William T. Simpson

Abstract: The x‐ray form factors for a bonded hydrogen in the hydrogen molecule have been calculated for a spherical approximation to the bonded atom. These factors may be better suited for the least‐squares refinement of x‐ray diffraction data from organic molecular crystals than those for the isolated hydrogen atom. It has been shown that within the spherical approximation for the bonded hydrogens in H2, a least‐squares refinement of the atomic positions will result in a bond length (Re value) short of neutron diffraction or spectroscopic values. The spherical atoms are optimally positioned 0.07 A off each proton into the bond. A nonspherical density for the bonded hydrogen atom in the hydrogen molecule has also been defined and the corresponding complex scattering factors have been calculated. The electronic density for the hydrogen molecule in these calculations was based on a modified form of the Kolos—Roothaan wavefunction for H2. Scattering calculations were made tractable by expansion of a plane wave in spheroidal wavefunctions.

Abstract: The interpretation of the absorption spectrum of a molecular crystal requires a knowledge of the strength of resonance force electrostatic interaction between molecules in the crystal as compared to an energy parameter characterizing the molecular vibrational level pattern. Depending on the relative magnitudes of these energy terms the spectrum reflects absorption by the crystal as a whole or by independent, though oriented molecules. These two familiar cases are examined from both stationary‐state and time‐dependent points of view with the object of defining the energy parameters. The resonance force interaction term is a theoretically derived quantity, the crystal electronic band width; the comparable vibrational energy term is shown to be the width of the total electronic band comprising all the vibronic transitions for an isolated molecule. Particular attention is given to the case in which these terms are nearly equal.

Abstract: Sigma‐bond electronic transitions in alkanes have been investigated both experimentally and theoretically. Vacuum ultraviolet spectra of a large number of alkanes have been recorded in the range 50 to 94 kK and absorption intensities as well as frequencies have been determined. The implications of vapor‐phase conformational isomerism are discussed. A qualitative interpretation of the observed spectra has been developed using independent‐systems theory in connection with a simple model in which excitations in only C–C bonds are considered. A reasonably good topological matching between theory and experiment is achieved. A quantitative theory in which C–H bond excitations are included has also been developed. With this theory parameters are derived from the spectra of methane and ethane which prove sufficient to describe the higher alkanes without recourse to ``fitting.'' The theoretical procedures used are outlined in the text and described in greater detail in an Appendix.

Abstract: The failure of certain LCAO molecular orbital calculations of the spectrum of porphine is attributed to the incorrect assumption of complete ``aromaticity.'' A division of the π‐electron system into non‐interacting parts is supported by the fact that the dynamical properties of the divided system, as calculated by the free‐electron model, are in approximate agreement with experiment. A proof of the equivalency of the LCAO method (with overlap neglected) and an approximation to the free‐electron method is indicated for certain dynamical systems.

Abstract: Three more or less structured bands (at 55, 60, and 68 kK in propionaldehyde) are located in compounds containing carbonyl. These bands are tentatively assigned, respectively, as n′→π; n→σ* (n→3s); and π→π*.The π→π* is tentatively located in compounds containing carboxyl, e.g, at about 67 kK in formic acid and acetic acid, and is followed as it red shifts in methyl formate and finally to 58 kK in formamide. The band is considered as perturbed carbonyl with some charge transfer until the perturbation becomes too strong—as it evidently is in dimethyl formamide where the band comes at 51 kK. Here one sees (from the absence of a Brooker deviation) that the molecule must be regarded as allylic. Other bands in carboxyl are tentatively identified as being analogous to the 55‐ and 60‐kK bands in propionaldehyde, though the possibility of a second π→π* absorption in the region studied cannot be ruled out. The spectra of formic acid and acetic acid dimer are presented and interpreted.

Abstract: In the last few years the analysis of molecular crystal structures using tools based on Hirshfeld surfaces has rapidly gained in popularity. This approach represents an attempt to venture beyond the current paradigm—internuclear distances and angles, crystal packing diagrams with molecules represented via various models, and the identification of close contacts deemed to be important—and to view molecules as “organic wholes”, thereby fundamentally altering the discussion of intermolecular interactions through an unbiased identification of all close contacts.

Abstract: The review opens by presenting the absorption spectra for three series of porphyrins derived from the basic skeleton: (a) compounds obtained by simple substitution; (b) compounds obtained by reduction of one or more pyrrole rings; and (c) compounds obtained from fusion of aromatic rings onto the basic skeleton. The spectra are discussed in terms of a four orbital model—that is intensity changes and energy shifts are related to the properties of two top filled and two lowest empty pi orbitals. Emission spectra of metal porphyrins are then discussed, three metal series being distinguished: (1) In closed shell metals, the continuous enhancement of phosphorescence at the expense of fluorescence is attributed to spin-orbit coupling. (2) In paramagnetic metals, observed effects are attributed to the existence of a state at the same energy as the usual triplet but with multiplicity the same as the ground state; its intensity is ascribed to exchange interactions. (3) In diamagnetic metals with unfilled d shells, peculiar emission properties are attributed to enhanced spin orbit coupling due to low lying metal triplets. The review closes by discussing n-π transitions and triplet-triplet spectra.

Abstract: J-aggregates are of significant interest for organic materials conceived by supramolecular approaches. Their discovery in the 1930s represents one of the most important milestones in dye chemistry as well as the germination of supramolecular chemistry. The intriguing optical properties of J-aggregates (in particular, very narrow red-shifted absorption bands with respect to those of the monomer and their ability to delocalize and migrate excitons) as well as their prospect for applications have motivated scientists to become involved in this field, and numerous contributions have been published. This Review provides an overview on the J-aggregates of a broad variety of dyes (including cyanines, porphyrins, phthalocyanines, and perylene bisimides) created by using supramolecular construction principles, and discusses their optical and photophysical properties as well as their potential applications. Thus, this Review is intended to be of interest to the supramolecular, photochemistry, and materials science communities.

Abstract: Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules; this electronic excitation is subsequently transferred to a suitable acceptor. For example, in photosynthesis, antenna complexes capture sunlight and direct the energy to reaction centres that then carry out the associated chemistry. In this Review, we describe the principles learned from studies of various natural antenna complexes and suggest how to elucidate strategies for designing light-harvesting systems. We envisage that such systems will be used for solar fuel production, to direct and regulate excitation energy flow using molecular organizations that facilitate feedback and control, or to transfer excitons over long distances. Also described are the notable properties of light-harvesting chromophores, spatial-energetic landscapes, the roles of excitonic states and quantum coherence, as well as how antennas are regulated and photoprotected.

Abstract: It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.