Solid-state Photochemical and Photomechanical Studies of Nanostructures and Microstructures of Anthracene Derivatives
ABSTRACT OF THE DISSERTATION
Solid State Photochemical and Photomechanical Studies of Nanostructures and Microstructures of Anthracene Derivatives
Doctor of Philosophy, Graduate Program in Chemistry
University of California, Riverside, December 2011
Dr. Christopher Bardeen, Chairperson
Photoactuators are devices made of photoreactive materials, which undergo shape changes to generate mechanical motions upon irradiation. Research on photoreactive materials has attracted much attention; this dissertation mainly focuses on photoreactive molecular crystals, which provide us great freedom to modify photoresponse through organic derivatization.
Various anthracene derivatives were synthesized and fabricated into nano- and micro-scale structures; multiple measurements, including Atomic Force Microscopy, solid-state Nuclear Magnetic Resonance, X-ray diffraction, etcetera, were applied with three goals: (1) maximize the photoresponse; (2) develop new modes of photoresponse; (3) enhance the reversibility of the photoresponse.
To maximize the photoresponse, we engineered the crystal structures and packing motifs of anthracene esters with diverse-sized substituents. The largest expansion of nanorods we obtained was 25% by length. We also tried to establish the relationship between the macroscopic photoresponse and the molecular-level structural changes, so that the photoresponse of molecular crystals may be predicted based on their crystal structures. A detailed study of the photochemistry of 9-tertbutyl-anthracene-carboxylic-acid-ester (9TBAE) shows that the solid-state photoproduct is a metastable intermediate, which slowly converts into another stable equilibrium form. This is a general phenomenon among anthracene esters. Thus, the metastable structure is the key for predicting the macroscopic photoresponse. Further studies are required to obtain the crystal structure of this crucial intermediate.
Our proposal to develop new modes of photoresponse is through morphology control. Using the floating drop method, we managed to grow 9-anthracene carboxylic acid (9AC) into single-crystalline microribbons, which generate reversible photo-induced twisting motions under uniform irradiation. The interfacial strain between the unreacted monomer and the photodimer regions within the ribbon is the driving force for the twisting.
To enhance the reversibility of the photoresponse, we tried to use the steric effects to accelerate the dissociation of the photodimer. However, only one out of six derivatives of 9AC, 10-flouro-9-anthracenecarboxylic-acid, shows the reversible photoreactivity yet with much longer response time. Attempts to self-consistently rationalize observed trends in terms of excited state lifetimes or steric effects were only partly successful. Balancing factors like electronic relaxation, steric interactions, and crystal packing presents a challenge for engineering photoactive solid-state materials based on molecular crystals.