UC Davis

Considerable research into organic small molecules (OSM) have taken place due to its cheap large scale fabrication cost compared to inorganic semiconductors, tunable optical and electronic properties, and its potential to be designed flexible, transparent, and light weight. OSM are a class of organic semiconductors that form extended crystals. These crystals are held together by weak Van der Waals forces which allow large amplitude inter- and intra- molecular motions. The motions cause large electronic coupling fluctuations, which results in unfavorable electronic properties. With the infinite design space afforded to OSM, it is difficult to make a distinction between molecular design and detrimental molecular motions. Thus, it is important to have full knowledge of the phonon spectrum to design molecules with favorable electronic properties. Inelastic neutron scattering (INS) is a spectroscopy technique well-suited to study the dynamics of OSM. Here, we use INS in conjunction with density functional theory to highlight the phonon modes that are detrimental to the electronic coupling. Design principles are considered to reduce the infinite design space that dampens those detrimental phonons. We then expand the capabilities of INS by designing and building a sample changer stick, and two photochemistry sample sticks. The sample changer stick can simultaneously cool four samples, and measure each individually. The first photochemistry sample stick uses an LED above the sample container within the closed-cycle refrigerator on the VISION neutron spectrometer at Oak Ridge National Laboratory. The second incorporates a laser and tumbler sample mixing system to increase the density of photoexcited sample. We show simple test cases using an acene and 405 nm photoresin for the two designs. Lastly, an experimental and computational study of an multiexciton generation mechanism, singlet fission (SF), in tetracene and pentacene are studied with varying excitation energies. The findings suggest the electronic states shifts due to the excimer deformations leading to energetically favorable SF.