UC Riverside

A mechanophore is a molecule which displays sensitivity toward some applied force. Mechanophores show promise for applications in smart materials and nanomachines. Identifying structural motifs in photoactive molecules which enhance pressure sensitivity has been one focus of this research. It was demonstrated that 9-tert-butylanthracene dissolved in Zeonex polymer shows increased rates of back reaction upon applying mild pressures (< 1.5 GPa) in a diamond anvil cell. While strained rings can enhance pressure sensitivity, it is not the most effective handle for tuning mechanophoric properties. This work highlighted that large structural changes in the photoproduct are also required to bring about the desired pressure sensitivity.

Intermolecular interactions cause assemblies of conjugated chromophores to behave distinctly different from the monomer. Delocalized excited states which lead to novel photophysics are examined in systems of varying complexity from covalently linked chromophores to molecular crystals. In covalent dimers of simple organic molecules like terthiophene and anthracene, a sulfur atom is used to link the chromophores, and the oxidation state of the sulfur is found to modulate the electronic coupling leading to photophysics and photochemistry which are controlled by the electronic structure of the linker. An added advantage of using the sulfur linker is that the geometry between molecules remains constant regardless of the oxidation state, allowing purely electronic effects to be isolated, demonstrating that chemical modification of a single atom can dramatically alter the excited state behavior of a molecular assembly. In molecular crystals, delocalized excited states can lead to technologically important multiexciton processes such as fission and fusion. In crystalline rubrene NIR-to-visible upconversion is observed without the use of extrinsic sensitizers, a process which may be facilitated by low energy intermolecular states. The mechanism of singlet fission in crystalline tetracene is also explored particularly regarding the correlated triplet pair state. Evidence of this spin superposition state is manifest by temporal oscillations in the photoluminescence decay of tetracene. Based on the temperature dependence, the triplets diffuse independently throughout the crystal while maintaining spin coherence. These results could have important implications for strategies which seek to use triplet states to enhance device efficiencies.