Low temperature phosphorescence

The low temperature phosphorescence spectrum of benzophenone (see Figure 1) has a well-resolved structure, in which the splitting corresponds to the carbonyl vibrational frequencies. From this structure, one can determine that the triplet energy of benzophenone is approximately 69 keal/mol. (70)... 

Table 1. Low-temperature phosphorescence properties of fresh stains on cloth...

Remarkably, one study [19] reported that ODPM rearrangement is also possible from the singlet excited state, but this is a matter of debate. For the triplet excited state of these (3,y-unsaturated ketones, it was clearly pointed out that it is nn in origin based on low temperature phosphorescence measurements, theoretical and mechanistic studies [20-23]. Furtheron, these phosphorescene and mechanistic studies indicated that the 37i7i excited state is approx. 8-12kcal/mol lower to the corresponding 3nn excited state [25]. 

FIGURE 15-13 Dewar flask and cell lor low-temperature phosphorescence measurements. The optical path traverses the unsilvered part of the flask. 

Molecules A" can also emit, and thereby return to the ground state. This process is also forbidden as for the absorptive transition [3]. Therefore its probability is very small. It is called P- or low-temperature phosphorescence (P in Fig. 1.1) ... 

An IL assignment is frequently supported by the appearance of a vibrational structure of the low-temperature phosphorescence. 

Turro to be a strong phosphor at room temperature in deaerated P-cyclodextrin solution (28). Indeed, halonaphthalenes were among the examples employed by early workers to establish the favourable heavy atom effect on low temperature phosphorescence (36,37). The other essential piece of inspiration came from... 

SS-RTP exhibits in all cases poorer detection limits than low-temperature phosphorescence. However, it is potentially useful as a routine analytical method because (1) no cryogenic equipment, expensive and rather cumbersome to use, is needed (2) no time-consuming degassing of the solvent is mandatory and (3) chromatographic separations can be performed on the substrate before the analysis. These features make SS-RTP particularly suitable to exploit new detection schemes. So, SS-RTP is a convenient means of observing delayed fluorescence in those... 

Low-volume micro cells are available for situations in which sample volumes are limited. Several companies make flow cells for fluorescence detection in chromatography and in continuous flow analysis. Samplehandling accessories include micro-plate readers, microscope attachments, and fiber-optic probes. Low-volume cells are often used for room-temperature phosphorescence and for chemiluminescence. Special cells and sample handling are needed for low-temperature phosphorescence measurements. 

Fig. 3. Energy levels of a molecule. G ground state, E, first electronic state, Es upper electronic state. Tj first triplet state. Small numbers indicate the first few vibrational levels of each electronic state. G-Ei and G-Ej are the transitions in absorption. Es-Ei radiationless transition that precedes fluorescence. Ei-G transitions in fluorescence. G-T, absorption transition to the first triplet. Ti-G transition from the first triplet state to the ground state, believed to be responsible for the low temperature phosphorescence. Allowed transitions in full line. Forbidden transitions are dotted.

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