Chromophoric Dyads for the Light-Driven Generation of Hydrogen: Investigation of Factors in the Design of Multicomponent Photosensitizers for Proton Reduction

Abstract

Two new dyads have been synthesized and studied as photosensitizers for the light-driven generation of H2 from aqueous protons. One of the dyads, Dy-1, consists of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine benzenedithiolate (bdt) charge transfer (CT) chromophore, denoted as PtN2S2. The two components are connected through an amide linkage on the bdt side of the PtN2S2 complex. The second dyad, Dy-2, contains a diketopyrrolopyrrole dye that is linked directly to the acetylide ligands of a Pt diimine bis(arylacetylide) CT chromophore. The two dyads, as well as the Pt diimine bis(arylacetylide) CT chromophore, were attached to platinized TiO2 via phosphonate groups on the diimine through sonication of the corresponding esters, and each system was examined for photosensitizer effectiveness in photochemical generation of H2 from aqueous protons and electrons supplied by ascorbic acid. Of the three photosensitizers, Dy-1 is the most active under 530 nm radiation with an initial turnover frequency of 260 h(-1) and a total of 6770 turnovers over 60 h of irradiation. When a "white" LED light source is used, samples with Dy-2 and the Pt diimine bis(arylacetylide) chromophore, while not as effective as Dy-1, perform relatively better. A key conclusion is that the presence of a strongly absorbing organic dye increases dyad photosensitizer effectiveness only if the energy of the CT excited state lies below that of the organic dye's lowest excited state; if not, the organic dye does not improve the effectiveness of the CT chromophore for promoting electron transfer and the light-driven generation of H2. The nature of the spacer between the organic dye and the charge transfer chromophore also plays a role in the effectiveness of using dyads to improve light-driven energy-storing reactions.

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

“…Comparisons of the UV/Vis spectra in tetrahydrofuran (THF) solutions of ZnP‐dyad, YD2‐ o ‐C8, and Bodipy‐dye before and after sonication in the presence of platinized TiO 2 , were used to determine the extent of binding of the PS onto the TiO 2 particles (the corresponding spectra are displayed in Figures S20–S24.) The relatively low dye‐loading values (≈40–70 %) as compared to the previous reports were probably owing to the larger molecular size of PSs in this study. In short, the H 2 generation experiments were conducted in aqueous solutions at pH 4.0, with AA (0.5 m ) serving as the hole scavenger.…”

“…Most of these PSs contain noble metals such as ruthenium(II), iridium(III), and platinum(II) metal centers . However, these charge‐transfer complexes typically suffer from inefficient photon capture because of their poor absorption in the blue–green region of the spectrum and low molar absorptivities (≈7–15×10 3 m −1 cm −1 ) . In 2015, Zheng et al.…”

Directly Coupled Versus Spectator Linkers on Diimine PtII Acetylides—Change the Structure, Keep the Function?

“…This result is consistent with the LUMO being destabilised with the less electron‐withdrawing groups, as follows from the absorption data presented above. The data also confirm that the LUMO is largely localised on the bipyridine ligand, as is the case with similar complexes, a notion also confirmed by the DFT calculations (Figure ).…”

“…Pt II bipy acetylide complexes, which intensely absorb light across the visible range of the spectrum, have been well studied with regard to their photophysical properties, yet surprisingly few examples of these type of complexes as photosensitisers are reported . A series of Pt II bipy acetylide dyes anchored through carboxylate linkers to thin‐film nanocrystalline TiO 2 has been used in DSSCs .…”

“…A series of Pt II bipy acetylide dyes anchored through carboxylate linkers to thin‐film nanocrystalline TiO 2 has been used in DSSCs . Pt bipy acetylide attached to TiO 2 through a directly coupled phosphonate group has been used in light‐driven generation of H 2 from aqueous protons …”