All organic photoactive liquid crystalline materials for 3D optical data storage – 3D-ODS

Recently, we demonstrated in Rennes, in collaboration with the University of Leuven, that bipyrimidine derivatives (D2d symmetry) functionalized with four promesogenic groups bearing chiral alkyl chains are capable of self-organizing into thin liquid crystal films exhibiting exceptional second harmonic generation (SHG) properties, comparable to those of inorganic crystalline materials such as KH₂PO₄ (KDP). The research developed within this project has therefore focused on the integration of coumarin derivatives, as two-photon photodimerizable fragments, into these liquid crystal matrices to locally and durably modulate the SHG signal within voxels, thus enabling data storage in the form of 3D bits.

As a first approach, we attempted to substitute one of the four promesogenic groups bearing chiral carbon chains with a coumarin fragment. Two grafting strategies were explored: one via direct alkene bond formation, and the other via a Sonogashira coupling. With the first route, we observed that grafting the coumarin fragment onto the bipyrimidine core via a Knoevenagel condensation under basic conditions led to the hydrolysis of the coumarin. The second strategy, involving a Sonogashira coupling with a coumarin-alkyne, was more promising but faced purification difficulties.

To overcome these issues, we simplified the molecular design by developing dissymmetric molecules based on a pyrimidine core, bearing a promesogenic fragment on one side and a coumarin fragment on the other. This approach proved more successful, and an original series of 12 new molecules was synthesized. Their thermal properties, self-organization behavior, and linear and nonlinear optical properties were fully characterized. One compound, which exhibited both liquid crystal properties and promising SHG activity, was selected for information storage in thin films. We successfully encoded information onto a thin film via one- or three-photon absorption. However, the stored data could not be erased using irradiation at lower wavelengths.

To understand this lack of reversibility, model compounds derived from coumarin were synthesized. Spectroscopic studies revealed that connecting the coumarin moiety to the rest of the molecule via an alkyne linker suppressed the photodimerization reaction. The observed writing process was therefore attributed to photodegradation, as confirmed by spectroscopic studies conducted in the presence and absence of oxygen.

To restore the photodimerization properties of the coumarin unit, a new series of molecules was synthesized, in which the coumarin fragment was connected to the pyrimidine moiety via a non-conjugated ether linkage.

To meet the objectives of the project, we initially focused on the synthesis of symmetrically tetrafunctionalized bipyrimidines, investigating how the position and nature of the peripheral chains grafted onto the bipyrimidine cores affect their thermal and optical properties. A library of 18 new symmetrically tetrafunctionalized bipyrimidines was synthesized from three series bearing structurally diverse promesogenic units. These studies showed that functionalization at the 4-position with chiral chains is essential to maximize the SHG signal, and that extending the π-conjugation enhances the luminescence of the compounds, which in turn hampers the observation of strong SHG signals in solution and solid state. The work also revealed a strong interdependence between the nature, number, length, and position of the alkyl chains, affecting both the emergence of liquid crystalline properties and nonlinear optical responses.

We then turned to the synthesis of an asymmetrically tetrasubstituted bipyrimidine bearing three promesogenic units and one coumarin. This asymmetric design was intended to preserve non-centrosymmetry at the supramolecular level while introducing a photoactive unit for data storage. However, the challenges encountered in obtaining the coumarin-functionalized bipyrimidine led to a revision of the initially planned structure. Indeed, the asymmetric tetrasubstitution of BPM generated numerous side products that were difficult to separate.

In this context, a new series of asymmetrically disubstituted pyrimidine-core compounds was developed, featuring one promesogenic arm functionalized with various alkyl chains (chiral and linear) and another arm bearing a coumarin unit. In total, 12 new pyrimidine-core compounds were synthesized. Among them, the compound PM(1)C(yne)-3,4,5-C16 showed the best compromise between favorable thermal and optical properties. As initially envisioned, it was possible to stably write information in a thin liquid crystal film via 1PA and 3PA UV irradiation, and to read it using SHG microscopy.

However, the information storage process proved to be irreversible, contrary to expectations. The writing process was not triggered by photodimerization of the coumarin moiety, but rather by other irreversible optical processes. Our most recent efforts have thus focused on restoring the photodimerization capability of the coumarin, by incorporating non-conjugated coumarin units into the pyrimidine core. Unfortunately, this strategy was unsuccessful, as the molecule’s photodegradation process continued to dominate over photoisomerization.

Multiple strategies are envisioned. Within this framework, a new molecule with an asymmetrical pyrimidine core bearing a different photochrome could be envisioned. This type of compound could serve as an alternative to the coumarin initially considered in the project. Indeed, the photoisomerization reaction is intramolecular and could occur prior to the photodegradation processes observed in the compounds studied during this project.