Effect of regioregularity and role of heteroatom on the chiral behavior of oligo(heteroalkyl thiophene)s.

Novel optically active oligothiophenes bearing electron-donating chiral side chains have been prepared by synthetic methods suitable to achieve regioregular head-to-tail and head-to-head/tail-to-tail derivatives. In particular, the chiral (S)-(2-methyl)butyl moiety was linked at position 3 of the thiophene ring through heteroatoms, such as S or O, to evaluate its effect on the macro molecular aggregation and, consequently, on the chiroptical properties of the material in the solid state. The materials have been fully characterized and investigated by optical and chiroptical methods upon aggregation both from the solution and as cast films. Compared with the related head-to-tail and head-to-head/tail-to-tail poly(3-alkyl)thiophene derivatives, with the same optically active moiety directly linked to the ring and possessing a higher polymerization degree, the chiroptical properties of the newly synthesized oligomers were significant, or even better, and provided insight into the role of intrachain-interchain interactions between the heteroatom and the thienyl sulfur atom.

Chirality induced by microaggregation of regioregular polythiophenes with side-chain optically active substituent.

Two novel optically active terthiophene and bithiophene monomers bearing at the C-ß position of thiophene ring an enantiomerically pure chiral alkyl group, namely 3,3″-di[2-((S)-(+)-2-methylbutoxy)ethyl]-2,2':5',2″-terthiophene and 3,3'-di[2-((S)-(+)-2-methylbutoxy)ethyl]-2:2'-bithiophene, have been synthesized and characterized. Being symmetrically substituted, their polymerization adopting simple and economic method, such as the chemical oxidation with iron (III) trichloride, allowed to obtain regioregular macromolecules. The synthesized polymers have been investigated by CD in solution as well as in the microaggregated state upon gradual addition of a poor solvent. The results display different CD behaviors related to their chemical structure.

Synthesis and chiroptical properties of chiral azoaromatic dendrimers with a C3-symmetrical core.

New chiral azoaromatic dendrimeric systems have been synthesized starting from 1,3,5-benzenetricarbonyl trichloride as the core molecule. The simultaneous presence of the (S)-3-hydroxy pyrrolidinyl ring as the optically active moiety and the azobenzene donor-acceptor conjugated system as the photochromic group with permanent dipole moment, makes these systems potentially interesting as materials for advanced applications in nanotechnologies. All the compounds obtained have been characterized with particular attention to the effects induced by changing the electron-withdrawing group in the chromophoric moiety and to their optical activity. A strong nonlinear enhancement of chiroptical properties related to the number of chiral units linked to the symmetrical core is observed in these derivatives, which indicates the presence of conformationally chiral substructures.

Supramolecular chirality and reversible chiroptical switching in new chiral liquid-crystal azopolymers.

The synthesis of chiral liquid-crystalline polymers of well-controlled structure (linear and three-armed star-shaped) with distinct average chain lengths and low polydispersity was achieved by atom transfer radical polymerisation (ATRP) of a new optically active monomer (S)-4-[6-(2-methacryloyloxypropanoyloxy)hexyloxy)]-4'-ethoxyazobenzene [(S)-ML6A], containing the L-lactic residue of one absolute configuration in the side-chain. All the obtained polymeric samples, characterised by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarised optical microscopy (POM), exhibit a smectic A(1/2) (fully interdigitated) liquid-crystalline phase and high cleaning points, with transition temperatures dependent on the average polymerisation degree and the macromolecular structure. The chirality originated at the molecular level by the asymmetric functionality of the L-lactic acid residue provides the polymers, in the smectic phase, of highly homogeneous conformations with a prevailing chirality related to the presence of H-aggregates having conformational dissymmetry of one prevailing screw-sense. By irradiating with circularly polarised light (CPL), it is possible to photomodulate the chiroptical properties of these intrinsically chiral polymeric thin films. Upon irradiation with left-handed CPL (l-CPL), the circular dichroism (CD) spectra of the films show enhancement of ellipticity and a net inversion of sign. The effect is reversible and the mirror image of the CD spectrum can be restored by pumping with right-handed CPL radiation (r-CPL). The results show the ability of l-CPL to invert the supramolecular chirality of the materials and demonstrate the essential role of azoaromatic aggregates.

Chiral interactions in azobenzene dimers: a combined experimental and theoretical study.

To investigate interchromophore interactions in azobenzene polymers, we have undertaken a thorough spectroscopic analysis of the azodye [(S)-3-pivaloyloxy-1-(4'-nitro-4-azobenzene)pyrrolidine] by modeling the repeating unit of poly[(S)-3-methacryloyloxy-1-(4'-nitro-4-azobenzene)pyrrolidine) and its dimeric derivative whose synthesis is presented here. The analysis of the electronic and Raman spectra of the azodye in several solvents is based on a previously proposed model for polar chromophores in solution. Electronic and CD spectra of the dimeric unit are collected and analyzed within the framework of a new model. On the basis of the information collected from the spectroscopic analysis of the solvated dye, this model accounts for interchromophore interactions in the dimer. The large CD signal measured for the dimer (amounting to about a third of the signal measured for the polymer) suggests the presence of important chiral interactions in the dimeric unit, and is modeled in terms of a right-handed relative orientation of the two chromophores.

Photomodulation of the chiroptical properties of new chiral methacrylic polymers with side chain azobenzene moieties.

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.