Experimental probes of excited state character in supercritical fluids

Solute-solute and solute-solvent clustering of N,Ndimethylaminobenzonitrile and ethyl N,N-dimethylaminobenzoate in supercritical fluoroform, supercritical carbon dioxide, and supercritical fluoroform-carbon dioxide mixtures are quantitatively evaluated from the dependence of the relative contributions of the locally excited (LE) state and the twisted intramolecular charge transfer (TICT) emissions on supercritical fluid composition and pressure. Microscopic and macroscopic solvation in these mixtures can be rationalized with an Onsager reaction field model. Photophysical measurements on the singlet excited state of tetraphenylethylene (1p* state) in supercritical fluid mixtures similarly indicate high local polarity with inhomogeneous clustering about a dipolar excited state. Fluorescence lifetimes of 9-cyanoanthracene in the low and nearcritical density regions of a supercritical fluid show that the relationship between the emissive radiative rate constant and the solvent refractive index in supercritical fluid mixtures differs appreciably from that observed in liquid solvents.