Co-sensitization of colloidal TiO2with two dyes

    Squarylium cyanine dyes (SQ) have been widely used as efficient photosensitizers due to the possession of strong and intense absortion band in the visible and near-infrared region [17].

Scheme 1. Squarilium cianine

In order to improve the light harvesting efficiency of photosensitizer, SQ can be incorporated with RuL2(NCS)2 (where L=2,2'-bipyridyl-4,4'-dicarboxylate) to sensitize nanocrystalline TiO2 electrodes. FTIR spectrum provides strong evidence of the co-adsorption of SQ and RuL2(NCS)2 on nanocrystalline TiO2 electrodes [17].

    New stretch bands cannot be found in this spectrum, indicating that the SQ and RuL2(NCS)2 co-adsorbed on the nanocrystalline TiO2 film electrodes without chemical reactions [17].
Interstingly, the IPCE values of the the TiO2 electrodes sensitized by the dye mixtures became larger than those of RuL2(NCS)2 sensitized electrodes [17].
In order to understand the mechanism of the association effects of SQ and RuL2(NCS)2 on the photosensitization of nanocrystalline TiO2 electrodes, the absorption spectra of SQ in ethanol solution containing RuL2(NCS)2 or TiO2 colloids were measured [17].

Fig. 1. Absorption spectra of the mixtures of SQ and RuL2(NCS)2 at different molar proportions: (a) in ethanol solution; (b) adsorbed on the nanocrystalline TiO2 electrodes [17].

   By exciting with 590 nm light, a strong fluorescence band centered at 646 nm was observed in the SQ emission spectra. The fluorescence intensities decreased on growing addition of the concentration of RuL2(NCS)2 implying that the fluorescence was quenched by RuL2(NCS)2 molecules [17].

Fig. 2. Emission spectra of SQ in ethanol solution at the presence of RuL2(NCS)2. Molar proportions (SQ:RuL2(NCS)2): (a) 1:0; (b) 6:1; (c) 2:1; (d) 1:1; (e) 1:5; (f) 1:10; (g) 1:20; (h) 1:50; and (i) 1:100 mol/mol. Excitation by 590 nm light [17].

   Addition of TiO2 colloids to the SQ ethanol solution also resulted in quenching SQ fluorescence as shown in figure, in which the fluorescence intensities were suppressed gradually by the increase of concentration of TiO2 colloids [17].

Fig. 3. Emission spectra of SQ in ethanol solution at presence of TiO2 colloids: (a) 0.00; (b) 0.18; (c) 0.27; (d) 0.35; (e) 0.50; and (f) 0.75*10−3 M. Excitation by 590 nm light [17].

    The fluorescence quenching effects shown in figures indicate occurrence of electron transfer from excited state of SQ to the RuL2(NCS)2 molecules or TiO2 colloids [17].
   Comparison of the redox potential of SQ (E0 -1.45 V) with the redox potential of RuL2(NCS)2 (E0 -1.30 V) the feasible electron transfer from the higher energy level of excited states of SQ to the lower energy level of RuL2(NCS)2 can be demonstrated. Squarylium cyanine as a co-adsorbate functions as a sensitizer by injecting electrons into the conduction band of TiO2 and, more importantly, as TiO2-to- RuL2(NCS)2 back electron transfer (BET) blocker by reducing the one electron oxidation product RuL2(NCS)2radical+ [17],[18].

   In summary an electron transfer from excited SQ to RuL2(NCS)2 or TiO2 led to the improvement of the photoelectric behavior of nanocrystalline TiO2 electrodes [17].

Fig. 4. Processes in a nanocrystalline TiO2 electrode.

   Fig. 4 illustrates the processes involved in a nanocrystalline TiO2 electrode. ET is the electron transfer from the first excited state of RuL2(NCS)2 (S1) to valence band (VB) of TiO2. BET is the back electron transfer from the VB of TiO2 to the ground state of RuL2(NCS)2 (S0). The electron transfer from the first excited state (S1) of squarylium cyanine (SQ) to the ground state S0 of RuL2(NCS)2 is the BET blocker. CB is the conduction band of TiO2. ET is very fast, it proceeds on a few tens of femtoseconds and it has a quantum efficiency close to unity. BET is much slower, on the timescale of microseconds. The BET blocker is quite fast, on the timescale of 300 ps, (obviously faster than BET), while RuL2(NCS)2 reduction by I-/I3- electrolyte proceeds on a few tens of nanoseconds. Electron injection from SQ to TiO2 (not shown in figure) is slower than that from RuL2(NCS)2: as sensitizer RuL2(NCS)2 is better than SQ [18].