2.1. UV-Vis Absorption and Chiroptical Properties in the Solid State
The UV-Vis absorption data of the studied polymers as amorphous thin films are given in Table 1
. As expected, the absorption spectra of all these azobenzene derivatives are very similar to each other, showing, in the spectral region 700–190 nm, two absorption bands (Figure 2
bottom): the more intense one is centered at about 425 nm and is due to the n
–π*, π–π* and internal charge transfer electronic transitions of the conjugated azoaromatic chromophore; the other one, positioned around 275 nm, is related to the π–π* electronic transition of the aromatic ring [47
The registered spectra appear quite similar to the previously reported ones for the same systems in DMA dilute solutions [47
] even though the first absorption band results are blue-shifted by about 20–25 nm (Table 1
). In agreement with several reports [52
], such a behavior suggests the formation of H-type chromophoric aggregates (which consist of an intramolecular parallel arrangement of the azobenzene electric dipoles) forced by the structural constraints of the macromolecules [43
] as a result of solvent removal. The blue shift is even more evident if we consider the spectrum of the monomeric model compound (S
)-3-pivaloyloxy-1-(4′-cyano-4-azobenzene) pyrrolidine [(S
] (Table 1
) which lacks any structural restriction. Thus, the further shift observed upon passing from solution to solid state can be ascribed to an increase of the H-type chromophoric aggregates as a consequence of the removal of the solvent and the collapse of the polymer chains to form the film.
The CD spectra of the amorphous polymers in the solid state are also very similar to those in dilute solutions [47
] (Table 2
and Figure 2
up). The two enantiomeric homopolymers, indeed, show an exciton splitting of opposite sign originated by cooperative interactions between side-chain azochromophores disposed in a mutual chiral geometry of one prevailing and opposite handedness at the wavelength of the maximum of the visible absorption band [55
]. Such a behavior confirms that the two homopolymers assume, also in the solid state, at least for chain sections, enantiomeric conformations of one prevailing screw sense related to the absolute configuration of the starting monomer.
By decreasing the relative content of repeating units of one prevailing configuration, the intensity of the dichroic couplet is reduced, as demonstrated by the CD spectra of copol (R
)-75 and (S
)-75 (Figure 2
up); as expected, no dichroic signals are present in the spectrum of the racemic one [copol (rac
The same trend was previously shown by these materials in diluted DMA solution [48
], thus demonstrating that the dipolar interactions between chiral groups of identical absolute configuration, which are responsible for this phenomenon, are maintained (freezing) also in the solid state.
The amorphous thin films of the two enantiomeric homopolymers and the copol (rac
) were heated in air for a given amount of time at constant temperature, then rapidly cooled at room temperature and their CD and UV-vis spectra recorded. Each sample was submitted to several such thermal cycles, applying progressively higher temperatures every cycle, as many times as required for two subsequent spectra to not show any further change. As expected, no variations on CD and UV-vis spectra were detected by annealing the samples for 60 min at temperatures far from their glass transition temperature (Tg
about 185–192 °C determined on the polymers as a powder) [47
], e.g., lower than 170 °C.
At a higher temperature (200 °C) a significant change of the UV-vis and CD bands of the homopolymers is observed; such a behavior can be ascribed to some kind of thermal transition experienced by the thin film samples. As an example, Figure 3
and Figure 4
display the UV-vis and CD spectra of the thin films of the two homopolymers recorded at different times upon annealing at 200 °C, just above their glass transition temperatures.
It is evident that, by annealing at a temperature close to the Tg, the absolute intensity of all the CD bands progressively and considerably enhances, and saturates at a value that is at least 20 times larger than that of the native films. Although their resulting shape and position are unchanged, the cross-over point of the exciton couplet in the Visible region progressively moves towards shorter wavelengths (about 15–20 nm). As expected, no changes were registered on the CD spectra of the annealed copol (rac) which remains silent.
It is worth noting that remarkable modification of the UV-vis absorption spectra of the polymer films were also recorded (Figure 3
and Figure 4
bottom). Upon annealing, indeed, the visible absorption band shows a significant intensity reduction together with a further 8–10 nm blue shift of the band maximum with respect to the native film. The shift of the visible absorption band to shorter wavelengths, observed after prolonged thermal treatments of the film, suggests an enhancement of the degree of the chromophore aggregation (H-type aggregation) [43
]. This reorganization of the dipolar interactions between chromophores in the side chain could also explain the dramatic changes of the CD spectra reported above.
It is worth noting that all the annealed films, as well as the related native ones, were optically isotropic (no LC phases were detected, thus no LC chiral suprastructures with macrodomains conformationally ordered are present).
Previous studies demonstrated that the optical activity of these materials is not related to the presence of a predominant configuration of the stereogenic centers in the backbone but is essentially of conformational origin [46
]. Moreover, investigations on analogous azobenzene chiral methacrylic polymers [49
] highlighted that the CD bands are strongly dependent on the average polymerization degree of the macromolecules, tending to attain the highest and constant amplitude at Xn
around 20–25 (Figure 5
), and short chain sections are already able to contribute to the overall optical activity.
In particular, it was demonstrated that the interactions between adjacent side-chain chromophores having conformational dissymmetry of one prevailing screw sense for short chain sections already constitute a relevant contribution to the overall chirality manifestations of the material, in accordance with the results previously achieved by investigating the spectroscopic and chiroptical properties of a similar dimeric derivative containing two photochromic chiral moieties [51
], the smallest section of polymer where interchromophore interactions can be present.
Taking into account that the films are subjected to a temperature slightly above their glass transition, a rearrangement of the azobenzene chromophores can be envisaged, driven by the enhanced chain mobility allowing for a more thermodynamically favored chiral organization of the same chirality sign of the starting one. The strong reorganization of the dipolar interactions in the solid state subsequently brings in an increase of the macromolecular suprastructure and/or of the chain sections characterized by a helical structure that could explain the observed chiral amplification, as previously described by Saxena et al., for polysilane film [57
] and by Zou et al., for azobenzene-substituted polydiacetylenes [58
]. The enhanced optical activity of the amorphous thin films upon annealing could thus be ascribed to the formation of nanodomains of chromophores aggregated (H–type) in a chiral conformation with a prevailing helicity whose chirality is driven by the intrinsic optical activity of the macromolecules and which are frozen in the solid state as idealized in Figure 6
To evaluate the evolution of chiral conformations assumed by the macromolecules in the solid state during annealing, we report in Figure 7
and Figure 8
the ellipticity registered at a wavelength close to the maxima of the two dichroic bands constituting the excitonic couplet, as a function of heating time.
The CD signals tends to increase their amplitude progressively (with a quasilinear behavior) as a function of annealing time up to 80 min, where the amplitude reaches an almost constant value. Stemming from the knowledge that the chiroptical properties of interacting chromophores strongly depend on their dihedral angle and relative distance (R) (by a factor of about 1/R6
), as stated by the model of electrostatic dipolar interchromophore interactions adopted to describe the CD spectra [56
], this fact can be invoked to explain the previously discussed behaviour. In fact, by increasing the number of repeating units included within an ordered section, R also increases and consequently the interactions of a given chromophore progressively decay from the first neighbouring azobenzene, to the following one, and so forth, with convergence of the intensity of the CD signals to one asymptotic value.
An evaluation of the persistence with time of the conformational arrangement assumed by the macromolecules after annealing at a temperature around the Tg was made by keeping the film at room temperature for almost 6 months. After this time, the appearance of the UV and CD spectra is the same as that shown by annealing films for 75 min, thus suggesting that the above-mentioned thermal transition takes place rapidly at the Tg and does not produce further structural changes at room temperature. In conclusion, the thermal stability displayed by these chiral materials suggests that the resulting thermally induced chiroptical properties are stable at room temperature for a long period of time.
Though some light has still to be shed on the mechanism underlying the chiral amplification shown by these materials after thermal treatment, to the best of the authors’ knowledge this is the first time this effect has been reported for polymers which are not liquid crystals.