Ruthenomesogens for Nonlinear Optics

Recent interest in the emerging sciences of molecular electronics and photonics has inspired many studies with transition metal complexes as the basis of new functional materials. Two especially active areas are metal-containing liquid crystals (aka metallomesogens) and nonlinear optical (NLO) properties. Liquid crystals (LCs) lacking metal atoms have long proved useful, primarily in computer monitors and other display devices. Current NLO devices use mostly inorganic crystals, but organic materials hold great promise for many purposes including optical data processing and biological imaging. While purely organic compounds have been studied most intensively, transition metal complexes are also very interesting due to greater structural diversity and extensive possibilities for combining NLO behaviour with properties such as reversible redox chemistry, magnetism, etc.

We propose to create a range of new ruthenium compounds designed to combine very large NLO responses with ferroelectric LC (FLC) properties. FLCs are attractive for NLO purposes because they contain bulk polar order and can therefore display quadratic effects such as second harmonic generation (SHG, aka frequency doubling). The ready processability of LCs into thin films makes them well suited to applications in opto-electronics. While purely organic FLCs are relatively well studied, work with ferroelectric metallomesogens is scarce and extremely few such substances that show also NLO behaviour have been reported. In addition, ruthenium compounds have been under-explored as mesogenic materials, especially thermotropic substances that show temperature-dependent phase changes.

We will prepare ruthenium complexes that are expected to form classical calamitic mesophases, and also bent-core derivatives. The optical properties of these new dyes will be amenable to reversible, redox and/or photo-induced switching. The project will involve collaborative studies with experts on the physical properties of FLCs in Bilbao and also with leading research groups in Europe and the USA. Together with facilities in Manchester, these interactions will allow us to both measure and understand by computer modeling the optical properties of our new compounds. Our work is driven primarily by scientific curiosity, but an important goal is to create new compounds and materials which may be useful in a range of optical technologies.

Keywords describing areas of proposal: Synthetic Chemistry, Transition Metal Chemistry, Nonlinear Optics, Liquid Crystals

The proposed work will generate new fundamental scientific knowledge relevant to the development of opto-electronic technologies. An increasing level of reliance on optical data processing in computing and telecommunications systems requires new materials, and NLO effects are likely to make major contributions. Furthermore, other related applications such as SHG imaging of biological systems and THz wave generation have emerged in recent years. With our collaborators, we have established a uniquely multifaceted experimental/theoretical approach to molecular NLO research, allowing us to achieve a number of notable advances. Our papers concerning the redox-induced switching of NLO responses have been especially highly cited and influential. We propose to study an unexplored class of compounds (thermotropic charge-neutral ruthenium complexes) with potential for producing FLCs that show large bulk quadratic NLO activities.

The PDRA to be employed on this project will gain extensive new experiences and skills derived from working in an inherently challenging and interdisciplinary field. Interactions with our various international collaborators will serve to further expand the PDRA's knowledge and professional contacts base. Our collaborators also benefit from opportunities to carry out measurements on various new compounds and materials.

These studies are fundamental in nature, but could afford major advances with respect to technological applications. Both metal-based NLO chromophores and metallomesogens have been investigated relatively intensively for several decades, but have yet to lead to practical applications. While we can not promise device-ready materials within a 3 year timeframe, the proven technological relevance of LCs provides a strong basis for optimism in the longer term.

Society in general has benefited immensely in terms of quality of life from new electronics technologies over recent years, including LC displays and optical data transmission and storage. In this context, compounds and materials (semiconductors, dyes, etc.) that originate from chemistry research are essential. The potential for further innovations will depend critically on the availability and exploitation of new materials, such as those we propose to investigate.