Project Summary/Abstract The long-term goal of this project is to interrogate the dynamics of membrane voltage in the context of intact brains with high spatiotemporal resolution. Optical methods to dissect neuronal activity promise to revolutionize our understanding of the brain at the cellular and circuit level; however, our understanding remains incomplete due, in part, to a lack of tools that can report directly on neuronal activity with sufficient speed and sensitivity. We propose to use the power of molecular design and organic chemistry to develop and apply new optical tools for monitoring membrane potential with unprecedented speed and sensitivity in intact brains and without disruptive capacitive load associated with other classes of voltage indicators. We plan to exploit photoinduced electron transfer (PeT) through molecular wires as a versatile platform for optical voltage sensing. We will build a palette of colors for optical voltage sensing that extends into the near infrared regions of the electromagnetic spectrum; we will create new voltage sensors with exceptionally high two-photon absorption cross sections for use in thick tissue and intact brains; and we will explore methods for genetically targeting and localizing ultra- sensitive fluorescent voltage sensors to neurons of interest. Throughout, development of molecular tools will be closely wed to applications in neurons and tissues, and we will apply these tools to understand how membrane potential dynamics change in both healthy and neurological disease states.