Relating structure and photoelectrochemical properties: electron injection by structurally and theoretically characterized transition metal-doped phenanthroline–polyoxotitanate nanoparticles

Abstract

Whereas a large number of sensitized polyoxotitanate clusters have been reported, information on the electrochemical properties of the fully structurally defined nanoparticles is not available. Bridging of this gap will allow a systematic analysis of the relation between sensitizer-cluster binding geometry, electronic structure and electron injection properties. Ti17O28(O(i)Pr)16(Fe(II)Phen)2 is a member of a doubly-doped series of nanoclusters in which the phenanthroline is attached to the surface-located transition metal atom. The visible spectrum of a dichloromethane solution of the studied sample shows a series of absorption bands in the 400-900 nm region. Theoretical DOS and TDDFT calculations indicate that the bands in increasing wavelength order correspond essentially to metal-to-core charge transfer (MCCT) at ∼460 nm, metal-to-ligand charge transfer (MLCT) at ∼520 nm and d-d metal-atom transitions. Exposure of a thin layer of the sample to light in a photoelectrochemical cell produces an electric current in the 400 to ∼640 nm region. The fit of the wavelength range of the electron injection with the results of the calculations suggests that charge injection into the FTO anode occurs both from the TiO cluster and from the phenanthroline ligand. Injection from the phenanthroline via the cluster orbitals is ruled out by the lower energy of the phenanthroline orbitals.

How Does Substitutional Doping Affect Visible Light Absorption in a Series of Homodisperse Ti11 Polyoxotitanate Nanoparticles?

“…[15] Compound HOMO-LUMO separation HSE06/6-31G** 2 ,i nw hich part of the transfer occurs through the barep henanthroline ligand. [16] A cathodic current, indicatingh ole injection, observed in similar experimentsw ith 4-nitrophenyl-acetylacetonate functionalized Ti 17 O 24 (OPri) 20 nanoparticles, hasbeen reported elsewhere. [17] …”

“…The Fe complexes stand out by showingapronounced absorption up to about 800 nm fort he more concentrated solutions. This absorption band with maximum around 560 nm is ac onse- 16 cluster; [5] (bottom)t he TiMnClc luster.T i-purple;O-red;C-g rey;Mncyan;Cl-brightg reen. Hydrogen atoms are omitted for clarity.…”

“…An MX 2 salt, or its hydride, was heated with as ignificant excess of Ti(OiPr) 4 at 150 8Cf or 96 ha nd then cooled to 40 8Cf or 48 h. When cooled to room temperature, crystals were collected by filtration, washed with 2-propanol and dried in anitrogen atmosphere. To complement the series, the previously reported Ti 12 O 16 (OiPr) 16 compound [5] was also synthesized. Full synthetic details are given in Section 1S in the Supporting Information.…”

Novel properties and potential applications of functional ligand-modified polyoxotitanate cages

“…22 (Figure 7a). 23 Coppens and co-workers 27 Due to the lack of direct bonding between Phen and Ti centres, it is difficult to investigate the physical effect of Phen coordination on POT cores themselves (i.e., in the absence of a transition metal ion). The photocurrent from these Phen-M-POT cages is transition-metal specific, the photocurrent being readily observed in the Co-and Fe-doped cages 10 and 12, but much weaker in the related Cd-doped counterpart 11, and completely absent from the Mn cage 13.…”

“…[7][8][9] Resembling the fragments of bulk TiO 2 , POT cages of the type [Ti x O y (OR) z ] (OR = alkoxide), consisting of Ti x O y inorganic titanium oxide cores encapsulated within an alkoxide ligand periphery, can be considered as models for studying the structural chemistry of bulk TiO 2 , including its crystal growth mechanism, 10,11 the electronic and structural effects of heterometallic doping, [12][13][14][15][16][17][18][19][20][21] and the influence of surface functional ligand modification. [22][23][24][25][26][27][28][29][30][31] While their excellent solubility in common organic solvents allows POTs to be studied using various standard methods (e.g., NMR, 32-34 mass spectrometry 35 ), single-crystal X-ray diffraction is perhaps the preeminent tool for their characterisation (allowing unambiguous characterisation of structural features which can be related to that of bulk TiO 2 itself). We and others have recently reviewed advances in the area of metal-doped cages (M-POTs), with the emphasis on their structures, photochemistry and applications.…”