There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

In this review, we highlight the use of organic photoredox catalysts in a myriad of synthetic transformations with a range of applications. This overview is arranged by catalyst class where the photophysics and electrochemical characteristics of each is discussed to underscore the differences and advantages to each type of single electron redox agent. We highlight both net reductive and oxidative as well as redox neutral transformations that can be accomplished using purely organic photoredox-active catalysts. An overview of the basic photophysics and electron transfer theory is presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.

Organic Photoredox Catalysis

Recent experiments, advances in theory, and analogies to other complex systems such as glasses and spin glasses yield insight into protein dynamics. The basis of the understanding is the observation that the energy landscape is complex: Proteins can assume a large number of nearly isoenergetic conformations (conformational substates). The concepts that emerge from studies of the conformational substates and the motions between them permit a quantitative discussion of one simple reaction, the binding of small ligands such as carbon monoxide to myoglobin.

https://science.sciencemag.org/content/sci/254/5038/1598.full.pdf

The energy landscapes and motions of proteins.

Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength. The reverse cis →transisomerization can be driven by light or occurs thermally in the dark. Azobenzene's photochromatic properties make it an ideal component of numerous molecular devices and functional materials. Despite the abundance of application-driven research, azobenzene photochemistry and the isomerization mechanism remain topics of investigation. Additional substituents on the azobenzene ring system change the spectroscopic properties and isomerization mechanism. This critical review details the studies completed to date on the 3 main classes of azobenzene derivatives. Understanding the differences in photochemistry, which originate from substitution, is imperative in exploiting azobenzene in the desired applications.

Photoisomerization in different classes of azobenzene

Fourier transform infrartd (FTIR) spectroscopy is an established tool for the structural character- ization of proteins. However, many potential pitfalls exist for the unwary investigator. In this review we critically assess the application of FIlR spectroscopy to the determination of protein structure by (1) outlining the principles underlying protein secondary structure determination by FZZR spectroscopy. (2) highhghting the situations in which FZZR spectroscopy should be considered the technique of choice, (3) discussing the manner in which experiments should be conducted to derive as much physiologically relevant information as possible, and (4) outlining current methods for the determination of secondary structure from infrared spectm of proteins,

The Use and Misuse of FTIR Spectroscopy in the Determination of Protein Structure

Femtosecond time-resolved UV−visible absorption spectroscopy has been used to study the UV photochemistry of trans-azobenzene (t-AB) in solution at 30 °C. Photolysis of t-AB at 303 nm results in transient absorption at 370−450 nm, the decay of which can be fitted by a sum of two exponential components. The shorter-lived component has a lifetime of 0.9 ± 0.2 ps in hexane, cyclohexane, and hexadecane and 1.2 ± 0.2 ps in acetonitrile; this is attributed to the S2(ππ*) excited state of t-AB. The longer-lived component has a lifetime which is similar to the recovery time of the ground-state absorption of t-AB at 303 nm, found to be 13 ± 1 ps in hexane, cyclohexane, and hexadecane and 16 ± 1 ps in acetonitrile. This longer-time-scale process is attributed to the internal conversion of an intermediate excited state, S†, into ground state t-AB, and this intermediate is tentatively assigned as a twisted conformer of excited t-AB on the S2 or S1 potential energy surface. The vibrational relaxation of hot t-AB molec...

Femtosecond Time-Resolved UV−Visible Absorption Spectroscopy of trans-Azobenzene in Solution

Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength

The iron-carbonyl geometries in carboxymyoglobin (MbCO) and carboxyhemoglobin (HbCO) in ambient temperature solution have been investigated using picosecond time-resolved infrared spectroscopy. Polarized infrared and visible beams were used to monitor the change in infrared absorbance of the bound CO stretch bands on photodissociation of the ligand. The ratio of the change in absorbance for perpendicular and parallel relative polarizations of the photolysis and infrared probe beams is directly related to the angle between the ligand bond axis and the normal to the heme plane. Ratios, and hence the angles, have been obtained for the configurations giving rise to the principal CO stretch infrared absorption bands of HbCO and MbCO: 18 degrees for the 1951 cm-1 band of HbCO; 20 degrees and 35 degrees, respectively, for the 1944 cm-1 and 1933 cm-1 bands of MbCO. Structures consistent with x-ray diffraction and the picosecond experiments reported here are proposed for MbCO and HbCO in which the Fe-C bond tilts to the heme normal and the Fe-C-O angle differs significantly from 180 degrees.

https://www.pnas.org/content/pnas/85/14/5062.full.pdf

Iron-carbonyl bond geometries of carboxymyoglobin and carboxyhemoglobin in solution determined by picosecond time-resolved infrared spectroscopy

Ultrafast time-resolved electronic absorption spectroscopy has been used to study the photochemistry of trans-azobenzene and trans-1, a derivative in which azobenzene is capped by an azacrown ether, on UV excitation to the S2(ππ*) state. Excitation of trans-1 results in transient absorption which decays with a dominant component of lifetime ca. 2.6 ps and in bleaching of the ground-state UV absorption band which recovers on a similar time scale. In contrast, excitation of trans-azobenzene results in transient absorption which decays with a dominant component with a shorter lifetime of ca. 1 ps, and in bleaching which recovers on a much longer time scale of ca. 18 ps. The recovery of the ground-state UV absorption band is not complete in either case, and the ultrafast data indicate that the quantum yield of trans-to-cis photoisomerization of 1 is approximately twice that of azobenzene. These observations demonstrate that the restricted rotational freedom of the phenyl groups in trans-1 has a significant ef...

John N. Moore

Papers

Femtosecond Time-Resolved UV−Visible Absorption Spectroscopy of trans-Azobenzene in Solution

Femtosecond time-resolved UV-visible absorption spectroscopy of trans-azobenzene: dependence on excitation wavelength

Iron-carbonyl bond geometries of carboxymyoglobin and carboxyhemoglobin in solution determined by picosecond time-resolved infrared spectroscopy

Photoisomerization of a Capped Azobenzene in Solution Probed by Ultrafast Time-Resolved Electronic Absorption Spectroscopy

A new method for picosecond time-resolved infrared spectroscopy: applications to CO photodissociation from iron porphyrins