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Chemical Physics Letters 422 (2006) 547–551

Functional properties of fluorescent poly(amidoamine) dendrimersin nematic liquid crystalline media

Ivo Grabchev a,*, Seher Sali a, Jean-Marc Chovelon b

a Institute of Polymers, Bulgarian Academy of Sciences, 103A, Acad. G. Bonchev, str, Sofia 1113, Bulgariab Universite Claude Bernard – Lyon 1, Laboratoire d’Application de la Chimie a l’Environnement (LACE), CNRS - UMR 5634, boulevard du 11

novembre 1918, 69622 Villeurbanne Cedex, France

Received 14 December 2005; in final form 15 February 2006Available online 28 February 2006

Abstract

The effectiveness of poly(amidoamine) dendrimers from zero generation as a fluorescent guest for liquid crystal displays of the ‘guest–host’ type is discussed on the basis of their absorption and fluorescent properties. It has been shown that the dendrimers at concentrationof 0.3 wt% do not destabilize the orientation of the liquid crystal matrix. The orientation order parameters SA and SF depends on thenature of the substituent at C-4 position of the 1,8-naphthalimide. The effect that poly(amidoamine) dendrimers have upon the phasetransition temperature and the electro-optical properties of the LC/dendrimer mixtures has been also presented. All investigationsreported have been carried out in surface stabilized display cells.� 2006 Elsevier B.V. All rights reserved.

1. Introduction

During the last few years, coloured liquid crystal dis-plays (LCDs), containing fluorescent guest molecules, dis-solved in low molar mass nematic liquid crystal host,have been investigated intensively with regard to theirapplication in electro-optical devices. This type of LCDswas first described by Heilmeier and Zanoni [1]. Theabsorption and fluorescence intensity of these systemscould be controlled by changing the orientation of theguest and host molecules by means of the electric field[2]. Such displays can work in passive and active mode.

At present, one of the most important problems for thepractical utilization of fluorophores in the LC displays isthe choice of suitable fluorophores with intensive fluores-cence, high photostability and good solubility in LCs.

Linear fluorescent polymers have been recently reportedto have a novel use as luminophores in liquid crystal systemsof the ‘guest–host’ type [3–6]. Hyperbranched macromole-

0009-2614/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2006.02.080

* Corresponding author.E-mail address: grabchev@polymer.bas.bg (I. Grabchev).

cules have been also discussed to be suitable componentsfor LC systems. In these terms fluorescent dendrimers havebeen of particular interest. They are monodispers, tridimen-sional polymer structures comprising functional groups athigh concentration that can be functionalized with differentfluorophores [7–10]. Among them the poly(amidoamine)(PAMAM)s are a novel class commercial dendrimers witha well defined molecular composition and functional aminoend-groups [11]. Our first studies on the design, synthesisand photophysical properties of some new PAMAM deriv-atives, comprising 1,8-naphthalimide units in their periph-ery were published previously [12–15].

To our knowledge the fluorescent PAMAM dendrimershave not been investigated in ‘guest–host’ liquid crystallinesystems. Here, we report on the potential utilization of fivenovel PAMAM dendrimers from zero generation, havingfunctionalized with 1,8-naphthalimide fluorophores in theperiphery for colouring liquid crystal ZLI 1840 displaysof the ‘guest–host’ type. The results are discussed on thebasis of the spectral properties and the effect that thePAMAM dendrimers have upon the phase transition tem-perature and the orientation order parameter ÆP2æ of the

548 I. Grabchev et al. / Chemical Physics Letters 422 (2006) 547–551

liquid crystal, evaluated by polarized UV–Vis absorptionand fluorescent spectroscopy in surface stabilized displaycells. Additionally, the effect of the PAMAM dendrimerson the electro-optical properties of the liquid crystal hasbeen examined.

2. Experimental part

The investigated PAMAM dendrimers D1–D5 in the LChost are of the following structure presented in Scheme 1.Their synthesis has been recently described [12,14,15].

The LC mixture ZLI 1840, supplied by MERCK(Darmstadt, Germany), was used as liquid crystalline host.It exhibits a stable nematic phase over a broad temperaturerange (�15 to 90 �C) and high positive dielectric anisotropy[16]. The PAMAM dendrimers D1–D5 were initiallyscreened for solubility in the LC. For further studies thedendrimers were dissolved at a concentration of 0.3 wt%,which was suitable for spectroscopic evaluation of theorder parameter and simultaneously guaranteed appropri-ate constant ratio [17]. LC/dendrimer mixtures were stud-ied in ‘sandwich’ cells of 20 lm thickness. The mixturesformed thin oriented layers between two glass plates withan area of 2 · 3 cm. The uniform planar orientation ofthe LC/dendrimer systems was achieved by coating the cellsurfaces with polyamide layers that were additionallyrubbed. The orientation order parameters of the dendri-mers dissolved in LC were evaluated from the polarizedcomponents of the absorption and fluorescence spectra.Neutral UV polarizer was used both for the absorptionand fluorescence measurements.

The polarized UV–Vis absorption spectra were mea-sured with UVIKON 930 spectrophotometer (KONTRONinstruments). The fluorescence intensity measurementswere performed on a FP-6500 Jasco spectrofluorometer.The fluorescence spectra were measured in p geometry(the exciting light beam was perpendicular to the cell sur-face and the fluorescence light emerging perpendicular tothe surface from the same side of the cell was detected).The studies were carried out with both the pure LC (baseline) and the LC/dendrimer mixtures.

N

NH

N

NH

N

O

O

A

AO

O

O

O

A= NHC2H5 (D1), NHC6H13 (D2), NHCH piperidino (D5)

Scheme 1. Molecular structure of PAMAM dendrimers used as guest fluorophNHCH2CH@CH2 (D3), NHCH2CH2N(CH3)2 (D4), piperidino (D5).

3. Results and discussion

3.1. Photophysical properties of dendrimers D1–D5 in

tetrahydrofuran solution

In non-polar tetrahydrofuran (THF) solution (e = 7.58)the fluorescent PAMAM dendrimers D1–D5 functional-ized with 1,8-naphthalimides exhibits yellow-green colourwith absorption maxima at kA = 410–433 nm and fluores-cence maxima at kF = 511–516 nm.

For all dendrimers the extinction coefficients in the vis-ible region are higher than 10000 l mol�1 cm�1, indicatingthat the long wavelength band of the absorption spectrais a band of charge transfer /CT/, due to p, p* electrontransfer during the S0! S1 transition.

The Stokes shift is a parameter, indicating the differencein the properties and structure of the fluorescent com-pounds in the ground state S0 and the first excited stateS1 and is calculated according to

mA � mF ¼ ð1=kA � 1=kFÞ � 107 cm�1. ð1ÞThe values of the Stokes shift for dendrimers D1–D5 are

in the 3367–4820 cm�1 region. The higher Stokes shift hasbeen observed for D5 probably due to the possible confor-mational changes in the piperidino cycle.

For the fluorescent ‘guest–host’ liquid crystal systemsthe quantum yield of fluorescence is an importantparameter for their application in LC display technolo-gies. The fluorescence efficiency of dendrimers D1–D5is estimated according to the quantum yield UF, calcu-lated on the basis of their absorption and fluorescencespectra

UF ¼ Ust

Su

Sst

Ast

Au

n2Du

n2Dst

; ð2Þ

where UF is the emission quantum yield of the sample, Ust

the emission quantum yield of the standard, Ast and Au

represent the absorbance of the standard and sample atthe excited wavelength, respectively, while Sst and Su arethe integrated emission band areas of the standard andsample respectively, and nDst and nDu is the solvent refrac-

N

HN

HN

N

N

O

O

A

A

O

O

O

O

2CH=CH2 (D3), NHCH2CH2N(CH3)2 (D4),

ores in ZLI 1840 liquid crystal host. A = NHC2H5 (D1), NHC6H13 (D2),

Table 2Absorption and fluorescence maxima and order parameters of dendrimersD1–D5 in ZLI 1840

Dendrimer kA (nm) kF (nm) mA � mF (cm�1) SA SF

1 429 511 3740 0.52 0.502 432 516 3768 0.55 0.533 428 512 3833 0.51 0.494 426 509 3827 0.50 0.485 408 503 4629 0.41 0.38

I. Grabchev et al. / Chemical Physics Letters 422 (2006) 547–551 549

tive index of the standard and sample, u and st refer the un-known and standard, respectively.

It is seen that the dendrimers D1–D5 have a quantumyield in the range UF = 0.17–0.46. Dendrimer D5 has thelowest quantum fluorescence yield (UF = 0.17) that couldbe ascribed to the conformational changes in the dendrimerstructure during the excitation and exclusively in the piperi-dino cycle, which has well pronounced flexibility whereasthe higher quantum yield was observed for D4. In this case,the 1,8-naphthalimide core units are subjected to photoin-duced electron transfer (PET) from the nitrogen atoms ofthe central amino groups of the dendrimer on one hand,and to other hand from the terminal amino groups ofN,N-dimethylamino groups where the latter PET effect isa major factor [12].

3.2. Photophysical properties of dendrimers D1–D5 in ZLI

1840 LC mixture

Table 2 presents the spectral characteristics of the dendri-mers D1–D5 in liquid crystal ZLI 1840. The absorption (kA)and fluorescence (kF) maxima, and Stokes shift (mA � mF), aregiven. In LC all PAMAM dendrimers retain their yellow-green colour with absorption maxima in the range kA =408–432 nm and fluorescent maxima ranging kF = 503–516 nm. For all dendrimer/LC mixtures a negligible hypso-chromic shift with respect to the maxima in tetrahydrofuransolution has been observed. This effect might be due to theviscosity and different polarity of the media. Another plausi-ble reason might be the fact that intermolecular hydrogenbonds can be formed between the dendrimer and the LCmatrix. In this case the hydrogen bonds are formed betweenH-atoms of the NH substituent at C-4 position and amidgroups (NHCO) from the PAMAM whit the non-sharedelectron pairs from the CN and C@O groups in ZLI 1840.The formed hydrogen bonds lead to a change in the polariza-tion of the chromophore system decreasing the electrondonor–acceptor interaction which leads to the negligible hyp-sochromic shift observed of kA and kF (see Tables 1 and 2).

The Stokes shifts range from 3740 to 4629 cm�1 thehighest value being observed for D5, which is identical tothat running the experiments in tetrahydrofuran solution.That means that in both media the conformation changesin the dendrimer chromophoric systems of D1–D4 are lesspronounced. The exact determination of the quantum fluo-rescence yield for a dendrimer embedded into an orientedmatrix is, however, very difficult to determine because

Table 1Photophysical characteristics of D1–D5 in tetrahydrofuran (see text)

Dendrimers kA (nm) e (l mol�1 cm�1) kF (nm) mA � mF

(cm�1)UF

D1 433 35000 515 3677 0.24D2 432 35100 515 3730 0.26D3 430 35200 516 3876 0.21D4 428 34800 516 3985 0.46D5 410 35600 511 4820 0.17

many factors, including anisotropy, must be taken intoaccount. The results obtained in tetrahydrofuran solutionshow that the dendrimers have good quantum efficiencythat could be retained in oriented LC matrix.

3.3. Ordering in dendrimer/liquid crystal systems

It has been found that substances added to a nematicLC affect its orientational order. The method based onpolarized absorption or fluorescence spectroscopy is partic-ularly convenient for measuring the orientation orderparameters in LC systems involving dichroic fluorophores.The orientation of the guest molecules in the host LC sys-tems depends on their molecular structure [18], concentra-tion [19] and the molecular interactions between themolecules of the dye ‘guest’ and LC ‘host’ [20].

The orientational order parameter SA of the dendrimersin LC can be estimated by measuring the polarized absor-bencies at kA [21,22]:

SA ¼Ak � A?

Ak þ 2A?1� 3

2sin2 b

� ��1

ð3Þ

Here, Ak and A^ are the corresponding absorbencies at kA

max, at parallel and perpendicular orientation of the pola-rizer according to the macroscopic orientation of the LC,and b is the angle between the long molecular axis of the1,8-naphthalimide chromophores from the PAMAMdendrimer and the direction of its absorption transitionmoment.

At b = 0 Eq. (3) reduces to

SA ¼ ðAk � A?Þ=ðAk þ 2A?Þ. ð4Þ

Assuming that the lifetime of the excited state of thefluorescent dyes is greater than the rotational correlationtime, Bauer et al. [23] showed that the order parameterSF of the dendrimers can also be calculated from the fluo-rescence measurements

SF ¼ ðF k � F ?Þ=ðF k þ 2F ?Þ; ð5Þ

where Fk and F^ are the corresponding fluorescence inten-sities in polarized light at k max with the polarization direc-tion parallel and perpendicular to the macroscopicorientation of LC sample. This equation is valid only ford = 0�, where d is the angle between the absorption andthe emission oscillators.

The angles b and d for the new PAMAM dendrimersunder study are still unknown. However, in first approxi-

550 I. Grabchev et al. / Chemical Physics Letters 422 (2006) 547–551

mation we can use Eqs. (4) and (5) to estimate the orienta-tion of the dendrimers in the LC matrix, to fix b = d = 0�.

Dichroism measurements provide information on theorientation of the dendrimer molecules in LC matrix. Alldendrimer samples D1–D5 under study are dichroic, indi-cating that the dendrimer molecules are roughly alignedalong the director, perpendicular to the surfaces (seeFig. 1).

Table 2 summarizes the values of orientation orderparameters obtained from the absorption and fluorescencemeasurements at T = 298 K. As seen from the data pre-sented in Table 2 the substituents affect both order param-eters SA = 0.41–0.55 and SF = 0.38–0.53. The results listedevidence the effect of the molecular structure of the fluores-cent dendrimers on their orientation in a liquid crystallinematrix. In the case of D2 having the longest alkyl chain atC-4 position of the 1,8-naphthalimide the increase in theorientation order parameter is related to the elongationof the ‘guest’ molecules.

In the case of SF, the values are lower than thoseobtained by polarized absorption spectroscopy (SA > SF).These differences can be on account of: (i) the existenceof non-zero intramolecular angles between the absorptionand emission oscillators, (ii) the intramolecular energytransfer processes which can depolarize the fluorescencein an unpredictable manner and lead to misinterpretationof the results, and (iii) the interactions between the LC mol-ecules and the cell surface [24,25].

3.4. Effect of ‘guest’ dendrimers upon the phase transitionbehaviour of ZLI 1840 liquid crystal

The addition of a non-mesogenic solute to nematic LCchanges its normal nematic–isotropic phase transition tem-perature, in most cases causing its decrease. Another spe-cific feature of the phase transition is the appearance oftwo-phase regions predicted by theory and observed exper-imentally [26].

400 450 500 550 6000.0

0.2

0.4

0.6

0.8

= __

_

= _

FA

FA

Fl

serouc

cnei en

sne tity

/ U.

A.

sbA

ocnabre

Wavelength / nm

0

200

400

600

800

Fig. 1. Polarized absorption (A) and fluorescence (F) spectra of D2 innematic LC ZLI 1840.

The results from the temperature investigations on pureLC and on binary dendrimer/LC mixtures are given inTable 3. The following temperature characteristics of thenematic–isotropic phase transition are determined: TN, atwhich the first drop of the isotropic liquid appears, andTI, at which the last drop of the nematic disappears. Theirdifference, (TI � TN), is the range of the two-phase region,and DTN and DTI are the shifts of TN and TI with respectto the relevant temperatures of pure LC. TNI =1/2(TN + TI) is the average temperature of the nematic–isotropic transition of the dye/LC mixtures and DTN isits shift with respect to that of pure LC.

The temperature investigations of the binary systemsdendrimer/LC show that dendrimers decrease feebly bothphase transition temperatures TN (0.6–1.3 K) and TI

(0.4–0.6 K) of pure LC (Table 3). This behaviour is inaccordance with theoretical predictions (based on Helf-and’s lattice model) describing the effect in which flexiblePAMAM guests have upon the nematic–isotropic phasetransition of low molar LCs [27].

3.5. Electro-optical properties of LC/dendrimer

From technical point of view the reorientation of LCunder applied voltage is very important. The effect of reori-entation can be investigated by measuring absorptionchange of initially oriented LC/dendrimer mixture in LCdisplay cells.

The parallel absorption component, Ak, decreases withapplying higher voltage while the perpendicular parameter,A^, does not vary. This observation is typical for ‘guest–host’ LC displays which contain a LC with positive dielectricanisotropy. At a high voltage, the parallel and perpendicularcomponent should be equal. We report the voltage givingx% of absorbance for A = Ak � A^ normalised to 100% inthe form:

V X ða; T Þ;where a is the angle measured to the sell normal (degrees)and T is the temperature.

We regard the displays to be completely OFF or ON if[2]:

OFF : V < V 90ð0�; T ÞON : V > V 10ð0�; T Þ.

Table 3Effect of dendrimers D1–D5 upon the temperatures TN and TI of thenematic–isotropic phase transition of liquid crystal ZLI 1840 (in K)

Systems TN TI TI � TN DTN DTI TNI DTNI

LC 362.5 368.5 6.0 – – 365.5 –LC/dendrimer 1 361.9 368.1 6.2 �0.6 �0.4 365.0 �0.5LC/dendrimer 2 361.4 368.0 6.6 �1.1 �0.5 364.7 �0.8LC/dendrimer 3 361.2 367.9 6.7 �1.3 �0.6 364.6 �0.9LC/dendrimer 4 361.6 367.9 6.7 �1.2 �0.6 364.8 �0.7LC/dendrimer 5 361.2 368.1 6.9 �1.3 �0.4 364.7 �0.8

0 2 4 6 8 100.0

0.1

0.2

0.3

0.4

0.5

0.6

SA

U / V

Dendrimer 2 Dendrimer 1

Fig. 2. Order parameters SA for LC/dendrimers as a function of theapplied voltage U.

Table 4Characterization data of LC/dendrimer systems

Systems V90 (0�, 25 �C)DV = ±0.1 V

V10 (0�, 25 �C)DV = ±0.1 V

Ak(OFF)/Ak(ON)

LC/dendrimer 1 1.59 4.44 8.90LC/dendrimer 2 1.32 6.99 8.16

I. Grabchev et al. / Chemical Physics Letters 422 (2006) 547–551 551

Fig. 2 presents the dependence of the values of orderparameter SA for the dendrimers D1 and D4 in LC on theapplied voltage. The threshold effect is observed at about0.7–1.1 V. It is seen that the above mentioned dendrimershave different influence on the applied reorientation volt-age. The values of V90 are similar found to be between1.32–1.59 V, until the values of V10 for LC/D1 is larger.In both cases the contrast ratio is good and can accept thechange of the absorbance by on–off control of the appliedvoltage measured at the absorption maxima.

The value obtained for V10 (0�, 25 �C) and V90 (0�,25 �C) and the contrast ratio Ak(OFF)/Ak(ON) for LC/dendrimer systems are presented in Table 4.

4. Conclusion

In this Letter, the photophysical results coming fromsystems composed of commercial nematic liquid crystalZLI 1840 and fluorescent PAMAM dendrimers from zerogeneration functionalized with 1,8-naphthalimide in theirperiphery have been shown for the first time. Their thermo-dynamical and electro-optical properties have also beeninvestigated with regard to their usage in liquid crystal dis-

plays. The orientation of the dendrimers in LC is related tothe substituent in C-4 position of the 1,8-naphthalimidesbonded to the PAMAM structure. The results obtainedhave shown that the fluorescence PAMAM dendrimerscan be suitable components of colour liquid crystal displayoperating both in passive and active modes.

Acknowledgements

The work was supported by the bilateral cooperationbetween Bulgarian Academy of Science and CNRS-Franceand partially by a grant from the National Science Foun-dation of Bulgaria (CH-1512/2005).

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