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Carbazole-Derived Group of Uniform Materials Based on Organic Salts: Solid State Fluorescent Analogues of Ionic Liquids for Potential Applications in Organic-Based Blue Light-Emitting Diodes Noureen Siraj,† Farhana Hasan,†,# Susmita Das,*,†,# Lucy W. Kiruri,† Karen E. Steege Gall,‡ Gary A. Baker,⊥ and Isiah M. Warner*,† †

Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States HORIBA Scientific, 3880 Park Avenue Edison, New Jersey 08820, United States ⊥ Department of Chemistry, University of MissouriColumbia, Columbia, Missouri 65211-7600, United States ‡

S Supporting Information *

ABSTRACT: In this study, we report synthesis and characterization of novel carbazole-based group of uniform materials based on organic salts (GUMBOS), as well as potential applications of these compounds. These organic-based compounds exhibit high thermal stability (decomposition temperatures in the range of 395−432 °C) and photostability. In addition, these compounds have appreciably high fluorescence quantum yields (73−99%) with broad emissions in the visible region and quantum yields which depend on the GUMBOS counteranion. The physicochemical, optical, and electrochemical properties of these materials are investigated and detailed here. Evaluation of band gap values (3.4 eV), HOMO−LUMO energy levels, and measured fluorescence quantum yields as compared to carbazole suggest potential use in organic light-emitting diodes. Computational results are found to be complementary to experimental results, and calculated band gaps are in agreement with experimentally obtain values.

1. INTRODUCTION Over the last several decades, ionic liquids (ILs) have gained increasing interest of researchers due to their unusual and applicable properties.1 These molecules have been used in many different fields to replace conventional organic solvents and have also been referred to as green solvents due to their low volatility.2,3 The general tunability of these molecules has led to emergence of task-specific ionic liquids that are designed to incorporate desired characteristics for specific applications. In recent years, our group has introduced a new class of solid phase materials designated by the acronym GUMBOS (group of uniform materials based on organic salts). GUMBOS, which are solid state versions of ILs, exhibit a broad range of melting points (25−250 °C). In addition to retaining the most interesting properties of ILs such as tunability, high thermal stability, and nonflammability, GUMBOS have been shown to have multifaceted applications including biomedical imaging,4 photovoltaics,5 and antimicrobial agents,6,7 as well as other applications.8 In the present work, we report on the synthesis of novel organic semiconductor-based GUMBOS that exhibit desired characteristics for use in organic light emitting diodes (OLEDs) and other optoelectronic applications. Carbazole derivatives have been widely exploited for their electronic and optical properties and are extensively used in optoelectronic devices.9 These applications are realized as a result of their semiconductor properties, transporting ability, and great thermal characteristics. In this regard, many different © 2014 American Chemical Society

derivatives with extended conjugation as well as polymer components have been synthesized to incorporate the amorphous characteristics with high thermal stability needed for OLEDs applications.10−17 Examination of the literature indicates that several bulky carbazole-based molecules have been reported with increased conjugation achieved via long synthetic approaches, ultimately leading to rather expensive compounds.18 Moreover, many of these synthetic procedures are quite complicated and tedious, including a number of steps which result in low yields. In regard to organic-based OLEDS, a relatively small molecule for this use has been reported by Tang and VanSlyke.19 This achievement has revealed new opportunities for small organic-based compounds which can be efficiently used as optoelectronic materials. We note that there are also a few reports which cite modest increases in OLEDs efficiency by use of imidazolium ionic liquids.20 Furthermore, the role of ionic liquids in enhancement of charge transport and improvement in efficiency of OLEDs has also been reported.20 We have undertaken the present study with these important characteristics of carbazole and the significant contributions of ionic liquids to optoelectronics in mind. The major aim of the present study is to synthesize a low-cost, highly efficient Received: November 1, 2013 Revised: December 20, 2013 Published: January 9, 2014 2312

dx.doi.org/10.1021/jp410784v | J. Phys. Chem. C 2014, 118, 2312−2320

The Journal of Physical Chemistry C

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fluorescent material for potential use in OLEDs. In this regard, a carbazole-based GUMBOS was prepared by introduction of an imidazolium ring onto the third carbon of the carbazole unit and use of iodide as the counteranion. We have found very few examples of carbazole-based ionic liquids in the literature. In addition, these few reports are severely lacking in information regarding spectral, electrochemical, and thermal properties of these materials. For example, carbazole having imidazolium at the tail of the alkyl chain has been reported as having the properties of a surfactant.21 A novel ionic conductor, carbazoleimidazoleiodide solid electrolyte, has also been synthesized as a triiodide transportation material for use in solid state dye-sensitized solar cells (SDSC) by Midya et al.22 A similar synthetic procedure has been adopted for the present study. In this regard, synthesis of a new derivative of carbazole via simple attachment of various groups at the 3, 6, and Nposition of carbazole is easily implemented. Thus, this approach was adapted for the current synthesis to obtain carbazoleimidazole-based GUMBOS with preferred characteristics such as amorphous morphologies, appropriate redox potentials, high fluorescence quantum yields in the visible region, and great thermal and photostability. Three different derivatives of the carbazoleimidazole-based cation were synthesized using trifluoromethanesulfonate ([OTf]), bis(trifluoromethanesulfonyl)imide ([NTf 2 ]), and bis(pentafluoroethylsulfonyl)imide ([BETI]) as the counteranions. These three anions were chosen to investigate their effects on the physicochemical properties of the parent compound as a result of increasing trifluoromethane chain with increasing hydrophobicity. On the basis of previous studies, the counterions [NTf2] and [BETI] are likely to impart higher thermal and photostabilities.23 The present study was designed to employ simple synthesis of carbazoleimidazole-based GUMBOS. Such GUMBOS are expected to provide broad fluorescence emission due to extensive conjugation between the carbazole and the imidazole unit, with good quantum yields, suitable band gaps, high thermal and photostability, and excellent prospects for applications in optoelectronics. Bulky carbazole derivatives have been employed in the past as hole transport materials as well as emissive materials in OLEDs owing to their excellent holetransporting properties and high thermal, morphological, and photostability.24,18,25,26 Recently, researchers have expressed great interest in the synthesis of materials which emit at low wavelengths in order to develop a white light source by incorporating other colored materials. However, it is not easy to obtain a material with blue emission for OLEDs fabrication due to several previously reported problems.25,17 The carbazole-based GUMBOS described in this study exhibit characteristics such as amorphous properties, thermal stabilities, appropriate band gap values, strong broad fluorescence emission, and unexpectedly high quantum yields with appreciably good photostabililties. All these properties together constitute an appropriate combination for their potential applicability in OLEDs as blue-emitting or hole transport materials.

dimethylformamide were purchased from Sigma Aldrich and used as received. Imidazole was purchased from Fluka. Hexane and methanol (MeOH) were purchased from OmniSolv, dicholoromethane (DCM) was from J.T Baker, and diethyl ether was purchased from Fisher Scientific. Triply deionized water (18.2 MΩ cm) was obtained by use of an Elga model PURELAB ultra water-filtration system and was used for all ion exchange reactions. 2.2. Instrumentation. The thermal decomposition temperature of each compound was measured by use of a Hi Res Modulated TGA 2950 Thermogravimetric Analyzer TA Instrument. Absorbance measurements were performed using a Shimadzu UV- 3101PC and a UV−vis−near-IR scanning spectrometer (Shimadzu, Columbia, MD). Fluorescence studies were performed using a Fluorolog-3 spectrofluorimeter (model FL3-22TAU3; HORIBA Scientific, Edison, NJ). A 0.4 cm path length quartz cuvette (Starna Cells) was used to collect fluorescence and absorbance against an identical cell filled with solvent as the blank. Fluorescence studies were all performed using right angle geometry. Quantum yields were measured using an integrated sphere, and measurements of quantum yields were conducted on a HORIBA Scientific Quanta φ accessory (150 mm diameter) coupled with Spex Fluorolog-3 spectrofluorimeter (model FL3-22TAU3; HORIBA Scientific, Edison, NJ). Quantum yields were measured using a stoppered quartz cuvette of 1 cm path length (Starna Cells). Fluorescence lifetimes were measured on a FluoroCube, spectrofluorimeter (model FluoroCube, HORIBA Scientific, Edison, NJ) using the time domain mode. A picosecond pulsed LED excitation source of 273 nm was used and emission collected at 385 nm in MeOH and at 440 nm in DCM with a TBX detector. The time-correlated single photon counting (TSCPC) mode was used for data acquisition with a resolution of 7 ps/Channel. Quartz glass was purchased from SPI supplies and used to prepare solid films. Solid films were prepared using Gamma High Voltage Research, Inc., coupled with a Harvard apparatus to simultaneously control the voltage and flow rate. Films were characterized by use of scanning electron microscopy (SEM) and fluorescence microscopy. The photostabilities of GUMBOS containing hydrophobic anions were studied over a period of 3000 s at 275 nm with emission and excitation slit widths of 14 nm. Temperature-dependent fluorescence studies were also performed over the temperature range of 20−60 °C with 5° intervals to investigate temperature-dependent changes in fluorescence, as well as reversibility of these properties upon cooling. Cyclic voltammetric measurements were performed using an Autolab/EAs 2 computer-controlled electrochemical system equipped with a potentiostat (model PGSTAT 302N) and GPES (version 4.9.007) software. Cyclic voltammograms (CVs) of these three compounds were recorded separately by using Pt as a working and counter electrode. The working electrode was polished using wet filter paper prior to any experiments. The reference electrode was Ag/AgCl, while ferrocene was used as an internal reference electrode. The supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) prepared in an organic solvent (DCM). The potential window was determined by running the CV of the supporting electrolyte solution followed by performance of cyclic voltammetry on the GUMBOS and ferrocene. These measurements were performed at different

2. EXPERIMENTAL METHOD 2.1. Materials. Carbazole, N-bromosuccinimide, sodium hydride, 2-ethylhexyl bromide, 1,10-phenanthroline, sodium sulfate, sodium trifluoromethanesulfonate (NaOTf), lithium bis(trifluoromethylsulfonyl)imide (LiNTf2 ), lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), iodomethane, and 2313



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Figure 1. Synthesis scheme and structure of GUMBOS.

3.1. Thermal Gravimetric Analysis (TGA). Samples were heated gradually from room temperature to 600 °C at a rate of 10 °C min−1. Values of the onset temperature were determined using TA universal analysis software, and these values were reported as the decomposition temperature (Td). Examination of TGA data indicated that these carbazole-based salts possessed good thermal stability, with decomposition temperatures (Td) ranging from 395 to 432 °C. The results obtained for various anions with the same carbazoleimidazolium cation showed that thermal stability was greatly enhanced with [BETI], [NTf2], or [OTf], as compared to the iodide-based parent compound. These results clearly demonstrated that the Td was primarily dependent on the anion, with hydrophobic anions exhibiting high thermal stability as depicted in Table 1.

scan rates. Cyclic voltammograms were analyzed to determine peak potentials, which were later used to calculate band gaps. 2.3. Computational Details. The Gaussian 09 program27 was utilized for calculations in the present study. The geometric structures of compounds were visualized using GaussView 5.0. The ground state geometries of GUMBOS including both counterions were first optimized using density function theory (DFT)28 and postoptimized using time-dependent density function theory (TDDFT) to calculate the transition energies. The hybrid DFT Becke’s three-parameter nonlocal exchange functional,29,30 with a correlation function similar to Lee− Yang−Parr31 (B3LYP), was used for all calculations. A diffuse function basis set of 6-31+G(d,p)32,33 was employed. The choice of basis set with polarized (for heavy and hydrogen atoms) and diffuse functions was made for a better description of electrons relatively far from the nucleus as well as success of B3LYP/6-31+G(d,p) in a similar study.34 Vibrational frequencies were analyzed in order to confirm the optimized structures as a local minima. Optimized structures were used for TDDFT using the same model chemistry (B3LYP/6-31+G(d,p)).

Table 1. Anion-Dependent Decomposition Temperatures Measured for Carbazole-Based GUMBOS

3. SYNTHESIS AND CHARACTERIZATION Carbazoleimidazolium iodide (CII) was synthesized following a protocol described in the literature.22 Details of this procedure were presented in the Supporting Information. Various derivatives were prepared by use of a simple anion exchange method. Iodide ion from CII was replaced with organic hydrophobic anions through a simple anion exchange procedure. This reaction was performed in a biphasic solution, where CII was dissolved in dichloromethane (DCM) and highly concentrated solutions of other salts were prepared using DI water. Carbazoleimidazolium trifluoromethanesulfonate [CI][OTf] was prepared by using the corresponding sodium salt, whereas carbazoleimidazolium bis(trifluoromethylsulfonyl)imide [CI][NTf2] and carbazoleimidazolium bis(pentafluoroethylsulfonyl)imide [CI][BETI] were synthesized by use of their lithium salts. After stirring for 3−4 days, the lower layer of DCM was separated from water, and later the DCM layer was washed with water several times to remove the byproduct (lithium or sodium salt of iodide) which is highly soluble in water. DCM was evaporated under high vacuum and freeze-dried to remove small amounts of water. These compounds were characterized by use of ESI-MS, H NMR, and 19F-NMR. The synthesis scheme and structures of the cation and anions are shown in Figures 1 and Supporting Information S1.

GUMBOS

Td/°C

CII [CI][OTf] [CI][NTf2] [CI][BETI]

310 395 432 417

The final decomposition temperature of GUMBOS containing [BETI] and [NTf2] anions were comparable. These results were consistent with previous studies where [NTf2] and [BETI] counteranions exhibited higher and comparable thermal stabilities.1,35,36 [CI][NTf2] and [CI][OTf] showed respectively almost 17% and 30% of weight loss before reaching the final decomposition temperature. The remaining residue of about 11−17% is attributed to the anions (Figure S2 of the Supporting Information). TGA plots are shown in Figure 2, and the data obtained from the onset are tabulated in Table 1.

4. RESULTS AND DISCUSSION 4.1. X-ray Diffraction (XRD). X-ray diffraction was used to estimate the morphology of the GUMBOS. Extensive research has been performed to design amorphous materials to avoid nonlinear optical activity from crystalline materials.37 It has been shown that molecules that exhibit packing difficulty show stable amorphous characteristic with high morphological stability.38 Thus, the materials derived in our studies should be amorphous due to frustrated packing in GUMBOS produced by use of bulky cations. Examination of XRD data showed a broad indistinguishable peak in the XRD spectrum 2314



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with different anions in a given solvent, Figure S5 of the Supporting Information. The band gap, which is designated as the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), was calculated from the onset wavelength of the lowest energy absorption peak. The onset wavelength is designated as the negative tangent line of the lowest energy absorption peak that intersects with a linear tangent line of the absorption tail (see Figure 4).

Figure 2. Thermogravimetric profile of CII, [CI][OTf], [CI][NTf2], and [CI][BETI].

which reveals the amorphous properties of these GUMBOS as depicted in Figure S3 of the Supporting Information. The amorphous properties of these GUMBOS are attributed to the presence of an ethylhexyl chain on the nitrogen of carbazole which decreases the chances of constricted packing of ions. 4.2. UV−Vis Spectroscopy. Absorption spectra of all GUMBOS were recorded and are shown in Figures 3 and

Figure 4. Absorption spectrum of concentrated solution of [CI][BETI] in DCM to show onset wavelength.

Absorption onset at higher wavelength corresponds to the minimum amount of energy which is required for excitation of the electron from HOMO to LUMO. In other words, this is the energy for electronic transition from ground to excited state. A 3.4 eV value of the band gap in DCM was determined from eq 1: 1240 Eg (eV) = λ(nm) (1) 4.3. Fluorescence Spectroscopy. Emission spectra were recorded at an excitation wavelength of 275 nm as depicted in Figure S6 of the Supporting Information. A broad emission spectrum with a λmax at 440 nm in DCM and 375 nm in methanol was observed for all carbazole-based GUMBOS (Figure 5). This broadness was attributed to the presence of the imidazole ring within the carbazole unit. Samanta and coworkers have reported an excitation wavelength-dependent fluorescence for imidazolium-based ILs.40 Similar behavior was observed in the current study as well, although the precise

Figure 3. Normalized absorption spectra of [CI][BETI] and carbazole in methanol.

Figures S4 and S5 of the Supporting Information. Solutions were prepared in methanol and DCM. The absorption spectrum of [CI][BETI] exhibited two highly intense peaks at 236 and 275 nm as well as two lower intensity bands at 335 and 350 nm. The peaks at 335 and 275 nm were attributed to first (S1) and second singlet (S2) excited states, respectively, as represented in the literature for carbazole and its different derivatives.39 As shown in Figure 3, formation of [CI][BETI] led to a peak shift from 290 nm (for pure carbazole) to 275 nm which is possibly due to the presence of quaternary nitrogen in the ring. The red shift of the first singlet excited state peak from 322 to 335 nm was observed in carbazole-based GUMBOS, as compared to carbazole. This shift is attributed to the extensive conjugated system. Thus, there was a significant increase in the energy gap between S1 and S2 in our GUMBOS compounds. All absorption peaks were attributed to the carbazoleimidazolium cation (Figure 3), and as expected, none were contributed by the anion. A very small shift of 5 nm was observed for a compound in two different solvents as depicted in Figure S4 of the Supporting Information. As expected, no peak shifts were observed for the carbazoleimidazolium cation when conjugated

Figure 5. Fluorescence emission of [CI][NTf2] in methanol and DCM, λex 275 nm. 2315



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origin of the emission is still a matter of debate.41 Figure S7 of the Supporting Information is a representation of the fluorescence emission spectrum of [CI][BETI], which overlaps with the fluorescence spectrum of carbazole and imidazole upon excitation at the same wavelength (275 nm). The peak between the two suggests that the broadness in the emission spectrum of [CI][BETI] arises from a combination of two units. Excitation spectra were measured using respective emission wavelengths of 385 and 440 nm in methanol and DCM. These spectral data are presented in Table 2. The emission and Table 2. Absorption, Emission Wavelength, Molar Extinction Coefficients, and Quantum Yields of GUMBOS GUMBOS

solvent

λabs/nm

CII

MeOH DCM MeOH DCM MeOH DCM MeOH DCM

275 280 273 280 273 281 273 280

[CI][OTf] [CI][NTf2] [CI][BETI]

ε/104 M−1 cm−1 2.34 1.51 6.75 9.52 2.60 1.68

λfluo/nm

% ϕfl

380 440 378 440 380 440 378 440

25 28

Figure 7. Absorption, excitation, and emission spectra of [CI][BETI] in DCM.

dyes as a result of ICT due to formation of C−N bonds.23 The broadness of the emission spectra could also be attributed to the formation of an ICT state which emits in the red region of the spectrum. 4.4. Quantum Yield Measurements. Absolute quantum yields were measured for all carbazole-based GUMBOS using an integrating sphere. The reported value of quantum yield for carbazole is 0.4,42 which is consistent with the value we obtained using the integrating sphere. Quantum yields were also obtained using a relative method employing carbazole as the standard. Both approaches showed very high quantum yields for GUMBOS with [OTf], [NTf2], and [BETI] counteranions. In this study, the reasons for enhanced measured quantum yields can be attributed to the large Stokes shift. High quantum yields with polymeric derivatives or with bulky organic compounds of carbazole have been previously reported in the literature.43,44 The primary advantage of GUMBOS-based materials is that we are able to achieve these enhanced quantum yields using small molecules with simple changes in counteranions. We note that we can also tune these quantum yields, as reflected in for the data presented in Table 2. 4.5. Fluorescence Lifetimes. The fluorescence lifetime measurements were performed in two solvents (MeOH and DCM). The fluorescence lifetime decays of the three carbazoleimidazole-based GUMBOS were best fit to a bi- or triple exponential decay, and the contributions to fluorescence in each case were determined to be primarily from two states (Tables 3 and Table S1 of the Supporting Information). The

94 73 99

Figure 6. Absorption and fluorescence emission spectra of [CI][BETI] in DCM, λex 275 nm.

excitation spectra were not mirror images (Figures 6 and Figure S8 of the Supporting Information). The larger bandwidth of the fluorescence emission spectrum was attributed to incorporation of the imidazolium emission into the emission spectrum, while such changes were not observed in the excitation spectra. A large Stokes shift of 105 nm was observed in DCM (Figure 7), which produces reduced fluorescence emission as a result of secondary inner-filter effects. A significant increase in Stokes shift was observed after addition of the imidazole ring onto carbazole and is the result of intramolecular charge transfer (ICT). The Stokes shift was calculated for each intermediate compound during synthesis (not shown here). After addition of the alkyl group at the N position, the Stokes shift was the same as observed in carbazole alone. However, it drastically increased after the addition of the imidazole group at the third position of the carbazole. Hence, the Stokes shift is attributed to the C−N bond of the carbon at the third position of carbazole which is attached to the nitrogen of imidazole. Such a large Stokes shift has been previously identified by our group in cyanine-based

Table 3. Lifetime Measurements of GUMBOS in DCM GUMBOS

τ1/ns

α1

τ2/ns

α2

τavg/ns

χred2


3.379 1.903 3.414

0.07 0.06 0.07

6.649 6.624 6.659

0.93 0.94 0.93

6.428 6.348 6.445

1.032 1.021 1.020

shorter lifetime component in each GUMBOS was attributed to emission from the excited singlet state (1.9−3.4 ns in DCM and 94−133 ps in MeOH), while the slower component was ascribed to emission from the charge transfer state (6.4 ns in DCM, 4.8 ns in MeOH).45 In methanol, the contribution from the third component is minor and could be the result of a back transition between the ICT and S1 state. In previous studies, it has been observed that substitution of an electron-withdrawing substituent at the third position leads to a drop in fluorescence 2316



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electrostatic, π−π stacking, and van der Waals interactions in GUMBOS produced a relatively homogeneous film as suggested in Figure 8 and Figure S9 of the Supporting Information. 4.7. Photo- and Thermal Stability Tests. Photostability and thermal stability are extremely important factors for any dyes developed for use in OLEDs. An emitting material with significantly high photo- and thermal stability would enhance the life and broaden the applications of such materials under a variety of conditions. All GUMBOS investigated in the present study exhibited extremely interesting properties in response to light exposure. An increase in photostability was actually observed for [CI][BETI], whereas [CI][NTf2] and [CI][OTf] underwent fairly stable fluorescence upon irradiation for more than 3000 s (see Figure 10). Such an increase in photostability was also observed in one of our previous studies and was attributed to irradiation-induced changes in aggregation.23

lifetime from 7.33 ns (unsubstituted carbazole) to 350 ps.39 Thus, it is likely that the enhanced conjugation due to substitution of an imidazolium unit leads to a decrease in fluorescence lifetime from the S1 state into the picosecond regime. The relatively shorter lifetime of the S1 state in [CI][NTf2] as compared to the other GUMBOS explains the lower quantum yield value of [CI][NTf2]. 4.6. Solid Film Studies. Solid films were prepared from each GUMBOS, and their spectral properties were studied. Various solution techniques (such as drop casting, spin coating, inkjet, and electrospray) were employed to obtain continuous, homogeneous, stable, and good solid films. For organic compounds, the vacuum deposition method is a wellestablished technique for acquiring thin films for OLEDs. However, since GUMBOS have low vapor pressures, this approach is not suitable for our materials. For our materials, we determined that electrospray methods produced good quality films and also offered the best size control of droplets.46 These solid films were then characterized by use of SEM and also by use of fluorescence microscopy, Figure S9 of the Supporting Information and Figure 8.

Figure 8. Epifluorescence image of a [CI][BETI] thin film on quartz glass.

Figure 10. Photostability of carbazole-based GUMBOS.

The fluorescence emission was studied for these thin films. In these experiments, red-shifted fluorescence emission maxima were observed which attributed to dye aggregation as tabulated in Table 4 (Figure 9). Intermolecular forces arising from

Examination of data from temperature-dependent fluorescence measurements suggested that the fluorescence emission intensity continuously decreased by 25% with an increase in temperature from 20 to 60 °C (Figure 11). However, this

Table 4. Fluorescence Emission Maxima in Solution and in Solid Film GUMBOS

λmax/nm solution (film)


378 (385) 380 (389) 378 (392)

Figure 11. Thermal stability of [CI][BETI] in methanol (Inset: a plot of fluorescence intensity against temperature).

sample showed recovery of its original fluorescence intensity after cooling back to 20 °C from 60 °C (Figure S10 of the Supporting Information). No change in the photoluminescence spectra were observed before and after heating. This study demonstrates that our GUMBOS compounds are quite stable toward heat and light.

Figure 9. Fluorescence emission of [CI][NTf2] in bulk and in solid film, λex 275 nm. 2317



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4.9. Computational Study. DFT/TDDFT calculations provide additional understanding of the structural, electrochemical, and optical properties of the GUMBOS studied here. Optimized geometries revealed planar carbazole substituents, while the imidazole moiety had a twist. In all systems investigated, the HOMO is located primarily at the carbazole substituent, and the LUMO distributed over the imidazolium moiety (Figure 12). The band gap computed using DFT/

4.8. Electrochemistry. Electrochemical properties of these GUMBOS were evaluated by use of cyclic voltammetry. All solutions were prepared in DCM, and 0.1 M TBAPF6 was used as the supporting electrolyte. Cyclic voltammograms were recorded at a scan rate of 0.1 V/s. The potential of the working electrode was scanned to a positive value within the solvent window limit in order to acquire the oxidation peak of the carbazole unit in these compounds. The measured cyclic voltammograms generally displayed oxidation peaks, reflecting the formation of a dication. CII exhibited multiple electron transfer processes, which were attributed to oxidation and reduction of iodide. This redox reaction was not seen in other GUMBOS having hydrophobic anions, i.e., [OTF], [NTf2], and [BETI] (Figure S12 of the Supporting Information). This also verifies that the products are pure and little or no iodide remains after the ion exchange reaction. We note that these potentials can be measured at the peak positions or at the peak onset. The values of oxidation potentials obtained were recalculated versus a ferrocene/ferrocenium internal reference electrode. The redox potential for Fc/Fc+ was measured using the cyclic voltammograms. Cyclic voltammograms were analyzed in order to determine anodic peak potentials, which were later used to calculate the highest occupied molecular orbital (HOMO) energy level using eq 2.47

Figure 12. Calculated (a) HOMO and (b) LUMO structures for [CI][BETI].

TDDFT is tabulated in Table 5. In all cases, these DFT calculations overestimated the HUMO−LUMO band gap, while the TDDFT results are in excellent agreement with the experimental results.

E HOMO(eV) = −le−[Epa( V vs Fc+/Fc) + 4.8(V Fc+ /Fc vs zero)]

(2)

These values were determined using a ferrocene reference, where Epa is the anodic potential. The energy of the HOMO is ultimately based on the absolute value of the normal hydrogen electrode (NHE). The values of HOMO energy levels for our GUMBOS were obtained using eq 2 and tabulated in Table 5. The band gap (Eg EC) is measured as the difference in energy level between LUMO and HOMO, i.e. Eg (eV) = −(E HOMO(eV) − E LUMO(eV))

5. CONCLUSIONS Carbazole-based GUMBOS have been synthesized using a very simple procedure. These GUMBOS exhibited high absorbance and excellent luminescence properties in combination with high quantum yields and excellent photo- and thermal stability. These compounds possess broad emission characteristics in the visible region and demonstrate good quantum yields. A blue emissive material with appropriate combination of properties has been achieved in a very small molecule without the need for synthesis of large molecules involving multiple steps and low yields. A very simple approach has been used to tune the physicochemical properties of these compounds. The tunability in quantum yields and thermal stability of GUMBOS was controlled by use of counteranions. Evaluation of the spectral and electrochemical properties, as well as computed band gaps, suggest the potential use of these compounds for optoelectronic applications and as emitting materials for use in OLEDs. Future work will involve device fabrication and examination of these GUMBOS as an emissive layer in OLEDs. The high chemical stability and photostability reported for these compounds are essential for long life required for OLEDs.

(3)

Table 5. Redox Potential, HOMO−LUMO Energies, and Experimental and Theoretical Band Gap of GUMBOS GUMBOS

E vs Fc/V

HOMO/eV

LUMO/eV

Ega/eV

Egb /eV

CII [CI][OTf] [CI][NTf2] [CI][BETI]

0.96 0.94 0.95 0.93

−5.76 −5.74 −5.75 −5.73

−2.36 −2.32 −2.36 −2.31

3.42 3.41 3.39 3.42

3.77/4.4 3.74/4.3 3.58/4.1

a Band gap is calculated by using onset wavelength. bBand gap is calculated by computational calculation, TDDFT/DFT.

The absorption spectral band gap and electrochemical HOMO energy levels were used to evaluate the LUMO energy level (eq 3). The lowest unoccupied molecular orbital (LUMO) energy levels were computed, and data are presented in Table 5. These values are quite similar for different GUMBOS, as the oxidation potential is primarily attributed to oxidation of the cationic carbazole unit since the anion does not have any redox characteristic. The HOMO energy levels of our GUMBOS are lower than the ITO HOMO energy level (4.70 eV), and the LUMO energy levels lie above the electron transport material (TBPI (1,3,5-tris(N-phenylbenzimidazol-2yl)benzene, LUMO 2.70 eV).18 From an electronic perspective, it is expected that carbazole-based GUMBOS can perform as potential emitters for use in OLEDs.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and synthesis scheme, XRD, TGA, absorption, and fluorescence emission spectra, SEM, cyclic voltammogram, lifetime data in methanol of GUMBOS and the complete reference for ref 27. This material is available free of charge via the Internet at http://pubs.acs.org. 2318



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AUTHOR INFORMATION

Corresponding Author

*Fax: 1-225-578-3971. Tel.: 1-225-578-2829. E-mail: iwarner@ lsu.edu. Author Contributions #

These two authors have equal contribution.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.S. acknowledges support by the National Science Foundation under grant no. CHE-1243916. The authors thank Dr. Randall Hall for discussion regarding computational study and Dr. Evgueni E. Nesterov for use of electrochemical instrumentation.

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REFERENCES

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