Electronic Circular Dichroism of Surface-Adsorbed Molecules by

Feb 26, 2014 - Means of Quantum Mechanics Capacitance Molecular Mechanics. Xin Li,*. ,† ... difference time-domain (FDTD) electrodynamics approach b...
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Electronic Circular Dichroism of Surface-Adsorbed Molecules by Means of Quantum Mechanics Capacitance Molecular Mechanics Xin Li,*,† Zilvinas Rinkevicius,†,‡ and Hans Ågren† †

Division of Theoretical Chemistry and Biology, School of Biotechnology, KTH Royal Institute of Technology, SE-10691 Stockholm, Sweden ‡ Swedish e-Science Research Centre, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden S Supporting Information *

ABSTRACT: To promote a more comprehensive understanding of the influence of metal−adsorbate interaction for molecules at metallo surfaces or metallo nanoparticles in solvent environments on their electronic circular dichroism (ECD) spectra, we evaluate the application of a recently derived quantum mechanics capacitance molecular mechanics (QMCMM) model for ECD. Using helicene absorbed on gold surfaces in protic and aprotic solvents as illustration, we elucidate the detailed effects on excitation energies, transition moments, rotatory strengths, orientation dependence of ECD spectra, and the different roles of aprotic and protic solvents and the induced charge distribution patterns on the surface. These changes are decomposed in terms of surface alone, solvent alone, and combined surface−solvent influence, and furthermore into the indirect contributions by the surface-induced restructuring of the helicene. Much of the salient changes of the ECD can be rationalized to the substantial redistribution of charge at the gold surface induced by the presence of the helicene. The study indicates that through the QMCMM model the effects of a metallic surface on the circular dichroism spectra of adsorbed organic molecules can be tackled by extended QM calculations coupled to polarizability−capacitance force fields for large metallic clusters representing surfaces or nanoparticles.

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nanoparticles by Corni and Tomasi,12 the heterogeneous solvation response theory by Jørgensen et al.,13 the finitedifference time-domain (FDTD) electrodynamics approach by Lopata and Neuhauser,14 the polarizable quantum mechanics/ molecular mechanics (QM/MM) method by Arcisaukaite et al.,15 and the discrete interaction model/quantum mechanics (DIM/QM) method by Jensen and co-workers.16−19 In particular, recent development of multiscale QM/MM models has been generalized to include metallic surfaces through parametrizations in terms of capacitance force fields for the metallic surface atoms.16−21 The implementation of linear response theory for this, quantum mechanics capacitance molecular mechanics (QMCMM) model,21 allows a direct assessment of a chiral molecule physisorbed on metal surfaces and has thus the capability to provide the above-mentioned structure−function design. In the present work, we outline the particularities of this theory when implemented for chiral systems and electric field induced circular dichroism (ECD) spectra. We demonstrate the theory by application on a prototype system, helicene on gold in a solvent environment, to address, both numerically and qualitatively, the direct influence of the gold surface, as well as the combined influence of the surface and the solvent environment on the ECD response of the helicene molecule. We further assess the results in terms of

INTRODUCTION Chiral photonics is an emerging field with great promise for technical applications in fields like optical signal processing, biosensing, and bioimaging.1−7 It mainly deals with enantioselective polarization control of linear and nonlinear optical functions.8−10 The optical properties of the surface groups and adsorbents can be altered by the presence of gold atoms through indirect effects, which provide signal changes and facilitate optical detection of events taking place inside biological systems. While new flexible materials with customtailored photonic functionality are required to realize the full potential of this emerging field, the efforts toward developing such materials with enhanced optical and magneto-optical activity have been hampered by the lack of fundamental understanding of the structure−response−functionality relationships. Rational-based design is thus wanted but requires insight of the underlying influence of the electronic structure, geometry of the chiral species, as well of the bulk environment on the optical properties. Understanding surface assisted chiral photonics is of special importance for future device technology as surfaces can act as both immobilizing templates and directly enhancing the chiral functions, thus promoting the need for quantum mechanical or hybrid quantum-classical descriptions of such effects and for the general optical response of physisorbed organic molecules on metal surfaces.11 So far, there have been a few mainstream computational approaches taking advantage of hybrid methods, for instance, the polarizable continuum description of metal © 2014 American Chemical Society

Received: December 9, 2013 Revised: February 25, 2014 Published: February 26, 2014 5833

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between QM and MM atoms as well as between atoms in the MM region is modeled using an empirical Lennard-Jones potential. According to the interaction scheme between the QM and MM regions, the targeted molecule placed in the QM region interacts with the permanent charges of the solvent molecules from the nonmetallic part of the MM region, and with induced charges and induced dipoles from the metallic part of the MM region. The charges and dipoles of the latter are determined in a self-consistent procedure where they adapt to the current state of the QM region (a detailed description of the QM and MM region interaction can be found in our previous paper on the DFT/CMM approach21). The outlined model of the heterogeneous MM region captures the essential physical features of the molecular system consisting from molecules physisorbed on a metal surface or a nanoparticle in vacuum or solvent environments, and thus includes the three main intermolecular forces: electrostatic, polarization, and van der Waals interactions between the molecules in the QM region, metal surface or nanoparticle in the MM region, and solvent molecules in the MM region. The calculation of the rotatory strength tensor for a molecule requires the determination of electric dipole−magnetic dipole and electric dipole−electric quadrupole polarizability residues:

electronic structure variations and the nature of the solvent and state some ramifications for further work in the field based on the results obtained.

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THEORY A. Rotatory Strength Tensor of Oriented Molecules. Circular dichroism, that is, the difference between absorption coefficients for left and right polarized light of an oriented molecule, is described by a rotatory strength tensor, which can be defined for a 0 → n excitation in a molecule as22,23 3 n ⟨0|p|n⟩ × ⟨n|pr|0⟩ 0R = − 2ωn0 (1.1) where ωn0 is the frequency of the 0 → n excitation, and p and r are the linear momentum and position vector operators, respectively. The rotatory strength tensor can be rewritten as the sum of symmetric and antisymmetric parts: n 0R

= 0nRmv + 0nRQv

(1.2)

which describe electric dipole−magnetic dipole and electric dipole−electric quadrupole polarizabilities, respectively. In the velocity gauge, the electric dipole−magnetic dipole contribution to the rotatory strength tensor is given by the following expression: n m 0R v

=

3 [⟨0|L|n⟩·⟨n|p|0⟩I − ⟨0|L|n⟩⟨n|p|0⟩] 4ωn0

lim ⟨⟨p; L⟩⟩ω , ω

ω → ω0n

(1.3)

lim ⟨⟨p; pr + rp⟩⟩ω , ω

while the electric dipole−electric quadrupole contribution to the rotatory strength tensor is given by n Q 0R v

=−

3 ⟨0|p|n⟩ × ⟨n|pr + rp|0⟩ 4ωn0

ω → ω0n

(1.5)

from which the matrix elements needed for the rotatory strength tensor are obtained. In the DFT/CMM approach, the heterogeneous environment contribution to the linear response residue has the following components (for details, see our paper on the DFT/CMM method21): (a) contribution from permanent charges in the nonmetallic MM region, which is contracted with the perturbed density of the QM region; (b) contribution from induced charges and dipoles in the metallic part of the MM region, which is determined for the unperturbed QM region density and contracted with the perturbed density of the QM region; and (c) contribution from the first-order perturbed charges and dipoles in the metallic part of the MM region, which is determined for the perturbed density of the QM region. Thus, the linear response function residue in the DFT/CMM approach includes the dynamic relaxation of the heterogeneous environment, so that the computations of the rotatory strength tensor include a description of the essential interaction between metal surface and adsorbate molecule in a solvent environment. C. Orientation Dependence of the Observed Circular Dichroism. In an isotropic environment, for example, a solution, the rotatory strength tensor of a molecule is subject to random reorientation due to the fast tumbling of the molecule of interest, and the experimental observation corresponds to the ensemble average over a large number of molecules. Therefore, only the average of the isotropic elements of the rotatory strength tensor is recorded in experimentally observed spectra. However, when adsorbed onto a planar surface, the orientation of the molecule is no longer random, leading to orientation-dependent circular dichroism. Assuming that the incoming light beam propagates along a direction u(θ,φ), the observed rotatory strength is expressed as22

(1.4)

where the angular momentum operator L = r × p is introduced. From the above expressions of electric dipole−magnetic dipole and electric dipole−electric quadrupole contributions to the rotatory strength tensor, it is clear that the first contribution solely defines the scalar rotatory strength. The n0RQv contribution vanishes for samples containing randomly oriented molecules, such as solutions or amorphous materials. However, for ordered samples, both contributions to the rotatory strength tensor must be considered. Taking this into account and to characterize ECD of molecules absorbed on metal surfaces, we do not limit our computation to the scalar rotatory strength, as used to characterize CD of molecules in solution, but compute the full rotatory strength tensor using the QMCMM or, more specifically, the DFT/CMM approach.21 B. Evaluation of Rotatory Strength Tensor in the DFT/ CMM Approach. The rotatory strength tensor n0R defined in eq 1.1 in the response theory formalism is computed as a set of residues of the linear response function. The computational procedure is well-known and is extensively described in previous works on CD of free or solvated molecules. In the DFT/CMM approach,21 the heterogeneous MM environment of a targeted molecule absorbed on a metallic surface is partitioned into a metallic part, which contains the metallic surface itself, and a nonmetallic part, which contains the solvent molecules surrounding the targeted molecule absorbed on the metallic surface. The heterogeneous MM part in the DFT/ CMM approach assumes a capacitance-polarization model for the electrostatics and polarization of the metallic part of the MM region, and distributed charges are used in the nonmetallic part of the MM region. Furthermore, van der Waals interaction 5834

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Table 1. Vertical Distance between the Four Groups and the Au(111) Surface (in Å), Together with Their Standard Deviations environment

terminal benzene ring

nitrogen atom

hydrogen atom 1

hydrogen atom 2

no solvent aprotic solvent protic solvent

3.286 ± 0.080 3.332 ± 0.103 3.335 ± 0.091

3.252 ± 0.242 3.141 ± 0.307 3.176 ± 0.283

3.300 ± 0.594 3.369 ± 0.503 2.949 ± 0.524

3.300 ± 0.621 3.361 ± 0.428 2.989 ± 0.583

n 0 Γ(θ ,

interaction between the gold surface and its periodic images. Each simulation was conducted under constant-NVT ensemble for 10 ns with the temperature maintained at 298 K by the velocity-rescaling algorithm,30 after which 64 snapshots were evenly extracted from the last 3.2 ns of the trajectory. From each snapshot, a cylinder centered at the aminohelicene was cut from the whole system as the input model for the subsequent QMCMM calculations. In this input model, the cutoff radius of the cylinder was set to 15 Å, and three layers of gold atoms were involved. We show in Figure S2 in the Supporting Information that the cutoff radius of 15 Å is sufficient to realistically describe the ECD spectra of aminohelicene on gold surface. Furthermore, solvent molecules on top of the surface (within 20 Å) were also included. In the QMCMM calculations, the aminohelicene was described by the parameter-free global hybrid PBE0 functional 31,32 and Ahlrichs’s triple-ζ basis set def-TZVP,33 the Au(111) surface described by metallic force field with polarization and capacitance taken into account, and the organic solvent described by discrete point charges as in the CGenFF. Twenty excited states were solved by the linear response timedependent (TD) DFT calculations to properly describe the bisignate shape of the ECD spectra. Such a QMCMM model enables charge transfer within the gold substrate and is thus expected to reflect the charge distribution and image charge effects of the gold surface. The computed ECD spectra were obtained by Gaussian convolution of the stick spectra with a full width at half-maximum (fwhm) of 0.2 eV and then averaged over the 64 snapshots to provide a statistical description under ambient temperature. See Figure S3 in the Supporting Information for the dependence of averaged ECD spectra on the number of snapshots. The GROMACS program package34 and the DALTON quantum chemistry program35 were used to carry out the MD simulations and QMCMM calculations, respectively.

φ) = [0nR xx cos2 φ + 0nR yy sin 2 φ + (0nR xy + 0nR yx)

sin φ cos φ] sin 2 θ + [(0nR xz + 0nR zx) cos φ + (0nR yz + 0nR zy) sin φ] sin θ cos θ + 0nR zz cos2 θ (1.6)

where θ and φ are the spherical polar angles of u in the coordinate system. In practice, the rotation of angle φ, the rotation around the normal direction of the surface, can be averaged to give 24

∫ ⎡⎣ Γ(θ , φ)⎤⎦dφ n

n 0 Γave(θ )

=

=

0

∫ dφ n 0 R xx

+ 0nR yy 2

sin 2 θ + 0nR zz cos2 θ

(1.7)

n 0Γave(θ)

Further, the average of over θ (note the weighting factor sin θ) gives the isotropic rotatory strength as the average of the diagonal elements of the rotatory tensor:

∫ ⎡⎣ Γave(θ) ⎤⎦sin θ dθ n

n 0 Γiso

■

=

0

∫ sin θ dθ

=

n 0 R xx

+ 0nR yy + 0nR zz 3 (1.8)

COMPUTATIONAL DETAILS Using 5-amino[6]helicene25 as an example, we here employ our QMCMM scheme to explore its adsorption onto an Au(111) surface under different circumstances to shed light on the effect of a metal surface on the ECD spectra of organic adsorbents. Four systems were studied: (1) an aminohelicene molecule in vacuum; (2) an aminohelicene molecule adsorbed onto Au(111) surface; (3) an aminohelicene molecule adsorbed onto Au(111) surface and solvated by trichlorobenzene; and (4) an aminohelicene molecule adsorbed onto Au(111) surface and solvated by octanoic acid. Trichlorobenzene is an aromatic and aprotic solvent, while octanoic acid is an aliphatic and protic solvent. For each system, the theoretical calculations can be divided into two parts, molecular dynamics (MD) simulations and subsequent QMCMM calculations. MD simulations were first carried out employing the GolPCHARMM force field26 for the Au(111) surface and the CHARMM general force field (CGenFF)27 for the organic constituents of the system including aminohelicene and the solvents. Here, the CGenFF is refined so that the quantum mechanically optimized geometry of the aminohelicene at the ωB97X-D/6-311G** level of theory28,29 is reproduced by force field energy minimization. We show in Figure S1 in the Supporting Information that the computed ECD spectrum of the aminohelicene at the molecular mechanical geometry is comparable to that obtained at the QM geometry and the experimental observations.25 The gold substrate consists of six layers with a cross-section area of 34.98 × 34.62 Å2, and periodic boundary conditions were applied to the system with a z-dimension 70.67 Å, which is sufficiently large to minimize the

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RESULTS AND DISCUSSION A. Geometry of the Adsorbed Aminohelicene. We first monitored the vertical distance between the helicene and the Au(111) surface during the MD simulations. The positions of four groups with respect to the Au(111) surface are listed in Table 1, the terminal benzene ring adsorbing on the gold surface and the nitrogen and hydrogen atoms in the amino group. It can be seen that the adsorption of aminohelicene onto Au(111) gives rise to comparable vertical distances for the terminal benzene ring and the nitrogen atom, and that the two hydrogen atoms on average appear at slightly higher positions. In the presence of solvent molecules, the terminal benzene ring is slightly lifted from the gold surface; meanwhile, the nitrogen atom shows greater affinity to the gold surface as reflected by the shorter vertical distances. Interestingly, the hydrogen atoms in the amino group are dragged away from the gold surface, while these hydrogen atoms stay closer to the gold surface in the presence of protic solvents. In both cases, they exhibit a large standard deviation, which is indicative of flipping between the pointing-up and the pointing-down conformations. In the

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centroids, resulting in a more relaxed geometry of aminohelicene. B. Electronic Structure and ECD Spectrum of Aminohelicene. The six frontier molecular orbitals (MOs) of the aminohelicene ranging from HOMO−2 to LUMO+2 are shown in Figure 2. Linear response TDDFT calculations show

presence of the aprotic solvent, the hydrogen atoms show a preference toward the pointing-up geometry with respect to the nitrogen atom, due to the strong interaction between nitrogen and gold surface. The protic solvent molecule is found to form a hydrogen bond from the top of the nitrogen atom of the aminohelicene, which forces the two hydrogen atoms to point downward. Comparison between the three environments suggests that the presence of the aprotic solvent (trichlorobenzene) gives rise to a stronger interaction between the nitrogen atom and the gold surface, while the presence of the protic solvent (octanoic acid) leads to a more direct contact between hydrogen atoms and the gold surface. We further investigated the structural character of the aminohelicene molecule by looking at the centroids of the aromatic rings. The geometry of the aminohelicene molecule can be effectively described by the three dihedral angles formed by the six centroids of the aromatic rings and the distance between the two terminal centroids (Figure 1). As listed in

Figure 2. Frontier molecular orbitals of aminohelicene.

that the HOMO→LUMO transition, which corresponds to charge transfer from the amino group to helicene, contributes dominantly to the S0→S1 excitation. The HOMO→LUMO+1 transition comprises the majority of the S0→S2 excitation, while the HOMO−1→LUMO transition contributes largely to the S0→S3 excitation. To analyze the contribution of each transition to the spectra, we here decompose each excitation into individual electronic transitions, as shown in Figure 3.

Figure 1. Centroids of the aromatic rings in the aminohelicene molecule and the dihedral angles defined by the centroids.

Table 2. Dihedral Angles and End-to-End Distance of the Centroids of the Aromatic Rings in the Aminohelicene Molecule, Together with Their Standard Deviations environment

α/deg

β/deg

γ/deg

d/Å

in vacuum on Au(111), no solvent on Au(111), aprotic solvent on Au(111), protic solvent

16.3 ± 3.7 8.9 ± 3.2

21.9 ± 4.5 23.2 ± 3.1

18.5 ± 4.7 20.6 ± 4.0

4.182 ± 0.150 4.161 ± 0.109

12.0 ± 4.0

26.0 ± 3.4

17.6 ± 3.7

4.241 ± 0.123

9.1 ± 3.9

24.3 ± 4.3

20.9 ± 4.0

4.219 ± 0.152

Table 2, the dihedral angle α, which is as large as 16.3° in vacuum, becomes much smaller upon adsorption onto Au(111) surface, due to the Au−π interaction between gold and the aromatic rings. Adsorption onto the gold surface in vacuum enlarged both dihedral angles β and γ in response to the squeezed dihedral angle α. The distance between the two terminal centroids also slightly decreased, presumably due to the enhanced π−π interaction induced by the presence of the gold surface. In the presence or absence of solvent molecules, the statistical average of dihedral angle α of the adsorbed aminohelicene remains at around 10° and is much smaller than β and γ, indicating that the lowest four aromatic rings are almost coplanar facilitating the interaction between the helicene and the Au(111) surface. The presence of aprotic solvents puts sizable effects on the dihedral angles, leading to an increase in α and β and a decrease in γ. However, the effects of protic solvents are much smaller. The presence of solvent molecules also slightly enlarges the distance between the two terminal

Figure 3. Decomposed circular dichroism spectra of aminohelicene.

Here, the HOMO→LUMO transition with the lowest excitation energy contributes very little to the spectra and is thus not shown. The HOMO−1→LUMO and HOMO→ LUMO+1 transitions contribute dominantly to the low-lying optical absorption bands with the strongest oscillator strength and the largest rotatory strength, and influences of these transitions are expected to put an observable change in the absorption and ECD spectra of aminohelicene. In particular, the amino group exhibits a strong interaction with the gold surface and surrounding protic solvent molecules, and its geometrical change, hydrogen-bonding, as well as gold− 5836

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Figure 4. Comparison between the (a) peak excitation energies and (b) maximal molar extinction coefficient εmax by pure QM and the hybrid QMCMM models at the TD-PBE0/def-TZVP level of theory.

Figure 5. Orientation dependence of ECD spectra for adsorbed aminohelicene (left) on Au(111) without solvent, (center) on Au(111) in trichlorobenzene solution, and (right) on Au(111) in octanoic acid solution. θS is the supplementary angle of the spherical polar angle θ.

nitrogen interaction may significantly affect the MO compositions of the optical excitations and in turn the ECD spectra. Calculations in vacuum suggest that the excitation energy and molar circular dichroism (Δε) of the aminohelicene at the positive peak of the ECD spectrum are 3.581 eV and 135.8 M−1 cm−1, respectively, comparable to the experimental observation at 334 nm (3.713 eV, Figure S1 in the Supporting Information).25 The positive part of the computed circular dichroism spectrum consists of contributions from the HOMO→LUMO+1 (34%) and the HOMO−1→LUMO (47%) transitions, where the latter contributes significantly to the rotatory strength as HOMO−1 and LUMO receive major contributions from the whole helicene skeleton. The HOMO− 2→LUMO+2 transition, lying at a higher energy level, contributes mainly to the negative part of the ECD spectrum (Figure 3). Unlike the positive part of the ECD spectrum, which consists of several main contributions from the low-lying electronic transitions, the negative part of the spectrum receives

contributions from much more electronic transitions with smaller rotatory strengths. C. Enhancement of Dipole Strength by Gold Surface. Upon adsorption onto the Au(111) surface, the transition electric and magnetic dipole moments are subject to change and in turn affect the resultant ECD spectra. Here, to examine the effects of adsorption on the ECD signals, we address the change in the dipole strength, the square of the transition electric dipole moment, which is closely related to the oscillator strength and the molar extinction coefficient. As shown in Figure 4a, the geometrical changes upon adsorption onto the gold surface give rise to small changes (within 0.03 eV) in the peak excitation energies of the absorption spectra, and the effects arising from interaction between aminohelicene and solvents are also subtle (∼0.01 eV) . However, when the gold substrate is involved in the QMCMM calculations, the resultant peak excitation energy of the absorption spectrum shows a redshift of 0.06−0.08 eV. Again, the inclusion of solvent molecules 5837

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Figure 6. Comparison between the (a) excitation energy at the positive Cotton effect and (b) maximal circular dichroism Δεmax by pure QM and the hybrid QMCMM models at the TD-PBE0/def-TZVP level of theory.

E. Influence on ECD Signals by Gold Surface and Solvent Molecules. Despite the restricted motions of adsorbed aminohelicene molecules on the gold surface, the isotropic rotatory strength is still of interest, because the isotropically averaged ECD spectra may provide information on the influence of gold nanoparticles on adsorbed organic molecules. As shown in Figure 6a, the gray bars indicate that the peak excitation energies do not deviate much (within 0.04 eV) when only the aminohelicene molecule is involved in the QM calculations. As compared to pure QM calculations of the aminohelicene molecule, the inclusion of the gold surface in QMCMM calculations gives rise to a significant red-shift of around 0.08 eV in the peak excitation energy and a decrease of 5−10 M−1 cm−1 in the computed molar circular dichroism. Further inclusion of solvent molecules in the QM/MM calculations adds only a negligible effect on the peak excitation energies; however, the presence of aprotic trichlorobenzene or the protic octanoic acid solvent molecules leads to opposite effects on the computed molar circular dichroism (Figure 6). Overall, the involvement of the Au(111) surface and/or solvent molecules leads to decreased peak excitation energies; aprotic solvents enhance the molar circular dichroism of aminohelicene as compared to that in vacuum, while protic solvents diminish the molar circular dichroism. F. Mirror-Image Charge Distribution of the Gold Surface. Finally, we investigated the charge distribution on the gold substrate by computing charge densities from twodimensional Gaussian distributions of point charges located at each gold atom in the first layer, as shown in Figure 7. It can be seen that in the absence of solvent or in the presence of protic solvent like octanoic acid, the charge distribution on gold surface exhibits a pattern perpendicular to the transition electric dipole moment of the aminohelicene, where negative charges are accumulated at the bottom of the aminohelicene molecule. This facilitates the S0→S3 excitation by increasing the composition of the HOMO−1→LUMO transition. Differently, in the presence of the aprotic trichlorobenzene solvent, the charge distribution of the gold surface shows a pattern, which is almost parallel to the direction of the transition electric dipole moment of aminohelicene. The composition of the HOMO− 1→LUMO transition in the S0 →S3 excitation is thus moderately enhanced, and the computed molar circular

in the QMCMM calculations has little influence on the peak excitation energy of the absorption peak. This indicates that the interaction between aminohelicene and the Au(111) surface leads to significant enhancement in the transition electric dipole moment and hence in the dipole strength, while the solvent molecules have limited roles in this process. A similar phenomenon was also observed for the maximal molar extinction coefficient εmax (Figure 4b). Geometrical changes of aminohelicene led to a slight increase in εmax, while the presence of the gold surface gives rise to a significant enhancement of around 2 × 104 M−1 cm−1. The presence of solvents also put little effect on εmax. Overall, the presence of the gold surface leads to a red-shifted absorption band and an enhanced absorbance. D. Orientation Dependence of ECD Spectra. The effects of a gold substrate on ECD spectra are complicated also because the thermal motions of the adsorbed aminohelicene molecules are restricted. The rotatory strength has to be presented as a tensor to take into account its orientation during the interaction with the incoming light beam. The direction of the incoming light beam can be described by the spherical polar angles θ and φ, where φ corresponds to rotation around the zaxis and θ describes the angle formed by the propagation vector of the incoming light beam and the z-axis. Taking the gold surface as a reference plane, we averaged the orientationdependent ECD spectra over φ (from 0 to 2π) to obtain the orientation dependence of ECD spectra on θS, the supplementary angle of θ, as shown in Figure 5. In fact, according to eq 1.7, the observed mean rotatory strength n0Γave(θ) depends on sin2 θ and cos2 θ, which are equivalent to sin2 θS and cos2 θS, respectively. Thus, the ECD spectra show the same dependence on θ and θS. As such, the orientation-dependent ECD spectra are plotted in the interval of θS ∈ [0,π/2] (Figure 5). In both the absence and the presence of solvent molecules, the ECD spectra of the adsorbed aminohelicene on Au(111) show very strong dependence on θS. When θS is close to 0°, the incoming light beam is perpendicular to the gold surface, and two negative Cotton effects are observed at 250 and 350 nm, respectively. As θS increases and approaches 90°, the Cotton effect at the longer wavelength gradually becomes positive; meanwhile, the magnitude of the Cotton effect at the shorter wavelength is diminished. 5838

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contributes to the negative part in the higher energy region. The presence of a gold substrate is found to result in a redshifted absorption band and an enhanced molar extinction coefficient. The ECD spectra of adsorbed aminohelicene are dependent on the direction of the incoming light beam, where an incoming light beam perpendicular to the gold surface results in two negative Cotton effects and a parallel incoming light beam gives rise to a bisignate ECD spectrum. Furthermore, the charge density distribution on the surface of the gold substrate shows a pattern parallel to the transition dipole moment of aminohelicene in the presence of an aprotic solvent and exhibits a pattern perpendicular to the transition dipole moment in the absence of solvent or in the presence of a protic solvent. The perpendicular pattern of charge distribution of the gold surface promotes an accumulation of negative charges at the bottom of the aminohelicene, facilitating the S0→ S3 excitation, which consists of mainly the HOMO−1→LUMO transition. The general conclusion confers that through the QMCMM model the effects of a metallic surface on the circular dichroism spectra of adsorbed organic molecules can now be tackled by high level QM calculations for the adsorbent and a polarizability−capacitance force field for a large cluster representing the metallic substrate. It allows us to pinpoint a number of effects, like the orientational dependence of ECD spectra and the different roles of aprotic and protic solvents on the charge distribution patterns of the surface. The QMCMM model can be expected to promote a more comprehensive understanding of metal−adsorbent interaction and future development of ligand-protected gold nanoparticles for applications in many areas.

Figure 7. Snapshots and surface charge distributions (in e/Å2) for aminohelicene adsorbed onto gold substrate (a) with no solvent, (b) in aprotic trichlorobenzen, and (c) in proticoctanoic acid. Only the first layers of solvent atoms are shown in (b) and (c). The red arrow represents the direction and magnitude of the transition electric dipole moment of aminohelicene.

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ASSOCIATED CONTENT

S Supporting Information *

Dependence of the computed ECD spectra on basis set, cutoff radius of gold substrate, number of snapshots, and type of metal. This material is available free of charge via the Internet at http://pubs.acs.org.

dichroism is thus slightly smaller than those obtained in the absence of solvent or in the presence of protic solvents.

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CONCLUSION We have in this work outlined the implementation of a recently derived quantum mechanics capacitance molecular mechanics theory for electronic circular dichroism (ECD) of surface adsorbed molecular species. The model is a generalization of conventional quantum mechanics/molecular mechanics that include metallic environments through parametrizations of the metallic surface atoms in terms of capacitance force fields. As illustration of the theory, we addressed the ECD spectral change of the aminohelicene molecule upon adsorption onto a gold surface in different solvent environments. Our conclusion from the study is 2-fold, one general and one specific. The specific conclusion concerns the studied system and the ECD spectra: The interaction between the Au(111) surface and aminohelicene results in a shorter distance between the nitrogen atom and the gold surface, and the helicene skeleton is found to exhibit a significant geometrical change facilitating the Au−π interaction. Such geometrical changes are further found to put effects on the circular dichroism spectra of aminohelicene. Decomposition of the spectra into contributions from individual electronic transitions reveals that the HOMO→LUMO+1 and HOMO−1→LUMO transitions contribute dominantly to the positive part of the circular dichroism spectra, while the HOMO−2→LUMO+2 transition

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Swedish National Infrastructure for Computing (SNIC) for providing computational resources for the project “Multiphysics Modeling of Molecular Materials”, SNIC 025/ 12-38. X.L. acknowledges support from the Carl Tryggers Foundation.

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REFERENCES

(1) Prasad, P. N. Nanophotonics; John Wiley & Sons, Inc.: Hoboken, NJ, 2004; pp 129−151. (2) Hentschel, M.; Schäferling, M.; Weiss, T.; Liu, N.; Giessen, H. Three-Dimensional Chiral Plasmonic Oligomers. Nano Lett. 2012, 12, 2542−2547. (3) Kong, X. Y.; Wang, Z. L. Spontaneous Polarization-Induced Nanohelixes, Nanosprings, and Nanorings of Piezoelectric Nanobelts. Nano Lett. 2003, 3, 1625−1631.

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