lighting, molecular, organic, solid state
General illumination consumes 22% of the electricity generated in the U.S. This huge proportion is partly due to the ubiquity of artificial lighting but also the inefficiency of converting electrical energy to light. Incandescent lightbulbs convert a mere 5% of the supplied power into light (most of the rest emerging as heat) whereas the more efficient fluorescent bulbs achieve about 20% efficiency. Improving the efficiency of these light sources is difficult since in all cases the emission of light is essentially a by-product of an energetic excitation process. In contrast, solid state lighting utilizes materials which directly convert electrical energy to light with little production of heat and therefore have the potential for far higher efficiency, with over 70% demonstrated in the infrared. New materials based on direct bandgap semiconductors and organic light emitters may permit this level of efficiency for general lighting. In both cases, however, understanding the nanoscale structure of the material is critical to achieving high efficiency. This is particularly evident in the case of organic molecular compounds, where weak inter-molecular interactions can permit the photophysical properties of a solid to be tuned by changing the chemical structure of the molecular building block.