Construction of high efficiency non-doped deep blue emitters based on phenanthroimidazole: remarkable substitution effects on the excited state properties and device performance

Abstract

Aryl-substituted phenanthroimidazoles (APIs) are beneficial due to their facile synthesis, thermal properties, high quantum yields, and exciton efficiencies obtained by a reverse intersystem crossing (RISC) process. However, it is puzzling how to combine high quantum yields, exciton utilizing ratios and color purity with stable blue-emitting compounds via coupling. Here, BPPI and N-BPPI are utilized as model compounds for understanding C2- and N1-substitution effects via constructing dimers in each coupling position. By integrating the information obtained from DFT calculations, photophysical analysis, and OLED performance, valuable guidance was obtained. C2-substituted groups typically offer a large orbital overlap between the LE states with large oscillator strengths, and play an important role in the maximum peak area and quantum yields. N1-substituted groups contribute to enhanced orbital coupling and cause excitons to transform freely between different excited states. Unexpected results from the decreased barriers of the N1-coupled system included the loss of PL efficiency and increased emission spectral width, which are important for efficiency and color purity of deep blue emitters. The substitution effects are consistent with most reported results. Therefore, this work may be useful for the generation of non-doped deep blue electroluminescent API-based materials.

Recent advances in twisted intramolecular charge transfer (TICT) fluorescence and related phenomena in materials chemistry

“…9a). 64,65 Electrons and holes possess degrees of freedom not only in orbitals, but also in their spin. Thus, to maximise the quantum efficiency of OLEDs, 25% of singlet excitons and 75% of triplet excitons, generated by arbitrary recombination of carriers, must be utilised as either fluorescence or phosphorescence.…”

Recent Progress in High‐Efficiency Blue‐Light‐Emitting Materials for Organic Light‐Emitting Diodes

“…The merits of blue‐shifted emission spectrum and narrow emission spectrum in addition potentially high efficiency of the HLCT mechanism based emitters encouraged the development of blue HLCT emitters. Since the first demonstration of enhanced fluorescent emission by the CT state assisted emission process, several blue emitters were reported . Initial blue fluorescent material identified as the HLCT emitter was N,N ‐diphenyl‐4′‐(1‐phenyl‐1 H ‐phenanthro[9,10‐ d ]imidazol‐2‐yl)biphenyl‐4‐amine (TPA‐PPI) with a twisted donor–acceptor structure .…”

Imidazole derivatives for efficient organic light-emitting diodes

“…A more balanced carrier mobility was realized in CN-BPPI due to the substitution of cyano (CN). The three isomers of BPPI, L-BPPI, Z-BPPI, and N-BPPI, were prepared by tuning the substituted position on the biphenyl from para-to meta-coupling and the N1 position [36,37]. Due to the conjugated degree change at the C2 position in the PI block, the fluorescent colors of L-BPPI (433 nm) and Z-BPPI (402 nm) showed an obvious blueshift in the films compared with that of BPPI (468 nm).…”