Photochemistry and Light Energy Conversion
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conference program exemplified the evolution of photochemistry and its expanding role in material science, biology, and light energy conversion. The Keynote lectures and parallel sessions covered both fundamental aspects as well as emerging areas such as plasmonics, solar cells, solar fuels, ultrafast spectroscopy, biophotonics, sensing, and imaging. The breadth of these topics was a stark reminder of the influence that photochemistry continues to play in shaping modern day energy research. Although the major research focus during 1960−1980 was on molecular systems and establishing mechanistic insights into photoinduced processes such as electron transfer, isomerization, charge transfer complexes, exciplexes, and so forth, these activities further led to the exploration of more complex systems involving polymers, micellar systems, solid surfaces, and so forth. In the 1980s, nanomaterials (e.g., semiconductor quantum dots) came into play. The spectroscopic tools once championed to probe the excited state behavior of molecular systems were quickly embraced to explore the photophysical and photochemical behavior of colloidal semiconductors (or semiconductor QDs) and metal nanoparticles (Figure 1).
ight-induced chemical transformations have been in the forefront of scientific research since the beginning of the last century. Fascination for colors was one of the initial driving forces for synthesizing new dyes and fluorescent materials. The early work in the previous century focused on establishing the basic principles of photochemistry, molecular design, synthesis of photoresponsive molecules, theoretical understanding of excited state processes, and development of spectroscopic tools to probe photoinduced processes. Many of these discoveries in photochemistry and related areas have been recognized through the awarding of several Nobel prizes. A few prominent Nobel Laureates whose work has influenced modern day light energy conversion research efforts include Albert Einstein (1921, photoelectric effect), C. V. Raman (1930, Raman spectroscopy), Melvin Calvin (1961, photosynthesis), George Porter and Ronald GW Norrish (1967, transient absorption spectroscopy), Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen (1987, molecules with structurespecific interactions), Rudy Marcus (1992, electron transfer reactions), Ahmed Zewail (1999, femtosecond spectroscopy), Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa (2000, conducting polymers), Eric Betzig, Stefan W. Hell, and William E. Moerner (2014, fluorescence microscopy), and JeanPierre Sauvage, J. Fraser Stoddart, and Bernard L. Feringa (2017, molecular machines). Initially, natural photosynthesis and silver halide photography motivated scientists to explore new areas of photochemical research. During the first oil crisis of the 1970s, scientists initiated exploration of photochemical pathways to convert light energy into chemical energy and electrical energy. For example, the principle of dye sensitization, which was deeply rooted in silver halide photography, led to the explosion of dyesensitized solar cell (DSSC) research. Other major applications of photochemistry were realized in the areas of energy upconversion, organic photovoltaics, organic light-emitting diodes (OLED), display devices, photodynamic therapy (PDT), and fluorescence imaging. The conversations with leading scientists Art Nozik (http://pubs.acs.org/doi/10.1021/ acsenergylett.6b00273), Tom Meyer (http://pubs.acs.org/doi/ 10.1021/acsenergylett.6b00482), C. N. R. Rao (http://pubs. acs.org/doi/abs/10.1021/acsenergylett.6b00503), Akira Fujishima (http://pubs.acs.org/doi/10.1021/acsenergylett.7b00483), Michael Grät zel (http://pubs.acs.org/doi/abs/10.1021/ acsenergylett.7b00523), and Allen Bard (http://pubs.acs.org/ doi/10.1021/acsenergylett.7b00566) published in our previous issues provide a chronology of scientific advances in light energy conversion since 1970. Recently, Professor Lin Chen (Senior Editor) and I had the opportunity to attend the 21st International Conference on Photochemistry (ICP) in Strasbourg, France. This meeting, which is held biannually, brings together researchers from around the world to discuss recent advances. The opening lecture, “From Charge Separation to Light-Driven Molecular Machines”, was delivered by Prof. Jean-Pierre Sauvage, 2016 Nobel Laureate from the University of Strasbourg. The © 2017 American Chemical Society
Figure 1. Comparison of the excited state processes of molecular systems and semiconductor QDs. Time-resolved transient absorption and emission spectroscopies are often used to characterize radiative (kr) and nonradiative (knr) decay pathways.
Today, light energy conversion is pursued through the design of nanostructured assemblies with semiconductor, metal, and molecular architectures. Implementation of these light energy harvesters in solar cells and solar fuel-generating devices requires a better understanding of photoinduced processes. The principles of photochemistry that include excited state interactions, singlet fission, charge separation, energy and electron transfer processes, thermodynamic considerations for achieving charge transfer, and surface chemistry remain the key for establishing the feasibility of newly designed light conversion systems. As we design more complex light harvesting systems (e.g., tandem structures, Z-scheme, etc.), one needs to take into account various interparticle, intermolecular, and interfacial interactions that may come into play. Despite the availability of extensive knowledge of photochemistry fundamentals, we often see poorly executed Published: September 8, 2017 2157
DOI: 10.1021/acsenergylett.7b00746 ACS Energy Lett. 2017, 2, 2157−2158
Editorial
http://pubs.acs.org/journal/aelccp
ACS Energy Letters
Editorial
research that ignores the accuracy of data analysis and interpretation of photochemical measurements (e.g., reporting quantum efficiencies or kinetic parameters). Researchers who are new to the field should make an effort to go through the early research papers and familiarize themselves with the basic concepts of photochemistry. The editors of ACS Energy Letters put great emphasis on the accuracy of the methodology and careful analysis of the results when evaluating papers related to solar cells and solar fuels. Natural photosynthesis still remains a great motivator to design light harvesting assemblies for next-generation artificial photosynthetic devices. With a well-thought-out research strategy and adoption of basic principles of photochemistry, we can tackle the challenges of light energy conversion.
Prashant V. Kamat, Editor-in-Chief, ACS Energy Letters
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University of Notre Dame, Notre Dame, Indiana 46556, United States
AUTHOR INFORMATION
ORCID
Prashant V. Kamat: 0000-0002-2465-6819 Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
2158
DOI: 10.1021/acsenergylett.7b00746 ACS Energy Lett. 2017, 2, 2157−2158
