Singlet photophysics

Previous expositions of photochemical laws have distinguished ptominentiy between states of singlet and triplet multiplicity (1). This distinction continues to be important with respect to photophysics of smaH organic molecules, but among inorganic and organometaHic compounds, states of other multiplicities, eg, doublet and quartet states (23), play an important role. Spin conservation characterizes electronic molecular excitations and localized... 

Unusual photophysical properties of polyazaanthracenes and polyazapentacenes having low values of calculated singlet-triplet energy gap 99PAC295. 

In Chapter 2 we discussed a number of techniques used to study the various photophysical and photochemical processes occurring in anthracene and similar molecules. In that discussion we were primarily interested in the singlet state. In this chapter we will discuss some of the techniques available for studying the photophysical and photochemical properties of the triplet state. Most of our discussion will be directed to the photochemistry of simple ketones. 

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

Figure 4. Electronic energy level diagram for anthracene illustrating the photophysical transitions (including reaction with the initiator from both the singlet and triplet states) and the associated kinetic constants.

PHOTOPHYSICAL ASPECTS OF ELECTROPHOSPHORESCENCE 4.2.1 Singlet-Triplet Excitons Ratio... 

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

On the other hand, there are other series of polyaers having sc-electronic chroaophore such as N-carbazolyl and 1-pyrenyl groups, whose photophysical properties are quite different fron the above polyaers and whose laser cheaistry is studied in detail. A relation aaong interchronophoric interaction, spectral shape and geonetrical structure in the excited singlet, triplet, cationic and anionic... 

The first porphyrinylsugar derivatives prepared by Frank and co-workers, the bis(D-glucosyl)isohematoporphyrin 2a and bis(D-galactosyl)isohemato-porphyrin 3a, exhibit an unusual water solubility for a porphyrin derivative and also maintain the photophysical properties of their precursor 1, namely singlet oxygen production.11... 

We saw in the last section that because of the rapid nature of vibrational relaxation and internal conversion between excited states an electronically-excited molecule will usually relax to the lowest vibrational level of the lowest excited singlet state. It is from the Si(v = 0) state that any subsequent photophysical or photochemical changes will generally occur (Kasha s rule). 

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