Hydrophilicity Control of Visible‐Light Hydrogen Evolution and Dynamics of the Charge‐Separated State in Dye/TiO 2 /Pt Hybrid Systems

Visible‐light‐driven H 2 evolution based on Dye/TiO 2 /Pt hybrid photocatalysts was investigated for a series of ( E )‐3‐(5′‐{4‐[bis(4‐R 1 ‐phenyl)amino]phenyl}‐4,4′‐(R 2 ) 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 R 1 , varying in the order MOD (R 1 =CH 3 OCH 2 , R 2 =H)≈ MO4D (R 1 =R 2 =CH 3 OCH 2 )> HD (R 1 =R 2 =H)> PD (R 1 =C 3 H 7 , R 2 =H). In the case of MOD /TiO 2 /Pt, the apparent quantum yield for photocatalyzed H 2 generation at 436 nm was 0.27±0.03. Transient absorption measurements for MOD ‐ or PD ‐grafted transparent films of TiO 2 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 R 1 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 H 2 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.