Global phase angle description of generalized two-dimensional correlation spectroscopy: 2. Its application to temperature-dependent infrared spectra of a Langmuir-Blodgett film of 2-dodecyl-7,7,8,8-tetracyanoquinodimethane

A spectral analysis based on the concept of a global phase map derived from generalized two-dimensional (2D) synchronous and asynchronous correlation spectra has been applied to actual experimental data to demonstrate its potential. As an application of a global 2D phase map, temperature-dependent infrared transmission spectra from 30 to 80°C of an eleven-layer Langmuir-Blodgett (LB) film of 2-dodecyl-7,7,8,8-tetracyanoquinodimethane (dodecyl-TCNQ) have been investigated. Although the generalized 2D correlation asynchronous map presents only a very weak signal due to the CH₃ asymmetric stretching vibration compared with much stronger signals due to the CH₂ stretching vibrations of dodecyl-TCNQ, the global 2D phase map clearly shows the distinct nonlinear response arising from the CH₃ asymmetric stretching vibration. The detection of nonlinear responses suggests that the active thermal movement of the alkyl-chain loses a trans-zigzag skeleton plane in the intermediate state above 80°C between the solid and melt states.