Unique determination of the –CN group tilt angle in Langmuir monolayers using sum-frequency polarization null angle and phase

The relative phase and amplitude ratio between the ssp and ppp polarization combinations of the vibrational sum-frequency generation (SFG) response can be uniquely and accurately determined by the polarization null angle (PNA) method for orientational analysis. With PNA measurement we show that the –CN group in the 4-n-pentyl-4′-cyanoterphenyl (5CT) Langmuir monolayer is tilted around 25° ± 2° from the interface normal, while that in the 4-n-octyl-4′-cyanobiphenyl (8CB) is tilted around 57° ± 2°, consistent with the significant differences in the phase diagrams and hydrogen bonding SFG spectra of the two Langmuir monolayers reported in the literature.