Computational Chemistry Modeling and Design of Photoswitchable Alignment Materials
for Optically Addressable Liquid Crystal Devices
An example of a single azobenzene-substituted methacrylate
repeat unit used in the simulations
Photoalignment technology, based on optically switchable "command surfaces," has been receiving increasing interest for liquid crystal (LC) optics and photonics device applications. Azobenzene compounds in the form of low-molar-mass, water-soluble salts deposited either directly on the substrate surface or after dispersion in a polymer binder have been almost exclusively employed for these applications. Ongoing research in the area follows a largely empirical materials design and development approach. This process is time consuming, labor intensive, and wasteful of costly, and potentially scarce, materials resources because of the need to synthesize a large number of compounds to establish trends in physical properties.
Recent advances in computational chemistry now afford unprecedented opportunities to develop predictive capabilities that will lead to new photoswitchable alignment layer materials with low switching energies, enhanced bistability, write/erase fatigue resistance, and high-laser-damage thresholds. The near-IR laser-damage resistance of coumarin-based, photoalignment layers developed at the Laboratory for Laser Energetics has made it possible to fabricate a wide variety of photoaligned LC devices for high-peak-power laser applications, including wave plates, beam shapers, apodizers, and radial polarization converters.*
In the work described here, computational methods based on the density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to determine the properties of a series of oligomeric methacrylate and acrylamide photoswitchable alignment layer materials intended as potential candidates for use in an optically switchable LC laser beam shaper. Twenty-two different terminal functional groups were computationally evaluated to determine their individual effects on the energy difference between the trans and cis isomerization-state energy levels (one of the three factors affecting bistability in photoswitchable alignment layers) when they were used as substituents on azobenzene cores linked through a four-carbon tether to methacrylate and acrylamide backbones. The effect of the alkyl tether connecting the chromophore to the oligomer backbone on the isomerization-state energy differences of the methacrylate and acrylamide oligomers was also investigated computationally. This work revealed the following key findings:
- When methoxy-substituted azobenzene chromophores are tethered to a methacrylate oligomer, lower energy differences between the trans and cis isomerization states occur for alkyl tether lengths of 5, 6, 8, 9, and 11 carbons. The C6 and C11 tethers produce the smallest energy difference, implying that they are a good choice for a photoalignment coating intended for bistable switching applications. For the same core and an alkyl tether length of C4, replacing the methoxy terminal group on the azobenzene core with alkyl groups up to C9 appears to have limited ability to lower the isomerization-state energy difference, while cyanate ester and 2-methoxy-N-(2-methylphenyl) acetamide terminal groups are highly effective in producing the smallest differences in trans–cis isomerization energy levels.
- Acrylamide oligomers tethered to a methoxy-substituted azobenzene chromophore show the smallest trans–cis isomerization energy differences for alkyl tethers containing 4, 5, 7, 8, and 9 carbon atoms, in some cases considerably smaller than those of the corresponding methacrylate oligomers. Unlike what was seen for methacrylate oligomers, replacing the methoxy group on the azobenzene core with C5 and C9 terminal alkyl groups shows a significant reduction in trans–cis isomerization-state energies. With the exception of C4 and C9 terminal groups, all of the acrylamide oligomers with alkyl-substituted azobenzene cores show consistently lower trans–cis isomerization energy state differences than do their methacrylate counterparts. Other terminal functional groups that show a large decrease in trans–cis isomerization-state energy differences are the 2-methoxy-N-(2-methylphenyl) acetamide group (62%) and the fluoroalkane terminal group (70%) as compared to an unsubstituted azobenzene core.
- With only a few exceptions, acrylamide oligomers as a group exhibit lower trans–cis isomerization energy differences than methacrylate oligomers with the same structure, making them (in the absence of other factors) preferred candidates for photoswitchable device applications where good bistability is required.
Considerable work remains in developing these computational tools and methodologies into a reliable, predictive capability for photoswitchable alignment layer design. The observed odd–even effect in the trans–cis isomerization energies as a function of tether chain length seen for methoxyazobenzene-methacrylate oligomer systems must be investigated, further evaluating longer tether lengths and on different oligomeric backbones (e.g., methacrylate, acrylamide, siloxane) to determine if it is specific to one oligomer class. Both the transition-state energy and the swept volume produced by motion of the chromopore pendant (both azobenzenes and spiropyrans) will be determined by transition-state modeling (DFT) and molecular dynamics simulations using the Jaguar and Desmond components of Shrödinger, Inc.'s Materials Science Suite, respectively. Highly intensive computational modeling of systems with up to 15 or more backbone segments, along with targeted synthesis and characterization of the most-promising candidate materials from these studies, will lead to both a more-detailed understanding of these materials systems and sufficient quantities of materials for characterization studies and device development activities.
*C. Dorrer et al., Opt. Lett. 36, 4035 (2011); K. L. Marshall et al., Proc. SPIE 8114, 81140P (2011); K. L. Marshall et al., Proc. SPIE 8475, 84750U (2012); K. L. Marshall et al., Proc. SPIE 7050, 70500L (2008).