Luigi Angiolini

The induction of chirality in achiral liquid crystalline azoaromatic polymers with circularly polarized light (CPL) has been studied on well-defined macromolecular derivatives with different molecular structures, synthesized by atom transfer radical polymerization (ATRP). Two methacrylic homopolymers have been investigated: a linear and a three arms star polymer. Both the polymeric samples are featured by low polydispersity and similar average polymerization degree, so that structure-properties correlations are made possible. Irradiation with CPL of one handed rotation sense of casted films and a planar cell of these polymers in the glassy nematic phase allowed to ascertain by UV-Vis and CD spectroscopy the formation of a supramolecular chiral structure with high chirality induced into the material. The results are interpreted in terms of the structural features of the polymers.

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. Chirality, 2010.

Corona poling behaviours of optically active photochromic copolymers derived from methyl methacrylate (MMA) and the methacrylic ester of ( S )-3- hydroxy pyrrolidine linked through the nitrogen atom to the highly conjugated photochromic 4'-(β-cyano-β-(methylsulfonyl)vinyl)-4-azobenzene moiety, have been investigated with the aim to evaluate the effect on the non-linear optical (NLO) properties originated by the presence of inactive side-chain MMA groups along the main chain. The NLO properties of in situ corona poled polymeric films have been studied by second harmonic generation (SHG) measurements. The d33 values of the investigated polymers were determined to be in the range 10-86 pm/V after corona poling. The temporal and thermal stability of the optimal SHG signals obtained after corona poling process of all the macromolecular materials has been investigated and compared. The results indicate that the maximum of these properties can be obtained at a molar content of photochromic units around 20- 40%.

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.

A novel optically‐active polymeric methacrylate, bearing the (S)‐3‐hydroxy pyrrolidinyl group linked through the nitrogen atom to an azopyridine chromophore, was synthesized and investigated in combination with a polymeric methacrylate incorporating the photoisomerizable spiropyran chromophore in the side‐chain. In the presence of acid, this system can be operated in solution to modulate the protonation of the azopyridine moieties by photoisomerizing the spiropyran component. In addition to UV‐vis spectroscopy, the proton transfer process occurring between the macromolecular components can be also followed by CD spectroscopy, thus behaving as a chiroptical switch. magnified image

Two novel optically active polymethacrylates bearing in the side chain a cyclic chiral group of one prevailing absolute configuration linked to the carbazole chromophore, deriving from the related monomers (S)‐(−)‐3‐methacryloyloxy‐N‐[3‐(9‐ethylcarbazolyl)]pyrrolidine [(S)‐(−)‐MECP] and (S)‐(+)‐2‐methacryloyloxy‐N‐[3‐(9‐ethylcarbazolyl)]succinimide [(S)‐(+)‐MECSI], have been prepared and characterized with the aim to obtain materials suitable to photoconductive applications. Poly[(S)‐(−)‐MECP] and poly[(S)‐(+)‐MECSI] exhibit remarkable thermal stability, with glass transition temperature around 200 °C and decomposition temperatures in the range 330–350 °C. Spectroscopic, thermal and chiroptical characterizations indicate the occurrence of dipolar interactions among the side chain moieties and the presence of chiral conformation at least for chain segments of the macromolecules. The photoconductive properties are discussed in terms of extent of conjugation in the side chain based on the electron‐acceptor or electron‐donator properties of the optically active ring linked to the carbazole group. magnified image