1136 Volume 59, Number 9, 2005 APPLIED SPECTROSCOPY0003-7028 / 05 / 5909-1136$2.00 / 0q 2005 Society for Applied Spectroscopy

Temperature-Dependent Raman Spectroscopic Study of theNematic Liquid Crystal 4-n-Pentyl-49-Cyanobiphenyl

DAE SEUNG KANG, KI-SUN KWON, SUNG IL KIM, MYOUNG-SEON GONG,SUNG SEOK A. SEO, TAE WON NOH, and SANG-WOO JOO*School of Electrical Engineering, Soongsil University, Seoul 156-743, Korea (D.S.K., K.-S.K.); Department of Chemistry, DankookUniversity, Cheonan 330-714, Korea (S.I.K., M.-S.G.); School of Physics, Seoul National University, Seoul 151-742, Korea(S.S.A.S., T.W.N.); and CAMDRC (Computer Aided Molecular Design Research Center), Department of Chemistry, SoongsilUniversity, Seoul 156-743, Korea (S.-W.J.)

The spectral behaviors of 4-n-pentyl-49-cyanobiphenyl (5CB) havebeen studied by means of temperature-dependent Raman spectros-copy in the range between 270 and 70 8C. The n(C[N) bands inthe Raman spectra were found splitting below the transition tem-perature from the solid to the nematic liquid crystalline phase at;24 8C, suggesting the existence of solid crystalline polymorphism.The interfacial structures of 5CB on metal plate surfaces have beenreexamined by surface-enhanced Raman scattering (SERS) at dif-ferent temperatures. On Ag and Au, the asymmetric shapes of then(C[N) bands suggest that there should exist different bindingschemes for 5CB on metal surfaces. These asymmetric bands in then(C[N) stretching region were found to vary in changing temper-ature.

Index Headings: Temperature-dependent Raman; 4-n-Pentyl-49-cy-anobiphenyl (5CB); Au; Ag; Adsorption; Surface-enhanced Ramanscattering; SERS.

INTRODUCTION

In the past few decades, there have been enormousadvances in applications of liquid crystals.1 Nematic liq-uid crystals are technologically important as a mesogenicunit for molecular engineering and mixture formation. 4-n-Pentyl-49-cyanobiphenyl (5CB), one of the simplest ne-matic liquid crystal materials, has been widely used as amixture or modified analog in the liquid crystal researcharea. The literature values of the solid–liquid crystal andliquid crystal–liquid transition temperatures are known tobe 24 and 35 8C, respectively, for 5CB.

Interfacial interactions between the molecules and sur-faces are assumed to play an important role in the align-ment and orientational order of liquid crystalline mate-rials.1a There have been several reports on orientationaldynamics of liquid crystals using time-domain spectro-scopic experiments2 and polarized Raman scattering.3,4

Structures and phase transitions of liquid crystals havebeen recently investigated using various techniques.5–11 Ithas been reported that a super-cooled structure12,13 or solidcrystalline polymorphism14,15 could exist for liquid crys-tals as temperature varies.

Adsorption of nitriles on metal substrates has attractedmuch interest due to its potential applications in manyareas including liquid crystal thin films.1 In organome-tallic chemistry, nitriles are generally known to have as-type coordination to metal atoms via the nitrogen lone-pair electrons.16 The compiled spectroscopic data17 have

Received 8 February 2005; accepted 5 July 2005.* Author to whom correspondence should be sent. E-mail: sjoo@ssu.

ac.kr.

led to a conclusion that a majority of aromatic nitrileswere adsorbed on silver surface via a C[N p system,however.

Surface-enhanced Raman scattering (SERS) as a sur-face vibration spectroscopic method has been utilized asa convenient technique to investigate self-assembledmonolayers (SAMs) on metal sufaces.18 Our recent SERSstudy has shown 4,49-dicyanobiphenyl to have a ratherflat geometry on Ag and Au surfaces,19 whereas the spec-tral evidence of 4-cyanobiphenyl in silver and gold nano-particle surfaces indicated that the biphenyl ring groupshould have a tilted orientation with respect to both thesurfaces.20 Although the SERS study of 5CB on Ag andAu electrodes have been reported recently, different spec-tral behaviors are not fully clarified yet.21

Temperature-dependent spectral behaviors on surfacescan provide the energetics or adsorption energy of ad-sorbates on metal surfaces.22 In this article, to better un-derstand interfacial structures and phase transitions of thenematic liquid crystalline materials, we performed a tem-perature-dependent Raman study of 5CB self-assembledmonolayers (SAMs) adsorbed on Ag and Au plate sur-faces as well as of its liquid crystalline state.

EXPERIMENTAL

Sample Preparations. 5CB was purchased from Sig-ma Aldrich. The Au SERS substrate was produced froman Au plate by an oxidation–reduction cycle using a CHInstrument 700A potentiostat. The Ag SERS substratewas prepared by eroding the Ag plate using a HNO3 so-lution. Before immersing the Ag and Au substrates in theethanolic solution of 5CB, the substrates were rinsed withexcess ethanol and dried in an N2 gas stream. After theself-assembly of 5CB, the substrates coated with theSAMs were rinsed with excess ethanol and dried with anN2 gas stream again.

Temperature-Dependent Polarized Microscopy Im-ages and Spectroscopic Measurements. Optical imagesof 5CB, depending on temperature, are obtained with acombination of an Olympus microscope (BH2-UMA)equipped with a cross-polarizer and a heating stage (Met-tler FP82HT). Raman spectra were obtained using a Ren-ishaw Raman confocal system model 1000 spectrometerequipped with an integral microscope (Leica DM LM).The 632.8 nm irradiation from a 35 mW air-cooled HeNelaser (Melles Griot Model 25 LHP 928) with a plasmaline rejection filter was used as the excitation source forthe Ag and Au SERS experiments. Temperature-depen-

APPLIED SPECTROSCOPY 1137

FIG. 1. Polarized microscopy images taken at different temperatures: (a) 270.0, (b) 240.0, (c) 220.0, (d) 0.0, (e) 20.0, (f) 23.0, (g) 26.0, (h)34.0, (i) 34.4, and (j) 34.7 8C.

FIG. 2. (a) OR (Ordinary Raman) spectrum of 5CB in the liquid stateand (b) SERS spectrum of 5CB on (c) silver and gold. The spectra aretaken at room temperature (25 8C).

dent Raman measurements could be performed using aLinkam THMS 600 thermal heating stage. The samplewas allowed to sit approximately 10 min before shiftingto different temperatures. Infrared spectra were obtainedusing a Bruker IFS 66 V/S spectrometer and a ZnSe win-dow. The 5CB sample was sandwiched between the twoquartz plates for passing the infrared beam. Thermal be-haviors were checked using a DSC Q-1000 differentialscanning calorimeter (TA instruments).

RESULTS AND DISCUSSION

Polarized Microscopy Images at Different Temper-atures. Figures 1a through 1j show representative polar-ized microscopy images of 5CB at different temperaturesfrom 270.0 to 34.7 8C. Considering that the phase tran-sitions for 5CB should occur at 24 and 35 8C, respec-tively, the phase transition from the solid to the nematicphase would be rather difficult to discern, whereas thatfrom the nematic to the isotropic phase could be easilymonitored from the images taken by polarized micros-copy. As shown in Fig. 1g, a rather bright image indi-

cated the phase change involving a melting process fromthe solid to the nematic phase. In contrast, the relativelydark areas in Figs. 1i and 1j appeared to be due to theisotropic phase. Figures 1a through 1e indicate a phasetransition even occurring at low temperatures.

Raman Spectra. In the OR (ordinary Raman) spec-trum as shown in Fig. 2a, the major bands associated withthe ring vibrational modes can be divided into four sym-metry species assuming a C2v point group. The A1 ringmodes appeared at 1002 (n12), 1056 (n18a), 1177 (n9a),1231 (n7a), 1519 (n19a), 1603 (n8a), and 3063 (n2) cm21.The bands at 773 (n10a) cm21 could be assigned to the A2

mode. The bands at 807 (n4) and 822 (n11) cm21 could beascribed to the B1 mode, whereas the bands at 634 (n6b)and 1282 (n3) cm21 could be ascribed to B2. Their peakpositions with appropriate vibration assignments are list-ed in Table I. It was rather straightforward to correlatethe OR bands with the Ag and Au SERS bands.23

As shown in Figs. 2b and 2c, the broad bands at 233and 236 cm21 could be ascribed to the metal–N stretchingbands. These strong features suggest that 5CB should ad-sorb on Ag and Au via the nitrogen atom of its cyanogroup. It is noteworthy that the ring C–H stretching bandswere identified at ;3065 cm21 both in the Au and AgSERS spectra. It has been well documented in the liter-ature that the presence of the ring C–H stretching bandin a SERS spectrum is indicative of a vertical (or at leasttilted) orientation of the aromatic ring moiety on a metalsubstrate.19,20

On the other hand, the ring breathing mode n12, whichappeared at 1002 cm21 in the OR spectrum, did not ex-hibit a substantial red shift in the Ag and Au SERS spec-tra with the increase of its bandwidth, as shown in Figs.2b and 2c. This result may indicate an insubstantial in-teraction of the benzene rings on the gold surface.20 Inaddition, the position of the n8a band varied from 1603cm21 in its neat state to 1602 and 1603 cm21 in the Agand Au SERS spectra, respectively. Most in-plane vibra-tional modes were not red shifted in the SERS spectra.It is noteworthy that both a substantial red shift and asignificant band broadening of the ring breathing modeswere observed for the case of 4-cyanobiphenyl where the

1138 Volume 59, Number 9, 2005

TABLE I. Spectral data and vibrational assignment of 5CB.a

OR Ag SERS Au SERS Assignmentb

In-Plane3063 3065 3067 2(A1)1603 1602 1603 8a(A1)1519 1525w 1519 19a(A1)1282 1282 1282 3(B2)1231 1236 1239 7a(A1)1177 1177 1177 9a(A1)1056 1072 1064 18a(A1)1002 1003 1008 12(A1)

634w 633w 634w 6b(B2)

Out-of-Plane822w 828w 814 11(B1)807 808 806 4(B1)773 775 780w 10a(A2)

Substituents2931 2927 2926 nas(CH2)2885 2865 2869 ns(CH2)2223 2237br 2240br

2223 2225 n(C[N)1317 gs(CH2)

519w 596w 569 das(CN)404 391 397 gs(CN)

233br 236br metal-N

a Units in cm21. Abbreviations: w; weak, br: broad.b In Wilson notation with symmetries based on C2v point group.

FIG. 3. n(C[N) stretching region between 2300 and 2150 cm21 in OR(Ordinary Raman) spectra for 5CB at varying temperatures.

molecule is assumed to have a tilted orientation on Agand Au.13 It is a matter of conjecture whether the alkylchain attached at the para position of 5CB would lead tomuch different orientation in comparison with that of 4-cyanobiphenyl.

The nas(CH2) and ns(CH2) bands appeared to be quitebroadened at ;2925 and ;2870 cm21 in Ag and AuSERS spectra. In addition, the two nas(CH2) and ns(CH2)bands on Au were found to be more enhanced than thoseon Ag. The band positions of both Ag and Au SERSspectra looked quite comparable except for the band at1317 cm21. This band, observed only in the Au SERSspectrum, could be ascribed to the gs(CH2) band. Al-though it was uncertain why these bands did not appearin the OR and Ag SERS spectra, it may suggest differentorientations of 5CB upon adsorption on Ag and Au. Thedetailed origins of different adsorption characteristics of5CB on Au and Ag are still not yet clear. Energetic orkinetic factors may also result in different adsorption be-haviors on Ag and Au.

n(C[N) Stretching Bands. The CN stretching fre-quencies at 2223 and 2225 cm21 in Figs. 2b and 2c werenot quite different from that at 2223 cm21 in the ORspectrum of a free state, shown in Fig. 2a. In addition,broad features were found at 2237 and 2240 cm21 on Agand Au, respectively. Considering the fact that then(C[N) bands were considerably broader in the SERSspectra, the cyano group seemed to directly interact withmetal surfaces. The full bandwidth of the n(C[N) bandof a free-state 5CB became broadened from 35 cm21 into;70 and ;68 cm21 upon adsorption on silver and goldsurfaces, respectively. The blue shifts of the n(C[N)band were explained in coordination chemistry by invok-ing the fact that the nitrogen lone-pair electrons have anantibonding character and the electron donation to themetal surfaces may increase the strength of the C[N

bond.16 In the previous spectroscopic study,17 however,such blue shifts on silver were mainly ascribed to thedisruption of conjugation induced by the p coordinationof the nitrile group to metals. Yet it seems a matter ofconjecture to dictate the exact adsorption mechanismfrom our SERS.

Figure 3 exhibits the C[N stretching region for 5CBwith varying temperature. It is noteworthy that then(C[N) band could be split to the two bands at 2223 and2232 cm21 between 210 and 20 8C. The peak positionswere found at ;2225 cm21 in the temperature settings of270, 240, 24, and 70 8C. The band observed at ;2226cm21 for 270 8C was only slightly shifted by ;2 cm21

to ;2224 cm21 for 70 8C. Their full-widths at half-max-imum (FWHM) were measured to be ;10 cm21 withoutmuch deviation in this temperature range. The FWHMvalue at 270 8C was measured to be broader by approx-imately 1 cm21 than that at 70 8C. Regarding the splitbands at 2232 and 2223 cm21 in the temperature regionsof 210 and 20 8C, neither of the peak positions or band-widths for the doublet appeared to change significantly.

It is not absolutely certain whether these splittings aredue to the existence of different phases of 5CB, since thetemperatures are quite below that of the solid–liquid crys-tal transition. Liquid crystal materials can be super-cooled below the thermodynamic melting point withoutthe onset of crystallization.2,17,18 It seems possible thatglasses and their associated super-cooled liquids couldoccur in the temperature region between 210 and 20 8C.Since the peak splitting is also observed for the heatingprocess as well as the cooling process, it is not absolutelycertain whether the splitting merely suggests the exis-tence of the super-cooled structure. It is also possible thatthis may be due to solid crystalline polymorphism.

In order to further characterize the phase transitions,we performed difference scanning calorimetry (DSC) andFourier transform infrared (FT-IR) spectroscopic mea-surements. The CN bands in FT-IR spectra also split at0, 10, and 20 8C, as shown in Fig. 4. The reason whythe splitting is not observed at 210 8C, as opposed to thecase of the Raman spectra, may be due to hysteresis. TheDSC data also indicated the phase transitions at low tem-

APPLIED SPECTROSCOPY 1139

FIG. 4. FT-IR spectra for 5CB at varying temperatures. The arrowsindicate the splitting CN bands.

FIG. 6. Ag SERS spectra for 5CB at varying temperatures.

FIG. 5. Differential scanning calorimetric diagram for heating andcooling of the 5CB sample. Arrows indicate the detection time. FIG. 7. Au SERS spectra for 5CB at varying temperatures.

peratures between 230 and 220 8C, as shown in Fig. 5and consistent with the previous report.15

As shown in Fig. 6, the n(C[N) bands in the Ag SERSspectrum looked quite asymmetric at low temperaturesbelow 30 8C. An additional broad feature at ;2237 cm21,along with the relatively narrow band at ;2223 cm21,suggests that there should exist different binding schemesfor 5CB on Ag surfaces. The broad features at ;2237cm21 were found to decrease at 30;37 8C but increaseagain at 50 8C. The features at ;2240 cm21 became quiteweakened at higher temperatures above 30 8C, in linewith the single peaks in the OR spectrum as shown inFig. 3. Since the broad feature at ;2237 cm21 appearedto exist at 230 and 50 8C, it is not absolutely certain yetwhether the feature at ;2237 cm21 should be related tothe band at ;2226 cm21 that is assumed to be caused bythe solid crystalline polymorphism. Although not shownhere, the spectra at 270 and 70 8C looked quite similarto those at 230 8C (240 8C for Au) and 30 8C, respec-tively. The Au SERS spectra showed similar behaviors,

as exhibited in Fig. 7. The additional feature at ;2240cm21 disappeared at 37 8C.

It is not absolutely certain whether additional broadfeatures observed in the n(C[N) stretching region couldbe interpreted as being due to adsorption at different crys-tal faces or the existence of several different complexes.The decrease in spectral intensity depending upon tem-perature indicated that the broad features at ;2237 cm21

should be due to the relatively unstable one in compari-son with that at ;2223 cm21. Since the interaction be-tween the nitrile moiety and roughened metal surfacesmay strongly perturb the liquid crystalline phase, it seemsdifficult to interpret the temperature-dependent SERSspectra. Due to the fact that the interaction between thenitrile moiety and roughened metal surfaces may stronglyperturb the liquid crystalline phase, it seems difficult tointerpret the temperature-dependent SERS spectra.

Since the experiment was conducted on the roughenedSERS substrates and the Raman spectra appeared to de-pend on the substrate condition, we have to mention thatour spectral data mainly provide information on the ad-sorption pathways occurring at the surface rather than onthe electronic interaction between the adsorbate and thesubstrate. It seems difficult to determine to what extent

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the observed SERS spectra is due to the adsorbate–sur-face interactions because of the heterogeneity of the me-tallic surface. Also, the surface concentrations and mo-lecular orientations may affect the intensity of each SERSband. Due to the broadening of SERS spectral lines fromthe adsorbate–metal interactions, it seems difficult toreach the resolution for the Raman spectra of neat 5CB.In this way, SERS spectra could not provide much in-formation on the phase transitions of 5CB as in the caseof the Raman spectrum of the neat sample. As previouslyreported,24 the orientation of adsorbates can be estimatedby determining the relative intensities of each vibrationalband. In our temperature-dependent spectra, it seemsproblematic to determine the adsorbate orientation bycomparing relative intensities because of the strong influ-ences of concentrations and surface conditions.

We plan to conduct electrochemical studies combinedwith infrared reflection absorption spectroscopy and the-oretical calculations in order to firmly understand thephase transitions in its liquid crystalline state and the ad-sorption characteristics and surface orientations of 5CBon Ag and Au.

CONCLUSION

The phase transitions of 4-n-pentyl-49-cyanobiphenyl(5CB) have been studied by means of temperature-de-pendent Raman spectroscopy. The n(C[N) stretchingpeaks were found to split in the temperature region be-tween 210 and 20 8C, suggesting the existence of solidcrystalline polymorphism. The interfacial structures of5CB on Ag and Au have been reinvestigated by surface-enhanced Raman scattering (SERS). On Ag and Au, quiteasymmetric shapes of the n(C[N) bands at several tem-peratures suggest that there should be different bindingschemes for 5CB.

ACKNOWLEDGMENTS

S.W.J. would like to thank Prof. Kwan Kim for his guidance on SERSstudies of self-assembled monolayers and Mr. Y.-K. Lee at NICEM forDSC measurements. This work was supported by Korea Research Foun-dation Grant (KRF-2004-015-C00226).

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