Photoactivity of MoleculeâTiO2 Clusters with Time-Dependent Density

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Photoactivity of Molecule-TiO Clusters with Time-Dependent Density-Functional Theory Eleonora Luppi, Ines Urdaneta, and Monica Calatayud J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b00477 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 19, 2016

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The Journal of Physical Chemistry

Photoactivity of Molecule-TiO2 Clusters with Time-Dependent Density-Functional Theory E. Luppi1,2, I. Urdaneta1,2, M. Calatayud1,2,3*

1

Sorbonne Universités, UPMC Univ Paris 06, UMR 7616, Laboratoire de Chimie Théorique, F-

75005, Paris, France 2

CNRS, UMR 7616, Laboratoire de Chimie Théorique, F-75005, Paris, France

3

Institut Universitaire de France, France

* corresponding author: Dr. Hab. Monica Calatayud Laboratoire de Chimie Théorique Université P. M. Curie 4, Place Jussieu case 137 75252 Paris, France Phone: +33 1 44 27 25 05 Fax: +33 1 44 27 41 17 e-mail: [email protected]

ACS Paragon Plus Environment

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Abstract The interaction of molecules with titanium oxide substrates may lead to substantial modifications of their optical properties, in particular a redshift of the absorption spectrum compared to bare titania. In the present paper we discuss the role of the interface between two molecules, catechol and dopamine, with gas-phase (TiO2)N clusters (N=2,4,6). We studied, for the interface, the bidentate modes (the molecule bonded to two Ti sites via its two oxygen sites), which was the most energetically favorable, followed by the chelated modes (the molecule bonded to one Ti site via its two oxygen sites), and the monodentate mode (the molecule bonded to one Ti site via one oxygen site). The absorption spectra were calculated with Time-Dependent Functional-Theory with CAM-B3LYP for the description of charge-transfer excitations. We observe a red shift of the molecule/cluster systems with respect to the molecules and clusters alone. Moreover, the chelated mode was found to present bands at lower energies than the other modes, making it the most interesting mode to tune the absorption edge of these systems.


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Introduction The key technological applications of nanostructured titania have prompted in the last years an intense research in fundamental and advanced science 1. Since the discovery of the water photosplitting properties of titania 2, much effort is conducted to unravel the relationship between structure and photoactivity of TiO2 materials. A variety of phenomena involve the response of titania and titania-modified nanosystems to light 3,4: ultraviolet (UV) radiation is responsible for the generation of electron-hole pairs with application in photochemical processes like water splitting or decomposition of environmentally harmful molecules. The photoactivity can be extended to the visible spectrum by doping the bulk materials or by sensitizing its surfaces with dyes adsorption 3,4, which results in the appearance of discrete levels in the band gap of the material. Interestingly, titania nanoparticles exhibit Surface Enhanced Raman Spectroscopy (SERS) activity in contact with biologically active molecules 5. The mechanism responsible for the SERS is related to the specific electronic structure of the molecule-nanoparticle adsorption system, in which a molecular state is present in the band gap of the substrate. This leads to important changes in the titania electronic structure, in particular a decrease of the band gap, and allows different excitation pathways involving both the molecule and the particle. Some important questions regarding the mechanisms related to titania spectroscopic response remain unsolved, in particular as concerns the connection between the geometry and the electronic structure. Catechol and dopamine molecules have been studied in the context of a specific spectroscopic activity when in contact with TiO2 nanoparticles or surfaces. Both are typical sensitizers exhibiting a type II mechanism i.e. the photoexcitation takes place between the molecular state and the nanoparticle 3. Experimentally the interaction of catechol with


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titania nanoparticles in aqueous solution results in an orange solution 6 whereas dopamine leads to red solutions 7. The binding of the molecules to the particles’ surface redshifts the absorption compared to the bare particles, and is characterized by a charge transfer effect between the ligand and the particle, usually attributed to the chelation of the surface undercoordinated titanium sites by the molecules 6-8 (see Figure 1 for a schematic view of the gas-phase molecules and their possible interaction geometries).

Figure 1: Schematic representation of the gas-phase molecules (catechol and dopamine) and the adsorption modes selected.

The presence of a molecular state in the band gap of the substrate is a key feature of type II photosystems, and such state is found in catechol and dopamine-TiO2 systems calculated by density functional theory 9-12. In order to take properly into account the role of the excited states in the optical response the use of better adapted theoretical methods is needed. Previous theoretical works in the literature reported excitation energies for catechol and dopamine interaction with TiO2 nanoclusters, obtained by Time-Dependent DensityFunctional Theory TD-DFT 13-15 or semi-empirical configuration interaction methods 16,17. In all these references the molecule is chelated to small TiO2 clusters, binding the two oxygens


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to a single Ti site. However, surface science experiments and periodic calculations indicate a higher stability for bidentate geometries i.e. binding the two oxygen sites to two different Ti sites 9-12,18,19. Catechol and dopamine interaction with titania has been studied theoretically with TiO2 cluster models 14,16,17,20, concluding that both bidentate and chelated models are stabilized. Few works deal with the role of the adsorption mode in the electronic structure of the systems. Li et al. 9 find that for catechol on rutile TiO2 (110) only the bidentate mode presents a state in the gap. Some of us found that bidentate, monodentate and chelated geometries of dopamine on anatase low index surfaces exhibit a molecular state in the gap, although at different energy depending on the mode and the slab termination 11. Sanz and coworkers obtained real-time TDDFT spectra for catechol interaction with (TiO2)N clusters (1 to 38 units) 14. De Angelis and coworkers calculated a squaraine dye, anchored by a benzoic group to an anatase slab model, and concluded that the solvent plays a key role in the adsorption/desorption equilibrium with impact in the TDDFT calculated spectrum 21.

In order to clarify the role of the interface geometry as regards the electronic structure and the corresponding optical response, in the present paper we will investigate the structural and optical properties of catechol and dopamine neutral molecules with neutral TiO2 clusters. We studied, for the interface, the bidentate mode (the molecule bonded to two Ti sites via its two oxygen sites), which demonstrated to be the most energetically favorable, followed by the chelated modes (the molecule bonded to one Ti site via its two oxygen sites), and the monodentate mode (the molecule bonded to one Ti site via one oxygen site). The absorption spectra were calculated with TDDFT with CAM-B3LYP for the description of charge-transfer excitations. Focus will be given to the study of irregular sites containing


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undercoordinated titanium, suggested to be responsible of the redshift absorption in modified titania nanoparticles 6-8. TiO2 neutral gas-phase clusters can be obtained experimentally: they show catalytic reactivity towards small organic molecules 22, also in the presence of visible light 23, and have been characterized theoretically with regards to their geometry, energetics and electronic structure 24-27. The size of the cluster models selected in the present work (a few units of TiO2) is below the size of real nanoparticles (few nm), ensuring the presence of undercoordinated reactive sites and enabling the use of state of the art of ab-initio methods (TD)DFT.

Methods and models All the calculations were carried out with the Gaussian09 suite 28. The free molecules, cluster and interaction systems have been first optimized in DFT at the B3LYP/6-311G* level. The TiO2 models used in the present work are neutral stoichiometric (TiO2)N (N=2,4,6) gas-phase clusters. Their geometries have been described in previous works at different computational levels 33-37. For the adsorption systems, one molecule (catechol or dopamine) is considered in the bidentate, chelated and monodentate modes, see Figure 1. Deprotonation of the OH groups is considered for all the systems, making Ti-Omolec bonds which is preferred to molecular adsorption 9,11,14,16,38. The H+ is transferred to terminal Ti=O groups to form Ti-OH groups since these terminal sites are found to be more reactive towards atomic H and H+ than two-fold oxygen sites 27,33. The bidentate and chelated modes exhibit two Ti-Omolec bonds (to two different Ti sites in the bidentate case, the same Ti in the chelated case) and two Ti-OH bonds, whereas the monodentate presents only one Ti-Omolec bond and one Ti-OH bond. The optimized geometries for the systems considered can be found in the supporting


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information. All of them were characterized as minima in the potential energy surface by a frequency analysis. They will be labelled in the following as NTiO2_molecule_mode where N refers to the size of the cluster (N: 2, 4 or 6 TiO2 units) and is equivalent to (TiO2)N; molecule is catechol (cat) or dopamine (dopa); mode refers to the anchoring geometry (bi: bidentate, chel: chelated, mono: monodentate). On top of these structures we performed TDDFT calculations to obtain excitation energies, transition dipole moments and oscillator strengths in order to construct the dynamic polarizability tensor () with i,j = x,y,z defined as 29,30 :

() =

Ψ |̂ |Ψ Ψ ̂ Ψ Ψ ̂ Ψ Ψ |̂ |Ψ − − + + −

where Ψ and Ψ are respectively the ground- and excited-state electronic wavefunction,

are the excitation energies and Ψ |̂ |Ψ are the transition dipole moments.

We are interested only in the spherical average of the polarizability tensor () = ∑

which can also be written as

( ) =

, −

where we neglected the infinitesimal shifts ∓ and we have introduced the oscillator strengths defined as

2 = | Ψ |̂ |Ψ | . 3


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Oscillator strengths and dipole moments give the probability of an electronic transition between states Ψ and Ψ corresponding to the excitation energy . Here, we analysed the imaginary part of () which gives access to the optical absorption spectrum of the systems. We performed all our TDDFT calculations using the long-range corrected functional CAMB3LYP and the basis set 6-311+G*. The only exception was made for the dopamine for which we used a more diffuse basis set 6-311++G*. Diffuse functions turned out to be necessary in order to accurately describe excitations. In fact, we verified that basis sets lacking of adequate diffuse functions do not correctly describe the excited states of these systems. Typically, without diffuse functions, basis sets lead to an overestimation of the low-energy excitations. Another important technical point in the calculations was the use of the functional CAM-B3LYP 31 to improve the description of charge transfer excitations. Moreover, CAM-B3LYP scheme has already proven to provide accurate results for excited states description of (TiO2)N (N=1-13) gas-phase clusters 24,32. Finally, for all the calculations we used 250 states to converge and a of 0.1 eV.

Results and discussion Geometry The optimized structures are displayed in Figures 2. The bare 2TiO2 cluster presents two sets of Ti-O distances: a short distance of 1.61 Å corresponding to terminal Ti=O titanyl groups, and a long one of 1.84 Å corresponding to four Ti-O bonds in the planar four-membered ring. The two titanium sites are three-fold coordinated. The adsorption of catechol in the


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bidentate mode results in the formation of a C2v symmetry complex with two equivalent TiOmolec bonds of distance 1.82 Å, the O-C bonds become identical with respect to the gasphase molecule (1.36 Å vs 1.38/1.36 Å in the gas-phase). The Ti-OH distances are of 1.80 Å. The two titanium sites become tetrahedrically coordinated and equivalent, and the fourmembered ring is no longer planar. The chelated geometry is constructed by binding the molecule to one titanium site and moving a titanyl oxygen to the other metal site, resulting in a Cs symmetry complex. The Ti-Omolec distance is 1.86 Å, C-O 1.37 Å. The four-membered ring is planar; the phenyl ring plane is tilted with respect to the Ti2O2 ring. Interestingly, starting from a C2v symmetry the optimization evolves to the Cs symmetry. The monodentate geometry is built from the bidentate one by protonating one Omolec. As a result the distance Ti-Omolec increases to 1.88 Å and 2.14 Å (for the OH group). Ti-O distances are 1.80 (Ti-OH) and 1.61 Å (Ti=O). Contrary to the bidentate mode, the phenyl ring plane is tilted from the Ti2O2 ring plane, the C-OH group is not coplanar with the phenyl ring, and the three-fold coordinated Ti interacts with the protonated oxygen site.


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Figure 2: optimized structures for the molecule-NTiO2 systems. Selected distances in Å, angles in degrees.

The 4TiO2 bare cluster presents C2v symmetry. The four titanium sites are four-fold coordinated, two of them with a tetrahedral environment (capped by titanyl groups), the other two with a much distorted chemical environment. The bidentate mode has been built on the distorted sites, after optimization the geometry resembles that of the 2TiO2 system as regards O-Ti, C-O and C-C distances. However, the environment around Ti is not tetrahedral, due to the lack of flexibility of the cluster. The chelated mode is formed in this cluster on a titanium site that becomes five-fold coordinated: two equivalent bonds of 1.82 Å with the


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molecule, two equivalent Ti-O bonds of 1.89 Å, and a longer Ti-O bond of 2.10 Å. The OmolecTi-Omolec is 87.1 degrees. The most stable monodentate mode is built by protonation of the bidentate mode, but evolves during the optimization to a monodentate chelated mode (angle Omolec-Ti-Omolec 76.1 degrees). The 6TiO2 cluster shows C2 symmetry. Two titanyl bonds are present, as well as an internal four-fold oxygen site; two titanyl sites are tetrahedral (titanyl capped), the other four are unsaturated bipyramidal. The bidentate mode has been built on two of these sites, and leads after optimization to some distortion of the cluster, by migration of the oxygen sites. The geometry is intermediate between the bidentate of the 2TiO2 and 4TiO2: one titanium site is almost tetrahedral, the other is distorted, the two Ti-Omolec distances are 1.82 Å and 1.83 Å respectively and the Omolec-Ti-Omolec angle is 81.3 degrees. The monodentate mode is built from the bidentate mode by protonation of one Omolec site. The titanium site connected to the molecule is in a distorted tetrahedral environment, and the molecule tilts with respect to the bidentate mode. Such tilting might be driven by the interaction of the OmolecH lone pairs with the three-fold coordinated titanium left (distance Ti-Omolec is 2.16 Å). The dopamine systems exhibit interaction geometries within 0.1 Å, 2 degrees from those of catechol. The presence of the (CH2)2NH2 group does not have an impact in the interface geometry. Note that in solution the amino group is protonated 7,39 and this might have a strong impact in the spectroscopic response.


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Energetics The interaction energy between the molecules and the clusters is displayed in Table 1. It is calculated as the electronic energy of the complex minus the sum of that of the bare cluster and the gas-phase molecule (negative values indicate favorable interaction). The values corrected with zero-point energy (ZPE) are also displayed, they do not involve significant change in the absolute value nor in the trends. Only the values corrected by ZPE will be discussed in the following. The interaction of TiO2 with catechol and dopamine is exothermic for all the cases studied. For the 2TiO2 cluster, the bidentate mode is exothermic by -5.10 eV, the chelated -4.82 eV and the monodentate mode, -3.58 eV. For the 4TiO2, the bidentate mode is exothermic by 3.53 eV, the chelated -3.77 eV and the monodentate mode -2.84 eV. For the 6TiO2, the bidentate mode is exothermic by -3.18 eV, the chelated -1.54 eV and the monodentate mode, -1.53 eV. The dopamine modes exhibit adsorption energy values very close to those of catechol and will not be commented. Several points arise from an analysis of the results in Table 1. First, the adsorption energy Eads values are large, in a range between -1.53 and -5.10 eV. This is especially true in the case of the 2TiO2 cluster, which is by its small size very reactive (two three-fold coordinated sites). For comparison, water dissociative adsorption is reported to be -2.59 eV 24 and H2 adsorption energy is -1.36 eV27 for the 2TiO2 naked cluster. Sanz and coworkers 14 report values for catechol interacting with 6TiO2 cluster: Eads=-1.08 eV (monodentate) and -1.89 eV (intermediate between bidentate and chelated); the H are considered as bonded to two-fold oxygen sites instead of titanyl groups as in the present work. Dopamine adsorption energy on regular anatase slabs lie between -1.84 eV and -0.67 eV 11. There are several reasons for


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such large interaction energies, in particular the fact that the most stable adsorption modes involve the deprotonation of 2 hydroxyl groups from catechol or dopamine; this is energetically favored because it leads to the formation of two Ti-Omolec and two Ti-OH bonds at the titanyl groups, both very stabilizing. Also, the formation of tetrahedral titanium, where initially only three-fold coordinated titanium was present, stabilizes the system. It is interesting to note that the formation of regular tetrahedra is energetically more favorable than that of distorted tetrahedra: the 2TiO2_cat_bi forms two regular tetrahedral around Ti (Eads=-5.10 eV) whereas 2TiO2_cat_chel forms a distorted tetrahedron (Eads=-4.82 eV).

Table 1. Interaction energy Eads in eV for the molecule-NTiO2 systems, EadsZPE zero-point energy corrected.

2TiO2

Eads

EadsZPE

Catechol bidentate

2TiO2_cat_bi

-5.02

-5.10

Catechol chelated

2TiO2_cat_chel

-4.77

-4.82

Catechol monodentate

2TiO2_cat_mono

-3.56

-3.58

Dopamine bidentate

2TiO2_dopa_bi

-5.05

-5.12

Dopamine chelated

2TiO2_dopa_chel

-4.79

-4.84

Dopamine monodentate

2TiO2_dopa_mono

-3.59

-3.60

Catechol bidentate

4TiO2_cat_bi

-3.46

-3.53

Catechol chelated

4TiO2_cat_chel

-3.70

-3.77


4TiO2_cat_mono

-2.83

-2.84

Dopamine bidentate

4TiO2_dopa_bi

-3.47

-3.53

Dopamine chelated

4TiO2_dopa_chel

-3.69

-3.76

4TiO2


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4TiO2_dopa_mono

-3.07

-3.11

Catechol bidentate

6TiO2_cat_bi

-3.09

-3.18

Catechol chelated

6TiO2_cat_chel

-1.43

-1.54


6TiO2_cat_mono

-1.49

-1.53

Dopamine bidentate

6TiO2_dopa_bi

-3.12

-3.19

Dopamine chelated

6TiO2_dopa_chel

-1.42

-1.53


6TiO2_dopa_mono

-1.53

-1.53

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6TiO2

Second, the adsorption energy becomes less endothermic as the size of the cluster increases. This can be explained by the higher reactivity of small clusters due to the presence of undercoordinated titanium sites: 2TiO2 presents two three-fold coordinated Ti, 4TiO2 and 6TiO2 interact via distorted four-fold Ti sites. Additionally, the interaction with the molecules may induce a geometric rearrangement of the bare cluster structure since it provides more oxygen sites to satisfy the undercoordinated titanium. An example of such structural flexibility is clearly observed in the 6TiO2 cluster that exhibits an internal four-fold coordinated oxygen in the bare system, and none in the adsorption complexes. Third, the most stable geometries of adsorption involve the formation of two Ti-Omolec bonds, either with two different Ti sites (bidentate mode is the most stable for 2TiO2 and 6TiO2 systems) or with one single Ti site (chelated mode is the most stable for 4TiO2). The monodentate mode, although exothermic, is less favorable, in agreement with previous data reported in the literature for titania clusters 14. The preference for a given geometry seems to be driven by the local geometry around the Ti sites after adsorption, although no general rule arises. Thus, the difference between 2TiO2_cat_bi and 2TiO2_cat_chel is 0.27 eV in favor


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of the former, whose final structure involves two regular tetrahedra around Ti sites with no tension. In the 6TiO2 the difference in energy in favor of the bidentate mode reaches 1.65 eV; here the presence in 6TiO2_cat_chel of a three-fold and a five-fold coordinated Ti involves a destabilization of the whole system. Interestingly, the chelated mode is preferred over the bidentate in 4TiO2 systems (the former is 0.23 eV more stable than the latter), probably due to the tension of the complex upon forming a five-fold and a distorted tetrahedron in 4TiO2_cat_bi. Earlier results on dopamine adsorption on regular anatase periodic slabs show a clear preference for bidentate modes (Eads in the -1.84/-0.67 eV range11, chelated modes are found to be endothermic). This results points to the stabilization of the chelated modes in the presence of surface defects and reactive sites, with respect to regular non-defective surfaces.

Absorption spectra The relation between geometry and optical response of molecules and clusters has been studied through the analysis of the absorption spectra and Natural Transition Orbitals (NTOs) 40. This permitted to investigate the interplay of bidentate, chelated and monodentate modes together with cluster sizes for optical excitations. In particular, in this analysis, we are mainly interested in the low energy region of the absorption spectra. In fact, our goal is to analyse how the different modes and cluster sizes can be tuned in order to modify the absorption edge. For this reason we focus on the peaks close to the edge, extracting the excitation energies contributing to these peaks. The character of the excitations was analysed through NTOs. NTOs permits a compact orbital representation for the electronic transition density matrix and allow a dramatic


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simplification in the qualitative description of an electronic transition. In fact, in most of the cases, the excitation energies are a sum of many contributions of the molecular orbitals which can be very difficult to analyse. Therefore, we decided to show the NTOs, which describe the optical excitations as a hole-particle associated with a given amplitude which gives the transition probability. In the following, the NTO from which an electron gets excited will be referred to as “hole”, and the NTO into which an electron is excited will be referred to as “electron”. In Fig. 3 we show the optical spectra of dopamine, catechol, 2TiO2, 4TiO2 and 6TiO2. We start by analysing the spectra of the dopamine and catechol which have a very similar shape. In the low-energy region, we observe that the most important peaks of dopamine are at 5.01 eV (0.0581), 5.71/5.74 eV (0.0396/0.0258) and 6.76/6.79 eV (0.1015/0.4318) with a shoulder at 6.42/6.52 eV (0.0359/0.1973). The oscillator strengths are reported in parenthesis. The catechol has the first peak at 5.07 eV (0.0475), a small peak at 5.82 eV (0.0240) and a very strong one which involve two different excitations at 6.66/6.85 eV (0.4679/0.5730). The energy positions of the peaks and the relative intensity (related to the oscillator strength) is very similar between the two molecules. Our results agree with those reported in the literature both for the experiment 41 and previous calculations 14,15.

Figure 3: Optical absorption spectra of dopamine, catechol, 2TiO2, 4TiO2 and 6TiO2 (left panel). Natural transition orbitals pairs of peak I (see inset) of dopamine and catechol: for each state the “hole” is on the left and “electron” is on the right, as defined in the text. The


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associated amplitudes are also reported (right panel).

The NTOs for the first peaks (see inset) are reported on the right of Fig. 3 for the molecules. We note that also the character of the excitations is very similar. The “hole” (left) and particle “right” are mainly localized on the rings with two different amplitudes associated. These amplitudes show a weak (0.18541/0.20575) and a strong (0.81573/0.79487) transition similar in magnitude for the dopamine/catechol respectively. In ref. 13 there are reported optical spectra on similar systems but with functionals B3LYP and PW91. The disagreement we obtain with respect some excitation energies are due to the fact that B3LYP and PW91 are not corrected for long-range interactions. This implies an overestimation of the excitations with respect to our results with CAM-B3LYP. Moreover, it is worth to note that another chromophore: the alizarin was studied13. Also in that case the two chromophores, catechol and alizarin, has similar spectra.


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In Fig. 3 we also report the spectra of the 2TiO2 , 4TiO2 and 6TiO2 clusters. The general shape of the spectrum seems to depend on their size. In fact, the spectrum of the 2TiO2 has sharp peaks similar to those of the molecules, with an absorption edge at excitations 4.23/4.30 eV (0.0014/0.0088). However, the strongest peak of 2TiO2 is in the same energy region of the absorption edge of the molecules. This peak is due to a degenerate excitation at 4.91 eV (0.0422/0.0214). In Fig. S1 the NTO of the first and second peak are shown. In particular, as the first peak is a contribution of more than one excitation, we only show the NTOs corresponding to 4.30 eV (0.0088) as similar results were obtained for 4.23 eV (0.0014). We obtained that the “hole” and the “electron” are delocalised on the ensemble of the cluster, mixing in a strong and a weak transition amplitudes configuration. Increasing the size of the clusters (4TiO2 and 6TiO2) we observe that the shape of the spectra changes with respect to 2TiO2. In fact, starting from the absorption edge of 4.3583 eV (0.0003) for 4TiO2 and of 4.6884 (0.0011) for 6TiO2, a “continuous” of excitations is present. This results in a broad-ranging spectrum which increases with the cluster size. The same trend is reported in ref. 14. We analysed the NTOs of these transitions which are a combination of many orbitals contributions where the “hole” and the ”electron” are delocalised over the entire system. Here, we show (Fig. S1) as an example the NTOs corresponding to the first highest excitation identified in the broadening continuum and indicated in the inset of Fig. 3. This excitation corresponds to 4.75 eV (0.0055) for 4TiO2 and to 5.03 eV (0.0045) for 6TiO2. In Fig. 4 we report the spectra for catechol in bidentate, chelated and monodentate modes with the 2TiO2. The absorption edge for the catechol in the bidentate mode starts at


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3.28/3.40 eV (0.0009/0.0025). However, a very intense peak appears only at 4.69 eV (0.2877). Instead, for the monodentate the absorption starts directly at high energy as the absorption edge is at 4.66 eV (0.0853). The absorption edge of the chelated mode starts around 3.33/3.6 eV (0.0001/0.0009) and the first strong peak is at 3.74 eV (0.1605), considerably lower with respect to the bidentate and the monodentate structures. Moreover, another strong peak at almost the same energy range of the bidentate and of the monodentate, is observed. This peak is related to the excitation at 4.75 eV (0.1015). The interaction of the molecule and the cluster red shift the absorption edge. This effect is particularly strong for the chelated mode. The nature of these transitions is shown on the right of Fig. 4 for the most important peaks. In particular, for the bidentate we just show the NTOs for the peak II as for peak I we obtain the same trend and it is two or three orders of magnitude smaller. All the excitations have a charge transfer character which also pointed out the importance of using CAM-B3LYP functional. The charge transfer excitation is visualised by a “hole” on the ring molecule and an “electron” delocalized over the cluster.


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Figure 4: Optical absorption spectra of catechol and 2TiO2 in bidentate, chelated and monodentate modes (left panel). Natural transition orbitals pairs of peak II of 2TiO2_cat_bi, peak I and II of 2TiO2_cat_chel and peak I of 2TiO2_cat_mono. For each state the “hole” is on the left and “electron” is on the right, as defined in the text. The associated amplitudes are also reported (right panel).

The dopamine with the 2TiO2 has the same trend, as shown in Fig. 5. However, the presence of the amine group complicates the charge transfer excitation. This is clear observing the overlap between two different transition amplitudes in the case of the bidentate mode. This depends on the fact that the “hole” has more degrees of freedom as it can be diffuse on the ring but also localised on the amine group. The fact is that dopamine and catechol adsorbed


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on clusters give almost the same results except some more complicated behaviour due to the amine group in the catechol. For this reason in the following we focus only on the dopamine systems.

Figure 5: Optical absorption spectra of dopamine and 2TiO2 in bidentate, chelated and monodentate modes (left panel). Natural transition orbitals pairs of peak II of 2TiO2_dopa_bi, peak I and II of 2TiO2_dopa_chel and peak I of 2TiO2_dopa_mono. For each state the “hole” is on the left and “electron” is on the right, as defined in the text. The associated amplitudes are also reported (right panel).

In Fig. 6 the spectra for dopamine adsorbed on 4TiO2 is shown. Note the influence of the 4TiO2 on the spectra: the bare cluster and the molecule-cluster complex show a more


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homogenous shape compared to the 2TiO2 case. For the bidentate mode the absorption edge starts at 3.21/3.39 eV (0.0036/0.0081). However, the first strong absorption is the sum of three excitations at 3.88 eV (0.0927), 3.99 eV (0.0461) and 4.09 eV (0.0236). We analysed the NTOs of all of them and we just show the most intense since the trend is the same for all excitations. For the monodentate mode the first peak is at 3.57 eV (0.0017) and some more intense excitations are observed at 3.81 eV (0.0226), 4.16 eV (0.0457) 4.38 eV (0.0357). It is interesting to note that in the case of the chelated mode an important red shift of the absorption edge is observed with a peak at 2.52/2.61 eV (0.0002/0.0101), with two other peaks at 3.30 eV (0.0542) and 3.97 eV (0.1154). The NTOs also in this case show that these excitations have charge transfer character and confirm for the III peak of the chelated the role of the amine group, which shows an overlap of “particle-hole” transition with a higher probability if the “hole” is on the ring.

Figure 6: Optical absorption spectra of dopamine and 4TiO2 in bidentate, chelated and monodentate modes (left panel). Natural transition orbitals pairs of peak II of 4TiO2_cat_bi, peak I, II and III of 4TiO2_cat_chel and peak I of 4TiO2_dopa_mono. For each state the “hole” is on the left and “electron” is on the right, as defined in the text. The associated amplitudes are also reported (right panel).


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The dopamine adsorbed on the 6TiO2 cluster show the effect of the cluster alone by the broadening of excitation energies as a continuous. However, the presence of the molecules makes appear some sharp peaks. In particular, it is confirmed that the chelated mode redshifts the absorption spectra, see Figure 7. A strong peak appears at 2.56 eV (0.0259). Instead, the bidentate mode has a weak absorption edge starting around 3.20 eV (0.0024), 3.50 eV (0.0020), 3.55 eV (0.0032), 3.66 eV (0.0035). The strongest peak is observed at higher energy, 4.26 eV (0.0746). In the case of the monodentate mode the absorption edge starts around 4.13 eV (0.0025) but the stronger peak is around 4.6 eV with many electronic transitions contributing at 4.65 eV (0.0108), 4.66 eV (0.0096), 4.72 eV (0.0005) and 4.74 eV (0.0196). Also in this case we only show the NTOs for the strongest transition since the


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others show similar behavior. The NTOs on the right of Fig. 7 confirm a strong charge transfer between the molecules and the clusters. However, in these cases we observed that the “hole” is delocalised over all the molecule and does not involve a particular interplay between the ring and the amine group.

Figure 7: Optical absorption spectra of dopamine and 6TiO2 in bidentate, chelated and monodentate modes (left panel). Natural transition orbitals pairs of peak II of 4TiO2_cat_bi, peak I, II and III of 4TiO2_cat_chel and peak I of 2TiO2_dopa_mono. For each state the “hole” is on the left and “electron” is on the right, as defined in the text. The associated amplitudes are also reported (right panel).


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Conclusion The interaction of catechol and dopamine with (TiO2)N clusters (N: 2, 4, 6) has been studied by (TD)-DFT state of the art methods. The interfaces between molecule and cluster have been characterized as regards their geometry, energetics and optical properties. The main results are summarized as follows. The behavior of catechol and neutral dopamine with titania clusters is very similar concerning geometry and energetics. The two molecules deprotonate in contact with TiO2 clusters and form Ti-O and OH bonds. The local topology of the cluster titanium sites explains the stability of the systems: undercoordinated titanium sites are the most reactive. All the complexes studied are exothermic with respect to the gas-phase isolated systems, with interaction energies in the range -5.10/-1.53 eV. The adsorption energies become less exothermic with increasing the cluster size. Adsorption may induce structural rearrangement of the bare cluster. Bidentate (formation of two O-Ti bonds with two different sites) and chelated (formation of two O-Ti bonds with one single site) are energetically the most favorable, while monocoordinated species are less stable. The formation of regular tetrahedral environment is energetically favorable. The stability of chelated modes is enhanced by the presence of defective undercoordinated sites. The calculated optical spectra of the interacting systems show unambiguously the redshift of the absorption compared to isolated molecules and clusters. Larger clusters’ spectra broaden the peaks over a larger range of energies and the overall spectra appear as more continuous compared to small clusters. The chelated mode exhibits the lowest transition energies; bidentate and monocoordinated modes show similar behavior. The transitions are of charge-transfer character and involve both the molecules’ ring (hole) and the cluster


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(particle). The behavior of catechol and dopamine is similar as regards the energy of the transitions, but dopamine presents a more complex behavior due to the amine group that may also contribute as hole.

Supporting information Optimized geometries for the systems calculated. Natural transition orbital representations of the bare clusters. This material is available free of charge via the Internet at http://pubs.acs.org

Acknowledgements This work was performed using HPC resources from GENCI- CINES/IDRIS (Grants 2014x2013082131, 2015-x2014082131) and the CCRE-DSI of Université P. M. Curie. Dr. B. Diawara is warmly acknowledged for the Modelview visualization program.


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Photoactivity of MoleculeâTiO2 Clusters with Time-Dependent Density

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