UC Riverside

Anthracene derivatives can undergo photochemical reactions under light irradiation, like a [4+4] photodimerization and a Dewar isomerization. When these molecules are made into micro- and nano-sized crystals, some of them exhibit interesting photomechanical motions and deformations like bending, twisting, curling, and expanding. Both the internal and external environmental effects were found to significantly influence the photomechanical behaviors and photochemical reactions of these molecules in the solid-state. One goal of this research is to better understand the relationship between dynamics of chemical reactions and crystal photomechanical behavior they produce, and to see if these reaction rates can be enhanced by internal or external factors.

9-tert-butylanthracene (9TBA) undergoes a Dewar isomerization in solution and in polymers, but produced no photomechanical response in the crystal, even though it generates a significant volume change and a highly strained molecular structure after photoisomerization. But when this molecule is embedded in a polymer, a strong pressure dependence of the photoisomerization was observed in both forward and reverse reactions, providing us a possible way to engineer more pressure sensitive molecules in the future.

We investigated the photodimerization process of 9-methylanthracene (9MA) in polycrystalline thin films. It was found that both the photochemical reaction and nonradiative relaxation rates increase as more photoproduct is formed, which we denoted as an autocatalytic pathway. The local environment of a 9MA molecule changes as the surrounding molecules react. We derived rigorous equations and extended a current model (the Finke-Watzky Model) to describe the non-exponential reactant decay, and we were able to predict its time-dependent photoluminescence evolution over the course of the reaction, which showed agreement with the experimental results. The presence of autocatalytic reaction dynamics in a molecular crystal system may play a role in its photomechanical response. This new chemical reaction pathway is different from the single exponential process seen in photoresponsive soft polymer systems.

Besides internal factors, like intra- and intermolecular interactions, we also studied the external environmental effects on molecular crystals. Ionic surfactants like cetyltrimethylammonium bromide (CTAB) was discovered to dramatically promote both the photomechanical motion and the photochemical reaction rates of dimethyl-2-(3-anthracen-9-yl)allylidene)malonate (DMAAM) nanocrystals. These results may help illustrate how surface effects can enhance crystal reactivity. Future work will involve incorporating the photochemical processes with photomechanical behaviors and exploring more interesting mechanical motions with various shapes and sizes of molecular crystals.