Triplet photophysical properties

Neat Polymer Films. It is interesting to compare the triplet photophysical properties of poly(N-vinylcarbazole) (PVCA)(16) on the one hand and poly(l-vinylnaphthalene) (PIVN)(17) on the other when each is examined as a pure polymer film. Both polymers exhibit a prominent excimer phosphorescence band as well as a distinct delayed fluorescence emission. In addition, the delayed fluorescence arises by a process of triplet-triplet annihilation for both polymers. Furthermore, the luminescence decay kinetics suggest that equilibria of the type... 

Unusual photophysical properties of polyazaanthracenes and polyazapentacenes having low values of calculated singlet-triplet energy gap 99PAC295. 

The photophysical properties of 1-8 have been studied in different solvents (PhMe, CH2CI2, and CH3CN). The lifetimes of the lowest triplet excited states are summarized in Table 1. 

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

On the other hand, there are other series of polyaers having sc-electronic chroaophore such as N-carbazolyl and 1-pyrenyl groups, whose photophysical properties are quite different fron the above polyaers and whose laser cheaistry is studied in detail. A relation aaong interchronophoric interaction, spectral shape and geonetrical structure in the excited singlet, triplet, cationic and anionic... 

A few examples to render tetrapyrrolic compounds less phototoxic can be found in the hterature. In one approach, carotenoid structures were employed for the synthesis of some carotenoporphyrin derivatives [92-94]. Figure 8 shows two stuctures by way of example. Due to similar photophysical properties of the two structural components, the excited triplet state of the porphyrin is quenched by the carotenoid moiety, thus inhibiting the formation of singlet oxygen, while its fluorescence capabilities are still preserved. Biodistribution studies revealed enhanced uptake into tumour tissue [39,93,95]. However, microscopy studies have shown that such compounds are associated with connective tissues in the tumors rather than with cancerous cells indicating low specificities for mahgnant transformation [96]. 

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand [L58]3- have been synthesized and structurally characterized (Fig. 15), and their photophysical properties studied (90,91). The bimetallic cryptates only form with the lanthanides from gadolinium to lutetium due to the lanthanide contraction. The triplet energy of the ligand (ca. 16,500 cm-1) is too low to populate the terbium excited state. The aqueous lifetime of the emission from the europium complex is less than 0.5 ms, due in part to the coordination of a solvent molecule in solution. A recent development is the study of d-f heterobimetallic complexes of this ligand (92) the Zn-Ln complexes show improved photophysical properties over the homobinuclear and mononuclear complexes, although only data in acetonitrile have been reported to date. 

Two-substituted naphthalene thienyl oligomers have been characterized for their photophysical properties (09PCCP8706). The compound 7 showed absorption at 405 nm and an emission at 475 and 502 nm. The fluorescence quantum yield was 0.27 and tf 0.56 ns. The quantum yield for the internal conversion was 0.05, while the triplet quantum yield resulted to be 0.68. 

In this chapter we have described the photophysics and photochemistry of C6o/C70 and of fullerene derivatives. On the one hand, C6o and C70 show quite similar photophysical properties. On the other hand, fullerene derivatives show partly different photophysical properties compared to pristine C6o and C70 caused by pertuba-tion of the fullerene s TT-electron system. These properties are influenced by (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in case of multiple adducts by the addition pattern. As shown in the last part of this chapter, photochemical reactions of C60/C70 are very useful to obtain fullerene derivatives. In general, the photoinduced functionalization methods of C60/C70 are based on electron transfer activation leading to radical ions or energy transfer processes either by direct excitation of the fullerenes or the reaction partner. In the latter case, both singlet and triplet species are involved whereas most of the reactions of electronically excited fullerenes proceed via the triplet states due to their efficient intersystem crossing. 

III. OPTICAL AND PHOTOPHYSICAL PROPERTIES A. Energy Gap Law for Triplet States... 

The four meso positions of porphyrin have been substituted with several O-containing cycles, including benzodioxolan, benzodioxane, benzo-12-crown-4, benzo-15-crown-5, and benzo-18-crown-6 ethers (fig. 22) in an attempt to determine the influence of coordinated alkali ions on the Ybm photophysical properties (Korovin et al., 2001). The triplet state in the [Yb(10)(acac)] complexes lies in the range 13 775-13 955 cm-1 and upon excitation at 532 nm, the 5Fs/2 2F7/2 transition is seen for all five complexes. 

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