Exciton fission

Triplet-triplet Interaction, Singlet Exciton Fission... 

The singlet exciton fission [/s in Eq. (80)] is a process reducing the concentration of singlet excitons, thus, it should show up in a low fluorescence efficiency of a material. According to Eq. (82) the materials with 2ET less or comparable with Es are expected to reveal an efficient fission process. Such relations hold for example in solid pentacene CESl=1.9eV,... 

E i1 = 0.9eV), tetracene CEg1 = 2.37eV, Ex,— 1-27 eV) and rubrene (Sg1=2.33eV, Ex,— 1-2 eV), and their fluorescence quantum yield is known to be very low at room temperature, strongly increasing as temperature decreases. Examples are shown in Fig. 34. In the case of neat tetracene and rubrene, the fluorescence efficiency increases as the temperature is lowered because of the suppression of the thermally activated singlet exciton fission channel [cf. y s in (80) and (82)]. 

Due to the spin conserving rule, the rate constants of some excitonic interactions are subject to modulation by weak magnetic fields (less than IT). The magnetic field modulation of the singlet exciton fission rate [see Scheme (80)] can be experimentally observed as a magnetic field effect on prompt fluorescence [192,200,201,273-278]. The magnetic field sensitive triplet-triplet (80) and triplet-doublet (110) annihilation... 

Table 3 Singlet Exciton Fission Parameters11 for a Tetracene Single Crystal at Two Different Hydrostatic Pressures...

The complementary process to fusion is exciton fission or splitting (Fig. 6.25). Flere, an exciton decays into two other excitons of correspondingly lower energies, for example... 

Fig. 6.25 Exciton fission, schematically, a The excitation takes place from the singlet absorption region of the crystal. The singlet exciton splits into two triplet excitons. Their emission is observed, b When the energy of the excited S-state is somewhat smaller than twice the energy T-, then exciton fission can be activated by the thermal energy AE. After [43].

The ground state bleach signal decayed with the same lifetime as the singlet exciton population in the dilute case, but showed a doubling in magnitude over the first 10 ns in the concentrated solution, indicative of exciton fission from a localized state. 

Congreve DN, Lee J, Thompson NJ, Hontz E, Yost SR, Reusswig PD, Bahlke ME, Reineke S, Van Voorhis T, Baldo MA (2013) External quantum efficiency above 100 % in a singlet-exciton-fission-based organic photovoltaic cells. Science 340 334-337... 

Wilson MWB, Rao A, Ehrler B, Friend RH (2013) Singlet exciton fission in polycrystalline pentacene from photophysics toward devices. Accounts Chem Res 46 1330-1338... 

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