vibrational and surface-enhanced raman spectra of 1,6-diphenyl-1,3,5-hexatriene

Vibrational and Surface-Enhanced Raman Spectra of1,6-Diphenyl-1,3,5-hexatriene

IGOR O. OSORIO-ROMAN, VICTOR VARGAS C., and RICARDO F. AROCA*Laboratory of Luminescence and Molecular Structure, Department of Chemistry, Faculty of Science, University of Chile, Casilla 653, Santiago-

Chile (I.O.O.-R., V.V.C.); and Materials and Surface Science Group, Faculty of Sciences, University of Windsor, Windsor, ON, Canada, N9B 3P4

(R.F.A.)

The vibrational spectra and surface-enhanced Raman scattering (SERS)

of 1,6-diphenyl-1,3,5-hexatriene (DPH) are discussed. The fundamental

vibrational frequencies, overtones, and combinations observed in the

infrared and Raman spectra of DPH are reported. The interpretation of

the observed vibrational spectra was supported by a complete geometry

optimization, followed by vibrational frequency and intensity computa-

tions for the cis- and trans- isomers of the DPH using density functional

theory at the B3LYP/6-31G(d,p) level of theory. Because the molecule is

photo-chemically active on Ag metal surfaces, the best SERS results for

silver islands were obtained at low temperature and low energy density of

the exciting laser line. DPH SERS on Au films was obtained at room

temperature.

Index Headings: Infrared spectroscopy; Raman spectroscopy; Surface-

enhanced Raman scattering; SERS.

INTRODUCTION

The photophysics and photochemistry of diphenyl-polyeneshave been under scrutiny because these molecules areconsidered to be models for retinyl-polyenes that are relatedto vitamin A and the visual pigments.1 Among numerousmembers of this class, 1,6-diphenyl-1,3,5-hexatriene (DPH) isthe most extensively employed as a fluorescence probe forstudies of molecular order, dynamic behavior, and micro-fluidity.2,3

A detailed study of the electronic-vibrational spectra ofpolyenes including the DPH molecule can be found in the workof Baranov et al.4 Semi-empirical methods were used forcomputation of the vibrational structure of the electronicspectrum and the determination of the parameters of theexcited-state potential energy surface. The electronic andphotochemical characterization of DPH has also been report-ed.5–7 In an earlier publication, Moskovits and Dillela8 reportedthe fluorescence and surface-enhanced Raman scattering(SERS) spectra of DPH deposited onto rough silver surfacesat a temperature of 12 K. However, the complete vibrationalanalysis of DPH required for future fluorescence, photo-dynamic, and photochemical studies is not available. There-fore, the first part of this report provides a vibrational analysisof the infrared and Raman spectra of the DPH material. After acomplete assignment is achieved, the SERS9 spectra wereobtained at 93 K on silver and on gold at room temperature.

EXPERIMENTAL

1,6-Diphenyl-1,3,5-hexatriene (DPH) was purchased fromAldrich in the highest purity available, 98%, and was usedwithout any additional purification. Solvents used from Aldrich

were high-performance liquid chromatography (HPLC) grade.The theoretical calculations were carried out using Gaussian‘03 for Unix.10 Geometry optimization, harmonic frequencies,and intensities were computed at the B3LYP/6-31G(d)11 levelof density functional theory (DFT), and the frequencies werescaled using a 0.9614 scaling factor.12 For comparison withexperimental spectra, simulated infrared and Raman spectrawere created using Gaussian band shape with a full-width athalf-maximum (FWHM) of 5 cm�1.

Ultraviolet (UV)-visible absorption spectra were recordedfor all DPH solutions and metal island films in a Cary 50 scanUV-visible spectrophotometer. Atomic force microscopy(AFM) images were recorded using a Digital InstrumentsNanoScope IV, operating in tapping mode with an nþ-silicontip. Images were collected with high resolution (512 lines perscan) at a scan rate of 0.5 Hz. Micro-Raman scattering spectrawere collected using either a Renishaw InVia system with laserexcitation at 514.5 nm, 632.8 nm, and 785 nm, or a Renishawsystem 2000 with excitation laser line at 442 nm. Both systemswere equipped with a Peltier charge-coupled device (CCD)detector and Leica microscope. All measurements were madein a backscattering geometry, using a 503 microscopeobjective with a numerical aperture value of 0.75. The infraredtransmission spectra were obtained on a Bruker Equinox 55.Gold and silver island films of 6 nm mass thickness wereprepared in a Balzer BSV 080 glow discharge evaporation unit.The metal island films were fabricated onto preheated (200 8C)glass microscope slides (2947 Corning). During the deposition,the background pressure was nominally 10�6 torr and thedeposition rate (0.5 A/s) was monitored using an XTC Inficonquartz crystal oscillation.

The SERS measurements on silver island films were carriedout at 93 K (�180 8C) using a 600 LINKAN THMS heating–cooling stage and a 503 microscope objective to focus the laserbeam onto a spot approximately 1.0 lm2. SERS on gold islandfilms were measured at room temperature as well as at 93 K.

RESULTS AND DISCUSSION

Infrared and Raman Spectra. The infrared and Ramanvibrational spectra of the DPH were recorded, computed, andassigned. The trans-DPH belongs to the C2h molecular pointgroup with 34(C18H16) atoms and 96 vibrational fundamentals.The total number of species of symmetry is given by Cvib ¼33ag þ 15bg þ 16au þ 32bu.

Trans-DPH has a center of symmetry and the mutualexclusion rule applies for the Raman and infrared activities. Itis expected that the ag modes would dominate the Ramanspectrum. The measured infrared transmission and Ramanspectra for the solid DPH are shown in Fig. 1. Although thespectra are in the solid state, where significant deviations from

Received 8 February 2007; accepted 11 June 2007.* Author to whom correspondence should be sent. E-mail: [email protected].

Volume 61, Number 9, 2007 APPLIED SPECTROSCOPY 10010003-7028/07/6109-1001$2.00/0

� 2007 Society for Applied Spectroscopy

the symmetric model could be observed, the mutual exclusionrule seems to hold. However, distortions from the regular transstructure of the hexatriene are apparent in the IR and Ramanspectra by the appearance of additional bands close to the mainbands in the regions below 1600 cm�1 and 1100 cm�1.However, the relative intensity of these additional bands isalmost negligible.13 The band assignment is based on the twomain moieties present in the DPH molecule, the phenyl rings14

and the hexatriene.13 Direct support in the interpretation wasprovided by the calculated spectra as shown in Fig. 2, and alisting of the characteristic vibrational frequencies, includingcalculated intensities and observed band centers, is given inTable I (Raman active species) and Table II (infrared activemodes). The agreement between the DFT calculated spectra asshown in Fig. 2 and the solid state is quite remarkable, given

the fact that the computed spectra should represent the gas-phase spectra.

The Raman spectrum presented in Fig. 1 was recorded at632.8 nm and was used to construct Table I. The latter is thenormal Raman scattering because DPH does not absorb above390 nm. Confirming the nature of the scattering, the samerelative intensities were obtained with 514.5 nm excitation, asshown in Fig. 3. This result, in addition, is used to convey thefact that the normally very weak overtone and combinationbands are clearly observed in the Raman spectrum of DPH.

The electronic absorption of DPH in dichloromethane isshown in Fig. 4, with a UV-vis maximum at 362 nm. Thehighest occupied molecular orbital (HOMO) and the lowestunoccupied molecular orbital (LUMO) are also shown in Fig. 4to illustrate the change in electron density around the centralbonds in the LUMO. The calculated energy of the HOMO is�5.03 eV and that of the LUMO is�1.84 eV, giving an energygap of 3.19 eV or 388 nm, in agreement with the observation.The minimized theoretical structure for DPH is that of theplanar trans isomer.

The Raman spectrum in Fig. 1 shows characteristic bands ofthe aromatic ring stretching modes and the C¼C stretching ofthe hexatriene frame. These modes are seen at 1605 cm�1, 1591cm�1, 1585 cm�1, and 1567 cm�1. The 1605 cm�1 and 1567cm�1 vibrations have a large contribution from the C¼C of thehexatriene frame. Other bands with strong relative intensity arethe C–C stretching and C–H bending of the hexatriene frame,recorded at 1143 cm�1 and 1256 cm�1 with a FWHM ofapproximately 10 cm�1. The previous assignments aresupported by the corresponding calculated vibrations forDPH (scaled), at 1616 cm�1, 1594 cm�1, 1582 cm�1, 1564cm�1, 1142 cm�1, and 1247 cm�1, respectively, which are ingood agreement with the observation.

The high-frequency C–H stretching mode at 3061 cm�1 is a

FIG. 1. Infrared and Raman spectra of 1,6-diphenyl-1,3,5-hexatriene (DPH).

FIG. 2. Experimental and calculated Raman and Infrared spectra of DPH.

1002 Volume 61, Number 9, 2007

characteristic vibration of the phenyl ring observed with a

FWHM of 13 cm�1 in the Raman spectrum shown in the Fig. 3.

In the same spectral region, an overtone is observed at 3176

cm�1 (xovertone ¼ 2xe � 6xexe)15 that may correspond to a

fundamental mode (a in Fig. 3) observed at 1591 cm�1. The

latter could also be a combination of the fundamental modes at

1585 cm�1 and 1591 cm�1 that also add to 3176 cm�1.

Notably, overtones and combinations have a broader FWHM

(26 cm�1) that is double the width of the corresponding

fundamentals. The combination of the very strong ring mode

with the two most intense bands below 1500 cm�1 is also seen

at 1585þ 1143 (c)¼ 2728 cm�1 and 1585þ 1256 (b)¼ 2841

cm�1.

A characteristic band in the Raman spectrum of DPH is the

ring breathing frequency (phenyl ring16) and is seen here at 999

cm�1, and the corresponding combination is observed at 1585

TABLE I. Vibrational Raman bands in wavenumber (cm�1) of DPH.Calculated Raman intensities in A4/amu.

Symmetry

CalculatedExperimental

AssignmentFrequency Intensity FrequencyRelativeintensity

bg 35 2 Def. out-of-planea

ag 91 2 C–C¼C bendinga

bg 113 15 159 0.00 Def. out-of-planea

ag 181 1 190 0.20 C–C¼C bendinga

bg 248 4 Def. out-of-planea

bg 319 4 318 0.43 Def. out-of-planea

ag 329 1 C–C¼C bendinga

bg 397 1 488 0.35 Twist out-of-planeb

bg 497 2 502 0.37 C–H waggingb



ag 611 40 621 0.31 Ring deformationbg 675 10 660 0.43 C–H waggingb


bg 809 10 807 0.25 C–H wagginga,b

ag 817 20 823 0.30 Ring breathingbg 821 7 833 0.27 C–H waggingb

bg 874 50 877 1.20 C–H wagginga,b

bg 891 70 889 0.27 Ring def. þC–H wagga

bg 926 1 936 0.26 C–H wagginga



ag 975 1100 999 2.57 Ring breathingag 1018 130 1005 0.21 Ring def. þ

C–H bendingb

ag 1069 90 1035 0.13 Ring def. þC–H bendingb

ag 1142 11890 1143 8.71 C–C stretchinga

ag 1147 260 1156 0.21 C–H bendingb

ag 1170 1130 1180 2.96 C–H bendingb

ag 1204 610 1212 1.19 C–C stretchingag 1247 10290 1256 11.45 C–H bendinga

ag 1287 3ag 1295 180 1292 1.12 C–H bendinga

ag 1317 200 1304 1.98 C–H bendinga

ag 1322 830 1325 1.26 C–H bendingag 1437 770 1444 1.19 C¼C stretchingag 1485 1800 1488 1.29 C¼C stretchingag 1564 5530 1567 4.17 C¼C stretchinga

ag 1582 45430 1585 100.00 C¼C stretchingag 1594 2190 1591 83.44 C¼C stretchingag 1616 13440 1605 4.43 C¼C stretchinga

ag 3020 10 C–H stretchinga

ag 3026 200 3022 0.72 C–H stretchinga

ag 3040 40 C–H stretchinga

ag 3055 100 C–H stretchingb

ag 3061 200 3034 0.22 C–H stretchingb


ag 3080 300 C–H stretchingb


a Hexatriene vibration.b Ring vibration.

TABLE II. Vibrational fundamental modes in the infrared spectrum ofDPH in wavenumber (cm�1). Calculated absolute intensity in km/mol.

Symmetry

CalculatedExperimental

AssignmentFrequency Intensity FrequencyRelativeintensity

au 18 1.00 Deformation-out-of-planea

au 26 1.00 Torsiona

bu 30 1.00 Deformation-in-planea

au 111 1.00 Deformation-out-of-planea

bu 202 1.00 Deformation-in-planea

au 252 1.00 Torsiona

au 344 1.00 355 6.10 Torsiona

bu 379 15.50 364 3.40 Ring deformationau 398 1.00 387 6.10 Twist ring-out-of

planebu 460 3.20 474 3.30 C–C¼C bendinga

au 509 8.80 517 23.70 Twist ring-out-ofplane

bu 609 1.20 603 5.30 Ring deformationbu 620 20.50 619 4.10 Ring deformationau 677 41.70 694 62.80 C–H waggingb

au 740 64.30 757 80.00 C–H waggingb

au 816 1.00 C–H waggingb

bu 833 1.90 824 1.00 Ring breathingau 848 1.70 842 5.00 C–C¼C bendinga

au 888 4.00 886 11.70 C–H waggingb

au 918 4.30 912 10.00 C–H wagginga

au 926 1.00 929 13.60 C–H waggingb

au 954 1.00 957 8.60 C–H waggingb

bu 975 1.00 977 2.00 Ring deformationau 1005 75.70 1004 100.00 C–H wagginga

bu 1017 4.60 1013 15.90 Ring def.þ C–Hbendingb

bu 1070 7.30 1072 11.80 Ring def.þ C–Hbendingb

bu 1137 2.30 C–C stretchinga

bu 1147 1.00 1152 1.00 C–H bendinga

bu 1170 1.00 1177 5.00 C–H bendingb

bu 1192 1.00 1180 10.00 C–H bendinga

bu 1214 1.00 1214 1.00 C–H bendinga

bu 1289 2.80 1291 5.00 C–H bendinga

bu 1300 1.00 1298 8.00 C–H bendinga

bu 1321 1.00 1315 5.00 C–H bendingb

bu 1355 3.30 1351 5.20 C–H bendinga

bu 1441 11.20 1448 30.00 C–H bendingb

bu 1486 41.50 1491 30.00 C¼C stretchingb

bu 1568 2.10 1573 10.00 Ring stretchingbu 1594 36.00 1593 20.20 Ring stretchingbu 1634 1.00 1634 7.20 C¼C stretchingb

bu 3023 3.10 2852 1.20 C–H stretchinga



bu 3055 24.20 C–H stretchingb

bu 3062 1.60 C–H stretchingb

bu 3071 33.30 3058 1.00 C–H stretchingb



a Hexatriene vibration.b Ring vibration.

APPLIED SPECTROSCOPY 1003

þ999¼2584 cm�1, supporting the assignment. The weak bandat 889 cm�1 gives rise to an overtone observed at 1778 cm�1

and may be assigned to a phenyl ring mode with largecontribution of C–H bending.

In the infrared spectrum several C–H stretching frequenciesare observed at 3081 cm�1 (w), 3058 cm�1 (w), and 3015 cm�1

(middle intensity) that can be associated with the phenyl rings.The weak band at 3000 cm�1 is assigned to the C–H stretchingof the hexatriene moiety. The strongest IR bands are observedat 1004 cm�1, 757 cm�1, and 694 cm�1 and can be assigned toC–H angle deformation modes. Four infrared bands at 1634cm�1, 1593 cm�1, 1573 cm�1, and 1491 cm�1 clearlycorrespond to C¼C or ring stretching modes and are observedwith medium relative intensity (see Fig. 1). Ring deformationsmixed with C–H bending also give rise to bands with mediumrelative intensity at 886 cm�1, 1013 cm�1, and 1072 cm�1. Agroup of observed vibrational frequencies are assigned as in orout of the plane, referring to the molecular plane generated bythe trans structure.

It can be seen that characteristic modes of hexatrienediscussed by Panchenko et al.,13 in a theoretical ab initio

investigation of the vibrational spectra of isomers of ethyleneincluding all-trans-hexa-1,3,5-triene, are clearly seen in thespectrum of DPH. The band at 1569 cm�1 assigned to C¼Cstretching of the hexatriene moiety corresponds to thehexatriene mode at 1555 cm�1. Notably, the assignment ofthe central C¼C stretching in the cis isomer of free hexatrienecorresponds to a vibrational band observed at 1537 cm�1 that isnot seen in the DPH spectra. The assignments of normal modesstrongly localized on the C–C stretching and C–H bending arealso in good agreement with the DPH spectrum. The summaryof this comparison is given in Table III.

Surface-Enhanced Raman Scattering. The SERS studieson Ag and Au islands were carried out by casting 5 lL of DPH(10�5 M) onto silver and gold nanostructures at roomtemperature and allowing for solvent evaporation. The surfaceplasmon spectra of the metal island films can be seen in Fig. 4,and the AFM images of the metal island films are shown inFig. 5.

At room temperature, SERS spectra on silver and gold filmswere recorded using the 785 nm laser line with a power at thesample of approximately 30 lW. The SERS experiments on Agislands did not produce a reproducible spectrum of DPH,

FIG. 3. Fundamentals, overtones, and combinations in the Raman spectrum ofDPH excited with the 514.5 nm laser line.

FIG. 4. Electronic absorption spectrum of DPH in dichloromethane, and plasmon absorption of silver and gold islands. The HOMO and LUMO molecular orbitalsare also shown with the calculated electronic transition.

TABLE III. Vibrational comparison of hexatriene (trans, cis) and of thehexatriene moiety in DPH.

Assignmenta

Hexatrieneb

AssignmentHexatrienein DPHet,T,tc t,C,tc

C¼C stretching, v 1651 1650 C¼C stretching 1605C¼C stretching, cm 1555 1537 C¼C stretching, cm 1567C–C stretching, v 1132 1195 C–C stretching, v 1143C–C stretching, v 1204 1085 C–C stretching, v 1212C–H bending, mv 1276 1270 C–H bending, all framed 1256C–H bending, mv 1278 1271 C–H bending, all framed 1292

a Abbreviations: v, vinyl; cm, central/middle; mv, middle/vinyl.b Ref. 12.c t,T,t is for all-trans; t,C,t is for the isomer cis rotated at the central bond.d ‘‘all frame’’ is for all C–H of the hexatriene.e Raman bands of the hexatriene moiety.


probably due to a photodegradation of the adsorbate. Using thesame experimental conditions, the SERS spectrum on goldisland films was obtained and is shown in Fig. 6. The SERSspectrum is very similar to that of the solid spectrum with oneimportant difference: the full-width at half-maximum(FWHM). For instance, the FWHM of the 1259 cm�1 bandin the solid was 9 cm�1, while that of the same band at 1254cm�1 in the SERS spectrum is 27 cm�1. This trend is observedfor all the vibrational frequencies. The large FWHM of 62cm�1 for the 1589 cm�1 band in the SERS spectrum may bedue to the overlapping of several bands in the ring stretchingregion, with contributions from the double bond (C¼C) of thehexatriene and the ring stretching modes. Because the SERSspectrum had the same bands and a similar trend of relativeintensities compared to the normal Raman spectra, it isconcluded that the observed average SERS spectrum doesnot reflect a specific molecular orientation at the surface andcorresponds to a random distribution of physisorbed DPH

molecules on the Au surface.17,18 However, a weak SERS bandat 1534 cm�1 is observed (see Fig. 6) that could be interpretedas due to the C¼C stretching in the cis isomer of the hexatrienemoiety, which is characterized by a vibrational band observedat 1537 cm�1.13 The latter would indicate the presence of thecis isomer of DPH on the gold nanostructures. The catalyticproperties of the metallic nanostructure19 would bring aboutisomerization and photodecomposition of the molecule at roomtemperature.

Surface-Enhanced Raman Scattering at Low Tempera-ture. Since SERS spectra of DPH had been recorded at verylow temperatures,8 these conditions provide a pathway toovercome the problem of photodegradation of DPH on Agmetal films. SERS spectra were successfully recorded in a low-temperature cell using liquid nitrogen to achieve a temperatureof 93 K, or �180 8C. The SERS spectra on Ag island filmswere carried out by casting 5 lL of DPH (10�5M) and allowingfor solvent evaporation at room temperature. Since the samplesare prepared at room temperature and then placed in the lowtemperature cell, it can be concluded that the photodegradationon silver is laser induced. The DPH SERS spectra presented inFig. 7 were obtained with laser lines at 514.5 nm and 442 nmwith a maximum power of approximately 30 lW.

The fingerprint spectra (showing only a few characteristicmodes) obtained at low temperature on Ag were similar to theSERS spectra on Au and in close agreement with the Ramanspectrum of the solid, as can be seen in Fig. 7. Thecharacteristic C–H bending mode at 1256 cm�1 (hexatriene),C–C stretching at 1143 cm�1 (hexatriene), and ring breathing at999 cm�1 are in good agreement with the bands obtained on thesilver island films with the 514.5 nm and 442 nm excitationlines. Notably, the band observed at 1534 cm�1 in the room-temperature SERS spectrum on Au was not observed in theSERS spectra presented in Fig. 7. In addition, the very strongband in the 1590 cm�1 region observed with excitation at 442nm and 514.5 nm due to the double bond (C¼C) stretchingmodes has a bandwidth value of approximately 33 cm�1. TheseFWHM values are lower than the values found for the SERS

FIG. 5. Atomic force microscopy (AFM) image of silver and gold nanostructures.

FIG. 6. SERS of the DPH on gold islands at room temperature and the Ramanspectrum of the solid DPH recorded using the 785 nm laser line.

APPLIED SPECTROSCOPY 1005

spectrum on the gold metal films at room temperature (1589cm�1, bandwidth of 62 cm�1). It can be speculated that thelarge FWHM in this DPH SERS band is caused byphotoisomerization at room temperature, and such effects areabsent in the spectra recorded at low temperature. Therefore,low energy laser excitation of DPH on Au nanostructures atroom temperature may form small amounts of cis-DPH,leading to the sprouting of the band at 1534 cm�1 and theincrease of the spectral bandwidth. A more detailed study onthe increase of the FWHM in SERS spectra of a series ofmolecules is underway.

CONCLUSION

The vibrational spectra of solid DPH are reported and theassignment of the observed infrared and Raman bands isproposed. The DFT calculated infrared and Raman spectra arein good agreement with the experiment, supporting thevibrational assignment of normal modes. The average SERSspectra of DPH on gold island films and silver island filmswere obtained. Reproducible SERS spectra on Ag can only beattained at low temperature, hindering the photodegradation of

DPH. The SERS spectra of DPH on Au hint the formation ofthe cis-isomer of DPH. The enhanced SERS spectra areconsistent with an average electromagnetic enhancement of arandom distribution of molecules on metal nanostructures.

ACKNOWLEDGMENTS

Financial support and fellowship from CONICYT of Chile and NSERC ofCanada are gratefully acknowledged.

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FIG. 7. SERS of the DPH on an evaporated silver island film at �180 8C.Lasers lines at 442 nm and 514.5 nm. For comparison, the Raman of DPH solidobtained with the 442 nm laser line is also included.


vibrational and surface-enhanced raman spectra of 1,6-diphenyl-1,3,5-hexatriene

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