Mechanism, photochemical phosphorescence

In recent years luminescence nomenclature has become confusing within the literature and in practice. Luminescence involves both phosphorescence and fluorescence phenomena. While luminescence is the appropriate term when the specific photochemical mechanism is unknown, fluorescence is far more prevalent in practice. Moreover, the acronym LIE has historically inferred laser -induced fluorescence however, in recent years it has evolved to the more general term light -induced fluorescence due to the various light sources found within laboratory and real-time instruments. Within this chapter fluorescence and LIE are interchangeable terms. 

The energy of a single photon is obviously insufficient to ionize an organic compound. As early as the nineteen forties (3, 4), however, it -was observed that Wurster blue cation radical is produced by photoirradiation of 3-methylpentane glass containing N,N-tetramethyl p-phenylenediamine (TMPD) at 77° K. The recent detailed study of this system by electric conductivity measurement (5, 6) and electronic spectroscopy (7) provided conclusive evidence that the ionization is brought about via excitation to the triplet state followed by successive photoabsorption at the triplet state. This mechanism is supported by the facts that the life-time of the photochemical intermediate is identical with that of phosphorescence and the formation of Wurster blue, and that phosphorescence is inhibited in the presence of triplet scavengers. 

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

An Example The Phosphorescence of Dithiosuccinimide Many thio-carbonyls have photostable excited (n > ji ) and (ti —> ti ) states that tend to relax by photophysical rather than photochemical processes.177,178 Recently, the electronic spectra of dithioimides have been under experimental and theoretical investigation.179-181 The spin-forbidden radiative decay of the lowest-lying triplet state of dithiosuccinimide may serve as an example to illustrate the results of the previous sections. Experimentally a lifetime of 0.10 0.01 ms was determined for the Ti state.179 This value has been corrected for solvent effects, but the transition may include radiative as well as nonradiative depletion mechanisms. 

FIGURE 1. A schematic photochemical mechanism, showing some of the possible elementary transformations. For the purpose of illustration, it is assumed that the states A and A2 have the same multiplicity, and correspond to the ground and lowest excited singlet states of most organic molecules. The state A] would then represent the lowest triplet state. Thus 21 and 11 are radiative transitions, fluorescence and phosphorescence, respectively, and 23 and 13 (intersystem crossing) and 22 (internal conversion) are nonradiative. All of 8, C, D, and F are chemical species distinct from A. Only vibrationally equilibrated electronic states are included in this mechanism (see discussion in Section III.A.l). 

The questions to be addressed in this context have historically been characterized as photophysical and photochemical, categories whose boundaries are sometimes ill-defined. The photophysical steps include intrastate vibrational relaxation, photon emission (fluorescence and phosphorescence), and interstate radiationless transitions (internal conversion between states of the same multiplicity and intersystem crossing between states of different multiplicity). The major aim in this area is to determine the rates of the individual steps and the relationship between molecular structure and these rates. Other goals are to identify the photoactive state and to detail the reaction mechanism. 

B2) Tttrro, N. J., and R. Engel Quenching of Biacetyl Fluorescence and Phosphorescence. A New Mechanism for Quenching of Ketone Excited States. Mol. Photochem. 1, 143 (1969). 

Although considerable evidence > is presented for involvement of protein triplet states in this phenomenon of afterglow, the mechanism is not that usually associated with the production of phosphorescence. Rather, in the above instances the UV irradiation causes photochemical damage resulting in the production of free radicals 9>i ). The afterglow is the result of luminescence from chemically excited states formed upon radical recombinations. 

Many materials benefit from analysis at cryogenic temperatures. At 77 K, enhanced fluorescence and phosphorescence are seen, and low-temperature studies are used for elucidation of the mechanisms of photochemical reactions, characterizing bandgap changes in semiconductors and other applications. 

In many cases, the proper description of the rates and mechanisms of photochemical reactions also requires knowledge of processes such as fluorescence and phosphorescence that can deactivate an excited state before the reaction has a chance to occur. Electronic absorption takes place in about 10 -10 s, and because fluorescence lifetimes are typically 10" -10 s, an excited singlet state can initiate very fast photochemical reactions in the range from femtoseconds (10 s, the time it takes to excite a molecule) to picoseconds (10 s, the lifetime of the excited state). Examples of such ultrafast reactions are the initial events... 

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