Triplet-excitons

In previous sections a hidden assumption was made that an excitation of a molecule does not change its spin. Since the majority of molecules have spin zero in the ground state, this assumption is valid only for singlet excitons, i.e. excitons which are created by transitions of crystal molecules in one of their excited singlet states. 

At the same time we must have in mind that for the majority of aromatic molecular crystals such as, for example, naphthalene, the lowest molecular electron excited state is a triplet state. In these circumstances triplet excitons exhibit a number of specific properties, which has been used for their study. 

We notice that singlet-triplet transitions in molecules are forbidden by spin selection rules and the lifetime of triplet states in crystals exceeds by many orders 

10A representation is called real when, in a linear function space corresponding to it, a basis can be chosen such that matrices of all operators of this representation are real. 

11 In the case of triplet excitons this problem was discussed by Sternlicht and McConnell (26). 

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Kolubayev T, Geacintov N E, Paillotin G and Breton J 1985 Domain sizes in chloroplasts and chlorophyll-protein complexes probed by fluorescence yield quenching induced by singlet-triplet exciton annihilation Biochimica Biophys. Acta 808 66-76... 

The impurities may capture this migrating exciton and lose its excess energy. The mutual annihilation of two or more triplet excitons occurs in the same polymer chain and delayed fluorescence is observed. 

The zero Field splitting (ZFS) parameters D and of the triplet exciton are ... 

Figure 7-30. A-PADMR spectra of P3UT films, measured at [lie resonant fields for spin-1/2 species (solid line) and triplet excitons (dashed line).

The lowest excited states in molecular crystals are singlet and triplet excitons [3]. Since it costs coulombic energy to transfer an electron that has been excited optically from the HOMO (highest occupied molecular orbital) to the LUMC)... 

AR Brown, K Pichler, NC Greenham, DDC Bradley, RH Friend, and AB Holmes, Optical spectroscopy of triplet excitons and charged excitations in poly(p-phenylenevinylene) light-emitting diodes, Chem. Phys. Lett., 210 61-66, 1993. 

Z Shuai, D Beljonne, RJ Silbey, and JL Bredas, Singlet and triplet excitons formation rates in conjugated polymer light-emitting diodes, Phys. Rev. Lett., 84 131-134, 2000. 

M Wohlgenannt, K Tandon, S Mazumdar, S Ramasesha, and ZV Vardeny, Formation cross-sections of singlet and triplet excitons in TT-conjugated polymers, Nature, 409 494-497, 2001. 

B Hu, Y Wu, Z Zhang, S Dai, and J Shen, Effects of ferromagnetic nanowires on singlet and triplet exciton fractions in fluorescent and phosphorescent organic semiconductors, Appl. Phys. Lett., 88 022114, 2006. 

F. Chen, G. He, Y. Yang, Triplet exciton confinement in phosphorescent polymer light-emitting diodes, Appl. Phys. Lett., 82 1006-1008 (2003). 

PHOTOPHYSICAL ASPECTS OF ELECTROPHOSPHORESCENCE 4.2.1 Singlet-Triplet Excitons Ratio... 

FC Chen, SC Chang, G He, S Pyo, Y Yang, M Kurotaki, and J Kido, Energy transfer and triplet exciton confinement in polymeric electrophosphorescence devices, J. Polym. Sci. B Polym. Phys., 41 2681-2690, 2003. 

Exciton decay When an exciton decays radiatively a photon is emitted. When the excitons form in fluorescent materials radiative decay is limited to singlet excitons and emission occurs close to the recombination region [7] of the OLED due to the relatively short lifetime of the excited state (of the order of 10 ns). For phosphorescent materials, emission can occur from triplet excitons. Due to the longer excited state lifetime (of the order of hundreds of nanoseconds), triplet excitons can diffuse further before decaying. 

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