Optical response sensitive to the assembly in a molecular material: ultrathin film with a vanishing electronic absorption.

Optical properties of single-component molecular materials can often be described in terms of modest perturbations on the responses of the molecular building blocks; in specific systems, the modifications may be more pronounced. In all cases, however, the effects are broadly independent of the form of assembly, such as thin films, nanostructures and crystals, even though variations may exist at the quantitative level. We present a very different situation encountered with an amphiphilic molecule possessing a strongly polar head group that shows similar electronic absorption spectra in the solution as well as the bulk solid state, but a dramatically different one in ultrathin Langmuir-Blodgett (LB) films. The lowest energy absorption is switched off completely in the latter. It is demonstrated further that a metastable form of the LB film exhibiting the normal molecular spectrum can be generated, although its lowest energy absorption decays and vanishes over a period of time. The Langmuir films formed at the air/water interface and the corresponding LB films are examined through pressure-area isotherms, microscopy and spectroscopy. The experimental observations coupled with models constructed through computational investigations allowed us to interpret the unusual optical responses, which are a result of the specific supramolecular 2D assembly in the LB film and the impact of the neighboring molecular dipoles on the intramolecular charge transfer transition. The present study illustrates that selective molecular design and directed assembly can be exploited to elicit dramatic changes in physical responses of molecular materials and realize novel effects in molecular nanostructures.