Photochromic Performance of Donor-Acceptor Stenhouse Adducts in Polymer Binders and Solution

Donor-acceptor Stenhouse adducts (DASAs) are an exciting new class of negative photochromic compounds, offering high colouration and rapid photo-switching speeds, however the relationship between binder properties and photo-switching behaviour of DASAs in films is poorly understood. The determination and analysis of rate constants of seven DASAs in five polymer binders showed a complex relationship between the glass transition temperature (T-g) of the binder, the solubility parameters of the matrix and the push-pull nature of the donor and acceptor moieties. The rate constants of photo-switching steps that require large degrees of molecular motion such as the actinic and thermal carbon-carbon double bond rotations (k(bleach) and k(-1)) were found to be highly dependent on the T-g of the polymer binder, with higher T-g binders restricting these processes, resulting in slower switching rates and impacting changes in colour intensity. Correlation of rate constants with the Hansen solubility parameter for solvents and binders revealed strong relationships between the sum of the polarity and hydrogen bonding parameters (delta P + delta H) and the actinic and thermal carbon-carbon double bond rotations (k(bleach) and k(-1)). This is attributed to changes in the zwitterionic resonance contributions of the triene with increases in these two solubility parameters.