UC Berkeley

The movement of excitons is fundamental to many light harvesting systems. This dissertation describes the development of a new technique to study the migration of excitons in complex photoactive materials on the relevant spatial and temporal scales. The spatial heterogeneity of many light harvesting systems and the movement of excitons both occur on spatial scales that are difficult to measure due to the optical diffraction limit. This limit places a lower bound of approximately 200 nm on the spatial resolution of measurements with visible light. Exciton migration lengths tend to be approximately 5-20 nm and the structural heterogeneity of the relevant materials can be on the order of 10s of nm. Stimulated emission depletion microscopy (STED) circumvents the optical diffraction limit by trimming exciton populations via stimulated emission depletion. Herein is described a temporally resolved adaptation of STED for the purpose of studying exciton migration dynamics in complex photoactive systems. In a basic iteration of STED, an initial diffraction limited excitation spot is created with a laser pulse tuned to the absorption of the material. A second laser pulse, tuned to the emission spectrum of the material, is used to quench excitons and narrow the exciton distribution which can result in exciton distributions far smaller the diffraction limit. STED usually uses robust, small molecule chromophores as contrast agents. STED imaging of a material with dense and morphologically complex packing of chromophores presents a unique set of challenges. Those challenges are compacted when adding temporal resolution with a second STED pulse.

STED was used to image nanoparticles of an organic conjugated polymer semiconductor, poly(2,5-di(hexyloxy)cyanoterephthalylidene) (CN-PPV). The most significant challenge in imaging CN-PPV with STED was the relatively strong two photon absorption (2PA) cross section which had the potential to interfere with STED-based measurements. After careful tuning of the spectral and temporal characteristics of the STED pulse to minimize 2PA and maximize stimulated emission depletion, a pulse modulation scheme was used to subtract the remaining fluorescence signal from STED micrographs of CN-PPV nanoparticles. The results were STED images of CN-PPV nanoparticles with resolution of less than 90 nm.

The measurement of exciton migration via STED required temporal resolution. To achieve temporal resolution in STED-based measurements, a second STED pulse was added to the experiment. The second STED pulse was used to probe the expansion of the exciton distribution created by the excitation pulse and the first STED pulse. This new technique, time resolved ultrafast stimulated emission depletion (TRUSTED) and was used to measure an exciton migration length of 16 ± 2 nm in a thin film of CN-PPV.

Preliminary work into STED imaging of biological photosynthetic membranes from Spinacia oleracea is presented in the last chapter. The photophysical properties of these samples were far less amenable to STED then those of CN-PPV. The parameters of the STED pulse were iterated to find those best suited to depleting excitons via stimulated emission. The STED pulse parameters that most effectively quenched excitons were used to image thylakoid membrane samples from the grana of Spinacia oleracea which resulted in observed improvement in STED image resolution when compared to traditional epifluorescence microscopy. The irregular form of these membranes made definitive resolution claims untenable but a summary of the STED pulse parameters that were tried is provided to inform those that wish to continue this research. The development of TRUSTED gives researchers a new tool to study exciton migration and has the potential to correlate exciton migration dynamics to local structure.