Photochemical action plots

The study of biological responses to specific wavelengths dates back to the late 19th century. Research primarily focused on assessing photodamage from solar radiation using broad-band lamps and narrow filters. These studies quantified effects such as cell viability,{{Cite journal |last1=Neuman |first1=Keir C. |last2=Chadd |first2=Edmund H. |last3=Liou |first3=Grace F. |last4=Bergman |first4=Keren |last5=Block |first5=Steven M. |date=November 1999 |title=Characterization of Photodamage to Escherichia coli in Optical Traps |journal=Biophysical Journal |language=en |volume=77 |issue=5 |pages=2856–2863 |doi=10.1016/S0006-3495(99)77117-1 |pmc=1300557 |pmid=10545383|bibcode=1999BpJ....77.2856N }} production of erythema,{{Cite journal |last1=Schmalwieser |first1=Alois W. |last2=Wallisch |first2=Silvia |last3=Diffey |first3=Brian |date=December 2012 |title=A library of action spectra for erythema and pigmentation |url=https://link.springer.com/10.1039/c1pp05271c |journal=Photochemical & Photobiological Sciences |language=en |volume=11 |issue=2 |pages=251–268 |doi=10.1039/c1pp05271c |pmid=22194032 |s2cid=205797837 |issn=1474-905X}} vitamin D3 degradation,{{Cite journal |last1=MacLaughlin |first1=J. A. |last2=Anderson |first2=R. R. |last3=Holick |first3=M. F. |date=1982-05-28 |title=Spectral Character of Sunlight Modulates Photosynthesis of Previtamin D 3 and Its Photoisomers in Human Skin |url=https://www.science.org/doi/10.1126/science.6281884 |journal=Science |language=en |volume=216 |issue=4549 |pages=1001–1003 |doi=10.1126/science.6281884 |pmid=6281884 |issn=0036-8075}}{{Cite journal |last1=Norval |first1=Mary |last2=Björn |first2=Lars Olof |last3=de Gruijl |first3=Frank R. |date=January 2010 |title=Is the action spectrum for the UV-induced production of previtamin D3 in human skin correct? |url=https://link.springer.com/10.1039/b9pp00012g |journal=Photochemical & Photobiological Sciences |language=en |volume=9 |issue=1 |pages=11–17 |doi=10.1039/b9pp00012g |pmid=20062839 |issn=1474-905X}} DNA changes,{{Cite journal |last1=Setlow |first1=Richard B. |last2=Setlow |first2=Jane K. |date=June 1972 |title=Effects of Radiation on Polynucleotides |url=https://www.annualreviews.org/doi/10.1146/annurev.bb.01.060172.001453 |journal=Annual Review of Biophysics and Bioengineering |language=en |volume=1 |issue=1 |pages=293–346 |doi=10.1146/annurev.bb.01.060172.001453 |pmid=4567755 |issn=0084-6589}}{{Cite journal |last1=Freeman |first1=S E |last2=Hacham |first2=H |last3=Gange |first3=R W |last4=Maytum |first4=D J |last5=Sutherland |first5=J C |last6=Sutherland |first6=B M |date=July 1989 |title=Wavelength dependence of pyrimidine dimer formation in DNA of human skin irradiated in situ with ultraviolet light. |journal=Proceedings of the National Academy of Sciences |language=en |volume=86 |issue=14 |pages=5605–5609 |doi=10.1073/pnas.86.14.5605 |issn=0027-8424 |pmc=297671 |pmid=2748607 |bibcode=1989PNAS...86.5605F |doi-access=free }} and skin cancer appearance.{{Cite journal |last1=de Gruijl |first1=F. R. |last2=Sterenborg |first2=H. J. |last3=Forbes |first3=P. D. |last4=Davies |first4=R. E. |last5=Cole |first5=C. |last6=Kelfkens |first6=G. |last7=van Weelden |first7=H. |last8=Slaper |first8=H. |last9=van der Leun |first9=J. C. |date=1993-01-01 |title=Wavelength dependence of skin cancer induction by ultraviolet irradiation of albino hairless mice |url=https://aacrjournals.org/cancerres/article/53/1/53/498742/Wavelength-Dependence-of-Skin-Cancer-Induction-by |journal=Cancer Research |volume=53 |issue=1 |pages=53–60 |issn=0008-5472 |pmid=8416751 |access-date=2023-12-16}} The first biological action spectrum was recorded by Engelmann, who used a prism to produce different colors of light and then illuminated cladophora in a bacteria suspension. He discovered the effects of different light wavelengths on photosynthesis, marking the first recorded action spectrum of photosynthesis.{{Cite book |last=Mcgraw-Hill |first=Tata |url=https://books.google.com/books?id=yrTt7ufgdtYC&pg=PA311 |title=Question Bank In Biology For Class Xi |publisher=McGraw-Hill Education (India) Pvt Limited |isbn=978-0-07-026383-3 |language=en}}

Critical evaluations of active wavelength regions in these studies helped identify contributing chromophores to processes such as photosynthesis. These chromophores are key for converting solar energy into chemical energy, with their absorption closely matching the rate of photosynthesis, usually determined by oxygen production or carbon fixation.{{Cite journal |date=1836-12-31 |title=XIII. On the action of light upon plants, and of plants upon the atmosphere |url=https://royalsocietypublishing.org/doi/10.1098/rstl.1836.0015 |journal=Philosophical Transactions of the Royal Society of London |language=en |volume=126 |pages=149–175 |doi=10.1098/rstl.1836.0015 |s2cid=186209183 |issn=0261-0523}} This correlation led to the discovery of chlorophyll as a key chromophore in plant growth. Such studies have also been instrumental in identifying DNA as the core genetic material,{{Cite journal |last=Gates |first=Frederick L. |date=1930-09-20 |title=A Study of the Bactericidal Action of Ultra Violet Light |url=https://rupress.org/jgp/article/14/1/31/11237/A-STUDY-OF-THE-BACTERICIDAL-ACTION-OF-ULTRA-VIOLET |journal=Journal of General Physiology |language=en |volume=14 |issue=1 |pages=31–42 |doi=10.1085/jgp.14.1.31 |pmid=19872573 |pmc=2141090 |issn=1540-7748}} key wavelengths leading to skin cancer,{{Cite journal |last1=Setlow |first1=R B |last2=Grist |first2=E |last3=Thompson |first3=K |last4=Woodhead |first4=A D |date=1993-07-15 |title=Wavelengths effective in induction of malignant melanoma. |journal=Proceedings of the National Academy of Sciences |language=en |volume=90 |issue=14 |pages=6666–6670 |doi=10.1073/pnas.90.14.6666 |issn=0027-8424 |pmc=46993 |pmid=8341684 |bibcode=1993PNAS...90.6666S |doi-access=free }} the transparent optical window of biological tissue,{{Cite journal |last1=Anderson |first1=R. Rox |last2=Parrish |first2=John A. |date=July 1981 |title=The Optics of Human Skin |journal=Journal of Investigative Dermatology |language=en |volume=77 |issue=1 |pages=13–19 |doi=10.1111/1523-1747.ep12479191|pmid=7252245 |doi-access=free }} and the influence of color on circadian rhythms.{{Cite journal |last1=Brainard |first1=George C. |last2=Hanifin |first2=John P. |last3=Greeson |first3=Jeffrey M. |last4=Byrne |first4=Brenda |last5=Glickman |first5=Gena |last6=Gerner |first6=Edward |last7=Rollag |first7=Mark D. |date=2001-08-15 |title=Action Spectrum for Melatonin Regulation in Humans: Evidence for a Novel Circadian Photoreceptor |journal=The Journal of Neuroscience |language=en |volume=21 |issue=16 |pages=6405–6412 |doi=10.1523/JNEUROSCI.21-16-06405.2001 |issn=0270-6474 |pmc=6763155 |pmid=11487664}}

In the late 20th century, action spectra became essential in developing optical devices for photocatalysis{{Cite journal |last1=Melchionna |first1=Michele |last2=Fornasiero |first2=Paolo |date=2020-05-15 |title=Updates on the Roadmap for Photocatalysis |journal=ACS Catalysis |language=en |volume=10 |issue=10 |pages=5493–5501 |doi=10.1021/acscatal.0c01204 |issn=2155-5435|doi-access=free |hdl=11368/2979800 |hdl-access=free }} and photovoltaics,{{Cite journal |last1=Nayak |first1=Pabitra K. |last2=Mahesh |first2=Suhas |last3=Snaith |first3=Henry J. |last4=Cahen |first4=David |date=2019-03-28 |title=Photovoltaic solar cell technologies: analysing the state of the art |url=https://www.nature.com/articles/s41578-019-0097-0 |journal=Nature Reviews Materials |language=en |volume=4 |issue=4 |pages=269–285 |doi=10.1038/s41578-019-0097-0 |bibcode=2019NatRM...4..269N |s2cid=141233525 |issn=2058-8437}} particularly in measuring photocurrent efficiency at various wavelengths. These studies have been vital in understanding primary contributors to photocurrent generation,{{Cite journal |last1=Pettersson |first1=Leif A. A. |last2=Roman |first2=Lucimara S. |last3=Inganäs |first3=Olle |date=1999-07-01 |title=Modeling photocurrent action spectra of photovoltaic devices based on organic thin films |url=https://pubs.aip.org/jap/article/86/1/487/489720/Modeling-photocurrent-action-spectra-of |journal=Journal of Applied Physics |language=en |volume=86 |issue=1 |pages=487–496 |doi=10.1063/1.370757 |bibcode=1999JAP....86..487P |issn=0021-8979}}{{Cite journal |last1=Terao |first1=Yuhki |last2=Sasabe |first2=Hiroyuki |last3=Adachi |first3=Chihaya |date=2007-03-05 |title=Correlation of hole mobility, exciton diffusion length, and solar cell characteristics in phthalocyanine/fullerene organic solar cells |url=https://pubs.aip.org/apl/article/90/10/103515/333034/Correlation-of-hole-mobility-exciton-diffusion |journal=Applied Physics Letters |language=en |volume=90 |issue=10 |doi=10.1063/1.2711525 |bibcode=2007ApPhL..90j3515T |issn=0003-6951}} leading to advancements in materials,{{Cite journal |last1=Cushing |first1=Scott K. |last2=Li |first2=Jiangtian |last3=Meng |first3=Fanke |last4=Senty |first4=Tess R. |last5=Suri |first5=Savan |last6=Zhi |first6=Mingjia |last7=Li |first7=Ming |last8=Bristow |first8=Alan D. |last9=Wu |first9=Nianqiang |date=2012-09-12 |title=Photocatalytic Activity Enhanced by Plasmonic Resonant Energy Transfer from Metal to Semiconductor |url=https://pubs.acs.org/doi/10.1021/ja305603t |journal=Journal of the American Chemical Society |language=en |volume=134 |issue=36 |pages=15033–15041 |doi=10.1021/ja305603t |pmid=22891916 |issn=0002-7863}}{{Cite journal |last1=Kuang |first1=Daibin |last2=Uchida |first2=Satoshi |last3=Humphry-Baker |first3=Robin |last4=Zakeeruddin |first4=Shaik M. |last5=Grätzel |first5=Michael |date=2008-02-22 |title=Organic Dye-Sensitized Ionic Liquid Based Solar Cells: Remarkable Enhancement in Performance through Molecular Design of Indoline Sensitizers |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.200705225 |journal=Angewandte Chemie International Edition |language=en |volume=47 |issue=10 |pages=1923–1927 |doi=10.1002/anie.200705225 |pmid=18214873 |issn=1433-7851}} morphologies,{{Cite journal |last1=Sun |first1=Baoquan |last2=Snaith |first2=Henry J. |last3=Dhoot |first3=Anoop S. |last4=Westenhoff |first4=Sebastian |last5=Greenham |first5=Neil C. |date=2005-01-01 |title=Vertically segregated hybrid blends for photovoltaic devices with improved efficiency |url=https://pubs.aip.org/jap/article/97/1/014914/896687/Vertically-segregated-hybrid-blends-for |journal=Journal of Applied Physics |language=en |volume=97 |issue=1 |pages=014914–014914–6 |doi=10.1063/1.1804613 |bibcode=2005JAP....97a4914S |issn=0021-8979}}{{Cite journal |last1=Wang |first1=Zhong-Sheng |last2=Kawauchi |first2=Hiroshi |last3=Kashima |first3=Takeo |last4=Arakawa |first4=Hironori |date=July 2004 |title=Significant influence of TiO2 photoelectrode morphology on the energy conversion efficiency of N719 dye-sensitized solar cell |url=https://linkinghub.elsevier.com/retrieve/pii/S0010854504000530 |journal=Coordination Chemistry Reviews |language=en |volume=248 |issue=13–14 |pages=1381–1389 |doi=10.1016/j.ccr.2004.03.006}} and device designs{{Cite journal |last1=Ghosh |first1=Amal K. |last2=Morel |first2=Don L. |last3=Feng |first3=Tom |last4=Shaw |first4=Robert F. |last5=Rowe |first5=Charles A. |date=1974-01-01 |title=Photovoltaic and rectification properties of Al/Mg phthalocyanine/Ag Schottky-barrier cells |url=https://pubs.aip.org/jap/article/45/1/230/171261/Photovoltaic-and-rectification-properties-of-Al-Mg |journal=Journal of Applied Physics |language=en |volume=45 |issue=1 |pages=230–236 |doi=10.1063/1.1662965 |bibcode=1974JAP....45..230G |issn=0021-8979}}{{Cite journal |last1=Thompson |first1=Barry C. |last2=Kim |first2=Young-Gi |last3=Reynolds |first3=John R. |date=2005-06-01 |title=Spectral Broadening in MEH-PPV:PCBM-Based Photovoltaic Devices via Blending with a Narrow Band Gap Cyanovinylene−Dioxythiophene Polymer |url=https://pubs.acs.org/doi/10.1021/ma0505934 |journal=Macromolecules |language=en |volume=38 |issue=13 |pages=5359–5362 |doi=10.1021/ma0505934 |bibcode=2005MaMol..38.5359T |issn=0024-9297}} for improved solar energy capture and utilization.

In photochemistry, action spectra have been mainly used in photodissociation studies. These involve a monochromatic light source, often a laser, coupled with a mass spectrometer to record wavelength-dependent ion dissociation in gaseous phases.{{Cite journal |last1=Dunbar |first1=Robert C. |last2=Teng |first2=Harry Ho I. |last3=Fu |first3=Emil W. |date=October 1979 |title=Photodissociation spectroscopy of halogen-substituted benzene ions |url=https://pubs.acs.org/doi/abs/10.1021/ja00516a004 |journal=Journal of the American Chemical Society |language=en |volume=101 |issue=22 |pages=6506–6510 |doi=10.1021/ja00516a004 |issn=0002-7863}} These spectra help identify contributing chromophores in molecular systems,{{Citation |last1=Polfer |first1=Nicolas C. |title=Infrared Photodissociation of Biomolecular Ions |date=2013 |url=http://dx.doi.org/10.1007/978-3-319-01252-0_4 |work=Lecture Notes in Chemistry |pages=71–91 |access-date=2023-12-16 |place=Cham |publisher=Springer International Publishing |isbn=978-3-319-01251-3 |last2=Stedwell |first2=Corey N.|doi=10.1007/978-3-319-01252-0_4 }}{{Cite journal |last1=Uleanya |first1=Kelechi O. |last2=Dessent |first2=Caroline E. H. |date=2021 |title=Investigating the mapping of chromophore excitations onto the electron detachment spectrum: photodissociation spectroscopy of iodide ion–thiouracil clusters |url=http://xlink.rsc.org/?DOI=D0CP05920J |journal=Physical Chemistry Chemical Physics |language=en |volume=23 |issue=2 |pages=1021–1030 |doi=10.1039/D0CP05920J |pmid=33428696 |bibcode=2021PCCP...23.1021U |s2cid=231587688 |issn=1463-9076}} characterize radical generation and unstable isomers,{{Cite journal |last1=Cabré |first1=Gisela |last2=Garrido-Charles |first2=Aida |last3=Moreno |first3=Miquel |last4=Bosch |first4=Miquel |last5=Porta-de-la-Riva |first5=Montserrat |last6=Krieg |first6=Michael |last7=Gascón-Moya |first7=Marta |last8=Camarero |first8=Núria |last9=Gelabert |first9=Ricard |last10=Lluch |first10=José M. |last11=Busqué |first11=Félix |last12=Hernando |first12=Jordi |last13=Gorostiza |first13=Pau |last14=Alibés |first14=Ramon |date=2019-02-22 |title=Rationally designed azobenzene photoswitches for efficient two-photon neuronal excitation |journal=Nature Communications |language=en |volume=10 |issue=1 |page=907 |doi=10.1038/s41467-019-08796-9 |issn=2041-1723 |pmc=6385291 |pmid=30796228|bibcode=2019NatCo..10..907C }}{{Cite journal |last1=Marlton |first1=Samuel J. P. |last2=McKinnon |first2=Benjamin I. |last3=Ucur |first3=Boris |last4=Bezzina |first4=James P. |last5=Blanksby |first5=Stephen J. |last6=Trevitt |first6=Adam J. |date=2020-05-21 |title=Discrimination between Protonation Isomers of Quinazoline by Ion Mobility and UV-Photodissociation Action Spectroscopy |url=https://pubs.acs.org/doi/10.1021/acs.jpclett.0c01009 |journal=The Journal of Physical Chemistry Letters |language=en |volume=11 |issue=10 |pages=4226–4231 |doi=10.1021/acs.jpclett.0c01009 |pmid=32368922 |s2cid=218505627 |issn=1948-7185}} and understand higher state electron dynamics.{{Cite journal |last1=Wellman |first1=Sydney M. J. |last2=Jockusch |first2=Rebecca A. |date=2015-06-18 |title=Moving in on the Action: An Experimental Comparison of Fluorescence Excitation and Photodissociation Action Spectroscopy |url=https://pubs.acs.org/doi/10.1021/acs.jpca.5b04835 |journal=The Journal of Physical Chemistry A |language=en |volume=119 |issue=24 |pages=6333–6338 |doi=10.1021/acs.jpca.5b04835 |pmid=26020810 |bibcode=2015JPCA..119.6333W |issn=1089-5639}}{{Cite journal |last1=Wellman |first1=Sydney M. J. |last2=Jockusch |first2=Rebecca A. |date=2017-06-07 |title=Tuning the Intrinsic Photophysical Properties of Chlorophyll a |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/chem.201605167 |journal=Chemistry – A European Journal |language=en |volume=23 |issue=32 |pages=7728–7736 |doi=10.1002/chem.201605167 |pmid=27976433 |issn=0947-6539}}

The field underwent a transformation when a team led by Barner-Kowollik and Gescheidt recorded the first modern-day photochemical action plot using a tuneable monochromatic nanosecond pulsed laser system, discovering a strong mismatch between photochemical reactivity and absorptivity and marking a critical advancement in mapping wavelength-dependent conversions in photoinduced polymerizations. Following this, numerous photochemical action plots have been recorded in various molecular and polymerization systems.{{Cite journal |last1=Marschner |first1=David E. |last2=Frisch |first2=Hendrik |last3=Offenloch |first3=Janin T. |last4=Tuten |first4=Bryan T. |last5=Becer |first5=C. Remzi |last6=Walther |first6=Andreas |last7=Goldmann |first7=Anja S. |last8=Tzvetkova |first8=Pavleta |last9=Barner-Kowollik |first9=Christopher |date=2018-05-22 |title=Visible Light [2 + 2] Cycloadditions for Reversible Polymer Ligation |url=https://pubs.acs.org/doi/10.1021/acs.macromol.8b00613 |journal=Macromolecules |language=en |volume=51 |issue=10 |pages=3802–3807 |doi=10.1021/acs.macromol.8b00613 |bibcode=2018MaMol..51.3802M |issn=0024-9297}}{{Cite journal |last1=Menzel |first1=Jan P. |last2=Noble |first2=Benjamin B. |last3=Lauer |first3=Andrea |last4=Coote |first4=Michelle L. |last5=Blinco |first5=James P. |last6=Barner-Kowollik |first6=Christopher |date=2017-11-08 |title=Wavelength Dependence of Light-Induced Cycloadditions |url=https://pubs.acs.org/doi/10.1021/jacs.7b08047 |journal=Journal of the American Chemical Society |language=en |volume=139 |issue=44 |pages=15812–15820 |doi=10.1021/jacs.7b08047 |pmid=29024596 |issn=0002-7863|hdl=1885/209117 |hdl-access=free }}

Key differences between traditional (biological) action spectra and modern photochemical action plots lie in the precision resolution of wavelengths (monochromaticity) and that an exact number of photons at each wavelength is applied coupled with the fact that covalent bond forming reactions were investigated for the first time.{{Cite journal |last1=Fast |first1=David E. |last2=Lauer |first2=Andrea |last3=Menzel |first3=Jan P. |last4=Kelterer |first4=Anne-Marie |last5=Gescheidt |first5=Georg |last6=Barner-Kowollik |first6=Christopher |date=2017-03-14 |title=Wavelength-Dependent Photochemistry of Oxime Ester Photoinitiators |url=https://pubs.acs.org/doi/10.1021/acs.macromol.7b00089 |journal=Macromolecules |language=en |volume=50 |issue=5 |pages=1815–1823 |doi=10.1021/acs.macromol.7b00089 |bibcode=2017MaMol..50.1815F |issn=0024-9297}}File:Action Plot.pngIn the field of photochemical analysis, it is common to measure the extinction of chemicals with high precision, often at the sub-nanometer scale, using UV/Vis spectroscopy. To understand fundamental relationships between a chemical's absorbance and its photoreactivity, a detailed analysis of the reactivity at a similar level of resolution is required.{{Cite journal |last1=Bandyopadhyay |first1=Disha |last2=Ji |first2=Yiming |last3=Yee |first3=Daryl W. |date=August 2024 |title=Lights, chemistry, action plots: Rethinking photoresin design in additive manufacturing |url=https://linkinghub.elsevier.com/retrieve/pii/S2590238524003382 |journal=Matter |volume=7 |issue=8 |pages=2745–2747 |doi=10.1016/j.matt.2024.06.019 |issn=2590-2385}} Traditional methods using broadly emitting light sources or filters have inherent limitations in resolving true wavelength dependence in photoreactivity.{{Cite journal |date=1925-06-02 |title=Studies on the biological action of light |journal=Proceedings of the Royal Society of London. Series B, Containing Papers of a Biological Character |language=en |volume=98 |issue=688 |pages=171–187 |doi=10.1098/rspb.1925.0029 |issn=0950-1193|doi-access=free }}{{Cite web |title=Optica Publishing Group |url=https://opg.optica.org/josa/viewmedia.cfm?uri=josa-20-8-433&html=true |access-date=2024-09-02 |website=opg.optica.org}}{{Cite journal |last1=Haxo |first1=F. T. |last2=Blinks |first2=L. R. |date=1950-03-20 |title=Photosynthetic Action Spectra of Marine Algae |url=https://rupress.org/jgp/article/33/4/389/12271/PHOTOSYNTHETIC-ACTION-SPECTRA-OF-MARINE-ALGAE |journal=Journal of General Physiology |language=en |volume=33 |issue=4 |pages=389–422 |doi=10.1085/jgp.33.4.389 |issn=1540-7748 |pmc=2147193 |pmid=15406376}}{{Cite journal |last1=Myers |first1=Jack |last2=French |first2=C. S. |date=1960-03-01 |title=Evidences from Action Spectra for a Specific Participation of Chlorophyll b in Photosynthesis |url=https://rupress.org/jgp/article/43/4/723/30499/Evidences-from-Action-Spectra-for-a-Specific |journal=The Journal of General Physiology |language=en |volume=43 |issue=4 |pages=723–736 |doi=10.1085/jgp.43.4.723 |issn=1540-7748 |pmc=2195029 |pmid=14425592}} To record an action plot, a wavelength-tuneable laser system is employed, capable of delivering a stable number of photons at each wavelength.{{Cite journal |last1=Walden |first1=Sarah L. |last2=Carroll |first2=Joshua A. |last3=Unterreiner |first3=Andreas-Neil |last4=Barner-Kowollik |first4=Christopher |date=January 2024 |title=Photochemical Action Plots Reveal the Fundamental Mismatch Between Absorptivity and Photochemical Reactivity |journal=Advanced Science |language=en |volume=11 |issue=3 |pages=e2306014 |doi=10.1002/advs.202306014 |issn=2198-3844 |pmc=10797470 |pmid=37937391}} The photoreactive reaction mixture is divided into aliquots and subjected to monochromatic light independently. The photochemical process' yield or conversion is subsequently measured using sensors like UV-Vis absorption or nuclear magnetic resonance (NMR) frequency changes.

A key finding of modern photochemical action plots is that the absorption spectrum of a photoreactive molecule or reaction mixture correlates poorly with photochemical reactivity as a function of wavelength in many cases. Initial studies showed a significant red-shift in photopolymerization yield compared to the absorption spectrum of the employed photoinitiators, which showed extremely low absorptivity in those regions. This mismatch between absorption spectra and photochemical action plots has by now been observed in a wide array of photoreactive systems.{{Cite journal |last1=Ma |first1=Congkai |last2=Han |first2=Ting |last3=Efstathiou |first3=Spyridon |last4=Marathianos |first4=Arkadios |last5=Houck |first5=Hannes A. |last6=Haddleton |first6=David M. |date=2022-11-22 |title=Aggregation-Induced Emission Poly(meth)acrylates for Photopatterning via Wavelength-Dependent Visible-Light-Regulated Controlled Radical Polymerization in Batch and Flow Conditions |journal=Macromolecules |language=en |volume=55 |issue=22 |pages=9908–9917 |doi=10.1021/acs.macromol.2c01413 |issn=0024-9297 |pmc=9686136 |pmid=36438594|bibcode=2022MaMol..55.9908M }}{{Cite journal |last1=Reeves |first1=Jennifer A. |last2=De Alwis Watuthanthrige |first2=Nethmi |last3=Boyer |first3=Cyrille |last4=Konkolewicz |first4=Dominik |date=November 2019 |title=Intrinsic and Catalyzed Photochemistry of Phenylvinylketone for Wavelength-Sensitive Controlled Polymerization |url=https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/cptc.201900052 |journal=ChemPhotoChem |language=en |volume=3 |issue=11 |pages=1171–1179 |doi=10.1002/cptc.201900052 |s2cid=155141292 |issn=2367-0932}}{{Cite journal |last1=Irshadeen |first1=Ishrath Mohamed |last2=Walden |first2=Sarah L. |last3=Wegener |first3=Martin |last4=Truong |first4=Vinh X. |last5=Frisch |first5=Hendrik |last6=Blinco |first6=James P. |last7=Barner-Kowollik |first7=Christopher |date=2021-12-22 |title=Action Plots in Action: In-Depth Insights into Photochemical Reactivity |url=https://pubs.acs.org/doi/10.1021/jacs.1c09419 |journal=Journal of the American Chemical Society |language=en |volume=143 |issue=50 |pages=21113–21126 |doi=10.1021/jacs.1c09419 |pmid=34859671 |s2cid=244880552 |issn=0002-7863}} A prominent example is the photoinduced [2+2] cycloaddition of the stilbene derivative, styrypyrene, which exhibited an 80 nm discrepancy between the action plot and absorption spectrum. Current research focuses on understanding the reasons behind these frequently observed mismatches.

For photochemical applications, the consequences of the absorptivity/reactivity mismatch are far reaching, as only photochemical action plots can reveal the most effective wavelength for a given process, moving away from the past paradigm that absorption spectra provide guidance for selecting the most effective wavelength.

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