Ab initio, DFT vibrational calculations and SERRS ...

Vibrational Spectroscopy 41 (2006) 90–96 www.elsevier.com/locate/vibspec

Ab initio, DFT vibrational calculations and SERRS study of Rhodamine 123 adsorbed on colloidal silver particles Jyotirmoy Sarkar a, Joydeep Chowdhury b, Prabir Pal a, G.B. Talapatra a,* a

Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India b Department of Physics, Sammilani Mahavidyalaya, Baghajatin Station, E.M. Bypass, Kolkata 700 075, India Received 8 November 2005; received in revised form 4 January 2006; accepted 4 January 2006 Available online 5 April 2006

Abstract The adsorption of biologically important Rhodamine 123 molecule on silver colloids has been investigated by Surface enhanced resonance Raman scattering (SERRS) spectroscopy. The SERRS spectra are compared with its FTIR spectrum and normal Raman spectrum (NRS) in solution. The optimized structural parameters and computed vibrational wavenumbers have been estimated from ab initio (Hatree–Fock) and density functional theory (DFT) calculations. Some vibrational modes have been reassigned. From frontier orbital theory, the direction of charge transfer (CT) mechanism is concluded. # 2006 Elsevier B.V. All rights reserved. Keywords: SERS spectra; DFT calculations; RHF calculations; Rhodamine 123

1. Introduction Surface enhanced Raman scattering (SERS) and Surface enhanced resonance Raman scattering (SERRS) are now wellestablished techniques, which provide useful information’s about the nature and orientation of adsorbed molecular species and the adsorbate–metal interaction mechanism [1]. The origin of SERS is broadly explained in terms of electromagnetic [2] and chemical interactions [3,4]. To obtain SERRS, an analyte containing a chromophore is adsorbed onto a suitable roughened metal surface. A laser excitation frequency is chosen to coincide with the absorption frequency of the chromophore to give molecular resonance and the surface roughness of the metal is set so that the surface plasmon is in resonance at the same frequency to create surface enhancement [5]. Recently ab initio and density functional theory (DFT) calculations are successfully utilized to explain the adsorption behavior of molecules on metal surface and assignment of vibrational modes in Raman spectroscopy. The availability of a range of computational tools affords the experimentalist to use computational methods hand-in-hand with experiment to understand the structural and spectroscopic details of molecules [6,7].

* Corresponding author. Tel.: +91 33 24734971; fax: +91 33 24732805. E-mail address: [email protected] (G.B. Talapatra). 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.01.012

Rhodamine 123 (Rh123) is a biologically important molecule. It has been extensively employed as a fluorescence stain of mitochondria in living cell [8,9]. Its halogenated analogues are used as phosensitizer in Photodynamic therapy [10]. Photophysical properties of this biologically significant molecule and its application as photosentsitizers are reported elsewhere [11]. Detailed ab initio and DFT analysis of this molecule is rare. The enormous biological importance of Rh123 forces us to reinvestigate detailed experimental and theoretical vibrational analysis and SERRS spectra of the molecule. In this paper we investigate the adsorptive behavior of Rh123 on the colloidal silver surface and the nature of charge transfer between the molecule and the metal using FTIR and SERRS spectra together with ab initio and DFT calculations. This study may be helpful to understand the role of this molecule at biological interfaces. 2. Experimental and computational procedures 2.1. Chemicals and procedure Rhodamine 123 (Rh123) was purchased from Aldrich Chemical Co. and used after checking the purity by HPLC. Stable silver sol was prepared by the process of Creighton et al. [12]. The stable yellowish sol thus prepared shows a single

J. Sarkar et al. / Vibrational Spectroscopy 41 (2006) 90–96

extinction maximum at 394 nm and was aged for 2 weeks before being used in the experiment. The size of the silver particles in this sol is known to be in the range 1–50 nm [13]. All required solutions were prepared with distilled and deionized water from a Milli-Q-plus system of M/S Millipore Corporation, USA. Dilute HCl are used to adjust the pH value of the mixture, which was measured by a pH meter (Systronics pH meter model MK-VI, India) 2.2. Instrumentation Raman spectra were recorded by a Spex double monochromator (Model 1403) fitted with a holographic grating of 1800 grooves/mm and a cooled photomultiplier tube (Model R928/115) from Hamamatsu Photonics, Japan. The operation of the photon counter and data acquisition and analysis were controlled by Spex Datamate 1B. The acquisition time by the spectral element was 0.5 s. SERRS are taken at 514.5 nm excitation using Spectra Physics Ar+ ion laser (model 2020-05). To avoid the strong fluorescence background normal Raman spectra are recorded with excitation at 609 nm using a dye laser (model Spectra Physics: 375B) pumped with the Argon ion laser. Laser power used for excitation was 0.05 W. The scattered light was focused onto the entrance slit of width of 4 cm1. The accuracy in the measurement was 1 cm1 for strong and sharp bands and slightly less for other bands. The FTIR spectra of the powder samples were taken in a KBr pellet using a Nicolet Magna-IR 750 spectrometer series II. The resolution of the infrared bands was about 4 cm1 for sharp bands and slightly less for broader bands. All the spectra reported in the figures are original raw data directly transferred from the instrument and processed using the Microcal origin version 6.0. They are presented even without single smoothing. 2.3. Theoretical calculations The theoretical calculations were carried out using Gaussian 98 program for windows [14]. Optimization of the molecular structures and the calculations of the vibrational frequencies for the optimized structures were done by restricted Hartree–Fock (RHF) and density functional level of theory (DFT). The B3LYP functional [15] was used to the DFT calculations. The Pople split valance basis set [16] 6–31G are performed for both RHF and DFT level of theory. 3. Results and discussion 3.1. Molecular geometry of Rh123 The molecular structure of cationic Rh123 was optimized by ab initio RHF and DFT levels of theories. Selected optimized structural parameters of Rh123 molecule are listed in Table 1. In the process of geometry optimization for the fully relaxed method, convergence of all the calculations and the absence of imaginary values in the wavenumbers confirmed the attainment of local minima on the potential energy surface. It is observed from Table 1 the dihedral angles C6–C5–O10–C9 and O10–C9–

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Table 1 Relevant structural parameters of Rh123 molecule calculated from RHF/6-31G and B3LYP/6-31G levels of theories ˚) Bond lengths (A

C1–C2 C5–C6 C2–H27 C1–N25 C4–C7 C5–O10 N26–C13 N26–H43 C7–C15 C15–C20 C20–C21 C21–C22 C21–O23 O23–C24

RHF

DFT

1.426 1.371 1.072 1.346 1.406 1.359 1.346 0.992 1.499 1.401 1.482 1.219 1.331 1.455

1.433 1.382 1.085 1.359 1.416 1.383 1.359 1.008 1.498 1.416 1.484 1.243 1.365 1.475

Bond angles (8)

C6–C1–C2 C6–C1–N25 C6–C5–O10 C1–C2–H27 C4–C7–C8 C5–O10–C9 H43–N26–H42 C15–C20–C21 C15–C20–C19 C21–O23–C24 O23–C21–O22

RHF

DFT

119.17 121.01 117.20 118.99 119.69 122.88 117.08 120.42 119.54 120.23 122.64

119.02 121.04 116.58 119.01 119.34 121.07 117.23 119.73 119.66 117.24 122.64

Dihedral angles (8)

C6–C1–N25–H40 C4–C7–C15–C20 C7–C8–C11–C12 C19–C20–C21–O23

RHF

DFT

179.83 92.41 178.83 0.0

179.86 92.40 178.89 0.0

C14–C13 are nearly 1808. These indicate that the xanthene ring moiety of Rh123 molecule is almost planar in its electronic ground state. The estimated dihedral angles C6–C1–N25–H40 and C14–C13–N26–H42 are also 1808. These indicate that the externally attached amino groups lie in the plane of the xanthene ring moiety of the molecule. Geometry optimizations, using the above-mentioned level of theories further suggest that the angle between the xanthene plane and its adjacent phenyl-ring plane is nearly perpendicular (92.48/92.38 in the RHF/DFT), whereas the angle in the crystal structure is 87.968 [8]. This is in accordance with the recently reported structural parameter of Rhodamine 6G (R6G) molecule [17]. The DFT optimized structure of Rh123 molecule is shown in Fig. 1. 3.2. Normal Raman, FTIR spectra and vibrational analysis of Rh123 Rhodamine 123 molecule has 43 atoms; hence it has 123 fundamental vibrations. It belongs to CS point group and

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Fig. 1. The optimized structure of Rhodamine 123 molecule obtained from B3LYP/6-31G level of theory.

consequently 83 planer (A0 species) and 40 non-planer (A00 species) fundamental vibrations are expected to appear both in the Raman and in the FTIR spectra. The normal Raman spectrum of Rh123 molecule at 3.0 104 M aqueous solution is shown in Fig. 4(c). Fig. 4(a) and (b) represent the theoretical NRS spectrum of Rh123 molecule calculated from ab initio (RHF) and DFT levels of theories, respectively. It is to be emphasized that the calculated Raman spectrum represents vibrational signatures of molecules in their gas phase. Hence, the experimentally observed Raman spectrum of the solid and solution may differ significantly from the calculated spectrum [18]. Moreover, the ab initio (RHF) calculations typically predict larger harmonic vibrational wavenumbers than the ones observed experimentally [19]. Thus, the restricted Hartree–Fock (RHF) vibrational wavenumbers presented in Table 2 have been uniformly scaled by the scaling factor of 0.8953 [20]. In DFT calculations, the B3LYP functional also tends to overestimate the fundamental modes compared to the experimentally observed values [20,21]. In order to obtain a considerable better agreement with the experimental data, scaling factors have to be used [20,21]. Thus a scaling factor of 0.9614 has been uniformly applied to the B3LYP calculated wavenumbers [20]. The observed disagreement between the

Table 2 Observed and calculated Raman, IR and SERS bands of Rh123 molecule and their tentative assigments FTIR (observed)

1649 1592

1541 1513 1475 1409 1362 1278 1235 1185 1165 1132 1081 958 916 848 812 762 747 705 662 632 594 570 457

NRS solution (observed)

1642 1591 1578 1558 1548 1518 1500

m w sh m sh w s

1361 s 1272 m 1179 w 1169 m

940 w 914 w

NRS (calculated) RHF

DFT

1639 1596 1583 1557 1532 1516 1496 1466 1388 1334 1290 1244 1195 1161 1139 1088 962 944 923 855 804 749 703 665 628 602

1645 1606 1584 1559 1538 1515 1498 1474 1404 1360 1278 1237 1195 1171 1121 1072 960 938 915 850 820 760 748 707 659 620 602

450 410 339

459 420 351

757 m

628 m

418 s 343 s

Abbreviations: XR, xanthene ring; PHR, phenyl ring; o.p., out of plane.

SERS

Tentative assignment

Symmetry species

1650 1601

XR stretching XR stretching PHR stretching

A0 A0 A0

XR stretching XR stretching XR stretching XR stretching XR stretching XR stretching C–H bending of XR C–H bending of XR C–H bending of XR

A0 A0 A0 A0 A0 A0 A0 A0 A0

C–H bending of XR PHR stretching C–H bending of XR XR + PHR stretching XR + PHR stretching C–H o.p. bending of XR C–H o.p. bending of XR

A0 A0 A0 A0 A0 A00 A00

711 671 636 597

C–H o.p. bending of XR C–H o.p. bending of PR C–H o.p. bending of XR XR + PHR stretching –NH2 wagging

A00 A00 A00 A0 A00

447 420 352

C–H o.p. bending of XR XR deformation + NH2 oscillation XR stretching

A00 A0 A0

1561 1548 1507 1362 1357 1273 1195

1081 946 920

765


Fig. 2. FTIR spectrum of Rh123 molecule of neat powder in KBr pellet.

theory and the experiment could be a consequence of the anharmonicity and of the general tendency of the quantum chemical methods to overestimate the force constants at the exact equilibrium geometry [22]. Nevertheless, after applying the respective scaling factors on the ab initio and DFT normal mode calculations, as one can see from Table 2, the theoretical

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calculations reproduce the experimental data well and allow us to assign the vibrational modes. The observed FTIR spectrum of the powder sample in a KBr pellet is shown in Fig. 2. Table 2 lists the FTIR, NRS and SERRS band frequencies of the molecule along with their tentative assignments and symmetry species. In assigning the vibrational frequencies of Rh123 molecule, the visual inspection of the normal modes animated from the output files of ab initio and DFT calculations using Gauss View 3.0 and Molekel 4.2 [23,24] program have been considered. The available literature concerning the vibrational assignment of this [25] and related molecules [17,26] are also consulted. The modes principally arising from the xanthene, phenyl rings and the externally attached –NH2 group of Rh123 molecule are identified. Interesting observation can be drawn regarding the assignments of some vibrational bands centered at around 1500 (calcd. 1498/1496 cm1 in DFT/RHF) and 1592 cm1 (calcd. 1606/1596 cm1 in DFT/RHF). The 1592 cm1 mode is strong in the FTIR but appears as weak but prominent band in the observed NRS. The other band at 1500 cm1 is strong in the experimental NRS but does not appear in the FTIR spectra. The visual inspection of these normal modes indicates that the vibrations principally represent

Fig. 3. Cartesian displacement and calculated (B3LYP/6-31G) vibrational modes of Rh123. The numbers in the parentheses referred to the experimental value of the assigned band.

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the stretching vibrations of the xanthene ring moiety of the molecule. Previously these bands were ascribed to externally attached phenyl ring stretching vibration of Rh123 molecule. There is a discrepancy in the assignment of another band centered at 1272 cm1 (calcd. 1278/1290 cm1 in DFT/RHF). This band is prominent both in the Raman and in the FTIR spectra and has been assigned to the in-plane C–H bend of the xanthene ring moiety of the molecule. Previously, this band was assigned to the C–O–C stretching mode of the xanthene ring of Rh123 molecule [25]. Inference can be drawn regarding the assignments of the vibrational bands centred at around 662 cm1 (calcd. 659/665 cm1 in DFT/RHF), 705 cm1 (calcd. 707/703 cm1 in DFT/RHF), 812 cm1 (calcd. 820/ 804 cm1 in DFT/RHF) and 848 cm1 (calcd. 850/855 cm1 in DFT/RHF). All these bands are absent in the Raman spectrum but appear in FTIR. Generally out-of-plane modes appear strong in the infrared and weak in the Raman [27,7]. Considering this fact, 662, 812, 848 and 705 cm1 bands are ascribed to C–H out-of-plane bend of the xanthene ring and the phenyl ring, respectively, of Rh123 molecule. The visual inspection of the above modes also substantiates this conjecture. The Cartesian displacements and normal modes of some selected vibrations calculated from DFT calculation are shown in Fig. 3. 3.3. SERRS spectra of Rh123 The SERRS spectra of Rh123 molecule at 3.0 107 M adsorbate concentration at pH 2 with 514.5 nm excitation is shown in Fig. 4(d) . The SERRS spectra oh Rh123, recorded under identical experimental condition at neutral pH, is characterized by strong fluorescence background overshadowing the Raman signal. The fluorescence background is reduced significantly at pH 2 with the appearance of sharp Raman bands. Detailed analysis of the pH dependent SERRS spectra of Rh123 molecule adsorbed on silver hydrosol are reported elsewhere [25]. Scanning through SERS spectra immediately reveal that the prominent SERS bands principally represent the in-plane vibrational modes of the xanthene and the phenyl rings moieties of Rh123 molecule. Significant enhancements are observed for the bands centered at around 1650, 636 and 352 cm1 in the SERRS spectra assigned to the stretching vibrations of the xanthene ring moiety of the molecule. In fact these bands also show a 8–10 cm1 blue shift in comparison with its NRS counterpart in solution. Interestingly we observe that the vibrational signatures principally contributing from external phenyl ring moiety of the molecule in the SERES spectra exhibit the same band shapes and show very small or no shift compared to its NRS counterpart in solution. These observations may signify considerable interaction of the xanthene ring moiety and weaker or no interaction of external phenyl ring moiety of Rh123 molecule with the colloidal silver surface. The xanthene ring moiety of the molecule, however, can bind to the silver surface through the lone pair electrons of oxygen (O10) and nitrogen (N25 and N26) atoms of the externally attached amino group. In order to find the probable adsorptive sites of the

Fig. 4. Background-corrected (a) the theoretical gas-phase Raman spectrum calculated using RHF method; (b) using DFT method; (c) Raman spectra of aqueous solution (3.0 104 M) of Rh123 at pH 6.5 for lexc = 609 nm; (d) SERS spectrum of 3.0 107 M Rh123 adsorbed in silver hydrosol at pH 2 for lexc = 514.5 nm.

molecule on the silver surface, we estimate the negative charge density on each of these atoms [28,29]. The higher is the negative charge density on the atom, the higher is the probability of it to act as an adsorptive site for silver substrate. Theoretical results estimated from DFT/RHF ab initio calculations show that the


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Mullikan atomic charges on the N25, N26, and O10 atoms are 0.788/0.980, 0.788/0.980, and 0.577/0.821, respectively. Thus the negative charge density is observed to be appreciable on both the nitrogen atoms as well as on the oxygen atom of the xanthene ring moiety of the molecule. The theoretical results, therefore predict active involvement of N25, N26, and O10 atoms of Rh123 molecule in the adsorption process, though the existence of Ag–N or Ag–O stretching vibrations are not recorded in the SERS spectra. 3.4. Electronic absorption spectra of silver colloid with added Rh123 Fig. 5 shows the room temperature UV–vis electronic absorption spectra of Ag sol, as prepared, before and after the addition of Rh123 molecule at 3.0 107 M adsorbate concentration. The pure stable silver sol shows a single extinction maximum at 394 nm. When 3.0 107 M Rh123 is added to the sol at pH 2, the extinction maximum at 394 nm diminishes with the appearance of a broad hump in the longer wavelength region. The appearance of this broad hump at longer wavelength region is attributed to the coagulation of colloidal silver particles in the presence of the adsorbed molecules [30,31]. Alternatively such a band has been ascribed to a charge transfer (CT) band due to molecule–metal interaction [13,32,33]. The frontier orbital theory plays a significant role in order to understand the CT mechanism of SERS and SERRS [34,35]. Two types of CT mechanisms are predicted. One is molecule to metal and other is metal to molecule. Molecule to metal CT excitation occurs when an electron is transferred from highest occupied molecular orbital (HOMO) of the adsorbate to the Fermi level (EF) of the metal. Conversely, transfer of an electron from EF of the metal to the lowest unoccupied molecular orbital (LUMO) results in metal to molecule charge transfer. In order to introspect the direction of CT interaction, the HOMO, LUMO, LUMO + 1 energies of Rh123 molecule have been estimated from the DFT calculation. The theoretical results show that the HOMO, LUMO, and LUMO + 1 energies of the molecule are 8.71, 5.71 and 4.22 eV,

Fig. 6. Calculated (i) HOMO (ii) LUMO (iii) LUMO + 1 of Rh123 with B3LYP/6-31 G (Isocontour 0.02 a.u.).

respectively, which are energetically much lower than the EF of the silver (+5.48 eV) [36]. Hence, we conclude that metal to molecule CT interaction is more preferred in our case. The electron is probably transferred from metal to the LUMO of the molecule. The HOMO, LUMO and LUMO + 1 orbitals of the molecule are shown in Fig. 6. It is observed that the HOMO and LUMO are mainly localized in the xanthene ring moiety and LUMO + 1 is localized on the phenyl ring moiety of Rh123 molecule. The transfer of electron from the Fermi level of silver to the LUMO of the molecule may perturb the electronic charge density in the xanthene ring of the molecule. This may results in the blue shift of certain SERRS bands principally contributing from the xanthene ring moiety of Rh123 molecule. Interestingly, the SERR spectrum of the molecule is characterized by the enhancement of totally symmetric A0 species. This may indicate that Albrecht ‘A’ term i.e. the Frank–Condon term may play a dominant role in CT interaction [4,37]. 4. Conclusion

Fig. 5. Room temperature UV–vis absorption spectra of (a) pure silver sol; (b) sol with added Rh123 (concentration 3.0 107 M) at pH 2.

The adsorption behavior on colloidal silver particles of enormous biologically importance Rhodamine 123 molecules has been investigated by SERRS spectroscopy. The observed Raman and infrared bands of this molecule are satisfactorily assigned on the basis of ab initio (RHF) and DFT calculations employing a high-level basis set. Some vibrational modes of the free molecule have been reassigned on the basis of above mentioned visualization program. From HOMO, LUMO energies calculations, we conclude that metal to molecule CT interaction is more preferred in this case and enhancement calculations predicts that in the CT interaction, Albrecht ‘A’ term play a significant role.

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Acknowledgements We thank DST, Government of India (Project no.: IDP/Sen/ 94/03) for partial financial support. J.C. thanks the University Grants Commission (UGC), Government of India for financial support through the minor research project (MRP Project no.: PSW-089/03-04). References [1] P.K. Chang, T.E. Furtak, Surface Enhanced Raman Scattering, Plenum Press, New York, 1982. [2] J.A. Sanchez-Gil, J.V. Garcia-Ramos, Chem. Phys. Lett. 367 (2003) 361. [3] P. Kambhampati, M.C. foster, A. Campion, J. Chem. Phys. 110 (2000) 551. [4] J.R. Lombardi, R.L. Birke, T. Lu, J. Xu, J. Chem. Phys. 84 (1986) 4174. [5] M. Campbell, S. Lecomte, W.E. Smith, J. Raman Spectrosc. 30 (1999) 37. [6] T. Iliescu, D. Maniu, V. Chis, F.D. Irimie, Cs. Paizs, M. Tosa, Chem. Phys. 310 (2005) 189. [7] J. Sarkar, J. Chowdhury, M. Ghosh, R. De, G.B. Talapatra, J. Phys. Chem. B 109 (2005) 12861. [8] I.C. Summerhayes, T.J. Lampidis, S.D. Bernal, J.J. Nadakavukaren, K.K. Nadakavukaren, E.L. Shepard, L.B. Chen, Proc. Natl. Acad. Sci. U.S.A. 79 (1982) 5292. [9] L. Villeneuve, P. Pal, G. Durocher, D. Girard, R. Giasson, L. Blanchard, L. Gaboury, J. Fluoresc. 6 (1996) 209. [10] B.W. Henderson, T.J. Douglherty, Photochem. Photobiol. 55 (1992) 145. [11] P. Pal, H. Zeng, G. Durocher, D. Girad, T. Li, A.K. Gupta, R. Giasson, L. Blanchard, L. Gaboury, A. Balassy, C. Turmel, A. Laperriere, L. Villeneuve, Photochem. Photobiol. 63 (1996) 161. [12] J.A. Creighton, C.G. Blatchford, M.G. Albrecht, J. Chem. Soc. Faraday Trans. 275 (1979) 790. [13] J. Chowdhury, M. Ghosh, T.N. Misra, Spectochim. Acta A 56 (2000) 2107.

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