A 1998 study by Koene, Loy and Thompson introduced a family of asymmetric triaryldiamines — molecules in which two different diarylamine groups (diphenylamine, phenyl-m-tolylamine, naphthylphenylamine, iminostilbene, iminodibenzyl, or carbazole) are bound to opposite ends of a 1,4-phenylene or 4,4'-biphenylene core — and showed that the whole-molecule asymmetry suppresses crystallization and yields stable amorphous glasses with glass transition temperatures up to 125 °C, substantially above the standard hole-transport materials TPD and NPD. However, the study stopped at synthesis, thermal, electrochemical, and photoluminescent characterization: no hole mobility was measured, no organic light-emitting devices were fabricated with these compounds, and the origin of the distinctive low-energy (500–530 nm) green emission of the iminostilbene-containing derivatives remained assigned only tentatively to an intramolecular transition within the iminostilbene group. The surviving open problem is to determine the charge-transport and device-level behavior of these specific asymmetric triaryldiamine hole-transport layers (hole mobility, efficiency, turn-on voltage, operational stability) so that their superior thermal stability can be meaningfully compared with TPD/NPD, and establish whether the low-energy emission is a locally excited intramolecular transition of the iminostilbene chromophore or a charge-transfer state.

Background

In an organic light-emitting device (OLED), a hole-transporting layer (HTL) is an amorphous organic film that carries positive charges (holes, i.e. radical-cation carriers on amine nitrogen centers) from the anode toward the emitting layer. The workhorse HTL materials of the 1990s were symmetric benzidine-type diamines such as TPD (N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine) and NPD (N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine). Their weakness is morphological: glass transition temperatures (Tg, the temperature at which an amorphous solid softens, measured here by differential scanning calorimetry, DSC) of about 65 °C for TPD and 95 °C for NPD, with a tendency to crystallize during device operation, which destroys the device. Iminostilbene is 5H-dibenz[b,f]azepine, a tricyclic system of two benzene rings fused to a seven-membered azepine ring containing a vinylene bridge (a dibenzo-fused enamine when N-arylated); iminodibenzyl is its saturated 10,11-dihydro analogue; carbazole is a dibenzo-fused pyrrole. Koene, Loy and Thompson (Chem. Mater. 1998, 10, 2235) synthesized a series of asymmetric triaryldiamines, Ar1N–C6H4–NAr2 and Ar1N–biphenyl–NAr2, in which the two arylamine substituents flanking the core are different. They showed by DSC that this whole-molecule asymmetry suppresses crystallization: all the asymmetric diamines form stable glasses, with Tg up to 125 °C (e.g. the iminostilbene–carbazole–biphenyl and iminostilbene–iminodibenzyl–biphenyl compounds), whereas the symmetric 4,4'-bis(carbazolyl)biphenyl shows no glass transition at all. Cyclic voltammetry (an electrochemical technique whose first oxidation potential estimates the HOMO energy, the highest-occupied-molecular-orbital level that governs hole injection) gave oxidation potentials spanning both sides of the common HTL materials. Photoluminescence of the iminostilbene-containing biphenyl derivatives shows a blue/violet band (380–410 nm) plus a low-energy green band (500–530 nm; the symmetric bis(iminostilbenyl)biphenyl emits only the green band); dilution experiments excluded excimers (emissive dimers formed between an excited and a ground-state molecule), and lifetimes of 5.5–6.5 ns identified singlet emission, leading the authors to assign the green band to an intramolecular transition within the iminostilbene group. What the paper did not do is equally important: it reports no time-of-flight (TOF, the standard method in which photogenerated carriers drift across a micrometre-thick film under a bias, yielding the mobility in cm^2 V^-1 s^-1) or space-charge-limited-current mobility measurement, and no OLED device data, even though the compounds are proposed as HTLs and the title promises their use 'for Organic Light-Emitting Devices'. Meanwhile the general context was being built by others: Shirota's school measured TOF hole mobilities of symmetric benzidine-family amorphous molecular glasses and demonstrated thermally stable OLEDs with high-Tg HTLs, and Wong and co-workers later measured TOF hole mobilities of spirobifluorene-based triaryldiamines (spirobifluorene: two fluorene units joined at a shared spiro carbon, giving a rigid, orthogonally twisted three-dimensional molecular core) and correlated them with OLED performance — but never for this specific asymmetric-diamine family. The source paper therefore leaves two linked, well-defined questions that the field has not picked up: whether the improved thermal stability of these exact compounds translates into superior transport and device characteristics, and what the electronic origin of their low-energy emission is (a locally excited transition confined to the iminostilbene chromophore versus an intramolecular charge-transfer state, i.e. an excited state with substantial electron density shifted from the electron-rich diarylamine donor toward the azepine/stilbene acceptor, which in N-aryl enamines is a common alternative).

Problem Statement

For the asymmetric triaryldiamines introduced by Koene, Loy and Thompson (Chem. Mater. 1998, 10, 2235) — compounds of the form Ar1N–C6H4–NAr2 and Ar1N–biphenyl–NAr2 with different diarylamine groups on the two sides of the core (Ar = diphenylamine, phenyl-m-tolylamine, 1- or 2-naphthylphenylamine, iminostilbene, iminodibenzyl, carbazole), which include the iminostilbene–carbazole–biphenyl and iminostilbene–iminodibenzyl–biphenyl glasses with Tg = 125 °C and at least one member on each of the two cores (1,4-phenylene and 4,4'-biphenylene) — two linked questions remain open: (1) determine their charge-transport and device-level characteristics as hole-transport layers in OLEDs: measure the hole mobility of the amorphous films and its field and temperature dependence, and fabricate and characterize OLEDs using these compounds as the HTL (current–voltage–luminance behaviour, turn-on voltage, luminous or external quantum efficiency, and operational lifetime), so that it can be decided — by direct comparison with TPD- and NPD-based control devices — whether the compounds' higher Tg, suppressed crystallization, and bracketing oxidation potentials actually translate into superior device performance and operational stability; and (2) establish the electronic origin of the low-energy emission band at 500–530 nm observed in the iminostilbene-containing derivatives (and dominating the spectrum of bis(iminostilbenyl)biphenyl), which the source assigned only tentatively to an intramolecular transition within the iminostilbene group: determine whether this band is a locally excited transition of the iminostilbene chromophore or an intramolecular charge-transfer state involving the diarylamine donor and the azepine/stilbene acceptor. Both parts address the same material system; the first delivers the missing structure–property link between thermal stability and device function, and the second the missing excited-state assignment that governs whether these compounds could also serve as emitters rather than pure transporters.

References

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