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. 2024 Jan;11(3):e2306014.
doi: 10.1002/advs.202306014. Epub 2023 Nov 8.

Photochemical Action Plots Reveal the Fundamental Mismatch Between Absorptivity and Photochemical Reactivity

Affiliations

Photochemical Action Plots Reveal the Fundamental Mismatch Between Absorptivity and Photochemical Reactivity

Sarah L Walden et al. Adv Sci (Weinh). 2024 Jan.

Abstract

Over the last years, the authors' laboratory has employed monochromatic tuneable laser systems to reveal a fundamental mismatch between the absorptivity of a chromophore and its photochemical reactivity for the vast majority of covalent bond forming reactions as well as specific bond cleavage reactions. In the general chemistry community, however, the long-held assumption pervades that effective photochemical reactions are obtained in situations where there is strong overlap between the absorption spectrum and the excitation wavelength. The current Perspective illustrates that the absorption spectrum of a molecule only provides information about electronic excitations and remains entirely silent on other energy redistribution mechanisms that follow, which critically influence photochemical reactivity. Future avenues of enquiry on how action plots can be understood are proposed and the importance of action plots for tailoring photochemical applications with never-before-seen precision is explored.

Keywords: absorption; action plot; photochemistry; photophysics; wavelength-dependent reactivity.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
A) Schematic of our action plot apparatus where a tuneable, monochromatic light source is directed from below into a transparent vial containing the reaction solution. The energy incident upon the sample is carefully tuned to ensure an identical photon input at each wavelength of interest. B,C) Comparison of the UV‐vis spectra (insets: zoom into 375/390 to 450 nm) and the wavelength‐dependent conversion of methyl methacrylate initiated by two oxime‐based free‐radical photoinitiators at constant photon count (60 µmol) at each irradiation wavelength. Adapted with permission.[ 10 ] Copyright 2017, American Chemical Society (ACS).
Figure 2
Figure 2
A) Action plot of styrylquinoxaline in water indicating strongly red‐shifted reactivity into the blue and green light regions and B) schematic diagram of the application of 455 nm blue light induced [2 + 2] cycloaddition of styrylquinoxaline for DNA labeling and C) fluorescence readout. a) Adapted with permission.[ 16 ] Copyright 2020, Springer Nature. b,c) Adapted with permission.[ 17 ] Copyright 2023, Royal Society of Chemistry.
Figure 3
Figure 3
Example of two‐color photochemistry. A) Action plot measurements on the cycloaddition and cycloreversion of styrylpyrene highlighting that dissociation supresses the dimerization conversion at short wavelengths.[ 28 ] B) Schematic diagram of orthogonal activation of two chromophores in a single photoresist, in this case the two chromophores are styrylpyrene and o‐methylbenzaldehyde (oMBA). C) Action plot measurements of oMBA reaction with N‐ethylmaleimide showing efficient reaction in the wavelength region where styrylpyrene dimerization is supressed.[ 29 ]
Figure 4
Figure 4
Jablonski diagram illustrating molecular excitation through absorption of a single photon to a specific vibrational energy. Radiative and non‐radiative pathways exist for the return to the ground state. The energy levels, along with available pathways and their propensities, are unique to each molecule. Singlet states from ground (S0) to states higher (Sn) increase in energy, while Tn represents triplet states. Each band corresponds to a vibrational energy level within that molecular state. Rotational energies are not shown for each vibrational energy.

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