Visualizing The Origin Of Rotational Entropy Effects In Coadsorbed Systems

We demonstrate that significant entropic effects govern the stability of rotationally constrained molecular complexes on surfaces. Using a complementary combination of low-temperature scanning tunneling microscopy, temperature programmed desorption, and density functional theory calculations, we probe the energetics of surface adsorbed molecular rotors as well as their corresponding rotationally hindered complexes. The results reveal that absorbate-absorbate interactions can significantly decrease the rotational entropy associated with such complexes leading to lower than expected desorption temperatures.