Helicenes as All-in-One Organic Materials for Application in OLEDs: Synthesis and Diverse Applications of Carbo- and Aza[5]helical Diamines

A set of eight helical diamines were designed and synthesized to demonstrate their relevance as all ‐ in ‐ one materials for multifarious applications in organic light ‐ emitting diodes (OLEDs), that is, as hole ‐ transporting materials (HTMs), EMs, bifunctional hole transporting + emissive materials, and host materials. Azahelical diamines function very well as HTMs. Indeed, with high Tg values (127 – 214 °C), they are superior alternatives to popular N,N′ ‐ di(1 ‐ naphthyl) ‐ N,N′ ‐ diphenyl ‐ (1,1′ ‐ biphenyl) ‐ 4,4′ ‐ diamine (NPB). All the helical diamines exhibit emissive properties when employed in nondoped as well as doped devices, the performance characteristics being superior in the latter. One of the carbohelical diamines (CHTPA) serves the dual function of hole transport as well as emission in simple double ‐ layer devices; the efficiencies observed were better by quite some margin than those of other emissive helicenes reported. The twisting endows helical diamines with significantly high triplet energies such that they also function as host materials for red and green phosphors, that is, [Ir(btp)2acac] (btp=2 ‐ (2′ ‐ benzothienyl)pyridine; acac=acetylacetonate) and [Ir(ppy)3] (ppy=2 ‐ phenylpyridine), respectively. The results of device fabrications demonstrate how helicity/ helical scaffold may be diligently exploited to create molecular systems for maneuvering diverse applications in OLEDs.

Not only aesthetically beautiful, but also functionally charming! Helicity as a design element allows creation of new organic materials for multifarious applications in organic light ‐ emitting diodes (OLEDs). Helical diamines are shown to function as hole ‐ transporting (HTM), emissive (EM), hole ‐ transporting +emissive, and host materials (see figure).

amines; fluorescence; helical structures; luminescence; thin films

Helicity is all ‐ pervasive. From gigantic galaxies to microscopic DNA, it prevails in wide facets of our existence and imagination. In the molecular world, helicity continues to enjoy unstinted privilege for its aesthetic charm. Creation of helical compounds with unique structural attributes and properties remains an unrelenting pursuit. The recent literature reveals a surge of interest in helical structures for exploration as molecular springs,[4] solenoids,[5] tweezers,[6] motors,[7] IR ‐ sensing materials,[8] dye ‐ sensitized solar cell (DSSC) materials,[9] liquid crystals,[10] NLO materials,[11] optoelectronic materials,[12] and so on. In the realm of organic light ‐ emitting diodes (OLEDs), which have captured the market of lighting and display currently, one witnesses an explosion in the development of materials for application in devices over the past decade; OLEDs offer unrivaled advantages from the points of view of production cost, power consumption, wide ‐ angle viewability, contrast ratio, opportunities for flexible displays, and so on. The hunt for newer materials with properties that surpass the existing ones is an incessant quest. Given how advanced the research in OLEDs is, one surprisingly finds only a few scattered reports on exploitation of helicene ‐ based materials in OLEDs.[23] Some helical systems explored by different groups for electroluminescence are shown in Figure [NaN] .

We have been concerned with control of macroscopic order and disorder in a bottom ‐ up approach involving de novo design of molecular systems and their structural manipulations.[29] , [34] The marvelous allure of helical structures and their utility as steric scaffolds for controlling photochromism in our recent investigations were the motivations to develop materials with all ‐ in ‐ one attributes for application in OLEDs. Accordingly, we designed eight new helical diamines, shown in Figure [NaN] , based on carbo[5]helicene and monoaza[5]helicene cores by two ‐ fold substitution at the termini with groups such as carbazole, 3,6 ‐ di ‐ tert ‐ butylcarbazole, diphenylamine, triphenylamine, and phenylcarbazole. Herein, we report that the helical diamines in Figure [NaN] can be readily synthesized and that they exhibit photophysical, electrochemical, and thermal properties suitable for a range of applications in OLED devices. It is shown that 1) their fluorescence properties allow exploitation as emissive materials (EMs); 2) high HOMO levels as a consequence of electron ‐ richness permit utility as hole ‐ transporting materials (HTMs); 3) electron ‐ rich nature as well as emissive behavior can be exploited to develop bifunctional materials with hole ‐ transporting as well as emissive properties, whereby device fabrication is simplified; and 4) the twisted structures endow them with triplet energies that are significantly high enough as to be suitable as host materials for red and green phosphorescent dopants, that is, [Ir(btp)2acac] and [Ir(ppy)3], respectively. Applicability of a small set of diamines, featuring helicity as a design element, is demonstrated for multifarious applications in OLEDs.

All the molecular systems in Figure [NaN] were synthesized based on three key reactions, namely, 1) Wittig olefination to furnish diarylethylenes; 2) oxidative photocyclization of the latter to afford dibromo ‐ substituted helicenes; and 3) Pd0 ‐ catalyzed Suzuki and Buchwald – Hartwig coupling of dibromo ‐ helicenes with appropriate boronic acids and diarylamines, respectively. The synthetic routes for all the helical diamines in Figure [NaN] are shown in Scheme [NaN] . All the compounds were characterized by comprehensive spectroscopic data (see the Supporting Information).

Furthermore, the structures of helical diamines were unequivocally established by single crystal X ‐ ray structure determinations for at least two cases, namely, CHCZL and CHDPA; good quality single crystals of the latter were obtained by slow evaporation of their solutions in CHCl3 over two days. The perspective drawings of the molecular structures of CHCZL and CHDPA exhibited in Figure [NaN] show that the helical scaffold is significantly twisted in both cases. Whereas the peripheral carbazole groups are almost perpendicularly oriented in the case of CHCZL, the aryl groups of nitrogen centers appear more propeller ‐ like in CHDPA, as is observed typically for simple triarylamines. Clearly, the conjugation in the helical scaffold as well as that between the scaffold and peripheral groups are minimal to manifest in high band gap energies (see below).

Crystallinity is, in principle, not expected for as ‐ synthesized compounds that display amorphous property. Presumably, the solvent molecules play a significant role in engineering crystal growth with the solvent molecules, at times, included in the crystal lattices, leading to lattice inclusion compounds. This, indeed, is the scenario observed for CHDPA. Our efforts to crystallize azahelical diamines were unsuccessful, presumably because of their poor solubility.

UV/Vis absorption spectra of carbo ‐ and azahelical diamines recorded in dilute DCM solutions (ca. 1×10−5 m) are shown in Figure [NaN]  a and 4b, respectively. Insofar as the absorption spectra of carbohelical diamines, that is, CHCZL, CHDPA, and CHTPA, are concerned, they are completely different from each other. The absorption spectrum of CHCZL is structured with three distinct peaks at 294, 328 and 341 nm. In contrast, the absorption spectrum of CHDPA is quite broad in nature with a maximum around 300 nm, whereas that of CHTPA has a maximum at 310 nm associated with a broad shoulder starting from 340 nm. Insofar as UV/Vis absorption spectra of AHCZL, AHDPA, and AHTPA are concerned, one observes certain similarities with their carbohelical analogues. For example, the absorption spectrum of AHCZL is also structured and has three major peaks at 294, 312, and 399 nm. On the contrary, the absorption spectrum of AHDPA has two maxima at 308 and 363 nm, as opposed to one broad maximum for CHDPA. As for AHTPA, one observes a broad spectral feature with a maximum at about 323 nm. For the rest of the azahelical diamines, that is, AHCZLt and AHPCZL, one observes striking similarities between the absorption spectra for the two. They are characterized by three bands in a narrow range of 294 – 298 nm, 311 – 319 nm and 401 – 402 nm. Overall, broad absorption features are observed for the non ‐ carbazole compounds with flexible amino groups, whereas structured absorptions are a signature for carbazole ‐ functionalized helical diamines. The band gap energies calculated from the red edge absorption cut ‐ off for AHDPA, CHDPA, and CHTPA are in the range of 2.79 – 2.87 eV. For all the other compounds, the band gap energies are above 2.90 eV (Table [NaN] ).

Photophysical, electrochemical and thermal properties of the carbo ‐ and azahelical diamines.

1 [a] Absorption and fluorescence spectra were recorded in dilute DCM solutions (ca. 10−5 m). [b] Band gap energies were calculated from red edge absorption onset values using the formula E=hc/λ. [c] Triplet energies were determined from the 0 – 0 transitions in the phosphorescence spectra recorded in 2 ‐ MeTHF at 77 K. [d] Lifetimes were determined by time ‐ correlated single photon counting. [e] Quantum yields were determined for excitation at 341 nm relative to anthracene as the standard. [f] HOMO energies were calculated from oxidation potentials in the CV spectra. [g] LUMO energies were calculated by subtracting the band gap energies from HOMO energies. [h] From DSC. [i] From TGA.

Fluorescence spectra of all diamines recorded in dilute DCM solutions (ca. 1×10−5 m) for excitation at 341 nm are shown in Figure 4 c and [NaN]  d. Within the carbohelical diamines, the fluorescence spectrum of CHCZL is significantly blue ‐ shifted relative to those of CHDPA and CHTPA. The emission of CHCZL is structured with maxima at 420 and 435 nm, whereas the same for CHDPA and CHTPA are broad and almost superimposable with maxima at about 459 nm; the only difference between the emission spectra of the two is that the emission profile for CHTPA is slightly broader than that of CHDPA. Progression from carbo ‐ to azahelical diamines brings about similarities as well as differences between the analogous diamine pairs, that is, CHCZL versus AHCZL, CHDPA versus AHDPA, and CHTPA versus AHTPA. The emission profile of AHCZL is also structured with maxima at 413 and 433, which are marginally blue ‐ shifted relative to those of CHCZL. On the contrary, the emission profile of AHDPA is red ‐ shifted by about 15 nm relative to that of CHDPA; the emission maximum for AHDPA lies at 474 nm. Insofar as the triphenylamino ‐ derivative is concerned, one observes that the emission of AHTPA is not only blue ‐ shifted by about 15 nm relative to that of CHTPA, but also shows a shoulder at 425 nm in addition to the maximum at 444 nm. The remaining carbazole compounds, that is, AHCZLt and AHPCZL, display features akin to AHCZL, and are characterized by structured emissions with two closely associated maxima in the range of 410 – 411 nm and 432 – 433 nm. Fluorescence quantum yields of the compounds in DCM determined relative to anthracene as the standard were found to vary from 4.5 – 34.6 % (Table [NaN] ). Fluorescence lifetimes of all diamines measured by single photon counting are collected in Table [NaN] , and were found to range between 4.98 – 7.19 ns.

Phosphorescence spectra of all the carbo ‐ and azahelical diamines recorded in dilute 2 ‐ MeTHF solutions at 77 K are shown in Figure [NaN]  a and 5 b, respectively. Triplet energies of all the compounds calculated based on 0 – 0 transitions in their phosphorescence spectra are collected in Table [NaN] . For carbohelical diamines, that is, CHCZL, CHDPA, and CHTPA, the triplet energies were found to vary between 2.31 – 2.37 eV. In contrast, analogous azahelical diamines, that is, AHCZL, AHDPA, and AHTPA, were found to display slightly higher triplet energies, which range between 2.37 – 2.54 eV (Table [NaN] ); the remaining two higher azahelical analogues, that is, AHCZLt and AHPCZL, were found to possess similar triplet energies. Clearly, the triplet energies are somewhat higher for azahelical diamines than for the corresponding carbohelical diamines, and highest for AHCZL and AHCZLt, which are 2.54 and 2.55 eV, respectively. Presumably, electron ‐ richness of azahelical scaffold reduces conjugation between helical core and carbazole groups. By the same token, the lower triplet energy of AHPCZL relative to its lower analogue, that is, AHCZL, can be reconciled by more conjugation between helical scaffold and the amino groups.

Electrochemical properties of the carbo ‐ and azahelical diamines were examined by cyclic voltammetry with nBu4NPF6 as the supporting electrolyte in DCM. Analyses of the cyclic voltammograms (CV) reveal that the helicenes display only oxidation, but no reduction peaks within the potential window of the operation (Supporting Information, Figure S1). The CV of CHCZL is irreversible, whereas the CVs of CHDPA and CHTPA are quasi ‐ reversible. Insofar as azahelical diamines are concerned, the CV of AHCZL is irreversible, whereas those of AHDPA and AHTPA are completely reversible. Clearly, non ‐ fused amino ‐ derivatives based on azahelicene, that is, AHDPA and AHTPA, display better electrochemical behavior than their carbohelical analogues, that is, CHDPA and CHTPA. The CV profiles of CHTPA and AHTPA are shown for comparison in Figure [NaN] . The higher azahelical homologues, that is, AHCZLt and AHPCZL, display different electrochemical features (Supporting Information, Figure S1). Whereas CV of AHCZLt is completely reversible, insertion of phenyl spacer as in AHPCZL leads to irreversible electrochemical behavior. Overall, the following generalized observations may be made with regard to their electrochemical behavior: first, nonfused di ‐ /triarylamino ‐ functionalized diamines, that is, CHDPA, CHTPA, AHTPA, and AHDPA, are endowed with stable electrochemical properties. Second, fused carbazole derivatives, that is, CHCZL, AHPCZL, and AHCZL, do not display desirable electrochemical stability; this indeed is the case with many other reported carbazole ‐ based materials.[45] Last, introduction of bulky tert ‐ butyl groups at 3 and 6 positions of each of the carbazole groups at the periphery significantly improves the electrochemical stability; indeed, the CV of AHCZLt is reversible, whereas that of AHPCZL is completely irreversible. Presumably, polymerization at 3 and 6 positions of the reactive carbazole radical species produced under electrochemical oxidation is precluded by the presence of bulky tert ‐ butyl groups.[48]

HOMO energies of all carbo ‐ and azahelical diamines calculated relative to ferrocene as the standard (HOMO energy of ferrocene is −4.8 eV with respect to vacuum) using the onset/half ‐ cell oxidation potentials determined from the CVs are collected in Table [NaN] . The HOMO energies of carbohelical diamines were found to be in the range of 5.14 – 5.51 eV, whereas those of azahelical diamines ranged between 5.04 – 5.40 eV. Of course, electron ‐ richness of azahelical diamines is responsible for the slightly elevated HOMO levels of the azahelical diamines relative to their carbohelical diamine analogues. The HOMO energies of the two higher azahelical diamines, that is, AHCZLt and AHPCZL, were determined to be 5.09 and 5.34 eV, respectively; it should be noted that the HOMOs of popular HTMs fall in this range. LUMO energies were calculated by subtraction of band gap energies from the HOMO energies, and were found to fall in the range of 2.10 – 2.58 eV (Table [NaN] ).

Good thermal stability is essential for a material to be applied in OLED devices to withstand decomposition during vacuum sublimation as well as joule heating that occurs on prolonged device operation. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) at a heating rate of 10 °C min−1 under inert atmosphere were performed to examine thermal properties of the helical diamines. All as ‐ synthesized carbo ‐ as well as azahelical compounds exhibited excellent thermal stabilities with decomposition temperatures (Td) well above 400 °C (Table [NaN] ; Supporting Information, Figure S2), with the exception of CHDPA for which the Td is 383 °C. The Td values of CHCZL and CHTPA are 453 and 514 °C, respectively. Clearly, Td values of the carbohelical diamines are a function of molecular weight. The glass transition temperatures (Tg) for CHCZL, CHDPA, and CHTPA are 164, 115, and 130 °C, respectively, (Supporting Information, Figure S3). Whereas higher Tg for CHTPA compared to that for CHDPA is due to differences in the molecular weight, a much higher Tg for CHCZL than for CHTPA, despite lower molecular weight of the former, should be understood from the rigidity of the carbazole moieties. The azahelical diamines AHCZL and AHDPA display higher Td values than their corresponding carbohelical analogues, that is, CHCZL and CHDPA (Table [NaN] ), although Td of AHTPA is lower than that of CHTPA. A similar trend in the Tg values was observed for AHCZL, AHDPA, and AHTPA as well with the Tg values of AHCZL and AHDPA being the highest and the lowest, respectively. A closer analysis reveals that the Tg values of azahelical diamines are slightly higher than those of carbohelical diamines (CHCZL vs. AHCZL, CHDPA vs. AHDPA and CHTPA vs. AHTPA, Table [NaN] ). The higher azahelical analogues, that is, AHCZLt and AHPCZL, display significantly high thermal stabilities in terms of both Td and Tg (Table [NaN] ). It should be emphasized that the Tg values of all compounds are higher than those of other commercially available popular diarylamines such as N,N′ ‐ di(1 ‐ naphthyl) ‐ N,N′ ‐ diphenyl ‐ (1,1′ ‐ biphenyl) ‐ 4,4′ ‐ diamine (NPB, Tg≈95 °C),N,N′ ‐ bis(3 ‐ methylphenyl) ‐ N,N′ ‐ diphenylbenzidine (TPD, Tg≈60 °C), 1,3 ‐ bis(N ‐ carbazolyl)benzene (mCP, Tg≈60 °C), 4,4′ ‐ bis(N ‐ carbazolyl) ‐ 1,1′ ‐ biphenyl (CBP, Tg≈62 °C), and so on. Evidently, high Tg is indispensible for formation of pinhole ‐ free stable glasses, which crucially determine the device longevity.[53]

Comprehensive studies of photophysical, electrochemical and thermal properties of the carbo ‐ and azahelical diamines clearly suggest that these compounds can be exploited for multifarious applications in OLEDs. Thus, electroluminescence properties of the helical diamines were examined by fabrication of different types of devices in which the compounds function as 1) hole ‐ transporting materials; 2) emissive materials; 3) hole ‐ transporting as well as emissive materials; and 4) host materials. Application of helical diamines as each of these materials is dealt with separately in the following.

The high HOMO energies (5.04 – 5.40 eV) of azahelical diamines in particular are appealing for their applicability as HTMs. Unfortunately, fabrication of simple double layer devices with AHTPA and tris(8 ‐ hydroxyquinolinato)aluminum (Alq3) yielded very poor results; presumably, a large gap (ca. 0.7 eV) between HOMOs of the two employed materials curtail charge transport leading to abysmal results. Therefore, an emissive material of a higher HOMO level, namely, 3,6 ‐ bis(triphenylamino)phenanthrene (PTPA) with its HOMO at 5.3 eV, was considered. Multilayer devices of the following configuration were fabricated: (A) ITO/azahelical diamine (40 nm)/PTPA (20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (150 nm) to examine hole transport properties of the azahelical diamines, in which ITO functions as an anode, PTPA serves as an emissive material, 2,2′,2“ ‐ (1,3,5 ‐ benzinetriyl)tris(1 ‐ phenyl ‐ 1H ‐ benzimidazole) (TPBI) as an electron ‐ transporting material, and LiF/Al as a composite cathode. A control device in which azahelical diamine was replaced with commercial NPB was also fabricated to allow comparison of the hole transport abilities. The results of the fabricated devices are collected in Table [NaN] . I ‐ V ‐ L characteristics, typical EL spectra and energy level diagram are shown in Figure [NaN] .

Electroluminescence data for the devices with azahelical diamines as HTMs [a].

2 [a] The device configuration followed was: A) ITO/azahelical diamine (40 nm)/PTPA (20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (150 nm). [b] Turn ‐ on voltage [V]. [c] Maximum external quantum efficiency [%]. [d] Maximum power efficiency [lm W−1]. [e] Maximum luminance efficiency [cd A−1]. [f] Maximum luminance achieved [cd m−2]. [g] λmax (EL) [nm]. [h] 1931 chromaticity coordinates measured at 6 V.

A cursory glance of the data in Table [NaN] compellingly brings out the fact that the hole ‐ transporting abilities of azahelical diamines are superior to those of NPB in the device configuration employed. The obvious advantages that the azahelical diamines offer over NPB are as follows. First, external quantum efficiencies and luminous efficiencies for devices fabricated with azahelical diamines are higher than those based on NPB, not only at the peak positions, but also over a wide range of current densities (Supporting Information, Figure S4); the power efficiency of NPB ‐ based devices is, however, better than those based on azalelical diamines at higher current densities. Second, with the exception of AHTPA, devices fabricated with other HTMs produce maximum luminance values that are higher than those observed for the device with NPB as a HTM. Third, although the λmax is same (420 nm) for all devices, devices fabricated with azahelical diamines as HTMs emit blue light with slightly lower CIE coordinates than that obtained with the NPB ‐ based device (Table [NaN] ). Given that NPB has a Tg of only 95 °C, the azahelical compounds with significantly higher Tg values ranging between 127 – 214 °C constitute excellent alternatives to popular NPB.

As discussed earlier, the helical diamines are fluorescent with moderate quantum yields. Their utility as emissive materials in nondoped as well as doped devices was investigated by fabrication of devices with the following four different configurations: (B) ITO/NPB (40 nm)/helicene (10 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm), (C) ITO/NPB (40 nm)/helicene (10 nm)/PBD (40 nm)/LiF (1 nm)/Al (100 nm) and (D) ITO/NPB (40 nm)/MADN:helicene (5 %, 40 nm)/TPBI (35 nm)/LiF (1 nm)/Al (100 nm) and (E) ITO/NPB (40 nm)/CBP:helicene (5 %, 40 nm)/TPBI (35 nm)/LiF (1 nm)/Al (100 nm), in which ITO functions as an anode, NPB as a hole ‐ transporting material, TPBI and 2 ‐ (4 ‐ tert ‐ butylphenyl) ‐ 5 ‐ (4 ‐ biphenylyl) ‐ 1,3,4 ‐ oxadiazole (PBD) serve as electron ‐ transporting as well as hole ‐ blocking materials, 2 ‐ methyl ‐ 9,10 ‐ bis(naphthalen ‐ 2 ‐ yl)anthracene (MADN) and CBP as fluorescent hosts and LiF/Al as the composite cathode; of course, helicene serves as an emissive material in all cases. The configurations B and C correspond to nondoped devices, whereas D and E refer to the doped devices. The results obtained from the best devices from a limited set are collected in Table [NaN] . The I ‐ V ‐ L profiles, resultant electroluminescence spectra for device B and energy level diagram are shown in Figure [NaN] , whereas those for devices D and E are given in Figure S5 (Supporting Information).

Electroluminescence data for the OLED devices fabricated with the helicenes as EMs.

3 [a] B – E refer to the device configurations: B) ITO/NPB (40 nm)/helicene (10 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm); C) ITO/NPB (40 nm)/helicene (10 nm)/PBD (35 nm)/LiF (1 nm)/Al (100 nm); D) ITO/NPB (40 nm)/MADN:helicene (5 %, 40 nm)/TPBI (35 nm)/LiF (1 nm)/Al (100 nm); E) ITO/NPB (40 nm)/CBP:helicene (5 %, 40 nm)/TPBI (35 nm)/LiF (1 nm)/Al (100 nm). [b] Turn ‐ on voltage [V]. [c] Maximum external quantum efficiency [%]. [d] Maximum power efficiency [lm W−1]. [e] Maximum luminance efficiency (cd A−1). [f] Maximum luminance achieved [cd m−2]. [g] λmax (EL) [nm]. [h] 1931 chromaticity coordinates measured at 8 V.

As can be perused from the results in Table [NaN] , the helical diamines indeed function as emissive materials in both nondoped (B and C) and doped (D and E) devices. The turn ‐ on voltages are low (ca. 3.0 – 4.0 V) for the carbohelical diamines, whereas they are slightly higher for the azahelical diamines (ca. 3.5 – 6.5 V). In general, the efficiencies obtained with nondoped devices are unremarkable, whereas they are found to be much improved in doped devices. In nondoped devices, the efficiencies obtained with CHTPA are better than those with the azahelical analogue, that is, AHTPA, whereas those obtained with CHCZL are poorer than those with azahelical AHCZL; CHDPA and AHDPA exhibit comparable device performance results (Table [NaN] ). Insofar as nondoped devices are concerned, the best performance in terms of efficiencies is exhibited by AHPCZL in configuration C; the device yielded maximum external quantum efficiency, power efficiency, luminous efficiency and luminance of 1.56 %, 3.06 lm W−1, 4.39 cd A−1 and 2940 cd m−2, respectively. However, emission emanating from the device in this instance cannot be ascribed solely to AHPCZL, as bluish ‐ white emission with CIE coordinates of (0.25, 0.39) was observed, which is presumably contributed by the exciplexes formed at the interface with electron transport layer (ETL) (Supporting Information, Figure S6). In contrast, the best performance in doped devices is exhibited by CHTPA, which yielded maximum external quantum efficiency, power efficiency, luminous efficiency and luminance of 2.54 %, 3.72 cd A−1, 3.34 lm W−1 and 1810 cd m−2, respectively.

From the results of a number of experiments with all the helical diamines, we determined that CHTPA allows better dual property. Therefore, efforts were focused more on CHTPA for dual function as a hole ‐ transporting as well as emissive material. The devices fabricated were: (F) ITO/CHTPA (70 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm), (G) ITO/CHTPA (70 nm)/BCP (40 nm)/LiF (1 nm)/Al (100 nm), (H) ITO/CHTPA (70 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) and (I) ITO/CHTPA (70 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm). The results of device performances are collected in Table [NaN] . The I ‐ V ‐ L profile and EL spectrum are typically shown for the device G in Figure 9 a and [NaN]  b, respectively. In all cases, CHTPA serves the dual purpose of hole ‐ transport as well as emission, that is, as a bifunctional material, thereby eliminating the requirement of separate layers for hole transport and emission. Quite remarkable is the fact that the performance characteristics of CHTPA are better when no additional hole ‐ injection layer (m ‐ MTDATA) is present (devices F1, G1 and H1, Supporting Information, Table S1); this indeed points to a significant interfacial stability of CHTPA over ITO. While the CHTPA/TPBI device yielded a maximum brightness of 4960 cd m−12, the most efficient device was found to be CHTPA/BCP, which produced maximum external quantum, luminous and power efficiencies of 2.75 %, 3.99 cd A−1 and 3.13 lm W−1, respectively. It is noteworthy that the applicability of helical diamines as bifunctional materials is heretofore unknown, although they have been investigated as emissive materials under doping conditions. Indeed, the fact that helical diamines can function as bifunctional hole ‐ transporting as well as emissive materials in simple double layer devices is demonstrated for the first time.[56] Efficiencies obtained in the device configuration G is the highest by quite some margin amongst other helical amines reported so far and also amongst those considered herein. The merit lies in the fact that a simple nondoped double layer device structure gives such an excellent performance as opposed to complicated doped device strategies reported so far.

Device performance results involving carbohelical diamines employed as HTMs as well as EMs.

4 [a] F – J refer to the device configurations: F) ITO/CHTPA (70 nm)/TPBI (40 nm)/LiF (1 nm)/Al (150 nm), G) ITO/CHTPA or CHDPA or CHCZL (70 nm)/BCP (40 nm)/LiF (1 nm)/Al (100 nm), H) ITO/CHTPA (70 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm), I) ITO/CHTPA (70 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), J) ITO/CHDPA or CHCZL (40 nm)/CHTPA (20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (150 nm). [b] Turn ‐ on voltage [V]. [c] Maximum external quantum efficiency [%]. [d] Maximum power efficiency [lm W−1]. [e] Maximum luminance efficiency [cd A−1]. [f] Maximum luminance achieved [cd m−2]. [g] λmax (EL) [nm]. [h] 1931 chromaticity coordinates measured at 8 V. [i] 1931 chromaticity coordinates recorded at 13 V.

Based on our observation that CHTPA/BCP ‐ based device yielded maximum efficiencies among other devices with different ETLs, we fabricated similar devices with CHCZL and CHDPA as well. Poor quantum yields of emission curtailed meaningful device results. Both CHCZL and CHDPA led to poor efficiencies (device G, Table [NaN] ); of course, low HOMO level for CHCZL appears to be responsible for the observed abysmal performance, which renders hole injection difficult (Supporting Information, Figure S7). Remarkably, both perform nicely as typical HTMs when emissive layer was changed to CHTPA (device J, Table [NaN] ).

In general, the triplet energy of an organic compound depends crucially on the extent of conjugation. Although carbazole, triphenylamine, and diphenylamine are well ‐ known for their high triplet energies, the latter depend on overall conjugation subsequent to their attachment to the carbo ‐ and monoaza[5]helicene cores. As can be seen from the results in Table [NaN] , the triplet energies of all helical diamines are respectable. Twisting as a consequence of unique helical core scaffolds is evidently responsible for the observed high triplet energies of the diamines, given that triplet energy of planar pentacene is as low as 1.0 eV.[66] Thus, all helical diamines qualify for application as host materials for the red dopant, that is, [Ir(btp)2acac]. Further, the triplet energies for AHCZL and AHCZLt are >2.5 eV such that they also qualify as hosts for the green dopant, that is, [Ir(ppy)3]; note that the triplet energy for the latter is 2.42 eV.[67] Insofar as the devices for red emission are concerned, a device of configuration (K): ITO/NPB (40 nm)/CHCZL:[Ir(btp)2acac] (10 %, 20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (100 nm) was constructed. This device yielded maximum external quantum efficiency, luminous efficiency, power efficiency and luminance of 4.70 %, 3.47 cd A−1, 5.33 lm W−1 and 2640 cd m−2, respectively, (Table [NaN] ). Analogous devices were also fabricated for azahelical carbazole compounds, that is, AHCZL, AHCZLt, and AHPCZL, with the following configuration: (L) ITO/NPB (40 nm)/AHCZL:[Ir(btp)2acac] (20 nm)/PBD (40 nm)/LiF (2 nm)/Al (150 nm). The device performance results are collected in Table [NaN] . The typical I ‐ V ‐ L profile and EL spectrum for the red emission are shown in Figure [NaN] . The best performance was observed for the devices fabricated with AHPCZL. This compound led to maximum external quantum efficiency, luminous efficiency, power efficiency and luminance of 4.83 %, 4.66 cd A−1, 2.93 lm W−1 and 1560 cd m−2, respectively.

Electroluminescence data for the devices in which helical diamines serve as host materials.

5 [a] K, L, and M refer to the device configurations: K) ITO/NPB (40 nm)/CHCZL:[Ir(btp)2acac] (10 %, 20 nm)/TPBI (35 nm)/LiF (1 nm)/Al (100 nm), L) ITO/NPB (40 nm)/AHCZL:[Ir(btp)2acac] (20 nm)/PBD (40 nm)/LiF (2 nm)/Al (150 nm), and M) ITO/NPB (40 nm)/TCTA (10 nm)/AHCZL:[Ir(ppy)3] (20 nm)/PBD (40 nm)/LiF (2 nm)/Al (150 nm). [b] Turn ‐ on voltage [V]. [c] Maximum external quantum efficiency [%]. [d] Maximum power efficiency [lm W−1]. [e] Maximum luminance efficiency [cd A−1]. [f] Maximum luminance achieved [cd m−2]. [g] λmax (EL) [nm]. [h] 1931 chromaticity coordinates measured at 8 V.

As mentioned earlier, the triplet energies of AHCZL and AHCZLt are higher than that of the green dopant, that is, [Ir(ppy)3]. Indeed, when a device of configuration M: ITO/NPB (40 nm)/TCTA (10 nm)/AHCZL:[Ir(ppy)3] (20 nm)/PBD (40 nm)/LiF (2 nm)/Al (150 nm) was fabricated in which AHCZL was employed as the host material, respectable device performance results were obtained (Table [NaN] ). The device produced pure green emission that is characteristic of [Ir(ppy)3]; I ‐ V ‐ L characteristics and EL spectrum of the device M are shown in Figure [NaN] . The device produced a maximum luminous efficiency of 23.0 cd A−1 at 5 V; corresponding external quantum efficiency and power efficiency were 8.72 % and 18.2 lm W−1, respectively. Further, the maximum brightness obtained was 4200 cd m−2 at 11.5 V, which is significantly higher than those obtained from the devices fabricated with the red dopant, that is, [Ir(btp)2acac]. Clearly, energy transfer to the green dopant is more efficient when compared with that of the red dopant.

Although unremarkable, the above results compellingly demonstrate that the helical compounds can indeed serve as host materials for red PhOLED devices. Amongst carbo ‐ and azahelical systems, the latter exhibit higher triplet energies than those of the carbohelical systems. Based on the observation that AHCZL and AHCZLt possess triplet energies significantly higher than those of the others, these compounds were exploited as host materials for green dopant, that is [Ir(ppy)3]. The device performance results with AHCZL as a host for [Ir(ppy)3] are, however, moderate, (Table [NaN] ). Notwithstanding the fact that the device performance results with carbo ‐ and azahelical diamines as host materials do not compare with the best materials reported so far, the fact that helicity can be exploited for creation of new host materials is compellingly borne out.

A set of eight helical dimines based on carbo ‐ and aza[5]helicene scaffolds was rationally designed and synthesized for diverse applications in OLEDs; the syntheses of all helicenes were readily accomplished by Suzuki and Buchwald – Hartwig couplings as key reactions. The X ‐ ray crystal structure determinations for CHCZL and CHDPA reveal that the helical scaffold is significantly twisted. All the diamines were found to exhibit high thermal stabilities, as evidenced by their Td values in the range of 383 – 514 °C. The Tg values of as ‐ synthesized diamines were found to be moderate to very high (115 – 214 °C) with the higher values generally observed for azahelical diamines. It should be remarked that the Tg values of all diamines are significantly higher than those of popular commercial diarylamines such as NPB, TPD, CBP, and so on. Their high Tg values are ascribed to the rigidity and twisting induced by the helical scaffold. Insofar as their photophysical properties are concerned, their quantum yields of emission are found to be in the range of 4.5 – 34.6 % with their band gap energies lying between 2.79 – 2.99 eV. The triplet energies of carbohelical diamines are found to be 2.3 – 2.4 eV, whereas their aza ‐ analogues exhibit slightly higher energies.

Based on suitable HOMO – LUMO energies, emission properties and triplet energies, the helical diamines were employed as 1) hole ‐ transporting materials (HTMs); 2) emissive materials (EMs); 3) bifunctional materials, that is, hole transport+emission; and 4) host materials in the devices to elicit electroluminescence. The azahelical diamines are demonstrated to serve as excellent HTMs with performance results that are superior to those obtained with popular NPB. They are shown to be applicable as EMs, and better under doping conditions. It is shown that carbohelical CHTPA can serve the dual purpose of hole transport as well as emission in simple double layer devices. Indeed, the results constitute first demonstration of a bifunctional property for helicenes. Furthermore, the performance efficiencies (external quantum efficiency: 2.75 %; luminous efficiency: 3.99 cd A−1; power efficiency: 3.13 lm W−1) of a CHTPA ‐ based double layer device are the best amongst those of all helicenes reported so far. The high triplet energies of helical diamines were exploited to fabricate red PhOLED devices in which they serve as host materials. It is also shown that AHCZL (based on its triplet energy of 2.54 eV) can be utilized as a host material for green PhOLED devices as well. Indeed, when doped with [Ir(ppy)3], AHCZL is shown to exhibit respectable performance as a host material; the devices fabricated are shown to lead to maximum luminous efficiency and luminance of 29.0 cd A−1 and 4200 cd m−2, respectively. In a nutshell, diverse applications as materials in OLEDs have been demonstrated for a small set of diamines based on helicenes, which have long elicited interest as aesthetic marvels. Given their ease of synthesis, the present results exemplify the potential for exploitation of helicity as a design element in the creation of organic materials with new properties in different areas.

2,13 ‐ Dibromopentahelicene: 6 ‐ Bromophenanthrene ‐ 3 ‐ carbaldehyde was synthesized following a previously reported procedure.[68] A 50 % aqueous solution of NaOH at 0 °C was added dropwise to a mixture of 6 ‐ bromophenanthrene ‐ 3 ‐ carbaldehyde (0.050 g, 0.18 mmol) and (4 ‐ bromobenzyl)triphenylphosphonium bromide (0.10 g, 0.20 mmol) in 5 mL distilled DCM (0.2 mL). After 10 min of stirring at this temperature, the reaction mixture was warmed to room temperature and continued stirring for 3 h. At the end of this period, water was added to the reaction mixture and extracted twice with DCM. The combined organic extract was dried over anhydrous Na2SO4 and stripped off solvent to obtain the crude product. The latter was subjected to a short pad silica gel filtration to isolate a mixture of cis and trans isomers; yield 96 % (0.078 g).

The thus ‐ derived mixture of geometrical isomers was dissolved in toluene (200 mL) and iodine (0.03 g, 0.11 mmol) was added. The resulting dilute solution was subjected to irradiation with 350 nm radiation in a photoreactor for 2 d. At the end of this period, toluene was removed under reduced pressure to obtain the crude solid, which was dissolved in chloroform and washed with sodium thiosulfate solution. The combined organic extract was dried over anhydrous Na2SO4, filtered, and evaporated to obtain the crude product, which was subjected to silica gel column chromatography to isolate the 2,13 ‐ dibromopentahelicene as a yellowish powder; yield 86 % (0.065 g). M.p. 227 °C; 1H NMR (CDCl3, 400 MHz): δ=7.64 (dd, J1=8.72 Hz, J2=2.32 Hz, 2 H), 7.82 (d, J=8.72 Hz 2 H), 7.87 (s, 6 H), 8.68 ppm (d, J=1.84 Hz, 2 H); 13C NMR (CDCl3, 100 MHz): δ=118.9, 125.7, 126.8, 127.2, 127.8, 129.5, 129.7, 130.9, 131.1, 131.5, 132.7 ppm; IR (KBr): ν˜ =3040, 1614, 1590, 1474, 1424, 1391, 1294 cm−1; EI ‐ MS+: m/z: 433.9307 [M+] [C22H12Br2+].

CHCZL: An oven ‐ dried pressure tube was charged with dry toluene (5 mL) and degassed thoroughly by bubbling N2 gas for 10 min. Dibromopentahelicene (0.05 g, 0.11 mmol), carbazole (0.04 g, 0.25 mmol), sodium tert ‐ butoxide (0.05 g, 0.44 mmol), P(tBu)3 (2 μL, 0.002 mmol), and [Pd(OAc)2] (0.004 g, 0.002 mmol) were then added. Subsequently, the pressure tube was capped tightly under nitrogen atmosphere, and the contents were heated at 100 °C for 48 h. At the end of this period, the pressure tube was cooled to room temperature and the solvent was removed in vacuo. The crude reaction mixture was extracted with chloroform three times, and the combined extract was dried over anhydrous Na2SO4. The crude product obtained subsequent to evaporation of the solvent was purified by silica gel column chromatography to afford CHCZL as a colorless solid; yield 94 % (0.065 g). M.p. 343 °C; 1H NMR (CDCl3, 400 MHz): δ=6.75 – 6.86 (m, 8 H), 7.12 – 7.15 (m, 4 H), 7.39 (dd, J1=8.72 Hz, J2=1.84 Hz, 2 H), 7.98 – 8.01 (m, 8 H), 8.07 (d, J=7.76 Hz, 4 H), 8.88 ppm (d, J=1.84 Hz, 2 H); 13C NMR (CDCl3, 125 MHz): δ=109.5, 119.8, 123.1, 125.8, 125.9, 126.6, 126.8, 127.5, 127.7, 130.0, 131.5, 131.7, 132.8, 134.7, 140.8 ppm; IR (KBr): ν˜ =3047, 1610, 1594, 1514, 1492, 1477, 1450, 1331, 1311 cm−1; ESI ‐ MS+: m/z: 609.2335 [M++H] [C46H29N2+].

CHDPA: In an oven ‐ dried pressure tube, dry toluene (10 mL) was degassed thoroughly by bubbling N2 gas for 10 min. Dibromopentahelicene (0.47 g, 1.08 mmol), diphenylamine (0.55 g, 3.26 mmol), sodium tert ‐ butoxide (0.48 g, 4.32 mmol), P(tBu)3 (26 μL, 0.11 mmol) and [Pd(OAc)2] (0.024 g, 0.11 mmol) were later introduced. Subsequently, the pressure tube was capped tightly under nitrogen and the contents were heated at 100 °C for 48 h. At the end of this period, the pressure tube was cooled to room temperature and the solvent removed in vacuo. The crude mixture was extracted three times with chloroform, and the combined organic extract was dried over anhydrous Na2SO4. Evaporation of the solvent under vacuum led to the crude product, which was purified by silica gel column chromatography to afford CHDPA as a yellowish ‐ green solid, yield 82 % (0.55 g). M.p. 262 °C; 1H NMR (CDCl3, 500 MHz): δ=6.88 (t, J=7.45 Hz, 4 H), 7.01 – 7.09 (m, 16 H), 7.22 (dd, J1=8.60 Hz, J2=1.75 Hz, 2 H), 7.66 (d, J=8.00 Hz, 2 H), 7.72 – 7.76 (m, 6 H), 8.23 ppm (d, J=2.30 Hz, 2 H); 13C NMR (CDCl3, 125 MHz): δ=121.7, 122.7, 124.2, 124.4, 124.6, 126.4, 126.95, 126.99, 128.83, 128.88, 129.0, 132.0, 132.3, 144.2, 147.3 ppm; IR (KBr): ν˜ =3034, 1587, 1509, 1489, 1437, 1313 cm−1; ESI ‐ MS+: m/z: 613.2645 [M++H] [C46H33N2+].

CHTPA: Toluene (3 mL), ethanol (2 mL) and distilled water (1 mL) were added to an oven ‐ dried pressure tube. The solvent mixture was degassed thoroughly by bubbling N2 gas for 10 min. Subsequently, dibromopentahelicene (0.20 g, 0.45 mmol), (4 ‐ (diphenylamino)phenyl)boronic acid (0.53 g, 1.83 mmol), NaOH (0.11 g, 2.74 mmol) and [Pd(PPh3)4] (0.24 g, 0.09 mmol) were added, and the pressure tube was capped tightly under N2 gas. The contents were heated at 110 °C for 48 h. At the end of the period, the pressure tube was cooled to room temperature, and toluene and ethanol were removed in vacuo. The resultant residue was extracted three times with chloroform and the combined organic extract was dried over anhydrous Na2SO4. Evaporation of the solvent led to the crude product, which was subjected to silica gel column chromatography to afford pure CHTPA as a yellowish ‐ green solid; yield 90 % (0.32 g); M.p. 287 °C; 1H NMR (CDCl3, 500 MHz): δ=6.94 (d, J=8.55 Hz, 4 H), 6.99 – 7.05 (m, 12 H), 7.20 – 7.26 (m, 12 H), 7.75 (d, J=8.60 Hz, 2 H), 7.84 – 7.88 (m, 4 H), 7.92 (d, J=8.60 Hz, 2 H), 7.99 (d, J=8.00 Hz, 2 H), 8.89 ppm (s, 2 H); 13C NMR (CDCl3, 125 MHz): δ=122.8, 123.9, 124.3, 125.1, 126.2, 126.6, 127.11, 127.17, 127.3, 127.9, 128.6, 129.2, 131.1, 131.6, 132.6, 135.0, 136.6, 146.9, 147.6 ppm; IR (KBr): ν˜ =3034, 1589, 1519, 1492, 1437, 1313 cm−1; ESI ‐ MS+: m/z: 765.3273 [M++H] [C58H41N2+].

• 6 ‐ Bromo ‐ 9 ‐ methyl ‐ 9H ‐ carbazole ‐ 3 ‐ carbaldehyde: 9 ‐ Methyl ‐ 9H ‐ carbazole ‐ 3 ‐ carbaldehyde was synthesized following a reported procedure.[69] A two ‐ necked round bottom flask fitted with a CaCl2 guard tube was charged with 9 ‐ methyl ‐ 9H ‐ carbazole ‐ 3 ‐ carbaldehyde (0.50 g, 2.39 mmol) and N,N ‐ dimethylformamide (15 mL). The clear solution was cooled to 0 °C and N ‐ bromosuccinimide (0.45 g, 2.51 mmol) was introduced in small portions. Subsequently, the reaction mixture was allowed to warm up to room temperature and stir for 3 h. At the end of this period, water was added to the reaction mixture, and the solid residue formed was filtered and washed thoroughly with water to obtain 6 ‐ bromo ‐ 9 ‐ methyl ‐ 9H ‐ carbazole ‐ 3 ‐ carbaldehyde as an off ‐ white solid; yield 98 % (0.67 g); M.p. 145 °C; 1H NMR (CDCl3, 500 MHz): δ=3.86 (s, 3 H), 7.30 (d, J=8.60 Hz, 1 H), 7.45 (d, J=8.60 Hz, 1 H), 7.61 (dd, J1=8.60 Hz, J2=2.30 Hz, 1 H), 8.03 (dd, J1=8.60 Hz, J2=1.70 Hz, 1 H), 8.22 (d, J=2.30 Hz, 1 H), 8.51 (d, J=1.70 Hz, 1 H), 10.08 ppm (s, 1 H); 13C NMR (CDCl3, 125 MHz): δ=29.5, 109.1, 110.6, 113.3, 121.9, 123.4, 124.2, 124.5, 127.6, 128.9, 129.5, 140.3, 144.6, 191.5 ppm; IR (KBr): ν˜ =2800, 2707, 1688, 1665, 1626, 1591, 1570, 1493, 1480, 1451, 1364, 1332 cm−1; EI ‐ MS+: m/z: 286.9937 [M+] [C14H10BrNO+].
• 2,12 ‐ Dibromo ‐ 9 ‐ methyl ‐ 9H ‐ naphtho[2,1 ‐ c]carbazole: An aqueous solution of NaOH (50 %) was added dropwise at 0 °C to a mixture of 6 ‐ bromo ‐ 9 ‐ methyl ‐ 9H ‐ carbazole ‐ 3 ‐ carbaldehyde (0.35 g, 1.23 mmol) and (4 ‐ bromobenzyl)triphenylphosphonium bromide (0.76 g, 1.48 mmol) in distilled DCM (1 mL). After stirring for 10 min at this temperature, the reaction mixture was warmed to room temperature and the stirring was continued for additional 3 h. At the end of this period, water was added to the reaction mixture and the organic matter was extracted twice with DCM. The combined organic extract was dried over anhydrous Na2SO4 and stripped off solvent to obtain the crude product, which was subjected to a short pad silica gel filtration to afford a mixture of cis and trans isomers, yield 93 % (0.50 g). This mixture of geometrical isomers was dissolved in toluene (700 mL) and iodine (0.20 g, 0.79 mmol) was added. The resulting dilute solution was irradiated in a photoreactor fitted with 350 nm radiation for 2 d. At the end of this period, toluene was removed under reduced pressure to obtain the crude solid, which was dissolved in chloroform and washed with sodium thiosulfate solution. The combined chloroform solution was dried over anhydrous Na2SO4, filtered and evaporated to obtain the crude product, which was subjected to silica gel column chromatography to isolate the desired dibromo ‐ substituted azahelical compound, that is, 2,12 ‐ dibromo ‐ 9 ‐ methyl ‐ 9H ‐ naphtho[2,1 ‐ c]carbazole, as a yellowish powder, yield 77 % (0.38 g); M.p. 216 °C; 1H NMR (CDCl3, 500 MHz): δ=3.93 (s, 3 H), 7.38 (d, J=8.85 Hz, 1 H), 7.59 (dd, J1=8.55 Hz, J2=1.85 Hz, 1 H), 7.66 (t, J=7.95 Hz, 2 H), 7.74 (dd, J1=8.55 Hz, J2=2.20 Hz, 1 H), 7.81 (d, J=6.10 Hz, 1 H), 7.83 (d, J=6.10 Hz, 1 H), 7.88 (d, J=8.85 Hz, 1 H), 8.91 (d, J=1.55 Hz, 1 H), 9.44 ppm (d, J=1.25 Hz, 1 H); 13C NMR (CDCl3, 125 MHz): δ=29.5, 110.37, 110.45, 111.6, 115.4, 118.5, 123.6, 124.9, 125.8, 126.8, 127.5, 127.7, 127.8, 129.4, 129.6, 129.9, 130.5, 131.5, 139.3, 141.1 ppm; IR (KBr): ν˜ =2927, 1581, 1516, 1471, 1438, 1360, 1335, 1301 cm−1; EI ‐ MS+: m/z: 436.9415 [M+] [C21H13Br2N+].

AHCZL: An oven ‐ dried pressure tube was cooled under N2 gas and charged with dry toluene (15 mL). The latter was degassed thoroughly by bubbling N2 gas for 10 min. Subsequently, 2,12 ‐ dibromo ‐ 9 ‐ methyl ‐ 9H ‐ naphtho[2,1 ‐ c]carbazole (0.60 g, 1.36 mmol), carbazole (0.50 g, 3.00 mmol), sodium tert ‐ butoxide (0.61 g, 5.46 mmol), P(tBu)3 (100 μL, 0.27 mmol) and [Pd(OAc)2] (0.06 g, 0.27 mmol) were added, and the pressure tube was capped tightly under nitrogen. The contents were heated at 100 °C for 48 h. At the end of this period, the pressure tube was cooled to room temperature and the solvent was removed in vacuo. The crude mixture was extracted three times with chloroform, and the combined organic extract was dried over anhydrous Na2SO4, filtered and evaporated under reduced pressure to obtain the crude product, which was purified by silica gel column chromatography to afford AHCZL as an off ‐ white solid; yield 82 % (0.68 g); M.p.>350 °C; 1H NMR (CDCl3, 500 MHz): δ=4.09 (s, 3 H), 6.74 (t, J=7.63 Hz, 2 H), 6.81 (d, J=8.55 Hz, 2 H), 6.92 – 6.97 (m, 6 H), 7.13 (t, J=7.02 Hz, 2 H), 7.35 (dd, J1=8.55 Hz, J2=2.45 Hz, 1 H), 7.56 (dd, J1=8.55 Hz, J2=2.15 Hz, 1 H), 7.62 (d, J=8.55 Hz, 1 H), 7.84 (d, J=8.55 Hz, 1 H), 7.87 (d, J=9.15 Hz, 1 H), 7.91 (d, J=7.30 Hz, 2 H), 8.00 (d, J=9.15 Hz, 1 H), 8.05 – 8.07 (m, 3 H), 8.11 (d, J=7.95 Hz, 1 H), 8.84 (d, J=1.85 Hz, 1 H), 9.52 ppm (d, J=1.25 Hz, 1 H); 13C NMR (CDCl3, 125 MHz): δ=29.7, 109.12, 109.17, 110.2, 110.6, 116.4, 119.2, 119.5, 119.78, 119.82, 122.1, 123.0, 123.9, 124.1, 125.3, 125.43, 125.45, 126.5, 126.8, 127.5, 127.8, 128.2, 128.7, 129.7, 130.4, 132.5, 134.4, 139.9, 141.5, 141.6, 141.9 ppm; IR (KBr): ν˜ =3044, 2927, 1608, 1592, 1511, 1491, 1476, 1450, 1361, 1333, 1312 cm−1; ESI ‐ MS+: m/z: 612.2444 [M++H] [C45H30N3+].

AHDPA: A similar C−N coupling protocol as described for the synthesis of AHCZL was followed for the synthesis of AHDPA. The reaction between 2,12 ‐ dibromo ‐ 9 ‐ methyl ‐ 9H ‐ naphtho[2,1 ‐ c]carbazole (0.60 g, 1.36 mmol) and diphenylamine (0.51 g, 3.00 mmol) in the presence of sodium tert ‐ butoxide (0.61 g, 5.46 mmol), P(tBu)3 (108 μL, 0.27 mmol) and [Pd(OAc)2] (0.06 g, 0.27 mmol) led to AHDPA, which was isolated as yellow solid after routine work ‐ up and column chromatography; yield 86 % (0.72 g); M.p. 240 °C; 1H NMR (CDCl3, 400 MHz): δ=3.94 (s, 3 H), 6.89 – 6.98 (m, 10 H), 7.11 – 7.15 (m, 9 H), 7.27 – 7.34 (m, 3 H), 7.41 (d, J=8.68 Hz, 1 H), 7.64 (dd, J1=8.72 Hz, J2=2.76 Hz, 2 H), 7.73 (d, J=8.24 Hz, 1 H), 7.81 (d, J=8.68 Hz, 1 H), 7.87 (d, J=8.72 Hz, 1 H), 8.70 (s, 1 H), 9.01 ppm (d, J=1.84 Hz, 1 H); 13C NMR (CDCl3, 100 MHz): δ=29.5, 109.7, 110.0, 116.4, 121.1, 122.21, 122.25, 122.4, 122.5, 123.5, 124.0, 124.5, 125.7, 125.8, 126.2, 127.2, 127.3, 128.7, 129.0, 129.2, 129.5, 130.6, 138.0, 144.4, 147.7 ppm; IR (KBr): ν˜ =3036, 2925, 1586, 1490, 1450, 1430, 1367, 1341, 1309 cm−1; ESI ‐ MS+: m/z: 615.2678 [M+] [C45H33N3+].

AHTPA: An oven ‐ dried pressure tube was cooled to room temperature under N2 gas and charged with toluene (12 mL), ethanol (8 mL) and distilled water (4 mL). The solvent mixture was degassed thoroughly by bubbling N2 gas for 10 min. Subsequently, 2,12 ‐ dibromo ‐ 9 ‐ methyl ‐ 9H ‐ naphtho[2,1 ‐ c]carbazole (0.80 g, 1.82 mmol), (4 ‐ (diphenylamino)phenyl)boronic acid (2.10 g, 7.28 mmol), NaOH (0.43 g, 10.93 mmol) and [Pd(PPh3)4] (0.42 g, 0.36 mmol) were added, and the pressure tube was capped tightly under N2 gas. The contents were heated at 100 °C for 48 h. At the end of this period, the pressure tube was cooled to room temperature, and the solvent was removed in vacuo. The residue was extracted three times with chloroform and the combined organic extract was dried over anhydrous Na2SO4 and filtered. Evaporation of the solvent led to the crude product, which was subjected to silica gel column chromatography to afford AHTPA as an yellow solid; yield 95 % (1.31 g); M.p. 278 °C; 1H NMR (CDCl3, 500 MHz): δ=4.05 (s, 3 H), 6.94 – 7.07 (m, 16 H), 7.15 – 7.25 (m, 10 H), 7.54 (d, J=8.55 Hz, 2 H), 7.59 (d, J=8.55 Hz, 1 H), 7.72 – 7.76 (m, 3 H), 7.83 – 7.86 (m, 2 H), 7.95 (d, J=8.55 Hz, 1 H), 8.02 (d, J=7.95 Hz, 1 H), 9.00 (s, 1 H), 9.67 ppm (s, 1 H); 13C NMR (CDCl3, 125 MHz): δ=29.6, 109.1, 110.0, 116.9, 121.9, 122.5, 123.0, 123.4, 123.7, 124.1, 124.2, 124.4, 124.6, 124.7, 125.8, 126.3, 127.29, 127.34, 127.6, 128.0, 128.1, 128.2, 129.1, 129.3, 129.8, 131.7, 131.9, 135.1, 136.6, 137.0, 140.0, 141.3, 146.2, 147.1, 147.6, 147.9 ppm; IR (KBr): ν˜ =3036, 2925, 1587, 1514, 1492, 1483, 1456, 1362, 1311 cm−1; ESI ‐ MS+: m/z: 767.3302 [M+] [C57H41N3+].

AHCZLt: A similar C−N coupling protocol as that described for the synthesis of AHCZL was followed for the synthesis of AHCZLt. Reaction between 2,12 ‐ dibromo ‐ 9 ‐ methyl ‐ 9H ‐ naphtho[2,1 ‐ c]carbazole (0.47 g, 1.08 mmol) and 3,6 ‐ di ‐ tert ‐ butylcarbazole (0.66 g, 2.38 mmol) in the presence of sodium tert ‐ butoxide (0.48 g, 4.32 mmol), P(tBu)3 (88 μL, 0.22 mmol) and [Pd(OAc)2] (0.05 g, 0.22 mmol) led to AHCZLt as a colorless solid after routine work ‐ up followed by column chromatography; yield 75 % (0.67 g). M.p.>350 °C; 1H NMR (CDCl3, 500 MHz): δ=1.35 (s, 18 H), 1.43 (s, 18 H), 4.09 (s, 3 H), 6.90 (dd, J1=8.55 Hz, J2=1.80 Hz, 2 H), 6.96 (d, J=8.55 Hz, 2 H), 7.01 – 7.05 (m, 4 H), 7.39 (dd, J1=8.55 Hz, J2=1.85 Hz, 1 H), 7.53 (dd, J1=8.55 Hz, J2=1.55 Hz, 1 H), 7.60 (d, J=8.55 Hz, 1 H), 7.85 (t, J=8.22 Hz, 2 H), 7.97 – 8.02 (m, 5 H), 8.07 (t, J=7.95 Hz, 2 H), 9.11 (s, 1 H), 9.73 ppm (s, 1 H); 13C NMR (CDCl3, 125 MHz): δ=29.7, 31.9, 32.0, 34.51, 34.56, 109.1, 109.3, 110.1, 110.5, 115.96, 115.98, 116.5, 121.1, 123.1, 123.22, 123.25, 123.4, 123.91, 123.95, 124.6, 125.3, 126.5, 127.6, 127.8, 128.0, 129.7, 129.8, 130.4, 132.2, 135.5, 139.4, 140.17, 140.26, 141.6, 141.9, 142.2 ppm; IR (KBr): ν˜ =3045, 2956, 2901, 2864, 1612, 1588, 1567, 1511, 1488, 1474, 1362, 1325 cm−1; ESI ‐ MS+: m/z: 836.4943 [M++H] [C61H62N3+].

AHPCZL: A similar Suzuki coupling protocol as that described for the synthesis of AHTPA was followed for the synthesis of AHPCZL. The reaction of 2,12 ‐ dibromo ‐ 9 ‐ methyl ‐ 9H ‐ naphtho[2,1 ‐ c]carbazole (0.80 g, 1.82 mmol) with (4(4 ‐ (9H ‐ carbazol ‐ 9 ‐ yl)phenyl)boronic acid (2.08 g, 7.28 mmol) in the presence of NaOH (0.43 g, 10.93 mmol) and [Pd(PPh3)4] (0.42 g, 0.36 mmol) followed by work ‐ up afforded the crude product. The latter was suspended in methanol (25 mL), sonicated for 10 min, filtered and air ‐ dried to afford AHPCZL as an off ‐ white solid; yield 94 % (1.30 g). M.p. 217 °C; 1H NMR (CDCl3, 400 MHz): δ=4.10 (s, 3 H), 6.96 – 7.00 (m, 2 H), 7.11 – 7.22 (m, 8 H), 7.37 (d, J=8.08 Hz, 2 H), 7.41 (d, J=8.44 Hz, 2 H), 7.58 (d, J=8.44 Hz, 2 H), 7.61 (d, J=8.40 Hz, 2 H), 7.70 (d, J=8.40 Hz, 1 H), 7.79 (d, J=8.80 Hz, 1 H), 7.82 (d, J=8.44 Hz, 1 H), 7.88 (dd, J1=8.44 Hz, J2=1.68 Hz, 1 H), 7.93 – 8.02 (m, 5 H), 8.04 (d, J=7.68 Hz, 2 H), 8.10 – 8.14 (m, 3 H), 9.21 (d, J=1.48 Hz, 1 H), 9.90 ppm (d, J=1.08 Hz, 1 H); 13C NMR (CDCl3, 125 MHz): δ=29.7, 109.5, 109.6, 109.7, 110.2, 116.9, 119.7, 120.0, 120.2, 122.3, 123.2, 123.5, 123.8, 124.4, 124.6, 125.8, 125.9, 126.9, 127.33, 127.35, 127.6, 127.7, 127.9, 128.1, 128.54, 128.59, 128.9, 129.8, 131.2, 132.5, 136.1, 136.3, 137.0, 140.5, 140.7, 140.8, 141.3, 141.5 ppm; IR (KBr, cm−1): ν˜ =3040, 2926, 1598, 1584, 1519, 1500, 1479, 1451, 1362, 1334, 1316 cm−1; ESI ‐ MS+: m/z: 763.2986 [M+] [C57H37N3+].

CCDC 1452879 (CHCZL) and 1452880 (CHDPA) contain the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centre These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif.

J.N.M. is thankful to SERB, New Delhi, for generous financial support. S.J. and A.K.; Brunner, A. v. Dijken, H. Börner, J. J. A. M. Bastiaansen, N. M. M. Kiggen, B. M. W. Langeveld, J. Am. Chem. Soc. 2004, 126, 6035 – 6042M. are grateful to CSIR and UGC, New Delhi, respectively, for senior research fellowships. We acknowledge the optoelectronic device fabrication and testing by the scientific instrument facility at the Institute of Chemistry, Academia Sinica, Taipei.

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re ‐ organized for online delivery, but are not copy ‐ edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Graph: Structures of the helical compounds previously exploited in OLEDs.

Graph: Structures of the target helical diamines. Note that the crossing point between the single bonds in the bay region of the helical scaffold of each structure does not correspond to a quaternary carbon.

Graph: Synthesis of dibromopentahelicenes and subsequent functionalizations Suzuki/Buchwald – Hartwig amination reactions. Reagents and conditions: a) carbazole, [Pd(OAc)2], P(tBu)3, tBuONa, dry toluene, 100 °C, 48 h; b) diphenylamine, [Pd(OAc)2], P(tBu)3, tBuONa, dry toluene, 100 °C, 48 h; c) (4 ‐ (diphenylamino)phenyl)boronic acid, [Pd(PPh3)4], NaOH, toluene/ethanol/water (3:2:1), 100 °C, 48 h; d) 3,6 ‐ di ‐ tert ‐ butylcarbazole, [Pd(PPh3)4], NaOH, toluene/ethanol/water (3:2:1), 100 °C, 36 h; e) (4 ‐ (9H ‐ carbazol ‐ 9 ‐ yl)phenyl)boronic acid, [Pd(PPh3)4], NaOH, toluene/ethanol/water (3:2:1), 100 °C, 36 h.

Graph: X ‐ ray determined ORTEP molecular structures of CHCZL (a) and CHDPA (b). The latter includes chloroform in its crystal lattice (see the Supporting Information).

Graph: Normalized absorption spectra of a) carbohelical and b) azahelical diamines, and fluorescence spectra of c) carbohelical and d) azahelical diamines. The fluorescence spectra were recorded for excitation at 341 nm.

Graph: Phosphorescence spectra of a) carbohelical and b) azahelical diamines in 2 ‐ methyltetrahydrofuran (ca. 1×10−5 m) at 77 K.

Graph: The cyclic voltammograms of representative helical diamines, namely, CHTPA and AHTPA.

Graph: a) Current density versus voltage and b) luminance versus voltage profiles for the devices of configuration A fabricated with azahelical diamine/NPB as an HTM and PTPA as an EM; c) typical EL spectrum of the devices of configuration A; d) energy level diagram of the HOMOs and LUMOs of the materials employed in the devices of configuration A.

Graph: a) Current density versus voltage and b) luminance versus voltage profiles for the devices of configuration B; c) typical EL spectra; d) alignment of HOMO and LUMO levels of the materials employed in the devices of configuration B.

Graph: a) I ‐ V ‐ L profile and b) typical EL spectrum for the devices of configuration G in which CHTPA serves as an HTM as well as an EM.

Graph: a) Current density versus voltage and b) luminance versus voltage profiles for the devices of configurations K and L in which helical diamines serve as host materials for the red dopant, that is, [Ir(btp)2acac]; c) typical EL spectrum for the emission of red dopant, that is, [Ir(btp)2acac]; d) energy level diagram for the red PhOLED device fabricated with AHPCZL as a host material.

Graph: a) I ‐ V ‐ L profile and b) typical EL spectrum of device M in which AHCZL serves as a host material for the green dopant, that is, [Ir(ppy)3].

Graph: Supplementary