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Symmetric Liquid Crystal Dimers Containing aLuminescent Mesogen: Synthesis, MesomorphicBehavior, and Optical PropertiesLuminita Marin a , Andrei Zabulica a & Mitica Sava aa “Petru Poni” Institute of Macromolecular Chemistry, Iasi, RomaniaAccepted author version posted online: 12 Oct 2011.Version of record first published: 07 Dec2012.

To cite this article: Luminita Marin , Andrei Zabulica & Mitica Sava (2013): Symmetric Liquid Crystal Dimers Containing aLuminescent Mesogen: Synthesis, Mesomorphic Behavior, and Optical Properties, Soft Materials, 11:1, 32-39

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Soft Materials (2013) 11, 32–39Copyright © Taylor & Francis Group, LLCISSN: 1539-445X print / 1539-4468 onlineDOI: 10.1080/1539445X.2011.567434

Symmetric Liquid Crystal Dimers Containing a LuminescentMesogen: Synthesis, Mesomorphic Behavior, and OpticalProperties

LUMINITA MARIN*, ANDREI ZABULICA, and MITICA SAVA

“Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania

Received 24 January 2011, Accepted 27 February 2011

New thermotropic liquid crystalline dimers containing chromophoric units into calamitic mesogens have been synthesized and studied.Cyan, pyrrole, indole and diphenyl moieties were chosen as chromophoric units. The thermotropic behavior of the dimers was monitored bypolarized light microscopy, differential scanning calorimetry, and variable temperature X-ray diffraction measurements. The photophysicalproperties were studied by UV-vis and photoluminescence spectroscopy. All the dimers have mesomorphic behavior and emit violet, indigo,or blue light, in solution and film.

Keywords: Azomethine, Dimer, Liquid crystal, Cyan, Pyrrole, Indole, Diphenyl

Introduction

The symmetric dimeric liquid-crystalline compounds formed bytwo identical mesogenic units linked via a single flexible spacerare interesting compounds investigated as model compounds forliquid-crystalline polymers or for their unusual mesomorphism,different from that of the corresponding monomers (1, 2). Forthese reasons, the dimers attract considerable attention, focusingon the synthesis of new dimeric compounds, their mesomorphicproperties, the phase transition kinetics or the studying of thestructure-property relationships (2–7). Among the liquid crys-tal dimers, those containing an azomethine linkage, particularly,attracted considerable attention due to their rich mesomorphism(8–10). On the other hand, the azomethine connection is easilyattainable in mild reaction conditions, with high yields and purityand the compounds containing it have good properties: high ther-mal stability, thermotropic liquid crystalline potential, ability toform organometallic complexes, and semiconducting properties(11–15). Apart from this, polyazomethine derivatives are furtheradvantageous because they can be both p- and n-doped, due totheir reversible oxidation and reduction, respectively (16).

Due to the development of luminescent materials as a hottopic of current interest in energy production or storage, from apractical point of view, obtaining functional thermotropic liquidcrystalline compounds (TLC) by incorporating new properties(such as luminescence) into the same molecule, in addition toits inherent self-organization is an interesting development. This

*Address correspondence to: Luminita Marin, “Petru Poni” Instituteof Macromolecular Chemistry, 41A Grigore Ghica Voda Alley,700487 Iasi, Romania. Email: lmarin@icmpp.ro

kind of compounds can be used as electron transporting layersin the development of organic light-emitting diodes (OLEDs),organic field effect transistors (OFET), or other devices (17–21). Due to their twisted conformation, conventional azomethinecontaining mesogens do not display luminescence properties.However, protonation or complexation of the CH=N linkageenhances its planarity and influences optoelectronic properties:for example, photo- and electro- luminescence. Another strategyto improve π–electron delocalization is to introduce an electron-withdrawing unit, usually a heterocyclic ring, in the moleculararchitecture of the mesogenic core (11).

For all these reasons, we considered it interesting to synthe-size new dimeric compounds containing an azomethine connec-tion and various chromophoric units as cyan, pyrrole, indole,and biphenyl. Herein, we report the synthesis, mesomorphic,and optical properties of four new dimers containing variouschromophores, designed as model compounds for luminescentpolymers.

Experimental Section

Reagents

p-Cyano-benzaldehyde 97%, biphenyl-4-carboxaldehyde 98%,dimethyl formamide (DMF) 99%, and ethyl acetate 99%were purchased from Fluka and used as received. 1-Acetyl-3-indolaldehyde and 2-pyrrolecarbaldehyde from Riedel de Haenwere recrystallized from ethanol.

1,2-bis[2-(4-aminophenoxy)ethoxy]ethane (DD3) was pre-pared and structurally characterized in our laboratory accordingto published procedures (22).

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Luminescent Liquid Crystalline Dimers 33

General Procedure for the Preparation of Azomethine Dimers

The synthesis of the new dimeric azomethines was performedby a condensation reaction of the appropriate aldehyde with 1,2-bis[2-(4-aminophenoxy)ethoxy]ethane (DD3) diamine taken in a2/1 molar ratio. To avoid the mono-reacted compound forma-tion, the aldehyde was used in 10% excess. The reagents weredissolved in DMF or ethanol to form a 15–20% solution and thereaction mixture was heated and stirred for 9–20 hours, in aninert atmosphere. The solvent and reaction time have been cho-sen so as to obtain a high yield. After cooling, the crystallineproduct was separated by filtration and purified by recrystal-lization. Finally, the compounds dried overnight in vacuum at60◦C. Pure, dimeric compounds were obtained as yellowish crys-talline or powder solids in yields of 70–80%. Their structure wasconfirmed by FTIR and 1H-NMR spectroscopy.

The synthesis reaction and the dimer codes are given inScheme 1. The synthesis data for the new dimeric azomethinesare collected in Table 1.

CAzD

FTIR ν: cm−1(KBr): 3081–3041 (=C–H stretch of the aromaticrings), 2915–2878 (C–H stretch of the aliphatic chains), 2223(-C≡N stretch), 1622 (-C=N- stretch), 1587, 1506, 1452(C-C ring stretch), 1248 (C–O–C stretch), 841 (absorption bandassigned to 1,4 – phenylene ring).

1H – NMR (d-DMSO, δ, ppm): 8.74 (s, 2H, CH=N), 8.07,8.05 (d, 4H, ortho to -CN), 7.96, 7.94 (d, 4H, ortho to CH=N),7.35, 7.33 (d, 4H, ortho to N=CH), 7.02, 7.00 (d, 4H, ortho to-O-), 4.12 (t, 4H, Ar–O–CH2–CH2–), 3.77 (t, 4H, Ar–O–CH2–CH2–), 3.64 (t, 4H, O–CH2–CH2-O-)

OO

OO

NH2

H2NCHO +2R

OO

OO

NNC

HR

CH

R

Code CAzD IAzD PAzD DAzD

RNC

N

OH3C

N

H

Scheme 1. Synthesis of dimers.

Table 1. Reaction Conditions for Dimer Synthesis

CodeReactionsolvent

Reactiontime (h)

Temperature(◦C)

Recrystallizationsolvent

CAzD DMF 14 110 ethylacetateIAzD Ethanol 20 80 acetonePAzD Ethanol 12 80 ethanolDAzD DMF 9 110 ethanol

IAzD

FTIR ν: cm−1(KBr): 3138 (=C–H stretch of the aromatic rings),2925–2867 (C–H stretch of the aliphatic chains), 1771 (-C=Ostretch), 1624 (-C=N- stretch), 1585, 1501, 1449 (C-C ringstretch), 1244 (C–O–C stretch), 832 (absorption band assignedto 1,4 – phenylene ring).

1H – NMR (d-DMSO, δ, ppm): 8.48 (s, 2H, CH=N), 8.81 (s,2H, ortho to -CH=N-), 8.55, 8.53 (d, 2H, H7 in indole), 8.43,8.41(d, 2H, H4 in indole), 7.50, 7.48, 7.46, 7.44, 7.42 (m, 4H,H5 and H6 in indole), 7.34, 7.32 (d, 4H, ortho to -CN), 7.34,7.32 (d, 4H, ortho to -N=CH), 7.08, 7.05 (d, 4H, ortho to -O-),4.19, 4.18, 4.17 (t, 4H, Ar–O–CH2–CH2–), 3.85, 3.84, 3.83 (t,4H, Ar–O–CH2–CH2–), 3.71 (s, 4H, O–CH2–CH2-O-), 2.75 (t,3H, -CH3).

PAzD

FTIR ν: cm−1(KBr): 3217 (N–H stretch of the pyrrole ring),2928–2875 (C–H stretch of the aliphatic chains), 1621 (-C=N-stretch), 1594, 1502, 1452 (C-C ring stretch), 1241 (C–O–Cstretch), 831 (absorption band assigned to 1,4 – phenylene ring).

1H – NMR (d-DMSO, δ, ppm): 11.65(s, 2H, H-N in pyrrole),8.29 (s, 2H, CH=N), 7.16, 7.13 (d, 4H, ortho to -N=CH), 6.98(s, 2H, ortho to N in pyrrole), 6.95, 6.93 (d, 4H, ortho to -O-),6.64 (d, 2H, ortho to -CH=N-), 6.19, 6.18, 6.17 (t, 2H, meta to -CH=N-), 4.09, 4.08, 4.07 (t, 4H, Ar–O–CH2–CH2–), 3.76, 3.75,3.74 (t, 4H, Ar–O–CH2–CH2–), 3.62 (s, 4H, O–CH2–CH2-O-).

DAzD

FTIR ν: cm−1(KBr): 3055, 3033 (=C–H stretch of the aromaticrings), 2940–2886 (C–H stretch of the aliphatic chains), 1621(-C=N- stretch), 1605, 1577, 1500 (C-C ring stretch), 1252 (C–O–C stretch), 839 (absorption band assigned to 1,4 – phenylenering).

1H – NMR (d-DMSO, δ, ppm): 8.63 (s, 2H, CH=N), 7.97,7.95 (d, 4H, ortho to -CH=N-), 7.77, 7.75 (d, 4H, meta to -CH=N-), 7.71, 7.69 (d, 4H, ortho to phenyl), 7.49, 7.47, 7.45(t,4H, meta to phenyl), 7.40, 7.38, 7.36 (t, 2H, para to phenyl), 7.27,7.25 (d, 4H, ortho to -N=CH), 6.99, 6.97 (d, 4H, ortho to -O-),4.14, 4.13, 4.12 (t, 4H, Ar–O–CH2–CH2–), 3.79, 3.78, 3.77 (t,4H, Ar–O–CH2–CH2–), 3.65 (s, 4H, O–CH2–CH2–O-).

Characterization

Infrared (IR) spectra were recorded on a FT-IR Bruker Vertex70 Spectrophotometer in the transmission mode, by using KBrpellets.

Proton nuclear magnetic resonance (1H-NMR) spectra wererecorded with a BRUKER Avance DRX 400 MHz spectrometerusing DMSO-d6 as solvent and tetramethylsilane (TMS) as inter-nal reference. Chemical shifts are reported in parts per million(ppm).

UV–vis absorption and photoluminescence spectra wererecorded on a Carl Zeiss Jena SPECORD M42 spectrophoto-meter and a Perkin Elmer LS 55 spectrophotometer, respectively,in solution and film, using 10 mm quartz cells and glass plates,respectively.

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34 Marin et al.

Differential scanning calorimetry (DSC) measurements wereperformed with a Pyris Diamond DSC, Perkin Elmer USA sys-tem, under nitrogen atmosphere (nitrogen flow 120 ml/min,sample mass 3–4 mg). The transition temperatures were read atthe top of the endothermic and exothermic peaks.

The mesophases of the thermotropic dimers were studied byobserving the textures with an Olympus BH-2 polarized lightmicroscope under cross polarizers with a THMS 600 hot stageand LINKAM TP92 temperature control system.

Wide Angle X-Ray Diffraction (WAXD) was performed on aBruker D8 Advance diffractometer, using the Ni-filtered Cu-Kα

radiation (λ = 0.1541 nm). A MRI-WRTC temperature chamber(with nitrogen inert atmosphere) and a MRI-TCPU1 TemperatureControl and Power Unit were used. The working conditions were36 kV and 30 mA. All diffractograms were investigated in therange of 2–40◦ (2 theta degrees), at different temperatures. Theinitial samples for X-ray measurements were crystalline or pow-der solids as obtained by recrystallization. All diffractograms arereported as observed.

Results and Discussions

Four new symmetric azomethine dimers were synthesizedvia a condensation reaction of a flexible diamine DD3 withfour different aldehydes containing chromophoric units(Scheme 1).

The Fourier transform infrared (FTIR) spectroscopic analysesand 1H nuclear magnetic resonance (NMR) spectroscopy con-firmed the successful synthesis of the dimers. All FTIR spectraclearly show an intensive characteristic absorption band around1621–1624 cm −1 due to the newly formed azomethine bond,while the bands characteristic for the functional groups of thereagents (the aldehyde group around 1670 cm −1 and the aminegroup around 3350 cm −1, respectively), disappear. The FTIRspectrum of the DAzD dimer is given as an example in Fig. 1.

In the 1H-NMR spectra, chemical shifting of all the protonswas detected, and the ratio values of their integrals perfectlyagree with the calculated ones for the proposed structures.The 1H-NMR spectrum of the PAzD is given as an examplein Fig. 2.

4000 3000 2000 1000

Wavenumber (cm–1)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Tra

nsm

itta

nce

3055.1

43033.9

2 2940.3

82886.3

7

1621.1

11605.6

81500.5

7

1252.7

2

839.0

1

Fig. 1. FTIR spectrum of DAzD dimer.

Fig. 2. 1H-NMR spectrum of PAzD dimer.

Thermotropic Behavior

The thermotropic behavior of the prepared compounds has beenstudied by polarizing optical microscopy (POM), differentialscanning calorimetry (DSC), and variable temperature wideangle X-ray diffraction measurements (WAXD). The DSC curveswere registered during two successive heating-cooling scans, at aheating/cooling rate of 10◦C/min. The POM and X-ray measure-ments were carried out under the same temperature conditions,in order to make sure that the samples experience the samethermal history. The thermodynamic parameters (the transitiontemperatures, enthalpies, and entropies of transition) of all thesynthesized dimers are enclosed in Table 2.

For the DAzD dimer, all the DSC curves exhibit three peaks(Fig. 3) which were attributed based on the POM observationsand X-ray measurements.

In the first heating scan, the POM observation did notreveal significant changes corresponding to the first endother-mic peak, while at the temperature corresponding to the sec-ond endothermic peak, a clear, fluid nematic texture withthreads appeared, which further transformed into an isotropicliquid (at a temperature corresponding to the highest tem-perature endothermic peak). Based on these observations, thefirst endothermic peak was ascribed to a solid–solid transi-tion, the second endothermic peak to a crystalline–nematic

Table 2. Transition Temperatures (◦C), Enthalpies (KJ/mol) andthe Dimensionless Value of �S/R of Studied Dimers

Code T/◦C(�H/kJ mol−1, �S/R)

DAzD 1H: Cr1 201(7.4, 1.9) Cr2 214(52.4, 12.9) N 221(1.3,0.3) I1C: I 220(1.5, 0.3) N 189(2.6, 0.7) Sm 184(33,8.7) Cr

CAzD 1H: Cr 174(61.8, 16.6) I1C: I 156(2.2, 0.6) N139(51.6, 15) Cr

IAzD 1H: Cr 150 M 176(109.8, 29.4) IPAzD 1H: Cr 110 M 140(46.6, 13.5) I

Cr: crystalline phase; N: nematic mesophase; Sm: smectic mesophase; I:isotropic phase; M: plastic mesophase.

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Luminescent Liquid Crystalline Dimers 35

140 160 180 200 220 240

2H

1H

1C

2C

Temperature (°C)

Hea

t F

low

Endo U

p (

mW

)

Fig. 3. DSC curves of DAzD dimer (1H: first heating scan; 1C: firstcooling scan; 2H: second heating scan; 2C: second cooling scan).

phase transition and the last one to a nematic–isotropictransition.

In the first cooling scan, nematic droplets appeared from theisotropic liquid, forming by coalescence an instable Schlierentexture (Fig. 4a), which very rapidly transformed into anematic texture with threads (Fig. 4b), and, at further cool-ing, changed into a fan-shaped texture (Fig. 4c), which crys-tallized (Fig. 4d). Taking into account these observations, theDSC exothermic peaks were properly ascribed to isotropic–nematic, nematic–smectic, and smectic–crystalline transitions,respectively (Table 2). The second heating/cooling cycle issimilar to the first one; the only difference being the weakerfirst endothermic peak, in second heating scan, due to thedifference between the crystalline state obtained by recrys-tallization from solvent and crystallization from melted state,respectively. The enthalpy and entropy changes registered byDSC are in agreement with the values usually observed fortypical crystalline–nematic, nematic–isotropic, nematic–smectic,and smectic–crystalline transitions, confirming the presence of asmectic mesophase and a nematic one (3, 23). The solid–liquidcrystal or liquid crystal–solid transitions involved much moreenergy than the liquid crystal–isotropic liquid or liquid crystal–liquid crystal transitions. One aspect of the DSC thermogram ofthe DAzD is that the crystallization point is much lower than itsmelting point, an effect which is called hysteresis (24). In thiscase, hysteresis is an inherent physical property, as solids aremore oriented than liquids, which indicates a more difficult ori-enting of the molecules while cooling, especially into complexcrystalline structures.

a) DAzD, 1C, 222°C, 200x, nematic dropletswhich form by coalescence Schlieren texture

b) DAzD, 1C, 220°C, 200x, nematic texture withthreads

c) DAzD, 1C, 188°C, 200x, fan-shaped texture d) DAzD, 1C, 186°C, 200x, fan-shaped texturetransform in crystalline

Fig. 4. POM textures of DAzD, during first cooling scan (1C) (colorfigure available online).

To further clarify the morphologic changes taking placeduring the phase transitions, variable temperature wide anglediffraction measurements were carried out at temperaturessituated between two endothermic/exothermic peaks.

As it can be seen in Fig. 5, the diffraction pattern of the DAzDpowder, as was obtained by recrystallization from DMF, exhib-ited small peaks in the smaller angle domain (2–15, 2θ degree)and some intensive peaks in the wide angle region, indicating alayered crystalline structure (Fig. 5a). In the first heating scan,the diffraction pattern registered at a temperature between thefirst and the second endothermic peaks shows some changes;all the reflections slightly shift to the smaller angle correspond-ing to the higher values of the interlayer and inter-mesogenic

0 5 10 15

Diffraction angle (2 theta)

a)

b)

c)

Inte

nsity

(a.

u.)

20 25 30 35

Fig. 5. WAXD patterns of DAzD at a) room temperature, b) 205◦C,c) 218◦C (color figure available online).

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36 Marin et al.

distance (Fig. 5b). On the other hand, the intensity of the reflec-tions changes slightly: increases for the small angle reflectionsand decreases for the wide angle reflections, indicating reorga-nizations into a supramolecular architecture. The most probably,these changes are the result of the rotational movements of theethyleneoxy groups when the latter gain enough energy dueto the temperature increase. To accommodate the movements,the mesogenic cores undergo rotational tilts by intermolecularforces. The movements are collective, being propagated in closepacked architecture (25). Taking into consideration the smallenthalpy and the entropy change of this transition, the insignif-icant changes observed in the POM and the changes of the X-raydiffractogram we can ascribe the first transition in the first heat-ing scan of the DAzD, to a crystal–soft crystal transition (26) dueto a conformational polymorphism.

At the temperature corresponding to the second DSC tem-perature range, the diffraction pattern shows, for the threadedtexture, drastic changes; all the reflections disappear, indicatingthe absence of any correlation among molecules, which suggestsa nematic mesophase (Fig. 5c). In the first cooling scan, at thetemperature corresponding to the nematic texture with threads,the X-ray diffraction pattern did not exhibit any reflection, alsoindicating a nematic mesophase. Due to the short thermal stabil-ity range of the smectic mesophase and to equipment limitations(long data acquisition time, 2θ angle higher than 2◦), the X-raypattern of the smectic mesophase could not be registered, andthus the morphology of the fan-shaped texture could not be stud-ied. Based on the POM observation of the fan-shaped texture, wecan assume a smectic A mesophase to occur. Thus, by correlat-ing DSC, POM, and WAXD measurements, we can assert thatthe DAzD dimer is an enantiotropic nematogen and, in addition,it shows a monotropic smectic mesophase.

The IAzD and PAzD dimers show a similar thermotropicbehavior consisting of the occurrence, in first heating scan, underpolarized light, of a fine viscous texture (Fig. 6a, c); the tex-ture viscosity decreases when the temperature increases (Fig. 6a,b), and, at further heating, the sample suffers a transition to theisotropic phase. In the cooling scan, the isotropic sample froze,without occurrence of any mesophase or crystalline phase. Thisbehavior was also observed for dimers or polymers containingheterocyclic rings into their mesogens (2, 27–30). By quench-ing the sample from the mesophase state, an anisotropic glasswas obtained (Fig. 6d). The anisotropic glass has very good glassadherence, and very good screeching resistance.

Corresponding to the thermal stability range of thismesophase, both dimer samples show overlapped peaks on theDSC curve, forming a broad endotherm (Fig. 7) accompa-nied by important enthalpy and entropy changes (Table 2). Theendotherm starting point corresponds, in the POM, to the occur-rence of the fine texture under shear (Fig. 6a), while the highesttemperature endotherm maximum corresponds to the isotropiza-tion. Because, no other clear change of texture was observed forthe first endotherm maxima, we considered the thermal stabilityrange of this mesophase being comprised between the startingtemperature of the broad endotherm and its highest tempera-ture maximum (Table 2). No exothermic peaks were observed incooling scan, confirming the freezing of the sample in isotropicstate. To further clarify the nature of the mesophase, variabletemperature X-ray diffraction measurements were carried out,

a) PAzD: 1H, 105°C, 200x b) PAzD: 1H, 135°C, 200x

c) IAzD: 1H, 176°C, 200x d) IAzD: 1°C, RT, 200x

Fig. 6. POM microphotographs of the IAzD and PAzD (color figureavailable online).

50 100 150

IAzD, 1H

IAzD, 1C

PAzD, 1H

PAzD, 1C

200

Temperature (°C)

Hea

t F

low

Endo U

p (

mW

)

Fig. 7. The DSC curves of IAzD and PAzD (1H: first heating; 1C:first cooling).

at different temperatures into the thermal stability mesophaserange. As it can be seen in Fig. 8, the diffractogram pat-tern registered progressively at increasing temperatures, suffer

0 5 10 15

Diffraction angle (2θ)

Inte

nsity

(a.

u.)

150 °C

138 °C

135 °C

125 °C

RT

20 25 30 35

Fig. 8. Diffraction patterns of PAzD, in the first heating, at differenttemperatures.

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Luminescent Liquid Crystalline Dimers 37

small changes, consisting of reflection shifting to smaller angleand weak peak intensity increase or decrease, indicating smallreorganizations of the supramolecular architecture. These data,correlated with POM observations (which reveal a birefringenttexture with increasing fluidity at increasing temperature), andDSC data (which indicate that a relatively high transition energyis involved), indicate the formation of an ordered mesophase. Themesophase maintains the crystalline structure order, as formedduring the recrystallization from solution, but, upon tempera-ture increasing, the ethylenoxy chains gain enough energy fora rotational motion and, upon further temperature increasing, fora translational motion and rotation of the molecules, generatingthe good fluidity of the sample. This mesophase, named plasticmesophase (2, 27–30) retains the order of the crystalline phaseby virtue of strong attractive interactions among mesogens, butthere is a degree of disorder due to the conformational disor-der of the flexible chains and the translational movements of themolecules.

The thermotropic behavior of the CAzD was published else-where (2). It is a monotropic nematogen. Its thermodynamicparameters are enclosed in Table 2, for comparison reasons.

Photophysical Properties

Photophysical properties of the synthesized dimers have beeninvestigated using ultraviolet-visible and photoluminescencespectroscopy of the dimer diluted solutions and films, in order tohave an insight into the electronic structure generated by differentchromophores.

The electronic spectra of the dimers were detected in highlydiluted DMF solution (10−5 mol/l), in order to avoid the inter-molecular interactions; thus, isolated molecule absorption couldbe considered.

Taking into account the structure of the mesogenic units ofthe dimers which contain lone pairs of electrons (N, O) andmultiple bonds (-CH=N-, -C≡N, -C=C-, C=O), which allowan extent conjugation, generating π - molecular orbitals, froma theoretical point of view, the dimer absorption is based onelectron transitions from the highest energy bonding π -orbital(HOMO) to the lowest energy antibonding π∗-orbital (LUMO)(31). Increased conjugation brings the HOMO and LUMOorbitals closer together; therefore, there is less energy requiredto carry out the electron promotion, and the wavelength whichprovides this energy is increased correspondingly.

The UV-vis absorption spectrum of the dimers in solution ischaracterized by two absorption bands: an absorption band atlonger wavelength in the 310–430 nm range corresponding to the

300 350 400 450–0,1

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

Absorb

ance

Wavelength (nm)

CAzD DAzD IAzD PAzD

Fig. 9. UV-vis spectra of studied dimers.

π–π∗ transition of the conjugated mesogens, while the shorterwavelength band (λmax: 280–300 nm) originates in the electronictransition of the individual aromatic units (31–35). The UV-visspectra of the all dimer solutions in DMF are shown in Fig. 9. Allparameters obtained from these spectra are enclosed in Table 3.

Upon comparing the absorption maximum of the DMF dimersolutions, we can arrange the dimers in the order of theirincreasing conjugation: CAzD>DAzD>PAzD>IAzD. The bestconjugation is found in the mesogen containing a –CN group,which can be explained by the better co-planarity of the meso-gen containing a short end-group, in comparison with twistedaromatic/heterocyclic units. In the electronic spectrum of thedimer films obtained by casting from the DMF 0.5% solution,a considerable red shift of the UV-vis absorption maximum(Table 3) was observed, along with a decrease in the over-all absorption intensity. This observation could indicate thatthe planar molecule conformations in the DMF solution weremaintained in the film casted from the same solvent; moreover,the molecules have an increased planarity in solid state relatedto the DMF solution, induced by the stronger intermolecularinteractions in the solid film versus the interactions with thesolvent (31). The optical band gap of the solution and film sam-ples, calculated for the longer wavelength absorption maximumand absorption edge was found to be in range 3.07–3.73 and2.25–3.27, respectively, values which are close by typical bandgap values of organic semiconductors (32).

The photoluminescence spectrum of the dimer solutions showthat the maximum of the photoluminescence emission peak hasbeen located at 413–450 nm, corresponding to the photon energy

Table 3. Photophysical Properties of Studied Dimers

Codeλmax (absorption)

DMF /filmλmax(emission)

DMF /filmEg

1

DMF/filmEg

2

DMF/filmStockes shiftDMF/film

DAzD 296; 342/381 427/419 3.6/3.25 3.07/2.25 85/39CAzD 280; 356/385 418/446 3.48/3.22 2.94/2.75 62/61IAzD 304; 332/379 413/414 3.73/3.27 3.15/2.75 81/35PAzD 293; 338/400 450/449 3.66/3.1 3.15/2.7 112/49

1Eg calculated from higher wavelength maximum of absorption, in eV; 2Eg calculated from the absorption edge, in eV.

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38 Marin et al.

of 2.75– 3.0 eV (Table 3). Thus, the CN- and indole containingdimers emitted violet light; the diphenyl containing dimer emit-ted indigo light, while the pyrrole containing dimer emitted bluelight. The intensity of the photoluminescence peak changes whenthe wavelength of the exciting light changes, but its maximummaintains the same values (Fig. 10).

The photoluminescence spectra of the dimer films showalmost the same color of the emitted light as in solution, exceptfor the CN-end group mesogen containing dimer, which emitsviolet light in solution but blue light in film (Fig. 11). Theemission intensity in solution and film is almost the same forthe dimers, except for, once again, the dimer containing a–CNgroup, whose photoluminescence intensity in film is higher thanin solution.

Stokes shift calculated for dimer samples in solution andfilm shows close values for CAzD and smaller values, in filmsversus solutions, for the other ones (Table 3). This behaviorillustrates that less energy loss by nonradiative transitionsoccurs during the electron transition, from ground to excitedstate, in films. This interesting phenomenon can be explained

350 400 450 500 550–100

0

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300

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700

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nsity (

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.)

Wavelength (nm)

CAzD, 280 DAzD, 296 DAzD, 342 IAzD, 332 PAzD, 292 PAzD, 338

Fig. 10. Photoluminescence of the DMF solutions of dimers, excitedat different wavelength values.

400 450 500 5500

100

200

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500

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CAzD, 280 DAzD, 342 IAzD, 306 PAzD, 338

Fig. 11. Photoluminescence spectra of the dimer films.

by compact aggregation of the mesogens, with restriction ofthe intramolecular rotation preponderant over the π–π stakinginteraction. Thus, the nonradiative channels are blocked and thephotoluminescence intensity of dimers enhanced.

Usually, many luminophores are highly luminescent in dilutesolutions but their high emissions are often quenched in thesolid state due to the aggregation of their chromophoric unitsin the condensed phase (36). As it can be observed in Fig. 10and 11, the emission intensity of the studied dimers is maintainedor boosted by aggregation; the CAzD dimer can be consid-ered AIEE [“aggregation induced emission enhancement” (37)]active. Such compounds could be regarded as competitive can-didates for practical use as highly emissive materials, and theyseem especially ideal for violet or blue organic light-emittingdiodes. Moreover, the CH=N linkage into dimer molecules favornew development of chemo- or biosensors for detecting metalcations, biomolecules, organic vapors (38–40), or the preparationof dynamers for adaptive materials with a well-defined responseto external stimuli or environmental factors (41, 42).

Conclusions

New thermotropic liquid crystalline dimers containing CN-,pyrrole, indole, or diphenyl chromophoric moieties have beensuccessfully synthesized. The CN- containing dimer is amonotropic nematogen, the diphenyl containing dimer is anenantiotropic nematogen and, in addition, a monotropic smec-tic mesophase present, while the pyrrole and indole containingdimers show a plastic mesophase. By quenching PAzD and IAzDsamples from the mesophase state, anisotropic glass with goodmechanical properties was obtained. All the studied dimers areluminescent materials, in solution and films, emitting violet,indigo, or blue light. A drastic decrease of the Stokes shift in filmversus solution prove a blocking of the nonradiative channels dueto the compact stacking of the mesogens and indicate that thesemesogens as real candidates for optoelectronic applications.

Acknowledgments

The financial support of European Social Fund, “Cristofor I.Simionescu” Postdoctoral Fellowship Program (ID POSDRU/

89/1.5/S/55216), Sectoral Operational Program HumanResources Development 2007–2013 is acknowledged.

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