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Photoinduced Ultrafast Charge Transfer and Charge Migration in Small Gold Clusters Passivated by a Chromophoric Ligand Valérie Schwanen, and Francoise Remacle Nano Lett., Just Accepted Manuscript • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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Photoinduced Ultrafast Charge Transfer And Charge Migration In Small Gold Clusters Passivated By A Chromophoric Ligand Valérie Schwanen and Francoise Remacle* Theoretical Physical Chemistry, UR MOLSYS, University of Liège, B4000 Liège, Belgium

*

Corresponding author. Email [email protected], fax : + 32 4 3663413 ACS Paragon Plus Environment

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Abstract Since the development of attopulses, charge migration induced by short optical pulses has been extensively investigated. We report a computational purely electronic dynamical study of ultrafast few fs charge transfer and charge migration in realistic passivated stoichiometric Au11 and Au20 gold nanoclusters functionalized by a bipyridine ligand. We show that a net significant amount of electronic charge (0.1 to 0.4 |e| where |e| is the electron charge) is permanently transferred from the bipyridine chromophore to the gold cluster during the short 5-6 fs UV-VIS strong pulse. This electron transfer to the metallic core is induced by the optical excitation of electronic states with a partial charge transfer character involving the chromophore, before the onset of nuclei motion. In addition the photoexcitation by the strong fs pulse builds a non-equilibrium electronic density that beats between the chromophore and the metallic core around the average of the transferred value. Modular systems made of a donor chromophore that can be photoexcited in the UV-VIS range coupled to an efficient acceptor that could trap the charge are of interest for applications to nanodevices. Our study provides understanding on the very early, purely electronic dynamics built by the fs optical excitation and the initial charge separation step.

Keywords: charge transfer, charge migration, gold cluster, attochemistry, electronic quantum dynamics

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The recent progress in the engineering of short, few cycle optical atto pulses1-5 has made possible to investigate ultrafast dynamics in molecular systems on short, sub and few fs time scales. Electron localization and charge migration have been probed in diatomic and larger molecules using various pump-probe schemes involving atto and fs optical pulses.3, 6-21 Special emphasis has been devoted both theoretically22-29 and experimentally13,

20, 21

to ultrafast

charge migration in modular systems, made of a donor chromophore that can be photoexcited in the UV-VIS range and an efficient acceptor that could trap the charge. Controlling the early steps of electronic charge migration and separation on a fs time scale will allow exploiting ultrafast processes in natural30 and designed transition metal complexes31 and at the interface of organic heterojunctions32, 33 and hybrid nanostructures.34, 35 The time scales involved in charge separation in redox chemistry are usually longer, on the order of tens of ps, all the way to the µs and ms range, because the transfer and the trapping of charge on an acceptor site typically involves a significant rearrangement of the nuclei and transitions between singlet and triplet states.36 , 37 The development of pump-probe schemes on a sub femtosecond time scale allows investigating the early steps of photoinduced electronic dynamics and if charge transfer can be engineered from tailoring the non-equilibrium electronic density resulting from exciting the system on an ultrashort time scale before the onset of the response of the nuclei.22 We report a computational dynamical study of ultrafast charge transfer and charge migration in passivated stoichiometric gold nanoclusters functionalized by a neutral substituted bipyridine ligand. We show that it is possible to engineer a permanent charge transfer from the bipyridine chromophore to the gold metallic core by tailoring a short strong few fs UV-VIS pulse to access electronic states with a partial chromophore-metallic core charge transfer character. The dynamical simulations demonstrate that a net significant amount of electronic charge (0.1 to 0.4 |e| where |e| is the electron charge) is transferred during the few fs pulse from the bipyridine ligand to the gold cluster before the onset of significant nuclei motion. The photoexcitation by the strong short fs pulse builds a non-equilibrium electronic density that beats between the two moieties around the average of electronic charge that is ACS Paragon Plus Environment

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permanently transferred. The beating periods of the coherent charge migration are determined by the transition frequencies between the electronic states accessed during the pulse. We designed our model systems starting from two gold clusters Au11(PPh3)7Cl338 and Au20(TBBT)1639 that have been experimentally synthesized and characterized. The two clusters have significantly different passivation shells, which influences both their electronic and structural properties. The Au11 cluster possesses a mixed ligand shell consisting of both electron withdrawing (chlorine) and electron donating (phosphine) ligands around a C3v metallic core. The passivation shell of the Au20 cluster consists of electron withdrawing thiolate ligands that tend to form -S-Au-S- staple motifs.40 As a result, the metallic core of the Au20 cluster exhibits a Au7 kernel surrounded by –S-Au-Sstaples. To build modular systems that can be suitably photoexcited, we substituted a chlorine in the ligand shell of the Au11 cluster and a thiol ligand in the Au20 one by a S-(CH2)n-(4,4’)bipyridine-CH3 chromophore. Two lengths of the alkane linker, -(CH2)2- and –(CH2)4-, were studied for both clusters. Ligand exchange41 can be easily achieved by choosing stoichiometric amounts of reactants such that on average only one ligand per cluster is exchanged, as has recently been reported for pyrene42 on Au25(SR)18. Our choice is motivated by the recent experimental43-45 and theoretical studies46-49 of small stoichiometric gold nanoclusters (NCs) with a diameter of less than 2nm, also called “aspicules”,40. They present several advantages for applications in catalysis,50 in biomedicine and biosensing,51 or in photovoltaics.52 Gold NCs can be crystallized and are small enough to be put in the gas phase by electrospray ionization techniques that allows to characterize their stoichiometry.53-55 Their photophysical properties can be studied both in solution and in the gas phase, which opens the way to subject them to ultrashort few fs pulses. Few nanometer gold NCs are molecular-like compounds with discrete energy levels and HOMOLUMO (H-L) gaps of a few eV,56 that exhibit optical properties57-59 significantly different from the surface plasmon resonance observed in larger gold nanoparticles (NPs).

60-62

By properly choosing a


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substituting ligand so that its H-L gap matches the H-L gap of the gold cluster, it is possible to engineer electronic coupling between the core and the ligand.63 Recently, new photo-responsive materials based on gold NCs, such as a Au25 with full azobenzene coverage,64 have been reported. Devadas and coworkers42 experimentally studied the electron transfer from the gold core to the pyrene molecule in the Au25(C6S)17PyS system, which causes the quenching of the pyrene’s fluorescence. The structural and electronic properties of this system were theoretically investigated by Perrier at al,63, 65 as well as the photoactivity of a dithienylethene moiety grafted on gold NCs66 and NPs.67 Previous ultrafast electron dynamics photochemistry studies on gold NCs mostly focused on the investigation of the relaxation dynamics of the excited gold core by time-resolved spectroscopy measurements.68,

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Theoretical efforts were dedicated to the understanding of the relaxation

mechanism of the excited Au25(SR)18- cluster,70 whose picosecond relaxation dynamics has been experimentally investigated by several groups.71-73 Zhou et al. identified a picosecond internal charge transfer between the semi-ring shell and the Au7 core upon photoexcitation for the Au20(SR)16 system.74 On the other hand, femtosecond charge transfer dynamics in self-assembled monolayer of cyanoterminated molecules on a gold substrate has recently been reported.75 Subsequent theoretical work with a gold substrate model illustrated that the electron transfer time scale depends on the length of the aliphatic chain76 while simulations on small gold clusters showed that the ultrafast charge transfer dynamics is insensitive to the cluster size.77 Our aim here is to investigate the ultrafast non equilibrium electronic dynamics that results from the photoexcitation of a chromophore grafted on a gold NC by a short, strong 5-6 fs UV-VIS optical pulse. Such pulses are becoming available.78,

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The dynamical simulations are carried out by solving the

time-dependent Schroedinger equation for the frozen ONIOM equilibrium geometry of the ground electronic state (GS) of the cluster. On the time scale of the simulations reported below, we do not expect the cluster to undergo significant geometry changes. The gold atoms are heavy and slow to respond to the change in electronic density. The substituted bipyridine ligand is planar in its neutral and ACS Paragon Plus Environment

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+1 charge state and becomes twisted in the dication80. In the neutral clusters studied here, the bipyridine moiety remains planar and we do not expect the charge transfer to trigger significant nuclear rearrangement of the chromophore. To build the model systems, we started from the crystallographic structure of each cluster, i.e., Au11(PR3)7Cl338 and Au20(SR)1639, in which we substituted either a chlorine or a thiol ligand by the chromophore S-(CH2)n-(4,4’)bipyridine-CH3 group, n

= 2 and 4. In order to compute the

photoexcitation dynamics for a realistic orientation of the chromophore with respect to the gold core, we first determined the equilibrium geometry of the substituted clusters at the CAM-B3LYP81/UFF ONIOM level with Gaussian09,82 using the phenyl R groups for Au11 and TBBT (-SPh-t-Bu) for Au20 (for details, see Supplementary Information (SI), section 1). In the ONIOM equilibrium geometry the substitution by the chromophore ligand slightly distorts the gold core of the undecagold cluster which loses the approximate C3v symmetry of the crystallographic structure. In both clusters, the substituted (4,4’) bipyridine moiety is planar and aligns perpendicularly to the metallic core since its folding on the gold core is hindered by the full ligand coverage used in the ONIOM procedure. We then substituted the R groups of the phosphine ligands in Au11 by H atoms and the R groups of the thiolate ligands in Au20 by CH3 in the ONIOM equilibrium geometry to reduce the cost of the computation of the excited electronic states. Two linker lengths were investigated for each cluster, a (CH2)2- linker called ‘short’ in the following and a -(CH2)4- linker called ‘long’. We focus in the main text on Au11short and Au20long. For completeness, all the details about Au11long and Au20short are given in the SI. The geometries of Au11(PH3)7Cl2S-(CH2)2-(4,4’)bipyridine-CH3 (‘Au11short’), and Au20(SCH3)15S-(CH2)4-(4,4’)bipyridine-CH3 (‘Au20long’) are reported in Figures 1a and 1b respectively (see Figures S1a and S1b for ‘Au11long’ and ‘Au20short’). The partial charge on the metallic core of Au11short is about -0.2 |e|, due to its mixed electron donating – electron withdrawing passivation shell. The partial charge on the gold atoms in Au20long is 3.5 |e| (SI, Table S1). This large positive value is due to the gold atoms involved in staple motifs (3.6 |e|) that compensate for the ACS Paragon Plus Environment

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electron withdraw from thiol ligands (-2.6 |e|), which leaves a -0.1 |e| partial charge on the Au7 kernel. The partial charge on the bipyridine subunit is about -0.45 |e| for all systems. The partial charges in Au11long and Au20short are similar (SI, Table S1).

Figure 1. Geometries of the electronic ground state for Au11short (a) and Au20long (b) and the molecular frame orientation. The x direction is roughly parallel to the bipyridine-gold core distance. Colour code: Au-yellow, Cl-green, S-light blue, P-orange, N-dark blue, C-grey and H-white. Schematic energy level structure (with a degeneracy threshold of 0.13eV) of the frontier MO’s for Au11short (c) and Au20long (d). Isocontours of selected MO’s are shown as inset (isovalue 0.02 e/Å3). See Figure S1 for Au11long and Au20short. An essential feature in order to be able to engineer a net charge transfer from the chromophore to the gold core by photoexcitation is that the energy of the HOMO of the free bipyridine ligand lies within the HOMO-LUMO gap of the non-modified gold clusters. The patterns of the frontier molecular orbitals of the two model clusters Au11short and Au20long, computed at the CAM-B3LYP level as described above, are plotted in Figures 1c and 1d (see Figure S1c and S1d for Au11long and Au20short). On can see that for both clusters the HOMO is localised on the bipyridine group which results in a value of the HOMO-LUMO gap of the model systems smaller than the HOMO-LUMO gap of the nonACS Paragon Plus Environment

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modified clusters. In addition, the energies of the MO’s localized on the gold core are basically left unchanged by the substitution of the chromophore in the ligand shell. The Au11 cluster possesses Super-Atomic Molecular Orbitals49 (SAMO) in the frontier region, on the contrary to Au20 cluster. In both clusters, the lowest unoccupied MO’s are localised on the metallic core. We therefore expect charge transfer excitations from the chromophore to the metallic core to play an important role in the optical spectra of the clusters. The stationary excited electronic states were computed with the linear response time-dependent density functional theory (TD-DFT) as implemented in Gaussian09.82 This level of electronic structure theory, while known to have drawbacks for the accurate determination of charge transfer excited states83-88 allows computing the large band of field-free electronic excited states of a different character (local and charge transfer (CT)) that is needed to carry out the dynamical simulations. Hybrid functionals generally yield a better description of the CT excited states. The stability of the excitation energies was examined using two hybrid functionals, CAM-B3LYP81 and wB97XD89 (Figures S2 and S3). As expected, the energies of the CT states were found to be functional-dependent whereas the excitation energies of the local excited states remained similar. In order to describe as well as possible the relative energies of local and CT excited states, the amount of long-range exact Hartree-Fock exchange in the CAM-B3LYP functional was therefore tuned (SI, section 3).90 The tuning of the functional also ensures that all the electronic excited states of our field-free basis set are below the computed ionization potential of the clusters. From the tuning procedure, we determined an optimal fraction of 80% of long-range exact HF exchange for Au11 and 82% for Au20. We computed the value of the delocalization Tozer parameter91 Λ for the excited states of interest in order to check the validity of this approach (Tables S3 and S4). The pure local excited states on the gold core (or on the chromophore) are optically active, while the pure chromophore to gold core CT excited states are optically inactive. Several excited states possess a


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mixed nature with only a fraction of CT character, and several of those mixed excited states are optically active. Those mixed CT-local excited states are the states of interest in our study.

Figure 2. TD-DFT computed stick spectra of the optical activity and of the CT character of the excited states of (a) Au11short, (b) Au20short, (c) Au11long and (d) Au20long. The red vertical lines give the oscillator strength of each excited state (left axis) and the CT character is quantified by the electron density difference on the substituted bipyridine ligand in the excited state with respect to the ground state, green vertical lines (right axis). The UV-Vis spectra (black curves) are obtained by a convolution of the stick spectrum with a Gaussian line shape (σ=0.085 eV) to account for broadening effects. Figure 2 summarizes the optical activity and CT character of the excited electronic states of the four model clusters, Au11short and long in Figure 2a and 2c, Au20short and long in Figure 2b and 2d. All spectra exhibit two absorption bands, one centred at 350 nm and one at 300 nm. In both bands the fraction of mixed CT-local excited states (ES) is smaller for the long than for the short linker, while in general the fraction of purely local or purely CT excited states increases for the long ligand. The highest absorption band corresponds to the π − π * transition of the bipyridine chromophore and is the most intense for all clusters. In that band, they are few ES of a mixed local-CT character and they correspond to a local excitation on the chromophore and a CT from the chromophore to the gold core. The character of the excited states in the first band is markedly different. The local excitations are ACS Paragon Plus Environment

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localized on the gold core and there is a larger fraction of mixed CT-local ES, with a CT character from the chromophore to the gold core as in the highest band. The relative intensity of the lowest band is higher for Au11, which reflect the higher density of local states due to the presence of the occupied and unoccupied SAMO’s, see Figures 1 and S1. The relative excitation energies of the states and their mixed local-CT nature were also computed for a band of 30 field-free excited states of the smaller cluster, Au11short, at the correlated CAS-SCF average level.92 The active space comprises eight active electrons in 14 MO’s in which we included the LUMO of the chromophore (Table S5). The transition energies of the local excited states are on the average higher in energy by about 0.6 eV than the TDDFT results (Table S6) which is expected for a CAS average computation. The CAS average CT excitations energies are higher by at least 1.0 eV when compared with the TD-DFT values, which is also expected since the TD-DFT is well-known to underestimate the excitation energy of CT states.84, 93 The nature of the first dozen of ES is mostly local on the gold core whereas the higher excited states possess a significant fraction of CT character (from 10% to 80% of CT, see Table S6). Thus, at the CAS level, the mixed local-CT states are also present in the band of excited states below the IP albeit higher in energy. The computed UV-Vis spectrum is shown in Figure S4. The analysis of the nature of the excited electronic states computed with both TDDFT and CAS-SCF allows for tailoring the short exciting pulse to induce a significant amount of charge transfer between the chromophore and the gold core. The purely electronic dynamics is computed by solving the timedependent Schroedinger equation for the electronic Hamiltonian in the basis of the field free electronic states.24, 94 The Hamiltonian includes the coupling to the electric field of the short pulse in the dipole approximation (SI, section 6). In the simulations reported below, we used short 5-6 fs optical pulses with a carrier frequency ω in the UV-VIS spectral range and a Gaussian envelope of width σ .  −(t − t0 )2  E(t ) = f 0 E exp   cos(ωt + φ ) 2  2σ 

(1)


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In eq (1), φ is the carrier envelop phase that is set to zero in the simulations below, E the polarisation vector of the electric field and f 0 its strength. The carrier frequency can be tuned to target the excited states exhibiting the mixed local-CT character characterized above (see Figure 2). One can also choose the polarization direction of the pulse to target specific states. In this work, since we aim at exciting states of the neutral, we restrict the maximum of the intensity of the field, f 0 , below 10 TW/cm2 to avoid multiphoton excitation and ionization. Control using the carrier envelope phase, φ , is limited to essentially one cycle pulses,95, 96 which is not the case for the fs UV-Vis pulses used in this work. The pulse builds a coherent superposition, Ψ (r , t ) = ∑ j c j (t ) ψ j (r ) , of field-free electronic ES

ψ j (r ) , which exhibit either a local or a mixed CT – local character as discussed above. The nature and the relative amplitudes, c j (t ) , of the field-free electronic ES ψ j (r ) in the coherent electronic wave-packet determines the amount of charge transfer and the coherences in the non-equilibrium electronic density built at the end of the pulse. After the end of the pulse, the time evolution is fieldfree and the amplitudes of the excited electronic states in the wave function are given by the expression , where c j ( t F ) is the amplitude of the jth state at the end of the pulse and Ej 2

its field free energy. After the pulse is over, the weights of the states c j (t ) are stationary. However, in addition to a stationary term, the one electron density matrix

(2)

and the dipole moment

(3)


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exhibit a non stationary term,

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24, 94, 97

the second term on the rhs of eqs. (2) and (3) above (see SI,

section 6), that is due to the electronic coherences built by the pulse. In eq (2) ρ ii (r ) is the stationary one-electron density matrix of the electronic state ψ i (r ) and ρ ij (r ) the transition density matrix between electronic states ψ i (r ) and ψ j (r ) , whereas in eq (3) µii is the permanent dipole moment of the electronic state ψ i (r ) and µij the transition dipole moment between the electronic states

ψ i (r ) and ψ j (r ) . The non stationary coherent terms beat with periods given by the transition frequencies between the electronic states in the wave packet. In the case of the modular systems that we study, we expect a non-negligible amount of charge transfer between the metallic core and the chromophore when the excited states of the coherent wave packet possess a charge transfer character. In order to assess the amount of charge transfer between the chromophore ligand and the rest of the cluster, we monitor the time-evolution of the electron density on the chromophore subunit as follows:

(4) chr , ρ iichr are the stationary densities localized on the chromophore subunit in the ground state and ρ GS

in the i th excited state respectively and ρijchr the transition density on the chromophore between two field-free state ψ i (r ) and ψ j (r )

(SI, section 7). The first term on the rhs of eq (4) gives the

permanent charge transfer with respect to the initial electron density of the ground state. This amount of charge transfer is the average value around which the electron density beats after the end of the pulse, as described by the second term of the rhs of eq (4) (the interference term). We performed the purely electronic dynamic simulations induced by linearly polarized pulses in the UV-VIS spectral range for a field strength of 0.005 a.u. (which corresponds to a maximum intensity of 8.775 1011 W/cm2) in order to significantly populate the excited states while avoiding multiphoton ACS Paragon Plus Environment

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ionization of the complex. The dynamical simulations are carried out for oriented molecules which is realistic since the clusters have sufficiently large permanent dipole moments (1.76 D for Au11short, 1.73 D for Au11long, 2.03 D for Au20short and 1.70 D for Au20long).98-100 Based on the analysis of the absorption spectra shown in Figure 2 above, the wavelength of the pulse is chosen to be resonant either with the lowest band of excited states (350nm, 3.54 eV) or with the second band of ES (300 nm, 4.13 eV). In order to selectively excite one of these bands of ES, the width, σ in eq (1), of the pulse Gaussian envelope was chosen to be 2.8 fs. which matches what is currently experimentally accessible in this spectral range. The duration σ = 2.8 fs corresponds to a FWHM of the Gaussian envelope in energy equal to 3.47 eV. As such pulses contain several cycles, the value of the CEP, φ in eq (1), is not expected to influence the dynamics and set to zero throughout this work. Using a shorter exciting pulse (σ=1.4fs) gives similar trends for the amount of charge transfer. However, since shorter pulses are broader in energy, they do not allow fully selectively exciting one band or the other with the two carrier wavelengths. We show in Figure 3 the results for the excitation of the first band in Au11short and the second band in Au20long. The results for Au20short and Au11long are plotted in Figure S6. The coherent wave-packet built at the end of the pulse is composed of several excited states of different character. As expected from the analysis of the character of the excited states, in the Au11short system, since the lowest absorption band is targeted, the wave-packet consists mostly of excited states with a character ranging from 50% of CT (with local excitation on the gold core) to pure CT, as well as a large amount of states involving a small fraction of CT (Figure 3a). The wave-packet resulting from the excitation of the second band of Au20long is mainly composed of ES with less than 20% of CT whose local component is localized on the chromophore (Figure 3b), as well as smaller fractions of purely local or CT ES. The amount of charge transfer is monitored by the time-evolution of the electron density on the chromophore subunit, eq (4). As can be seen from Figures 3e and 3f, the permanent charge depletion of


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the chromophore (stationary term of eq (4)) is about 0.1 |e| in Au20long and 0.4 |e| in Au11short. The electron density beats around these average values of CT with frequencies that are determined by the energy differences between the states populated in the wave-packet. The faster beating (about 1 fs) is related to interferences between the GS and the ES whereas the slower beating corresponds to interferences between ES close in energy. Due to the large amplitude of the slow beating, the total electron density depletion on the chromophore varies significantly during its time evolution. This motion of the electron density occurs at long time scale with respect to the purely electron dynamics time scales and would certainly be modulated by the motion of the nuclei a few dozens of fs after the end of the pulse. However, the permanent charge transfer is rapid and occurs during the 5-6 fs pulse. We do not expect extensive motion of the nuclei because the gold atoms are heavy, the gold core is a good electron acceptor and the substituted bipyridine ligand remains planar in its neutral and cationic states. These trends for the charge transfer are the same for Au20short and Au11long (Figure S6e and Table 1).


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Figure 3. Purely electronic dynamics simulations on Au11short and Au20long for the basis of ES computed by TD-DFT. (a,b) Time-evolution of the weights, ci ( t ) , of the stationary electronic excited 2

states in the coherent wave-packet built by the interaction with pulse. States of similar charge transfer (CT) character are added in one curve for clarity. (c,d) Time-evolution of the electric dipole moment, see eq (3), along the three coordinates (x,y,z) of the molecular frame (Figure 1). The stationary value of the electric dipole moment along the x axis (core-chromophore direction) is shown in black dashed lines. (e)-(f) Time-evolution of the electron density difference on the chromophore subunit with respect to its initial electron density in the electronic ground state. Solid lines: total electron density difference (eq (4)), dashed lines: permanent charge transfer (stationary term of eq (4)). The time-profile of the pulse, with a strength of 0.005 a.u. and a Gaussian envelope of σ=2.8 fs (see eq. (1), showed by the black curves, is indicative and not at scale. Wavelength and polarization of the pulses are 350nm (3.54 eV) along the x axis for Au11short and 300nm (4.13 eV) along the y axis for Au20long. The electric dipole moment along the y- and z-axis of the clusters oscillate around the value of the GS (Figures 3c and 3d) which indicates that there is a beating of the electron density but no net charge transfer along those directions. On the other hand, the average value of the electric dipole moment along the x-axis, which is the core-chromophore direction, is significantly shifted by about 16 D towards more positive values for Au11short (Figure 3c) and by about 10 D towards negative values for Au20long (Figure 3d). Given the orientation of the clusters in the molecular frame, these shifts confirm that a significant amount of charge is transferred from the chromophore to the metallic core. Table 1. Permanent charge transfer after the end of the exciting pulse in the four model clusters. Au11short Au11long Au20short Au20long x

-0.385

-0.115

-0.489

-0.073

y

-0.080

-0.114

-0.061

-0.013

z

-0.259

-0.076

-0.001

-0.000

3.54 eV (350nm)


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x

-0.119

-0.022

-0.121

-0.041

y

-0.214

-0.018

-0.199

-0.129

z

-0.032

-0.024

-0.012

-0.003

4.13 eV (300nm) The permanent charge transfer is given as the electron density difference on the chromophore subunit with respect to its initial electron density in the electronic ground state (stationary term of eq (4)). The pulse is resonant with the first ( ω = 350nm ) or the second ( ω = 300nm ) band of ES and polarized along the three axis of the clusters’ molecular frame (see Figures 1 and S1) The results of Figures 3 and S6 correspond to one direction of polarization of the pulse in the molecular frame. Changing the polarization of the pulse along the three axis of the molecular frame while keeping the same values for the other parameters modifies the populations of the excited states in the coherent wave-packet at the end of the pulse which in turn influences the amount of CT of the system depending on the proportion of mixed CT-local ES that are excited. In general, as can be seen from Table 1, there is more charge transfer in both bands for the short length of the ligand than for the long one for all polarization directions of the exciting pulse. The amount of charge transfer upon changing the polarization direction of the pulse depends on the nature of the local component of the two bands of excited states. When the first band of excited states around 350nm is targeted, a significant amount of charge is transferred for the three polarization directions. The local component of the ES of the first band is localized on the metallic core for all model systems. Several of the mixed CT-local excited states of this band are optically active for a polarization direction along any of the three axis of the molecular frame in the Au11 clusters, indicating that the significant CT could be observed experimentally even for a random orientation of the clusters in the experimental setup. In Au20short and Au20long, the ES of the first band are optically inactive for the z polarization direction due to the absence of SAMO’s in the frontier MO’s of the clusters, which leads to no CT and only 5% depletion of the GS for a 350nm-pulse polarized along the z-axis. The higher band of excited states, around 300nm, whose local component is on the chromophore, is subject to local selection rules for the excitation of its π system. The HOMO-LUMO transition of the bipyridine moiety is in principle allowed for any polarization in the (x,y) plane but strictly forbidden ACS Paragon Plus Environment

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for a polarization along the z-axis in the molecular frame. The mixed local-CT ES of interest are thus not populated for the z-axis polarization and there is no CT between the chromophore and the core, which is clearly illustrated for the excitation of the second band in Au11short and Au20short (see Table 1). The CT for an x-axis polarization on Au20long is smaller as the mixed ES of interest are only transiently populated and the GS is less depleted (Figure S7). In the case of Au11long, there is no significant effect of polarization of the pulse on the amount of CT for an excitation of the second band, as the ES around 300nm with local excitation on the chromophore are purely local and do not contribute to the CT. To summarize, we expect to experimentally observe an averaged CT for a random orientation of the clusters when the carrier wavelength is resonant with the states of the first absorption band for the short linkers. The optimisation of the amount CT would require to orient the Au20 clusters. The net charge transfer in Au11short is confirmed by the electron dynamics simulation in the basis of average CAS-SCF field-free excited states. The results are reported in Figure 4 for a pulse whose carrier wavelength (275 nm) is chosen a little shorter than that used in Figure 3 for the TD-DFT excited states in order to be resonant with the lowest mixed CT-local excited states that exhibit a significant CT character and a local transition on the gold cluster. The polarization of the pulse is along the x axis (core-chromophore direction) and the y axis of the cluster’s molecular frame and the duration of the pulse is the same as used in Figure 3.


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Figure 4. Purely electronic dynamics simulations for Au11short in the basis of field-free excited states computed at the CAS-SCF average level, induced by a pulse polarized (a,c) along the x axis (corechromophore direction) and (b,d) along the y axis. (a,b) Time-evolution of the weights ci ( t ) of the 2

stationary electronic excited states in the coherent wave-packet. States of similar CT character are added in one curve for clarity. (c,d) Time-evolution of the electric dipole moment, see eq (3), along the three axis (x,y,z) of the molecular frame, Figure 1. The stationary value of the electric dipole moment along the x-axis is showed by the black dashed lines. The time-profile of the pulse, with a strength of 0.005 a.u, a Gaussian envelope of σ=2.8 fs and a wavelength of 275nm is shown by the black curves (not at scale). Similarly to the TD-DFT case, several excited states of mixed local-CT character are accessed with a charge transfer character ranging from 10% to 50% (Figures 4a, b). For both polarization directions, the electric dipole moment along the y and z axis oscillates around an average value that is not significantly shifted with respect to the value of the GS, which indicates that there is not a significant amount of charge transfer along these directions (Figures 4c, d). The average value of the electric dipole moment along the x axis is significantly shifted towards positive values (6-8 D) in both simulations. The amplitude of this shift is about half the one computed with the TD-DFT basis of states


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for a polarization of the pulse along the x-axis and the amount of electron density depletion on the chromophore subunit is about 0.2 |e|. Changing the polarization direction of the pulse modifies the nature of the coherent wave-packet, as ES with a larger CT character are more selectively populated with the y-axis polarization (Figure 4b), but does not modify the overall charge transfer behaviour as shown in Figure 4c and d, which confirms the TD-DFT results that in the case of the Au11 clusters a significant charge transfer occurs for the three polarization directions parallel to the molecular frame. In summary, our simulations shed light on the early purely electronic steps of the charge separation in gold clusters functionalized by a chromophore ligand when they are excited by a strong few fs laser pulse. We show that a net charge transfer of 0.1 to 0.4 |e| can be triggered from the chromophore to the metallic core by a 5-6fs UV-Vis pulse on the model systems Au11(PH3)7Cl2S-(CH2)n-(4,4’)bipyridinCH3 and Au20(SCH3)15S-(CH2)n-(4,4’)bipyridin-CH3, n=2,4. After the end of the pulse, a coherent non equilibrium electronic density is built and a non-negligible charge migration occurs between the two subunits, with beatings around the net value of electronic density transferred during the pulse. We show that the efficiency of the net charge transfer is due to the presence of excited states with mixed localCT character. The latter results from the intercalation of the HOMO of the bipyridine ligand in the H-L gap of the gold clusters, with local excitations localized either on the chromophore or on the gold core. For accessed excited states with a local component on the bipyridine chromophore the amount of charge transfer is sensitive to the polarization direction of the pulse due to local optical selection rules. The mixed nature of the excited states found at the TD-DFT level is confirmed by the CAS-average computation. Further investigation will need to include the response of nuclei to non-equilibrium electron density motion. The amount of charge transfer could also be optimized using optimal control.29 Gold nanoclusters are interesting building blocks for charge transfer devices because they can act as sensitizers for solar cells52, 101 due to their ability to inject electrons in TiO2 102 103 Our work paves the way for designing new devices for optoelectronic applications by grafting chromophore-functionalized gold clusters on semi conducting substrates. ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting information. Detailed information about the geometry optimizations: ONIOM procedures, partial charges; TD-DFT electronic structure computations: optical spectra for different functionals, parametrization of CAM-B3LYP, and diagnostic test of the ES accuracy; CAS-SCF electronic structure computations: natural orbitals, ES charge transfer character estimation. Detailed description of the electronic dynamics computation, transition densities and transition dipoles, nature of the electronic excited states and time-evolution of the charge transfer. Time-evolution of the populations, dipoles, and CT for Au11long (350nm, y direction), Au20short (350nm, x direction) and Au20long (300nm, x direction). ACKNOWLEDGMENT This work is supported by the Fonds de la Recherche Fondamentale Collective (T.0132.16) and by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award #DE-SC0012628. We benefited from our participation in the COST action CM1405 MOLIM. FR and VS thank FNRS (Fonds National de la Recherche Scientifique), Belgium, for its support and for access to computational resources through the Consortium des Equipements de Calcul Intensif (CECI) FNRS 2.5020.11.

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(98) Ghafur, O.; Rouzee, A.; Gijsbertsen, A.; Siu, W. K.; Stolte, S.; Vrakking, M. J. J. Nat Phys 2009, 5, 289-293. (99) Holmegaard, L.; Nielsen, J. H.; Nevo, I.; Stapelfeldt, H.; Filsinger, F.; Kupper, J.; Meijer, G. Phys. Rev. Lett. 2009, 102, 023001. (100) Kraus, P. M.; Rupenyan, A.; Wörner, H. J. Phys. Rev. Lett. 2012, 109, 233903. (101) Choi, H.; Chen, Y.-S.; Stamplecoskie, K. G.; Kamat, P. V. J. Phys. Chem. Lett. 2015, 6, 217223. (102) Furube, A.; Du, L.; Hara, K.; Katoh, R.; Tachiya, M. J. Am. Chem. Soc. 2007, 129, 1485214853. (103) Long, R.; Prezhdo, O. V. J. Am. Chem. Soc. 2014, 136, 4343-4354.


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Photoinduced Ultrafast Charge Transfer and ... - ACS Publications

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