both one-photon (OPA) and two-photon absorption (TPA) of each
chromophoric subunits leading to cooperative enhancement of TPA properties and improved transparency.
Quantum dots have been shown to provide a particularly effective approach to bright nano-objects for bioimaging due to
their unique optical properties, including their robust and size-dependent fluorescence. However these "hard"
nanoparticles raised a number of questions related to toxicity, biocompatibility and/or environmental issues. In that
context, we have developed a new class of "soft" fully organic alternative nanoparticles (i.e. organic nanodots). Our
approach relies on the confinement of a large number of organic chromophores within spherical nano-objects of
controlled size and structure, by embedding them within non-toxic and biocompatible dendrimeric architectures. This
highly modular strategy yielded organic nanodots of different sizes, colors and nature (lipophilic and hydrophilic). In
contrast with quantum dots (QDs), their emission color does not depend on their size, but only on the nature of their
constituting chromophoric subunits and their relative arrangement. Several series of nanodots of few nanometers in
diameter have been studied, exhibiting exceptional one and two-photon brightness and often outperforming the best
quantum dots. Nanodots offer major promises for bio and nanophotonics.
While structure-properties relationships are quite actively and successfully investigated at the molecular level of
engineering of optical nonlinear responses, supramolecular
structure-property relationships are an appealing field. The
realization that interchromophoric interactions between strongly polar/polarizable NLO chromophores can significantly
affect the NLO response of each chromophoric unit as well as promote associations has opened new dimensions for
molecular design. Several elegant routes have been implemented to hinder or counterbalance dipole-dipole interactions
between dipolar NLO chromophores for the elaboration of second-order materials (for SHG or electro-optical
modulation). At opposite, we have implemented a reverse strategy by confining discrete numbers of NLO push-pull
chromophores in close proximity within covalent organic nanoclusters with the aim to exploit interchromophoric
interactions in order to achieve enhanced NLO responses. As a proof of concept, we present here the investigation of
two-series of multichromophoric covalent assemblies built from NLO push-pull chromophores showing that cooperative
enhancement can be achieved both for second-order optical responses (first hyperpolarizabilities) or third-order
responses (two-photon absorption cross-sections).
Stimulated emission depletion (STED) and single molecule fluorescence correlation spectroscopy (FCS) are used to
determine stimulated emission cross-sections and investigate non-radiative relaxation in a branched quadrupolar
chromophore (OM77). The results are used as inputs to simulations of single molecule STED by which the feasibility of
STED control of the single molecule fluorescence cycle can be assessed. Single molecule STED in OM77 is shown to be
readily achievable; however its effectiveness in reducing triplet trapping is apparently mediated by fast non-radiative
relaxation processes other than intersystem crossing and rapid quenching of the triplet state in a non-deoxygenated
environment.
Semiconductor quantum dots are recognized to provide a particularly effective approach to bright nano-objects for
bioimaging. However, these inorganic systems suffer from several drawbacks such as toxicity, dispersity, blinking ...
and raise a number of questions with respect to environmental issues. With this in mind, we have developed an
innovative route towards purely organic nanodots showing exceptional one and two-photon brightness by confining a
large number of optimized fluorophores within nano-objects of defined and controlled structure. These novel "soft"
nano-objects offer major promises for bio and nanophotonics.
Investigations of the enhancement process in two-photon absorption organic dendrimers are presented in order to give further details of the mechanism of enhancement. Organic dendrimers based on the branched octupolar trimer building block molecule are studied by time-resolved fluorescence, transient absorption, and three-pulse photon echo peak shift measurements. The time resolved measurements suggest that the fundamental (spectroscopic) unit of excitation in these dendrimers is larger than the trimer building block molecule. These results suggest that it is possible to build dendrimers with enhanced two-photon absorption properties beyond the trimer molecule situation.
Molecular two-photon absorption (TPA) has attracted a lot of interest over recent years due to the many applications it offers both in biological imaging and in material science, constantly needing new optimized molecules with large TPA cross-sections. Various structures and functional groups have been studied; however, the use of electron-withdrawing boron groups has not been fully examined yet. As such compounds are known to lead to interesting photoluminescence and nonlinear optical (NLO) properties, we have investigated the TPA properties of a novel series of A-π-A quadrupoles, based on dimesitylborons as acceptor end-groups. Our experimental study reveals that intramolecular charge transfer is a crucial point in these TPA fluorophores, and can be modulated via changing the planarity of the molecule. We have obtained such planar molecules using vinylene spacers, which can release the steric hindrance close to the dimesitylboron end-group. The series of NLO-phores described here is promising for optical power limiting, with excellent TPA/transparency trade-off, and the work has highlighted that perfluorophenylene could be a key component for the future of TPA.
In this paper we measure three-photon absorption spectra and cross sections of two new series of molecules, linear bis-difluorenylamine-substituted oligofluorenylene-vinylenes (N-Fl3 series) and triphenylamine-branched phenylacetylene dendrimers (N-PA3 series). Using our model described before, we evaluate the size of coherent domains in these two series and compare them to those in dendrimers with nitrogen branching center and stilbene branches. This allows us to draw the conclusions about the effect of dimensionality (linear oligomers versus dendrimers), topology of the dendrimer core (3-arm versus 4-arm core), and branch structure on the size of coherent domains, and, hence the strength of cooperative enhancement of multiphoton absorption in macromolecules.
Structurally related chromophores of different symmetry (dipolar, V-shaped, octupolar) are investigated and compared for elucidation of the combined role of branching and charge symmetry on absorption, photoluminescence and two-photon absorption (TPA). Their design is based on the assembly of one, two or three π-conjugated dipolar branches on a central core. Two series of branched structures obtained from a central triphenylamine core and dipolar branches having different charge-transfer characters are investigated: photophysical properties are studied and TPA spectra are determined through two-photon excited fluorescence experiments using fs pulses in the 700-1000 nm range. Calculations based on time-dependent quantum-chemical approaches, as well as the Frenkel exciton model, complement experimental findings. Experiments and theory reveal that a multidimensional intramolecular charge transfer takes place from the central electron-donating moiety to the periphery of the branched molecules upon excitation, whereas fluorescence stems from a dipolar branch. Symmetry and inter-branch electronic coupling are found to be responsible for amplification of the TPA response of branched compounds with respect to their monomeric analogues. In particular, an enhancement is observed in regions where the TPA bands overlap, and TPA activation is obtained in spectral regions where the dipolar analogue is almost two-photon transparent. Thus, appropriate tuning of the number of branches, of the coupling between them, and modulation of intramolecular charge transfer from core to periphery open the way for substantial improvement of TPA efficiency or TPA induction in desired spectral regions.
Over the last two decades, a substantial effort has been devoted to the design of molecules with enhanced NLO responses. It has become increasingly clear over recent years that multipolar structures offer challenging possibilities in this respect. In particular, the octupolar framework provides an interesting route towards enhanced NLO responses and improved nonlinearity-transparency trade-off. In this perspective, we have implemented an innovative route based on octupolar structures derived from the boroxine ring. By grafting three electron-donating appendices on the electron-deficient boroxine core, octupolar quasi-planar molecules displaying markedly improved nonlinearity-transparency trade-off, as compared to the prototypical octupole (TATB) or the extensively studied triazine derivatives, were designed. This route indeed led to octupolar molecules showing beta(0) values (from calculations and solution measurements) larger than that of TIATB while remaining blue-shifted by nearly 100 nm and totally transparent in the visible region. Combined experimental and theoretical investigations reveal that this behavior is related to a periphery-to-core intramolecular charge transfer phenomenon in relation with the low-aromaticity and electron-withdrawing character of the boroxine ring. This study opens a new route for molecular engineering of transparent octupolar derivatives for NLO, including the design of effective materials for SHG in the visible-blue region.
A series of structurally-related multipolar chromophores of different symmetry (dipolar, quadrupolar, octupolar, dendritic...), and shape (rod-like, Y-shaped...) propeller-shaped, were investigated for optical power limiting based on multiphoton absorption processes. Their design is based on the functionalization of nanoscale linear or branched conjugated backbones with electro-active (i.e. electron-releasing or electron-withdrawing) peripheral and core/node groups. Their two-photon absorption (TPA) spectra were determined by investigating their two-photon-excited fluorescence properties in the NIR region using pulsed excitation in the femtosecond regime. These studies provide evidence that the charge symmetry plays an important role, the quadrupolar chromophores leading to giant TPA cross-sections in the visible red. Furthermore, modulation of the nonlinear absorptivity/transparency/photostability trade-off can be achieved by playing on the nature of the electroactive groups and of the spacers. Interestingly, higher-order charge symmetries and branched structures provide an innovative route for TPA amplification and/or spectral broadening in the NIR region.
Two-photon fluorescence polarisation and stimulated emission depletion dynamics are investigated in three high two-photon cross-section push-push polyenes: OM62, LP79 and OM77 and compared to the behaviour of a standard fluorophore (rhodamine 6G). Two-photon fluorescence anisotropy measurements (R(0) and Omega) were undertaken using picosecond time-correlated single photon counting (TCSPC). For OM62 and LP79 these are consistent with a diagonal two-dimensional transition tensor with SXX>SYY. For OM77 the contribution of off-diagonal elements (SXY & SYX) appears significant. Two-photon fluorescence anisotropy decay data is combined with streak camera measurements of excited state population depletion to determine stimulated emission cross-sections and ground state vibrational relaxation times. Cross-sections for STED in all three polyenes were found to be significantly higher than those for rhodamine 6G. The efficiency of STED is however dependent on the degree to which the S1→S0 transition is saturated by the DUMP pulse; this is mediated by fast ground state vibrational relaxation. Of the three polyenes, LP79 is seen to combine a large stimulated emission cross-section (c.a. 13σ(r6G)) with rapid ground state relaxation (τR=240fs).
Nowadays, it seems evident that a unique nonlinear optical (NLO)material cannot offer simultaneously linear transparency,colour neutrality and broadband optical limiting efficiency at the performance levels required for sensor and eye protection against all laser threats.Several combinations of NLO materials were investigated last few years, including multicell or multilayer geometries.
The approach presented here combines multiphoton absorption with nonlinear scattering. For that purpose, singlewall carbon nanotubes are suspended in various solutions of multiphoton absorbing chromophores. Such combinations allow us to obtain optical limiters of high linear transmittance and excellent colour neutrality. Broadband optical limiting is expected from the association of these two broadband materials,and enhanced optical limiting efficiency is expected from cumulative effects in the nanosecond regime.
We report here on the optical limiting studies performed with nanosecond laser pulses on several families of multiphoton absorbers in chloroform,with carbon nanotubes suspended in the solutions. The performances of these samples are compared with those of simple multiphoton absorber solutions and carbon nanotube suspensions, and the differences observed are interpreted in terms of cumulative NLO effects and adverse aggregation phenomenon. Ways to optimise stability of the suspensions are also experimented and discussed.
We have synthesized efficient two-photon absorbing fluorene and dithienothiophene (DTT) derivatives with electron donors (D) or electron acceptors (A) which are attached symmetrically or asymmetrically at the both end of conjugated links, forming D-π-D and D-π-A structures. The two-photon absorption (TPA) cross-section values
(σ) of these chromophores were evaluated by the nonlinear transmission measurements using 8 ns laser pulse, and also by the two-photon-induced fluorescence method with 80 fs pulse laser. The σ values obtained were larger by the nanosecond pulse laser than that by the femtosecond laser. This result suggests that the relative contribution of the excited state absorption becomes more manifested when longer time scale pulses are used. The strong nonlinear absorption including TPA of all these materials induced an efficient optical power limiting (OPL) activity. The trend in OPL behavior agreed well with the size of the TPA intensity.
Novel conjugated chromophores were designed and investigated for optical power limitation based on multiphoton absorption processes. Their design is based on the push-push functionalization of a semi-rigid elongated system derived from the extension of biphenyl cores. Biphenyl moieties with tunable twist angle were examined. Phenylene-vinylene rods were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency. These derivatives combine wide linear transparency and enhanced nonlinear absorptivities in the visible range. Pump-probe Kerr ellipsometry indicates large excited-state absorption cross-sections (with typical σe values of 5 10-16 cm2) while nanosecond nonlinear transmission measurements and optical limitation experiments reveal very strong nonlinear absorption that can be fitted by a three-photon absorption process (leading to α3 values up to 18000 cm3 GW-2). Such behavior results from a sequential multiphoton process involving excited-state absorption subsequent to two-photon excitation (with typical σ2 values of 5 10-20 cm4 GW-1). Both the linear transparency, the photostability and the nonlinear absorption spectral characteristics of these derivatives can be tuned by playing on the biphenyl twist angle. As a result, chromophores combining good linear transparency and enhanced nonlinear absorptivities in the visible range have been obtained.
Our aim has been the design of optimized NLO-phores with very high two-photon absorption (TPA) cross-sections (s2) in the red-NIR region, while maintaining high linear transparency and high fluorescence quantum yield. Our molecular engineering strategy is based on the push-push or pull-pull functionalization of semi-rigid nanoscale conjugated systems. The central building blocks were selected as rigid units that may assist quadrupolar intramolecular charge transfer by acting either as a (weak) donor or acceptor core. Quadrupolar molecules derived either from a phenyl unit, a rigidified biphenyl moiety or a fused bithiophene unit have been considered. Conjugated oligomers made of phenylene-vinylene and/or phenylene-ethynylene units were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency/fluorescence. The TPA cross-sections were determined by investigating the two-photon-excited fluorescence properties using a Ti:sapphire laser delivering fs pulses. Both the nature of the end groups and of the core moiety play an important role in determining the TPA spectra. In addition, by adjusting the length and nature of the conjugated extensor, both amplification and spectral tuning of TPA cross-sections can be achieved. As a result, push-push fluorophores which demonstrate giant TPA cross-sections (up to 3000 GM) in the visible red, high fluorescence quantum yields and good transparency in the visible range have been obtained.
The octupolar framework provides a promising route towards molecular compounds combining enhanced NLO responses and improved nonlinearity-transparency trade-off. In this perspective, we have designed original three-branched boomerang-shaped nanoscale molecules. Their molecular design is based on the grafting of three conjugated blades bearing either an electron-withdrawing or an electron-releasing end group on a triphenylbenzene core which can act as a (weak) donor or acceptor counterpart. We selected oligomeric phenylene-vinylene conjugated rods to allow for efficient charge transfer between the center and the periphery of the molecule while preserving transparency. Based on this strategy, we have prepared homologous nanoscale molecules with size varying between 2 and 5 nm. These molecules exhibit a definite solvatochromic behavior, in consistence with a multidimensional intramolecular charge transfer (MDICT) taking place between the core and the peripheral groups. Large first-order hyperpolarizabilities could be achieved by taking advantage and boosting of the MDICT phenomenon while maintaining wide transparency in the visible region (up to ||β|| = 800 10-30 e.s.u., with λmax = 377 nm). The superlinear dependence of β on size and their concave shape make elongated analogues attractive candidates for future developments.
A multichromophoric nanoassembly was designed by gathering seven push-pull chromophores on a β-cyclodextrin assembling unit via covalent linkers. Such supermolecule provides a valuable model for the investigation of confinement effects on the linear and nonlinear optical properties of push-pull chromophores in the condensed phase. Push-pull chromophores display a significant ground-state dipole, thus promoting dipolar interactions that are expected to influence both the conformation and the optical properties of the multichromophoric assembly. In this perspective, the photophysical and nonlinear optical properties of the mutichromophoric bundle were investigated and compared to those of the monomeric chromophore. The absorption, circular dichroism and fluorescence investigations provide evidence that the push-pull chromophores do not behave as isolated independent chromophores within the multichromophoric assembly. The nanoscale supermolecule is hypsochromically and significantly hypochromically shifted with respect to its monomeric analogue. In addition, the close proximity promotes excitonic coupling, as well as excimer formation phenomena. The nanoscopic assembly also shows a very large dipolar moment (μ = 38 D), and a significant molecular first-order hyperpolarisability, which reveal a spontaneous sheaf-type self-arrangement of the dipolar chromophores within the supermolecule. Such chiral hyperpolar nanoassemblies are promising candidates as model systems for nanophotonics.
Novel microscopies based on nonlinear optical (NLO) phenomena are attracting increasing interest in the biology community owing to their potentialities in the area of real-time, non-damaging imaging of biological systems. In particular, second-harmonic generation (SHG) and two-photon excited fluorescence (TPEF) are NLO phenomena that scale with excitation intensity squared, and thus give rise to an intrinsic 3-dimensional resolution when used in microscopic imaging. In this perspective, we have implemented a molecular engineering approach toward NLO-probes specifically designed for SHG and/or TPEF imaging of cellular membranes. We have designed nanoscale rod-like fluorophores showing very large TPEF cross-sections in the visible red, outperforming standard fluorophores such as fluorescein by up to two orders of magnitude. Bolaamphiphilic derivatives combining high TPEF cross-sections and affinity for cellular membranes were prepared. Their incorporation into model or cell membranes can be monitored by TPEF microscopy. Amphiphilic push-pull chromophores showing both high TPA and SHG cross-sections in the near-IR region were designed as NLO-probes for imaging of biological membranes by simultaneous SHG and TPEF microscopy. These NLO-phores offer intriguing potentialities for imaging of fundamental biological processes such as adhesion, fusion or for reporting of membrane electrical potentials.
Recent reports of push-pull dipolar and quadrupolar chromophores with enhanced two-photon absorption have generated considerable interest in these two molecular systems. Two photon absorption is related to the imaginary part of the two-photon resonant cubic hyperpolarizability Im[(gamma) ((omega) )]. In this work, we have described both push-pull dipolar and quadrupolar chromophores using multi valence-bond states models based on measurable parameters of the valence-bond forms. We have derived analytical expressions of their non-resonant static cubic hyperpolarizability (gamma) (0) and of Im[(gamma) ((omega) )]. Comparison between the transparency / Im[(gamma) ((omega) )] trade-off and Im[(gamma) ((omega) )] / (gamma) (0) correlation helps understand the advantages and drawbacks of each of these two push-pull systems. Furthermore by understanding how the valence-bond parameters are related to the molecular structure and its environment, it is possible to predict how Im[(gamma) ((omega) )] will be affected by changing either the conjugation size, the donor-acceptor pair or the solvent polarity for both of these push-pull systems. The results of this study suggest common guidelines for the molecular engineering of both the push-pull dipolar and quadrupolar chromophores.
Second-harmonic generation (SHG) and two-photon excited fluorescence (TPEF) are nonlinear optical (NLO) phenomena that scale with excitation intensity squared, and hence give rise to an intrinsic 3-dimensional resolution when used in microscopic imaging. TPEF microscopy has gained widespread popularity in the biology community whereas SHG microscopy promises to be a powerful tool because of its sensitivity to local asymmetry. We have implemented an approach toward the design of NLO-probes specifically adapted for SHG and/or TPEF imaging of biological membranes. Our strategy is based on the design of nanoscale amphiphilic NLO-phores. We have prepared symmetrical bolaamphiphilic fluorophores combining very high two-photon absorption (TPA) cross-sections in the visible red region and affinity for cellular membranes. Their incorporation and orientation in lipid membranes can be monitored via TPEF anisotropy. We have also prepared amphiphilic push-pull chromophores exhibiting both large TPA cross-sections and very large first hyperpolarizabilities in the near-IR region. These NLO-probes have proved to be particularly useful for imaging of biological membranes by simultaneous SHG and TPEF microscopy and offer attractive prospects for real-time imaging of fundamental biological processes such as adhesion, fusion or reporting of membrane potentials.
A large effort has been devoted to the preparation of organic polymeric materials for electro-optic modulation and more recently for cascading based processes. These materials contain push-pull chromophores either incorporated as guest in a high Tg polymeric matrix (doped polymers) or grafted onto the polymeric matrix. These systems present several advantages but require significant improvement at the molecular level- by designing optimized chromophores with very large molecular figure of merit specific to each application targeted. The sol-gel route was used to prepare hybrid organic-inorganic materials, for the fabrication of amorphous solids of various shapes (bulk, think films...). The results obtained on optimized chromophore-doped poled thin films emphasize that intermolecular interactions have to be taken into account, as already pointed out by Dalton and coworkers. By combining a molecular engineering strategy for getting large molecular figure of merit and by controlling the intermolecular dipole-dipole interactions via both tuning the push-pull chromophore concentration and the incorporation screening carbazole moieties in high concentration. This strategy allows us to obtain a r33 of about 50 pm/V at 831 nm for a new optimized chromophore structure. In parallel, these thin films are being processed to be used as passive components for integrated optics.
We have developed hybrid organic-inorganic materials based upon the incorporation of nonlinear chromophores in a rigid amorphous inorganic matrix. Functionalized thin films have been prepared using the sol-gel route allowing for mild synthesis conditions. Push-pull chromophores were either incorporated as guests or grafted on a silica based backbone via a spacer. Orientation of the dipolar chromophores within the materials was performed using the Corona technique. By playing on the structure of the push-pull chromophores and on its orientational stability inside the matrix, functionalized materials with large electro-optic coefficients and excellent stability have been designed. By combining both appropriate push-pull chromophores and photoconductors grafted on the matrix, materials showing interesting photorefractive properties can be obtained without requiring the application of an external electric field. Both the presence of a strong internal field in the poled materials and the occurrence of photo-assisted orientational birefringence plays a significant role.
A pre-requisite to obtain polymers with a large photorefractive response is to design non-linear optical chromophores with a large figure of merit. This figure depends on the glass transition temperature of the material. We present a theoretical investigation that shows which are the important molecular parameters that control the magnitude of the figure of merit either in a low-Tg or in a high-Tg polymer. Derivation of the figures of merit for various push-pull molecules show a molecular engineering strategy can be successfully implemented to yield very large figures of merit. This approach is supported by an experimental investigation based on electro-optical absorption measurements.
In this work, we have investigated theoretically the structure/hyperpolarizability correlations of push-pull molecules sing a two-form two-state model. For this, we have defined a parameter MIX characterizing the mixing between the two-limiting resonance forms and thus ruling the molecular structure and polarization. Also, we have analyzed the solvent effect on the structure and on the polarizabilities of push-pull molecules using the Onsager reaction field theory. We have shown that the dependences of the linear and nonlinear polarizabilities on the solvent dielectric constant look similar to the structure/(hyper)polarizability correlations. Finally, we have investigated experimentally the solvent effect on a series of push-pull polyenes of increasing length. In particular, we have determined the scalar (mu) g(Beta) (0) product (where (mu) g is the ground-state dipole and (Beta) (0) the vector part of static quadratic hyperpolarizability tensor) in different solvents using the electric-field-induced-second-harmonic generation technique. For the different chain lengths, we have obtained a mapping of the positive (mu) g(Beta) (0) peak. This leads for the longest compound of the series to a record high value of (mu) g(Beta) (0) product in chloroform.
A wide choice of push-pull polyenes and carotenoids of increasing length (up to 30 angstrom) and bearing various donor and acceptor end groups has been synthesized in order to investigate the chain-length dependence of their quadratic hyperpolarizability (beta) . (beta) measurements have been performed using the electric field induced second harmonic generation (EFISH) technique. In each series of homologous compounds, the lengthening of the conjugation path results in a pronounced increase in (mu) (beta) values. This behavior leads to very large static (mu) (beta) (0) values for the longest molecules and can be modeled by (mu) (beta) (0) equals kna relationships with respect to the number n of double bonds in the polyenic chain. The exponent value (a) was found to depend on the end groups. The replacement of a double bond by a triple bond in the middle of the polyenic chain results in a blue shift of the charge transfer absorption band and leads to smaller (mu) (beta) (0) values. However, this phenomenon is modulated by the end groups.
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