Molecular phosphorescence

Spectroscopists observed that molecules dissolved in rigid matrices gave both short-lived and long-lived emissions which were called fluorescence and phosphorescence, respectively. In 1944, Lewis and Kasha [25] proposed that molecular phosphorescence came from a triplet state and was long-lived because of the well known spin selection rule AS = 0, i.e. interactions with a light wave or with the surroundings do not readily change the spin of the electrons. 

The basic design of instrumentation for monitoring molecular fluorescence and molecular phosphorescence is similar to that found for other spectroscopies. The most significant differences are discussed in the following sections. 

Molecular Phosphorescence Instrumentation for molecular phosphorescence must discriminate between phosphorescence and fluorescence. Since the lifetime for fluorescence is much shorter than that for phosphorescence, discrimination is easily achieved by incorporating a delay between exciting and measuring phosphorescent emission. A typical instrumental design is shown in Figure 10.46. As shown... 

Block diagram for molecular phosphorescence spectrometer with inset showing how choppers are used to isolate excitation and emission. 

If the ligand triplet lies more than 2-3 kcal below the emitting level of the ions, energy is not transferred and broad molecular phosphorescence characteristic of the ligand is observed. Few other schemes are also visualized depending on the relative energy levels of ligand and the metal ions, such as ... 

Figure 120 The spectra of the two electroluminescent devices (I and II) containing organic phosphors, Ir(ppy)3 (a) (adapted from Ref. 304), and PtOEP (b) (see Ref. 493a, reprinted from Ref. 493a, Copyright 1998 Macmillan Publishers Ltd. [http //www.nature. com/]). The latter is compared with the EL spectrum of a device with no phosphor inside (III). For the chemical structures of the phosphors, see Fig. 31. The spectra from device I and II are characteristic of molecular phosphorescence as clearly seen from their comparison at different voltages with the PL spectrum (a). The DCM2-doped Alq3 layer of device III becomes dominated by their phosphorescene from the PtOEP-doped Alq3 layer in device II.

It appears that dissociation of tetrakis chelates in solution to give free ligand anions (which show phosphorescence lifetimes long with respect to those of ion fluorescence) (4) is responsible for reports of the observation of both molecular phosphorescence and ion fluorescence from solutions of certain europium -diketonates, the unequal lifetimes of which led to the suggestion that energy transfer to the europium ion came from a different triplet level (or a higher triplet level) than that from which phosphorescence is observed. 

Discuss the major reasons why molecular phosphorescence spectrometry has not been as widely used as molecular fluorescence spectrometry. 

The terbium complexes have by far the most intense fluorescence. For terbium, the state at 20,500 cm is the only ion resonance level below the 22,210 cm" triplet state of napy (8, 9, 22), The assignments for the flve observed transitions agree with several reports 23, 24), No band attributable to ligand molecular phosphorescence was observed for any of these stronger fluorescent complexes. 

Absorption then emission Emitted hght, / Atomic fluorescence Molecular fluorescence Molecular phosphorescence... 

Emission of UV/VIS radiation Molecular fluorescence, molecular phosphorescence Detection of small quantities (aromatic compounds and natural products, analysis of gels and glasses, determination of organic and inorganic species by tagging ... 

The nature of the emissive transition. Typically, allowed transitions, such as fluorescence or direct bandgap semiconductor emission, have radiative lifetimes between a few and a hundred ns. Forbidden transitions, such as molecular phosphorescence, have radiative lifetimes longer than ps, usually much longer. 

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