Alberto Marini

What is Solvatochromism

Solvatochromism is commonly used in many fields of chemical and biological research to study bulk and local polarity in macrosystems (membranes, etc.), or even the conformation and binding of proteins. Despite its wide use, solvatochromism still remains a largely unknown phenomenon due to the extremely complex coupling of many different interactions and dynamical processes which characterize it. In this study we analyze the influence of different solvents on the photophysical properties of selected charge-transfer probes (4-AP, PRODAN, and FR0). The purpose is to achieve a microscopic understanding of the intermolecular effects which govern the absorption and fluorescence properties of solvated molecular probes, such as solvent-induced structural modifications, polarization effects, solubility, solute-solvent hydrogen-bonding interactions, and solute aggregation. To this aim we have exploited a time dependent density functional theory (TDDFT) approach coupled to complementary solvation approaches (continuum, discrete and mixed discrete and continuum). Such an integration has allowed us to clearly disentangle the complex interplay between specific and nonspecific interactions of the solvent with the probes and show that strong H-bonding effects not only can lead to large solvatochromic shifts but also can affect the nature of the emitting species with resulting reduction of the quantum yield.

Polarity-sensitive fluorescent probes in lipid bilayers: bridging spectroscopic behavior and microenvironment properties.

We have studied the emission features of the fluorescent polarity-sensitive probes known as Prodan and Laurdan in a liquid-crystalline DPPC bilayer. To this purpose, we have combined high-level quantum mechanical electronic structure calculations with a molecular field theory for the positional-orientational-conformational distribution of the probes, in their ground and excited states, inside of the lipid bilayer, taking into account at both levels the nonuniformity and anisotropy of the environment. Thus, we can interpret the features of the fluorescence spectra of Prodan and Laurdan in relation to the position and orientation of their chromophore in the bilayer. We have found that the environment polarity is not sufficient to explain the large red shifts experimentally observed and that specific effects due to hydrogen bonding must be considered. We show that the orientation of the probe is important in determining the accessibility to water of the H-bond-acceptor group; in the case of Laurdan interesting conformational effects are highlighted.

Orientational Order of Difluorinated Liquid Crystals : A Comparative 13C-NMR, Optical, and Dielectric Study in Nematic and Smectic B Phases

Structural and orientational order properties of 3Cy2CyBF2 and of 5CyCy2BF2 have been investigated by means of (13)C-NMR, optical, and dielectric spectroscopy methods. In the case of NMR, order parameters have been independently obtained from the analysis of either (13)C-(19)F dipolar couplings or (13)C chemical shift anisotropies, both measured from (13)C-{(1)H} NMR static spectra. The assignment of the (13)C resonances has been carried out thanks to the comparison with solution state spectra and DFT calculations, and the relevant geometrical parameters and (13)C chemical shift tensors needed to derive orientational order parameters have been calculated by DFT methods. In the analysis of (13)C-(19)F dipolar couplings, empirical corrections for vibrations and anisotropic scalar couplings have been included. Dielectric measurements have been performed over a broad frequency range for two orientations of the nematic director with respect to the measuring field. At low frequencies (static case) a positive dielectric anisotropy has been determined, which has enabled the calculation of the order parameters according to a well-tested procedure. At high frequencies the dielectric anisotropy changes its sign, a property which can be useful in designing a dual addressing display. The nematic order parameter determined from optical, dielectric, and NMR methods have been compared: their trends with temperature are very similar, apart from some slight shifts, and were analyzed by Haller and Chirtoc models. The differences among the results obtained by the four methods have been discussed in detail, also with reference to the assumptions and approximations used in each case, and to the results recently reported for similar fluorinated nematogens. The presence of a non-negligible order biaxiality has been related to the presence of a CH2CH2 bridging group, linking one cyclohexylic unit with either the other cyclohexyl or the phenyl ring.

NMR studies of the ferroelectric SmC* phase

In this work an attempt is made to compare and rationalise the structural and dynamic behaviour of some ferroelectric rod-like mesogens studied by our group in recent years, mainly by means of 2H- and 13C-nuclear magnetic resonance (NMR) spectroscopy. This comparison concerning the local orientational order, the average molecular conformation and the chiral smectic C (SmC*) phase structure, with a brief overview on the reorientational dynamic properties, allowed us to identify some common behaviour of these ferroelectric rod-like mesogens when decreasing the temperature from lower ordered phases to and within the SmC* phase. The main results obtained by means of NMR studies on these mesogens are rationalised and discussed in comparison with detailed discrete Fourier transform computations of their molecular conformations (in the limit of isolated molecules) with the purpose of enlightening the role of intermolecular and packing interactions in the ferroelectric phase with respect to the smectic A and nematic phases.

Orientational Order of Fluorinated Mesogens Containing the 1,3,2-Dioxaborinane Ring: A Multidisciplinary Approach

Orientational order properties of two fluorinated liquid crystals containing the 1,3,2-dioxaborinane ring have been investigated by means of optical, dielectric, and (13)C and (19)F NMR spectroscopies. The optical birefringence and dielectric anisotropy values determined in the mesomorphic phases were analyzed in terms of well-established theoretical models to obtain the order parameters relative to the principal axis of the polarizability tensor and molecular dipole moment, respectively. A large set of data, including (13)C and (19)F chemical shift anisotropies and (13)C-(19)F and (1)H-(19)F couplings relative to nuclei on the aromatic rings, was acquired in the NMR experiments and analyzed to determine local order parameters (principal order parameter and biaxiality) for different rigid fragments of the mesogen aromatic core using advanced DFT methods for the calculation of geometrical parameters and chemical shift tensors. A critical analysis of the dependence of the order parameters on the data set employed and on the theoretical assumptions and approximations has also been performed. The orientational order parameters obtained using the different techniques are compared and discussed in relation to the reference frame associated with the anisotropic properties monitored.

Dielectric properties of selected laterally fluoro-substituted 4,4′′-dialkyl, dialkoxy and alkyl-alkoxy [1:1′;4′:1′′]terphenyls

Measurements of the complex permittivity, ε* = ε′ – iε″, within the frequency range 200 Hz to 10 MHz for 15 laterally fluoro-substituted terphenyls have been conducted. In most cases the substances exhibited the nematic phase over a broad temperature range. All substances were characterised by negative dielectric anisotropy, and are potentially useful for vertical alignment mode systems. The static permittivity tensor components have been analysed in relation to the dipole structure of the molecules. Dielectric relaxation processes observed in the liquid crystalline (LC) and solid rotator (R) phases (obtained by slow cooling of the samples) are characterised by calculation of the relaxation times and activation barriers. The rotation motions around the short axes are typical for LC phases, whereas rotations about the long axes, accompanied in some cases by internal motions, are present in the R phase.

Conformational Changes at Mesophase Transitions in a Ferroelectric Liquid Crystal by Comparative DFT Computational and 13C NMR Study

In this work, we report a detailed investigation on both the conformational and the orientational ordering properties of a ferroelectric liquid crystal mesogen, namely, M10/**, through the combination of high resolution solid state (13)C NMR and density functional theory (DFT) computational methods. The trends of the observed (13)C chemical shift in the blue, cholesteric, and ferroelectric SmC* phases of M10/** were analyzed in terms of conformational changes occurring in the flexible parts of the molecule. In particular, we focused on the aliphatic alpha methylenoxy carbons because of their high sensitivity to mesophase environment, as evidenced by experimental (13)C chemical shift anisotropy (CSA). DFT computation of the chemical shift tensors as a function of geometrical parameters, such as dihedral angles, put in evidence significant changes in the average conformation at the mesophase transitions. The conformations predicted by DFT have been validated by comparing the calculated (13)C chemical shifts with those experimentally observed for the alkoxylic carbons, whose relative orientation plays a key role in establishing the overall conformation of the molecule in each liquid crystalline phase. Furthermore, the orientational order parameters of the relevant flexible fragments were calculated and found to be in good agreement with those characterizing similar systems, thus validating our approach.

2H, 13C NMR and Ab Initio Calculations Applied to the SmC* Phase: Methodology and Case Studies

Recently several liquid crystals having a chiral smectic C phase (SmC*) have been studied by means of NMR spectroscopy combined with ab initio calculations. The advantage of exploiting different experimental techiques, such as 2H and 13C NMR, and the possibility to calculate relevant spectroscopic quantities, such as 13C chemical shift tensors, by means of ab initio methods, is here discussed based on recent results obtained on several ferroelectric liquid crystal compounds. Orientational order parameters, conformational and structural features can be determined in a self consistent way by applying this combined approach.

From the Molecular Structure to Spectroscopic and Material Properties: Computational Investigation of a Bent‐Core Nematic Liquid Crystal

We present a computational investigation of the nematic phase of the bent-core liquid crystal A131. We use an integrated approach that bridges density functional theory calculations of molecular geometry and torsional potentials to elastic properties through the molecular conformational and orientational distribution function. This unique capability to simultaneously access different length scales enables us to consistently describe molecular and material properties. We can reassign (13)C NMR chemical shifts and analyze the dependence of phase properties on molecular shape. Focusing on the elastic constants we can draw some general conclusions on the unconventional behavior of bent-core nematics and highlight the crucial role of a properly-bent shape.

Integrated NMR and computational study of push-pull NLO probes: interplay of solvent and structural effects.

In this study we combined QM calculations and NMR measurements to understand at a detailed level the complex interplay of structural/electronic properties with the effects of the solvent in the NLO activity of push-pull systems, quantified in terms of variations of the static hyperpolarizability. Different parameters (bond lengths and bond length alternation, vibrational frequencies, electronic charge distribution) are introduced and tested to rationalize both the solvent sensitivity of three molecular systems (namely, p-nitroaniline, ethyl 4-ammino benzoate, and 5-nitro-1H-indole) and the differences among them. This analysis has finally allowed us to establish a clear correlation between the charge transfer behavior of the systems, their NLO properties, and NMR parameters also validating simplified but effective chemical analyses based on resonance limit forms.