Journal of Molecular Structure 1058 (2014) 130–135

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Structural characterization, absorption and photoluminescence studyof symmetrical azomethines with long aliphatic chains

0022-2860/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.molstruc.2013.10.067

⇑ Corresponding author. Tel.: +48 71 328 30 61.E-mail address: a.iwan@iel.wroc.pl (A. Iwan).

Agnieszka Iwan a,⇑, Ewa Schab-Balcerzak b, Marzena Grucela-Zajac b, Lukasz Skorka c

a Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 55/61 Street, 50-369 Wroclaw, Polandb Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice, Polandc Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland

h i g h l i g h t s

�We analyze the influence of diimine structure on their photoluminescence properties.� Imine emitted blue, violet or green light in solution.� The theoretical electrochemical energy band gap of diimines was observed at in the range of 3.65–4.08 eV.� Some of the diimines seems to be promising potential candidates for OLEDs.

a r t i c l e i n f o

Article history:Received 30 July 2013Received in revised form 4 October 2013Accepted 29 October 2013Available online 9 November 2013

Keywords:PhotoluminescenceTheoretical calculationsDiiminesAzomethinesLiquid crystals

a b s t r a c t

In this study, we investigated structural and optical properties of three symmetrical azomethinesbis(4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzylidene)benzene-1,4-diamine(SAz1), bis(4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy)benzylidene)biphenyl-4,40-diamine (SAz2) and 4,40-methylenebis(N-(4-(octadecyloxy)benzylidene)benzenamine (SAz3). Electronicproperties, such as orbital energies and resulting energy gap of the three symmetrical azomethines werecalculated theoretically by density functional theory (DFT). The photoluminescence (PL) and absorptionUV–vis properties of the azomethines were investigated in chloroform solution. The effect of excitationwavelength and concentration on the PL properties was detected as well. Azomethines emitted violet,blue or green light. The highest PL intensity was found for SAz1.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The photoluminescence (PL) of organic compounds isessentially based on localized electronic systems within organicmolecules. In order to produce an electronically excited state, amolecule must absorb energy equal to or greater than the energydifference between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO)[1,2]. Incorporation of hetero atoms such as N, O, S within thep-conjugated systems usually causes the red (bathochromic) shiftof absorption and emission peaks. Moreover, attachment of elec-tron donating (D) groups (e.g. –NH2, –OCH3) or electron accepting(A) ones (e.g. –CN, –NO2) causes red shifts of the UV–vis and PLspectra.

Tuning the chemical structure, band gap of conjugatedcompounds (oligomer and polymers) and designing LEDs emitting

selected colors remains a challenging issue. Mainly, threestrategies are reported for tuning the emission color of organiccompounds: (i) changing the main chain molecular structure, (ii)blending a light emitting compound with another nonemissivepolymer e.g. poly(methylmethacrylate) (PMMA) and (iii) dopinge.g. [3–20]. Additionally, the effects of excitation wavelength, con-centration, film thickness and kind of solvent on the photolumines-cence (PL) properties of the organic compounds are investigated.

Among various group of organic compounds investigated asluminescence materials azomethines (imines, Schiff bases) are alsoanalyzed e.g. [6,15,21–37]. Some papers are dedicated to investiga-tion optical, mainly PL properties of soluble, liquid crystalline (LC)azomethines e.g. [38–46]. For example, the Marin group [42–45]investigated photoluminescence of liquid crystalline azomethineswith various chromophore units (pyrrole, indole, tiophene,fluorene and biphenyl). Investigated compounds emitted blue orgreen light [43].

Our previously work showed that liquid crystalline, unsymmet-rical azomethines obtained from 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,

A. Iwan et al. / Journal of Molecular Structure 1058 (2014) 130–135 131

10,11,11,11-heptadecafluoroundecyloxy) benzaldehyde and dif-ferent aromatic amines exhibited photoluminescence propertiesin the range of 359–451 nm [37,38]. Symmetrical thermotropicimine prepared via condensation of biphenyl-4-carboxaldehydewith poly(1,4-butanediol)bis(4-aminobenzoate) exhibited lightemission at 360 nm [39], while symmetrical azomethines obtainedfrom terephthaldicarboxaldehyde and aliphatic amines exhibitedgreen light [40,41].

In this paper, the optical (absorption and emission in UV–visrange) properties and electronic properties, that is, orbital energiesand resulting energy gap calculated theoretically of the three sym-metrical azomethines are presented. Additionally, the relationshipbetween azomethine structures and their photoluminescence (PL)along with the influence of excitation wavelength and concentra-tion on the PL are studied.

2. Experimental

2.1. Materials and methods

All compounds were prepared using the previously describedand characterized method [46].

UV–vis absorption spectra were recorded in chloroform solu-tion using a Lambda Bio 40 Perkin Elmer spectrophotometer. ThePL spectra of the azomethines in chloroform solution were ob-tained on a VARIAN Cary Eclipse Fluorescence Spectrophotometer.

DFT calculations were carried out using Gaussian09 RevisionD.01 [47] package and employing hybrid B3LYP [16,17,48–50] ex-change correlation potential combined with 6-31G(d,p) basis set.Ground-state geometries were fully optimized until a stable localminimum was found, which was confirmed by normal-mode anal-ysis (no imaginary frequencies were present). Initial structures

Fig. 1. Chemical structure of the inves

were constrained to the highest possible symmetry point groupwhich were: Ci for SAz1 and C2 for SAz2 and SAz3. In order to im-prove numerical accuracy of the calculations (possible energyoscillations during SCF due to many sp3 carbons) two-electronintegrals and their derivatives were calculated in a modified wayemploying the pruned (99,590) integration grid consisting of 99 ra-dial shells and 590 angular points per shell. The oscillator strengthsand energies of the vertical singlet excitations were calculatedemploying time-dependant version (TD) of DFT [51–57] and againat the same level of theory (vide supra). In order to account for thesolvent effect Polarizable Continuum Model (PCM) was applied[58]. Due to huge computational costs ground state optimizedgeometries were used in these calculations. The TD-DFT resultswere retrieved from output files using GaussSum 2.2 [59]. Molec-ular orbital plots were generated from .cube files with Gabedit2.4.6 [60].

3. Results and discussion

The chemical structures of the three symmetrical azomethinesinvestigated in this research are presented in Fig. 1.

Azomethines SAz1 and SAz2 were prepared from 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecyloxy) benz-aldehyde and two diamines such as 1,4-phenylenediamine (SAz1)and 4,40-diaminobiphenyl (benzidine) (SAz2). Azomethine SAz3was obtained from 4-octadecyloxybenzaldehyde and 4,40-diamino-diphenylmethane. All compounds investigated in this article weredescribed previously [46], however, their absorption UV–vis andphotoluminescence properties along with theoretical calculations,which are the subject of this work, were not presented. SAz1 andSAz2 exhibited liquid crystalline (LC) behavior with one LC meso-phase contrary to SAz3 which does not display LC properties as

tigated azomethines SAz1–SAz3.

Fig. 2. Conformation twisting in the investigated azomethines after fluorinesubstitution.

Fig. 3. HOMO/LUMO contours calculated at B3LYP/6-31G(d,p) level of theory(isovalue = 0.03) for SAz1–SAz3.

250 300 350 400 450 500 5500.0

0.2

0.4

0.6

0.8

Abs

orba

nce

[a.u

.]

Wavelength [nm]

SAz1 SAz2 SAz3

Fig. 4. UV–vis spectra of azomethines SAz1–SAz3 in CHCl3 solution.

Table 1UV–vis absorption data (kabs. and molar extinction coefficients, e) of the azomethinesSAz1–SAz3 in chloroform solution.

Code kabs (nm) e (L mol�1 cm�1)

SAz1 282, 344 29,220, 29,350SAz2 289, 346 25,490, 22,700SAz3 284, 323 82,700, 68,660

132 A. Iwan et al. / Journal of Molecular Structure 1058 (2014) 130–135

determine by differential scanning calorimetry (DSC), X-ray dif-fraction (SAXRD, WAXRD) and polarized optical microscopy(POM) measurements [46]. Azomethine SAz1 exhibited smectic A(SmA) mesophase, while for compound SAz2 SmC mesophasewas confirmed. Along with increase the number of phenyl ringsin azomethines SAz1 and SAz2 increase of clearing point tempera-ture along with increase the temperature of crystallization was ob-served [46]. Temperatures of crystallization and isotropisation ofSAz1 and SAz2 were found at around 70, 193 �C and 206, 313 �C,respectively [46]. Moreover, these azomethines were investigatedpreviously by us as a thermodetectors [61]. However, thesecompounds did not exhibit thermoluminescence properties underirradiation of Co-60 gamma-rays after the dose of 1–2 Gy [61].

3.1. DFT calculations

Geometries of all molecules were optimized in vacuum in astep-wise way using first simple azomethine core units and anabove specified symmetry point groups in order to determine a fairapproximation for further calculations. Then propoxy chains wereadded and then the geometry was again optimized. At the finalstage the fluorinated chains were attached and the procedurewas repeated. This was aimed at finding a reliable geometry andto account for possible conformation changes. It was found thatfor simple hydrocarbon chains (SAz3) a stable conformation isreached when all sp3 centers align in an antiperiplanar way whatmay have been expected. However when fluorinated chains areconsidered, the geometry optimization led to a structure twistedabout axis parallel to the chain (SAz2 and SAz3). These phenomenaare presented in Fig. 2.

The key parameter of organic materials is their HOMO/LUMOenergy levels and the corresponding band-gap (Eg). It is always cru-cial to fairly adjust their energies in order to suit the requirementsof certain applications. As it may have been expected the HOMO/LUMO distribution is mainly concerned with the central, aromaticpart of each molecule (see Fig. 3).

Table 3Comparison of the calculated singlet HOMO–LUMO transitions with estimatedtheoretical band gap.

Code Eg (eV) Excitation energy (eV) Excitation energy (nm) kabs (nm)

SAz1 3.63 3.14 394.71 344SAz2 3.65 3.17 391.21 346SAz3 4.09 3.56 348.13 323

Table 4Photoluminescence data of the azomethines SAz1–SAz3 in chloroform solution(c = 1 � 10�4 mol/L) under two excitation wavelengths.

Code kexc.1 (nm) kexc.2 (nm) kemis.1 (nm) kemis.2 (nm)

SAz1 282 344 406, 430 408, 426SAz2 289 346 450 447SAz3 284 323 517 525

A. Iwan et al. / Journal of Molecular Structure 1058 (2014) 130–135 133

The calculated HOMO levels are very close to each other andoscillate around �5.3 eV. On the contrary, there are some differ-ences in the LUMO levels for ach azomethine. SAz1 and SAz2 pos-ses almost the same level: �1.66 eV and �1.68 respectively,however the LUMO of SAz3 is somehow destabilized possiblydue to the incorporation of bridging methylene group and equals�1.27 eV. The calculated HOMO level for these imines is quite closeto that of materials currently used in hole-transporting layer [62].The energy band gaps ranged from 3.63 eV for SAz1 and 3.65 eV forSAz2 to 4.09 eV for SAz3.

Considering relationship between chemical structure and elec-tronic properties it was found that symmetrical imines SAz1 andSAz2 independently on the number of phenyl rings coming fromdiamine applied (see Fig. 3) exhibited lower energy gap comparingto SAz3.

Moreover, the Cartesian coordinates of all optimized geometriesfrom Gaussian output were added as supporting information.

Bold data indicate the main peak

3.2. UV–vis investigations

The UV–vis absorption spectra of three azomethines SAz1–SAz3were investigated in chloroform solution. The azomethines SAz1–SAz3 exhibited two well defined absorption bands in the range of323–346 nm and 282–294 nm. The absorption spectra of the allazomethines are shown in Fig. 4 and the corresponding absorptionwavelength maxima and molar extinction coefficients (e) valuesare given in Table 1.

As can be seen from Table 1 the absorption wavelength maximaand molar extinction coefficient values of the investigated azo-methines changes from 346 to 282 nm and from 22,700 to29,220 L/mol cm, respectively with change the chemical structureof the azomethine.

The origin of the electronic transitions was also analyzed bymeans of time dependant DFT. In this case it is a beautiful exampleof the importance of symmetry-related selection rules for theinvestigated compounds. Due to the presence of the center of inv-ertion in SAz1 the commonly known Laporte rule will only allowtransition between states of different symmetry with respect tothe center. The list of the most pronounced transitions with respectto their oscillator strengths is presented in Table 2.

What is important to note is that in each case there is very littlemixing of different transitions, so that no specific decompositionlike natural transition orbitals is required to associate the holeand the electron. In SAz1 the first band is associated entirely withthe strongly favored HOMO–LUMO transition (since HOMO be-longs to Ag and LUMO to Au irreducible representation). Similarly,in the rest of azomethines the HOMO–LUMO transition is highlypronounced which is reflected in very high value of the oscillatorstrength. Since HOMO is mainly associated with the phenylenerings along the molecules and LUMO mainly has contribution from

Table 2Results of TD-DFT calculations of the electronic transitions within SAz1–SAz3 along witcontribution.

Code Wavelength Oscillator strength

SAz1 394.71 1.6552300.38 0.3042266.53 0.4801

SAz2 391.21 1.9761299.97 0.1553291.05 0.7817

SAz3 348.13 1.4045335.83 0.3234314.27 0.1574289.40 0.5804287.90 0.2484

the C–N bond the HOMO–LUMO transition may be approximatelyconsidered as p?n� electron transfer. The transitions that give risethe second, higher energy band involve orbitals located of theperipheral phenylene rings being residuals of the aldehyde usedfor the syntesis. Their nature on the other hand may be consideredas p ? p� transitions. The excitation energies retrieved from singlepoint TD-DFT calculations were compared with theoretical bandgaps of the molecules and with experimental data collected duringUV–vis measurements. These data is presented in Table 3.

As it can be clearly stated the simple HOMO–LUMO gap is a verypoor approximation of the ground-to-first excited state transition.The values of HOMO–LUMO gap are ca. 0.5 eV bigger than the cor-responding excitation energies. On the other hand the excitationenergies in nanometers are also slightly overestimated in compar-ison to experimental data by ca. 25–35 nm, but within acceptablelimits. This proves that the presented level of theory gives an accu-rate and reliable picture of the nature of the electronic transitions.

Increase the aromatic azomethine unit length (increase of thenumber of phenyl rings) as in SAz2 in comparison with SAz1 didnot have any influence on absorption shift in UV–vis spectra. Onthe other hand, the introduction of a methylene unit betweenthe phenyl rings as in SAz3 in comparison with SAz2 results in a23 nm hypsochromic shift of the kmax.C=N band from 346 nm to323 nm due the lack of conjugation present in the central part ofthe SAz3 (see Table 1).

3.3. Photoluminescence investigations

3.3.1. Effect of excitation wavelength on PLThe photoluminescence (PL) properties of the azomethines

were studied for 1 � 10�4 mol/L chloroform solution under twoexcitation wavelengths and data obtained are given in Table 4.

h their energy, oscillator strength, symmetry of the excited state and major orbital

Symmetry Major contribution

Singlet-AU HOMO ? LUMO (97%)Singlet-AU H-2 ? LUMO (82%)Singlet-AU H-1 ? L + 1 (93%)

Singlet-B HOMO ? LUMO (96%)Singlet-B H-3 ? L + 1 (�15%), H-2 ? LUMO (60%)Singlet-B H-1 ? L + 1 (85%)

Singlet-B H-1 ? L + 1 (�14%) HOMO ? LUMO (79%)Singlet-A H-1 ? LUMO (�12%) HOMO ? L + 1 (76%)Singlet-B H-1 ? L + 1 (79%) HOMO ? LUMO (17%)Singlet-B H-3 ? L + 1 (�29%) H-2 ? LUMO (45%)Singlet-B H-3 ? LUMO (�34%) H-2 ? L + 1 (38%)

300 400 500 600 700 8000

100

200

300

PL in

tens

ity [a

.u.]

Wavelength [nm]

ex 282 nm ex 344 nm

SAz1(a)

400 500 600 700 8000

100

200

300

400

500

600

700

800

PL in

tens

ity [a

.u.]

Wavelength [nm]

c=1x10-4 mol/L c=1x10-5 mol/L

SAz1λex= 344 nm

(b)

Fig. 5. Photoluminescence spectra of the azomethine SAz1 (a) under differentexcitation wavelength (c1 = 10�4 mol/L) and (b) with different concentration.

Table 5Photoluminescence data of the azomethines SAz1–SAz3 in chloroform solution withdifferent concentration.

Code kexc.2 (nm) kemis.2 (nm)

c1 = 10�4 mol/L c2 = 10�5 mol/L

SAz1 344 408, 426 383, 403, 423SAz2 346 447 425SAz3 323 525 498

Bold data indicate the main peak.

Fig. 6. Photographs of the symmetrical azomethines SAz1–SAz3 in chloroformsolution under excitation 360 nm.

134 A. Iwan et al. / Journal of Molecular Structure 1058 (2014) 130–135

Photoluminescence of the azomethines were investigated underexcitation in maximum of absorption bands.

First, along with an increase of excitation wavelength, anincrease of the relative intensity of the photoluminescence of sym-metrical azomethine SAz1 was observed. For other azomethines nochanges in relative PL intensity were found along with an increasethe excitation wavelength.

The spectra of the azomethines SAz2 and SAz3 each show oneemission band in the 450–517 nm range under excitation�285 nm, and in the 447–525 nm range under excitation 323and 346 nm, respectively. Azomethine SAz1 exhibited under282 and 344 nm excitation wavelength two bands in the 406–430 nm range (see Table 4). The PL emissions bands of the azo-methines apparently were red shifted along with increase thenumber of phenyl rings (SAz1 and SAz2) or introduction ofmethylene group (SAz3). Moreover, together with increase ofexcitation wavelength slight (3–4 nm) hypsochromic shift wasobserved for SAz1 and SAz2, and bathochromic shift (8 nm)was found in the case of SAz3.

Symmetrical azomethines exhibited different color of theemitted light depend on the chemical structure of the com-pound. SAz3 exhibited emission of violet light while SAz3 emit-ted green light. Only compound SAz2 was found as a blueemitter with the maximum of emission band at 447 nm underexcitation at 346 nm (see Table 4). The highest PL intensity(analyzed in a.u.) was found for SAz1. The emission spectra ofthe azomethine SAz1, as an example (c1 = 10�4 mol/L) areshown in Fig. 5.

3.3.2. The effect of concentration on PLThe concentration effect on PL in chloroform solution of the

azomethines shows significant spectral changes in the relativeemission intensities and maximum of emission band with decreaseof the compound concentration. The PL peak wavelength for allazomethines with two different concentrations (c1 = 10�4 mol/L,c2 = 10�5 mol/L) are shown in Table 5. The emission spectra ofthe azomethine SAz1 with different concentration, as an exampleare shown in Fig. 5.

It is known that the PL concentration influence strongly thephotoluminescence properties and the mechanism of quenchingis still not widely investigated [63,64]. In our case the intermolec-ular distances between azomethine molecules are probably betterat the concentration 10�5 mol/L and consequently PL spectra aremore intensive than for the c = 10�4 mol/L. For all azomethines, to-gether with decrease of concentration (from 10�4 to 10�5 mol/L),blue shift of emission band maximum was observed. Small (3–5 nm) hypsochromic shift of maximum emission band was de-tected for SAz1 along with decrease of concentration. CompoundsSAz2 and SAz3 exhibited 22 and 27 nm, respectively hypsochromicshift in maximum of emission band along with decrease the con-centration. Fig. 6 depicts the photographs of imines emission inchloroform solution.

Azomethines SAz1 and SAz3 were investigated in our previ-ously work [46] as materials for OLEDs. The current–voltage curvesof two devices with following architecture ITO/azomethine/Alq3/Aland ITO/TiO2/azomethine/Alq3/Al, before and after irradiation withlight (halogen lamp, about 1000 W/m2), were studied. Ourmeasurements showed significant photo-generation of charges(electrons) in all investigated devices under illumination [46].

4. Conclusion

In this work the optical properties in chloroform solution ofthree symmetrical azomethines with aliphatic chains were studied.The influence of both chemical structure and experimentalcondition such as concentration and excitation wavelength on pho-toluminescence properties of imines was tested. It was found that

A. Iwan et al. / Journal of Molecular Structure 1058 (2014) 130–135 135

the increase of phenyl rings number as well as introduction ofmethylene group between phenyl rings in imine core resulted inbathoochromic shift of the position of emission band maximum.Thus, the azomethine with one (SAz1) or two (SAz2) phenyl ringsbetween imine linkages emitted violet or blue light, respectively.Whereas, imine with methylene unit (SAz3) exhibited green lightemission. The highest PL intensity exhibited SAz1. The lack of sig-nificant influence of excitation wavelength on color of emittedlight was detected. On the other hand, lowering of the imines con-centration in solution affected the position of emission band max-imum and intensity of emitted light. What is worth noting, thestudied compounds revealed a very close correlation with the the-oretical excitations. Moreover, a strong influence of the symmetrywas indicated and a very pure hole–electron pairs were calculated.It should be stressed that two azomethines with one or two phenylrings between imine units showed liquid crystalline properties inwide range of temperatures (above 100 �C). It can be concludedtaking into account our previously [46] and presently reported re-sults that investigated imines may be considered as potentialmaterials for stable organic optoelectronic devices such as OLEDs.

Acknowledgements

The Gaussian09 calculations were carried out in the WroclawCentre for Networking and Supercomputing, WCSS, Wroclaw,Poland. http://www.wcss.wroc.pl, under calculational Grant No.283. Lukasz Skorka would like to acknowledge a financial supportfrom the TEAM project No. TEAM/2011-8/6, which is operatedwithin the Fundation for the Polish Science Team Programme con-financed by the EU European Regional Developmnet Fund.

Appendix A. Supplementary material

\Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.10.067.

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