In the 21st century, the world faces more serious problems with energy, the environment, food, and disease than ever before. To come up with these fundamental issues, light could be one of the ideal solutions because it is one of nature’s mechanisms for carrying energy and information from one place to another with wavelength and frequency. Understanding how that light is efficiently absorbed and transformed into more useful forms of energy would give insight into how we can design photoactive materials to harness and control light more effectively. Along with that, my group has researched the molecular design strategy for efficient light harvesting toward organic solar cells and photodynamic therapy. To mimic natural photosystems, my group successfully developed vectorial electron transfer in a single sensitizer by integrating strong and relatively weak electronic couplings. Our optimization strategy of electronic coupling for inducing vectorial electron transfer could be potentially generalizable and improve the photovoltaic performance of DSCs. Next, I will present a molecular design strategy for an efficient generation of reactive oxygen species (ROS) for the applications of photodynamic therapy. The controlling and understanding of ROS give insight into the clue of cell death mechanism or cell recovery by ROS, which will be helpful to overcome chronic or pathogenic diseases. For this aim, our group has investigated the Ir(III) complex & organic photocatalyst-based photosensitizers for the generation of ROS, the understanding of the dysfunctions of proteins by ROS, and their cell death mechanism for real clinical applications toward cancer