Hydrophilicity Control of Visible‐Light Hydrogen Evolution and Dynamics of the Charge‐Separated State in Dye/TiO2/Pt Hybrid Systems

Visible‐light‐driven H₂ evolution based on Dye/TiO₂/Pt hybrid photocatalysts was investigated for a series of (E)‐3‐(5′‐{4‐[bis(4‐R¹‐phenyl)amino]phenyl}‐4,4′‐(R²)₂‐2,2′‐bithiophen‐5‐yl)‐2‐cyanoacrylic acid dyes. Efficiencies of hydrogen evolution from aqueous suspensions in the presence of ethylenediaminetetraacetic acid as electron donor under illumination at λ>420 nm were found to considerably depend on the hydrophilic character of R¹, varying in the order MOD (R¹=CH₃OCH₂, R²=H)≈ MO4D (R¹=R²=CH₃OCH₂)> HD (R¹=R²=H)> PD (R¹=C₃H₇, R²=H). In the case of MOD /TiO₂/Pt, the apparent quantum yield for photocatalyzed H₂ generation at 436 nm was 0.27±0.03. Transient absorption measurements for MOD ‐ or PD ‐grafted transparent films of TiO₂ nanoparticles dipped into water at pH 3 commonly revealed ultrafast formation (<100 fs) of the dye radical cation (Dye^.+) followed by multicomponent decays, which involve minor fast decays (<5 ps) almost independent of R¹ and major slower decays with significant differences between the two samples: 1) the early decay of the major components for MOD is about 2.5 times slower than that for PD and 2) a redshift of the spectrum occurred for MOD with a time constant of 17 ps, but not for PD . The substituent effects on H₂ generation as well as on transient behavior have been discussed in terms substituent‐dependent charge recombination (CR) of Dye^.+ with electrons in bulk, inner‐trap, and/or interstitial‐trap states, arising from different solvent reorganization.