James E. Rainbolt

Synthesis and application of pyridine-based ambipolar hosts: control of charge balance in organic light-emitting devices by chemical structure modification.

We studied the influence of a pyridine moiety versus a phenyl moiety when introduced in the molecular design of an ambipolar host. These pyridine-based host materials for organic light-emitting diodes (OLEDs) were synthesized in three to five steps from commercially available starting materials. The isomeric hosts have similar HOMO/LUMO energies; however, data from OLEDs fabricated using the above host materials demonstrate that small structural modification of the host results in significant changes in its carrier-transporting characteristics.

Anhydrous tertiary alkanolamines as hybrid chemical and physical CO2 capture reagents with pressure-swing regeneration

Anhydrous DMEA, DEEA and DIPEA are found to absorb carbon dioxide under pressure via chemical binding and physical absorption. The chemical CO2-bound derivatives of these materials are zwitterionic alkylcarbonate salts which are characterized by high-pressure 13C NMR. DMEA, DEEA and DIPEA absorb 20 wt.%, 17 wt.% and 16 wt.% carbon dioxide, respectively, at 300 psig (20.6 ATM). An increasing chemical carbon dioxide uptake capacity trend of DMEA > DEEA > DIPEA is observed while the physical CO2 absorption trend is DIPEA > DEEA > DMEA. DMEA captures up to 45 mole % (20 wt.%) of CO2 at 500 psig via both chemical binding and physical absorption. The amount of chemically bound and physically absorbed CO2 is directly linked to the CO2 pressure over the liquid. The zwitterion DMEA-CO2 regenerates CO2 and DMEA upon depressurization, allowing for an economical pressure swing regeneration rather than thermal regeneration. DMEA absorbs/releases CO2 repeatedly with no decline in capacity.

Chemically selective gas sweetening without thermal-swing regeneration

Natural gas purifications using chemically selective hydrogen sulfide (H2S) sorbents could be more efficient if chemical selectivity for H2S could be maintained without thermal regeneration of the sorbent. We used tertiary alkanolamines to reversibly capture H2S in the absence of water to produce hydrosulfide-based ionic liquids in high yield. These alkanolammonium hydrosulfide ionic liquids release H2S by exposure to inert gas or by mild heating. H2S can be rapidly and nearly quantitatively released at ambient temperature from the alkanolammonium hydrosulfide ionic liquids by the addition of nonpolar antisolvents, some of which naturally phase separate from the spent alkanolamine. The antisolvent-induced regeneration of the alkanolamine potentially allows an efficient H2S gas scrubbing process that is chemically selective and can be operated continuously at or near ambient temperature.

Emission zone control in blue organic electrophosphorescent devices through chemical modification of host materials

We report blue organic light-emitting devices with iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate as an emitter doped into a series of phosphine oxide-based host materials that have significantly different charge transport properties: 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), N-(4-diphenylphosphoryl phenyl) carbazole (PO12), 9-[6-(diphenylphosphoryl)pyridin-3-yl]-9H-carbazole (HM-A5), and 6-(diphenylphosphoryl)-N,N-diphenylpyridin-3-amine (HM-A6). Depending on the nature of the host material, the location of the emission zone can be moved within the emissive layer from the hole transport layer interface to the electron-transport layer interface. The charge transport properties of the materials were evaluated using single carrier devices.

Synthesis and characterization of p-type conductivity dopant 2-(3-(adamantan-1-yl)propyl)-3,5,6-trifluoro-7,7,8,8-tetracyanoquinodimethane

We report the synthesis and characterization of 2-(3-(adamantan-1-yl)propyl)-3,5,6-trifluoro-7,7,8,8-tetracyanoquinodimethane (F3TCNQ-Ad1), a substituted analog of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), designed for p-type conductivity doping. The dopant is designed as a model for substituted alternatives to F4TCNQ that maintain similar electronic properties with the goal of engineering dopants with superior fabrication characteristics over F4TCNQ. We describe the design strategy for F3TCNQ-Ad1 based on molecular modeling predictions that substitution of a single fluorine atom of F4TCNQ has little effect on the electronic properties of the molecule. Photophysical and electrochemical characterization reveal that the adamantyl substituent in F3TCNQ-Ad1 does not significantly alter the electronic properties of the substituted dopant relative to F4TCNQ. Unfortunately, F3TCNQ-Ad1 degrades under standard sublimation conditions, preventing sublimation deposition processing. Instead, hole-only devices were made via solution-processing of the p-doped films with the structure glass/ITO/2.3 × 103 A PVK:(MTDATA:dopant)/2.0 × 102 A Au/1.0 × 103 A Al, where dopant is either F4TCNQ or F3TCNQ-Ad1. We demonstrate that F3TCNQ-Ad1 increased the conductivity of the films by at least 1000 times compared to an undoped device.

Blue phosphorescent organic light-emitting devices utilizing cesium–carbonate-doped 2,4,6-tris(2′,4′-difluoro-[1,1′-biphenyl]-4-yl)-1,3,5-triazine

We report an alternative, high-yielding synthesis for the known compound 2,4,6-tris(2′,4′-difluoro-[1,1′-biphenyl]-4-yl)-1,3,5-triazine (tris-(dFB)Tz). The energy of the lowest unoccupied molecular orbital (ELUMO) for tris-(dFB)Tz is estimated to be −3.5 eV from electrochemical measurements. The deep ELUMO of tris-(dFB)Tz affords a material with excellent electron acceptor characteristics for use in n-doped electron transport layers. Tris-(dFB)Tz shows a four order of magnitude increase in the number of carriers on doping with 8 wt. % Cs2CO3. Enhanced electron injection was also observed on doping with Cs2CO3, which eliminated the necessity for a separate LiF electron injection layer. Blue phosphorescent organic light-emitting devices (OLEDs) were fabricated using n-doped tris-(dFB)Tz electron transport layers. OLEDs with thick (700-A) Cs2CO3-doped tris-(dFB)Tz electron transport layers had lower operating voltages than OLEDS with an undoped electron transport layer of bis(diphenylphosphoryl)dibenzothiophene (PO15), which has previously been used in low-voltage, high-efficiency OLEDs. The tris-(dFB)Tz results indicate that aromatic substituted triazines may be promising materials for use as electron acceptors in n-doped organic electronic systems.

Design of new anchored p-dopants for high power efficiency OLEDs

Conductivity doping of charge transporting layers is becoming increasingly attractive for improving power efficiency in OLEDs. However, the number of commercially available organic molecular p-dopants is limited. The electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4-TCNQ) is the most utilized p-dopant. F4-TCNQ can be used as a dopant for most hole transporting materials (HTM), but it is very volatile, which makes it difficult for vacuum processing, and has a low sticking coefficient. Here we present the design of novel anchored molecular dopants based on the TCNQ core. We first review how the reduction potential of TCNQ core is affected by substitution with alkyl groups of different electronic properties. Electron donating groups have negative effect on the reduction potential of the acceptor. However, attaching electron withdrawing groups such as halogens counteracts the effect of electron donating groups. Using gas phase theoretical calculations we determined that trifluorinated TCNQ can be anchored through a σ-coupled alkyl chain to an inert molecular anchor without sacrificing the electron affinity.

Substituent effects on the geometric and electronic properties of tetracyano-p-quinodimethane (TCNQ): a theoretical study

Electron acceptors are classes of molecules that are important in organic devices as they help to improve the conductivity of organic semiconducting molecules by forming p-type complexes or anion radical complexes. These molecules can be doped into hole transporting materials to provide good ohmic contact with the anode and to improve the carrier density of the hole transport layer. This results in organic light-emitting devices with low driving voltages and high power efficiencies. In this study, we investigate a series of tetracyano-p-quinodimethane derivatives with substituents expected to facilitate the electron-acceptor capabilities of the quinones using density functional theory (DFT) and time-dependent DFT (TDDFT). As expected, the cyano substitution stabilises both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels and increases the adiabatic electron affinity of the dopants. The stabilisation effect on the LUMO levels is greater, and as a result the narrowing of the HOMO–LUMO gap is seen. This fact was further confirmed by TDDFT studies, energy of the computed S1 ← S0 transition red shifted upon CN substitution. However, perturbation to the ground-state geometry is negligible, and all anionic structures exhibit aromatisation independent of substitution. Our study suggests that the substituted derivatives reported herein show promise as conductivity dopants.

Controlling charge transport in blue organic light-emitting devices by chemical functionalization of host materials

We report the photophysical characterization, computational results, and device properties for ambipolar phosphine oxide-based host materials that were chemically functionalized to control the charge transport. We study the effects of structural modifications of phosphine oxide hosts on the charge balance in the emissive zone of organic light-emitting devices (OLEDs). Significant changes in charge transport within the emissive layer are observed upon introduction of functional groups, such as pyridine and carbazole, into the organic phosphine oxide host structure. We demonstrate that rational design of host materials allows for the control of charge balance in the emissive zone of OLEDs.

P-188: Molecular Engineering of Host Materials for Blue Phosphorescent OLEDs: Past, Present and Future

We report molecular design considerations for blue phosphorescent host materials, as well as propose design rules necessary to build ambipolar hosts and thus reach charge balance in a device. Our beginning developments are presented followed by the evolution of the original design to our state-of-the-art, with the help of computational modeling.