Interactions between Spiropyrans and Room-Temperature Ionic Liquids: Photochromism and Solvatochromism

A series of imidazolium-based room-temperature ionic liquids (RTILs) containing anions from organic carboxylic acids were prepared. A set of dye probes, including Reichardt’s dye (30), 4-nitrioaniline, and N,N-diethyl-4-nitroaniline, were used to determine the E T(30) scales and the Kamlet−Taft parameters (π*, α, and β) of the RTILs. On the basis of the polarity properties, these RTILs were categorized into three groups: group A with β > 0.9, α < 0.9; group B with β < 0.9, α < 0.9; and group C with β < 0.9, α > 0.9. Interactions of these RTILs with four photochromic spiropyran derivatives (SP-I, SP-II, SP-III, and SP-IV) were investigated. It was found that the spiropyrans could present photochromism (positive or negative) or not, depending mainly on the polarity properties of the RTILs and also on the structure itself. A new spectroscopic method based on the molecular transition energy of the spiropyran probes (E SP) was proposed to determine the polarity of those protic or fluorine-containing RTILs, which were failed with the Reichardt’s dye (30) probe.