SERS Spectra of Alizarin AnionâAgn (n = 2, 4, 14 ... - ACS Publications

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The SERS spectra of Alizarin anion-Ag (n=2, 4, 14) Systems: TDDFT calculation and comparison with the experiment Cristiana Lofrumento, Elena Platania, Marilena Ricci, Maurizio Becucci, and Emilio M. Castellucci J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12321 • Publication Date (Web): 19 May 2016 Downloaded from http://pubs.acs.org on May 20, 2016

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The Journal of Physical Chemistry

The SERS Spectra of Alizarin Anion - Agn (n=2, 4, 14) Systems: TDDFT calculation and comparison with experiment Cristiana Lofrumento,† Elena Platania,†,¶ Marilena Ricci,† Maurizio Becucci,∗,†,‡ and Emilio M. Castellucci†,‡ †Dept. of Chemistry "Ugo Schiff", University of Florence, Scientific Campus, via della Lastruccia 9, 50019-Sesto F.no, Italy ‡European Laboratory for Non Linear Spectroscopy (LENS), University of Florence, Scientific Campus, via N. Carrara 1, 50019-Sesto F.no, Italy ¶Current address: Department of Chemistry, University of Oslo, Sem Sælands vei 26, 0371, Oslo, Norway E-mail: [email protected] Phone: +39 055 457 3089

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Abstract Using density functional theory (DFT) and various cluster models for the simulation of the interaction between alizarin and silver nanoparticles, we calculated the SERS spectra of the AZ anion-Agn (n=2, 4, 14) complexes and compared the results with experiment. The analysis of the calculated SERS spectra helped distinguishing the contribution of the chemical and electromagnetic mechanisms to the spectral enhancement, under the assumption that the excitation energies of the clusters are comparable with the local plasmon energies of nanoparticles. The results show a certain dependence of the relative Raman intensities and peak positions on the silver cluster size. Calculation of UV-VIS transition energies and Raman spectra of the complexes have been performed under the assumption that the AZ anion is bounded to the silver clusters through the oxygen atoms of the C=O groups in 1,9 positions, in a edge-on perpendicular orientation. The calculated SERS spectra show an acceptable similarity with the experimental SERS spectra carried out with excitation at 632 nm. The results of the calculation under pre-resonance condition with respect to chromophore located and cluster located excitations are compatible with mechanisms of enhancement acting on different parts of the AZ anion molecule. The calculations were performed using the B3LYP hybrid density functional. A 6-31g(d) basis set for H, C, O and LANL2DZ basis set for Ag were used. Vertical excitation energies and the corresponding oscillator strengths were calculated by means of time dependent density functional theory (TDDFT).

Introduction Alizarin, 1,2-dihydroxyanthraquinone, has for a long time garnered great attention in the field of conservation and in medicine due to its interesting photoactivity and chromatic properties. Miliani et al. 1 characterized the absorption and fluorescence spectra of alizarin and its monoand di-ionized species as a function of pH. Alizarin is the most important constituent of madder lake, where alizarin is linked to a metal mordant, usually Al based, in order to be fixed on a substrate such as a fabric or wood. As a dye, alizarin has been extensively employed in Asia, Egypt and in pre-colombian America since ancient times for dyeing textiles. Today, AZ is found 2 ACS Paragon Plus Environment

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in artists’ paints as synthetic madder. As a biologically active molecule, AZ has remarkable antigenotoxic activity as it is an inhibitor of the human recombinant cytochrome P450 isozymes. AZ, as a component of food, can act against the action of carcinogens. AZ has been considered an active sensitizer for ultrafast photoinduced electron transfer in association with semiconductors, mainly TiO2 2,3 . Theoretical DFT analyses of the UV-Vis absorption and emission properties of AZ 4,5 and its complexes with different metal atoms 4,6 have been reported. In recent years many experimental surface enhanced Raman scattering (SERS) spectra of alizarin and its mono- and di-anion species adsorbed on silver nanoparticles have been published 7–14 . DFT theoretical analyses of the vibrational spectroscopic properties of alizarin and its anions were also carried out by limiting the calculation to the bare molecular species 8–10,15 . Baran et al. 9 also reported on the calculated static Raman spectra of the AZ-Ag1 complex. Lofrumento et al. 15 simulated the static normal Raman spectra (NRS) and the resonance Raman (RR) spectra of AZ and its anions. Under the reasonable assumption that no major changes are induced in the molecular species by adsorption on nanoparticle surfaces, a comparison with the experimental SERS spectra obtained at different laser wavelength excitations was attempted. Although the simulation by Lofrumento et al. 15 of the static Raman and RR spectra of alizarin and its anionic species has provided useful elements for the description of the mechanisms of the enhancement occurring in the SERS spectra, it made clear the necessity of a more complete treatment which should consider the whole metal-molecule systems. A thorough theoretical study of the whole molecule-metal system is then in order to obtain a better understanding of the spectroscopic properties of the SERS spectra and of the importance of the different mechanisms, i.e., electromagnetic and chemical, which are taken as responsible of the spectral enhancements observed. The electromagnetic mechanism results from the excitation of the localized surface plasmon resonances (LSPR) induced by the incident laser light. The effect is independent on the nature of the molecule and usually is largely responsible of the SERS enhancement. The chemical



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mechanisms, which arise from the close chemical adsorption of the molecule on the metal surface, can be separated into different contributions, i.e., a non resonant (static) chemical mechanism, a charge transfer (CT) mechanism and a resonant mechanism within the adsorbed molecule itself. This last effect is especially important as it can provide a boost to the SERS intensity due to the electromagnetic enhancement mechanism. The CT chemical mechanism was first devised after the observation that Raman resonances of the adsorbed molecules could be obtained by applying different electric potentials to the silver electrodes. The CT excitation can occur in either direction, depending on the relative energies of the metal Fermi level and the HOMO and LUMO energies of the adsorbed molecule 16–18 . The modeling of the electromagnetic effect on the SERS spectra is a relatively established field of study, whereas a debate is still on as of the contribution of the different chemical mechanisms to the overall SERS enhancement and the Raman spectral shape. Recently, SERS surface active sites, such as silver ad-atoms and ad-clusters of metal interacting with adsorbates, have been adopted as useful models for the understanding of the SERS enhancement mechanisms 19 . The cluster model, in conjunction with DFT calculation, was employed for the characterization of a variety of molecule-metal cluster complex systems and to predict their excitation energies and SERS spectra 20–24 . It has also been proposed that silver clusters, with different number of atoms, can be used to simulate the silver nanoparticle to which the organic molecule is adsorbed 19,20,25 . More recently, the time dependent density functional theory (TDDFT) method has been used to predict the dependence of SERS on the excitation wavelength 19,26,27 . In our study we dealt with the alizarin anion (AZ anion) interacting with silver clusters to form molecule-metal complexes. The goal of the study was the obtainment of the SERS spectra of the whole molecule-cluster systems and their dependence on the cluster dimension, for a better comparison with the experimental SERS spectra. The cluster model’s ability to reproduce the SERS enhancement factors has been also evaluated. The whole study went through different steps. First a structure optimization of the Agn clusters was performed. For each cluster a complex with the alizarin ion molecule was sketched


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whose structure was further fully optimized. The fully optimized geometries were further adopted in the transition energies and Raman spectra calculations. Wavelength independent (static) and wavelength dependent SERS spectra were obtained for each complex. In the second case pre-resonance excitations were chosen to simulate either molecular resonance (SE[R]RS) or plasmon resonance (SERS) excitation, respectively.

Computational details Calculation was performed using the Gaussian 09 suite of programs 28 . We first proceeded to optimize the alizarin anion structure using the DFT/B3LYP functional with a 6-31g(d) basis set. The optimized structure of the mono-anionic form of alizarin, which has been used in this study, is shown in Fig. 1, where the standard atom numbering and reference axis system have been adopted. The structures of the Ag2 , Ag4 and Ag14 were also optimized with the same functional but with a LANL2DZ basis set. The structure of the n = 14 silver cluster corresponds to the primitive cell of silver metal, i.e., face centered cubic. Y

10

4

5

3

6

2

7

X

1

9

8

Z

Figure 1:

Optimized structure of the ionic form of alizarin; hydroxyl in position 1 has been ionized

The alizarin anion-silver cluster complexes, hereafter AZ anion-Agn (n=2, 4, 14), initial geometries have the AZ anion C = O groups in 1, 9 position (see Fig. 1) facing the silver



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cluster, an interaction geometry corresponding to that proposed by Ca˜ namares et al. 8 and Baran et al. 9 for alizarin adsorbed on the silver nanoparticle surface. The basis sets 6-31g(d) for C H O atoms and LANL2DZ for Ag were used. Geometry optimization of the AZ anionAgn complexes, the calculation of static Raman and SERS spectra of the complexes were performed with the DFT method at the level of hybrid density functional B3LYP. The vertical excitation energies of the AZ anion-Agn complex systems were calculated with time-dependent density functional theory (TDDFT). The calculation was extended over 30 excited states. All geometries were fully optimized except for AZ anion-Ag14 . Here only the Ag atoms facing close the C = O groups of the AZ anion moiety in position 1, 9 were allowed to follow the optimization process. The rest of the cluster structure was kept frozen during the process, a procedure proposed by Ding et al. 29 in the case of complexes with large silver clusters. Although the complex could not correspond to a global minimum, the reasonably fast convergence of the quantum chemical calculations and the absence of imaginary vibrational frequencies (see below) are strong indications that the structure corresponds or is close enough, to a local minimum and not to a saddle point of the potential energy surface (PES). SERS spectra were calculated under pre-resonance condition with respect to excitation energy maxima corresponding to transitions occurring on the molecular AZ anion chromophore group of the complex, thus obtaining the surface enhanced resonance Raman spectra or SE[R]RS, where [R] stands for molecular resonance. This can be named as a chemical enhancement mechanism. Pre-resonance Raman spectra calculation has been also performed with respect to the strongest calculated electronic transitions of the metal cluster engaged in the molecule-metal complex. This calculation is intended to simulate the SERS enhancement mechanism due to the localized surface plasmons, holding however that these excitations are electronic transitions of the metal clusters. A scaling factor for vibrational frequencies of 0.9768 was used throughout. On the basis of the optimized geometries, the static/dynamic Raman scattering activity of the vibrational modes (scattering factors Si ) can be directly obtained from calculation, and then the absolute Raman intensities were estimated by the differential Raman scattering cross section (DRSCS) given by:


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(

h (˜ νo − ν˜i )4 Si dσ )i = (2π)4 2 dΩ 8π c˜ νi 1 − exp(−hc˜ νi /kB T ) 45

(1)

where ν˜o and ν˜i are the wavenumbers of incident light and the ith vibrational mode, respec4

tively. Si is the static/dynamic Raman scattering factor (in Å /amu) with respect to the ith vibrational mode, where static means static field perturbation corresponding to normal Raman spectrum (NRS), while dynamic means the time-harmonic field perturbation corresponding to the frequency-dependent Raman spectrum (SERS).

Result and Discussion Geometry and excitation energies of silver clusters Many experimental and theoretical studies have been addressed to the structural, electronic and optical properties of small silver clusters 30 . Geometry optimizations were performed for the Agn clusters with n= 2, 4, 14, assuming for n = 4 a planar D2h structure and for n = 14 a cubic Td structure corresponding to the fcc structure of the elementary cell of the Ag crystal lattice. The optimized cluster structures were employed further for the calculation of their excitation energies. The most important lowest energy excitations for the Agn (n = 2, 4, 14) clusters are listed in Table S1 of the Supporting Information section. The agreement with the calculated UV-Vis spectra of Ag2 and Ag4 reported by Bonaˇ ci´ c-Koutecky et al. 30 and Jensen et al. 20 seems quite satisfactory. No reasonable comparison was found for the Ag14 cluster calculated absorption spectrum.

Geometry and excitation energies of the AZ anion-silver cluster complexes The full geometry optimization of the complexes (for the AZ anion-Ag14 complex, an almost full optimization, as described above) resulted in the structures depicted in Fig. 2. After optimization, the alizarin ion moiety remains planar in the complexes. In the starting schemes 7 ACS Paragon Plus Environment


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for AZ anion-Agn , with n = 2, 4, the clusters were positioned coplanar with respect to the AZ anion molecule. The optimized structures exhibit the following schemes: the Ag2 dimer is tilted with respect to the molecule plane but now pointing with one Ag atom halfway between the two C = O groups in position 1, 9; the Ag4 cluster plane revolved almost 90◦ with respect to the molecule plane. The Ag14 cluster position resulted slightly offset with respect to the AZ anion molecular plane. The structures of all the complexes correspond or are, at least, close to PES minima. The interaction between the AZ anion and the silver clusters leads to the formation of chelate rings engaging one silver atom of the cluster and the two oxygen atoms belonging to the carbonyl groups in position 1 and 9. The distances between Ag and O were almost the same (see Fig. 4). The final Ag-O distances for the O atoms of the carbonyl groups in position 1 and 9 are as follows: 2.359 and 2.489 Å, 2.326 and 2.424 Å, 2.312 and 2.399 Å, for the Ag2 , Ag4 and Ag14 cluster, respectively.

b)

a)

c)

Figure 2: Optimized geometries of the AZ anion-Agn (n = 2, 4, 14) complexes

The optimized structures were further employed for the calculation of the excitation energies via a vertical excitation TDDFT calculation. Fig. 3 presents the simulated absorption spectra of the different AZ anion-Agn (n = 2, 4, 14) complexes. The calculated excitation energies occur 50 nm to the blue of the experimental maximum absorption of the AZ anion molecule in solution at pH 8-9, that occurs at about 550 nm 1,8 . The convoluted shape of the absorption spectra, especially those regarding the transitions in the clusters, changes as a function of the


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cluster size. Where the chromophore excitation energies do not change, those relating to clusters increase in number and mix intimately with those of the anthraquinone chromophore. This has to be taken into account in the pre-resonance calculation of the SERS spectra, especially in the case of the AZ anion-Ag14 complex.

c )

In te n s ity ( a r b . u n its )

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b )

a )

4 0 0

4 5 0

5 0 0

W

5 5 0

6 0 0

6 5 0

a v e le n g th /n m

Figure 3: Plot of the calculated excitation energies of the AZ anion-Agn complexes. A FWHM of 0.2 eV corresponding to about 50 nm @ 500 nm was applied to the spectra. A shift to the red of 50 nm was also applied to the spectra to comply with the experimental maximum of AZ anion absorption. Arrows indicate the pre-resonance wavelengths: thicker arrows=pre-resonance with AZ anion chromophore excitation energies; thin arrows=pre-resonance with cluster located excitation energies; a) AZ anion-Ag2 , b) AZ anion-Ag4 , c) AZ anion-Ag14

The interactions between the alizarin anion and the silver clusters lead to new states which correspond to excitations from the Agn cluster MOs to the molecule MOs or vice-versa. These new charge-transfer (CT) states exhibited very small oscillator strengths. The energies of the main CT transitions are reported on Table 1. Here the MOs involved in the excitation, the corresponding energy/wavelength and oscillator strength, the detailed orbital origin of the electronic transitions in terms of the dominant (see coefficients) orbital character and the molecular orbital localizations are reported. A calculation of dynamic polarizabilities, in preresonance conditions with respect to the main CT excitation energies, should bring about a Raman signal enhancement of chemical type in the SERS spectra. However, due to the very small oscillator strengths of the CT transitions a rather tiny enhancement was expected in the SERS spectra. For this reason we did not run pre-resonance SERS spectra calculation pointing 9 ACS Paragon Plus Environment


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explicitly to these transitions. In Table 1 some calculated optical excitations of the AZ anion-Agn complexes with a predominant silver cluster (Ag) and molecular (AZ anion) character are reported. Although the calculation corresponds to the whole system, i. e., metal cluster plus molecule, we tried to distinguish the transitions inside the cluster from those pertaining mainly to the molecular chromophore. This can be done by a careful inspection of the charge distribution of the MOs involved in the excitations (see for example Fig. S1 of the Supporting Information section). The results are shown in the right most column of Table 1. Table 1:

Most intense molecular and cluster excitation energies for the different AZ anion-Agn complexes as obtained from the TDDFT calculation. MOs stands for molecular orbitals, H for HOMO, L for LUMO, f is the oscillator strength

Complex

MOs

AZ anion-Ag2

81→82 80→82 81→83 81→84 100→101 99→101 100→104 100→103 98→102 98→101 195→199 192→196 194→199 194→200 195→201 192→199 192→200 193→202 193→201

AZ anion-Ag4

AZ anion-Ag14

Energy/wavelength (eV/nm) 1.52/816 2.45/506 2.61/459 3.03/409 1.39/889 2.47/501 2.58/480 2.62/474 2.74/451 3.21/386 1.63/759 1.80/688 1.84/671 1.92/644 2.17/572 2.48/499 2.48/498 2.58/480 2.66/466

f 0.0012 0.1327 0.0005 0.5562 0.0018 0.1269 0.0004 0.4966 0.4291 0.0029 0.0007 0.0041 0.0011 0.0201 0.0304 0.0788 0.0048 0.2169 0.1172

Wave function 0.706|H→L> 0.697|H-1→L> 0.702|H→L+1> 0.682|H→L+2> 0.704|H→L> 0.697|H-1→L> 0.702|H→L+3> 0.466|H→L+2> 0.519|H-1→L+1> 0.504|H-2→L> 0.703|H→L+3> 0.701|H-3→L> 0.703|H-1→L+3> 0.642|H-1→L> 0.587|H→L+5> 0.688|H-3→L+3> 0.693|H-3→L+4> 0.504|H-2→L+6> 0.595|H-2→L+5>

Description AZ anion→Ag AZ anion→AZ anion Ag→AZ anion Ag→Ag Ag→AZ anion AZ anion→AZ anion Ag→AZ anion Ag→Ag Ag→Ag Ag→AZ anion Ag→AZ anion AZ anion→Ag Ag→AZ anion Ag→Ag Ag→Ag AZ anion→AZ anion AZ anion→Ag Ag→Ag Ag→Ag

SERS spectra calculation Although plenty of spectroscopic Raman data exist on AZ and its anions, as reported in the Introduction section, very few calculations exist on the SERS spectra of the whole moleculemetal cluster system. To our knowledge only Baran et al. 9 reported on calculation of the static Raman spectrum of the AZ-Ag1 complex. On the other side a rich literature exists on electronic structure methods for the study of SERS spectra of molecule-metal cluster systems 19,20,25,31,32 . We applied this approach to the case study of alizarin adsorbed on silver 10 ACS Paragon Plus Environment

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metal nanoparticles, to simulate, by the cluster model of molecule-metal interfacial complexes, its spectroscopic and structural properties. The simulation of the SERS spectra of AZ anion adsorbed on silver clusters should help evaluating the importance of the three different types of enhancement mechanisms: ground state (chemical), molecular resonance (chemical) and electromagnetic (physical). The first mechanism is attributed to ground state interaction after complexation of the molecule with the metal, the second is due to the amplification of the Raman signal when exciting in pre-resonance condition with respect to an electronic transition of the anthraquinone chromophore and the third amplification mechanism is linked to the augmentation of the incident and scattered electric fields after the interaction of excitation light with the localized surface plasmons. The calculated static SERS spectra of the AZ anion-Agn complexes are shown on Fig. 4 together with the calculated static (NRS) spectrum of the lone AZ anion species. The calculated spectra show a fair agreement with the experimental spectra obtained far from resonance by Ca˜ namares et al. 8 , Lofrumento et al. 15 and Baran et al. 9 at alkaline pH.

d )


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c )

b )

a )

2 0 0

4 0 0

6 0 0

W

8 0 0

1 0 0 0

1 2 0 0

a v e n u m b e r/c m

1 4 0 0

1 6 0 0

1 8 0 0

-1

Figure 4:

The calculated static NRS spectra of AZ anion and SERS (SE-NRS) spectra of the AZ anion-Agn complexes. Bands are given a Lorentzian width of 10 cm1 : a) AZ anion; b) AZ anion-Ag2 ; c) AZ anion-Ag4 and d) AZ anion-Ag14

It is worth mentioning that in our calculation the solvent effect has not been considered and this can be the origin of the observed frequency and intensity differences in the comparison with 11 ACS Paragon Plus Environment


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the experimental data. As far as the calculated frequencies of the AZ anion-Agn complexes are concerned we made a comparison of our results with those reported by Baran et al. 9 for the alizarin-Ag1 complex. The agreement between the spectra was unsatisfactory, especially in the case of the major features of the spectra. The calculated SERS spectra of the AZ anion-Agn complexes show a band at about 180 cm−1 , which can be assigned to the Ag-O stretching mode. The experimental value is often reported at little higher frequency 10 . In fact, due to the chelate ring structure involving one Ag atom and two oxygen atoms, more than one Ag-O vibration is expected as can be deduced from the potential energy distribution (PED) analysis, in terms of internal coordinates, of the low frequency modes of the AZ anion-Agn complexes 9 . Table 2:

Shifts (∆ν, cm−1 ) and Enhancement Factors (EFs) for selected bands of the calculated SERS spectra of AZ anion-Agn complexes, with respect to the bands of the NRS spectrum of the AZ anion. The calculation of the EFs has been performed on the basis of the Raman activity (A4 /amu) of the bands. Numbers in parentheses in the PED column indicate atom positions (see Fig. 1).

AZ anion-Ag2 AZ anion-Ag4 EF Freq.(*) ∆ν static 570nm 445nm ∆ν static 1241 11 2 1634 198 12 2 1252 7 6 6434 860 7 6 1290 3 1 307 116 3 1 1314 -5 2 3939 482 -5 2 1367 7 6 309 758 6 5 1410 3 2 1074 51 2 2 1443 -5 1 640 157 -6 1 1459 3 4 2246 365 3 3 1479 4 2 2851 378 5 2 1521 -12 1 342 6 -16 1 1574 -40 8 2177 61 -40 9 1588 1 3 1911 350 1 4 1632 -2 1 349 932 -1 3 1658 -16 4 374 428 -18 5 1671 3 39 5183 1658 5 40

EF

AZ anion-Ag14 EF 570nm 480nm ∆ν static 570nm 480nm PED 1016 561 11 2 589 1152 ring+δCH+δO-H 3790 1199 7 9 2724 2094 ring+δCH 100 1387 2 1 45 592 ring+δCH+νC-O(2) 2419 422 -6 1 1138 804 ring+δCH+δO-H 203 337 5 14 1011 662 ring+δCH 638 444 1 2 338 357 ring+δCH 502 4021 -8 2 656 5066 ring+δCH+δO-H 985 7898 3 4 437 7164 ring+δCH+δO-H 1814 138 5 2 824 267 ring+δCH 214 818 -25 1 168 565 ring+νC=O(1)+δO-H+δCH 1919 8208 -42 8 1386 1702 ring+δCH+νC=O(1,9)+δCH 889 4591 -2 7 1484 11594 ring+δCH+νC=O(1,9,10) 273 8359 -4 10 2893 8043 ring+δCH+νC=O(1,9,10)+δO-H 269 3575 -23 3 669 623 ring+νC=O(1,9,10) 2992 25297 7 30 3898 23452 ring+νC=O(1,9,10)+δO-H

δ=in plane bending, ν=stretching (*)calculated NRS frequencies of AZ anion; Freq. in cm−1

As mentioned in previous sections, the SERS spectra calculation in pre-resonance condition with respect to molecular electronic transitions and cluster electronic transitions may both lead to enhancement of the Raman band intensities. Calculation of Raman spectra in pre-resonance conditions with respect to the electronic transitions which pertain to the clusters may simulate the SERS electromagnetic enhancement 20 . The detailed decomposition in enhancement mechanisms that we are claiming here appears reasonable if the electronic transitions were well 12 ACS Paragon Plus Environment

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Table 3: Average shifts of harmonic frequencies and average SERS Enhancement Factors (EFs) for the different AZ anion-Agn (n=2, 4, 14) complexes, for selected vibrations calculated in the spectral range 1200 - 1700 cm−1 Complex

∆ν (cm−1 )

EFstatic SERS

EFAZanion (∗ ) SERS

n ∗∗ EFAg SERS ( )

AZ anion-Ag2 AZ anion-Ag4 AZ anion-Ag14

-3 -3 -5

5 6 7

1985 1202 1218

453 4484 4521

(*) AZ anion chromophore located excitations

(**) Agn cluster located excitations

a )


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b )

c )

d )

2 5 0

5 0 0

7 5 0

W

1 0 0 0

1 2 5 0


-1

1 5 0 0

Figure 5:

1 7 5 0

Comparison between the calculated SERS spectra of the different AZ anion-Agn complexes with the experimental alizarin SERS spectrum in alkaline solution; all the spectra are obtained under pre-resonance condition with respect to the chromophore transitions (see Fig. 3). Bands are given a Lorentzian width of 10 cm−1 ; a) experimental SERS spectrum of alizarin in alkaline solution at 632 nm excitation and SERS spectra of b) AZ anion-Ag2 , c) AZ anion-Ag4 , d) AZ anion-Ag14 complex, respectively.



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separated in energy. Should this not be the case, i.e., the molecular based and metal based excitations are too close in energy, the separation of the different mechanisms seems not completely feasible. In fact, in the case of small clusters, i.e., AZ anion-Agn , n=2, 4, the excitations used to mimic the electromagnetic effect, i.e., the cluster excitations, can be better distinguished from those pertaining to the molecular moiety, than in the case of the larger cluster, i.e., AZ anion-Ag14 , as seen in Table 1. In Fig. 5 the calculated SERS spectra of the three AZ anion-Agn complexes are reported and confronted with the experiment performed at 632 nm excitation. In fact, the pre-resonance Raman calculation has been carried out at 570 nm, a wavelength which is close to that excitation and far from Agn cluster resonances. The agreement seems excellent for the n = 2, 4 clusters but less satisfactory for the larger, n=14, cluster complex. The reason could be that in the smaller complexes the clear distinction between molecular excitations and cluster excitations allows an unambiguous choice of the pre-resonance wavelength. Moreover, another spectral feature of the simulated spectrum catches the eyes, i.e., the lower enhancement of the bands above 1400 cm−1 . In the pre-resonance calculation at 440-480 nm, these bands are instead enhanced like the bands in the range 1200 and 1400 cm−1 (see Fig. 6). Here the comparison is made with the experimental SERS spectrum obtained far from molecular resonance 15 . The experimental SERS spectrum at 632 nm and the theoretical spectrum at 570 nm excitation can both be considered pre-resonance SE[R]RS spectra which involve vibrational modes of that part of the chromophore most interested by the molecular electronic transitions (see Fig. S1 of Supporting Information section). Here the molecular resonance amplification effect adds up to the simulated electromagnetic SERS enhancement. The vibrational modes we are dealing with here are the 1245-1256, 1315 and 1411 cm−1 which are described mostly as in plane ring and δ(CH) modes (see Table 2). The bands above 1400 cm−1 , likewise enhanced in the simulated SERS spectra at 440-480 nm pre-resonance, are essentially described as due to in-plane ring and ν(C = O) modes (see Table 2), which should feel more the close interaction with the Ag cluster atoms. The most prominent bands in this range are calculated at 1483, 1523 and 1634 cm−1 , respectively.


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a )


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b )

c )

d )

2 5 0

5 0 0

7 5 0

W

1 0 0 0

1 2 5 0


-1

1 5 0 0

1 7 5 0

Figure 6: Comparison between the calculated SERS spectra of the different AZ anion-Agn complexes and the experimental alizarin SERS spectrum in alkaline solution; all calculated spectra are obtained under pre-resonance condition (see Fig. 3). Bands are given a Lorentzian width of 10 cm−1 ; a) experimental SERS spectrum of alizarin in alkaline solution at 785 nm excitation and calculated SERS spectra of b) AZ anion-Ag2 , c) AZ anion-Ag4 , d) AZ anion-Ag14 complex, respectively.



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It is worth mentioning here that the pre-resonance SERS spectra calculation at 570 nm highlights only the enhancement of in-plane vibrational modes, as it should be in the case of 0

the Cs planar AZ anion species with totally symmetric modes of A symmetry. This agrees with the Raman scattering theory of Albrecht 33 , represented by the expression for polarizability α = A + B + C, where, in case of resonance Raman scattering, only totally symmetric modes are enhanced by the term A, the Frank-Condon contribution. The extension of this theory to the case of molecules adsorbed on metal surfaces due to Lombardi and Birke 34 , foresees that also non-totally symmetric modes, in this case A” , can be enhanced through the B and C terms, via an Herzberg-Teller vibrational coupling mechanism involving charge transfer excitations. The application of the derived Herzberg-Teller surface selection rules seems however useless for systems of low symmetry as the present case 34 . Nevertheless, assuming an edge-on adsorption geometry of the AZ anion on the surface, vibrational modes with in-plane polarizability tensor elements αY Y and αY X , will be enhanced by far more than the other modes, as dictated by the general surface selection rules 35 . The experimental SERS spectra of the AZ anionAgn complexes should then exhibit practically only in-plane vibrational modes, as is the case observed. It is a point in favor of the cluster model applied for the calculation of the SERS spectra of the AZ anion-cluster complexes that only in-plane modes exhibit relevant spectral enhancements.

Simulated enhancement factors (EFs) Both the electromagnetic and chemical enhancement mechanisms have been addressed by the cluster model and DFT calculation of the vibrational SERS spectra. A useful test for the model will be its ability in reproducing the enhancement factors and frequency shifts in the case of static, molecular resonance and cluster resonance enhanced SERS spectra. The frequency shifts with respect to the bare AZ anion frequencies and the calculated enhancement factors (EFs) are reported in Table 2. The definition of the EFs in Table 2 is straightforward. From left to right, for each complex, we have in the order: the static chemical, the molecule-cluster near


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molecular resonance and the molecule-cluster near cluster resonance enhancement, respectively. The EFs are obtained with respect to the calculated Raman activities of selected bands of the static NRS of the bare AZ anion. Raman scattering activity has been used thus disengaging from the wavelength dependence (DRSCS intensity) of the SERS spectra. The comparison with experimental frequency shifts and enhancements factors (EFs) is not easily feasible due to the difficulties in the determination of the experimental conditions for SERS and SE[R]RS quantitative measurements 36–38 . Moreover, in view of a possible comparison with experimentally determined EFs it should be taken into account that our SERS and SE[R]RS TDDFT calculations have been performed under near-resonant conditions whereas the experimental measurements are usually performed under resonance. There can be expected higher EFs in this case. A comparison with the experimentally determined spectral shifts is also not easy, as the reported frequency differences between the NRS spectra of the lone alizarin and the SERS spectra of adsorbed alizarin in alkaline solution occur within the instrumental spectral resolution 8,15 . To minimize the effect of variation in any specific vibrational mode we calculated the average EFs over the bands, for each complex and excitation. The average EFs for the various excitation wavelengths, i.e., the static, the Agn cluster located excitation, the AZ anion located excitation, Agn AZanion are reported in Table 3 under the headings: EFstatic SERS , EFSERS and EFSERS , respectively. The

results summarized in Table 3 show that the Raman properties depend clearly on cluster size. Both the frequency shifts and the static chemical enhancement factors increase, in absolute value, with the cluster dimension. The bands which exhibit the higher shifts, i.e., 1241, 1521, 1574 and 1658 cm−1 , correspond to vibrational modes whose PED shows a strong involvement of the O-H group in position 2 and of the C=O groups in positions 1, 9 (see Fig. 1 and Fig. 2). The spectral enhancements due to cluster excitations also show an increase with cluster dimension. The total enhancements for the complexes are between 4×102 -4×103 , with the AZ anion-Agn (n=4, 14) complexes exhibiting the higher enhancements. For pre-excitation at the molecular resonances, the enhancement does not change substantially with the cluster dimension, although a higher enhancement is obtained for the smaller complex, i.e., AZ anion-



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Ag2 . The enhancement trend in the case of cluster excitation is suggestive of the onset of a type of electromagnetic enhancement which should be fully operative at larger cluster dimension, a confirmation of the hypothesis that small silver clusters can be taken to simulate larger particles 20 . The total enhancement under molecular pre-resonance condition is also of the order of 103 , but clearly below that obtained with cluster excitation and practically unchanged with the cluster dimension. The almost constant molecular enhancement could indicate that the AZ anion moiety in the complex behaves practically as the lone AZ anion molecule. Besides, the vibrational modes belonging to the C = O groups in position 1, 9 are strongly enhanced (see Fig. 6 and Table 2) after cluster excitation. As suggested by Baran et al. 9 , from the point of view of the enhancement mechanisms, we can divide the AZ anion molecule into two parts. The vibrational modes of the part of the molecule facing the silver cluster have an enhancement mostly due to the interaction with the cluster silver atoms whereas the ring modes are prone to enhancement due to excitations in resonance with molecular π → π ∗ transitions within the chromophore.

Conclusion In this work we have presented a TDDFT study of the absorption and Raman scattering properties of alizarin anion (AZ anion) adsorbed on small silver clusters Agn (n = 2, 4, 14). The Agn metal clusters alone were also characterized in terms of structure and transition energies. A reasonable agreement with previous theoretical results has been found. Three mechanisms for the enhancement of the Raman spectra have been considered: the pure chemical enhancement mechanism which occurs far from any resonance of the AZ anion-Agn complex, an enhancement mechanism for molecular resonance excitation and an enhancement mechanism for localized plasmon excitation, the so called EM mechanism. These mechanisms were simulated by calculating the excitation wavelength independent SERS spectra (static) and the frequency dependent SERS spectra under pre-resonance condition with respect to calculated transition energies of the molecular and metallic cluster located resonances, respectively. In particular we


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have assumed that a local plasmon excitation can be mimicked by exciting in pre-resonance with calculated transition energies of the metal Agn clusters. The calculation in case of CT transition has not been carried out due to the small oscillator strength of these transitions. The spectra calculated in pre-resonance condition with respect to molecular excitation energies appear very similar to the experimental SE[R]RS spectra obtained at 632 nm excitation. In this case the modes whose bands are more enhanced involve in-plane oscillations of the AZ anion group not involving the C = O bonds in position 1, 9. On the contrary, in the calculation under pre-resonance condition with respect to cluster transitions, the bands of the the C = O groups, facing the cluster, were enhanced. The different enhancements obtained for calculation under pre-resonance with molecular transitions with respect to cluster transitions, suggest the possibility of two enhancement mechanisms acting on different parts of the molecule. The NRS spectra of the AZ anion-Agn complexes resemble that of the lone AZ anion, with enhancements which increase with cluster dimension. The same occurs for the SERS spectra of the complexes excited under pre-resonance condition with respect to cluster excitations. Molecular excitation instead gives rise to an almost constant enhancement throughout the various complexes. The results sustain the assumption that the energy excitations in small clusters can simulate the localized surface plasmons in large metal particles.

Acknowledgement This work was supported by the Italian MIUR (grant no. 20100329WPF_007), LASERLABEUROPE (grant agreement no. 284464, EC’s Seventh Framework Programme) and the Ente Cassa di Risparmio di Firenze (grant no. 2014-0405A2202.8044).

Supporting Information Available 1) The full citation of the reference Frisch et al. 28 2) Table S1 of SI: Calculated principal (most intense) excitation energies for the Agn (n=2, 4, 14) clusters 19 ACS Paragon Plus Environment


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3) Figure S1 of SI: Molecular orbitals of the AZ anion-Ag2 complex involved in the excitation HOMO-1→LUMO This material is available free of charge via the Internet at http://pubs.acs.org/.

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