Issue 12, 2020

Perspective – life and death of a photon: an intuitive non-equilibrium thermodynamic distinction between photochemistry and thermochemistry

Abstract

Neither the thermodynamically determined probability isotherm nor its kinetically manifest rate isotherm can be applied to photo-absorptive reactions such that the participants, including photons, may be treated as if they were chemical reactants. Photons and chemical reactants differ from each other fundamentally: firstly, a photon's energy is absolute and, in all instances of practical relevance to the present paper, independent of its surrounding electrochemical field, while the energy of a chemical reactant is relative and defined by its surrounding field; secondly, while both photons and chemical reactants can and do engage in entropy creation, only chemical reactants can engage in entropy exchange. Clarification of these differences requires identification and abandonment of fundamental historical errors in photochemical thought deriving from inappropriate overreach of analogies drawn between light and ideal gases, and including: treatment of photo-absorption as a reversible chemical reaction; attribution to light of thermal potential, or temperature (as distinct from the idealised abstraction of a ‘temperature signature’); attribution to light of exchangeable entropy content. We begin by addressing widespread misapprehensions concerning the perennially misunderstood concept of entropy and the frequently overlooked distinction between entropy creation and entropy exchange. Armed with these clarifications, we arrive at a useful perspective for understanding energy absorption and transfer in photosynthetic processes which, through the chemical ‘kidnapping’ of metastable excited states within structured metabolic pathways, achieves outcomes which the Second Law denies to thermal chemical reactions.

Graphical abstract: Perspective – life and death of a photon: an intuitive non-equilibrium thermodynamic distinction between photochemistry and thermochemistry

Article information

Article type
Perspective
Submitted
14 8 2020
Accepted
03 11 2020
First published
13 11 2020

Photochem. Photobiol. Sci., 2020,19, 1623-1629

Perspective – life and death of a photon: an intuitive non-equilibrium thermodynamic distinction between photochemistry and thermochemistry

B. Chapman and D. Loiselle, Photochem. Photobiol. Sci., 2020, 19, 1623 DOI: 10.1039/D0PP00274G

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