The main goal of this proposal is the construction of photorheological fluids based on supramolecular host-guest assisted photochemical reactions. Photorheological fluids are those whose rheological properties, such as viscosity or viscoelasticity, can be controlled by light.[1] Most photorheological fluids described in the literature rely on photoisomerization reactions.[2-4] Yet, less exploited photodimerization reactions have a large potential in photorheology since they involve formation of new covalent bonds allowing chain cross-linking leading to extended covalent networks. Photodimerization reactions in solution pose nevertheless several problems, such as the need for higher concentrations of photoreactive units and the formation of several cyclization isomers.[5] Using supramolecular receptors able to pre-orient the photoreactive molecules in a configuration that is conducive for photoreactions may help overcome these limitations and develop photorheological fluids operative at low concentrations of photoreactive units and with high photoregioselectivity. The photochemistry of dimerization reactions inside supramolecular receptors such as cyclodextrins,[6] cucurbiturils [7-9] or cavitands is rather well characterized [9] but its exploitation in photorheology is at its early stages [8] and embodies the essential of this proposal.

The photoreactive units, the structural vectors where they can be conveyed and adequate supramolecular receptors were identified and synthetic strategies outlined. Based on the acquired know-how of the team on the synthesis of photoactive polymers [10] and ionic liquids (ILs),[11-13] these systems were selected as matrices to incorporate the photodimerizable units. On the other hand, the team has expertise in the synthesis of several photodimerizable compounds, such as coumarins, [12-16] anthracenes,[17] cynnamic acids [12,17] and chalcones.[18,19] Cucurbit[8]uril and ?-cyclodextrin were selected as receptors for their known ability to form host-guest homoternary complexes with coumarins and anthracenes.[6-9,20]

To assess changes in the rheological properties (co-PI?s expertise) upon irradiation, a combination of three techniques was set up: steady-state and dynamic shear flow rheological measurements,[12,21] evaluation of phase transitions by DSC and diffusion coefficients by DRS;[22] and Hyper-Rayleigh Scattering spectroscopy to infer about the aggregation state of the system.[23-25]

The project develops around 5 tasks, the first two involving the syntheses of the polymer and IL-based systems; the third consisting in the physico-chemical characterization (thermodynamics and kinetics of the association; photochemistry); the fourth dealing with the study of the light-dependent rheological properties and the fifth concerning a case-specific application towards the use of the IL-based host-guest assemblies into photoresponsive liquid crystals, in collaboration with an expert in this field.[26]