Noble-metal-free BODIPY–cobaloxime photocatalysts for visible-light-driven hydrogen production

Abstract

In this study a series of supramolecular BODIPY-cobaloxime systems Co-Bn (n = 1-4): [{Co(dmgH)2Cl}{4,4-difluoro-8-(4-pyridyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene}] (Co-B1), [{Co(dmgH)2Cl}{4,4-difluoro-8-(4-pyridyl)-1,3,5,7-tetramethyl-2,6-diiodo-4-bora-3a,4a-diaza-s-indacene}] (Co-B2), [{Co(dmgH)2Cl}{4,4-difluoro-8-(3-pyridyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene}] (Co-B3), and [{Co(dmgH)2Cl}{4,4-difluoro-8-(3-pyridyl)-1,3,5,7-tetramethyl-2,6-diiodo-4-bora-3a,4a-diaza-s-indacene}] (Co-B4) (BODIPY = boron dipyrromethene, dmgH = dimethylglyoxime) have been synthesized by replacing one axial chlorine of cobaloxime moieties with the pyridine residues of BODIPYs, and structurally characterized. Absorption spectra show that the optical properties of the BODIPY-cobaloximes are essentially the sum of their constituent components, indicating weak interactions between the cobaloxime units and BODIPY chromophores in the ground state. If any, electronic communications may take place through the intramolecular electron transfer across their orthogonal structures. The possibility of intramolecular electron transfer is further supported by the results of the density functional theory (DFT) calculations at UB3LYP/LANL2DZ levels on Co-B2˙(-) and Co-B4˙(-), which show that the highest occupied molecular orbitals (HOMOs) possess predominantly BODIPY character, while the lowest unoccupied molecular orbitals (LUMOs) are located on the cobalt centers. The HOMO → LUMO transition is an electron-transfer process (BODIPY˙(-) radical anions → cobaloxime fragments). In view of the possible occurrence of electron transfer, these noble-metal-free BODIPY-cobaloximes are studied as single-component homogeneous photocatalysts for H2 generation in aqueous media. Under optimized conditions, the 2,6-diiodo BODIPY-sensitized cobaloxime Co-B4 that contains a meta-pyridyl at the 8-position of BODIPY presents excellent H2 photoproduction catalytic activity with a turnover number (TON) of 85, which is comparable to that of its analogue Co-B2 that has a para-pyridyl attached onto 2,6-diiodo BODIPY (TON = 82); however, both of the noniodinated BODIPY-sensitizer cobaloximes (Co-B1, Co-B3) exhibit a complete lack of activity under the same experimental conditions. These results show that the presence of heavy atoms in the core of BODIPY is essential for the catalytic process and reductive quenching pathways (namely, the intramolecular electron transfers from BODIPY˙(-) species to the cobalt centers) for these photocatalytically active systems of Co-Bn (n = 2 and 4) are thermodynamically feasible for the hydrogen-evolving reaction.

Photophysics of BODIPY Dyes as Readily-Designable Photosensitisers in Light-Driven Proton Reduction

“…This suggests the necessity of a strong intersystem crossing (ISC) to form a long-lived, high-yield triplet excited state for efficient photocatalysis. Further evidence for the important role of a triplet excited state was reported by the groups of Weare and Luo for intermolecular [16], as well as intramolecular [13,[15][16][17] BODIPY-cobaloxime systems. More recently, Eisenberg et al have used a combination of strongly light-absorbing BODIPYs and platinum diamine dithiolate charge transfer chromophores for sensitizing TiO2 in light-driven reduction of aqueous protons [18,19].…”

“…Recent developments include the implementation of non-noble metal components, or even metal-free compounds into these systems and several groups described the use of classical organic dyes, such as Fluorescein, dyes, such as Fluorescein, porphyrins, or Eosin Y, as photosensitizer species [4][5][6][7][8][9][10][11]. Moreover, in recent years, BODIPY (boron dipyrromethene; IUPAC name: 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have also attracted considerable attention as light-harvesting units in inter- [12] and intramolecular [13][14][15][16][17] photocatalyst systems for light-driven proton reduction. In a study of dye-sensitised TiO2/Pt systems, Eisenberg and McCamant showed that incorporation of heavy atoms such as bromine or iodine at the 2,6 position of the BODIPY results in a pronounced coupling between singlet and triplet states that greatly decreases fluorescence quantum yield and fluorescence lifetime compared to the parent halogen-free compound and results in a more active photosensitizer [12].…”

Efficient Bimolecular Mechanism of Photochemical Hydrogen Production Using Halogenated Boron-Dipyrromethene (Bodipy) Dyes and a Bis(dimethylglyoxime) Cobalt(III) Complex

“…7--12 Recently, several groups have published articles featuring dyads, in which Bodipy chromophores have been covalently bound to Co(dmg) 2 catalysts for application in solar energy conversion. [13][14][15][16][17] In these cases, the Bodipy contains a pyridine aryl group, which coordinates to Co(III) as a ligand on the catalyst.…”

“…18 The TONs with respect to chromophore reported 18 were greater than those reported for the earlier dyads. 15,16 Here, we present the results of a series of discrete Bodipy chromophores and the Co(dmg) 2 catalyst, in which the effects of halogenation, sterics, and component concentrations are examined. By optimizing the relative concentrations of the dye and catalyst we show that the system is strongly inhibited under conditions of excess catalyst, as was used in prior studies.…”

Structural and electronic characterization of multi-electron reduced naphthalene (BIAN) cobaloximes

“…18,19 Cobaloximes have also been incorporated into heterogeneous systems such as glassy carbon, 20 nickel oxide, 21 carbon nanotubes, 22 and gallium phosphide 23,24 substrates. Furthermore, cobaloximes have been explored in conjunction with homogeneous photosensitizers, [25][26][27][28] as components in aqueous systems, [29][30][31] as a functional catalyst on vesicle membrane surfaces, 32 and used as part of an artificial hydrogenase in a hybrid SwMb system. 33 Moreover, increasingly elaborate ligand designs have been devised and studied.…”