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

New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF₂) dye and a platinum diimine dithiolate (PtN₂S₂) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H₂ from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN₂S₂ 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 TiO₂ 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 PtN₂S₂ moieties), indicating little electronic coupling between them. Connection to TiO₂ nanoparticles is carried out by sonication leading to in situ attachment to TiO₂ without prior hydrolysis of the ester linking groups to acids. For H₂ generation studies, the TiO₂ particles are platinized (Pt-TiO₂) so that the light absorber (the dyad), the electron conduit (TiO₂), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H₂O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO₂ via a phosphonate or carboxylate attachment shows excellent light-driven H₂ production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼40,000 turnover numbers of H₂ over 12 d (with respect to dye).Water splitting into hydrogen and oxygen is the key energy-storing 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 half-reactions, the reduction of aqueous protons to H₂ and the oxidation of water to O₂ (2–13). Whereas some of these studies date back more than 30 y (14–22), recent progress on each half-reaction has been notable, particularly with regard to catalyst development and mechanistic understanding of each transformation (6, 23–29). In this paper, we focus on efforts dealing with the light-driven generation of H₂, 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 H₂, and an ultimate source of electrons in the form of an electron donor.Dating from the earliest work on the light-driven generation of H₂, 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 charge transfer state (³MLCT) (5, 7, 30–33). Recent studies have included analogous Ir(III) d⁶ systems based on phenylpyridine (ppy) ligands in place of bpy (34). With charge transfer excited states, the sensitizers are poised for photoinduced electron transfer following photon absorption and intersystem crossing (ISC). Another set of charge-transfer chromophores that have been used in related systems is [Pt(terpyridyl)(arylacetylide)]⁺ complexes that also have ³MLCT states (35–37). Despite their success with photoinduced electron transfer, all of these charge transfer (CT) complexes have absorptions that are too weak for efficient photon capture with molar extinction coefficients (ε) of 7–15 × 10³ M⁻¹⋅cm⁻¹, and the energies of their singlet absorbing states (¹MLCT) are too high (typically >2.6 eV with λ < 490 nm) for effective use of the solar spectrum. More strongly absorbing organic dyes with ε of ∼10⁵ M⁻¹⋅cm⁻¹ were also examined during early studies of the light-driven generation of H₂. Whereas those with heavier halogen substituents such as Rose Bengal and Eosin Y were found to promote H₂ formation because of facile ISC to long-lived ³ππ* states, they also exhibited poor photostability, decomposing within 3–5 h (38–41).Platinum diimine dithiolate complexes (PtN₂S₂) such as 1 constitute another class of charge transfer chromophores that are solution luminescent and undergo electron transfer quenching of both oxidative and reductive types (42–48). The excited state energies of these complexes are significantly lower than those of the Ru(bpy), Ir(ppy), and Pt(terpyridyl) complexes mentioned above and have an excited state that has been described as having both MLCT and ligand-to-ligand charge transfer (LLCT) character. In complexes such as 1, the highest occupied molecular orbital (HOMO) is of mixed character, being composed of substantial percentages of both Pt-based and dithiolate-based wavefunctions. As a consequence of the mixed character of the HOMO in PtN₂S₂, the excited states derived from HOMO to LUMO (lowest unoccupied molecular orbital) excitation have been labeled mixed-metal ligand-to-ligand′ charge transfers (MMLL′CT). Based on these charge transfer states, two of the PtN₂S₂ complexes, including 1 with R = COOH, were examined as photosensitizers for H₂ generation with TiO₂ as the electron conduit and Pt islands on the TiO₂ surface as the catalyst (49). The deprotonated carboxlyate groups of di(carboxy)bipyridine (dcbpy) served to link the PS to the platinized TiO₂ (Pt-TiO₂). The system produced hydrogen and proved to be stable for more than 70 h, using light of λ > 450 nm. However, activity of the system was low, based in part on the low absorptivity of the PtN₂S₂ chromophore and the small amount of the complex bound to TiO₂.To improve absorptivity of the PS in such H₂ generating systems, an approach was initiated in which a strongly absorbing organic dye was linked directly to the PtN₂S₂ moiety for more efficient photon absorption and subsequent energy transfer to the CT chromophore. Such a strategy had been adopted by Ziessel through the synthesis of a bipyridine linked to a dipyrromethene-BF₂ (Bodipy) dye similar to 2 and was examined for Ru complexes containing this ligand, resulting in sensitization of the lower-energy ³ππ* of the Bodipy part of the molecule (50). In a study by Lazarides et al. (51), the dye-CT dyad 3 that contained 2 and the dithiolate-linked Bodipy dyad 4 were synthesized and examined by absorption spectroscopy and transient absorption measurements to determine the potential success of photoinduced charge separation. In that study, it was determined that the absorption spectra of the two dyads are essentially the sum of each constituent chromophore and that the redox potentials of each component are virtually unaffected by bringing the two units together in a dyad. The measurements also showed that energy transfer from the ¹ππ* state of the Bodipy moiety to the ¹MMLL′CT state was energetically favorable and that for both 3 and 4, the dynamics progressed as shown in Scheme 1. Although the rates of both singlet energy transfer (SenT) and ISC for the two dyads were similar and proceeded in less than 1 ps, a significant difference was seen for the triplet energy transfer (TEnT) step with the time constant for 3 being 8 ps whereas that for 4 was 160 ps, indicating that for productive electron transfer to TiO₂, the arrangement in 4 with Bodipy attached to the dithiolate was preferred.Open in a separate windowScheme 1.Jablonski diagram of the Bodipy-PtN₂S₂ dyads showing the transitions between the different states (SEnT, singlet energy transfer; ISC, intersystem crossing; TEnT, triplet energy transfer).In this paper, the synthesis and characterization of dyads 4 with R = COOMe, P(O)(OEt)₂, CH₂(P(O)(OEt)₂) and an analogous phenanthroline (phenP) derivative 5 are described, along with studies of their ability to promote light-driven production of H₂ from aqueous protons in conjunction with Pt-TiO₂ as both electron conduit and catalyst. The benefit of attaching the strongly absorbing bodipy chromophore to the charge transfer PtN₂S₂ complex is discussed.