Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (gabs and glum) and on the application of ECD and CPL to derive structural information on aggregated states

Impact and amplification of chirality in the aggregation of leucine-appended poly(p-phenylene ethynylene) (PPE)

A leucine-appended poly(p-phenyleneethynylene) (PPE) was prepared in enantiomeric stereoregular (L-1 and D-1) and stereorandom (rac-1) forms. The solution aggregates of L-1, D-1, rac-1, and mixtures of L-1/D-1, were characterized by absorption, electronic circular dichroism and emission spectra. Both rac-1 and L-1/D-1 mixtures are more prone to aggregate than L-1 and D-1. Upon aggregating, the enantiomeric mixtures manifest an apparent majority-rules effect, which is mostly due to the greater tendency to form heterochiral aggregates with respect to homochiral ones. The impact of chirality on the aggregation behaviour of the aminoacid-appended PPE is demonstrated

Circularly polarized light at the mirror: Caveats and opportunities

Moving from the simple concept that reflection onto a mirror surface changes the handedness of circularly polarized light, we describe what happens to the emergent polarization in two different cases after reflection on a back mirror. In the first case, a regular emitter is taken into account, where reflection has the effect to destroy the emergent polarization. In the second case, we show what could happen when a hypothetical apparently non-reciprocal emitting material undergoes a similar experiment. These simple concepts have important implications in the design of efficient circularly polarized emitting devices

Spatially Resolved Chiroptical Spectroscopies Emphasizing Recent Applications to Thin Films of Chiral Organic Dyes

Instrumental techniques able to identify and structurally characterize the aggregation states in thin films of chiral organic π-conjugated materials, from the first-order supramolecular arrangement up to the microscopic and meso-scopic scale, are very helpful for clarifying structure-property relationships. Chiroptical imaging is currently gaining a central role, for its ability of mapping local supramolecular structures in thin films. The present review gives an overview of electronic circular dichroism imaging (ECDi), circularly polarized luminescence imaging (CPLi), and vibrational circular dichroism imaging (VCDi), with a focus on their applications on thin films of chiral organic dyes as case studies

Remarkable near-infrared chiroptical properties of chiral Yb, Tm and Er complexes

We carried out a study of absorption (CD) and emission (CPL) chiroptical properties in the NIR region of two sets of Yb, Tm and Er complexes. The two complexes include a D3 symmetric, [TMG-H+]3Ln(BINOLate)3 (Ln = Yb, Tm, Er; TMG = 1,1,3,3-tetramethylguanidine; BINOLate = 1,1'-bi-2-naphtholate), and a tetrakis, C4 symmetric, CsLn(hfbc)4 (Ln = Yb, Tm, Er; hfbc = 3-heptafluorobutylyrylcamphorate). The lanthanides studied gave access to three discrete energy domains, Yb (900-1040 nm), Tm (1180-1240 nm) and Er (1430-1600 nm) in which the chiroptical activity was assessed using gabs (and glum for Yb complexes). Exceptionally high discrimination between left and right circularly polarised light was observed, with values up to almost the theoretical maximum (±2)

The psoriatic shift induced by interleukin 17 is promptly reverted by a specific anti-IL-17A agent in a three-dimensional organotypic model of normal human skin culture

Interleukin 17A (IL-17A), mainly produced by the T helper subclass Th17, plays a key role in the psoriatic plaque formation and progression. The clinical effectiveness of anti-IL-17A agents is documented, but the early and specific mechanisms of their protection are not identified yet. The challenge of the present study is to investigate the possible reversal exerted by a specific anti-IL-17A agent on the psoriatic events induced by IL-17A in a three-dimensional organotypic model of normal human skin. Bioptic skin fragments obtained after aesthetic surgery of healthy women (n=5) were incubated with i) IL-17A biological inhibitor (anti-IL-17A), ii) IL-17A, iii) a combination of IL-17A and its specific IL-17A biological inhibitor (COMBO). A Control group was in parallel cultured and incubation lasted for 24 and 48 h epidermal-side-up at the air-liquid interface. All subjects were represented in all experimental groups at all considered time-points. Keratinocyte proliferation and the presence of epidermal Langerhans cells were quantitatively estimated. In parallel with transmission electron microscopy analysis, immunofluorescence studies for the epidermal distribution of keratin (K)10, K14, K16, K17, filaggrin/occludin, Toll-like Receptor 4, and Nuclear Factor kB were performed. IL-17A inhibited cell proliferation and induced K17 expression, while samples incubated with the anti-IL-17A agent were comparable to controls. In the COMBO group the IL-17A-induced effects were almost completely reverted. Our study, for the first time, elucidates the most specific psoriatic cellular events that can be partially affected or completely reverted by a specific anti-IL-17A agent during the early phases of the plaque onset and progression. On the whole, this work contributes to expand the knowledge of the psoriatic tableau

Efficient 1400-1600 nm Circularly Polarized Luminescence from a Tuned Chiral Erbium Complex

Novel chiral Er complexes based on both enantiomers of extended (i)PrPyBox (2,6-Bis[4-isopropyl-4,5-dihydrooxazol-2-yl)]pyridine) show strong near-infrared circularly polarized luminescence (CPL) within the 1400 to 1600 nm spectral region under 450 nm irradiation. CPL activity in this region, despite being particularly rare, would open the way to potential applications in the domain, e.g., of fiber-optic telecommunications and free-space long-distance optical communications employing circularly polarized light. Moreover, the long wavelength excitation is advantageous for applications in the field of (circularly polarized) microscopy and bioimaging

Unique Aggregation of Sterigmatocystin in Water Yields Strong and Specific Circular Dichroism Response Allowing Highly Sensitive and Selective Monitoring of Bio-Relevant Interactions

We demonstrated the hitherto unknown property of the mycotoxin sterigmatocystin (STC) to provide homogeneous solutions in aqueous medium by forming a unique aggregate type (not formed by analogous aflatoxins), characterized by exceptionally strong circular dichroism (CD) bands in the 300-400 nm range. Results showed that these CD bands do not originate from intrinsic STC chirality but are a specific property of a peculiar aggregation process similar to psi-DNA CD response. Transmission electron microscopy (TEM) experiments revealed a fine fiber network resembling a supramolecular gel structure with helical fibers. Thermodynamic studies of aggregates by differential scanning calorimetry (DSC) revealed high reversibility of the dominant aggregation process. We demonstrated that the novel STC psi-CD band at 345 nm could be applied at biorelevant conditions (100 nanomolar concentration) and even in marine-salt content conditions for specific and quantitative monitoring of STC. Also, we showed that STC strongly non-covalently interacts with ds-DNA with likely toxic effects, thus contrary to the previous belief requiring prior enzyme epoxidation

From Mesocates to Helicates: Structural, Magnetic and Chiro-Optical Studies on Nickel(II) Supramolecular Assemblies Derived from Tetradentate Schiff Bases

The systematic reactions of a family of tetradentate pyridyl/imine and quinolyl/imine racemic or enantiopure Schiff bases with Ni(NO3)(2) or Ni(ClO4)(2) in the presence of sodium azide yielded, as a function of the starting racemic, chiral or achiral base, a set of chiral, meso or achiral complexes. In all cases, the compounds consist of two Ni-II cations linked by a double azido bridge in its end-on coordination mode. All the dimers exhibit a mesocate supramolecular structure and one of them, the unprecedented mix of helicate and mesocate in 2:1 ratio. The transition from mesocate to helicate conformation has been reached by tuning the flexibility of the central spacers of the Schiff bases and the size of the substituents. Electronic circular dichroism (ECD) studies have been performed for two pairs of enantiomers and interpreted by means of DFT calculations. Susceptibility measurements show a ferromagnetic coupling between the Ni-II cations mediated by the end-on azido bridges

Novel Potent Muscarinic Receptor Antagonists: Investigation on the Nature of Lipophilic Substituents in the 5- and/or 6-Positions of the 1,4-Dioxane Nucleus

A series of novel 1,4-dioxane analogues of the muscarinic acetylcholine receptor (mAChR) antagonist 2 was synthesized and studied for their affinity at M1-M5 mAChRs. The 6-cyclohexyl-6-phenyl derivative 3b, with a cis configuration between the CH2N+(CH3)3 chain in the 2-position and the cyclohexyl moiety in the 6-position, showed pKi values for mAChRs higher than those of 2 and a selectivity profile analogous to that of the clinically approved drug oxybutynin. The study of the enantiomers of 3b and the corresponding tertiary amine 33b revealed that the eutomers are (2S,6S)-(-)-3b and (2S,6S)-(-)-33b, respectively. Docking simulations on the M3 mAChR-resolved structure rationalized the experimental observations. The quaternary ammonium function, which should prevent the crossing of the blood-brain barrier, and the high M3/M2 selectivity, which might limit cardiovascular side effects, make 3b a valuable starting point for the design of novel antagonists potentially useful in peripheral diseases in which M3 receptors are involved