| Abstract
| - Because of their unique photophysical properties, organic semiconductors have shown greatpromise in both light-emitting devices (LEDs) and photovoltaic systems. In particular, the question of spinstatistics looms large in these applications: the relative energetics and rates of formation for singlet versustriplet excited states can have a significant impact on device efficiency. In this Article, we study the singletand triplet charge-transfer (CT) configurations that can be thought of as the immediate precursors to theluminescent states in organic LEDs. In particular, we find that the CT singlet−triplet energy gap (ΔEST) oforganic dyes and oligomers depends sensitively on both the material and the relative orientation of thedonor/acceptor pair. Furthermore, in contrast with the commonly held view, we find that the singlet CTstates nearly always lie energetically below the triplet CT states (ΔEST< 0). This trend is attributed to twophysical sources. First, the relatively close contact between the donor and acceptor leads to a strong kineticexchange component that favors the singlet. Second, Coulombic attraction between the separated chargesfavors inner-sphere reorganization that brings the donor and acceptor closer together, further enhancingthe kinetic exchange effect. We discuss the implications of these results on the design of organic LEDs.
|
