Photocathodes beyond NiO: charge transfer dynamics in a π-conjugated polymer functionalized with Ru photosensitizers

A conductive polymer (poly(p-phenylenevinylene), PPV) was covalently modified with RuII complexes to develop an all-polymer photocathode as a conceptual alternative to dye-sensitized NiO, which is the current state-of-the-art photocathode in solar fuels research. Photocathodes require efficient light-induced charge-transfer processes and we investigated these processes within our photocathodes using spectroscopic and spectro-electrochemical techniques. Ultrafast hole-injection dynamics in the polymer were investigated by transient absorption spectroscopy and charge transfer at the electrode–electrolyte interface was examined with chopped-light chronoamperometry. Light-induced hole injection from the photosensitizers into the PPV backbone was observed within 10 ps and the resulting charge-separated state (CSS) recombined within ~ 5 ns. This is comparable to CSS lifetimes of conventional NiO-photocathodes. Chopped-light chronoamperometry indicates enhanced charge-transfer at the electrode–electrolyte interface upon sensitization of the PPV with the RuII complexes and p-type behavior of the photocathode. The results presented here show that the polymer backbone behaves like classical molecularly sensitized NiO photocathodes and operates as a hole accepting semiconductor. This in turn demonstrates the feasibility of all-polymer photocathodes for application in solar energy conversion.


Synthesis and characterization of the ruthenium complexes
Scheme S1. Structures of ruthenium complexes Ru1 and Ru2.
The degree of the azide functionalization of the PPV polymer was determined to be approximately 7.6% by 1 H NMR ( Figure S1(c)).
Moreover, the IR spectrum ( Figure S1(b)) of the PPV polymer show the characteristic peak at around 2100 cm -1 for the asymmetric stretching mode of the azide.
Parts of the polymer sample were analyzed by size exclusion chromatography (SEC: chloroform/iso-propanol/trimethylamine (94/2/4) as eluent, polystyrene as calibration) to determine an average molar mass Mn of 10,700 g/mol and a dispersity (Ð) of 2.33. 1 H NMR (300 MHz, CDCl3, δ, Figure S1  The coupling degree of the Ru2 functionalization with the azide functions of the PPV polymer was determined to be approximately 100% by 1 H NMR due to the disappearance of the signal at 3.24 ppm, which corresponds to the CH2 group next to the azide function ( Figure S3).
Parts of the polymer sample were analyzed by size exclusion chromatography (SEC: chloroform/iso-propanol/tri-

Electrochemistry and Spectroelectrochemistry Measurements
Electrochemical studies using cyclic voltammetry were carried out to obtain reduction and oxidation potentials of individual components of the PPV-Ru systems, which are used to estimate the driving force of possible charge injection processes.    As the PPV-Ru system will be used as a photocathode, UV/vis spectroelectrochemistry (SEC) measurements of PPV-Ru1 films were carried out to (again) find the "optical tag" of transient species, i.e. reduced diimine ligands (either bpy or phen ligand). Thin (~ 300 nm) and thick (~1.6 µm, Fig. S9) PPV-Ru1 films were prepared for the SEC measurements. The reductive SEC spectra at the reduction potential of Ru1 and the absorption difference spectra are depicted in Fig. S10. Compared to the spectra of reduced Ru1 in solution, it is clear that the absorption difference peak at 390 and 530 nm is shifted to 425 and 575 nm, respectively.

Transient Absorption Study in Solution and on Films
The fs-transient absorption (TA) study is used to unravel the light-induced charge transfer dynamics in PPV-Ru both in solution and on film. The transient absorption data of PPV as discussed in the main text show an initial ground-state bleach (GSB), stimulated emission (SE) at 530 (peak) and 565 nm (shoulder), and an excited-state absorption (ESA) band beyond 600 nm (Fig. S11a). Arbitrarily scaled absorption and emission spectra of PPV are also shown to discriminate GSB and SE contribution in the TA data. A global fit of the TA data using three decay components results in the decay-associated spectra shown in Fig. S11c. The spectral features associated with 1 show a negative OD signal at 450 -570 nm reflecting the decay of the excited state of PPV and is attributed to vibrational cooling. 3 The spectral signature of 2 is quite similar to 1 with a distinct peak at 520 nm. This process (2 = 72 ps) is assigned to interchain or intrachain energy transfer which typically falls on a characteristic timescale of tens of ps. 4 The following process associated with the time constant of hundreds of ps (3) reflects a concerted decay of SE and absorption beyond 750 nm (spectral signature of radical cation/polaron PPV •+ ), 5,6 and hence, it is assigned to exciton recombination. Figure S11. (a) fs-transient absorption spectra of PPV in CHCl3 at different delay times, (b) kinetic traces at different probe wavelengths, and (c) decay-associated spectra resulting from the global fit with three exponential decay function. The grey area depicts the corresponding inverted ground state absorption spectrum of PPV, which is arbitrarily scaled to fit the transient absorption data.     Normalized kinetic traces at different probe wavelengths and decay-associated spectra (DAS) resulting from the global fit with three time constants for (c,e) PPV-Ru1 and (d,f) PPV-Ru2. The pump pulses are centred at 480 nm. The grey spectra in panel (e) and (f) depict the differential absorption spectra of electrochemically reduced Ru1 and Ru2, respectively, which is arbitrarily scaled to fit the transient absorption data. ns-transient absorption (TA) unravels the spectroscopic signatures of the long-lived species in PPV and PPV-Ru upon photoexcitation. The ns-TA data of PPV and PPV-Ru1 are depicted in Figure S17a and Figure S18b, respectively. TA data of PPV and PPV-Ru1 shows a spectral signature beyond 750 nm, which can be assigned to either 3 PPV or polaron. 5,6 The lifetime of this state is found to be > 40 µs, which is in good agreement with literature. 7,8 Furthermore, similar to the fs-TA data processing for PPV-Ru in solution, the data obtained for PPV alone is then subtracted from the ns-TA signal of PPV-Ru1 ( Figure S17c). Prior to subtraction the data is normalized to its early time (t = 200 ns) signal amplitude at 520 nm for PPV-Ru1 at which there is no contribution of the transient absorption signal from Ru alone. Apparently, the resulting OD spectra in Figure S17c show no signal even at the early time scale of 200 ns. This result indicates that the charge separation process in PPV-Ru1 occurs in a few ns as revealed in fs-TA study. Therefore, the long-lived signal in PPV-Ru1 might root from the 3 PPV signal of the fragment of PPV-Ru1, in which the polymer backbone isn't loaded with the photosensitizers.  (Fig. S18a). Nonetheless, it should be noted that the negative OD peak at 625 nm contains contribution from both stimulated emission of 0-0 transition and the ground state bleach of the vibronic 0-1 transition. 3,9,10 A global fit of TA data using three decay components results in the decayassociated spectra shown in Fig. S18c. The spectral features associated with 1 show negative OD signal at 450 -570 nm reflecting the decay of the early relaxation originates from a strong coupling between electronic and vibrational states, and hence, the fast kinetic processes are attributed to delocalized exciton states (self-trapping, ~ 100 fs). 15 The spectral signature of 2 is marked with the decay of GSB, SE at 575 nm and GSB of vibronic 0-1 transition at 625 nm. Also, this process (2 = 11 ps) is assigned to interchain or intrachain energy transfer, which typically falls on a characteristic timescale of tens of ps 10 and the process associated with 3 is assigned to exciton recombination. Figure S18. (a) fs-transient absorption spectra of PPV film at different delay times, (b) kinetic traces at different probe wavelengths, and (c) decay-associated spectra resulting from the global fit with three exponential decay function. The grey area depicts the corresponding inverted ground state absorption spectrum of PPV, which is arbitrarily scaled to fit the transient absorption data.
Similar to the measurement in solution, the transient absorption spectra of Ru1 films exhibit a broad and rather unstructured ESA beyond 560 nm for Ru1 due to ligand-to-metal charge transfer transitions (-bpy → d) with a GSB centered at around 450 nm. The band at 385 nm for Ru1 is assigned to the * transition at the bpy •fragment. 11,12 Unlike transient absorption data in solution, the amplitude of the initial transient absorption signal decays by ca. 80% for both photosensitizers indicating that the 3 MLCT excited states typical for Ru tris-diimine complexes is short-lived in films. This short-lived 3 MLCT excited state can be due to oxygen quenching as the Ru1 films are exposed to ambient air. 13,14 A quantitative analysis by globally fitting with two-exponential decay functions and an infinite component results in characteristic time constants (1 = 15 ps, 2 = 300 ps for Ru1) and decay-associated spectra shown in Fig. S19c.  comparably low, i.e. 7.5% loading density. Figure S20 indicates that some differences in the transient absorption spectra of PPV-Ru1 are observed compared to the reference PPV data. The spectral shape at the red flank of the negative OD differs slightly due to spectral overlap of GSB contribution from PPV and contributions from photosensitizer-associated excited-state absorption. Also the zero-crossing (OD = 0) of PPV-Ru1 shifts to a shorter wavelength at early delay time (t = 1.5 ps) as compared to the reference PPV, i.e., from 708 to 670 nm for PPV-

Ru1.
Similar to the transient absorption data processing for PPV-Ru in solution, in order to analyse the light-induced charge transfer process across the PPV-Ru film, the data obtained for PPV alone is subtracted from the transient absorption signal of PPV-Ru. Prior to subtraction the data is normalized to its early time (t = 5 ps) signal amplitude at 560 nm for PPV-Ru1 at which there is no contribution of the transient absorption signal from Ru alone (see Figure S19a). Thus, similar to the analysis of TA data in solution, the subtraction allows us to unravel the excited state interactions between the photosensitizer and the PPV in the photoexcited PPV-Ru system. The subtracted, i.e. differential transient absorption spectra, i.e., OD = OD[PPV-Ru] -OD[PPV], is determined as follow.
The OD spectra are subjected to a global analysis using a multi-exponential fit. The decay-associated spectra (DAS) generated from a fit function consisting of a sum of three exponentials for PPV-Ru1 are shown in Figure   S21. Figure S21. Differential fs-transient absorption spectra of PPV-Ru1 film at different delay times (a,b). Kinetic traces at different probe wavelengths (c,d) and decay-associated spectra (DAS) (e,f) resulting from the global fit with two time constants for PPV-Ru1. The pump pulses are centred at 480 nm. Table S2. Characteristic time constants () from global fitting of differential fs-transient absorption data of PPV-Ru films from three different measurements.

Photoelectrochemical Measurements
It is already discussed in the main manuscript the linear scan voltammogram of the drop-casted films of PPV, PPV-Ru1 and PPV-Ru2, in which the photocurrent density is normalized to the maximum absorbance. Here is the original photocurrent density response obtained in the linear scan voltammogram.

Photostability Test of PPV-Ru System
To check the photostability of each component in PPV-Ru system during photoelectrochemical and transient absorption measurements, pre-and post-operando UV/vis absorption spectroscopy were carried out. The absorption spectra for PPV, PPVRu1, and PPVRu2 films were measured both before and after the chopped-light voltammetry (CLV, 10 min irradiation) and linear scan voltammetry (LSV, 1-2 min irradiation) experiments in the SEC cuvette without the electrolyte solution. To probe the photostability of Co III /Co II electrolyte, the absorption spectra of the electrolyte solution in the SEC cuvette without the electrodes were also measured before and after irradiation in both the CLV and LSV experiments. The results indicate that the electrolyte seemed to be stable in the relatively short time of the photoelectrochemical experiments as no changes were observed in the UV/vis spectra recorded pre-and post-operando (see Fig.  S24). To ensure that the data obtained from fs-transient absorption spectroscopy is reliable for analysis, here we show the UV/vis absorption spectra of Ru1, PPV, and PPV-Ru1. It should be noted that the sample solution as well as the film was moved during the scan allowing the pump-probe beam in the transient absorption measurement to hit the fresh sample area and hence, signals from the non-photodegraded sample volume were collected. As shown in Fig. S25, no change in absorption spectrum is observed for Ru1. However, a slight decay of absorption signal is observed for both PPV and PPV-Ru1 after fs-transient absorption measurement. The results already indicate the photo-instability of the PPV backbone. Figure S25. UV/vis spectra of (a) Ru1, (b) PPV and (c) PPVRu1 (7% loading) measured before and after fs-transient absorption spectroscopy measurement.