The Balance between Charge Mobility and Efficiency in All-Solution-Processed Organic Light-Emitting Diodes of Zn(II) Coordination Compounds/PFO Composites

Abstract

Zn(II) coordination compounds with asymmetric salicylidene ligands in poly [9,9-dioctylfluorenyl-2,7-diyl] (PFO) composites were used for all-solution-processable white organic light-emitting diode (WOLED) purposes. These Zn(II) complexes were strategically synthesized to optimize their electronic structures, starting from the widespread known salophen core. Vertical excitation energies and natural transition orbital (NTO) densities of Zn(salicylidenes) were evaluated using density functional theory/time-dependent density functional theory (DFT/TD-DFT) at the PBE0/6−311++G(d,p) level of theory, presenting good accuracy to their experimental optical properties. Diodes were assembled with Zn(salicylidenes) dispersed into a PFOconducting polymer matrix at 0.1, 0.5, 1.0, and 2.5 % mol•mol -1 with the architecture: ITO|PEDOT:PSS|PVK|PFO:Zn(salicylidene)|Ca|Al. The electrical properties of the diodes are dependent on the relative concentration of the coordination compounds in the polymer matrix, where an optimized performance was obtained using diluted samples. The modified Zn(salophen) derivatives (II and III) also presented greater performance compared to the precursor Zn(salophen) (I). In addition, interesting color tunability was obtained depending on the device composition, and for Zn(BTS) with 0.1 and 0.5%, we obtained coordinated color emission very similar to white (0.27, 0.36) and (0.29, 0.41), respectively. The observed color was a composition of the polymeric blue emission and green emission from the coordination compounds. The discussion of the efficiency was based on the determination of the carrier mobilities using the trap-charge limited current (TCLC) model. According to these values, the best performance of the diodes with Zn(BTS) is due to a pronounced increase of the charge mobility compared to the polymer itself and other coordination compounds.

All-Solution Processed Single-Layer WOLEDs Using [Pt(salicylidenes)] as Guests in a PFO Matrix

“…As we can see, by coordination with the metal, the salophen framework has undergone a considerable improvement in molecular planarity and symmetry, conferring molecular geometry that is close to the C 2v point group, as previously described elsewhere for Zn(II) coordination [ 51 , 53 ]. Nevertheless, in the case of the sal-3,4-ben ligand, regardless of the increase in planarity of the salophen moiety after coordination, its molecular structure still exhibits a non-symmetric system (C 1 point group), due to the presence of the benzoyl moiety.…”

“…For comparison, we also recorded the PL spectrum of neat PFO (represented by the black curves in Figure 4 ). This spectrum is characteristic of PFO in a solid state, exhibiting a mixture of α-phase (amorphous) and β-phase, with the zero-phonon band at λ amorphous = 425 nm and λ β-phase = 437 nm, respectively [ 53 , 58 , 59 ]. The PFO spectra in the composites are different, indicating that the presence of the Pt(II) compounds impacts the PFO’s morphology.…”

“…In addition, a greenish band appears, λ EL = 525 nm, which is due to the PFO chain aggregates [ 22 ]. This band is almost absent in the neat PFO [ 53 ]. These significant differences between the PFO emission profiles in the PL and EL spectra may be attributed to the different substrates used to process the emissive layer.…”

Theoretical insights into effects of solvent polarity on excited‐state N–H proton transfer behavior for a new fluorophore of 3‐tosylamino‐N‐cyclohexylphthalimide

“…[ 31–33 ] In addition, intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), and photo‐induced electron transfer (PET) are all in connection with hydrogen bonds. [ 34–40 ] Because Han and Zhao theoretically confirmed that hydrogen bonds are significantly enhanced after photoexcitation, the study of hydrogen bond dynamics has once again attracted wide attention. [ 41–46 ]…”

“…[31][32][33] In addition, intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), and photo-induced electron transfer (PET) are all in connection with hydrogen bonds. [34][35][36][37][38][39][40] Because Han and Zhao theoretically confirmed that hydrogen bonds are significantly enhanced after photoexcitation, the study of hydrogen bond dynamics has once again attracted wide attention. [41][42][43][44][45][46] Up to now, 3-hydroxychromone, 10-hydroxybenzoquinoline (HBQ), 3-hydroxyflavone (3-HF), fisetin, and other molecules that can react with ESIPT have been deeply explored.…”