Tuning the Photophysical and Electroluminescence Properties in

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Tuning Photophysical and Electroluminescence Properties in Asymmetrically Tetrasubstituted Bipolar Carbazoles by Functional Group Disposition Rajendra Kumar Konidena, K. R. Justin Thomas, Ambika Pathak, Deepak Kumar Dubey, Snehasis Sahoo, and Jwo-Huei Jou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Applied Materials & Interfaces

Tuning Photophysical and Electroluminescence Properties in Asymmetrically Tetrasubstituted Bipolar Carbazoles by Functional Group Disposition Rajendra Kumar Konidenaa, K. R. Justin Thomas*a, Ambika Pathaka, Deepak Kumar Dubey,b Snehasis Sahoob and Jwo-Huei Joub a

Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee – 247 667, India

b

Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan [email protected]

Keywords: Carbazole; Organic Light Emitting Diodes (OLED); Hybridized Local and Charge Transfer (HLCT); TDDFT Calculations; Absorption Spectra; Emission Spectra.

ABSTRACT Carbazole decorated with both donor and acceptor fragments offer a classical way to optimize bipolar functional properties. In this work, a series of carbazoles featuring triphenylamine donor and cyano acceptor are synthesized and their structure‒property relationship studied. The effects of connectivity and the chromophore number density on photophysical and electroluminescence properties are investigated. The position of triphenylamine donor on the 3,6-dicyanocarbazole nucleus significantly affected the photophysical and electroluminescent properties. The dye possessing triphenylamine on C2 and C7 displayed red shift in absorption when compared to the structural analogue with triphenylamine tethered to C1 and C8. The emission wavelength of the dyes are tunable from 1 ACS Paragon Plus Environment

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blue to green, by altering the position of triphenylamine and cyano substituents. All the dyes exhibited positive solvatochromism in emission attributable to the photo-induced intra molecular charge transfer from triphenylamine donor to cyano acceptor. However, the extent of charge transfer and the hybridization of local and charge transfer excited states is highly dependent on the position of triphenylamine and cyano groups on the carbazole nucleus. Dyes containing cyano substituents at C2 and C7 showed elongated excited state lifetime, broad emission and large Stokes shifts indicating the presence of higher charge transfer component in the excited state. The dyes displayed exceptional thermal stability with onset decomposition temperature (10% weight loss) > 350 °C. Electrochemical measurements revealed low oxidation potential for dyes containing triphenylamine at C3 and/or C6. Addition of cyano acceptor on carbazole led to stabilization of LUMO. Further, the materials were tested as emitting dopants in solution processable multilayer OLED and found to display deep-blue/sky-blue electroluminescence with external quantum efficiency as high as 6.5% for a deep-blue emitter (CIEy ~ 0.06).

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INTRODUCTION The systematic investigations on structure‒property relationships of organic π-conjugated materials become everlasting quest for scientific community due to their versatile applications in optoelectronic devices such as organic light emitting diodes (OLEDs),1-3 photovoltaics (OPVs),4,5 field effect transistors (OFETs),6,7 non-linear optics (NLOs)8,9 and in molecular sensors.10,11 Among them electronic devices employing organic materials as functional ingredients, OLEDs draw enormous interest from both the academic and industrial laboratories as a solution for large area flat panel displays and potential next generation solid state lighting source. Thus, much efforts have been devoted to establish the structure-property relationships of organic luminescent materials.1-3 Although, green and red emitting materials set their bench mark performances, efficient blue emitters with Commission Internationale de L’Éclairage (CIE) Coordinate of CIEy ≤ 0.08, narrow full width at half maxima (FWHM) > 50 nm and high external quantum efficiencies are scarce in the literature.12-16 Moreover, pure blue emitters are highly demanding for full color displays not only as one of the three basic colors, but also to reduce the power consumption and serve as a host to generate longer wavelength emissions.17,18 Therefore, understanding the structure-property relationships and the development of bipolar blue emitting materials received tremendous interest in recent years.19-21 Several reports demonstrated that the conjugation connectivity between donor and acceptor motifs and central core structure plays an important role in determining the photophysical, electrochemical, thermal and charge transport parameters of the materials.19-27 Hence, the choice of donor and acceptor motifs and their linking topology around central building block should be examined carefully. 9H-Carbazole is one of the favourite synthon for material chemists and is adopted as promising building block to construct functional materials for organic electronics owing to its commendable properties such as rigid molecular structure, amorphous nature, high thermal



and photochemical stability, good hole transporting ability and high triplet energy (~ 2.9 ev).28-30 Consequently, innumerable oligo‒/poly‒carbazoles have been documented as representative benchmark materials in OLEDs, OFETs, OPVs and in fluorescent sensors.10,11,28-51 Carbazole can be easily functionalized on C1, C2, C3, C6, C7, C8 and N‒ positions. To date, majority of the literature is focused on trifunctionalization (C3, C6 & Nand C2, C7 & N-) of carbazole core.28-37 Yet, relatively less amount of C1 & C8‒functionalized (ortho‒ to carbazole nitrogen) carbazoles are known in the literature, when C3 & C6‒positions are protected with alkyl units.49-51 Recently, poly-functionalized carbazoles (C1, C3 & C6‒tri and C1, C3, C6 & C8‒tetra) were explored and found to exhibit decent device performances in electroluminescent devices.38-48 However, C2, C3 & C7‒tri and C2, C3, C6 & C7‒tetra substituted carbazoles are relatively less explored when compared to the aforementioned functionalizations due to the requirement of tedious indirect synthetic protocols.43-45 Although, the structure-function relationships of symmetrically functionalized poly-substituted carbazoles featuring different electron rich chromophores are well furnished in the literature,38-44 same on unsymmetrically polyfunctionalized bipolar carbazoles containing donor and acceptor units with different conjugation connectivity remained elusive. Besides, cyano group is exemplified as one of the promising acceptors for electroluminescent applications, its simple molecular structure suppresses the non-radiative vibrational relaxations, structural alterations in the excited state and trigger thermally assisted delayed fluorescnce.52 It is easy to introduce into aromatic nucleus at specific positions and its electron-withdrawing effect result in changes in functional properties of the materials.53,54 Most recently, we reported an highly efficient (EQE > 6%) carbazole-based deep-blue emitter by introducing triphenylamine donor on C2 & C7 and cyano acceptor on C3 & C6 (C1) (Chart 1).45 We presumed that the introduction of triphenylamine donor and cyano acceptor on carbazole platform with different linking topology would be beneficial to alter the


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functional properties of the materials. With these motivations and our continuous interest on establishment of structure‒property relationships of poly-substituted carbazole based luminescent materials,43-45 in this report we present the functional properties of asymmetrically poly-substituted bipolar carbazoles featuring triphenylamine donor and cyano acceptor with different conjugation connectivity. Herein we report the synthesis, optical, electrochemical and electroluminescent properties of triphenylamine-functionalized dicyanocarbazoles (Chart 1). The position of cyano and triphenylamine groups are mutually exchanged to arrive at a series of compounds exhibiting interesting alternations in the absorption and emission properties owing the differences in the charge transfer and conjugation. This triphenylamine‒carbazole‒cyano hybrids offered us an opportunity to study their structure-property relationships periodically. In particular, the effects of conjugation connectivity and the chromophore number density on their fundamental properties were investigated. To understand the electronic structure of the dyes, time dependent density functional theory (TD-DFT) computations were also performed. The photophysical and electrochemical properties of the dyes are highly dependent on the linking topology. The dyes containing triphenylamine chromophores on C3 and/or C6 and cyano group on C2 & C7 of carbazole exhibited most red-shifted emission when compared to their congeners. All the dyes exhibited positive solvatochromism in emission spectra whereas their absorption profiles remain unaffected by solvent polarity suggestive of photo induced ICT from the arylamine donors to cyano acceptor in the excited state. However, the extent of charge transfer is dependent on the substitution pattern. Some of the compounds exhibited hybridization of charge transfer and local excited states which otherwise common in phenathroimidazole derivatives. The compound 9 showed high-lying HOMO and low-lying LUMO, which results in low band gap in the series. Furthermore, the materials were applied



as emitting dopants in solution processable multilayer OLED devices and found to exhibit promising device performances.

Chart 1. Chemical structures of the target dyes.

Scheme 1. Synthesis of C1,C3,C6,C8-substituted derivatives.


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Scheme 2. Synthesis of C2,C3,C6,C7-substituted derivatives.

RESULTS AND DISCUSSION Synthesis and Characterization The synthetic strategies employed to synthesize the target dyes (Chart 1) are presented in Schemes 1 & 2. The key starting materials such as 3,6-dibromo-9-butyl-9H-carbazole (1), 2,7-dibromo-9-(2-ethylhexyl)-9H-carbazole (6) and (4-(diphenylamino)phenyl)boronic acid were synthesized by following literature methods.55,56 Cyanation of intermediates 1 and 6 were accomplished by using 4.0 eq. of copper cyanide in N,N-dimethylformamide (DMF) to obtain 9-butyl-9H-carbazole-3,6-dicarbonitrile (2) and 9-(2-ethylhexyl)-9H-carbazole-2,7dicarbonitrile (7), respectively. The selective bromination was performed on 7 using 1.0 eq. of bromine in chloroform at room temperature to get 3-bromo-9-(2-ethylhexyl)-9Hcarbazole-2,7-dicarbonitrile (7a). Iodination was performed on intermediates 2 and 7 with suitable amounts of iodine and potassium iodate in AcOH: H2SO4 to get 9-butyl-1-iodo-9Hcarbazole-3,6-dicarbonitrile (3a), 9-butyl-1,8-diiodo-9H-carbazole-3,6-dicarbonitrile (3b) and 9-(2-ethylhexyl)-3,6-diiodo-9H-carbazole-2,7-dicarbonitrile (7b). Finally, all the halo (bromo/ido) intermediates 3a, 3b, 7a and 7b were allowed to react with (4(diphenylamino)phenyl)boronic acid under palladium‒catalyzed Suzuki‒Miyaura cross coupling reaction57 conditions to obtain the target dyes (4, 5, 8 and 9) in moderate to excellent yields. Unambiguously, all compounds were thoroughly characterized using 1H & 13

C NMR spectroscopy, high resolution mass spectroscopy (HRMS) and elemental analysis.

The analytical data matched well with the proposed molecular structures of the compounds. The dyes are colorless or pale yellow in color and are reasonably soluble in common organic solvents, but insoluble in alcohols.



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66000

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Wavelength (nm) Figure 1. Absorption spectra of the dyes recorded in 1.0 × 10-5 dichloromethane. Table 1. Optical properties of the dyes Dye

λmax, nm (εmax, M-1 cm-1 ×103)a

2

349 (sh), 332 (sh), 297 (25.5), 283 (47.2)

4 5 7

352 (sh), 301 (sh), 282 (74.2) 351 (sh), 302 (sh), 286 (100.0) 387 (sh), 371 (sh), 317 (52.5), 304 (28.6), 258 (77.0)

λem (sol/film) (nm),a (ΦF)b 354, 371, 388 (sh)/398, 319 (sh) (0.10/-) 447/444 (0.65/0.49) 452/455 (0.60/0.52) 396, 417 (sh) /431, 455 (sh) (0.30/-) 517/525 (0.31/0.24) 513/524 (0.28/0.21)

Stokes shift (cm-1)a 405

t (ns)d

6038 6366 587

1.91 2.29 -

-

398 (4.56), 318 (60.2), 264 (48.5) 5783 7.59 420 (6.23), 336 (sh), 320 (83.9), 280 4316 7.78 (59.6) 460/453 (0.80/0.74) 5361 3.23 C1 369 (56.1), 282 (77.6) a Measured for DCM solution. bAbsolute quantum yields determined by a calibrated 8 9

integrated sphere system in solution/doped film (5wt%:CBP). cMeasured for drop-cast film. d

Fluorescence decay lifetime (t).

Photophysical Properties


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The electronic absorption spectra of the compounds collected in dichloromethane solution are shown in Figure 1 and the relevant data listed in Table 1. All the compounds displayed multiple absorption bands originating from different π-π* and charge‒transfer transitions. The parent compounds 2 and 7 showed complex absorption profiles in the range of 288‒380 nm attributable to the π-π* electronic transitions originating from carbazole and cyano units. Compound 7 displayed red shift in absorption when compared to compound 2.58 Notably, the extent of π-conjugation is highly dependent on the position of attachment of triphenylamine chromophore to central carbazole unit. The absorption spectra of the dyes 4 and 5 are similar to the precursor 2, however showed significant enhancement in molar extinction coefficient. Absence of effect due to chromophore loading indicates that the electronic communication between the chromophores on C1 and C8 with central carbazole is not effective in the ground state. This could be due the large dihedral angles (> 55°) between the chromophores on C1 and C8 with central carbazole unit, which results in twisting of the chromophores out of the plane of carbazole (Figure S8). These results suggest that the extent of π-conjugation in C1,C3,C6,C8-substituted carbazoles is limited to the substituents on C3 and C6 only (vide infra). The electronic isolation of chromophores on C1 and C8 may be beneficial to design TADF materials. However, the molar extinction coefficients of the dyes increased progressively (5 > 4 > 2) is attributable to the increment in chromophore loading and also conforming the origin of absorption from 9-butyl-9H-carbazole-3,6-dicarbonitrile backbone. Interestingly, the change in the triphenylamine chromophore connectivity from C1 & C8 (5) to C2 & C7 (C1) produced a new different absorption bands at longer wavelength. The peak profile upto 350 nm is similar indicating the isolated chromophore are of identical nature. The longer wavelength absorption band centered at ~ 370 nm stems from the delocalized ππ* electronic transitions comprising of central carbazole and triphenylamine units at C2 & C7. The compound C1 showed red-shifted absorption profile when compared to its analogue



5 which is attributable to the favorable linear conjugation offered by the 2,7‒positions of carbazole in former and existence of relatively smaller dihedral angle between triphenylamine chromophore and central carbazole when compared to later. Besides, addition of auxiliary chromophores on C3 and/or C6 positions of parent compound 7, in the dyes 8 (C3) and 9 (C3 & C6) led to a new absorption band above ~398 nm with mediocre intensity is attributed to the intramolecular charge transfer (ICT) transition from triphenylamine donors to cyano acceptors on carbazole. The ICT band is red-shifted for di-substituted derivative (9) when compared to its mono-substituted analogue (8). The red-shift is justified by considering double auxochromic effect by triphenylamine chromophores in 9.59 However, the observed mediocre intensity for CT band indicates poor orbital overlap between the chromophores on C3 and/or C6 and central 2,7-dicyanocarbazole core (vide infra). Yet, the lower wavelength absorption band centered at above ~310 nm displayed broadening with high molar extinction coefficient compared to precursor compound 7, which may be originating from the delocalized π-π* electronic transition of central carbazole and the triphenylamine chromophores on C3 and/or C6.42 It is interesting to compare the absorption profiles of the structural isomers such as 9 and C1. In contrast to former, the charge transfer band is absent in C1 due to meta- like linkage of triphenylamine donor with respect to carbazole nitrogen. At the same time, the electron accepting ability of cyano acceptor (C3 & C6) in C1 is reduced by electron releasing carbazole nitrogen. However, the delocalized π-π* electronic transition band is red-shifted for C1 when compared to 9 due to the presence of triphenylamine chromophores on C2 & C7‒ positions of carbazole in the former. From these results it is reasonable to correlate that the effective conjugation length of triphenylamine substituted dicyanocarbazoles increases in the order C1 & C8‒substituion < C3 & C6‒substitution < C2 & C7‒substitution. It is interesting to compare the absorption profiles of the compound 9 with that of known compound namely (4,4'-(9-dodecayl-9H-carbazole-3,6-diyl)bis(N,N-


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diphenylaniline).42 Insertion of cyano substituent on C2 & C7 of central carbazole resulted in new red-shifted CT absorption for 9. It is also interesting to note that when compared to the tetra-substituted derivatives, C1 shows red-shifted absorption profiles (~ 35 nm).43 This indicates reduction in steric crowding between the chromophores in former compound and effective linear π-conjugation between the carbazole and triphenylamine chromophores along C2‒C7 axis in C1. 1.05

Normalized Emission Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 4 5 7 8 9 C1

0.90 0.75 0.60 0.45 0.30 0.15 0.00 330

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Wavelength (nm)

Figure 2. Emission spectra of the dyes recorded in dichloromethane. Similar to the absorption properties, the emission profiles of the dyes are highly dependent on the linking mode of triphenylamine chromophore and cyano acceptor on carbazole (Figure 2 and Table 1). With the exception of 8 and 9, all the compounds are emitting in blue to deep blue region on exposure to visible light, yet a red-shifted green emission was witnessed for 8 and 9. The emission profiles of the parent compounds 2 and 7 showed vibronic transitions attesting their rigidity of the molecular structure in the excited state. In contrast, chromophore substituted derivatives displayed red-shifted broad emission suggesting the redistribution of



electron density in the excited state. Though, the C1,C3,C6&C8-tetra substituted compounds (4 and 5) showed similar absorption profiles as that of the parent dye 2, their emission maximas showed red-shift. This illustrates the involvement of auxiliary chromophores in excited state properties. It is speculated that the excited state assumes more planar structure, thus giving room for extension of π-conjugation throughout the molecule. This is further confirmed by the large Stokes shift observed for the dyes 4 and 5 in a particular solvent when compared to parent dye 2, which fell in the range of 6038‒6366 cm-1. Generally, large Stokes shift for organic molecules suggest a significant structural reorientations or polarized excited state compared to its ground state.60 Similarly, the dyes 8 and 9 displayed > 117 nm redshifted emission as compared to parent dye 7. It is expected that the photo-excitation of former compounds leads to excited state intramolecular charge transfer from triphenylamine donors to cyano acceptors on carbazole. Though the molecules are non-polar in the ground state, it is probable that the electronic excitation induces significant change in dipole moment in the excited state.59 Further, it is interesting to establish the correlation between positional isomers such as 9 and C1. The emission maxima of the compound 9 showed ~53 nm bathochromic shift when compared to C1. This could be due to less efficient exited state ICT between the meta-substituted triphenylamine donor and para-substituted cyano acceptor with respect to carbazole nitrogen. Though the compound C1 showed red-shifted absorption when compared to its congener 5, they displayed similar emission profiles with same maxima. This result highlights that the excited state is similar in nature for both the compounds. Further, the effect of cyano substitution on emission profiles of the dyes is evident on comparing the data observed for 9 and C1 with that of known compounds without cyano functionality. Compounds 9 and C1 exhibited 98 and 34 nm red-shifted emission when compared to (4,4'(9-dodecayl-9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline) and 4,4'-(9-(2-ethylhexyl)-9Hcarbazole-2,7-diyl)bis(N,N-diphenylaniline), respectively.42,43 Further, it is interesting to note


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that the replacement of electron-rich triphenylamine chromophores on C2 & C7 of 2,3,7-tri and 2,3,6,7-tetrasubstituted carbazoles featuring triphenylamine chromophore with cyano acceptor in 8 & 9 resulted ~48 nm bathochromic shift.43 These results highlight the importance of cyano substitution on triphenylamine featured carbazoles in shifting the emission to longer wavelength region. 1.2

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Cyclohexane Toluene Trichloroethylene Butylether Triethylamine Ethylacetate Dichloromethane Tetrahydrofuran Acetonitrile N,N-Dimethylformamide

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Figure 3. Emission spectra of the dyes a) 5, b) 9 and c) C1 recorded in different solvents. To elucidate the response of ground and excited states of these dyes with solvent polarity, we performed solvatochromism study for all the dyes in different solvents with varying polarity ranging from non-polar cyclohexane (CH) to polar acetonitrile (ACN).61 The absorption and emission profiles of the dyes collected in different solvents are showed in Figures 3 & S1-S5. The relevant data are listed in Tables S1 & S2. The influence of solvent 13 ACS Paragon Plus Environment


polarity on absorption profile of the dyes is less significant indicating imperceptible ICT transitions in their ground state. In contrast, the emission profiles of the compounds displayed positive solvatochromism with increasing solvent polarity from CH to ACN. Interestingly, the vibronic features in non-polar solvents such as CH and TOL becomes broad and redshifted in polar solvents. This indicates that the initially formed non-polar local excited state (LE) is transformed into a polar excited state in polar medium. Generally, such kind of redshifted featureless emission profiles in polar solvents is attributed to the structural reorganization or charge transfer induced electronic perturbations in the excited state of the compounds.60 Particularly, large Stokes shifts in polar solvents suggest the presence of polar excited state. From these results, unambiguously the observed positive solvatochromism of the dyes is assigned to the photo induced ICT from triphenylamine donors to cyano acceptor on carbazole nucleus. Also, it is interesting to estimate the effect of substitution pattern on extent of charge transfer (Figure 3). Generally, the charge transfer in dipolar compounds can be estimated by considering the degree of bathochromic shift in emission profiles from nonpolar to polar solvents (∆λCH-ACN).60 The dyes containing triphenylamine donors on C3 and/or C6 and cyano acceptor on C2 & C7 (8 and 9) exhibited large degree of bathochromic shift (~ 92-100 nm) in the series when compared to their congeners (~ 70-72 nm) (Table S2) indicating the existence of pronounced ICT character in the former dyes. Interestingly, no significant changes were observed in ∆λCH-ACN for 5 and C1 derivatives, which indicates the presence of similar charge transfer character in the excited state for these dyes. The solvatochromism data of the dyes were analyzed by correlating orientation polarizability (∆f) with Stokes shift and estimated the excited state dipole moment according to the LippertMataga model (Figure 4).62 It can be noted that all the compounds displayed two-section linear relationship in low and high polarity of solvents with dipole moment < 16.24 D and > 24.55 D suggesting the presence of two different excited states such as local excited and


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charge transfer states, respectively. This kind of ICT property is expected to improve the EL parameters of the materials.62,63 The time-solved fluorescence studies revealed monoexponential decay for all the compounds with lifetime in nano second time scale indicating the absence of delayed fluorescence (TADF) in the emission (Figure S6).63 In the series, the compounds 8 & 9 displayed relatively long lifetime (< 7.5 ns) than their congeners attributed to the enhancement of the CT component in the emissive state. Further, the absolute photoluminescence quantum yields (PLQY) of the compounds measured in solution and 5wt% doped film in CBP by using calibrated integrating sphere method. The PLQY of the materials are dropped in doped films when compared to the solution state.16 In the series, the compound C1 exhibited high PLQY, where as the dyes 8 and 9 showed low PLQY attributed to pronounced intra molecular charge transfer in the excited state. The PLQY of the dyes decreases with increasing solvent polarity confirms dipolar character for these dyes. 13000

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Figure 4. Lippert-Mataga plots for the dyes. The solid state emission spectra of the compounds collected as the drop‒cast thin film are shown in Figure S7. The parent dyes exhibited 20‒35 nm red-shifted emission profiles in thin film than those recorded in dichloromethane solution. However, the PL spectra of 15 ACS Paragon Plus Environment


triphenylamine substituted derivatives showed close resemblance to their solutions. This result highlights the beneficial role of non-planar triarylamine functionality to inhibit the formation of molecular aggregates in solid state in the former dyes.

2 4 5 7 8 9 C1

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Tempereture (°C)

Figure 5. Thermogravimetric traces of the dyes.

Thermal properties Thermal stability of the compounds was estimated by thermogravimetric analysis (TGA) (Figures 5) under nitrogen atmosphere at a heating rate of 10 °C/min and pertinent data are tabulated in Table 2. All the compounds exhibited excellent thermal stability with thermal decomposition temperature (Td) ranging from 345 to 679 °C. The onset decomposition temperature corresponding to 10% weight loss (T10d) varied from 274-468 °C. The triphenylamine featured dyes displayed two-step decomposition pathway with marked thermal stability with high Td and T10d when compared to the parent dyes and is attributable to their non-planar triphenylamine functionality. The weight loss in the first step between 360 and 515 °C may be attributed to degradation of the fragile alkyl chains and other labile units, while the second step between 565 and 715 °C corresponds to weight loss of aromatic residue 16 ACS Paragon Plus Environment

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and other stronger bonds. Enhancement in thermal stability of the organic materials due to the presence of triphenylamine chromophores has been adequately documented in the literature.42,43 The superior thermal stability of 5 and 9 over the mono-substituted analogues 4 and 8 is ascribed to the increased chromophore density. Interestingly, the substitution pattern also played a critical role in determining the thermal stability of these compounds. Consequently, the dye containing 3,6‒chromophore functionalization (9) exhibited high thermal robustness when compared to their 2,7‒ (C1) and 1,8‒triphenylamine functionalization (5). The severe steric strain between the chromophores and central carbazole on 1,8-positions may be responsible for the poor thermal stability of 5. Further, the role of cyano substitution in improving thermal stability of these dyes is evident on comparing the data for 9 and C1 with the compounds lacking cyano group i.e. 4,4'-(9dodecayl-9H-carbazole-3,6-diyl)bis(N,N-diphenylaniline) (T10d = 334 °C), 4,4'-(9-(2ethylhexyl)-9H-carbazole-2,7-diyl)bis(N,N-diphenylaniline) (Td = 603 °C), respectively.42,43 Also, the replacement of triphenylamine chromophore either on C2 & C7 or C3 & C6 of 2,3,6,7-tetrasubstituted carbazoles with simple cyano group boosted the thermal stability for the compounds 9 and C1.43 These results highlight the importance of cyano substituent in improving thermal stability of the dyes.



4 5 8 9 C1 Current (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

2µA -0.3

Ferrocene -0.1

0.1

0.3

0.5

0.7

0.9

+

Potential vs Fc/Fc (V)

Figure 6. Cyclic voltammograms of the dyes in dichloromethane. Electrochemical properties To understand the redox propensity of the newly synthesized compounds, the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were performed on the dyes in dichloromethane at a scan rate of 100 mV/sec. The representative CV plot of the dyes is displayed in Figure 6 and the relevant data listed in Table 2. The oxidation potentials of the dyes are quoted against the internal standard, ferrocene. All the compounds exhibited more positive oxidation potentials when compared to ferrocene suggesting the facile removal of electron from molecules. Generally, the triphenylamine featured dyes showed low oxidation potentials when compared to the parent dyes 2 and 6 attributable to the enhancement of electron richness on incorporation of triphenylamine chromophores. Interestingly, the oxidation propensity of the dyes is highly dependent on the conjugation connectivity of chromophore to the central carbazole unit. The decreasing order of first oxidation potential within a series follows the trend 9 < 8 < C1 < 5 < 4. The low oxidation


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potential for 9 in the series is attributed to the existence of effective conjugation between the triphenylamine chromophores on C3 & C6 and carbazole nitrogen (para-). But, in the case of its positional isomer C1 the direct involvement of carbazole nitrogen in electron delocalization is poor due to its meta‒like connectivity caused by the attachment of chromophores on C2 & C7, so it resulted in slightly high oxidation potential when compared to 9. The dyes 4 and 5 showed low oxidation propensity in the above series, this is mainly due to the steric crowding induced electronic deconjugation between the chromophores on C1 & C8 and central carbazole unit. Further, the di‒substituted derivatives (5 and 9) displayed relatively high oxidation ability when compared to their mono-substituted analogues (3 and 8) attributed to the increased electron richness of the former compounds due to the presence of additional chromophore unit. The HOMO energies of the dyes were estimated from the first oxidation potential and fell in the range of 5.27‒5.37 eV. The LUMO energies were calculated by subtracting the optical band gap (E0-0, eV) from the HOMO energy, which lies in the range from 2.09 to 2.54 eV. The E0-0 value was deduced from the intersection of absorption and emission spectra of the compounds. Generally, the HOMO and LUMO energies of the dyes are sensitive to the electron donating and accepting units tethered to the π-electronic system. Thus, the triphenylamine featured dyes showed high-lying HOMO in the series. Also, the effect of cyano substitution on LUMO of the dyes is evident on comparing the data observed for the dyes 9 and C1 with the compounds (4,4'-(9-dodecayl-9H-carbazole3,6-diyl)bis(N,N-diphenylaniline),

4,4'-(9-(2-ethylhexyl)-9H-carbazole-2,7-diyl)bis(N,N-

diphenylaniline).42,43 These results suggests that the insertion of cyano acceptor on triphenylamine featured carbazoles with different linking topology beneficial to stabilize the LUMO energy. Also, it is interesting to note that the replacement of electron rich triphenylamine chromophores on C2 & C7 or C3 & C6 of 2,3,7-tri and 2,3,6,7tetracarbazoles with cyano acceptor stabilized the LUMO energy well.43



Page 20 of 41

Table 2. Thermal and electrochemical characteristics of the dyes Eox, V (∆EP, HOMO (eV)c mV)b 303 110 1.10 5.90 2 350 155 0.57 (67) 5.37 4 414 175 0.55 (64) 5.35 5 274 125 1.29 6.09 7 395 165 0.49 (67) 5.29 8 468 185 0.48 (60) 5.27 9 454 165 0.52 (70) 5.32 C1 a Temperature corresponding to 10% weight loss. b Measured

Dye

Tonset ( °C)a

Tm (°C)

LUMO (eV)d 2.33 2.09 2.18 2.86 2.38 2.48 2.30 for 0.1M

E0‒0 (eV)e 3.57 3.28 3.17 3.23 2.91 2.79 3.02 dichloromethane

solutions and the potentials are quoted with reference to ferrocene internal standard. c HOMO = -(4.8 + Eox). d LUMO = HOMO-E0-0. e Optical band gap obtained from the intersection of normalized absorption and emission spectra. Theoretical investigations The geometries of the model compounds (approximated alkyl chains to methyl group) were optimized under vacuum by employing DFT method as implemented in Gaussian 0964 with the Becke’s three-parameter functional that was hybridized with the Lee−Yang−Parr correlation functional and 6-31G(d,p) basis set.65,66 The results of ground state absorption energies of the dyes estimated using time dependent DFT computations at the same theoretical level are listed in Table 3. The optimized geometries of the dyes 4 and 5 are shown in Figure S8 and the electronic distribution in the frontier molecular orbitals such as HOMO and LUMO of the dyes in the ground state displayed in Figure 7. Generally, the HOMO of the dyes is contributed by the appended chromophores and extended into the central core. While, with the exception of 8 and 9, the LUMO of the dyes is mainly localized on the central carbazole with minor contribution from the appended chromophores. The participation of cyano acceptor in constructing LUMO is minimal in these compounds. But in the case of 8 and 9 the LUMO is exclusively constituted by the central carbazole and cyano acceptor present on C2 & C7-positions. The well separated 20 ACS Paragon Plus Environment

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arrangement of HOMO and LUMO orbitals in the former compounds is symptomatic of efficient charge transfer during the HOMO to LUMO electronic excitation. The computed HOMO and LUMO energies of the dyes are in the range of -5.00 to -5.74 eV and -1.26 to 2.14 eV, respectively. The energy gap (Eg) fell in the range 2.92‒4.21 eV. This indicates that the introduction of triphenylamine chromophores on dicyanocarbazole core with different linking topology changes the HOMO and LUMO energies significantly. The dyes 8 and 9 showed high lying HOMO and low lying LUMO in the series, which resulted in low band gap (Eg) for them. The HOMO, LUMO energies and Eg values estimated for the dyes in gas phase is significantly deviated from the experimental results. This may be due to the solvent‒solute interactions in the later case. The trends in computed lowest energy vertical transitions of the compounds are in well agreement with the experimentally observed values.

Figure 7. Frontier molecular orbitals (HOMO and LUMO) for the model compounds computed at the B3LYP/6-31G(d,p) level. Table 3. Computed energies of prominent vertical transitions and their oscillator strengths for the model compounds of the dyes. Dye

λmax, nm

f

Configuration

4 5 8 9 C1

368 375 354 366 412

0.14 0.21 0.66 0.72 1.00

HOMO → LUMO (+98%) HOMO → LUMO (+96%) HOMO → LUMO+1 (+99%) HOMO → LUMO+1 (+91%) HOMO → LUMO (+97%)


HOMO (eV) -5.35 -5.27 -5.07 -5.00 -5.06

LUMO (eV) -1.59 -1.57 -2.14 -2.06 -1.66

Eg (eV)

3.76 3.69 2.92 2.94 3.40


Page 22 of 41

Further, the charge carrier mobility of organic π-conjugated oligomers may be determined by incoherent hopping mechanism, where the charge transport process is assumed as nonadiabatic charge transfer reaction between spatially separated neutral and ionic species denoted as below:67-69

M + M → M + M

(1)

M + M → M + M

(2)

Here, M represents the neutral state of the system, M+ and M‒ symbolize the cationic and anionic state, respectively. The charge transfer rate (Khole/eletron) can be accounted by standard Marcus‒Hush relation (Equation 3).70,71 K / =

π π /

!" − /

(3)

Where KB and h denotes the Boltzmann and Planck's constants respectively; T represents the standard temperature; V represents the electronic coupling matrix (charge transfer integral) between adjacent species which is largely governed by orbital overlap; $/ indicates the total reorganization energies of hole or electron. It is evident from the Marcus‒Hush relation, the charge transfer rate (Khole/eletron) is mainly dominated by the electronic coupling matrix (V) and the internal reorganization energies λ+/-. In general, it is tedious to determine the accurate electronic coupling matrix (V) values experimentally for organic small molecules because of its direct contact in amorphous state and it falls within the narrow ranges.67-69 Therefore, the charge mobility in this study can be directly correlated with the respective internal reorganization energies of hole and electron (λ+/-). The theoretically estimated internal reorganization energies (λ+/-), ionization potentials, electron affinities and triplet energies of the dyes at B3LYP/6-31G(d,p) level are listed in Table 4. It is understandable from the above equation, the charge transfer rate and internal reorganization 22 ACS Paragon Plus Environment

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energies are inversely proportional to each other, thus smaller value ($/ ) denotes higher charge mobility. Using this approximation, reasonable correlations have been established between molecular structure and charge mobility for organic materials in the literature.67-69 Here, the hole reorganization energies are highly dependent on the chromophore density. The di‒ substituted derivatives (5 and 9) displayed small λ+ value when compared to their mono‒substituted analogues (4 and 8) attributed to the increase in the chromophore loading. Among the series the dyes containing chromophore substitution on C3 and/or C6 (8 and 9) involves significant electron reorganization. This can be explained by looking into the LUMO of the dyes (Figure 7). In former dyes, the cyano acceptor exclusively participated in constructing the LUMO, while in 4, 5 and C1, it is delocalized on electron rich central carbazole unit. Further, we have estimated the triplet energy (ET) of the materials theoretically. Generally, an ideal host material for blue phosphorescent devices (PhOLEDs) require high triplet energy (> 2.8 eV). Interestingly, the dyes 4 and 5 showed high ET (> 2.8 eV) in the series is attributable to deconjugation of the triphenylamine chromophores from the central carbazole unit arising from the steric effect on C1 and/or C8 positions of carbazole. This result is in good agreement with the literature reports i.e introduction of aromatic chromophores on C1 & C8‒positions of carbazole improve the triplet energy (Table 4) when compared to those of C3 & C6‒ and C2 & C7‒ substituted derivatives due to their interrupted extended π-conjugation. This result suggests that the dyes 4 and 5 may be suitable candidates to use as bipolar hosts for blue PhOLEDs. Table 4. Computed reorganization energies for hole (λ+) and electron (λ−), ionization potentials (Ip), electron affinities (Ea) and excited state first triplet energy (ET) (All values are in eV) for optimized model dyes. Dye 4 5

&'( 6.50 6.15

&') 6.59 6.20

*(( 0.57 0.67

*() 0.32 0.45

λ+ 0.18 0.09


λ– ET 0.48 2.89 0.41 2.79


8 9 C1

6.23 5.85 5.88

6.31 5.89 5.92

0.95 1.02 0.83

0.78 0.83 0.59

0.15 0.08 0.07

Page 24 of 41

0.33 2.42 0.37 2.37 0.47 2.57

Electroluminescence properties In order to avoid the concentration quenching, the electroluminescence properties of the compounds were evaluated in solution processable multilayer OLED devices by dispersing them into a wide energy gap host 4,4’-bis(9H-carbazol-9-yl)biphenyl (CBP) with different doping concentrations (1wt%, 3wt% and 5wt%). The optimized device configuration is as follows: ITO/PEDOT:PSS/CBP+4 or 5 or 8 or 9 or C1/TPBi/LiF:Al. Here, poly- (3,4ethylene-dioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) and LiF served as hole transporting and electron injection layers, respectively. TPBi used as an electron transporting layer and its low lying HOMO (6.20 eV) can also effectively block the holes. The energy level alignment for the materials used in the devices is depicted in Figure 8.

Figure 8. Energy-level diagram of the compounds used for the fabrication of the devices (all the values are in eV relative to the vacuum level)


Page 25 of 41

300

350

1000

900

(a) 300

(b)

750

Current density (mA/cm )

200

150

2

2

Luminance (cd/m )

2


2

Luminance (cd/m )

800

250

600

400

200

0

100

4

5

6

7

8

9

10

11

Voltage (V)

1wt%:CBP 3wt%:CBP 5wt%:CBP

50

250 200

600

450

300

150

150

0 4

5

6

7

8

10


50 0

0

2

4

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0

1

2

3

4

Voltage (V) 350

5

6

7

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9

10

11

12

Voltage (V)

3500

2700

(c)

3000

250

(d)

2400

300 2

2


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Luminance (cd/m )

250 200 150

2000

1500

1000

500

0 0

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25

50

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1wt%+CBP 3wt%+CBP 5wt%+CBP

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2

1800 1500 1200 900 600 300 0 0

20

40

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60

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2


1wt%+CBP 3wt%+CBP 5wt%+CBP

50

0

0 0

Luminance (cd/m )

2100 2500

2

9

Voltage (V)

100

0


4

6

8

10

0

12

2

4

6

8

10

12

Voltage (V)

Voltage (V) 360

2400

320

(e)

2

Luminance (cd/m )

2000

280

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


240 200

1600

1200

800

400

0 0

160

50

100

150

200

250

2


120 80


40 0 0

2

4

6

8

10

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Voltage (V)

Figure 9. Current density-Voltage-Luminance plots of the diodes (I-V-L): a) 4, b) 5, c) 8, d) 9 and e) C1 The current density-voltage-luminance (I-V-L) plots of the devices displayed in Figure 9 and pertinent data are listed in Table 5. The dyes containing triphenylamine substitution on C3 and/or C6 (8 and 9) exhibited relatively low turn-on voltage and high current densities in



the series. This probably points an effective injection of charge carries in the molecular layers of them attributed to their low-lying LUMO and high lying HOMO energy levels suitable for better electron and hole injection from the adjacent ETL and HTL, respectively. Further, they displayed high luminance characteristics reflecting their balanced confinement of charge carriers in the emitting layer and effective capturing of electronically generated excitons in the CBP host matrix. The C2, C3, C6 & C7 modified derivatives (8, 9 & C1) exhibited superior device performance (Table 5) when compared to their congeners containing C1, C3, C6 & C8-substitution (4 & 5). This could be due to the low lying LUMO energy levels of the former dyes, which is well match with the adjacent TPBi ETL layer for better electron injection. In electroluminescence spectra (Figure 10), the devices based on 4, 5 and C1 exhibited deep-blue emission with CIE coordinates of 0.15 ≤ x ≤ 0.17, 0.05 ≤ y ≤ 0.09, while the dyes 8 and 9 witnessed sky-blue emission with CIE coordinates of 0.15 ≤ x ≤ 0.16, 0.17 ≤ y ≤ 0.32. The CIE color coordinates for the deep-blue devices (4, 5 and C1) are in good agreement with the blue emission (0.14, 0.08) prescribed by the National Television System Committee (NTSC), 1987. The achieved excellent color purity of these materials is attributed to their low FWHM (< 58 nm) of the EL spectra. The EL of the compounds are in close resemblance to their PL recorded in toluene solution, indicating that the EL is indeed form the desired emitting layer without excimer or exciplex emission. Interestingly, with increasing the doping ratio form 1wt% to 5wt%, no significant shift in peak maxima and CIE coordinates observed suggesting that the materials are aggregation resistant even at reasonably high doping concentrations. Moreover, with the exception of 8, no CBP emission was observed in the EL spectra suggesting a complete energy transfer from CBP host to dopant emitters. Overall, the device constructed with 1wt% of C1 exhibited superior performance in the series with a maximum luminescence of 1438 cd/m2, turn-on voltage of 5.1 V, current efficiency of 2.5 cd/A, power efficiency of 1.5 lm/W, CIE coordinates of (0.16,


Page 26 of 41

Page 27 of 41

0.06) and EQE of 6.5%. This is attributed to the utilization of both energies of LE and CT excitons (i.e HLCT).45 Moreover, the superior performance of C1 surpasses the reported phenanthromidazole based deep-blue HLCT emitters.14-16,62,63 1.05

1.05

Normalized EL Intensity



0.90

(a)

0.75 0.60 0.45 0.30

0.90


0.75

(b)

0.60 0.45 0.30 0.15

0.15

0.00

0.00 375

425

475

525

375

575

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525

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575

Wavelength (nm)

Wavelength (nm) 1.05

1.05


(c)

0.75


0.90


0.90


0.60 0.45 0.30 0.15

(d)

0.75 0.60 0.45 0.30 0.15

0.00

0.00 375

400

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Wavelength (nm)

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1.05


0.90

Normalized EL intesnity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(e)

0.75 0.60 0.45 0.30 0.15 0.00 380

425

470

515

560

605

Wavelength (nm)

Figure 10. EL spectra of the diodes in different doping concentrations: a) 4, b) 5, c) 8, d) 9 and e) C1.



Page 28 of 41

Table 5. Electroluminescence parameters of the devices Material Conc. [wt%] 4

5

8

9

C1

1 3 5 1 3 5 1 3 5 1 3 5 1 3 5

Driving voltage (V) 6.0 6.0 6.0 5.1 5.1 4.8 5.6 5.8 5.2 5.3 5.4 5.4 5.1 5.2 5.2

Power efficiency (lm/W) 0.4 0.5 0.5 0.4 0.5 0.5 1.7 2.3 4.0 1.7 3.3 3.8 1.5 1.8 2.3

Current efficiency (cd/A) 0.8 1.1 1.0 0.7 0.8 0.8 3.0 4.2 6.5 2.8 5.7 6.5 2.5 3.0 3.8

EQE (%) 2.2 2.8 2.8 2.8 2.9 2.3 3.4 3.8 3.9 1.7 2.8 3.1 6.5 5.7 4.8

Max. Luminance (cd/m2) 933 863 839 875 933 821 1994 2365 3065 1562 2484 2633 1438 2112 2218

EL max. (FWHM) (nm) 408 (56) 408 (60) 416 (52) 404 (52) 408 (56) 412 (56) 460 (76) 468 (72) 472 (74) 472 (72) 476 (72) 480 (70) 428 (48) 432 (55) 436 (56)

CIE (x,y) 0.17, 0.07 0.17, 0.07 0.17, 0.07 0.16, 0.05 0.16, 0.05 0.16, 0.05 0.15, 0.17 0.15, 0.21 0.16, 0.25 0.16, 0.23 0.16, 0.29 0.16, 0.32 0.16, 0.06 0.16, 0.07 0.16, 0.09

CONCLUSIONS In summary, we have developed a new class of poly-substituted bipolar carbazoles featuring triphenylamine donor and cyano acceptor by employing selective halogenation and Suzuki-Miyaura cross coupling reactions as key steps. The structure-property relationship of the compounds investigated systematically. The physiochemical properties of the compounds are significantly dependent on the linking topology. The C2 & C7 triphenylamine featured derivatives showed red‒shifted absorption in the series and is attributed to the high degree of linear π-conjugation. Yet, the addition of donors on C1 and/or C8‒positions improved the triplet energy. The emission wavelengths of the compounds are tuned from blue to green by varying the conjugation connectivity between donor and acceptor. Thus, the dyes containing donors on C3 and/or C6 and cyano acceptor on C2 & C7 (8 and 9) exhibited most red-shifted emissions in the series when compared to their congeners due to an efficient charge migration from donor to cyano acceptor. However, they exhibited low fluorescence quantum efficiencies. The absorption profiles of the dyes are less sensitive to the solvent polarity 28 ACS Paragon Plus Environment

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indicative of non-polar ground state for them. Yet, the emission spectra of the dyes showed positive solvatochromism is attributable to photoinduced ICT from donor to cyano acceptor in excited state. Interestingly, the extent of charge transfer is dominated by the substitution pattern. All the compounds exhibited excellent thermal stability, which increases further on increasing donor density. Electrochemical studies revealed low oxidation potentials for 3,6chromophore featured derivatives. In the series, the dyes 8 and 9 inherited low band gap. The applicability of these materials as dopant emitters in solution processable multilayer OLED devices

were

demonstrated.

The

compounds

exhibited

deep-blue/sky-blue

electroluminescence depending on the substitution pattern. The device fabricated with C1 showed superior performance in the series with a maximum luminescence of 1438 cd/m2, a current efficiency of 2.5 cd/A, a power efficiency of 1.5 lm/W, CIE of (0.16, 0.06) and EQE of 6.5%. Further, the utility of high triplet energy compounds (4 and 5) as hosts for blue PhOLEDs are under process in our laboratory. ASSOCIATED CONTENT Supporting information. Experimental section, absorption and emission spectra of the compounds recorded in different solvents. 1H and

13

C NMR spectra of the synthesized

compounds. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author E-mail: [email protected] ACKNOWLEDGEMENTS K.R.J.T is thankful Council of Scientific and Industrial Research (CSIR), New Delhi (02/(0230)/15/EMR-II) for financial support. R.K.K is thankful to CSIR, New Delhi for a Research Fellowship. A.P. is grateful for research fellowship from IIT Roorkee. Generous access to the instrumental facility at the Department of Chemistry and Institute



Instrumentation Centre is also gratefully acknowledged. We also acknowledge DST for the setup of ESI mass spectrometer via the FIST grant to the Chemistry Department, IIT Roorkee.

REFERENCES 1. Duan, L.; Hou, L.; Lee, T. W.; Qiao, J.; Zhang, D.; Dong,G.; Wang, L.; Qiu, Y. Solution Processable Small Molecules for Organic Light-Emitting Diodes. J. Mater. Chem. 2010, 20, 63926407. 2. Zhu, X. H.; Peng, J.; Cao, Y.; Roncali, J. Solution-Processable Single-Material Molecular Emitters for Organic Light-Emitting Devices. Chem. Soc. Rev. 2011, 40, 3509-3524. 3. Wong, M. Y.; Colman, E. Z. Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes. Adv. Mater. 2017, 29, 1605444-1605508. 4. Lin, Y.; Li, Y.; Zhan. X. Small Molecule Semiconductors for High-Efficiency Organic Photovoltaics. Chem. Soc. Rev. 2012, 41, 4245-4272. 5. Mishra, A.; Fischer, M. K. R.; Bäuerle, P. Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: from Structure: Property Relationships to Design Rules. Angew. Chem., Int. Ed. 2009, 48, 2474-2499. 6. Wang, C.; Dong, H.; Hu, Q.; Liu. Y.; Zhu, D. Semiconducting π-Conjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208-2267. 7. Li, J.; Zhou, K.; Liu, J.; Zhen, Y.; Liu, L.; Zhang, J.; Dong, H.; Zhang, X.; Jiang, L.; Hu, W. Aromatic Extension at 2,6-Positions of Anthracene toward an Elegant Strategy for Organic Semiconductors with Efficient Charge Transport and Strong Solid State Emission. J. Am. Chem. Soc. 2017, 139, 17261-17264. 8. Kawata, S.; Kawata, Y. Three-Dimensional Optical Data Storage Using Photochromic Materials. Chem. Rev. 2000, 100, 1777-17788 30 ACS Paragon Plus Environment

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