Singlet emission

Figure 9-15. Triplet absorption, singlet emission, and absorption (from left to right) of (from top to bottom) ni-LPPP [I44 and hexaphcnyl films 1145] at 90 K, and /n/ra-quater- and iciplienyl at 77 K in tetrahydro-2-methylfijran [146].

At least experiments with fluoranthene and other aromatic hydrocarbons having lower triplet energies than fluoranthene resulted only in fluoranthene singlet emission- this excited singlet appears to be generated directly and not by triplet-triplet annihilation. 

Comparison of Half-Wave Potential Increments and Singlet Emission Energies... 

V. Triplet-Singlet Emission from Phenanthrene Solutions. 341... 

V. Triplet—Singlet Emission from Phe nanthrene Solutions... 

Although at high rates of light absorption the square law dependence on rate of light absorption is no longer obeyed, the intensity of P-type delayed fluorescence is still proportional to the square of the intensity of triplet-singlet emission because the latter is always proportional to... 

The simple triplet-triplet quenching mechanism requires that at low rates of light absorption the intensity of delayed fluorescence should decay exponentially with a lifetime equal to one-half of that of the triplet in the same solution. Exponential decay of delayed fluorescence was, in fact, found with anthracene, naphthalene, and pyrene, but with these compounds the intensity of triplet-singlet emission in fluid solution was too weak to permit measurement of its lifetime. Preliminary measurements with ethanolic phenanthrene solutions at various temperatures indicated that the lifetime of delayed flourescence was at least approximately equal to one-half of the lifetime of the triplet-singlet emission.38 More recent measurements suggest that this rule is not obeyed under all conditions. In some solutions more rapid rates of decay of delayed fluorescence have been observed.64 Sufficient data have not been accumulated to advance a specific mechanism but it is suspected that the effect may be due to the formation of ionic species as a result of the interaction of the energetic phenanthrene triplets, and the subsequent reaction of the ions with the solvent and/or each other to produce excited singlet mole-... 

E-Type Delayed Fluorescence. (Produced by thermal activation of molecules from the triplet level to the upper singlet level.) The contour of its spectrum is identical with that of normal (short-lived) fluorescence. The intensity relative to that of the triplet-singlet emission decreases exponentially with the reciprocal of the absolute temperature and the activation energy is equal to the frequency difference between the two bands. The intensity is proportional to the first power of the rate of absorption of exciting light. The lifetime is the same as that of the triplet-singlet emission in the same solution under the same conditions. 

While the general applicability of the Cundall method to all situations may be questioned even when the proper energy relationships exist,34 there seem to be no reasonable doubts about its applicability to benzene.29,35 Neither of the two butenes quenches the singlet emission from benzene in the 1B2u state, and the triplet states of benzene which might be of importance all lie above the triplet state of the 2-butene (which should be nearly the same for both the cis and trans forms). 

A soluble silole-containing polyplatinayne 51 was prepared.47 As compared to 2,5-dibromo-l,l-diethylsilole (Xmax = 326 nm), the positions of the low-lying shoulder bands (Xmax = 504 nm in CH2C12) are remarkably red-shifted by 178 nm for 51 after the inclusion of heavy-metal chromophores. This is possibly due to the intramolecular D-A interaction between the electron-rich metal ethynyl unit and the electron-poor silole ring. The Ee value is impressive at 2.10 eV for 51, and it is significantly lowered by 1.0 eV relative to 53 (3.10 eV).48 Compound 51 is photoluminescent with the singlet emission band at 537 nm. No room temperature emission from the Ti state was detected over the measured spectral window. 

Excitation of room temperature deaerated solutions of selected heterocyclic-substituted platinum-1,2-enedithiolates leads to a dual emission, which is characteristic of this class of molecules (Fig. 3) (17-19, 22-24, 29-31). As can be seen in Fig. 3, the triplet emission is diffusionally quenched by oxygen while the singlet emission is insensitive to oxygen. 

Figure 188 PL and EL spectra of light-emitting diodes of the platinum-containing monomer and polymer at 290 K. The triplet emission is denoted by Ti and the singlet emission by Si. The percentual numbers provide the fraction of the numbers of singlet and triplet emitted photons with respect to the totally emitted photons (the larger numbers of Si and smaller numbers for Ti characterize the PL spectra). Reprinted by permission from Ref. 614. Copyright 2001 Macmillan Publishers, Ltd. [http //www.nature.com/].

The emission displayed in Fig. 12b has been assigned as a phosphorescence (triplet singlet emission) due to the relatively long decay time of 12 ps at 80 K in butyronitrile and of 5 ps at 293 K in acetonitrile. The quantum yield is reported to be as high as 30% at room temperature [53]. The structure in the emission spectrum is assigned to vibrational satellites as will be shown below (Sect. 4.2.4). 

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