The term ``sensitized luminescence'' in crystalline phosphors refers to the phenomenon whereby an impurity (activator, or emitter) is enabled to luminesce upon the absorption of light in a different type of center (sensitizer, or absorber) and upon the subsequent radiationless transfer of energy from the sensitizer to the activator The resonance theory of Forster, which involves only allowed transitions, is extended to include transfer by means of forbidden transitions which, it is concluded, are responsible for the transfer in all inorganic systems yet investigated The transfer mechanisms of importance are, in order of decreasing strength, the overlapping of the electric dipole fields of the sensitizer and the activator, the overlapping of the dipole field of the sensitizer with the quadrupole field of the activator, and exchange effects These mechanisms will give rise to ``sensitization'' of about 103−104, 102, and 30 lattice sites surrounding each sensitizer in typical systems The dependence of transfer efficiency upon sensitizer and activator concentrations and on temperature are discussed Application is made of the theory to experimental results on inorganic phosphors, and further experiments are suggested

A Theory of Sensitized Luminescence in Solids

The current state of understanding of molecular resonance energy transfer (RET) and recent developments in the field are reviewed. The development of more general theoretical approaches has uncovered some new principles underlying RET processes. This review brings many of these important new concepts together into a generalization of Forster's original theory. The conclusions of studies investigating the various approximations in Forster theory are summarized. Areas of present and future activity are discussed. The review covers Forster theory for donor-acceptor pairs and electronic coupling for singlet-singlet, triplet-triplet, and superexchange-mediated energy transfer. This includes the transition density picture of Coulombic coupling as well as electronic coupling between molecular aggregates (excitons). Spectral overlaps and ensemble energy transfer rates in disordered aggregates, the role of dielectric properties of the medium, weak versus strong coupling, and new models for energy transfer in complex molecular assemblies are also described.

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Long-Range Resonance Energy Transfer in Molecular Systems

CHAPTER 3 – Photosensitized Oxidation and Singlet Oxygen: Consequences in Biological Systems

The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing molecules (chromophores), which are also responsible for the subsequent funneling of the excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solutions for light harvesting. In this review, we describe the underlying photophysical principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET). First, optical properties of the individual pigment chromophores present in light-harvesting antenna complexes are introduced, and then we examine the collective behavior of pigment−pigment and pigment−protein interactions. The description of energy transfer, in particular multichromophoric antenna structures, is shown to vary depending on the spatial and energetic landscape, which dictates the relative coupling strength between constituent pigment molecules. In the latter half...

Light Absorption and Energy Transfer in the Antenna Complexes of Photosynthetic Organisms.

https://science.sciencemag.org/content/sci/162/3857/963.full.pdf

Mechanisms of Photosensitized Oxidation

The Molecular Biology of Photodynamic Action: Sensitized Photoautoxidations in Biological Systems1 ,2

Interactions of Chlorophyll in its Triplet State with Oxygen, Carotene, Etc.

An integrating sphere has been adapted to the measurement of the quantum yield of the fluorescence of solutions of pigments. The intensities of the fluorescence and of the absorbed light were determined separately with a thermopile.Chlorophylls a and b dissolved in various solvents have been studied with this apparatus over a range of concentrations, and the measured yields corrected for autoabsorption. The yield for chlorophyll a is 0.25 and is in general independent of solvent and excitation wavelength. The yield for chlorophyll b is 0.11 for ether solutions and 0.06 for methanol solutions. The absolute fluorescent yields of solutions of fluorescein, eosine, magdala red, and rubrene were also measured. These results are in reasonable agreement with the published values of the yields for the same compounds.

The Absolute Quantum Yields of the Fluorescence of Chlorophyll Solutions

Many of the apparently conflicting facts of the photochemistry as well as of the phosphorescence and of the fluorescence quenching of dyes can be given a rational and unified interpretation if it be assumed that an electronically excited dye molecule can go over, by a process of internal conversion, to the electronic ground state of a reactive energy‐rich tautomer. The endothermic reversion of such a tautomer to the electronically excited state of the original molecule would explain the weak, temperature dependent phosphorescence observed in some systems. In the presence of a suitable reducing agent the reactive tautomer may be reduced to a semiquinone. This process and the likely subsequent reactions are sufficient to explain the majority of dye‐sensitized photo‐oxidations as well as the photo‐bleaching of dyes by reducing agents. In the case of chlorophyll, and possibly a few other dyes, it appears to be necessary to assume that the tautomer and a normal dye molecule can undergo a process of disproportionation. The assumption of this step leads directly and simply to an explanation of the rapid reversible bleaching of chlorophyll which is consistent with the other known photochemical and optical properties of this substance. In addition to the more familiar reaction steps which have been assumed as follow reactions in setting up the various mechanisms, it is necessary to include a step in which the HO2 radical reduces a partially oxidized dye molecule (i.e., radical) back to the normal state. The several reaction steps assumed in these various schemes appear to be energetically feasible.

Robert Livingston

Papers

The Molecular Biology of Photodynamic Action: Sensitized Photoautoxidations in Biological Systems1 ,2

Interactions of Chlorophyll in its Triplet State with Oxygen, Carotene, Etc.

The Absolute Quantum Yields of the Fluorescence of Chlorophyll Solutions

Remarks on the Fluorescence, Phosphorescence and Photochemistry of Dyestuffs

A Flash-Photolytic Investigation of Rhodopsin at Low Temperatures