Sensitization of Nanocrystalline TiO₂ Anchored with Pendant Catechol Functionality Using a New Tetracyanato Ruthenium(II) Polypyridyl Complex

Abstract

We have synthesized a new photoactive ruthenium(II) complex having a pendant catechol functionality (K(2)[Ru(CN)(4)(L)] (1) (L is 4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol) for studying the dynamics of the interfacial electron transfer between nanoparticulate TiO(2) and the photoexcited states of this Ru(II) complex using femtosecond transient absorption spectroscopy. Steady-state absorption and emission studies revealed that the complex 1 showed a strong solvatochromic behavior in solvents or solvent mixtures of varying polarity. Our steady-state absorption studies further revealed that 1 is bound to TiO(2) surfaces through the catechol functionality, though 1 has two different types of functionalities (catecholate and cyanato) for binding to TiO(2) surfaces. The longer wavelength absorption band tail for 1, bound to TiO(2) through the proposed catecholate functionality, could also be explained on the basis of the DFT calculations. Dynamics of the interfacial electron transfer between 1 and TiO(2) nanoparticles was investigated by studying kinetics at various wavelengths in the visible and near-infrared region. Electron injection to the conduction band of the nanoparticulate TiO(2) was confirmed by detection of the conduction band electron in TiO(2) ([e(-)](TiO(2))(CB)) and cation radical of the adsorbed dye (1(*+)) in real time as monitored by transient absorption spectroscopy. A single exponential and pulse-width limited (<100 fs) electron injection was observed. Back electron transfer dynamics was determined by monitoring the decay kinetics of 1(*+) and [e(-)](TiO(2))(CB). This is the first report on ultrafast ET dynamics on TiO(2) nanoparticle surface using a solvatochromic sensitizer molecule.

Interfacial Electron Transfer Dynamics of Two Newly Synthesized Catecholate Bound RuII Polypyridyl‐Based Sensitizers on TiO2 Nanoparticle Surface – A Femtosecond Pump Probe Spectroscopic Study

“…We have observed that Ru II -and Os II -polypyridyl complexes with pendant catecholate moiety bind strongly to TiO 2 nanoparticles. [35][36][37]45,46] Figure 3 shows the optical absorption and emission spectra of 5 and 6 in aqueous solution in the presence and absence of TiO 2 nanoparticles. The spectral features observed in water are very similar to those in acetonitrile (Figure 1).…”

“…[47,48] This observation is in accordance with results reported earlier where we have shown that dyes bound to the TiO 2 surface through catecholate anchoring groups interact very strongly. [35,36,45,46,[49][50][51][52] To confirm the formation of the complex, we have constructed a Benesi-Hildebrand plot for 5 and 6 by recording the optical absorption spectrum of the complexes with increasing TiO 2 concentrations. From the Benesi-Hildebrand plot, equilibrium constants of 5.33 ϫ 10 3 and 4.28 ϫ 10 3 m -1 were determined for 5-TiO 2 and 6-TiO 2 , respectively.…”

“…Both spectra show two broad absorption features, one ranging from 510-780 nm and the other in the near IR region ranging from 830-1000 nm along with a bleach before 500 nm due to ground state MLCT absorption. These transient absorption bands are assigned to excited triplet state absorption, [35][36][37]46] because intersystem crossing in Ru IIpolypyridyl complexes is known to be very fast and is completed in 15 Ϯ 10 fs. [53] Thus, the singlet to triplet conversion is expected to have occurred within the pulse width of the instrument (Ͻ 100fs).…”

“…[64] In addition, our previous studies confirm that the transients obtained at different wavelengths are due to the charge separated species and that any contribution due to excited states is negligible. [35,36,46] The nonthermalized excited states involved in IET are presumably both the hot singlet and triplet states because the ISC process is known to be very fast in Ru II -polypyridyl complexes and this competes with the electron injection into TiO 2 . Complexes 5 and 6 are anchored to the TiO 2 surface through the catecholate functionality resulting in very strong electronic coupling, which is responsible for electron injection from the nonthermalized states, hence a single exponential and ultrafast electron injection.…”

“…Furthermore, dyes with carboxylate anchoring groups generally show biphasic electron injection. [17,34] Our previous reports show that ruthenium-polypyridyl-based sensitizer dyes with catecholate anchoring groups inject electrons into TiO 2 single exponentially and within instrumental pulse width (Ͻ 100 fs) [35][36][37] in contrast to dyes with carboxylate anchoring groups. In addition, catecholates have pK a values greater than 9.6 and, even at higher pH, the possibility of the dye molecules becoming desorbed from the surface of the semiconductor is excluded.…”