[Show abstract][Hide abstract] ABSTRACT: Fundamental photophysical properties of the phosphorescent organometallic complex Ir(btp)2(acac) doped in the polymeric matrices PVK, PFO, and PVB, respectively, are investigated. PVK and PFO are frequently used as host materials in organic light emitting diodes (OLEDs). By application of the laser spectroscopic techniques of phosphorescence line narrowing and persistent spectral hole burning – improved by a synchronous scan technique – we studied the zero-field splitting (ZFS) of the T1 state into the substates I, II, and III. Thus, we were able to probe the effects of the local environment of the emitter molecules in the different amorphous matrices. The magnitude of ZFS is determined by the extent of spin–orbit coupling (SOC) of the T1 state to metal-to-ligand charge transfer (MLCT) states. Only by mixings of MLCT singlets, a short-lived and intense emission of the triplet state to the singlet ground state becomes possible. Thus, sufficiently large ZFS is crucial for favorable luminescence properties of emitter complexes for OLED applications. The analysis of the spectral hole structure resulting from burning provides information about the ZFS values and their statistical (inhomogeneous) distribution in the amorphous matrices. For Ir(btp)2(acac), we found a significant value of ≈18 cm−1 for the splitting between the substates II and III for all three matrices. Interestingly, for PVK the width of the ZFS distribution is found to be ≈14 cm−1 – almost twice as large as for PFO and PVB. Consequently, for a considerable fraction of Ir(btp)2(acac) molecules in PVK, the ZFS is relatively small and thus, the effective SOC is weak. Therefore, it is indicated that a part of the emitter molecules shows a limited OLED performance.
[Show abstract][Hide abstract] ABSTRACT: The detection of protein-protein binding on microarrays using the fluorescence lifetime as a dynamic analytical parameter was investigated in a model system. The assay is based on Förster resonance energy transfer (FRET) and carried out with biotinylated Bovine Serum Albumin and streptavidin, labeled with the commonly used microarray dyes Alexa 555 and Alexa 647, respectively. This efficient FRET donor/acceptor pair was employed in a competitive assay format on three different microarray surfaces. The fluorescence was excited by 200ps laser pulses from a mode-locked and cavity-dumped argon-ion laser, adapted to an intensified CCD camera as detection unit allowing time resolution with subnanosecond precision. Lifetime maps were recorded according to the Rapid Lifetime Determination (RLD) scheme. Interaction between the proteins could clearly be detected on all formats and resulted in almost complete quenching on CEL Epoxy surfaces upon addition of excess streptavidin labeled the FRET acceptor dye. In this case, the fluorescence lifetimes dropped by 90%, whereas on ARChip Epoxy and ARChip Gel the reduction was 54% and 47%, respectively. Good linearity of the quenching curve was obtained in all cases. The method is applicable to all types of protein interaction analysis on microarrays, particularly in cases where evaluation of fluorescence intensity is prone to erroneous results and a more robust parameter is required.