Strategies for the Molecular Design of Donor–Acceptor-type Fluorescent Emitters for Efficient Deep Blue Organic Light Emitting Diodes

Abstract

We describe several strategies for the molecular design of high-efficiency blue fluorescent emitters. Asymmetric donor–acceptor (D–A) and symmetric A–D–A-type fluorescent emitters were designed with spiroacridine donors and diphenyltriazine acceptors. Substituting a toluene or xylene moiety for the phenyl group connecting the donor and acceptor and replacing the diphenylsilane group with a fluorene moiety resulted in a deeper blue emission without any losses in luminescence efficiency. On the basis of these substitutions, deep blue organic light emitting diodes (OLEDs) with Commission Internationale de L’Eclairage (CIE) coordinates of (0.149, 0.082) and an external quantum efficiency (EQE) of 7.7% were fabricated using a D–A-type emitter. Symmetrizing the D–A structure to an A–D–A structure increased the proportion of horizontally oriented emission dipoles in the organic film from 70 to 90%. OLEDs incorporating symmetric A–D–A-type emitters had EQEs as high as 8.5% due to increased outcoupling efficiencies and also showed deep blue emission with CIE coordinates of (0.142, 0.116). The molecular design strategies described herein can be applied to donor–acceptor-type fluorescent emitters for the fabrication of efficient deep blue OLEDs.

Straightforward Synthesis and Properties of Highly Fluorescent [5]‐ and [7]‐Helical Dispiroindeno[2,1‐c]fluorenes

“…In addition, steric factor of the spiro‐linkage prevents the close packing in the solid state, thus reducing the intermolecular interaction and maintaining the colour purity and spectral stability of fluorescence with usually much higher quantum yields than solely benzene ring composed compounds . For example, 9,9′‐spirobifluorenes (SBFs) have attracted considerable attention and were utilized in solar cells, organic light emitting diodes (OLEDs), transistors, lasers, and nonlinear optics . Equal attention has been focused on ortho ‐fused polycyclic aromatic compounds with a screw‐shaped π‐conjugated structure—helicenes.…”

Donor and Acceptor Fused 16,16‐Dimethyl‐11,16‐dihydrodibenzo[2,3:5,6]pyrrolizino[1,7‐ab]acridine as a Blue‐Emitting Chromophore for High External Quantum Efficiency and Long Lifetime

“…The external quantum efficiency (EQE) of D‐A type or D‐π‐A type blue fluorescent emitters was generally below 5% due to spin statistics limitation [ 23 ] and the blue emitters with above 5% EQE were mainly limited to pyrene or imidazole type materials. [ 24–31 ] Besides, the full width at half maximum (FWHM) was above 50 nm [ 31–35 ] and only very few emitters showed FWHM below 40 nm. [ 24 ] However, the D‐A and D‐π‐A type emitters also suffered from color purity due to intramolecular charge transfer (ICT) as it resulted in red‐shifted emission with broad spectrum.…”

Ternary Acceptor–Donor–Acceptor Asymmetrical Phenanthroimidazole Molecule for Highly Efficient Near‐Ultraviolet Electroluminescence with External Quantum Efficiency (EQE) >4 %

“…Another strategy is to exploit an asymmetrical molecular structure, which might not only strengthen the amorphous morphological stability, but also improve the efficiency by slightly changing the substitutes . Indeed, many reports demonstrate that molecular configuration exerts important influences on the excited state energy and thus emission color . Asymmetrical ternary acceptor (A)–donor (D)–acceptor (A) structures are often superior in carrier injection and transportation balance owing to independent and tunable charge affinities of the D and the A moieties …”

“…[14] Indeed, many reports demonstrate that molecular configuration exerts important influenceso nt he excited state energy and thuse mission color. [15][16] Asymmetrical ternary acceptor( A)-donor (D)-acceptor (A) structures are often superior in carrier injectiona nd transportation balance owing to independenta nd tunable charge affinities of the D and the Amoieties. [17][18][19] Along with these design strategies, we designed an ew near-UVe mitter,1 ,2-bis(4'- [9,10-d]i midazole (PPI-2BI, see Scheme 1), by asymmetrically attaching two bulky benzimidazole (BI) acceptoru nits to ap henanthroimidazole (PI) donor.…”