Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production

Abstract

The light-induced processes occurring in two dye-catalyst assemblies for light-driven hydrogen production were investigated by ultrafast transient absorption spectroscopy. These dyads consist of a push-pull organic dye based on a cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) bridge, covalently linked to two different H2-evolving cobalt catalysts. Whatever the nature of the latter, photoinduced intramolecular electron transfer from the excited state of the dye to the catalytic center was never observed. Instead, and in sharp contrast to the reference dye, a fast intersystem crossing (ISC) populates a long-lived triplet excited state, which in turn non-radiatively decays to the ground state. This study thus shows how the interplay of different structures in a dye-catalyst assembly can lead to unexpected excited state behavior and might open up new possibilities in the area of organic triplet sensitizers. More importantly, a reductive quenching mechanism with an external electron donor must be considered to drive hydrogen production with these dye-catalyst assemblies.

Spectroscopic Investigations Provide a Rationale for the Hydrogen-Evolving Activity of Dye-Sensitized Photocathodes Based on a Cobalt Tetraazamacrocyclic Catalyst

“…Applying a three-component exponential fit to the data, a characteristic time constant of τ1 < 1 ps is obtained for this process, in line with the ultrafast hole injection observed in literature for other organic dyes grafted on NiO. 15,25,[71][72][73][74][75][76][77][78][79] As this class of push-pull organic dyes are known to undergo a fast relaxation cascade to the relaxed ICT state on the same subps time scale (Figure S17), 40,50,70,71,[80][81][82] it seems likely that hole injection and the relaxation cascade occur simultaneously and that τ1 therefore reflects a convolution of the two. A second process, with a characteristic time constant 2 ≈ 8 ps, can be assigned to the decay of residual excited state to the ground state since the spectral change it causes is a loss of the typical features of the T2R dye in the excited state.…”

“…After laser pulse excitation and regardless of the potential applied to the electrode, the fs-TA spectra of T2R-and T2R-Cat1-sensitized NiO films initially show ground-state bleach (GSB) at 500 nm and excited-state absorption (ESA) at >550 nm, attributed to the vibrationally hot excited state of the dye (Figures 8 and S16 and S17). 70 This is followed by a fast recovery of GSB, accompanied by a blue shift of ESA to form a band with a maximum at 620 nm. Since the 620 nm band is characteristic of the one-electron reduced dye (Figure S18), this first process is assigned to hole injection, leading to the formation of the primary charge-separated state NiO + |T2R − (-Cat1).…”

Ultrafast Electron Transfer from CuInS2 Quantum Dots to a Molecular Catalyst for Hydrogen Production: Challenging Diffusion Limitations

“…36 Nevertheless, for photocatalysis, the modification at the para-pyridine position of the macrocycle did not adversely affect the catalyst's comparative performance, indicating that this site can be functionalized without impeding the catalytic mechanism, which validates modification at this position for improvement of this catalyst following rational design principles. 34,35,80 Similar TONs were recorded to those reported for CIS/ZnS, in spite of using hybrid passivation, and it is estimated that the quantum yields are also similar: initial quantum yields of H 2 production over the first 2 h in these experiments were calculated as 2−3% (see SI Section 4.1). This is interesting, as the shell has been described to reduce charge recombination and unproductive trapping.…”

“…Often inspired by biology, the coordination sphere around the metal can be tuned to improve the activity, selectivity, and stability in catalyzing the desired reaction, and anchoring groups can be included in the molecular structure to enable controlled integration onto materials. − One such catalyst is the cobalt tetraazamacrocyclic complex, [Co­(N 4 H)­Cl 2 ] + ( 1 ), which is an effective and robust, oxygen-tolerant catalyst for the hydrogen evolution reaction (HER) in both organic and aqueous conditions. 1 is reported to display notable activity, efficiency, and stability in both electrocatalytic and photocatalytic conditions. − Indeed, this catalyst has been successfully used with a diverse range of photosensitizers, including [Ru II (bpy) 3 ] 2+ , − the triazatriangulenium derivative organic dye (TATA + ), , and CdTe QDs, as well as in dye–catalyst assembly systems. , However, the excellent performances of 1 and its anchorable derivatives with glutathione-capped CIS/ZnS QDs, reported by the Wang and Collomb groups, are particularly noteworthy. ,, This combination of PS and molecular catalyst has reported turnover numbers (TONs) of up to 7700 at pH 5.0, with electron transfer (ET) occurring on the time scale of ∼1 ns in the case of an anchored derivative.…”

Diiodo-BODIPY Sensitizing of the [Mo3S13]2– Cluster for Noble-Metal-Free Visible-Light-Driven Hydrogen Evolution within a Polyampholytic Matrix

“…Artificial photosynthesis is one major approach that can overcome fossil fuel dependency, especially in the form of light-driven hydrogen evolution for solar fuel production. , However, most systems utilize precious-metal-based complexes as both their light-sensitive materials as well as their catalytically active sites, limiting their application to model systems for mechanistic investigations. − On the one hand, the development of catalysts composed of non-noble metals has led to a variety of different materials and molecular proton reduction systems, such as graphitic carbon nitride, , iron–iron hydrogenase mimics, − and thiomolybdate clusters. , On the other hand, development and application of photosensitizers (PSs) based on earth-abundant elements have been limited to a few examples as a result of their mostly hydrophobic nature and fast charge recombination. − One strategy to overcome these deficits relies on the involvement of macromolecular templating agents as soft matter matrices. Stupp and co-workers reported light-driven hydrogen evolution by thiomolybdate clusters sensitized by perylene monoimides (PMIs) in hydrogels based on poly­(diallyldimethylammonium) chloride (PDDA) .…”