Organic Triplet State Molecules

The magnitude of the rfs can be obtained directly from the powder spectmm when 1st order conditions apply (D << giMsB). These conditions are approximately satisfied for many triplet state organic molecules, such as biradicals and radical pairs and for several transition metal ions even at X-band. On the other hand the sign of the Tfs can only in exceptional cases (see Section 4.5) be determined at X-band. This can, however, be done more generally by measurements at high field by comparing the relative intensities of the different transitions, e.g. = 1 0 and ms = 0 -o- -1 for the triplet state. 

It has been found that certain dyes, such as methylene blue, eosin and Rose Bengal, sensitize the photo-oxidation of many organic compounds in solution [Refs. 242, 246-248, 328, 329, 371, 585, 639, 668]. More detailed investigations have shown that the photo-oxidation reaction takes place with the participation of singlet oxygen formed by the reaction between the dye excited to the triplet state and molecular oxygen. The energy of the triplet state dye molecules (Table 15) is sufficient to... 

Room-temperature phosphorescence in solution has been observed in organized media containing micelles. With micelles, the analyte is incorporated into the core of the micelle, which serves to protect the triplet state. Cyclodextrin molecules, which are... 

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

Ah initio programs attempt to compute the lowest-energy state of a specified multiplicity. Thus, calculations for different spin states will give the lowest-energy state and a few of the excited states. This is most often done to determine singlet-triplet gaps in organic molecules. 

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... 

In 1944, Lewis and Kasha (52) identified phosphorescence as a forbidden" transition from an excited triplet state to the ground singlet state and suggested the use of phosphorescence spectra to identify molecules. Since then, phosphorimetry has developed into a popular method of analysis that, when compared with fluorometry, is more sensitive for some organic molecules and often provides complimentary information about structure, reactivity, and environmental conditions (53). 

As mentioned in the introductory part of this section, quantum dots exhibit quite complex non-radiative relaxation dynamics. The non-radiative decay is not reproduced by a single exponential function, in contrast to triplet states of fluorescent organic molecules that exhibit monophasic exponential decay. In order to quantitatively analyze fluorescence correlation signals of quantum dots including such complex non-radiative decay, we adopted a fluorescence autocorrelation function including the decay component of a stretched exponential as represented by Eq. (8.11). 

Triplet state (cont d) intersystem crossing quantum yields, table of, 239-240 lifetime, 12 lowest triplet energies of carbonyls, table of, 224-225 of hydrocarbons, table of, 226 of various organic molecules, table of, 227... 

The quantum efficiency of fluorescence of a molecule is decided by the relative rates of fluorescence, internal conversion and intersystem crossing to the triplet state. Up to the present time it has proved impossible to predict these relative rates. Thus, whilst it is now possible to calculate theoretically the wavelengths of maximum absorption and of maximum fluorescence of an organic molecule, it remains impossible to predict which molecular structures will be strong fluorescers. Design of new FBAs still relies on semi-empirical knowledge plus the instinct of the research chemist. 

Organic Triplet State Molecules and the Dipolar Interaction... 

ESR Spectra of Biradicals, Triplet States, and other S> 1/2 Systems Table 6.1 ESR data for some organic triplet-state molecules... 

This process — also called sensibilisation — is of importance in preparative organic photochemistry 115) whenever a rate constant of intersystem crossing fcSl Tl for a given molecule is small or if direct irradiation of this molecule is inconvenient. This principle is illustrated in Figure 8 for benzo-phenone as donor and a diene as acceptor, the diene then isomerizing from the triplet state. 

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