Strain Engineering of Organic Semiconducting Molecules: A First Principles Study Feng Liu, Materials Science & Engineering, University of Utah Mechanical strain can induce structural changes on the molecular level. The impact can be counterintuitive in that the same absolute compressive strain can result in a red or blue shift of the vertical transition energies depending on the shape change. Also in going from a twisted to planar configuration in fluorenes on stretching, the elongated molecule adopts an ordered twisted conformation before planarizing on excitation. Transition Energy [eV] % Strain Excitation Uniaxial tensile and compressive strain can be induced in fluorene molecules by specific adsorption on a Si(100) substrate. The adsorption orientation is a more important factor in determining the molecular conformation than the starting geometry with consequences for the optoelectronic properties of molecular films.