carbazole-derived group of uniform materials based on organic salts: solid state fluorescent...

Carbazole-Derived Group of Uniform Materials Based on OrganicSalts: Solid State Fluorescent Analogues of Ionic Liquids for PotentialApplications in Organic-Based Blue Light-Emitting DiodesNoureen 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 MissouriColumbia, Columbia, Missouri 65211-7600, United States

*S Supporting Information

ABSTRACT: In this study, we report synthesis and characterization of novel carbazole-basedgroup of uniform materials based on organic salts (GUMBOS), as well as potential applicationsof 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 broademissions in the visible region and quantum yields which depend on the GUMBOScounteranion. The physicochemical, optical, and electrochemical properties of these materialsare investigated and detailed here. Evaluation of band gap values (3.4 eV), HOMO−LUMOenergy levels, and measured fluorescence quantum yields as compared to carbazole suggestpotential use in organic light-emitting diodes. Computational results are found to becomplementary to experimental results, and calculated band gaps are in agreement withexperimentally obtain values.

1. INTRODUCTION

Over the last several decades, ionic liquids (ILs) have gainedincreasing interest of researchers due to their unusual andapplicable properties.1 These molecules have been used inmany different fields to replace conventional organic solventsand have also been referred to as green solvents due to theirlow volatility.2,3 The general tunability of these molecules hasled to emergence of task-specific ionic liquids that are designedto 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 (groupof uniform materials based on organic salts). GUMBOS, whichare solid state versions of ILs, exhibit a broad range of meltingpoints (25−250 °C). In addition to retaining the mostinteresting properties of ILs such as tunability, high thermalstability, and nonflammability, GUMBOS have been shown tohave multifaceted applications including biomedical imaging,4

photovoltaics,5 and antimicrobial agents,6,7 as well as otherapplications.8 In the present work, we report on the synthesis ofnovel organic semiconductor-based GUMBOS that exhibitdesired 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 inoptoelectronic devices.9 These applications are realized as aresult of their semiconductor properties, transporting ability,and great thermal characteristics. In this regard, many different

derivatives with extended conjugation as well as polymercomponents have been synthesized to incorporate theamorphous characteristics with high thermal stability neededfor OLEDs applications.10−17 Examination of the literatureindicates that several bulky carbazole-based molecules havebeen reported with increased conjugation achieved via longsynthetic approaches, ultimately leading to rather expensivecompounds.18 Moreover, many of these synthetic proceduresare quite complicated and tedious, including a number of stepswhich result in low yields.In regard to organic-based OLEDS, a relatively small

molecule for this use has been reported by Tang andVanSlyke.19 This achievement has revealed new opportunitiesfor small organic-based compounds which can be efficientlyused as optoelectronic materials. We note that there are also afew reports which cite modest increases in OLEDs efficiency byuse of imidazolium ionic liquids.20 Furthermore, the role ofionic liquids in enhancement of charge transport andimprovement in efficiency of OLEDs has also been reported.20

We have undertaken the present study with these importantcharacteristics of carbazole and the significant contributions ofionic liquids to optoelectronics in mind. The major aim of thepresent study is to synthesize a low-cost, highly efficient

Received: November 1, 2013Revised: December 20, 2013Published: January 9, 2014

Article

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

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fluorescent material for potential use in OLEDs. In this regard,a carbazole-based GUMBOS was prepared by introduction ofan imidazolium ring onto the third carbon of the carbazole unitand use of iodide as the counteranion. We have found very fewexamples of carbazole-based ionic liquids in the literature. Inaddition, these few reports are severely lacking in informationregarding spectral, electrochemical, and thermal properties ofthese materials. For example, carbazole having imidazolium atthe tail of the alkyl chain has been reported as having theproperties of a surfactant.21 A novel ionic conductor,carbazoleimidazoleiodide solid electrolyte, has also beensynthesized as a triiodide transportation material for use insolid state dye-sensitized solar cells (SDSC) by Midya et al.22 Asimilar synthetic procedure has been adopted for the presentstudy. In this regard, synthesis of a new derivative of carbazolevia simple attachment of various groups at the 3, 6, and N-position of carbazole is easily implemented. Thus, this approachwas adapted for the current synthesis to obtain carbazoleimi-dazole-based GUMBOS with preferred characteristics such asamorphous morphologies, appropriate redox potentials, highfluorescence quantum yields in the visible region, and greatthermal and photostability. Three different derivatives of thecarbazoleimidazole-based cation were synthesized using tri-fl u o r o m e t h a n e s u l f o n a t e ( [ O T f ] ) , b i s -(trifluoromethanesulfonyl)imide ([NTf2]), and bis-(pentafluoroethylsulfonyl)imide ([BETI]) as the counter-anions. These three anions were chosen to investigate theireffects on the physicochemical properties of the parentcompound as a result of increasing trifluoromethane chainwith increasing hydrophobicity.On the basis of previous studies, the counterions [NTf2] and

[BETI] are likely to impart higher thermal and photo-stabilities.23 The present study was designed to employ simplesynthesis of carbazoleimidazole-based GUMBOS. Such GUM-BOS are expected to provide broad fluorescence emission dueto extensive conjugation between the carbazole and theimidazole unit, with good quantum yields, suitable band gaps,high thermal and photostability, and excellent prospects forapplications in optoelectronics. Bulky carbazole derivatives havebeen employed in the past as hole transport materials as well asemissive materials in OLEDs owing to their excellent hole-transporting properties and high thermal, morphological, andphotostability.24,18,25,26 Recently, researchers have expressedgreat interest in the synthesis of materials which emit at lowwavelengths in order to develop a white light source byincorporating other colored materials. However, it is not easy toobtain a material with blue emission for OLEDs fabrication dueto several previously reported problems.25,17

The carbazole-based GUMBOS described in this studyexhibit characteristics such as amorphous properties, thermalstabilities, appropriate band gap values, strong broadfluorescence emission, and unexpectedly high quantum yieldswith appreciably good photostabililties. All these propertiestogether constitute an appropriate combination for theirpotential applicability in OLEDs as blue-emitting or holetransport materials.

2. EXPERIMENTAL METHOD2.1. Materials. Carbazole, N-bromosuccinimide, sodium

hydride, 2-ethylhexyl bromide, 1,10-phenanthroline, sodiumsulfate, sodium trifluoromethanesulfonate (NaOTf), lithiumbis(trifluoromethylsulfonyl)imide (LiNTf2), lithium bis-(pentafluoroethylsulfonyl)imide (LiBETI), iodomethane, and

dimethylformamide were purchased from Sigma Aldrich andused as received. Imidazole was purchased from Fluka. Hexaneand methanol (MeOH) were purchased from OmniSolv,dicholoromethane (DCM) was from J.T Baker, and diethylether was purchased from Fisher Scientific. Triply deionizedwater (18.2 MΩ cm) was obtained by use of an Elga modelPURELAB ultra water-filtration system and was used for all ionexchange reactions.

2.2. Instrumentation. The thermal decomposition temper-ature of each compound was measured by use of a Hi ResModulated TGA 2950 Thermogravimetric Analyzer TAInstrument. Absorbance measurements were performed usinga Shimadzu UV- 3101PC and a UV−vis−near-IR scanningspectrometer (Shimadzu, Columbia, MD). Fluorescencestudies were performed using a Fluorolog-3 spectrofluorimeter(model FL3-22TAU3; HORIBA Scientific, Edison, NJ). A 0.4cm path length quartz cuvette (Starna Cells) was used to collectfluorescence and absorbance against an identical cell filled withsolvent as the blank. Fluorescence studies were all performedusing right angle geometry. Quantum yields were measuredusing an integrated sphere, and measurements of quantumyields were conducted on a HORIBA Scientific Quanta φaccessory (150 mm diameter) coupled with Spex Fluorolog-3spectrofluorimeter (model FL3-22TAU3; HORIBA Scientific,Edison, NJ). Quantum yields were measured using a stopperedquartz 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 pulsedLED excitation source of 273 nm was used and emissioncollected at 385 nm in MeOH and at 440 nm in DCM with aTBX detector. The time-correlated single photon counting(TSCPC) mode was used for data acquisition with a resolutionof 7 ps/Channel.Quartz glass was purchased from SPI supplies and used to

prepare solid films. Solid films were prepared using GammaHigh Voltage Research, Inc., coupled with a Harvard apparatusto simultaneously control the voltage and flow rate. Films werecharacterized by use of scanning electron microscopy (SEM)and fluorescence microscopy. The photostabilities of GUM-BOS containing hydrophobic anions were studied over a periodof 3000 s at 275 nm with emission and excitation slit widths of14 nm. Temperature-dependent fluorescence studies were alsoperformed over the temperature range of 20−60 °C with 5°intervals to investigate temperature-dependent changes influorescence, as well as reversibility of these properties uponcooling.Cyclic voltammetric measurements were performed using an

Autolab/EAs 2 computer-controlled electrochemical systemequipped with a potentiostat (model PGSTAT 302N) andGPES (version 4.9.007) software. Cyclic voltammograms(CVs) of these three compounds were recorded separately byusing Pt as a working and counter electrode. The workingelectrode was polished using wet filter paper prior to anyexperiments. The reference electrode was Ag/AgCl, whileferrocene was used as an internal reference electrode. Thesupporting electrolyte was 0.1 M tetrabutylammoniumhexafluorophosphate (TBAPF6) prepared in an organic solvent(DCM). The potential window was determined by running theCV of the supporting electrolyte solution followed byperformance of cyclic voltammetry on the GUMBOS andferrocene. These measurements were performed at different

The Journal of Physical Chemistry C Article

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scan rates. Cyclic voltammograms were analyzed to determinepeak 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 geometricstructures of compounds were visualized using GaussView 5.0.The ground state geometries of GUMBOS including bothcounterions were first optimized using density function theory(DFT)28 and postoptimized using time-dependent densityfunction theory (TDDFT) to calculate the transition energies.The hybrid DFT Becke’s three-parameter nonlocal exchangefunctional,29,30 with a correlation function similar to Lee−Yang−Parr31 (B3LYP), was used for all calculations. A diffusefunction basis set of 6-31+G(d,p)32,33 was employed. Thechoice of basis set with polarized (for heavy and hydrogenatoms) and diffuse functions was made for a better descriptionof electrons relatively far from the nucleus as well as success ofB3LYP/6-31+G(d,p) in a similar study.34 Vibrational frequen-cies were analyzed in order to confirm the optimized structuresas a local minima. Optimized structures were used for TDDFTusing the same model chemistry (B3LYP/6-31+G(d,p)).

3. SYNTHESIS AND CHARACTERIZATIONCarbazoleimidazolium iodide (CII) was synthesized following aprotocol described in the literature.22 Details of this procedurewere presented in the Supporting Information. Variousderivatives were prepared by use of a simple anion exchangemethod. Iodide ion from CII was replaced with organichydrophobic anions through a simple anion exchangeprocedure. This reaction was performed in a biphasic solution,where CII was dissolved in dichloromethane (DCM) andhighly concentrated solutions of other salts were prepared usingDI water. Carbazoleimidazolium trifluoromethanesulfonate[CI][OTf] was prepared by using the corresponding sodiums a l t , w h e r e a s c a r b a z o l e i m i d a z o l i u m b i s -(trifluoromethylsulfonyl)imide [CI][NTf2] and carbazoleimi-dazolium bis(pentafluoroethylsulfonyl)imide [CI][BETI] weresynthesized by use of their lithium salts. After stirring for 3−4days, the lower layer of DCM was separated from water, andlater the DCM layer was washed with water several times toremove the byproduct (lithium or sodium salt of iodide) whichis highly soluble in water. DCM was evaporated under highvacuum and freeze-dried to remove small amounts of water.These compounds were characterized by use of ESI-MS, HNMR, and 19F-NMR. The synthesis scheme and structures ofthe cation and anions are shown in Figures 1 and SupportingInformation S1.

3.1. Thermal Gravimetric Analysis (TGA). Samples wereheated gradually from room temperature to 600 °C at a rate of10 °C min−1. Values of the onset temperature were determinedusing TA universal analysis software, and these values werereported as the decomposition temperature (Td). Examinationof TGA data indicated that these carbazole-based saltspossessed good thermal stability, with decomposition temper-atures (Td) ranging from 395 to 432 °C. The results obtainedfor various anions with the same carbazoleimidazolium cationshowed that thermal stability was greatly enhanced with[BETI], [NTf2], or [OTf], as compared to the iodide-basedparent compound. These results clearly demonstrated that theTd was primarily dependent on the anion, with hydrophobicanions exhibiting high thermal stability as depicted in Table 1.

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

4. RESULTS AND DISCUSSION4.1. X-ray Diffraction (XRD). X-ray diffraction was used to

estimate the morphology of the GUMBOS. Extensive researchhas been performed to design amorphous materials to avoidnonlinear optical activity from crystalline materials.37 It hasbeen shown that molecules that exhibit packing difficulty showstable amorphous characteristic with high morphologicalstability.38 Thus, the materials derived in our studies shouldbe amorphous due to frustrated packing in GUMBOSproduced by use of bulky cations. Examination of XRD datashowed a broad indistinguishable peak in the XRD spectrum

Figure 1. Synthesis scheme and structure of GUMBOS.

Table 1. Anion-Dependent Decomposition TemperaturesMeasured for Carbazole-Based GUMBOS

GUMBOS Td/°C

CII 310[CI][OTf] 395[CI][NTf2] 432[CI][BETI] 417



which reveals the amorphous properties of these GUMBOS asdepicted in Figure S3 of the Supporting Information. Theamorphous properties of these GUMBOS are attributed to thepresence of an ethylhexyl chain on the nitrogen of carbazolewhich 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

Figures S4 and S5 of the Supporting Information. Solutionswere prepared in methanol and DCM. The absorptionspectrum of [CI][BETI] exhibited two highly intense peaksat 236 and 275 nm as well as two lower intensity bands at 335and 350 nm. The peaks at 335 and 275 nm were attributed tofirst (S1) and second singlet (S2) excited states, respectively, asrepresented in the literature for carbazole and its differentderivatives.39 As shown in Figure 3, formation of [CI][BETI]led to a peak shift from 290 nm (for pure carbazole) to 275 nmwhich is possibly due to the presence of quaternary nitrogen inthe ring. The red shift of the first singlet excited state peak from322 to 335 nm was observed in carbazole-based GUMBOS, ascompared to carbazole. This shift is attributed to the extensiveconjugated system. Thus, there was a significant increase in theenergy gap between S1 and S2 in our GUMBOS compounds.All absorption peaks were attributed to the carbazoleimidazo-lium cation (Figure 3), and as expected, none were contributedby the anion. A very small shift of 5 nm was observed for acompound in two different solvents as depicted in Figure S4 ofthe Supporting Information. As expected, no peak shifts wereobserved for the carbazoleimidazolium cation when conjugated

with different anions in a given solvent, Figure S5 of theSupporting Information.The band gap, which is designated as the difference between

the highest occupied molecular orbital (HOMO) and thelowest unoccupied molecular orbital (LUMO), was calculatedfrom the onset wavelength of the lowest energy absorptionpeak. The onset wavelength is designated as the negativetangent line of the lowest energy absorption peak that intersectswith a linear tangent line of the absorption tail (see Figure 4).

Absorption onset at higher wavelength corresponds to theminimum amount of energy which is required for excitation ofthe electron from HOMO to LUMO. In other words, this is theenergy for electronic transition from ground to excited state. A3.4 eV value of the band gap in DCM was determined from eq1:

λ=E (eV)

1240(nm)g

(1)

4.3. Fluorescence Spectroscopy. Emission spectra wererecorded at an excitation wavelength of 275 nm as depicted inFigure S6 of the Supporting Information. A broad emissionspectrum with a λmax at 440 nm in DCM and 375 nm inmethanol was observed for all carbazole-based GUMBOS(Figure 5). This broadness was attributed to the presence of theimidazole ring within the carbazole unit. Samanta and co-workers have reported an excitation wavelength-dependentfluorescence for imidazolium-based ILs.40 Similar behavior wasobserved in the current study as well, although the precise

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

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

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

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



origin of the emission is still a matter of debate.41 Figure S7 ofthe Supporting Information is a representation of thefluorescence emission spectrum of [CI][BETI], which overlapswith the fluorescence spectrum of carbazole and imidazoleupon excitation at the same wavelength (275 nm). The peakbetween the two suggests that the broadness in the emissionspectrum of [CI][BETI] arises from a combination of twounits.Excitation spectra were measured using respective emission

wavelengths of 385 and 440 nm in methanol and DCM. Thesespectral data are presented in Table 2. The emission and

excitation spectra were not mirror images (Figures 6 and FigureS8 of the Supporting Information). The larger bandwidth of thefluorescence emission spectrum was attributed to incorporationof the imidazolium emission into the emission spectrum, whilesuch changes were not observed in the excitation spectra. Alarge Stokes shift of 105 nm was observed in DCM (Figure 7),which produces reduced fluorescence emission as a result ofsecondary inner-filter effects. A significant increase in Stokesshift was observed after addition of the imidazole ring ontocarbazole and is the result of intramolecular charge transfer(ICT). The Stokes shift was calculated for each intermediatecompound during synthesis (not shown here). After addition ofthe alkyl group at the N position, the Stokes shift was the sameas observed in carbazole alone. However, it drastically increasedafter the addition of the imidazole group at the third position ofthe carbazole. Hence, the Stokes shift is attributed to the C−Nbond of the carbon at the third position of carbazole which isattached to the nitrogen of imidazole. Such a large Stokes shifthas been previously identified by our group in cyanine-based

dyes as a result of ICT due to formation of C−N bonds.23 Thebroadness of the emission spectra could also be attributed tothe formation of an ICT state which emits in the red region ofthe spectrum.

4.4. Quantum Yield Measurements. Absolute quantumyields were measured for all carbazole-based GUMBOS usingan integrating sphere. The reported value of quantum yield forcarbazole is 0.4,42 which is consistent with the value weobtained using the integrating sphere. Quantum yields werealso obtained using a relative method employing carbazole asthe standard. Both approaches showed very high quantumyields for GUMBOS with [OTf], [NTf2], and [BETI]counteranions. In this study, the reasons for enhancedmeasured quantum yields can be attributed to the large Stokesshift. High quantum yields with polymeric derivatives or withbulky organic compounds of carbazole have been previouslyreported in the literature.43,44 The primary advantage ofGUMBOS-based materials is that we are able to achievethese enhanced quantum yields using small molecules withsimple changes in counteranions. We note that we can alsotune these quantum yields, as reflected in for the data presentedin Table 2.

4.5. Fluorescence Lifetimes. The fluorescence lifetimemeasurements were performed in two solvents (MeOH andDCM). The fluorescence lifetime decays of the threecarbazoleimidazole-based GUMBOS were best fit to a bi- ortriple exponential decay, and the contributions to fluorescencein each case were determined to be primarily from two states(Tables 3 and Table S1 of the Supporting Information). The

shorter lifetime component in each GUMBOS was attributed toemission from the excited singlet state (1.9−3.4 ns in DCMand 94−133 ps in MeOH), while the slower component wasascribed to emission from the charge transfer state (6.4 ns inDCM, 4.8 ns in MeOH).45 In methanol, the contribution fromthe third component is minor and could be the result of a backtransition between the ICT and S1 state. In previous studies, ithas been observed that substitution of an electron-withdrawingsubstituent at the third position leads to a drop in fluorescence

Table 2. Absorption, Emission Wavelength, MolarExtinction Coefficients, and Quantum Yields of GUMBOS

GUMBOS solvent λabs/nm ε/104 M−1 cm−1 λfluo/nm % ϕfl

CII MeOH 275 380 25DCM 280 2.34 440 28

[CI][OTf] MeOH 273 1.51 378DCM 280 6.75 440 94

[CI][NTf2] MeOH 273 380DCM 281 9.52 440 73

[CI][BETI] MeOH 273 2.60 378DCM 280 1.68 440 99

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

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

Table 3. Lifetime Measurements of GUMBOS in DCM

GUMBOS τ1/ns α1 τ2/ns α2 τavg/ns χred2

[CI][OTf] 3.379 0.07 6.649 0.93 6.428 1.032[CI][NTf2] 1.903 0.06 6.624 0.94 6.348 1.021[CI][BETI] 3.414 0.07 6.659 0.93 6.445 1.020



lifetime from 7.33 ns (unsubstituted carbazole) to 350 ps.39

Thus, it is likely that the enhanced conjugation due tosubstitution of an imidazolium unit leads to a decrease influorescence lifetime from the S1 state into the picosecondregime. The relatively shorter lifetime of the S1 state in[CI][NTf2] as compared to the other GUMBOS explains thelower 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 organiccompounds, the vacuum deposition method is a well-established technique for acquiring thin films for OLEDs.However, since GUMBOS have low vapor pressures, thisapproach is not suitable for our materials. For our materials, wedetermined that electrospray methods produced good qualityfilms and also offered the best size control of droplets.46 Thesesolid films were then characterized by use of SEM and also byuse of fluorescence microscopy, Figure S9 of the SupportingInformation and Figure 8.

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

electrostatic, π−π stacking, and van der Waals interactions inGUMBOS produced a relatively homogeneous film assuggested in Figure 8 and Figure S9 of the SupportingInformation.

4.7. Photo- and Thermal Stability Tests. Photostabilityand thermal stability are extremely important factors for anydyes developed for use in OLEDs. An emitting material withsignificantly high photo- and thermal stability would enhancethe life and broaden the applications of such materials under avariety of conditions. All GUMBOS investigated in the presentstudy exhibited extremely interesting properties in response tolight exposure. An increase in photostability was actuallyobserved for [CI][BETI], whereas [CI][NTf2] and [CI][OTf]underwent fairly stable fluorescence upon irradiation for morethan 3000 s (see Figure 10). Such an increase in photostabilitywas also observed in one of our previous studies and wasattributed to irradiation-induced changes in aggregation.23

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

sample showed recovery of its original fluorescence intensityafter cooling back to 20 °C from 60 °C (Figure S10 of theSupporting Information). No change in the photoluminescencespectra were observed before and after heating. This studydemonstrates that our GUMBOS compounds are quite stabletoward heat and light.

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

Table 4. Fluorescence Emission Maxima in Solution and inSolid Film

GUMBOS λmax/nm solution (film)

[CI][OTf] 378 (385)[CI][NTf2] 380 (389)[CI][BETI] 378 (392)

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

Figure 10. Photostability of carbazole-based GUMBOS.

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



4.8. Electrochemistry. Electrochemical properties of theseGUMBOS were evaluated by use of cyclic voltammetry. Allsolutions were prepared in DCM, and 0.1 M TBAPF6 was usedas the supporting electrolyte. Cyclic voltammograms wererecorded at a scan rate of 0.1 V/s. The potential of the workingelectrode was scanned to a positive value within the solventwindow limit in order to acquire the oxidation peak of thecarbazole unit in these compounds. The measured cyclicvoltammograms generally displayed oxidation peaks, reflectingthe formation of a dication. CII exhibited multiple electrontransfer processes, which were attributed to oxidation andreduction of iodide. This redox reaction was not seen in otherGUMBOS having hydrophobic anions, i.e., [OTF], [NTf2], and[BETI] (Figure S12 of the Supporting Information). This alsoverifies that the products are pure and little or no iodideremains after the ion exchange reaction. We note that thesepotentials can be measured at the peak positions or at the peakonset. The values of oxidation potentials obtained wererecalculated versus a ferrocene/ferrocenium internal referenceelectrode. The redox potential for Fc/Fc+ was measured usingthe cyclic voltammograms. Cyclic voltammograms wereanalyzed in order to determine anodic peak potentials, whichwere later used to calculate the highest occupied molecularorbital (HOMO) energy level using eq 2.47

= −

+

− +

+

E E(eV) le [ ( V vs Fc /Fc)

4.8(V Fc /Fc vs zero)]

HOMO pa

(2)

These values were determined using a ferrocene reference,where Epa is the anodic potential. The energy of the HOMO isultimately based on the absolute value of the normal hydrogenelectrode (NHE). The values of HOMO energy levels for ourGUMBOS were obtained using eq 2 and tabulated in Table 5.The band gap (Eg EC) is measured as the difference in energylevel between LUMO and HOMO, i.e.

= − −E E E(eV) ( (eV) (eV))g HOMO LUMO (3)

The absorption spectral band gap and electrochemicalHOMO energy levels were used to evaluate the LUMO energylevel (eq 3). The lowest unoccupied molecular orbital(LUMO) energy levels were computed, and data are presentedin Table 5. These values are quite similar for differentGUMBOS, as the oxidation potential is primarily attributedto oxidation of the cationic carbazole unit since the anion doesnot have any redox characteristic. The HOMO energy levels ofour GUMBOS are lower than the ITO HOMO energy level(4.70 eV), and the LUMO energy levels lie above the electrontransport material (TBPI (1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene, LUMO 2.70 eV).18 From an electronic perspective,it is expected that carbazole-based GUMBOS can perform aspotential emitters for use in OLEDs.

4.9. Computational Study. DFT/TDDFT calculationsprovide additional understanding of the structural, electro-chemical, and optical properties of the GUMBOS studied here.Optimized geometries revealed planar carbazole substituents,while the imidazole moiety had a twist. In all systemsinvestigated, the HOMO is located primarily at the carbazolesubstituent, and the LUMO distributed over the imidazoliummoiety (Figure 12). The band gap computed using DFT/

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

5. CONCLUSIONS

Carbazole-based GUMBOS have been synthesized using a verysimple procedure. These GUMBOS exhibited high absorbanceand excellent luminescence properties in combination with highquantum yields and excellent photo- and thermal stability.These compounds possess broad emission characteristics in thevisible region and demonstrate good quantum yields. A blueemissive material with appropriate combination of propertieshas been achieved in a very small molecule without the need forsynthesis of large molecules involving multiple steps and lowyields. A very simple approach has been used to tune thephysicochemical properties of these compounds. The tunabilityin quantum yields and thermal stability of GUMBOS wascontrolled by use of counteranions. Evaluation of the spectraland electrochemical properties, as well as computed band gaps,suggest the potential use of these compounds for optoelec-tronic applications and as emitting materials for use in OLEDs.Future work will involve device fabrication and examination ofthese GUMBOS as an emissive layer in OLEDs. The highchemical stability and photostability reported for thesecompounds are essential for long life required for OLEDs.

■ ASSOCIATED CONTENT

*S Supporting InformationSynthesis and synthesis scheme, XRD, TGA, absorption, andfluorescence emission spectra, SEM, cyclic voltammogram,lifetime data in methanol of GUMBOS and the completereference for ref 27. This material is available free of charge viathe Internet at http://pubs.acs.org.

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

GUMBOS E vs Fc/V HOMO/eV LUMO/eV Ega/eV Eg

b /eV

CII 0.96 −5.76 −2.36 3.42[CI][OTf] 0.94 −5.74 −2.32 3.41 3.77/4.4[CI][NTf2] 0.95 −5.75 −2.36 3.39 3.74/4.3[CI][BETI] 0.93 −5.73 −2.31 3.42 3.58/4.1

aBand gap is calculated by using onset wavelength. bBand gap iscalculated by computational calculation, TDDFT/DFT.

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



http://pubs.acs.org

■ AUTHOR INFORMATIONCorresponding Author*Fax: 1-225-578-3971. Tel.: 1-225-578-2829. E-mail: [email protected].

Author Contributions#These two authors have equal contribution.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSN.S. acknowledges support by the National Science Foundationunder grant no. CHE-1243916. The authors thank Dr. RandallHall for discussion regarding computational study and Dr.Evgueni E. Nesterov for use of electrochemical instrumentation.

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carbazole-derived group of uniform materials based on organic salts: solid state fluorescent...

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