Dielectric Spectroscopy Studies of Conformational Relaxation Dynamics in Molecular Glass-Forming Liquids.

We review experimental results obtained with broadband dielectric spectroscopy concerning the relaxation times and activation energies of intramolecular conformational relaxation processes in small-molecule glass-formers. Such processes are due to the interconversion between different conformers of relatively flexible molecules, and generally involve conformational changes of flexible chain or ring moieties, or else the rigid rotation of planar groups, such as conjugated phenyl rings. Comparative analysis of molecules possessing the same (type of) functional group is carried out in order to test the possibility of assigning the dynamic conformational isomerism of given families of organic compounds to the motion of specific molecular subunits. These range from terminal halomethyl and acetyl/acetoxy groups to both rigid and flexible ring structures, such as the planar halobenzene cycles or the buckled saccharide and diazepine rings. A short section on polyesters provides a generalisation of these findings to synthetic macromolecules.

Amorphous solid dispersions of curcumin in a poly(ester amide): Antiplasticizing effect on the glass transition and macromolecular relaxation dynamics, and controlled release.

In order to exploit the pharmacological potential of natural bioactive molecules with low water solubility, such as curcumin, it is necessary to develop formulations, such as amorphous polymer dispersions, which allow a constant release rate and at the same time avoid possible toxicity effects of the crystalline form of the molecule under scrutiny. In this study, polymer dispersions of curcumin were obtained in PADAS, a biodegradable semicrystalline copolymer based on 1,12-dodecanediol, sebacic acid and alanine. The dispersions were fully characterized by means of differential scanning calorimetry and broadband dielectric spectroscopy, and the drug release profile was measured in a simulated body fluid. Amorphous homogeneous binary dispersions were obtained for curcumin mass fraction between 30 and 50%. Curcumin has significantly higher glass transition temperature Tg (≈ 347 K) than the polymer matrix (≈274-277 K depending on the molecular weight), and dispersions displayed Tg's intermediate between those of the pure amorphous components, implying that curcumin acts as an effective antiplasticizer for PADAS. Dielectric spectroscopy was employed to assess the relaxation dynamics of the binary dispersion with 30 wt% curcumin, as well as that of each (amorphous) component separately. The binary dispersion was characterized by a single structural relaxation, a single Johari-Goldstein process, and two local intramolecular processes, one for each component. Interestingly, the latter processes scaled with the Tg of the sample, indicating that they are viscosity-sensitive. In addition, both the pristine polymer and the dispersion exhibited an interfacial Maxwell-Wagner relaxation, likely due to spatial heterogeneities associated with phase disproportionation in this polymer. The release of curcumin from the dispersion in a simulated body fluid followed a Fickian diffusion profile, and 51% of the initial curcumin content was released in 48 h.

Eutectic Mixture Formation and Relaxation Dynamics of Coamorphous Mixtures of Two Benzodiazepine Drugs.

The formation of coamorphous mixtures of pharmaceuticals is an interesting strategy to improve the solubility and bioavailability of drugs, while at the same time enhancing the kinetic stability of the resulting binary glass and allowing the simultaneous administration of two active principles. In this contribution, we describe kinetically stable amorphous binary mixtures of two commercial active pharmaceutical ingredients, diazepam and nordazepam, of which the latter, besides being administered as a drug on its own, is also the main active metabolite of the other in the human body. We report the eutectic equilibrium-phase diagram of the binary mixture, which is found to be characterized by an experimental eutectic composition of 0.18 molar fraction of nordazepam, with a eutectic melting point of Te = 395.4 ± 1.2 K. The two compounds are barely miscible in the crystalline phase. The mechanically obtained mixtures were melted and supercooled to study the glass-transition and molecular-relaxation dynamics of amorphous mixtures at the corresponding concentration. The glass-transition temperature was always higher than room temperature and varied linearly with composition. The Te was lower than the onset of thermal decomposition of either compound (pure nordazepam decomposes upon melting and pure diazepam well above its melting point), thus implying that the eutectic liquid and glass can be obtained without any degradation of the drugs. The eutectic glass was kinetically stable against crystallization for at least a few months. The relaxation processes of the amorphous mixtures were studied by dielectric spectroscopy, which provided evidence for a single structural (α) relaxation, a single Johari-Goldstein (ß) relaxation, and a ring-inversion conformational relaxation of the diazepinic ring, occurring on the same timescale in both drugs. We further characterized both the binary mixtures and pure compounds by FTIR spectroscopy and first-principles density functional theory (DFT) simulations to analyze intermolecular interactions. The DFT calculations confirm the presence of strong attractive forces within the heteromolecular dimer, leading to large dimer interaction energies of the order of -0.1 eV.

Glass transition, crystallization kinetics, and inter-conformer relaxation dynamics of amorphous mitotane and related compounds.

We employ differential scanning calorimetry, broadband dielectric spectroscopy and optical microscopy to investigate the glass transition, molecular relaxation dynamics, and isothermal recrystallization kinetics of amorphous mitotane, the only drug approved for the pharmacological treatment of adrenocortical carcinoma. Amorphous mitotane displays a glass transition at Tg = 243 ± 1 K, characterized by relatively low fragility index of 68 ± 2. Besides the structural and Johari-Goldstein relaxations, amorphous mitotane displays an intramolecular relaxation with activation energy of 25 ± 1 kJ mol-1. The same relaxation process, with virtually the same activation energy and relaxation times, is observed in the closely-related o,p'-dichlorobenzophenone compound, which allows identifying it as the rotation of the chlorobenzene ring with the chlorine closest to the central carbon. Such conformational relaxation is active at human body temperature, and may thus be potentially relevant for the mechanism of action of the drug. Our study shows that the comparative study of the relaxation map of related molecular species is a powerful tool to identify and classify secondary relaxation processes. The amorphous drug is found to be unstable against recrystallization at as well as slightly below room temperature, and to display-two-dimensional growth with only sporadic nucleation, characterized by an Avrami kinetic exponent of 2.05 ± 0.05. The kinetic stability of the amorphous form of mitotane, observed at room temperature in micellar formulations, is therefore limited to the nanoconfined sample and is not observed in the bulk compound.

Drug-Biopolymer Dispersions: Morphology- and Temperature- Dependent (Anti)Plasticizer Effect of the Drug and Component-Specific Johari-Goldstein Relaxations.

Amorphous molecule-macromolecule mixtures are ubiquitous in polymer technology and are one of the most studied routes for the development of amorphous drug formulations. For these applications it is crucial to understand how the preparation method affects the properties of the mixtures. Here, we employ differential scanning calorimetry and broadband dielectric spectroscopy to investigate dispersions of a small-molecule drug (the Nordazepam anxiolytic) in biodegradable polylactide, both in the form of solvent-cast films and electrospun microfibres. We show that the dispersion of the same small-molecule compound can have opposite (plasticizing or antiplasticizing) effects on the segmental mobility of a biopolymer depending on preparation method, temperature, and polymer enantiomerism. We compare two different chiral forms of the polymer, namely, the enantiomeric pure, semicrystalline L-polymer (PLLA), and a random, fully amorphous copolymer containing both L and D monomers (PDLLA), both of which have lower glass transition temperature (Tg) than the drug. While the drug has a weak antiplasticizing effect on the films, consistent with its higher Tg, we find that it actually acts as a plasticizer for the PLLA microfibres, reducing their Tg by as much as 14 K at 30%-weight drug loading, namely, to a value that is lower than the Tg of fully amorphous films. The structural relaxation time of the samples similarly depends on chemical composition and morphology. Most mixtures displayed a single structural relaxation, as expected for homogeneous samples. In the PLLA microfibres, the presence of crystalline domains increases the structural relaxation time of the amorphous fraction, while the presence of the drug lowers the structural relaxation time of the (partially stretched) chains in the microfibres, increasing chain mobility well above that of the fully amorphous polymer matrix. Even fully amorphous homogeneous mixtures exhibit two distinct Johari-Goldstein relaxation processes, one for each chemical component. Our findings have important implications for the interpretation of the Johari-Goldstein process as well as for the physical stability and mechanical properties of microfibres with small-molecule additives.

Inter-enantiomer conversion dynamics and Johari-Goldstein relaxation of benzophenones.

We employ temperature- and pressure-dependent dielectric spectroscopy, as well as differential scanning calorimetry, to characterize benzophenone and the singly-substituted ortho-bromobenzophenone derivative in the liquid and glass states, and analyze the results in terms of the molecular conformations reported for these molecules. Despite the significantly higher mass of the brominated derivative, its dynamic and calorimetric glass transition temperatures are only ten degrees higher than those of benzophenone. The kinetic fragility index of the halogenated molecule is lower than that of the parent compound, and is found to decrease with increasing pressure. By a detailed analysis of the dielectric loss spectra, we provide evidence for the existence of a Johari-Goldstein (JG) relaxation in both compounds, thus settling the controversy concerning the possible lack of a JG process in benzophenone and confirming the universality of this dielectric loss feature in molecular glass-formers. Both compounds also display an intramolecular relaxation, whose characteristic timescale appears to be correlated with that of the cooperative structural relaxation associated with the glass transition. The limited molecular flexibility of ortho-bromobenzophenone allows identifying the intramolecular relaxation as the inter-enantiomeric conversion between two isoenergetic conformers of opposite chirality, which only differ in the sign of the angle between the brominated aryl ring and the coplanar phenyl-ketone subunit. The observation by dielectric spectroscopy of a similar relaxation also in liquid benzophenone indicates that the inter-enantiomer conversion between the two isoenergetic helicoidal ground-state conformers of opposite chirality occurs via a transition state characterized by a coplanar phenyl-ketone moiety.

Chloramphenicol loaded polylactide melt electrospun scaffolds for biomedical applications.

Melt electrospinning of polylactide (PLA) loaded with chloramphenicol (CAM) has been performed and characteristics of fibers, physical properties of scaffolds, CAM release behavior, antibacterial properties and biocompatibility have been evaluated. The interest of CAM loaded samples is nowadays enhanced for biomedical applications since this antibiotic has been demonstrated to be efficient for the treatment of cancer. Melt electrospinning has been selected as an ideal preparation process because it avoids the use of toxic solvents which are harmful to the environment and could be problematic for biomedical applications. The electrospinning process rendered fibers with a relatively large diameter (between 20 µm and 40 µm depending on the load) and minimum polymer degradation. Characteristics of melt electrospun scaffolds were also compared with those prepared by solution electrospinning. Differences consisted in a more sustained release and a higher biocompatibility for the melt processed samples. Bactericide effect was evaluated as an evidence of the maintenance of the CAM bioactivity after melt processing at high temperature and the slower release caused by the relatively high diameter of the constitutive fibers. Since pure CAM showed thermal degradation at temperatures relatively close to the PLA melting temperature, a complete analysis of the degradation process of pure CAM as well as of PLA samples loaded with CAM was performed. The Invariant Kinetic Parameters method allowed determining an initial decomposition step that followed an autoaccelatory Avrami model, and then an autocatalytic decomposition reaction took place for conversions higher than 50%. Dispersion in the PLA matrix enhances the thermal stability of the antibiotic, with an onset temperature of degradation that was higher by 16 °C in the melt-electrospun fibers than in the liquid state of pure CAM.

Comparative Physical Study of Three Pharmaceutically Active Benzodiazepine Derivatives: Crystalline versus Amorphous State and Crystallization Tendency.

Chemical derivatization and amorphization are two possible strategies to improve the solubility and bioavailability of drugs, which is a key issue for the pharmaceutical industry. In this contribution, we explore whether both strategies can be combined by studying how small differences in the molecular structure of three related pharmaceutical compounds affect their crystalline structure and melting point (Tm), the relaxation dynamics in the amorphous phase, and the glass transition temperature (Tg), as well as the tendency toward recrystallization. Three benzodiazepine derivatives of almost same molecular mass and structure (Diazepam, Nordazepam and Tetrazepam) were chosen as model compounds. Nordazepam is the only one that displays N-H···O hydrogen bonds both in crystalline and amorphous phases, which leads to a significantly higher Tm (by 70-80 K) and Tg (by 30-40 K) compared to those of Tetrazepam and Diazepam (which display similar values of characteristic temperatures). The relaxation dynamics in the amorphous phase, as determined experimentally using broadband dielectric spectroscopy, is dominated by a structural relaxation and a Johari-Goldstein secondary relaxation, both of which scale with the reduced temperature T/Tg. The kinetic fragility index is very low and virtually the same (mp ≈ 32) in all three compounds. Two more secondary relaxations are observed in the glass state: the slower of the two has virtually the same relaxation time and activation energy in all three compounds, and is assigned to the inter-enantiomer conversion dynamics of the flexible diazepine heterocycle between isoenergetic P and M conformations. We tentatively assign the fastest secondary relaxation, present only in Diazepam and Tetrazepam, to the rigid rotation of the fused diazepine-benzene double ring relative to the six-membered carbon ring. Such motion appears to be largely hindered in glassy Nordazepam, possibly due to the presence of the hydrogen bonds. Supercooled liquid Tetrazepam and Nordazepam are observed to crystallize into their stable crystalline form with an Avrami exponent close to unity indicating unidimensional growth with only sporadic nucleation, which allows a direct assessment of the crystal growth rate. Despite the very similar growth mode, the two derivatives exhibit very different kinetics for a fixed value of the reduced temperature and thus of the structural relaxation time, with Nordazepam displaying slower growth kinetics. Diazepam does not instead display any tendency toward recrystallization over short periods of time (even close to Tm). Both these observations in three very similar diazepine derivatives provide direct evidence that the kinetics of recrystallization of amorphous pharmaceuticals is not a universal function, at least in the supercooled liquid phase, of the structural or the conformational relaxation dynamics, and it is not simply correlated with related parameters such as the kinetic fragility or activation barrier of the structural relaxation. Only the crystal growth rate, and not the nucleation rate, shows a correlation with the presence or absence of hydrogen bonding.

A facile approach to hydrophilic oxidized fullerenes and their derivatives as cytotoxic agents and supports for nanobiocatalytic systems.

A facile, environment-friendly, versatile and reproducible approach to the successful oxidation of fullerenes (oxC60) and the formation of highly hydrophilic fullerene derivatives is introduced. This synthesis relies on the widely known Staudenmaier's method for the oxidation of graphite, to produce both epoxy and hydroxy groups on the surface of fullerenes (C60) and thereby improve the solubility of the fullerene in polar solvents (e.g. water). The presence of epoxy groups allows for further functionalization via nucleophilic substitution reactions to generate new fullerene derivatives, which can potentially lead to a wealth of applications in the areas of medicine, biology, and composite materials. In order to justify the potential of oxidized C60 derivatives for bio-applications, we investigated their cytotoxicity in vitro as well as their utilization as support in biocatalysis applications, taking the immobilization of laccase for the decolorization of synthetic industrial dyes as a trial case.

On the microscopic origin of the cryoprotective effect in lysine solutions.

The amino acid lysine has been shown to prevent water crystallization at low temperatures in saturated aqueous solutions [S. Cerveny and J. Swenson, Phys. Chem. Chem. Phys., 2014, 16, 22382-22390]. Here, we investigate two ratios of water and lysine (5.4 water molecules per lysine (saturated) and 11 water molecules per lysine) by means of the complementary use of computer simulations and neutron diffraction. By performing a detailed structural analysis we have been able to explain the anti-freeze properties of lysine by the strong hydrogen bond interactions of interstitial water molecules with lysine that prevent them from forming crystalline seeds. Additional water molecules beyond the 1 : 5.4 proportion are no longer tightly bonded to lysine and therefore are free to form crystals.

Amorphous binary dispersions of chloramphenicol in enantiomeric pure and racemic poly-lactic acid: Morphology, molecular relaxations, and controlled drug release.

We characterize amorphous solid dispersions (ASDs) of the Chloramphenicol antibiotic in two biodegradable polylactic acid polymers, namely a commercial sample of enantiomeric pure PLLA and a home-synthesized PDLLA copolymer, investigating in particular the effect of polylactic acid in stabilizing the amorphous form of the drug and controlling its release (e.g. for antitumoral purposes). Broadband dielectric spectroscopy and differential scanning calorimetry are employed to study the homogeneity, glass transition temperature and relaxation dynamics of solvent-casted ASD membranes with different drug concentrations. We observe improved physical stability of the ASDs with respect to the pure drug, as well as a plasticizing effect of the antibiotic on the polymer, well described by the Gordon-Taylor equation. The release of the active pharmaceutical ingredient from the films in a simulated body fluid is studied by UV/vis spectroscopy at two different drug concentrations (5 and 20% in weight). The amount of released drug is found to be proportional to the square root of time, with proportionality constant that is almost the same in both dispersions, despite the fact that the relaxation time and thus the viscosity of the two samples differ by four orders of magnitude at body temperature. Since the drug release kinetics does not display a significant dependence on the drug content in the carrier, it may be expected to remain roughly constant during longer release times.

Nose Temperature and Anticorrelation between Recrystallization Kinetics and Molecular Relaxation Dynamics in Amorphous Morniflumate at High Pressure.

We probe the dielectric response of the supercooled liquid phase of Morniflumate, a drug with anti-inflammatory and antipyretic properties, studying in particular the pressure and temperature dependence of the relaxation dynamics, glass transition temperature Tg, and recrystallization kinetics. Tg increases by roughly 20 K every 100 MPa at low applied pressure, where the ratio Tg/Tm has a constant value of ∼0.8 (Tm = melting point). Liquid Morniflumate displays two dielectric relaxations: the structural α relaxation associated with the collective reorientational motions, which become arrested at Tg, and a secondary relaxation likely corresponding to an intramolecular dynamics. The relaxation times of both processes scale approximately with the inverse reduced temperature Tg/T. Near room temperature and under an applied pressure of 50 MPa, supercooled Morniflumate recrystallizes in a characteristic time of few hours, with an Avrami exponent of 1.15. Under these conditions, the recrystallization rate is a nonmonotonic function of temperature, displaying a maximum at around 298 K, which can be taken to be the optimum crystal growth temperature Tnose. The ß relaxation becomes kinetically frozen at ambient temperature under an applied hydrostatic pressure higher than 320 MPa, suggesting that the Morniflumate glass should be kinetically stable under these conditions.

Tuning the Kinetic Stability of the Amorphous Phase of the Chloramphenicol Antibiotic.

We employ broadband dielectric spectroscopy to study the relaxation dynamics and crystallization kinetics of a broad-spectrum antibiotic, chloramphenicol, in its supercooled liquid form. Two dynamic processes are observed: the structural α relaxation, which becomes kinetically frozen at Tg = 302 ± 1 K, and an intramolecular secondary relaxation. Under isothermal conditions, the supercooled drug displays interconversion between different isomers, followed by recrystallization. Recrystallization follows the Avrami law with Avrami exponent n = 1.3 ± 0.1, consistent with a one-dimensional growth of crystalline platelets, as observed by electron microscopy. Exposure to humid atmosphere and subsequent heating to high temperature is found to degrade the compound. The partially degraded sample displays a much lower tendency to crystallize, likely because the presence of the degradation products results in spatial frustration. This sample exhibits enhanced conductivity and an additional relaxation, intermediate to the ones observed in the pure sample, which likely corresponds to the noncooperative dynamics of the main degradation product. We find that dispersing the antibiotic in polylactic acid results in an amorphous sample, which does not crystallize at room temperature for relatively long times.

Relaxation Dynamics vs Crystallization Kinetics in the Amorphous State: The Case of Stiripentol.

With the aim of finding a correlation between the crystallization kinetics and the molecular dynamics of a substance that would allow prediction of its crystallization time as a function of temperature for a given α relaxation time, we have studied stiripentol, an anticonvulsant drug. Stiripentol has been characterized in its supercooled liquid, amorphous (glass), and crystalline states by the concurrent use of broadband dielectric spectroscopy (BDS), differential scanning calorimetry, X-ray diffraction, and optical microscopy. BDS was employed to study both the dipolar molecular dynamics and the kinetics of crystallization from the melt. Three different molecular relaxation dynamics were identified: an α relaxation corresponding to the collective reorientation of the molecules and associated with the glass transition (Tg = 246.2 ± 0.5 K), a Johari Goldstein ß relaxation that can be associated with the single-molecule precursor of the α process, and a γ relaxation arising from intramolecular motions. Isothermal crystallization of Stiripentol was studied by means of BDS well above the glass transition (between 273 and 293 K), and it was observed under optical microscope at ambient conditions. Stiripentol did not exhibit any sign of polymorphism at ambient pressure, and it recrystallized from the melt into its stable crystalline form. The crystallization kinetics did not obey the Avrami law. Stiripentol displayed a very low nucleation rate, and drops of liquid stiripentol were observed to crystallize completely from a single nucleus before the appearance of new nuclei, so that the crystallite grew according to the morphology of the liquid domains, a fact that might explain the lack of validity of the Avrami law. Possible correlations between the crystallization kinetics and the molecular dynamics have been analyzed, finding that the crystallization time has a sublinear dependence on the cooperative relaxation time, as is the case in other substances reported in the scientific literature. This could suggest a general correlation of these parameters, at least at temperatures above Tg. The low nucleation rate is an interesting feature in the quest of possible mechanisms that allow enhancing the physical stability of amorphous drugs.

Genuine antiplasticizing effect of water on a glass-former drug.

Water is the most important plasticizer of biological and organic hydrophilic materials, which generally exhibit enhanced mechanical softness and molecular mobility upon hydration. The enhancement of the molecular dynamics upon mixing with water, which in glass-forming systems implies a lower glass transition temperature (T g ), is considered a universal result of hydration. In fact, even in the cases where hydration or humidification of an organic glass-forming sample result in stiffer mechanical properties, the molecular mobility of the sample almost always increases with increasing water content, and its T g decreases correspondingly. Here, we present an experimental report of a genuine antiplasticizing effect of water on the molecular dynamics of a small-molecule glass former. In detail, we show that addition of water to prilocaine, an active pharmaceutical ingredient, has the same effect as that of an applied pressure, namely, a decrease in mobility and an increase of T g . We assign the antiplasticizing effect to the formation of prilocaine-H 2 O dimers or complexes with enhanced hydrogen bonding interactions.

Thermodynamic Scaling of the Dynamics of a Strongly Hydrogen-Bonded Glass-Former.

We probe the temperature- and pressure-dependent specific volume (v) and dipolar dynamics of the amorphous phase (in both the supercooled liquid and glass states) of the ternidazole drug (TDZ). Three molecular dynamic processes are identified by means of dielectric spectroscopy, namely the α relaxation, which vitrifies at the glass transition, a Johari-Goldstein ß JG relaxation, and an intramolecular process associated with the relaxation motion of the propanol chain of the TDZ molecule. The lineshapes of dielectric spectra characterized by the same relaxation time (isochronal spectra) are virtually identical, within the studied temperature and pressure ranges, so that the time-temperature-pressure superposition principle holds for TDZ. The α and ß JG relaxation times fulfil the density-dependent thermodynamic scaling: master curves result when they are plotted against the thermodynamic quantity Tv γ , with thermodynamic exponent γ approximately equal to 2. These results show that the dynamics of TDZ, a system characterized by strong hydrogen bonding, is characterized by an isomorphism similar to that of van-der-Waals systems. The low value of γ can be rationalized in terms of the relatively weak density-dependence of the dynamics of hydrogen-bonded systems.

Collective relaxation dynamics and crystallization kinetics of the amorphous Biclotymol antiseptic.

We employ dielectric spectroscopy to monitor the relaxation dynamics and crystallization kinetics of the Biclotymol antiseptic in its amorphous phase. The glass transition temperature of the material as determined by dielectric spectroscopy is Tg = 290 ± 1K. The primary (α) relaxation dynamics is observed to follow a Vogel-Fulcher-Tammann temperature dependence, with a kinetic fragility index m = 86 ± 13, which classifies Biclotymol as a relatively fragile glass former. A secondary relaxation is also observed, corresponding to an intramolecular dynamic process of the non-rigid Biclotymol molecule. The crystallization kinetics, measured at four different temperatures above the glass transition temperature, follows an Avrami behavior with exponent virtually equal to n = 2, indicating one-dimensional crystallization into needle-like crystallites, as experimentally observed, with a time-constant nucleation rate. The activation barrier for crystallization is found to be Ea = 115 ± 22 kJ mol(-1).

Silicon-Chip-Based Dielectric Spectroscopy for Conductivity and Molecular Dynamics Studies of Organic Films.

Interdigital electrodes fabricated by standard lithography on silicon chips are employed to probe the dipolar molecular dynamics and electric conduction properties of thin rhodamine films grown with two different methods. The conductivity is due to electronic charge carriers, and at around room-temperature, it is higher by 1 order of magnitude in solution-deposited films than in thermally evaporated ones. The organic material exhibits two intrinsic dynamic processes, of which the one at higher temperature is due to the orientational motion of the dipole moment of the rhodamine units, while the one at lower temperature is due to the motion of a local dipole associated with the chlorine counterions and is absent in thermally evaporated films. Our results show that thin-film dielectric spectroscopy is an easily implementable and versatile tool to extract valuable information on thin organic films.

Large optical-frequency shift of molecular radiation via coherent coupling to an off-resonance plasmon.

We demonstrate coherent optical coupling between molecular and plasmon resonances that are well separated in energy. In the presence of metallic nanoparticles, the second harmonic spectrum of organic dyes no longer peaks at the absorption wavelength but is instead blueshifted by 25 nm towards the localized plasmon resonance. The phase of the light generated by the dyes displays a large modulation across the plasmon resonance and no change across the molecular one. The second harmonic signal contributed by the nanoparticles, which is peaked at the plasmon frequency when no molecules are present, similarly displays a shift towards the molecular resonance in their presence. A model based on the interplay of the nonlinear optical near fields is able to account for these observations.