Mechanism of Ir(ppy)3 Guest Exciton Formation with the Exciplex-Forming TCTA:TPBI Cohost within a Phosphorescent Organic Light-Emitting Diode Environment.

Cohosts based on hole transporting and electron transporting materials often act as exciplexes in the form of intermolecular charge transfer complexes. Indeed, exciplex-forming cohosts have been widely developed as the host materials for efficient phosphorescent organic light-emitting diodes (OLEDs). In host-guest systems of OLEDs, the guest can be excited by two competing mechanisms, namely, excitation energy transfer (EET) and charge transfer (CT). Experimentally, it has been reported that the EET mechanism is dominant and the excitons are primarily formed in the host first and then transferred to the guest in phosphorescent OLEDs based on exciplex-forming cohosts. With this, exciplex-forming cohosts are widely employed for avoiding the formation of trapped charge carriers in the phosphorescent guest. However, theoretical studies are still lacking toward elucidating the relative importance between EET and CT processes in exciting the guest molecules in such systems. Here, we obtain the kinetics of guest excitation processes in a few trimer model systems consisting of an exciplex-forming cohost pair and a phosphorescent guest. We adopt the Förster resonance energy transfer (FRET) rate constants for the electronic transitions between excited states toward solving kinetic master equations. The input parameters for calculating the FRET rate constants are obtained from density functional theory (DFT) and time-dependent DFT. The results show that while the EET mechanism is important, the CT mechanism may still play a significant role in guest excitations. In fact, the relative importance of CT over EET depends strongly on the location of the guest molecule relative to the cohost pair. This is understandable as both the coupling for EET and the interaction energy for CT are strongly influenced by the geometric constraints. Understanding the energy transfer pathways from the exciplex state of cohost to the emissive state of guest may provide insights for improving exciplex-forming materials adopted in OLEDs.

Considering both small and large scale motions of vascular endothelial growth factor (VEGF) is crucial for reliably predicting its binding affinities to DNA aptamers.

Vascular endothelial growth factor 165 (VEGF165), a predominant isoform of VEGF signal proteins, is an ideal target for developing drugs against various diseases. It is composed of a heparin binding domain (HBD) and a receptor binding domain (RBD), which are connected by a flexible linker. Among the two domains, RBD is utilized in binding with the signal transduction protein, the VEGF receptor (VEGFR). None the less for its pharmaceutical importance, structure-based studies for developing drugs has been severely hindered by the lack of its whole structure determination, mainly owing to the existence of the flexible linker. Fortunately, the utilization of computer simulation methods can offer a possibility to circumvent this difficult issue. Here, we employ ensemble docking in combination with the anisotropic network model analysis to examine the interactions between DNA aptamers and VEGF165. We model three-dimensional structures of aptamer variants based on their sequence information and perform docking calculations with the whole VEGF165 structure. Indeed, we show that we can closely reproduce the experimental binding affinity order among different DNA aptamer variants by inclusively considering the flexible nature of VEGF. In addition, we address how DNA aptamer that binds to HBD of VEGF165 impedes the interaction between VEGFR and VEGF165 through RBD, even though HBD and RBD are rather distant. The present study illustrates that the flexible docking scheme employed here can be applied to tricky cases that involve flexible proteins with undetermined structures, toward effectively predicting ligand binding affinities to such proteins.