Photoredox catalysis has emerged as a powerful platform for chemical synthesis, utilizing chromophore excited states as selective energy stores to surmount chemical activation barriers toward making desirable products. Developments in this field have pushed synthetic chemists to design and discover new photocatalysts with novel and impactful photoreactivity but also with uncharacterized excited states and only an approximate mechanistic understanding. This review highlights specific instances in which ultrafast spectroscopies dissected the photophysical and photochemical dynamics of new classes of photoredox catalysts and their photochemical reactions. After briefly introducing the photophysical processes and ultrafast spectroscopic methods central to this topic, the review describes selected recent examples that evoke distinct classes of photoredox catalysts with demonstrated synthetic utility and ultrafast spectroscopic characterization. This review cements the significant role of ultrafast spectroscopy in modern photocatalyzed organic transformations and institutionalizes the developing intersection of synthetic organic chemistry and physical chemistry.

Visible photons carry significantly more energy than the thermal energies typically used to overcome activation barriers in conventional chemistry. This thermodynamic advantage enables photochemical reactions that are inaccessible from electronic ground states. However, photochemistry also faces a kinetic challenge: excited states are inherently short-lived, necessitating rapid reactivity before their decay. In this Outlook, we explore the unique interplay of thermodynamics and kinetics in molecular photochemistry. We highlight current limits and knowledge gaps and propose directions for advancing the conceptual framework of photocatalysis. Topics include the design of photocatalysts with extreme redox potentials, the use of solvated electrons and visible-to-UV upconversion, and the potential to bypass Kasha's rule for higher-energy photochemical processes. Our aim is to survey strategies for pushing the boundaries of photocatalysis and to inspire future conceptual innovation in the field.

This chapter provides an overview of photochemistry. Life on Earth depends, both directly and indirectly, on the influence that light has on chemistry. Interactions of light with matter make up the subject of photochemistry. If there is one essential feature of photochemistry, it is probably the way in which excited states of atoms or molecules play a part in the processes occurring. The excited states of most concern to the photochemist are those in which the electrons have become distributed into a higher energy arrangement; the species are thus said to be electronically excited. The chapter then looks at the nature of light; wave mechanics and quantum numbers; the selection rules for optical absorption; the laws of photochemistry; and intramolecular energy transfer.

Predicting the outcome of a photochemical reaction is complicated by factors outside the scope of tradition theoretical chemistry. On such small scales, fluctuations of the environment render outcomes stochastic, nonadiabatic dynamics blur the separation between nuclear and electronic motion, and ultrafast relaxation renders typical assumptions of equilibrium inappropriate. This project aimed to elucidate and control nonadiabatic molecular dynamics in condensed phases. We developed accurate, efficient simulation tools to study irreversible molecular processes computational across a range of scales. We applied these methodological advances to study electron transfer under exotic conditions where spin and topology opened new pathways for reactivity. We also brought these ideas to bare on light harvesting systems, where energy and charge transport are coupled, and where long lifetimes can be deleterious to function. These advances helped to establish general principles for chemical efficiency and guide the design of nanoscale energy materials, molecular machines, and related technologies.

for specificity. Furthermore, photoacoustic measurements serve as a valuable research tool in the study of the heat evolved in photochemical reactions (see: The photoacoustic effect or optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically (modulated light) or as a single flash (pulsed light). The photoacoustic effect is quantified by measuring the formed sound (pressure changes) with appropriate detectors, such as microphon The… Energy storage (i.e. the fraction of light energy which is converted to chemical energy in the photosynthetic process; The extent and dynamics of the gas evolution and uptake from leaves or lichens. Most usually it is photosynthetic oxygen evolution which contributes to the photoacoustic signal; Carbon dioxide uptake is a slow process and does not show up in photoacoustic measurements. Under very specific conditions, however, the photoacoustic signal becomes transiently negative, presumably reflecting oxygen uptake. However, this needs more verification; Molecular volume changes, which occur during the primary steps of photosynthetic electron transfer. These measurements provided information related to the mechanism of photosynthesis, as well as give indications on the intactness and health of the specimen. Examples are: (a) the energetics of the primary electron transfer processes, obtained from the energy storage and molecular volume change measured under sub-microsecond flashes; (b) The characteristics of the 4-step oxidation cycle in photosystem II, obtained for leaves by monitoring photoacoustic pulsed signals and their oscillatory behavior under repetitive exciting light flashes; (c) the characteristics of photosystem I and photosystem II of photosynthesis (absorption spectrum, light distribution to the two photosystems) and their interactions. This is obtained by using continuously modulated light of a certain specific wavelength to excite the photoacoustic signal and measure changes in energy storage and oxygen evolution caused by background light at various chosen wavelengths. In general, photoacoustic measurements of energy storage require a reference sample for comparison. It is a sample with exactly the same light absorption (at the given excitation wavelength) but which completely degrades all the absorbed light into heat within the time resolution of the measurement. It is lucky that photosynthetic systems are self-calibrating, providing such a reference in one sample, as follows: One compares two signals: one, which is obtained with the probing modulated/pulsed light alone and the other when a steady non-modulated light (referred to as…