Tungsten Iodide Clusters as Singlet Oxygen Photosensitizers

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Tungsten Iodide Clusters as Singlet Oxygen Photosensitizers: Exploring the Domain of Resonant Energy Transfer at 1 eV Mikkel Bregnhøj, Kris Strunge, Rasmus Juhl Sørensen, Markus Stroebele, Thorsten Hummel, Hans-Juergen Meyer, Frank Jensen, and Peter R. Ogilby J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00541 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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The Journal of Physical Chemistry

Tungsten Iodide Clusters as Singlet Oxygen Photosensitizers: Exploring the Domain of Resonant Energy Transfer at 1 eV

Mikkel Bregnhøj,a Kris Strunge,a, Rasmus Juhl Sørensen,a Markus Ströbele,b Thorsten Hummel,b H.-Jürgen Meyer,b Frank Jensena and Peter R. Ogilbya*

a

Department of Chemistry, Aarhus University, 8000 Aarhus, Denmark.

b

Department of Inorganic Chemistry, University of Tübingen, 72076 Tübingen, Germany

* Corresponding Author: Tel: +45 8715 5927. E-mail: [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The photophysics of selected tungsten iodide clusters was examined with respect to their role as a photosensitizer for the production of singlet oxygen, O2(a1g). We examined all-iodo octahedral clusters, [W6I8(I6)]2-, and ligand-substituted octahedral clusters, [W6I8(L6)] 2-, in which the ligand, L, occupies the outer apical positions surrounding the cluster core. We also examined a square pyramidal cluster, [W5I8(I5)]-, in which the tungsten core was presumably more accessible to diffusional encounter with ground state oxygen, O2(X3g-). For the compounds examined, we find pronounced cluster-dependent changes in the yield of photosensitized O2(a1g) production. In particular, although the iodine encased octahedral cluster, [W6I8(I6)] 2-, is an efficient O2(a1g) sensitizer, the pyramidal cluster, [W5I8(I5)]-, does not make O2(a1g) at all. The latter provides fundamental insight into the important case where the sensitizer triplet state is nearly degenerate with the O2(X3g-) - O2(a1g) transition energy at 1 eV. Our data indicate that, even with near resonance, energy transfer to form O2(a1g) will not occur within the 3Sensitizer-O2(X3g-) encounter pair if other more efficient channels for energy dissipation are available.


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INTRODUCTION The photosensitized production of singlet oxygen, O2(a1g), is a phenomenon of interest to a broad cross section of the scientific community.1-3 On one hand, given the unique chemical reactivity of O2(a1g), certainly with a host of organic molecules,4 it is readily seen why one should be interested in methods by which O2(a1g) can be systematically produced. On the other hand, this photosystem involves fundamental molecular events relevant to a plethora of chemical, biological and physical processes.2 Although many molecules have an excited state whose energy exceeds the excitation energy of the O2(X3g-)-O2(a1g) transition, 94 kJ/mol or ~ 1 eV, an efficient O2(a1g) sensitizer must also have an excited state lifetime that is long enough to allow for the required collision with ground state oxygen, O2(X3g-).5 Even then, features of the excited state sensitizer-O2(X3g-) collision complex can preclude energy transfer to produce O2(a1g). For example, if this collision complex has an appreciable amount of charge transfer (CT) character, then two other channels of sensitizer deactivation can effectively compete against the energy transfer channel: (a) electron transfer to form the superoxide radical anion, and/or (b) CT-mediated non-radiative deactivation to re-form the ground state molecules.3, 5-8 However, through extensive studies, it has been demonstrated that the channel for energy transfer will be increasingly favored when the energy of the sensitizer triplet state decreases to approach a condition of resonance with the unique low-energy O2(X3g-)-O2(a1g) transition.3, 5 The use of organic molecules as O2(a1g) sensitizers has been extensively studied over the past ~50 years.3, 5, 9 The scientific community now has access to molecules that cover a wide range of properties: (a) efficiency of O2(a1g) production, as expressed in the O2(a1g) quantum yield, , (b) solubility, (c) biological compatibility, (d) excitation wavelength, and (e) photostability.9, 10 The 3 ACS Paragon Plus Environment


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ability of selected organometallic complexes to efficiently produce O2(a1g) has likewise been documented.9, 11, 12 In this context, the complexes to which we refer have a single metal atom or ion surrounded by an extended organic framework that plays a key role in defining the pertinent photophysical properties. Moving to the inorganic end of the spectrum, there is a wealth of data on metal particles/clusters to which an established organic sensitizer is attached.13-15 In these cases, however, the process of O2(a1g) sensitization still occurs in a complex between O2(X3g-) and a distinct localized excited state of the organic component, the latter having been produced by resonant energy transfer from the metal. The use of inorganic materials as the actual O2(a1g) sensitizer is, thus far, somewhat limited. In part, this may reflect reported values of  that are not large (~ < 0.1) from materials such as silver, platinum and gold nanoparticles,16, 17 gold nanoclusters,18 and CdSe and CdTe quantum dots.19, 20 Nevertheless,  values of ~ 0.7 – 1.0 have been reported for selected systems such as ligand-coated gold clusters21, 22 and silicon nanocrystals23 and, most interestingly, it has been reported that both the morphology and surface facets of gold and palladium nanocrystals are important factors in determining the efficiency of energy transfer to produce O2(a1g).24, 25 Thus, there is justification to carry forward in this field. Tungsten clusters, stabilized by -donating ligands such as halides, have been studied for many years.26 The general photophysical and electrochemical properties of hexanuclear tungsten clusters (e.g., [W6I8(I6)]2-), along with aspects of their electronic structure, have been of particular interest.27-29 Of relevance to our present study, hexanuclear iodide clusters of both tungsten and molybdenum, including those substituted with different ligands L in the apical positions,


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[W6I8(L6)]2- and [Mo6I8(L6)]2-, have been shown to photosensitize the production of O2(a1g) in relatively high yield ( ~ 0.6 – 0.9).30-32 To further elucidate the oxygen-dependent photophysics of such clusters, particularly events that contribute to the photosensitized production of O2(a1g), we set out to compare/contrast the behavior of a pentanuclear pyramidal cluster, [W5I8(I5)]-, with that of the octahedral cluster, [W6I8(L6)]2- (Figure 1). With an arguably naïve viewpoint, we wanted to change the extent to which the iodide ligands shield the tungsten core and, thus, allow for a different O2(X3g-)-cluster interaction as a consequence of oxygen’s approach through the “exposed” underside of the cluster. However, and as presented herein, we find that such structural and compositional changes have a greater effect on O2(a1g) production due to other things. In particular, and in comparison to selected organic photosensitizers, we find that the pentanuclear pyramidal cluster provides unique insight into events that occur within the 3Sensitizer-O2(X3g-) collision complex under conditions where the triplet energy of the sensitizer is nearly resonant with the O2(X3g-)-O2(a1g) transition. We now demonstrate that, despite near energy resonance, other deactivation pathways can still preclude the photosensitized production of O2(a1g).



Figure 1.

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Illustrations of compound 1, [W6I8(I6)]2-, and compound 7, [W5I8(I5)]-, based on crystal structures. In each case, the tungsten core is shown with blue spheres and the iodide ligands with purple spheres.

EXPERIMENTAL SECTION Cluster Synthesis and Characterization Compounds 1-5 were prepared using published procedures.29, 32, 33 Compound 6 was prepared accordingly, and the crystal structure refined (CCDC 1875476). W5I16 was the starting material for compound 7.34 It can be dissolved in ethanol, methanol or acetone, resulting in dark green solutions. Direct crystallization from solution resulted in dark green crystals unsuitable for singlecrystal structure refinements. The most probable cation in these cases could be H3O+ because the solvents contained small amounts of water. Crystallization using (Pr4N)I, (TBA)I, and (Ph4P)I resulted in the compounds (Pr4N)[W5I8(I5)]THF,35 (TBA)[W5I8(I5)], and (Ph4P)[W5I8(I5)], all 6 ACS Paragon Plus Environment

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containing the monoion [W5I8(I5)]- suitable for X-ray diffraction studies (Pr4N = tetrapropylammonium, TBA = tetrabutylammonium, and Ph4P = tetraphenylphosphonium). Sample Preparation and Stability All photophysical measurements were performed with samples dissolved in acetonitrile at 23 °C. In some cases, sonication for 30 min was used to facilitate dissolution. Any undissolved solid material was removed by filtration, and the sample absorbance at 400 nm was kept between 0.05 and 0.1. When dissolved in aerated or oxygenated acetonitrile, compounds 1-5 and 7 were completely stable under our working conditions. Although solutions of compound 6 were stable when kept in the dark or under an atmosphere of nitrogen, this compound degraded slowly when in the presence of light and oxygen. As such, we performed all experiments on compound 6 at the limit where no bleaching was evident. Gentle bubbling with a N2/O2 gas stream containing a known percentage of oxygen controlled the concentration of dissolved oxygen. The Henry’s law constant used to calculate the oxygen concentration was obtained using a value of [O2] = 2.42 ± 0.14 mM in air-saturated acetonitrile.36 Spectroscopic Measurements Absorption spectra were recorded on a Shimadzu UV3600 spectrometer. Time-gated emission spectra were recorded on a homebuilt instrument described elsewhere37, 38

and calibrated using a standard procedure.39 Emission quantum yields of the metal clusters were

determined using air-saturated solutions with 4-(diocyanomethylene)-2-methyl-6-(4dimethylaminostyryl)-4H-pyran, DMP, as the emitting standard: fluor(DMP in acetonitrile) = 0.44 ± 0.05.40 Phosphorescence lifetimes of the hexanuclear clusters 1-6 were measured at 700 nm using a 7 ACS Paragon Plus Environment


PMT operated in photon-counting mode. The red-shifted phosphorescence from the pyramidal cluster, compound 7, was recorded using the near-IR PMT also used for our O2(a1g) experiments (vide infra). In this case, spectral resolution was achieved using band pass filters instead of a monochromator. The equipment and procedure used to determine O2(a1g) quantum yields are described elsewhere.32, 41, 42 Briefly, samples were excited at 400 nm using the frequency-doubled output of a fs laser operating at 1 kHz. The time-resolved 1275 nm phosphorescence of O2(a1g) was isolated with a 1064 nm long-pass filter and a 1270 nm (fwhm 20 nm) band-pass filter, and the resultant light detected with a near-IR PMT (Hamamatsu). We used phenalenone dissolved in acetonitrile as the reference O2(a1g) sensitizer ( = 0.99 ± 0.0343, 44). DFT Calculations Calculations were performed on compounds 1, 2 and 7 using the Gaussian-16 program package45 with the B97X functional46 and the Def2-SVPD or Def2-TVZPPD basis sets47 with effective core potentials for I, W, and Mo. The results were not sensitive to the use of other exchange-correlation functionals, and the use of Def2-TVZPPD gave only small changes relative to Def2-SVPD. Solvent effects were modelled by the IEFPCM continuum method with parameters for acetonitrile, and excitation energies were calculated within the time-dependent density functional theory, TDDFT, framework.45, 48 Compounds 1, 2 and 7 all have singlet ground states. Molecular geometries were optimized within Oh (compounds 1 and 2) and C4v (compound 7) symmetry, and bond distances deviated by less than 0.05 Å compared to the experimental values obtained from X-ray diffraction.35 Frequency analyses showed the optimized structures to be minima on the potential energy surface within the above point groups. The lowest triplet states are Jahn-Teller distorted and were optimized without 8 ACS Paragon Plus Environment

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symmetry constraints. Calculated absorption and emission spectra in Figure 6 were plotted using a Gaussian convolution with a width of 0.2 eV.

RESULTS AND DISCUSSION Cluster Absorption and Emission Absorption and emission spectra measured for compounds 1 and 7 are shown in Figure 2. Spectra for all other compounds are in the Supporting Information (SI).



Figure 2.

Absorption (blue lines) and emission (red line) spectra for (A) compound 1, (TBA)2[W6I8(I6)], and (B) compound 7, [W5I8(I5)]-, dissolved in air-saturated acetonitrile. Note the different wavelength axes used for the respective compounds. For compound 7, emission was only detected at discrete wavelengths longer than 880 nm (see text). The inset in panel B shows the time-resolved emission trace recorded at 1325 nm (red arrow). Over the 1 s period shown, the decay is adequately represented by a single exponential function (superimposed line). At longer times, a second decay component becomes more apparent (see Figure 3).

For compound 1 and all the other hexanuclear clusters, the large Stokes shift and the lifetime and quenching data (Table 1) lead us to assign the emission spectrum to phosphorescence from the


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lowest energy triplet state of the clusters. This is consistent with published work on related compounds.31, 32, 49

Table 1. Photophysical parameters for clusters dissolved in acetonitrile. Compound

T(N2)a

kqb

(s)

(108 M-1s-1)

d

fTc

1

(TBA)2[W6I8(I6)]

23.6

10.1 ± 0.5

air 0.99

O2 1.00

air 0.67

O2 0.68

2

(TBA)2[Mo6I8(I6)]

95.8

1.7 ± 0.1

0.96

0.99

0.96

0.97

3

(TBA)2[Mo6I8(O3SC7H7)6]

232

0.8 ± 0.1

0.94

0.98

0.94

0.95

4

(TBA)2[W6I8(O3SC7H7)6]

40.2e

1.6 ± 0.3e

0.79

0.95

0.75

0.77

5

(TBA)2[W6I8(O3SC6H5)6]

39.6e

1.7 ± 0.3e

0.89

0.97

0.70

0.76

6

(PPh4)2[W6I8(NO3)6]

11.0

1.6 ± 0.1

0.73

0.93

0.29

0.41

7

(H3O)[W5I8(I5)]f

0.15e

1.9 ± 0.3e

0.06

0.24

Tungsten Iodide Clusters as Singlet Oxygen Photosensitizers

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