title: On the singlet-triplet splitting of geminate electron-hole pairs in organic semiconductors.
abstract: Because of their unique photophysical properties, organic semiconductors have shown great promise in both light-emitting devices (LEDs) and photovoltaic systems. In particular, the question of spin statistics looms large in these applications: the relative energetics and rates of formation for singlet versus triplet excited states can have a significant impact on device efficiency. In this Article, we study the singlet and triplet charge-transfer (CT) configurations that can be thought of as the immediate precursors to the luminescent states in organic LEDs. In particular, we find that the CT singlet-triplet energy gap (deltaE(ST)) of organic dyes and oligomers depends sensitively on both the material and the relative orientation of the donor/acceptor pair. Furthermore, in contrast with the commonly held view, we find that the singlet CT states nearly always lie energetically below the triplet CT states (deltaE(ST) < 0). This trend is attributed to two physical sources. First, the relatively close contact between the donor and acceptor leads to a strong kinetic exchange component that favors the singlet. Second, Coulombic attraction between the separated charges favors inner-sphere reorganization that brings the donor and acceptor closer together, further enhancing the kinetic exchange effect. We discuss the implications of these results on the design of organic LEDs.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA.
authors.forename: Seth
authors.initials: S
authors.lastname: Difley
authors.forename: David
authors.initials: D
authors.lastname: Beljonne
authors.forename: Troy
authors.initials: T
authors.lastname: Van Voorhis
mesh_terms: D004396:Coloring Agents / Q000737:chemistry*; D003198:Computer Simulation; D004583:Electrons; D007700:Kinetics; D008027:Light; D008163:Luminescent Measurements; D008956:Models, Chemical; D008970:Molecular Weight; D009942:Organometallic Compounds / Q000737:chemistry; D010777:Photochemistry; D012666:Semiconductors; D012680:Sensitivity and Specificity
chemical_list: D004396:Coloring Agents; D009942:Organometallic Compounds