& Helical Structures

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

Samik Jhulki,[a] Abhaya Kumar Mishra,[a] Tahsin J. Chow,*[b] and Jarugu Narasimha Moorthy*[a]

Abstract: A set of eight helical diamines were designed andsynthesized to demonstrate their relevance as all-in-one ma-terials for multifarious applications in organic light-emittingdiodes (OLEDs), that is, as hole-transporting materials(HTMs), EMs, bifunctional hole transporting + emissive ma-terials, and host materials. Azahelical diamines function verywell as HTMs. Indeed, with high Tg values (127–214 8C), theyare superior alternatives to popular N,N’-di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB). All the helical di-amines exhibit emissive properties when employed in non-doped as well as doped devices, the performance character-istics being superior in the latter. One of the carbohelical di-

amines (CHTPA) serves the dual function of hole transport aswell as emission in simple double-layer devices; the efficien-cies observed were better by quite some margin than thoseof other emissive helicenes reported. The twisting endowshelical diamines with significantly high triplet energies suchthat they also function as host materials for red and greenphosphors, that is, [Ir(btp)2acac] (btp = 2-(2’-benzothienyl)-pyridine; acac = acetylacetonate) and [Ir(ppy)3] (ppy = 2-phe-nylpyridine), respectively. The results of device fabricationsdemonstrate how helicity/ helical scaffold may be diligentlyexploited to create molecular systems for maneuvering di-verse applications in OLEDs.

Introduction

Helicity is all-pervasive. From gigantic galaxies to microscopicDNA, it prevails in wide facets of our existence and imagina-tion. In the molecular world, helicity continues to enjoy un-stinted privilege for its aesthetic charm.[1] Creation of helicalcompounds with unique structural attributes and propertiesremains an unrelenting pursuit.[1c] The recent literature revealsa surge of interest in helical structures for exploration as mo-lecular springs,[2] solenoids,[3] tweezers,[4] motors,[5] IR-sensingmaterials,[6] dye-sensitized solar cell (DSSC) materials,[7] liquidcrystals,[8] NLO materials,[9] optoelectronic materials,[10] and soon. In the realm of organic light-emitting diodes (OLEDs),which have captured the market of lighting and display cur-rently, one witnesses an explosion in the development of ma-terials for application in devices over the past decade;[11]

OLEDs offer unrivaled advantages from the points of view ofproduction cost, power consumption, wide-angle viewability,

contrast ratio, opportunities for flexible displays, and so on.The hunt for newer materials with properties that surpass theexisting ones is an incessant quest. Given how advanced theresearch in OLEDs is, one surprisingly finds only a few scat-tered reports on exploitation of helicene-based materials inOLEDs.[12] Some helical systems explored by different groupsfor electroluminescence are shown in Figure 1.

We have been concerned with control of macroscopic orderand disorder in a bottom-up approach involving de novodesign of molecular systems and their structural manipula-tions.[13, 14] The marvelous allure of helical structures and theirutility as steric scaffolds for controlling photochromism in ourrecent investigations[15] were the motivations to develop mate-rials with all-in-one attributes for application in OLEDs. Accord-ingly, we designed eight new helical diamines, shown inFigure 2, based on carbo[5]helicene and monoaza[5]helicenecores by two-fold substitution at the termini with groups suchas carbazole, 3,6-di-tert-butylcarbazole, diphenylamine, triphe-nylamine, and phenylcarbazole. Herein, we report that the heli-cal diamines in Figure 2 can be readily synthesized and that

Figure 1. Structures of the helical compounds previously exploited in OLEDs.

[a] S. Jhulki, A. K. Mishra, Prof. Dr. J. N. MoorthyDepartment of Chemistry, Indian Institute of TechnologyKanpur 208016 (India)Fax: (+ 91) 512-2596806E-mail : moorthy@iitk.ac.in

[b] Prof. Dr. T. J. ChowInstitute of Chemistry, Academia Sinica, TaipeiTaiwan 115 (Republic of China)E-mail : chowtj@gate.sinica.edu.tw

Supporting information for this article (including CV, TGA, DSC profiles, ELand efficiency plots for the devices constructed, and 1H and 13C NMR spec-tral reproductions of the compounds reported) is available on the WWWunder http://dx.doi.org/10.1002/chem.201600668.

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Full PaperDOI: 10.1002/chem.201600668

they exhibit photophysical, electrochemical, and thermal prop-erties suitable for a range of applications in OLED devices. It isshown that 1) their fluorescence properties allow exploitationas emissive materials (EMs); 2) high HOMO levels as a conse-quence of electron-richness permit utility as hole-transportingmaterials (HTMs); 3) electron-rich nature as well as emissive be-havior can be exploited to develop bifunctional materials withhole-transporting as well as emissive properties, wherebydevice fabrication is simplified; and 4) the twisted structuresendow them with triplet energies that are significantly highenough as to be suitable as host materials for red and greenphosphorescent dopants, that is, [Ir(btp)2acac] and [Ir(ppy)3] ,respectively. Applicability of a small set of diamines, featuringhelicity as a design element, is demonstrated for multifariousapplications in OLEDs.

Results and Discussion

Synthesis and crystal structure determinations

All the molecular systems in Figure 2 were synthesized basedon three key reactions, namely, 1) Wittig olefination to furnishdiarylethylenes; 2) oxidative photocyclization of the latter toafford dibromo-substituted helicenes; and 3) Pd0-catalyzedSuzuki and Buchwald–Hartwig coupling of dibromo-heliceneswith appropriate boronic acids and diarylamines, respectively.The synthetic routes for all the helical diamines in Figure 2 areshown in Scheme 1. All the compounds were characterized bycomprehensive spectroscopic data (see the Supporting Infor-mation).

Furthermore, the structures of helical diamines were un-equivocally established by single crystal X-ray structure deter-

minations for at least two cases, namely, CHCZL and CHDPA;good quality single crystals of the latter were obtained by slowevaporation of their solutions in CHCl3 over two days. The per-spective drawings of the molecular structures of CHCZL andCHDPA exhibited in Figure 3 show that the helical scaffold issignificantly twisted in both cases. Whereas the peripheral car-bazole groups are almost perpendicularly oriented in the caseof CHCZL, the aryl groups of nitrogen centers appear morepropeller-like in CHDPA, as is observed typically for simple tri-arylamines. Clearly, the conjugation in the helical scaffold aswell as that between the scaffold and peripheral groups areminimal to manifest in high band gap energies (see below).

Figure 2. Structures of the target helical diamines. Note that the crossingpoint between the single bonds in the bay region of the helical scaffold ofeach structure does not correspond to a quaternary carbon.

Scheme 1. Synthesis of dibromopentahelicenes and subsequent functionali-zations Suzuki/Buchwald–Hartwig amination reactions. Reagents and condi-tions: a) carbazole, [Pd(OAc)2] , P(tBu)3, tBuONa, dry toluene, 100 8C, 48 h;b) diphenylamine, [Pd(OAc)2] , P(tBu)3, tBuONa, dry toluene, 100 8C, 48 h;c) (4-(diphenylamino)phenyl)boronic acid, [Pd(PPh3)4] , NaOH, toluene/etha-nol/water (3:2:1), 100 8C, 48 h; d) 3,6-di-tert-butylcarbazole, [Pd(PPh3)4] ,NaOH, toluene/ethanol/water (3:2:1), 100 8C, 36 h; e) (4-(9H-carbazol-9-yl)-phenyl)boronic acid, [Pd(PPh3)4] , NaOH, toluene/ethanol/water (3:2:1),100 8C, 36 h.

Figure 3. X-ray determined ORTEP molecular structures of CHCZL (a) andCHDPA (b). The latter includes chloroform in its crystal lattice (see the Sup-porting Information).

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Crystallinity is, in principle, not expected for as-synthesizedcompounds that display amorphous property. Presumably, thesolvent molecules play a significant role in engineering crystalgrowth with the solvent molecules, at times, included in thecrystal lattices, leading to lattice inclusion compounds. This,indeed, is the scenario observed for CHDPA. Our efforts to crys-tallize azahelical diamines were unsuccessful, presumably be-cause of their poor solubility.

Photophysical properties

UV/Vis absorption spectra of carbo- and azahelical diamines re-corded in dilute DCM solutions (ca. 1 � 10�5

m) are shown inFigure 4 a and 4b, respectively. Insofar as the absorption spec-tra of carbohelical diamines, that is, CHCZL, CHDPA, andCHTPA, are concerned, they are completely different from eachother. The absorption spectrum of CHCZL is structured withthree distinct peaks at 294, 328 and 341 nm. In contrast, the

absorption spectrum of CHDPA is quite broad in nature witha maximum around 300 nm, whereas that of CHTPA has a maxi-mum at 310 nm associated with a broad shoulder startingfrom 340 nm. Insofar as UV/Vis absorption spectra of AHCZL,AHDPA, and AHTPA are concerned, one observes certain simi-larities with their carbohelical analogues. For example, the ab-sorption spectrum of AHCZL is also structured and has threemajor peaks at 294, 312, and 399 nm. On the contrary, the ab-sorption spectrum of AHDPA has two maxima at 308 and363 nm, as opposed to one broad maximum for CHDPA. As forAHTPA, one observes a broad spectral feature with a maximumat about 323 nm. For the rest of the azahelical diamines, thatis, AHCZLt and AHPCZL, one observes striking similarities be-tween the absorption spectra for the two. They are character-ized by three bands in a narrow range of 294–298 nm, 311–319 nm and 401–402 nm. Overall, broad absorption featuresare observed for the non-carbazole compounds with flexibleamino groups, whereas structured absorptions are a signaturefor carbazole-functionalized helical diamines. The band gap en-ergies calculated from the red edge absorption cut-off forAHDPA, CHDPA, and CHTPA are in the range of 2.79–2.87 eV.For all the other compounds, the band gap energies are above2.90 eV (Table 1).

Fluorescence spectra of all diamines recorded in dilute DCMsolutions (ca. 1 � 10�5

m) for excitation at 341 nm are shown inFigure 4 c and 4 d. Within the carbohelical diamines, the fluo-rescence spectrum of CHCZL is significantly blue-shifted rela-tive to those of CHDPA and CHTPA. The emission of CHCZL isstructured with maxima at 420 and 435 nm, whereas the samefor CHDPA and CHTPA are broad and almost superimposablewith maxima at about 459 nm; the only difference betweenthe emission spectra of the two is that the emission profile forCHTPA is slightly broader than that of CHDPA. Progressionfrom carbo- to azahelical diamines brings about similarities aswell as differences between the analogous diamine pairs, thatis, CHCZL versus AHCZL, CHDPA versus AHDPA, and CHTPAversus AHTPA. The emission profile of AHCZL is also structuredwith maxima at 413 and 433, which are marginally blue-shiftedrelative to those of CHCZL. On the contrary, the emission pro-file of AHDPA is red-shifted by about 15 nm relative to that of

Figure 4. Normalized absorption spectra of a) carbohelical and b) azahelicaldiamines, and fluorescence spectra of c) carbohelical and d) azahelical dia-mines. The fluorescence spectra were recorded for excitation at 341 nm.

Table 1. Photophysical, electrochemical and thermal properties of the carbo- and azahelical diamines.

Substrate lmax (UV)[a]

[nm]Eg

[b]/ET[c]

[eV]Lifetime[d]

[ns]lmax (PL)[a] soln[nm]

Ffl[e] soln

[%]HOMO[f]/LUMO[g]

[eV]Tg

[h]/Tm[h]/Td

[i]

[8C]

CHCZL 294, 328, 340 2.93/2.37 4.98 420, 435 4.5 5.51/2.58 164/343/453CHDPA 300 2.79/2.31 4.98 459 6.0 5.14/2.34 115/262/383CHTPA 310, 340 2.87/2.33 6.12 459 34.6 5.19/2.32 130/287/514AHCZL 294, 312, 399 2.99/2.54 5.40 413, 433 19.3 5.40/2.41 165/354/463AHDPA 308, 363 2.81/2.41 5.54 474 12.4 5.04/2.23 127/240/419AHTPA 323 2.95/2.37 6.64 425 (sh), 444 16.7 5.17/2.22 141/278/487AHCZLt 298, 319, 402 2.99/2.55 7.04 411, 433 29.1 5.34/2.35 214/440/457AHPCZL 294, 311, 402 2.99/2.40 7.19 410, 432 25.6 5.09/2.10 182/217/507

[a] Absorption and fluorescence spectra were recorded in dilute DCM solutions (ca. 10�5m). [b] Band gap energies were calculated from red edge absorp-

tion onset values using the formula E = hc/l. [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 rel-ative to anthracene as the standard. [f] HOMO energies were calculated from oxidation potentials in the CV spectra. [g] LUMO energies were calculated bysubtracting the band gap energies from HOMO energies. [h] From DSC. [i] From TGA.

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CHDPA; the emission maximum for AHDPA lies at 474 nm. In-sofar as the triphenylamino-derivative is concerned, one ob-serves that the emission of AHTPA is not only blue-shifted byabout 15 nm relative to that of CHTPA, but also showsa shoulder at 425 nm in addition to the maximum at 444 nm.The remaining carbazole compounds, that is, AHCZLt andAHPCZL, display features akin to AHCZL, and are characterizedby structured emissions with two closely associated maxima inthe range of 410–411 nm and 432–433 nm. Fluorescence quan-tum yields of the compounds in DCM determined relative toanthracene as the standard were found to vary from 4.5–34.6 % (Table 1). Fluorescence lifetimes of all diamines mea-sured by single photon counting are collected in Table 1, andwere found to range between 4.98–7.19 ns.

Phosphorescence spectra of all the carbo- and azahelical dia-mines recorded in dilute 2-MeTHF solutions at 77 K are shownin Figure 5 a and 5 b, respectively. Triplet energies of all the

compounds calculated based on 0–0 transitions in their phos-phorescence spectra are collected in Table 1. For carbohelicaldiamines, that is, CHCZL, CHDPA, and CHTPA, the triplet ener-gies were found to vary between 2.31–2.37 eV. In contrast,analogous azahelical diamines, that is, AHCZL, AHDPA, andAHTPA, were found to display slightly higher triplet energies,which range between 2.37–2.54 eV (Table 1); the remainingtwo higher azahelical analogues, that is, AHCZLt and AHPCZL,were found to possess similar triplet energies. Clearly, the trip-let energies are somewhat higher for azahelical diamines thanfor the corresponding carbohelical diamines, and highest forAHCZL and AHCZLt, which are 2.54 and 2.55 eV, respectively.Presumably, electron-richness of azahelical scaffold reducesconjugation between helical core and carbazole groups. By thesame token, the lower triplet energy of AHPCZL relative to itslower analogue, that is, AHCZL, can be reconciled by moreconjugation between helical scaffold and the amino groups.

Electrochemical properties

Electrochemical properties of the carbo- and azahelical dia-mines were examined by cyclic voltammetry with nBu4NPF6 asthe supporting electrolyte in DCM. Analyses of the cyclic vol-tammograms (CV) reveal that the helicenes display only oxida-tion, but no reduction peaks within the potential window ofthe operation (Supporting Information, Figure S1). The CV of

CHCZL is irreversible, whereas the CVs of CHDPA and CHTPAare quasi-reversible. Insofar as azahelical diamines are con-cerned, the CV of AHCZL is irreversible, whereas those ofAHDPA and AHTPA are completely reversible. Clearly, non-fused amino-derivatives based on azahelicene, that is, AHDPAand AHTPA, display better electrochemical behavior than theircarbohelical analogues, that is, CHDPA and CHTPA. The CV pro-files of CHTPA and AHTPA are shown for comparison inFigure 6. The higher azahelical homologues, that is, AHCZLt

and AHPCZL, display different electrochemical features (Sup-porting Information, Figure S1). Whereas CV of AHCZLt is com-pletely reversible, insertion of phenyl spacer as in AHPCZLleads to irreversible electrochemical behavior. Overall, the fol-lowing generalized observations may be made with regard totheir electrochemical behavior: first, nonfused di-/triarylamino-functionalized diamines, that is, CHDPA, CHTPA, AHTPA, andAHDPA, 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.[16] Last, introduction of bulky tert-butyl groupsat 3 and 6 positions of each of the carbazole groups at the pe-riphery significantly improves the electrochemical stability;indeed, the CV of AHCZLt is reversible, whereas that of AHPCZLis completely irreversible. Presumably, polymerization at 3 and6 positions of the reactive carbazole radical species producedunder electrochemical oxidation is precluded by the presenceof bulky tert-butyl groups.[17]

HOMO energies of all carbo- and azahelical diamines calcu-lated relative to ferrocene as the standard (HOMO energy offerrocene is �4.8 eV with respect to vacuum) using the onset/half-cell oxidation potentials determined from the CVs are col-lected in Table 1. The HOMO energies of carbohelical diamineswere found to be in the range of 5.14–5.51 eV, whereas thoseof azahelical diamines ranged between 5.04–5.40 eV. Of course,electron-richness of azahelical diamines is responsible for theslightly elevated HOMO levels of the azahelical diamines rela-tive to their carbohelical diamine analogues. The HOMO ener-gies of the two higher azahelical diamines, that is, AHCZLt andAHPCZL, were determined to be 5.09 and 5.34 eV, respectively;it should be noted that the HOMOs of popular HTMs fall inthis range. LUMO energies were calculated by subtraction ofband gap energies from the HOMO energies, and were foundto fall in the range of 2.10–2.58 eV (Table 1).

Figure 5. Phosphorescence spectra of a) carbohelical and b) azahelical dia-mines in 2-methyltetrahydrofuran (ca. 1 � 10�5

m) at 77 K.

Figure 6. The cyclic voltammograms of representative helical diamines,namely, CHTPA and AHTPA.

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Thermal properties

Good thermal stability is essential for a material to be appliedin OLED devices to withstand decomposition during vacuumsublimation as well as joule heating that occurs on prolongeddevice operation. Thermogravimetric analysis (TGA) and differ-ential scanning calorimetry (DSC) at a heating rate of10 8C min�1 under inert atmosphere were performed to exam-ine thermal properties of the helical diamines. All as-synthe-sized carbo- as well as azahelical compounds exhibited excel-lent thermal stabilities with decomposition temperatures (Td)well above 400 8C (Table 1; Supporting Information, Figure S2),with the exception of CHDPA for which the Td is 383 8C. The Td

values of CHCZL and CHTPA are 453 and 514 8C, respectively.Clearly, Td values of the carbohelical diamines are a function ofmolecular weight. The glass transition temperatures (Tg) forCHCZL, CHDPA, and CHTPA are 164, 115, and 130 8C, respec-tively, (Supporting Information, Figure S3). Whereas higher Tg

for CHTPA compared to that for CHDPA is due to differences inthe molecular weight, a much higher Tg for CHCZL than forCHTPA, despite lower molecular weight of the former, shouldbe understood from the rigidity of the carbazole moieties. Theazahelical diamines AHCZL and AHDPA display higher Td valuesthan their corresponding carbohelical analogues, that is,CHCZL and CHDPA (Table 1), although Td of AHTPA is lowerthan that of CHTPA. A similar trend in the Tg values was ob-served 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 aza-helical diamines are slightly higher than those of carbohelicaldiamines (CHCZL vs. AHCZL, CHDPA vs. AHDPA and CHTPA vs.AHTPA, Table 1). The higher azahelical analogues, that is,AHCZLt and AHPCZL, display significantly high thermal stabili-ties in terms of both Td and Tg (Table 1). It should be empha-sized that the Tg values of all compounds are higher thanthose of other commercially available popular diarylaminessuch as N,N’-di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-di-amine (NPB, Tg�95 8C),[18a] N,N’-bis(3-methylphenyl)-N,N’-diphe-nylbenzidine (TPD, Tg�60 8C),[18b] 1,3-bis(N-carbazolyl)benzene(mCP, Tg�60 8C),[18c] 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP, Tg

�62 8C),[18d] and so on. Evidently, high Tg is indispensible forformation of pinhole-free stable glasses, which crucially deter-mine the device longevity.[19]

Electroluminescence properties

Comprehensive studies of photophysical, electrochemical andthermal properties of the carbo- and azahelical diamines clear-ly suggest that these compounds can be exploited for multifar-ious applications in OLEDs. Thus, electroluminescence proper-ties of the helical diamines were examined by fabrication ofdifferent types of devices in which the compounds function as1) hole-transporting materials ; 2) emissive materials ; 3) hole-transporting as well as emissive materials; and 4) host materi-als. Application of helical diamines as each of these materials isdealt with separately in the following.

Azahelical diamines as HTMs

The high HOMO energies (5.04–5.40 eV) of azahelical diaminesin particular are appealing for their applicability as HTMs. Un-fortunately, fabrication of simple double layer devices withAHTPA and tris(8-hydroxyquinolinato)aluminum (Alq3) yieldedvery poor results; presumably, a large gap (ca. 0.7 eV) betweenHOMOs of the two employed materials curtail charge transportleading to abysmal results. Therefore, an emissive material ofa higher HOMO level, namely, 3,6-bis(triphenylamino)phenan-threne (PTPA) with its HOMO at 5.3 eV, was considered. Multi-layer 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 ofthe azahelical diamines, in which ITO functions as an anode,PTPA serves as an emissive material, 2,2’,2“-(1,3,5-benzinetriyl)t-ris(1-phenyl-1H-benzimidazole) (TPBI) as an electron-transport-ing material, and LiF/Al as a composite cathode. A controldevice in which azahelical diamine was replaced with commer-cial NPB was also fabricated to allow comparison of the holetransport abilities. The results of the fabricated devices are col-lected in Table 2. I-V-L characteristics, typical EL spectra andenergy level diagram are shown in Figure 7.

A cursory glance of the data in Table 2 compellingly bringsout the fact that the hole-transporting abilities of azahelical di-amines are superior to those of NPB in the device configura-tion employed. The obvious advantages that the azahelical dia-mines offer over NPB are as follows. First, external quantum ef-ficiencies and luminous efficiencies for devices fabricated withazahelical diamines are higher than those based on NPB, notonly at the peak positions, but also over a wide range of cur-rent densities (Supporting Information, Figure S4); the powerefficiency of NPB-based devices is, however, better than thosebased on azalelical diamines at higher current densities.Second, with the exception of AHTPA, devices fabricated withother HTMs produce maximum luminance values that arehigher than those observed for the device with NPB as a HTM.Third, although the lmax is same (420 nm) for all devices, devi-ces fabricated with azahelical diamines as HTMs emit blue light

Table 2. Electroluminescence data for the devices with azahelical dia-mines as HTMs[a] .

Substrate Von[b] hex

[c] hp[d] hl

[e] Lmax[f] lmax

[g] CIE[h] (x, y)

AHCZL 3.5 2.28 0.73 1.30 1740 420 0.16, 0.07AHDPA 3.5 2.23 1.33 1.48 1990 420 0.15, 0.08AHTPA 3.5 2.00 0.91 1.25 911 420 0.16, 0.08AHCZLt 3.5 2.37 1.58 1.76 1930 420 0.16, 0.07AHPCZL 4.0 2.43 1.18 1.69 2020 420 0.16, 0.08NPB 3.0 1.98 1.00 1.12 1590 420 0.17, 0.08

[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-onvoltage [V]. [c] Maximum external quantum efficiency [%]. [d] Maximumpower efficiency [lm W�1] . [e] Maximum luminance efficiency [cd A�1] .[f] Maximum luminance achieved [cd m�2] . [g] lmax (EL) [nm]. [h] 1931chromaticity coordinates measured at 6 V.

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with slightly lower CIE coordinates than that obtained with theNPB-based device (Table 2). Given that NPB has a Tg of only95 8C, the azahelical compounds with significantly higher Tg

values ranging between 127–214 8C constitute excellent alter-natives to popular NPB.

Helical diamines as EMs

As discussed earlier, the helical diamines are fluorescent withmoderate quantum yields. Their utility as emissive materials innondoped as well as doped devices was investigated by fabri-cation of devices with the following four different configura-tions: (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 asan anode, NPB as a hole-transporting material, TPBI and 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) serveas electron-transporting as well as hole-blocking materials, 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) and CBPas fluorescent hosts and LiF/Al as the composite cathode; ofcourse, helicene serves as an emissive material in all cases. Theconfigurations B and C correspond to nondoped devices,whereas D and E refer to the doped devices. The results ob-tained from the best devices from a limited set are collected inTable 3. The I-V-L profiles, resultant electroluminescence spec-tra for device B and energy level diagram are shown inFigure 8, whereas those for devices D and E are given in Fig-ure S5 (Supporting Information).

As can be perused from the results in Table 3, the helical dia-mines 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, whereasthey are slightly higher for the azahelical diamines (ca. 3.5–6.5 V). In general, the efficiencies obtained with nondoped de-vices are unremarkable, whereas they are found to be muchimproved in doped devices. In nondoped devices, the efficien-cies obtained with CHTPA are better than those with the aza-helical analogue, that is, AHTPA, whereas those obtained withCHCZL are poorer than those with azahelical AHCZL; CHDPAand AHDPA exhibit comparable device performance results

Figure 7. a) Current density versus voltage and b) luminance versus voltageprofiles for the devices of configuration A fabricated with azahelical dia-mine/NPB as an HTM and PTPA as an EM; c) typical EL spectrum of the devi-ces of configuration A; d) energy level diagram of the HOMOs and LUMOsof the materials employed in the devices of configuration A.

Table 3. Electroluminescence data for the OLED devices fabricated withthe helicenes as EMs.

Substrate Device[a] Von[b] hex

[c] hp[d] hl

[e] Lmax[f] lmax

[g] CIE[h] (x, y)

CHCZL B 4.0 0.25 0.32 0.41 1520 444 0.19, 0.19CHDPA B 3.0 0.33 0.31 0.39 2300 448 0.16, 0.15

D 4.0 1.13 1.11 1.41 1150 448 0.15, 0.11CHTPA B 3.0 1.05 1.40 1.33 3220 440 0.18, 0.16

D 3.5 2.54 3.34 3.72 1810 444 0.16, 0.10AHCZL C 4.5 1.04 0.91 1.53 1190 448 0.21, 0.18AHDPA B 4.0 0.48 0.42 0.87 2050 468 0.16, 0.25

C 3.5 0.77 1.00 1.43 1440 472 0.17, 0.24D 3.5 1.35 2.08 2.65 3040 472 0.17, 0.24

AHTPA B 3.5 0.87 0.91 1.06 2190 444 0.17, 0.14E 6.0 1.53 0.85 1.63 935 440 0.16, 0.09

AHPCZL C 3.5 1.56 3.06 4.39 2940 516 0.25, 0.39E 6.5 1.07 0.48 1.01 752 432 0.16, 0.09

[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 efficien-cy (cd A�1). [f] Maximum luminance achieved [cd m�2] . [g] lmax (EL) [nm].[h] 1931 chromaticity coordinates measured at 8 V.

Figure 8. a) Current density versus voltage and b) luminance versus voltageprofiles for the devices of configuration B; c) typical EL spectra; d) alignmentof HOMO and LUMO levels of the materials employed in the devices of con-figuration B.

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(Table 3). Insofar as nondoped devices are concerned, the bestperformance in terms of efficiencies is exhibited by AHPCZL inconfiguration C; the device yielded maximum external quan-tum efficiency, power efficiency, luminous efficiency and lumi-nance of 1.56 %, 3.06 lm W�1, 4.39 cd A�1 and 2940 cd m�2, re-spectively. However, emission emanating from the device inthis instance cannot be ascribed solely to AHPCZL, as bluish-white emission with CIE coordinates of (0.25, 0.39) was ob-served, which is presumably contributed by the exciplexesformed at the interface with electron transport layer (ETL)(Supporting Information, Figure S6). In contrast, the best per-formance in doped devices is exhibited by CHTPA, which yield-ed maximum external quantum efficiency, power efficiency, lu-minous efficiency and luminance of 2.54 %, 3.72 cd A�1,3.34 lm W�1 and 1810 cd m�2, respectively.

Carbohelical diamines as HTMs as well as EMs

From the results of a number of experiments with all the heli-cal diamines, we determined that CHTPA allows better dualproperty. Therefore, efforts were focused more on CHTPA fordual function as a hole-transporting as well as emissive materi-al. 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 ofdevice performances are collected in Table 4. The I-V-L profileand EL spectrum are typically shown for the device G in Fig-ure 9 a and 9 b, respectively. In all cases, CHTPA serves the dualpurpose of hole-transport as well as emission, that is, as a bi-functional material, thereby eliminating the requirement ofseparate layers for hole transport and emission. Quite remark-able is the fact that the performance characteristics of CHTPAare 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 stabilityof CHTPA over ITO. While the CHTPA/TPBI device yielded a max-imum brightness of 4960 cd m�12, the most efficient devicewas found to be CHTPA/BCP, which produced maximum exter-nal quantum, luminous and power efficiencies of 2.75 %,3.99 cd A�1 and 3.13 lm W�1, respectively. It is noteworthy thatthe applicability of helical diamines as bifunctional materials isheretofore unknown, although they have been investigated asemissive materials under doping conditions.[12b, d] Indeed, thefact that helical diamines can function as bifunctional hole-transporting as well as emissive materials in simple doublelayer devices is demonstrated for the first time.[20] Efficienciesobtained in the device configuration G is the highest by quitesome margin amongst other helical amines reported sofar[12b, d] and also amongst those considered herein. The meritlies in the fact that a simple nondoped double layer devicestructure gives such an excellent performance as opposed tocomplicated doped device strategies reported so far.[12b, d]

Based on our observation that CHTPA/BCP-based deviceyielded maximum efficiencies among other devices with differ-ent ETLs, we fabricated similar devices with CHCZL and CHDPAas well. Poor quantum yields of emission curtailed meaningfuldevice results. Both CHCZL and CHDPA led to poor efficiencies(device G, Table 4); of course, low HOMO level for CHCZL ap-pears to be responsible for the observed abysmal performance,which renders hole injection difficult (Supporting Information,Figure S7). Remarkably, both perform nicely as typical HTMswhen emissive layer was changed to CHTPA (device J, Table 4).

Helical diamines as host materials

In general, the triplet energy of an organic compound dependscrucially on the extent of conjugation. Although carbazole, tri-phenylamine, and diphenylamine are well-known for their hightriplet energies, the latter depend on overall conjugation sub-sequent to their attachment to the carbo- and monoaza[5]heli-cene cores. As can be seen from the results in Table 1, the trip-let energies of all helical diamines are respectable. Twisting asa consequence of unique helical core scaffolds is evidently re-sponsible for the observed high triplet energies of the dia-mines, given that triplet energy of planar pentacene is as lowas 1.0 eV.[21] Thus, all helical diamines qualify for application ashost materials for the red dopant, that is, [Ir(btp)2acac] . Further,the triplet energies for AHCZL and AHCZLt are >2.5 eV such

Table 4. Device performance results involving carbohelical diamines em-ployed as HTMs as well as EMs.

Substrate Device[a] Von[b] hex

[c] hp[d] hl

[e] Lmax[f] lmax

[g] CIE[h] (x, y)

CHCZL G 10.0 0.025 0.003 0.003 76 448 0.19, 0.15[i]

J 6.0 1.22 1.10 2.10 857 448 0.19, 0.20CHDPA G 4.5 0.26 0.32 0.46 616 448 0.16, 0.14

J 3.5 1.84 2.16 2.75 1810 444 0.18, 0.17CHTPA F 3.0 1.40 1.41 1.48 4960 448 0.16, 0.13

G 4.0 2.75 3.13 3.99 2170 452 0.16, 0.16H 4.0 2.14 2.34 2.98 2120 452 0.16, 0.16I 3.0 0.92 1.39 1.43 2000 448 0.17, 0.17

[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 powerefficiency [lm W�1] . [e] Maximum luminance efficiency [cd A�1] . [f] Maxi-mum luminance achieved [cd m�2] . [g] lmax (EL) [nm]. [h] 1931 chromatici-ty coordinates measured at 8 V. [i] 1931 chromaticity coordinates record-ed at 13 V.

Figure 9. a) I-V-L profile and b) typical EL spectrum for the devices of config-uration G in which CHTPA serves as an HTM as well as an EM.

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that they also qualify as hosts for the green dopant, that is,[Ir(ppy)3] ; note that the triplet energy for the latter is2.42 eV.[22] Insofar as the devices for red emission are con-cerned, 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 exter-nal quantum efficiency, luminous efficiency, power efficiencyand luminance of 4.70 %, 3.47 cd A�1, 5.33 lm W�1 and2640 cd m�2, respectively, (Table 5). Analogous devices werealso 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 col-lected in Table 5. The typical I-V-L profile and EL spectrum forthe red emission are shown in Figure 10. The best performance

was observed for the devices fabricated with AHPCZL. Thiscompound led to maximum external quantum efficiency, lumi-nous efficiency, power efficiency and luminance of 4.83 %,4.66 cd A�1, 2.93 lm W�1 and 1560 cd m�2, respectively.

As mentioned earlier, the triplet energies of AHCZL andAHCZLt 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 em-ployed as the host material, respectable device performanceresults were obtained (Table 5). The device produced puregreen emission that is characteristic of [Ir(ppy)3] ; I-V-L charac-teristics and EL spectrum of the device M are shown inFigure 11. The device produced a maximum luminous efficien-cy of 23.0 cd A�1 at 5 V; corresponding external quantum effi-ciency and power efficiency were 8.72 % and 18.2 lm W�1, re-spectively. Further, the maximum brightness obtained was4200 cd m�2 at 11.5 V, which is significantly higher than thoseobtained from the devices fabricated with the red dopant, thatis, [Ir(btp)2acac] . Clearly, energy transfer to the green dopant ismore efficient when compared with that of the red dopant.

Although unremarkable, the above results compellinglydemonstrate that the helical compounds can indeed serve ashost materials for red PhOLED devices. Amongst carbo- andazahelical systems, the latter exhibit higher triplet energiesthan those of the carbohelical systems. Based on the observa-tion that AHCZL and AHCZLt possess triplet energies signifi-cantly higher than those of the others, these compounds wereexploited 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 5). Notwithstandingthe fact that the device performance results with carbo- andazahelical diamines as host materials do not compare with thebest materials reported so far,[11j] the fact that helicity can beexploited for creation of new host materials is compellinglyborne out.

Conclusion

A set of eight helical dimines based on carbo- and aza[5]heli-cene scaffolds was rationally designed and synthesized for di-verse applications in OLEDs; the syntheses of all heliceneswere readily accomplished by Suzuki and Buchwald–Hartwig

Table 5. Electroluminescence data for the devices in which helical dia-mines serve as host materials.

Substrate Device[a] Von[b] hex

[c] hp[d] hl

[e] Lmax[f] lmax

[g] CIE[h] (x, y)

CHCZL K 4.5 4.70 3.47 5.53 2640 616 0.67, 0.32AHCZL L 4.0 3.93 3.04 3.89 1860 616 0.66, 0.32

M 4.5 8.72 18.2 29.0 4200 512 0.31, 0.61AHCZLt L 4.5 2.56 1.30 2.48 976 616 0.64, 0.31AHPCZL L 3.5 4.83 2.93 4.66 1560 616 0.66, 0.32

[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 effi-ciency [lm W�1] . [e] Maximum luminance efficiency [cd A�1] . [f] Maximumluminance achieved [cd m�2] . [g] lmax (EL) [nm]. [h] 1931 chromaticity co-ordinates measured at 8 V.

Figure 10. a) Current density versus voltage and b) luminance versus voltageprofiles for the devices of configurations K and L in which helical diaminesserve as host materials for the red dopant, that is, [Ir(btp)2acac]; c) typical ELspectrum for the emission of red dopant, that is, [Ir(btp)2acac] ; d) energylevel diagram for the red PhOLED device fabricated with AHPCZL as a hostmaterial.

Figure 11. a) I-V-L profile and b) typical EL spectrum of device M in whichAHCZL serves as a host material for the green dopant, that is, [Ir(ppy)3] .

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couplings as key reactions. The X-ray crystal structure determi-nations for CHCZL and CHDPA reveal that the helical scaffold issignificantly twisted. All the diamines were found to exhibithigh thermal stabilities, as evidenced by their Td values in therange of 383–514 8C. The Tg values of as-synthesized diamineswere found to be moderate to very high (115–214 8C) with thehigher values generally observed for azahelical diamines. Itshould be remarked that the Tg values of all diamines are sig-nificantly higher than those of popular commercial diaryla-mines such as NPB, TPD, CBP, and so on. Their high Tg valuesare ascribed to the rigidity and twisting induced by the helicalscaffold. Insofar as their photophysical properties are con-cerned, their quantum yields of emission are found to be inthe range of 4.5–34.6 % with their band gap energies lying be-tween 2.79–2.99 eV. The triplet energies of carbohelical dia-mines are found to be 2.3–2.4 eV, whereas their aza-analoguesexhibit slightly higher energies.

Based on suitable HOMO–LUMO energies, emission proper-ties and triplet energies, the helical diamines were employedas 1) hole-transporting materials (HTMs); 2) emissive materials(EMs); 3) bifunctional materials, that is, hole transport + emis-sion; and 4) host materials in the devices to elicit electrolumi-nescence. The azahelical diamines are demonstrated to serveas excellent HTMs with performance results that are superiorto those obtained with popular NPB. They are shown to be ap-plicable as EMs, and better under doping conditions. It isshown that carbohelical CHTPA can serve the dual purpose ofhole transport as well as emission in simple double layer devi-ces. Indeed, the results constitute first demonstration of a bi-functional property for helicenes. Furthermore, the per-formance efficiencies (external quantum efficiency: 2.75 %; lu-minous efficiency: 3.99 cd A�1; power efficiency: 3.13 lm W�1) ofa CHTPA-based double layer device are the best amongstthose of all helicenes reported so far. The high triplet energiesof helical diamines were exploited to fabricate red PhOLED de-vices in which they serve as host materials. It is also shownthat AHCZL (based on its triplet energy of 2.54 eV) can be uti-lized as a host material for green PhOLED devices as well.Indeed, when doped with [Ir(ppy)3] , AHCZL is shown to exhibitrespectable performance as a host material ; the devices fabri-cated are shown to lead to maximum luminous efficiency andluminance of 29.0 cd A�1 and 4200 cd m�2, respectively. In a nut-shell, diverse applications as materials in OLEDs have beendemonstrated for a small set of diamines based on helicenes,which have long elicited interest as aesthetic marvels. Giventheir ease of synthesis, the present results exemplify the po-tential for exploitation of helicity as a design element in thecreation of organic materials with new properties in differentareas.

Experimental Section

Syntheses

2,13-Dibromopentahelicene : 6-Bromophenanthrene-3-carbalde-hyde was synthesized following a previously reported procedure.[23]

A 50 % aqueous solution of NaOH at 0 8C 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 ofstirring at this temperature, the reaction mixture was warmed toroom temperature and continued stirring for 3 h. At the end ofthis period, water was added to the reaction mixture and extractedtwice with DCM. The combined organic extract was dried over an-hydrous Na2SO4 and stripped off solvent to obtain the crude prod-uct. The latter was subjected to a short pad silica gel filtration toisolate a mixture of cis and trans isomers; yield 96 % (0.078 g).

The thus-derived mixture of geometrical isomers was dissolved intoluene (200 mL) and iodine (0.03 g, 0.11 mmol) was added. The re-sulting dilute solution was subjected to irradiation with 350 nm ra-diation in a photoreactor for 2 d. At the end of this period, toluenewas removed under reduced pressure to obtain the crude solid,which was dissolved in chloroform and washed with sodium thio-sulfate solution. The combined organic extract was dried over an-hydrous Na2SO4, filtered, and evaporated to obtain the crude prod-uct, which was subjected to silica gel column chromatography toisolate the 2,13-dibromopentahelicene as a yellowish powder;yield 86 % (0.065 g). M.p. 227 8C; 1H NMR (CDCl3, 400 MHz): d= 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): d=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): n= 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. Di-bromopentahelicene (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 mL,0.002 mmol), and [Pd(OAc)2] (0.004 g, 0.002 mmol) were thenadded. Subsequently, the pressure tube was capped tightly undernitrogen atmosphere, and the contents were heated at 100 8C for48 h. At the end of this period, the pressure tube was cooled toroom temperature and the solvent was removed in vacuo. Thecrude reaction mixture was extracted with chloroform three times,and the combined extract was dried over anhydrous Na2SO4. Thecrude product obtained subsequent to evaporation of the solventwas purified by silica gel column chromatography to afford CHCZLas a colorless solid; yield 94 % (0.065 g). M.p. 343 8C; 1H NMR(CDCl3, 400 MHz): d= 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): d= 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): n=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) wasdegassed thoroughly by bubbling N2 gas for 10 min. Dibromopen-tahelicene (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 mL, 0.11 mmol)and [Pd(OAc)2] (0.024 g, 0.11 mmol) were later introduced. Subse-quently, the pressure tube was capped tightly under nitrogen andthe contents were heated at 100 8C for 48 h. At the end of thisperiod, the pressure tube was cooled to room temperature andthe solvent removed in vacuo. The crude mixture was extractedthree times with chloroform, and the combined organic extractwas dried over anhydrous Na2SO4. Evaporation of the solventunder vacuum led to the crude product, which was purified bysilica gel column chromatography to afford CHDPA as a yellowish-green solid, yield 82 % (0.55 g). M.p. 262 8C; 1H NMR (CDCl3,500 MHz): d= 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

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(m, 6 H), 8.23 ppm (d, J = 2.30 Hz, 2 H); 13C NMR (CDCl3, 125 MHz):d= 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): n= 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 mixturewas degassed thoroughly by bubbling N2 gas for 10 min. Subse-quently, dibromopentahelicene (0.20 g, 0.45 mmol), (4-(diphenyla-mino)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, andthe pressure tube was capped tightly under N2 gas. The contentswere heated at 110 8C for 48 h. At the end of the period, the pres-sure tube was cooled to room temperature, and toluene and etha-nol were removed in vacuo. The resultant residue was extractedthree times with chloroform and the combined organic extract wasdried over anhydrous Na2SO4. Evaporation of the solvent led to thecrude product, which was subjected to silica gel column chroma-tography to afford pure CHTPA as a yellowish-green solid; yield90 % (0.32 g); M.p. 287 8C; 1H NMR (CDCl3, 500 MHz): d= 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): d=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): n= 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 reportedprocedure.[24] A two-necked round bottom flask fitted with a CaCl2

guard tube was charged with 9-methyl-9H-carbazole-3-carbalde-hyde (0.50 g, 2.39 mmol) and N,N-dimethylformamide (15 mL). Theclear solution was cooled to 0 8C and N-bromosuccinimide (0.45 g,2.51 mmol) was introduced in small portions. Subsequently, the re-action mixture was allowed to warm up to room temperature andstir for 3 h. At the end of this period, water was added to the reac-tion mixture, and the solid residue formed was filtered and washedthoroughly 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 8C;1H NMR (CDCl3, 500 MHz): d= 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): d= 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): n= 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 aqueoussolution of NaOH (50 %) was added dropwise at 0 8C to a mixtureof 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 minat this temperature, the reaction mixture was warmed to roomtemperature and the stirring was continued for additional 3 h. Atthe end of this period, water was added to the reaction mixtureand the organic matter was extracted twice with DCM. The com-bined organic extract was dried over anhydrous Na2SO4 and strip-ped off solvent to obtain the crude product, which was subjectedto a short pad silica gel filtration to afford a mixture of cis andtrans isomers, yield 93 % (0.50 g). This mixture of geometrical iso-mers was dissolved in toluene (700 mL) and iodine (0.20 g,0.79 mmol) was added. The resulting dilute solution was irradiatedin a photoreactor fitted with 350 nm radiation for 2 d. At the end

of this period, toluene was removed under reduced pressure toobtain the crude solid, which was dissolved in chloroform andwashed with sodium thiosulfate solution. The combined chloro-form solution was dried over anhydrous Na2SO4, filtered andevaporated to obtain the crude product, which was subjected tosilica 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 8C; 1H NMR (CDCl3, 500 MHz): d= 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): d= 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): n= 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 andcharged with dry toluene (15 mL). The latter was degassed thor-oughly by bubbling N2 gas for 10 min. Subsequently, 2,12-dibro-mo-9-methyl-9H-naphtho[2,1-c]carbazole (0.60 g, 1.36 mmol), car-bazole (0.50 g, 3.00 mmol), sodium tert-butoxide (0.61 g,5.46 mmol), P(tBu)3 (100 mL, 0.27 mmol) and [Pd(OAc)2] (0.06 g,0.27 mmol) were added, and the pressure tube was capped tightlyunder nitrogen. The contents were heated at 100 8C for 48 h. Atthe end of this period, the pressure tube was cooled to room tem-perature and the solvent was removed in vacuo. The crude mixturewas extracted three times with chloroform, and the combined or-ganic extract was dried over anhydrous Na2SO4, filtered and evapo-rated under reduced pressure to obtain the crude product, whichwas purified by silica gel column chromatography to afford AHCZLas an off-white solid; yield 82 % (0.68 g); M.p.>350 8C; 1H NMR(CDCl3, 500 MHz): d= 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): d=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): n= 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 syn-thesis of AHCZL was followed for the synthesis of AHDPA. The re-action 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 thepresence of sodium tert-butoxide (0.61 g, 5.46 mmol), P(tBu)3

(108 mL, 0.27 mmol) and [Pd(OAc)2] (0.06 g, 0.27 mmol) led toAHDPA, which was isolated as yellow solid after routine work-upand column chromatography; yield 86 % (0.72 g); M.p. 240 8C;1H NMR (CDCl3, 400 MHz): d= 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): d= 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): n= 3036, 2925, 1586, 1490, 1450, 1430,1367, 1341, 1309 cm�1; ESI-MS+ : m/z : 615.2678 [M+] [C45H33N3

+] .

Chem. Eur. J. 2016, 22, 1 – 13 www.chemeurj.org � 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10&&

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AHTPA : An oven-dried pressure tube was cooled to room tempera-ture under N2 gas and charged with toluene (12 mL), ethanol(8 mL) and distilled water (4 mL). The solvent mixture was de-gassed 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 tightlyunder N2 gas. The contents were heated at 100 8C for 48 h. At theend of this period, the pressure tube was cooled to room tempera-ture, and the solvent was removed in vacuo. The residue was ex-tracted three times with chloroform and the combined organic ex-tract was dried over anhydrous Na2SO4 and filtered. Evaporation ofthe solvent led to the crude product, which was subjected to silicagel column chromatography to afford AHTPA as an yellow solid;yield 95 % (1.31 g); M.p. 278 8C; 1H NMR (CDCl3, 500 MHz): d= 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): d= 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): n= 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 thesynthesis of AHCZL was followed for the synthesis of AHCZLt. Reac-tion 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 mL, 0.22 mmol) and [Pd(OAc)2] (0.05 g,0.22 mmol) led to AHCZLt as a colorless solid after routine work-upfollowed by column chromatography; yield 75 % (0.67 g). M.p.>350 8C; 1H NMR (CDCl3, 500 MHz): d= 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): d=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): n=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 forthe 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 affordedthe crude product. The latter was suspended in methanol (25 mL),sonicated for 10 min, filtered and air-dried to afford AHPCZL as anoff-white solid; yield 94 % (1.30 g). M.p. 217 8C; 1H NMR (CDCl3,400 MHz): d= 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): d= 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): n= 3040, 2926, 1598,1584, 1519, 1500, 1479, 1451, 1362, 1334, 1316 cm�1; ESI-MS+ : m/z :763.2986 [M+] [C57H37N3

+] .

X-ray crystallography

CCDC 1452879 (CHCZL) and 1452880 (CHDPA) contain the supple-mentary crystallographic data for this paper. These data are provid-ed free of charge by The Cambridge Crystallographic Data CentreThese data can be obtained free of charge from The CambridgeCrystallographic Data Centre via www.ccdc.cam.ac.uk/data re-quest/cif.

Acknowledgements

J.N.M. is thankful to SERB, New Delhi, for generous financialsupport. S.J. and A.K. ; Brunner, A. v. Dijken, H. Bçrner, 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 ac-knowledge the optoelectronic device fabrication and testingby the scientific instrument facility at the Institute of Chemis-try, Academia Sinica, Taipei.

Keywords: amines · fluorescence · helical structures ·luminescence · thin films

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Received: February 12, 2016

Published online on && &&, 2016

Chem. Eur. J. 2016, 22, 1 – 13 www.chemeurj.org � 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim12&&

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& Helical Structures

S. Jhulki, A. K. Mishra, T. J. Chow,*J. N. Moorthy*

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Helicenes as All-in-One OrganicMaterials for Application in OLEDs:Synthesis and Diverse Applications ofCarbo- and Aza[5]helical Diamines

Not only aesthetically beautiful, butalso functionally charming! Helicity asa design element allows creation ofnew organic materials for multifariousapplications in organic light-emittingdiodes (OLEDs). Helical diamines areshown to function as hole-transporting(HTM), emissive (EM), hole-transportin-g + emissive, and host materials (seefigure).

Chem. Eur. J. 2016, 22, 1 – 13 www.chemeurj.org � 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim13 &&

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