Polarity of Liquid Interfaces by Second Harmonic Generation Spectroscopy

A spectroscopic method, based on the interface selectivity of second-harmonic generation, is used to obtain the polarity of liquid interfaces. In this paper the second-harmonic measurement of the spectrum of the polarity indicator molecule N,N‘-diethyl-p-nitroaniline (DEPNA) at the air/water interface demonstrates the method. Two different approaches are used to measure the intramolecular charge transfer (CT) absorption band position of DEPNA at the air/water interface. The DEPNA CT band blue-shifts from 429 nm in bulk water (polar solvent) to 359 nm in bulk hexane (nonpolar solvent) and 329 nm in the gas phase (no solvent). At the air/water interface, the charge transfer peak band maximum occurs at 373 nm, which indicates that the polarity of the air/water interface is similar to those of the bulk solvents carbon tetrachloride and butyl ether. The DEPNA results together with the results from another solvatochromic polarity indicator molecule, ET(30), which will be reported elsewhere, show that the polarity results of the air/water interface are general.