All-catecholate-stabilized black titanium-oxo clusters for efficient photothermal conversion

Abstract

The controlled synthesis of titanium-oxo clusters (TOCs) completely stabilized by organic dye ligands with high stability and superior light absorption remains a significant challenge. In this study, we report the syntheses of three atomically precise catechol (Cat)-functionalized TOCs, [Ti₂(Cat)₂(OEgO)₂(OEgOH)₂] (Ti2), [Ti₈O₅(Cat)₉(ⁱPrO)₄(ⁱPrOH)₂] (Ti8), and [Ti₁₆O₈(OH)₈(Cat)₂₀]·H₂O·PhMe (Ti16), using a solvent-induced strategy (HOEgOH = ethylene glycol; ⁱPrOH = isopropanol; PhMe = toluene). Interestingly, the TiO core of Ti16 is almost entirely enveloped by catechol ligands, making it the first all-catechol-protected high-nuclearity TOC. In contrast, Ti2 and Ti8 have four weakly coordinated ethylene glycol ligands and six weakly coordinated ⁱPrOH ligands, respectively, in addition to the catechol ligands. Ti16 is visually evident in its distinctively black appearance, which belongs to black TOCs (B-TOCs) and exhibits an ultralow optical band gap. Furthermore, Ti16 displays exceptional stability in various media/environments, including exposure to air, solvents, and both acidic and alkaline aqueous solutions due to its comprehensive protection by catechol ligands and rich intra-cluster supramolecular interactions. Ti16 has superior photoelectric response qualities and photothermal conversion capabilities compared to Ti2 and Ti8 due to its ultralow optical band gap and remarkable stability. This discovery not only represents a huge step forward in the creation of all-catecholate-protected B-TOCs with ultralow optical band gaps and outstanding stability, but it also gives key valuable mechanistic insights into their photothermal/electric applications.

Scheme 1. Schematic diagram of the synthesis for Ti2, Ti8, and Ti16 (HOEgOH = ethylene glycol; ⁱPrOH = isopropanol; PhMe = toluene; FA = formic acid).

Fig. 1. (a and b) Overall structure of Ti2. (c) Distribution of surface ligands on Ti2. (d) Binding modes of catecholate ligands. (e and f) Total structure of Ti8. (g) Distribution of surface ligands on Ti8. (h and i) Total structure of Ti16. (j) Distribution of surface ligands on Ti16. (k) The {Ti₁₆O₁₆} skeletal structure of Ti16 by removing all ligands. Color code: light green, Ti; red, O; gray, C.

Fig. 2. (a) Positive-ion mode ESI-MS of Ti8 dissolved in CH₂Cl₂ and insets show the experimental (light green) and simulated (light coral) isotopic distribution patterns for species 1a. (b) ¹H NMR spectrum of Ti16 in DMSO-d₆.

Fig. 3. PXRD pattern of Ti16 under different conditions. (a) Exposed to air for a month and soaked in common organic solvents for 48 hours. (b) Soaked in water solutions with pH ranging from 1 to 11 for 48 hours. (c) Solid-state UV-visible absorption spectra of Ti2, Ti8, and Ti16. (d) Tauc plots of Ti2, Ti8, and Ti16. (e) Transient photocurrent responses of Ti2, Ti8, and Ti16 under Xe lamp irradiation. (f) Electrochemical impedance spectroscopy (EIS) Nyquist plots of Ti2, Ti8, and Ti16.

Fig. 4. Iso-surface plots (iso-density value = 0.02 a.u.) of molecular orbitals of Ti16 and their corresponding energy levels.

Fig. 5. (a) Photothermal conversion of DMF solution of Ti2, Ti8, and Ti16. (b) Photothermal cycling measurement of DMF solution of Ti2, Ti8, and Ti16. (c) IR thermal images of DMF and DMF solutions of Ti2, Ti8, and Ti16. All measurements were conducted at a concentration of 0.60 mM upon 450 nm laser irradiation (0.6 W cm⁻²) for (a)–(c). (d) Photothermal conversion of Ti2, Ti8, and Ti16 crystals. Insets: thermal images of Ti16 at various irradiation times. (e) Photothermal cycling measurement of Ti2, Ti8, and Ti16 crystals. All measurements were conducted with 450 nm laser irradiation (0.6 W cm⁻²) at a distance of 15 cm for (d) and (e).