Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes

Abstract

New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF 2 ) dye and a platinum diimine dithiolate (PtN 2 S 2 ) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H 2 from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN 2 S 2 CT chromophore. Each of the new dyads contains either a bipyridine (bpy) or phenanthroline (phen) diimine with an attached functional group that is used for binding directly to TiO 2 nanoparticles, allowing rapid electron photoinjection into the semiconductor. The absorption spectra and cyclic voltammograms of the dyads show that the spectroscopic and electrochemical properties of the dyads are the sum of the individual chromophores (Bodipy and the PtN 2 S 2 moieties), indicating little electronic coupling between them. Connection to TiO 2 nanoparticles is carried out by sonication leading to in situ attachment to TiO 2 without prior hydrolysis of the ester linking groups to acids. For H 2 generation studies, the TiO 2 particles are platinized (Pt-TiO 2 ) so that the light absorber (the dyad), the electron conduit (TiO 2 ), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H 2 O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO 2 via a phosphonate or carboxylate attachment shows excellent light-driven H 2 production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼40,000 turnover numbers of H 2 over 12 d (with respect to dye).photochemistry | solar energy conversion | hydrogen | spectroscopy | synthesis W ater splitting into hydrogen and oxygen is the key energystoring reaction of artificial photosynthesis (AP) and one of the most promising long-term strategies for carbon-free energy on a potentially global scale (1). As a redox reaction, water splitting has been studied primarily in terms of its two halfreactions, the reduction of aqueous protons to H 2 and the oxidation of water to O 2 (2-13). Whereas some of these studies date back more than 30 y (14-22), recent progress on each halfreaction has been notable, particularly with regard to catalyst development and mechanistic understanding of each transformation (6,(23)(24)(25)(26)(27)(28)(29). In this paper, we focus on efforts dealing with the lightdriven generation of H 2 , which in its simplest form requires a light absorber or photosensitizer (PS) for electron-hole creation, a means or pathway for charge separation and electron transfer, an aqueous proton source, a catalyst for collecting electrons and protons and promoting their conversion to H 2 , and an ultimate source of electrons in the form of an electron donor.Dating from the earliest work on the light-driven generation of H 2 , the photosensitizer has most often been a Ru(II) complex with 2,2′-bipyridine (bpy) and/or related heterocyclic ligands having a long-lived triplet metal-to-ligand ...

Panchromatic Sensitization with ZnII Porphyrin‐Based Photosensitizers for Light‐Driven Hydrogen Production

“…From the TA result, a 2.3 ps time constant was assigned for the singlet energy transfer from the bodipy unit to the ZnP moiety, comparable to those (i.e., 0.51–0.73 ps) of the reported bodipy‐conjugated PtN 2 S 2 PSs. In the bodipy‐PtN 2 S 2 systems, the bodipy‐antenna was an effective energy‐transfer chromophore towards the PtN 2 S 2 charge‐transfer moiety, increasing the H 2 production efficiency . Accordingly, our dyad design should make a positive contribution towards H 2 generation compared to ZnP alone.…”

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

“…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]. A survey of the combined influence of BODIPY halogenation and introduction of sterically-demanding aryl groups on photocatalytic hydrogen production activity was presented by the same authors, showing the presence of a pronounced heavy atom effect as well as an enhanced dye stability with an increase in steric demand at the meso position [20].…”

Antenna molecule drives solar hydrogen generation

“…Since that time, alternative semiconductor materials have been identified that effectively harvest sunlight and split water more efficiently (4,5); however, the associated costs are not economically competitive with today's fossil fuels. In a step toward simple low-cost approaches to solar hydrogen generation, Zheng et al (6) report sustained H 2(g) generation over periods of 12 d with ∼40,000 turnovers by a dye molecule anchored to TiO 2 /Pt particles in an aqueous suspension. The turnover numbers are unprecedented for solar fuel production and may enable practically useful molecular approaches to solar hydrogen production.…”

“…Light absorption by "dyads" drives the newly discovered hydrogen generation (6). The particular dyad that proved to be the most efficient is shown in Fig.…”

“…For example, ascorbic acid is often used in fluorescence microscopy to prevent dye photobleaching; illuminated dyes are not as stable in its absence (8). Hence, as the authors correctly point out, the use of different sacrificial reagents and experimental conditions preclude a determination of how the Zheng dyads would perform in a complete water-splitting cell or why these dyads are so stable relative to others that have been reported in the literature (6). A critical step toward practical application will be to successfully integrate the light-driven H 2(g) formation with a nonsacrificial oxidation half-reaction.…”