Rhodamine-Platinum
Diimine Dithiolate Complex Dyads
as Efficient and Robust Photosensitizers for Light-Driven Aqueous
Proton Reduction to Hydrogen
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Abstract
Three new dyads consisting
of a rhodamine (RDM) dye linked covalently
to a Pt diimine dithiolate (PtN<sub>2</sub>S<sub>2</sub>) charge transfer
complex were synthesized and used as photosensitizers for the generation
of H<sub>2</sub> from aqueous protons. The three dyads differ only
in the substituents on the rhodamine amino groups, and are denoted
as <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b>. In acetonitrile, the three dyads show a strong absorption in the
visible region corresponding to the rhodamine π–π*
absorption as well as a mixed metal-dithiolate-to-diimine charge transfer
band characteristic of PtN<sub>2</sub>S<sub>2</sub> complexes. The
shift of the rhodamine π–π* absorption maxima in
going from <b>Pt-RDM1</b> to <b>Pt-RDM3</b> correlates
well with the HOMO–LUMO energy gap measured in electrochemical
experiments. Under white light irradiation, the dyads display both
high and robust activity for H<sub>2</sub> generation when attached
to platinized TiO<sub>2</sub> nanoparticles (Pt-TiO<sub>2</sub>).
After 40 h of irradiation, systems containing <b>Pt-RDM1</b>, <b>Pt-RDM2</b>, and <b>Pt-RDM3</b> exhibit turnover
numbers (TONs) of 33600, 42800, and 70700, respectively. Ultrafast
transient absorption spectroscopy reveals that energy transfer from
the rhodamine <sup>1</sup>π–π* state to the singlet
charge transfer (<sup>1</sup>CT) state of the PtN<sub>2</sub>S<sub>2</sub> chromophore occurs within 1 ps for all three dyads. Another
fast charge transfer process from the rhodamine <sup>1</sup>π–π*
state to a charge separated (CS) RDM<sup>(0•)</sup>-Pt<sup>(+•)</sup> state is also observed. Differences in the relative
activity of systems using the RDM-PtN<sub>2</sub>S<sub>2</sub> dyads
for H<sub>2</sub> generation correlate well with the relative energies
of the CS state and the PtN<sub>2</sub>S<sub>2</sub> <sup>3</sup>CT
state used for H<sub>2</sub> production. These findings show how one
can finely tune the excited state energy levels to direct excited
state population to the photochemically productive states, and highlight
the importance of judicious design of a photosensitizer dyad for light
absorption and photoinduced electron transfer for the photogeneration
of H<sub>2</sub> from aqueous protons
