Structural and photoresponsive behavior of a series of (phenylene)azobenzene-containing liquid-crystalline methacrylic copolymers

Abstract

A series of side-chain liquid-crystalline methacrylic copolymers, having various amounts of 4 -(4-decyloxyphenyl)-4-(hexyloxy)azobenzene as the mesogenic group, were synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction to determine their mesomorphic properties, and by UV−visible spectrometry to elucidate their optical absorption characteristics. Also, birefringence and surface relief gratings were photoinduced in the films using a laser beam of 413 nm. Copolymers with high mesogen contents display a polymorphic behavior of the smectic type. The smectic order tends to disappear as the content of mesogens in the copolymer decreases, going from extended lamellar domains of chromophores packed in a parallel position, typical for smectic liquid crystals, to small aggregates of chromophores dispersed in an amorphous matrix. Important hypsochromic shifts ( λ max ≈ 20 nm), associated with π interactions between neighboring aromatic groups, confirm the aggregation of chromophores in lamellar structures. The light-induced birefringence (0.005 − 0.06) is not as high as expected but it corroborates that there exists a tight interplay between photoinduced orientation and intrinsic molecular order. Surface relief gratings were recorded in copolymers with low mesogen content (15 − 30 mol%), showing by atomic force microscopy sinusoidal profiles with an average depth of around 150 nm.

Liquid‐crystalline and photo‐induced properties of P4VP quaternized with bromo derivative of dialkyloxy‐phenyleneazobenzene groups

“…Nevertheless, it seems that a coordinated motion of the close packed chromophores in P4VP‐100 produces a higher orientation (birefringence), despite the structural constrains. This result corroborate that there is a tight interplay between photo‐induced orientation and intrinsic molecular order in films, as indeed it was demonstrated to occur in other liquid crystal polymers where the prevalence of cooperative motions depends on the ability of chromophores to move in concert at specific experimental conditions . Although small, the obtained Δ n 30min values are comparable to those reported for other random and block liquid‐crystalline azopolymers …”

“…This means that for this low chromophore content polymer the interaction between the azobenzene groups is weak, responding to irradiation like independent units. For the other films (P4VP‐50 and P4VP‐100), the UV‐Vis spectra is shifted to lower wavelengths (hypsochromic shift), suggesting a major interaction between chromophores . Actually, hypsochromic or blue shifts are regularly associated to the formation of H‐aggregates, where chromophores respond to light not as independent units but coupled the ones to the others .…”

“…Actually, hypsochromic or blue shifts are regularly associated to the formation of H‐aggregates, where chromophores respond to light not as independent units but coupled the ones to the others . Such aggregates are lamellar structures (smectic phase already observed by XRD) whose extent may increase from the less (P4VP‐20) to the most (P4VP‐100) quaternized polymer . The molecular association can improve the light induced properties provided that the tightly held chromophores move in concert during irradiation.…”

“…It should be said that for random copolymers showing smectic phases at low chromophore content, the response can be of similar order to that of homopolymers, as recently demonstrated by Torres‐Rocha et al . on studying methacrylic copolymers bearing phenyleneazobenzene moieties . Within the lamellar domains, the interactions between these long phenyleneazobenzene groups are strong enough to stabilize the liquid crystal order and also favorable for a light‐induced coordinated molecular motion.…”

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

“…There are in addition several examples where the authors proved that their material was in a crystalline phase. ,− Nakano et al, for instance, observed the formation of SRG at the surface of a single crystal of 4-(dimethylamino)­azobenzene (Figure y). Several papers were aimed at comparing molecular materials to their poly­meric counter­parts. , Many examples have been des­cribed where the systems are in a liquid-crystal­line phase. ,,,,,,− Ruthenium acetyl­ides derivatives can also be used for the inscription of SRG. ,, These com­pounds are used in the field of non­linear optics because they are stable, easily prepared, and redox-responsive. Sahraoui et al synthesized such complexes with azobenzene derivatives (Figure z) and studied the ability to form relief gratings on these materials .…”