Modeling the Absorbance Properties of a Pyrene Chromophore

Article pubs.acs.org/JPCC

Modeling the Absorbance Properties of a Pyrene Chromophore Grafted onto a Au25 Nanocluster: A TD-DFT Study Arnaud Fihey,* François Maurel, and Aurélie Perrier* Laboratoire Interfaces, Traitements, Organisation et Dynamique des Systèmes (ITODYS), CNRS UMR 7086, Université Paris 7 Paris Diderot Sorbonne-Paris-Cité, Bâtiment Lavoisier, 15 rue Jean Antoine de Baïf, 75205 Paris Cedex 13, France S Supporting Information *

ABSTRACT: Using ab initio spectroscopic tools, we have studied the structural and electronic properties of a pyrene chromophore grafted onto a Au25 nanocluster synthesized by Devadas and co-workers [J. Phys. Chem. Lett. 2010, 1, 1497]. To simulate the electronic absorption spectra of this hybrid metallic/organic structure, we relied on a three-step approach: (1) Molecular Dynamics simulations based on Force Field Classical Mechanics, (2) geometry optimizations at the ZORABP86/TZP level, and (3) TD-DFT calculations with the CAM-B3LYP functional. This procedure allowed us to reproduce and rationalize the experimental observations. Because of small spatial overlap and energy matching between the organic and metallic frontier orbitals, the absorption spectrum of the hybrid system is a simple addition of the pyrene and nanocluster optical spectra. To tune the optical properties of the organic moiety and enable the electronic communication within the hybrid system, a modification of the pyrene skeleton is proposed. This study thus paves the way toward the simulation of UV−visible absorption spectra of hybrid metallic−organic systems.

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Au25(C6S)17PyS− system, represented in Figure 1, is synthesized from the hexane-thiolate capped gold cluster Au25(C6S)−18 by the exchange of one hexane-thiolate chain to a thiolated pyrene derivative (PySH, R1 = CO−NH−(CH2)2−SH and R2,3,4= H in Figure 2). Significant fluorescence quenching is observed for the pyrene, a π conjugated system constituting a potential candidate for organic light-emitting devices or solar cells applications.20 Time-resolved fluorescence upconversion and transient absorption measurements actually proved that an ultrafast electron transfer occurs from the Au25 NC to the attached pyrene. At the same time, the measurement of the optical absorption spectrum demonstrated that the optical properties of Au25 NC and PySH are unaltered within the hybrid system. This suggested that the organic and metallic moieties “do not have strong ground-state interactions”.18 Consequently, there is today a great challenge to understand the electronic communication within hybrid organic−inorganic systems and the possible electron and/or energy transfer between the chromophore and the metallic NC. In that framework, theoretical tools constitute a useful complement to experimental studies to design more efficient hybrid architectures. Thanks to the small size of the gold NC, it is possible to work within the Density Functional Theory (DFT) framework and its time-dependent extension (TD-DFT).21,22 However, one has to face two computational challenges. On the one hand, it is quite difficult to find an exchange-correlation functional that is able to satisfactorily describe the structural and electronic properties of both the metallic nanocluster and

INTRODUCTION In the course of developing electro-, photo-, or bioactive devices, the integration of molecular systems with the unique opto-electronic properties of gold nanoparticles (NPs) has attracted an ever-growing attention.1 The challenge consists of controlling and understanding the interactions between the NPs and the molecule to design new functional materials presenting emerging properties. For the last 10 years, consequent efforts have been devoted to the possibility of activating or deactivating the photophysical properties of a chromophore after grafting onto a NP.2−6 In particular, the presence of gold NP was shown to quench the fluorescence of organic molecules via energy transfer mechanisms.7−10 More recent works have considered the opportunity of using quantum-sized gold nanoclusters (NCs) presenting a subnanometer to ∼2 nm core size. Unlike large gold NP whose optical properties are dominated by surface plasmon resonances, gold NCs exhibit discrete electronic structure and molecule-like properties.11,12 Among the gold NC family, thiolate-protected Au25 nanoclusters (Au25(SR)18) have been extensively studied over the past decade because they possess high thermal and chemical stabilities.13−19 These Au25 NCs constitute promising candidates for nanostructured devices because they exhibit redox, photoluminescence, and catalytic activities. With the aim of elaborating photoresponsive molecule−NP hybrid devices, Au25 NCs functionalized by azobenzene derivative thiolates were recently synthesized.19 It was shown that the redox and optical absorption of the hybrid compound could be modulated after isomerization of the azobenzene photoswitches, thus proving the interplay between the NP and the organic moiety. In the same vein, Devadas and co-workers18 have recently synthesized a pyrene−Au25 hybrid system. The compound © 2014 American Chemical Society

Received: October 30, 2013 Revised: January 27, 2014 Published: January 28, 2014 4444

dx.doi.org/10.1021/jp410695v | J. Phys. Chem. C 2014, 118, 4444−4453

The Journal of Physical Chemistry C

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Figure 1. (a) Representation of the Au25(C6S)17PyS− system. (b) Snapshot extracted from MD simulation.

In this contribution, we are particularly interested in the UV−vis absorption spectrum of the Au25(C6S)17PyS− system. We aim to propose a protocol that is able to reproduce and rationalize the optical properties of the hybrid system in the Franck−Condon region. To this purpose, in a first stage, we define a computational methodology that can satisfactorily model both the structural and the electronic properties of the organic−inorganic hybrid system. This method relies on a three-step protocol based on Molecular Dynamics simulations to generate starting structures, DFT geometry optimizations, and TD-DFT calculations. Within this methodological framework, we next present an analysis of the absorption spectra of the hybrid system. In a last step, we modify the pyrene skeleton to design a hybrid compound presenting emerging UV−visible optical properties.

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Figure 2. (a) Structure of the Au25(SH)18− system. For clarity, the Au13 core is represented with balls and the Au12(SH)18 external shell with a tube representation (Au, yellow; S, pink; H, white). (b) Scheme of the pyrene bond labeling. For pyrene, Ri = H with i = 1−4, and for PySH, R1 = CO−NH−(CH2)2−SH and Ri = H with i = 2,3,4.

METHODOLOGY Conformational Analysis. To investigate the wide range of orientations that the pyrene moiety can adopt with respect to the metallic NC, Molecular Dynamics (MD) simulations have been performed. Such a systematic conformational study remaining out of range for DFT calculations, we used Force Field Molecular Mechanics. To this purpose, we considered the Au25 cluster grafted with 17 alkylthiolate chains (C6H13S) and a substituted thiolated pyrene molecule (PyS) represented in Figure 1. We applied the simulation methodology proposed by Malfreyt et al. to study the structural properties of selfassembled monolayers of alkanethiolates on gold.28,29 For the grafted molecules, we considered the Cornell force field AMBER, 30 while the Au parameters were taken from the work of Ayappa and co-workers.31 The partial charges of the different atoms were calculated within the DFT framework with the Gaussian 09 package.32 We considered Mulliken atomic charges computed with the PBE0 functional;33,34 the 6-31G(d) basis set has been applied for the C, S, and H atoms, and the relativistic double-ζ LANL2DZ effective core potential (ECP) and basis set was used for Au atoms.35 The latter basis set treats explicitly the 5s, 5p, 5d, and 6s electrons of Au. Hereafter, this basis set (BS) will be denoted as BS1. Simulations were run with the DL_POLY_MD package36,37 in a vacuum considering a unique Au25(C6H13S)17PyS− system

the conjugated molecule. On the other hand, to model and analyze the part of the UV−visible absorption spectrum involving electronic excitations localized on the conjugated molecule, one has to compute a large number of excited states. This is due to the large number of low energetic transitions corresponding to metal → metal excitations. Therefore, few theoretical studies have been dedicated to the computation of the optical properties of hybrid organic/inorganic systems. Previously, Corni and co-workers have proposed a computational strategy based on a hybrid quantum-mechanical/ continuum model (QM/CM) description to estimate the changes in the radiative and nonradiative decay rates of a chromophore near a surface.23−25 However, in these investigations, where the molecule is not directly grafted onto the metal NP, there are strong energy transfers between the organic and metallic moieties, but there is no formation of a “hybrid” system with orbitals delocalized on the overall compound. So far, for organic molecules directly grafted onto metallic aggregates, only hybrid structures with very small gold nanoclusters (for instance Au3, Au9, Au13) have been considered.26,27 4445



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the organic part, it has been shown that long-range corrections included in range-separated hybrid (RSH) functionals allow one to obtain an accurate estimate of charge transfer excitation energies (see, for instance, refs 50−52), a crucial problem for our systems. Besides, for the pyrene moiety, it is known to be a challenge to predict the correct ordering of excited states with TD-DFT.53,54 A recent study53 has shown that the CAMB3LYP RSH functional55 was able to obtain the correct ordering of the excitation energies and to reproduce the general trends in wavelengths observed experimentally. In the Theo4 scheme, we have thus selected the latter functional combined with BS1 and used the Gaussian 09 suite. Solvent effects are described with the PCM approach in the linear-response nonequilibrium model56 that is adequate for evaluating absorption spectra. To plot the different molecular orbitals, we systematically used a contour threshold of 0.03 au. To obtain convoluted absorption spectra, a broadening Gaussian with fwhm = 0.13 eV has been applied.

without periodic boundary conditions. The equations of motion were integrated using the Verlet leapfrog algorithm scheme at T = 298 K with a time step of 0.2 fs. We ran our simulations in the NVT ensemble using the Hoover thermostat with a coupling constant of 0.5 ps. A typical simulation run consisted of an equilibration period of 500 ps and a production phase of 10 ns. During these simulations, we froze the positions of the Au25 metallic core and the sulfur atoms. These positions correspond to the structure of the [Au25(SH)18]− structure optimized with the computational strategy described below (Theo1). We emphasize that MD simulations only constitute a tool to generate starting structures for the subsequent computational step. Geometry Optimization. The following three-step procedure has been applied: (i) Different snapshots were randomly extracted from MD simulations. To decrease the computational cost, the 17 (C6H13S) chains were then replaced by SH groups. (ii) For the subsequent Au25(SH)17PyS- systems, the structure of the PyS subunit is minimized with a fixed geometry for Au25SH17. (iii) All of the constraints are removed, and the geometry of the hybrid system is fully optimized. For the geometry optimizations, we have tested two different computational strategies. On the one hand, we have used the Amsterdam density functional program (ADF).38 Relativistic effects were taken into account using the zero-order regular approximation (ZORA), 39 and DFT calculations were performed with the BP8640,41 exchange correlation functional and a triple-ζ polarized (TZP) ZORA basis set of Slater-type orbitals for all atoms. The frozen core approximation was used to describe the inner shells of the Au atoms, the frozen core orbitals including up to the 4f orbitals. Conductor-like screening model (COSMO) is used for the solvent effect description.42,43 Hereafter, this computational scheme will be denoted as Theo1. This level of theory has been shown to provide structural parameters of gold clusters in good agreement with experiment.17,44 On the other hand, for the organic part, the PBE0/6311G(d,p) level of theory is known to yield accurate structural parameters for most conjugated molecules.45,46 Using the Gaussian 0932 package, we have completed this computational scheme (hereafter Theo2) by considering the relativistic double-ζ LANL2DZ ECP and basis set for Au atoms. This basis set (BS) combination will be referred to as BS2. The bulk solvent effects have been included by means of the Polarizable Continuum Model (PCM).47 Computation of the UV−Visible Absorption Spectrum. We have calculated the first low-lying electronic excited states using the vertical TD-DFT approximation. One must note that confronting vertical TD-DFT results to experimental λmax constitutes an approximate approach, a more physically reference being the 0−0 energy E0−0, that is, the absorption/ emission crossing point.48 However, the E0−0 determination implies the optimization of the excited states, which is not computationally feasible for our hybrid system. For the isolated pyrene, 20 excited states were computed, while in the case of metallic and hybrid systems, 200 states were taken into account. At this stage, we have also tested two methods. For the metallic part, the statistical average of orbital potentials (SAOP)49 implemented in ADF combined with the triple-ζ polarized basis set was shown to yield excitation energies within 0.15−0.20 eV of experimental values.44 This computational approach, which relies on the COSMO model to introduce the solvent effects, will be referred to as Theo3. For

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RESULTS AND DISCUSSION Choice of a Computational Scheme. Geometry Optimization. For the NC (pyrene, respectively), the structural parameters obtained from X-ray experiments and calculated with both Theo1 and Theo2 approaches (in vacuum) are compared in Table 1 (Table 2, respectively). For the NC, we Table 1. Comparison of the Main Geometrical Parameters of Au25(SH)18− Determined from X-ray Crystallographic Analysis15 with Calculated Structures Obtained with Theo1 (ADF: ZORA-BP86/TZP) and Theo2 (G09: PBE0/BS2) Computational Schemesa Theo1

mean distance Au−Au (core) mean distance Au−Au (shell) mean distance Au−S (shell)

Theo2

X-rayb (Å)

calcd (Å)

error (%)

calcd (Å)

error (%)

2.784

2.841

2.0

2.841

2.0

3.558

3.623

1.8

3.700

4.0

2.333

2.388

2.4

2.403

3.0

a The differences between measured and calculated values are given in %. bCrystal structure of Au25(SR)18 with R = phenylethyl group.

Table 2. Comparison of the Geometrical Parameters of the Pyrene Determined from X-ray Crystallographic Analysis58,59 with Calculated Structures Obtained with Theo1 and Theo2 Computational Schemesa Theo1 a b c d e f dM BLA (102)b

Theo2

X-ray (Å)

calcd (Å)

error (%)

calcd (Å)

error (%)

1.386 1.402 1.436 1.422 1.353 1.427 1.406 2.580

1.395 1.407 1.436 1.430 1.365 1.425 1.412 2.187

0.65 0.36 0.00 0.56 0.89 0.14 0.43 14.4

1.388 1.398 1.432 1.421 1.355 1.421 1.405 2.354

0.1 0.29 0.28 0.07 0.15 0.42 0.08 8.7

a

See Table 1 for more details and Figure 2 for the designation of the bond types. bIn angstroms.

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Table 3. Vertical Transitions Obtained with Theo3 (ADF: COSMO(dichloromethane)-ZORA-SAOP/TZP) and Theo4 (G09: PCM(dichloromethane)-CAM-B3LYP/BS1) Computational Schemes for the Au25(SH)18− and the Pyrene Systemsa exp. NC

pyrene a

Theo3

Theo4

λ

λ

ΔE

description

λ

ΔE

description

404 452 685 340

432 473 817 423

−0.20 −0.12 −0.29 −0.71

p(S),d(Au)→sp(Au) p(S),sp(Au),d(Au)→sp(Au) p(S),sp(Au)→sp(Au) HOMO→LUMO

331 384 617 335

+0.67 +0.49 +0.20 +0.05

p(S),d(Au)→sp(Au) p(S),sp(Au),d(Au)→sp(Au) p(S),sp(Au)→sp(Au) HOMO→LUMO

Wavelengths (λ, in nm), energetic difference between experimental and calculated excitation energies (ΔE, in eV), and state description are given.

Figure 3. (a) Definition of the angles indicating the position of the pyrene moiety with respect to the nanocluster θ1, θ2, and ϕ. (b) Representation of the three conformers (1, 2, 3) extracted from MD simulations.

have determined the structure of the Au25(SH)−18 system, which is represented in Figure 2a. This gold cluster presents a Ci symmetry and is based on a central icosahedral Au13 core surrounded by an exterior Au12(SH)18 shell.15 As shown in Table 1, the Au−Au core distance is reproduced with the same accuracy by Theo1 and Theo2 models, while for the exterior shell, Theo1 yields structural parameters in better agreement with experiment. For the pyrene molecule, we have investigated the structural properties of the system depicted in Figure 2b with Ri = H (i = 1−4). Following the theoretical work of Ottonelli et al. dedicated to pyrene-based systems,57 we have introduced two geometrical parameters: (i) the mean length dM, which is the average of the different C−C distances, and (ii) the bond length alternation BLA, which is defined in the foregoing work as: BLA =

1 N

To conclude, we can see that both computational strategies yield geometrical parameters in good agreement with experiments, for both the metallic and the organic moieties. We have thus decided to carry out all of the geometry optimizations with Theo1. This choice can be rationalized by the comparison of the computational burden: the Theo1:Theo2 computational time ratio actually reaches 1:15. TD-DFT Calculations. The results of the TD-DFT calculations performed with Theo3 and Theo4 computational schemes in dichloromethane are summarized in Table 3 for both the Au25(SH)−18 and the pyrene moieties optimized within the Theo1 framework. For the metallic nanocluster, both methods lead to an absorption spectrum dominated by three bands. These bands encompass several electronic transitions involving mainly Au sp and d orbitals. The agreement between Theo3 calculated absorption bands and experimental data is very satisfactory with an average signed error of −0.20 eV, while the excitation energies calculated with the Theo4 are overestimated (average error of 0.45 eV). For the pyrene molecule, the trends are opposite: the position of the maximum absorption band, which corresponds to a HOMO → LUMO electronic excitation, is well reproduced by Theo4 with a signed experiment-theory error of −0.05 eV but not by Theo3, which predicts a significantly too small λmax value (+0.71 eV error). Consequently, there is no method that can provide an accurate description of the optical properties of both the NC and the chromophore. In this work, we focus our attention on the modification of the absorption spectrum of the pyrene moiety after grafting on the metallic aggregate. We have thus chosen to work within the Theo4 framework, which is the method of

∑ (di − dM)2 i

(1)

with N being the number of C−C bonds and di being the ith bond length. The BLA is a measure of the π-electron delocalization: this index decreases with the conjugation. Table 2 shows a very good agreement between calculated bond parameters and X-ray data.58,59 Indeed, both Theo1 and Theo2 models yield structural differences between experimental and theoretical C−C distances lower than 1%. As expected, a closer examination of the results shows that the Theo2 computational scheme provides a slightly better description of the organic moiety. 4447



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choice to deal with organic chromophores. At the same time, despite the blue shift of the NC calculated spectra, this latter − scheme provides a satisfactory description of Au25(SH)18 electronic excitations, as shown by the convoluted absorption spectrum in the Supporting Information. Figure S-2 of the Supporting Information also shows the influence of the geometry optimization scheme on the optical properties. To go further, within the Theo4 framework, we have compared our results for the pyrene, obtained with the linearresponse (LR) approach, to the state-specific (SS) approximation60 in its nonequilibrium limit. Contrary to LR, the SS model accounts for the variations of the polarization of the medium following the electronic density rearrangements of the solute after absorption. We show that using the LR PCM approximation is sufficient to reproduce the absorption spectra, the computed λmax being, respectively, 335 nm with LR PCM and 328 nm with SS PCM. Study of the Au25(C6S)17PyS− System. Conformational Study. To describe the position of the pyrene moiety within the Au25(C6H13S)17PyS− system, we consider one dihedral angle ϕ that represents the orientation of the pyrene plane and two angles θ1 and θ2 that define the position of the pyrene carbon atom bridging the molecule to the NC. These different angles are depicted in Figure 3. Along the MD trajectory, the time evolution of these angles (given in Figure S-3 in the Supporting Information) shows that θ1 and θ2 are nearly constant; for example, the anchoring carbon atom of the pyrene can not move around. Indeed, the global motion of the −CO−NH− (CH2)2−S linker in the neighborhood of the NC is impeded by the (C6H13S) lateral chains. However, the value of ϕ dihedral angle shows strong variations: the pyrene, more distant from the NC, can adopt a wide range of plane orientation with respect to the NC. We have randomly extracted three different conformers and replaced the (C6H13S) chains by thiol groups. These structures (1, 2, 3) are represented in Figure 3, and the main energetic and geometrical properties obtained with Theo1 framework are given in Table 4. The comparison of the pyrene structures,

Figure 4. Absorbance spectra computed with the Theo4//Theo1 computational scheme in dichloromethane for (a) PySH (green line), (b) Au25(SH)−18 (black dotted line), and (c) Au25(SH)17PyS− (red line). The upper panel is the corresponding experimental plot reprinted with permission from ref 18. Copyright 2010 American Chemical Society.

PySH and Au25(SH)−18 optical features. In accordance with experimental observations,18 the absorbance spectrum of the hybrid structure is a simple addition of the pyrene and metallic NC optical properties. In the 250−320 and 340−800 nm ranges, the calculated UV−vis spectrum is dominated by metal localized excitations, and we recover perfectly the Au25(SH)−18 spectrum. In Figure 5, a close inspection of conformer 1 absorption spectrum reveals that the 330 nm band encompasses three different transitions. The 330 and 331 nm transitions correspond to electronic excitations localized on the NC moiety, whereas the 329 nm transition arises from a pyrene localized excitation resulting from a HOMO−3 → LUMO+2 electron promotion. Figure 6 shows that the corresponding orbitals are similar to the frontier orbitals of the isolated PySH system. As a reminder, for PySH ((a) in Figure 4), the main absorbance band peaking at 335 nm corresponds to a HOMO → LUMO excitation. Therefore, for Au25(SH)17PyS−, the 330 nm band involves electronic excitations localized either on the nanoparticle or on the pyrene moiety, but there is no mix between the organic and metallic transitions. In accordance with transient absorption studies,18 our calculations show that a 345 nm irradiation wavelength can excite both the Au cluster and the pyrene chromophore with roughly the same probability, given their comparable calculated absorbance intensities. We can thus conclude that there is no communication between the organic part and the metallic nanoparticle, and this yields the preservation of the structural and optical properties of the pyrene moiety (hereafter, the wavelength corresponding to an electronic excitation localized on the PyS moiety will be denoted as λPyS). This finding is also supported by the analysis of (i) the density of states, (ii) the charge transfer upon photo excitation, and (iii) the influence of the conformation on the optical properties. For (i), Figure S-4 in the Supporting Information

Table 4. Comparison of the Geometrical and Energetic Parameters of the Pyrene Moiety for Different Conformationsa isolated pyrene BLA θ1 θ1 ϕ Erel λPyS

2.232

335

1

2

3

2.219 117.6 83.8 −148.8 0.0 330

2.247 120.3 74.1 16.0 1.5 332

2.233 105.9 66.5 −32.6 1.9 336

The definition of the θ1, θ2, and ϕ angles is given in Figure 3. The BLA is in angstroms, the angles are in degrees, and the relative energy Erel with respect to conformer 1 is in kcal mol−1. λPyS (in nm) is the calculated absorption wavelength corresponding to the electronic excitation localized on the PyS moiety. a

isolated versus grafted onto the cluster, shows small variation of the BLA and thus trifling modification of the conjugation within the organic moiety after immobilization. As expected from the MD analysis, these three conformers primarily differ from the orientation ϕ of the pyrene plane with respect to the NC. Optical Properties. The computed UV−vis spectrum of the most stable conformer, 1, is shown in Figure 4 and compared to 4448



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Figure 5. Absorbance spectra computed with the Theo4//Theo1 computational scheme in dichloromethane for Au25(SH)17PyS−. The stick spectrum is also reported.

Molecular Design: Increasing the Communication between the NC and the Pyrene. The absence of electronic communication between the metallic and the organic moieties can be rationalized by an analysis of the fragment orbitals. In Figure 8, the molecular orbital energy diagram of the Au25(SH)−18 fragment is compared to the PyH frontier orbitals (for the fragment decomposition, we consider that the SH termination does not belong to the pyrene moiety but to the gold NC). Because of energetic considerations, the interaction between the NC and pyrene orbitals is weak, and, as previously described, we thus obtain a superposition of the fragments DOS. To create an interaction between the metallic and organic moieties, we have first reduced the distance between the NC and the pyrene core by modifying the linker at the R1 position. Indeed, we have directly connected the pyrene moiety to the NC. To optimize the energy match between the pyrene frontier orbitals and the NC “bands”, we then have modified the pyrene skeleton to tune the position of the HOMO−LUMO gap. Figure 8 shows that the NC presents several occupied molecular orbitals (encompassing the HOMO) in the [−5.75; −5.5 eV] range and a high density of virtual molecular orbitals above −0.5 eV. The energies of PyH frontier orbitals being lower than the positions of the NC occupied and virtual “bands”, we have introduced electron-donating substituents to destabilize both the HOMO and the LUMO. A recent theoretical work has demonstrated that the largest variation of the electronic properties is expected with the introduction of substituents at the R2, R3, and R4 positions depicted in Figure 2.20 One, two, or three NH2 substituents were thus introduced in these different positions. Figure 8 shows that the resulting biand trisubstituted NH2 compounds present an energy match for both the occupied and the unoccupied orbitals. Because of steric considerations, we could not consider the grafting of a trisubstituted pyrene moiety onto the metallic cluster, and we thus studied the electronic and optical properties of the bisubstituted compound Au25(SH)17Py(NH2)2S−. Figure 9 enables the comparison of the computed absorption spectrum of Py(NH2)2SH, Au25(SH)−18, and Au25(SH)17Py(NH2)2S−. The isolated Py(NH2)2SH presents a strong

Figure 6. Molecular orbitals of PySH (left) and Au25(SH)17PyS− (right).

shows that the density of state (DOS) of the hybrid structure is a perfect addition of the organic and metallic DOS. For (ii), the repartition of the Mulliken charge distribution between Au25(SH)17 and PyS is given in Figure 7 for the ground state and the excited state localized on the pyrene moiety. For the ground state, the NC bears the negative charge (∼ −1.1|e|), and upon irradiation with λPyS, the charge distribution remains constant. Therefore, in the Franck−Condon region, if one excites the pyrene moiety, there is no photoinduced charge transfer. One may note that for molecular systems including gold atoms, analysis of the Mulliken charge distribution is the population scheme generally considered.61,62 For (iii), the UV− vis optical properties for the 2 and 3 conformers depicted in Figure 3 have been calculated. As shown by Table 4, λPyS does not depend on the orientation of the PyS moiety with respect to the metallic NC (the convoluted absorption spectra of the three different conformers are compared in Supporting Information Figure S-5). This finding also corroborates the lack of communication within the hybrid system. 4449



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Figure 7. Mulliken charge distribution of the ground state and of the excited state(s) localized on the pyrene moiety for (a) Au25(SH)17PyS− and (b) Au25(SH)17Py(NH2)2S−. The Mulliken charges are, respectively, summed over the Au25(SH)17 nanoparticle and the organic moiety.

Figure 8. Orbital energy diagram of Au25(SH)18− and different substituted pyrene compounds.

absorption band at 356 nm corresponding to a HOMO → LUMO electronic excitation (the frontier orbitals are given in Figure 8). As expected, the absorbance spectrum of the hybrid structure is no longer a simple superposition of the pyrene and

NC optical features. Within the hybrid system, the UV−vis properties of the NC are mainly preserved, while the absorption band of the organic moiety disappears and a large absorption band with a shoulder appears in the 360−420 nm region. This 4450



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Figure 10. Molecular orbitals of Au25(SH)17Py(NH2)2S−.

and LUMO orbitals, the absorbance intensity of the 413 nm band is small but not negligible. The second transition involving some pyrene localized orbitals peaks at λPyS,2 = 373 nm and presents a strong absorbance intensity. It mainly corresponds to a HOMO−3 → LUMO+5. As depicted in Figure 10, the LUMO+5 is similar to the first virtual orbital of the NH2 disubstituted pyrene with some non-negligible contribution on the cluster moiety. The corresponding excited state presents a charge transfer character: Figure 7 shows that upon λPyS,2 excitation, there is a charge transfer of 0.12|e| from the pyrene to the NC. By modifying the pyrene skeleton, we thus expect to obtain a new functional material presenting emerging properties due to strong charge transfer character.

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CONCLUSIONS AND OUTLOOK The structure and the electronic properties of a hybrid pyrene− Au25(SH)−18 nanocluster system have been investigated using (Time-Dependent) Density Functional Theory. More precisely, we have set up a three-step computational protocol based on (1) Molecular Dynamics simulations to determine the position of the chromophore with respect to the metallic NC, (2) geometry optimizations of MD snapshots with the ZORA Hamiltonian within the BP86/TZP framework, and (3) computation of the UV−vis absorption spectrum using TDDFT with the CAM-B3LYP functional combined with 631G(d) and LANL2DZ basis sets. This computational protocol aims at describing the modification of the absorption spectrum of the chromophore after grafting onto the metallic NC. Our method allowed one to rationalize the experimental observations.18 For Au25(SH)17PyS−, the absorption spectrum of the hybrid system is a simple superposition of the pyrene and Au25 cluster optical spectra. Because of small spatial overlap and the lack of energy matching between the organic and metallic frontier orbitals, there is no frontier orbital interaction between the two systems and no excited state presenting a charge transfer character in the Franck−Condon region. By modifying the pyrene skeleton, that is, reducing the linker length and introducing NH2 electron-donating substituents, we have tuned the optical properties of the organic moiety and enabled the electronic communication within the hybrid system. The proposed hybrid system thus presents emerging absorbance optical properties with excited states presenting a pyrene → NC charge transfer character in the Franck−Condon region. From a synthesis point of view, the large curvature of the small NC enables the existence of a reserved space where the pyrene

Figure 9. (I) Superposition of the absorbance spectra calculated for (a) Py(NH2)2SH (green line), (b) Au25(SH)18− (black dotted line), and (c) Au25(SH)17Py(NH2)2S− (red line). (II) Detailed stick and convoluted absorbance spectrum of Au25(SH)17Py(NH2)2S−. See Figures 4 and 5 for more details.

large band encompasses several metal localized excitations and two transitions involving molecular orbitals localized on the pyrene derivative moiety. The λPyS,1 = 413 nm transition corresponds to a HOMO−3 → LUMO electron promotion. Figure 10 shows that the HOMO−3 mainly corresponds to the last occupied orbital of Py(NH2)2SH with some contributions on the cluster side, while the LUMO is a NC virtual orbital. The HOMO−3 → LUMO transition thus corresponds to a strong pyrene → NC charge transfer transition as also demonstrated by the analysis of the charge distribution upon photoexcitation in Figure 7. Upon λPyS,1 irradiation, the NC becomes more negatively charged, bearing a negative charge of −1.08|e| for the ground state and −1.77|e| for the excited state. A careful analysis of the variation of the Mulliken charges upon excitation shows that the transferred density is distributed on the entire nanocluster (see Supporting Information Figure S-6). Because of the weak spatial overlap between the HOMO−3 4451



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can be directly grafted onto the metallic moiety (see Supporting Information Figure S-7). Besides, these hybrid systems are reachable because the major difficulty arising from the direct grafting of the pyrene derivative on the gold NC has already been overcome with larger gold NP (5−8 nm size). 63 To investigate the UV−vis absorbance properties of hybrid organic−inorganic systems, we have thus pushed the limits of TD-DFT in terms of the system complexity and of number of excited states to compute. We have now to face two different challenges for ab initio spectroscopists: increasing the size of the metallic NP and studying the relaxation of the excited states to rationalize the observed fluorescence quenching phenomena in terms of energy and/or electron transfer.

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ASSOCIATED CONTENT

S Supporting Information *

Theoretical absorption spectrum of Au25(SH)18−; Molecular Dynamics simulation: time evolution of ϕ dihedral angle and (θ1, θ2) angles; density of states; influence of the conformer orientation on the absorbance spectrum: representation of the convoluted spectra; analysis of the transferred electronic density for Au25(SH)17Py(NH2)2S−; and representation of the Au25(C6H13S)17Py(NH2)2S− system. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: arnaud.fi[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research used resources of the GENCI-CINES/IDRIS (Grant c2011086680) and the ITODYS local cluster. We are grateful to Dr. Florent Barbault for his help concerning the manipulation of VMD visualization software.

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REFERENCES

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Modeling the Absorbance Properties of a Pyrene Chromophore

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