The correlated triplet-pair or "multi-exciton" state, 1 (T 1 T 1 ), represents a true double excitation in electronic structure terms. Following decoherence, SF ultimately results in two charge carriers (T 1 + T 1 ) for the price of a single photon. This photochemical two-for-one has the potential to overcome the thermodynamic limit on conversion efficiency for one-to-one processes. However, there are basic mechanistic questions that are still being investigated, including the role of low-energy CT states, 259,273-277 vibronic coherence, the nature of exciton/phonon couplings, and whether the 1 (T 1 T 1 ) state may represent a trap rather than an intermediate. In the electron-hole correlation plots shown in Fig. , the origin (r = 0) corresponds to zero net separation between electron and hole (r elec = r hole ), but the plots do not indicate significant probability there. Rather, the regions of highest probability in the xy plane are those around (x = 0, y = ±1 nm), indicative of charge separation between nearest-neighbor molecules, although the extent of F(r) indicates delocalization over as many as three molecules. This leads to an exciton length of > 5 Å, as determined by Eq. (5.8), with ≈ 50% CT charactert according to the definition in Eq. (5.9). In contrast, plots in the xz plane of the crystal indicate no delocalization in the z direction, which is attributable to the large intermolecular arising from bulky substituent groups.