Tailoring the Photophysics of First-row Transition Metal-based Chromophores for Light Capture and Conversion: Challenges and Opportunities

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Figure 1. P lot of the ground state recovery dynamics of [Fe(bpy) 3 ](PF 6 ) 2 in CH 3 CN solution as a function of temperature following 1 A 1 ® 1 MLCT excitation . The solid line corresponds to a fit of the data to an Arrhenius mod el, indicating an activation energy of 310 ± 15 cm -1 and an intercept (i.e., the rate constant in the limit of no barrier) of 230 ± 20 ps -1 . An analysis of these data in the context of semiclassical non - radiative decay theory is providing new insights into the factors controlling ultrafast dynamics in such systems. Tailoring the Photophysics of First - row Transition Metal - based Chromophores for Light Capture and Conversion: Challenges and Opportunities James K. McCusker Department of Chemistry, Michigan State University The conversion of light to chemical energy is one of the most fundamental processes on Earth. It is the basis of photosynthesis, in which light absorption results in the separation of charge that ultimately creates the chemical potential needed to drive ATP synthesis; an advantageous by - product of th is process is, of course, O 2 production. Ironically, photosynthesis is also the source of the biomass from which the fossil fuels that constitute the basis of society’s energy infrastructure are derived. The overwhelming majority of climate scientists are in agreement that it is the burning of these fossil fuels – in effect the re - release of what was sequestered carbon into the atmosphere – that is driving global climate change. Options for shifting away from fossil fuels as our primary energy source genera lly revolve around renewables such as wind, solar, biomass, nuclear, geothermal, and hydro: of these, the only renewable energy source that is limitless and carbon - free (at least in principle) is solar. The energy flux hitting the Earth is 120,000 TW: inte grated over a 24 - hour period, this translates to humankind’s total energy budget for an entire year. Despite the progress that has been made in the implementation of solar energy (due primarily to reductions in the cost of silicon), the intermittent nature of solar energy, the balance of systems costs that continue to represent a significant economic obstacle, combined with the fact that electricity constitutes only ~30% of the global energy footprint all underscore the need for continued research in solar energy conversion science. Fundamental research on solar energy conversion – which will ultimately lead to the next generation of solar energy technologies – has sought to replicate Nature’s solution through the creation of artificial constructs that mimic va rious aspect of photosynthesis. When considering large - scale (i.e., global) implementation of any solar energy conversion scheme, material availability becomes a critically important consideration in the light - capture part of the problem, particularly w hen one considers the projected two - to three - fold increase in energy de mand over the next 30 - 40 years. Unfortunately, virtually all of the molecule - based approaches for solar energy conversion that have been proven successful rely on some of the least abu ndant elements on earth. An obvious alternative is to employ chromophores based on earth - abundant materials: for transition metal - based approaches, this means moving away from the second - and third - row transition series elements (e.g., ruthenium) and devel op photoredox - active chromophores based on first - row, widely available metals like iron and copper. As our group first demonstrated in 2000, the central problem with this approach is that the charge - transfer excited states that lie at the heart of photo - in duced electron transfer chemistry exhibit sub - picosecond lifetimes (as compared to the microsecond lifetimes of their 2 nd - and 3 rd - row congeners). Our research program therefore focuses on understanding the factors that determine the dynamics associated wi th the excited states of first - row transition metal - based chromophores, with the ultimate goal of circumventing and/or redefining their intrinsic photophysical properties in order to make feasible their use as light - harvesting components in solar energy co nversion schemes . This seminar will describe the key experimental results establishing this paradigm, as well as survey several approaches that we are pursuing in an effort to broaden the utility of this class of chromophores for a wide range of solar ener gy and chemical transformations .