Nanostructuration Effect of Carbon‐Based Phenylene Vinylene Conjugated Porous Polymers on TiO2 Hybrid Materials for Artificial Photosynthesis

This work examines the role of polymer nanostructuration of phenylene vinylene (PV) conjugated porous polymers (PV‐CPPs) as highly active photocatalysts for both hydrogen production and CO2 photoreduction reactions. It is found that nanostructured PV‐CPP hybrids with TiO2 show a high increase in H2 production being the most active example, the n‐IEP‐20@T‐10 sample with an evolution rate of 3.24 mmol g−1 h−1 (ξ = 1.20%), that is, 8 times higher than that of its non‐nanostructured and 65‐fold higher than TiO2. In contrast, CO2 photoreduction in both nanostructured polymers shows a significant improvement in CH4 production compared with bare TiO2, and a clear change of selectivity toward C2+ products. In particular, C2+/C1 ratios are obtained with n‐IEP‐20‐based hybrids increased by one order of magnitude that obtained for TiO2. The beneficial effect of this synthetic strategy is associated with an increase of the dispersion on nanostructured CPPs over TiO2 leading to an improvement on the surface interaction between them that favors longer‐lived photogenerated carriers in spatially separated redox active sites, which favor the production and selectivity to highly electron demanding products. The use of these nanostructuration strategies opens new opportunities for the production of more processable polymers for different energy technologies.


Introduction
Global energy demand is on constant rise as a response to demographic and economic growth.Although this demand is fundamentally met by fossil fuels, the use of these reserves presents several disadvantages since they are limited, unevenly distributed across the planet, and the main cause of climate change.Solar fuels production may come as a promising solution as it provides a way of storing renewable solar energy, exploiting natural and available resources: H 2 O, CO 2 , N 2 , and sunlight. [1]In this regard, numerous inorganic semiconductors have been assayed as photocatalysts, being TiO 2 the benchmark as it is the most widely used material since the pioneering work of Honda and Fujishima in the early 70s. [2]Although many efforts have been made to optimize the performance of TiO 2 , two main drawbacks linger on: its high bandgap energy (≈3.2 eV) which limits TiO 2 absorption up to ultraviolet radiation, and its fast electron-hole recombination which limits the surface redox reactions. [3,4]oupling TiO 2 with a suitable bandgap semiconductor may overcome these limitations, by creating a new hybrid photocatalyst with better carrier mobility and a lower bandgap energy, which is able to absorb a bigger part of the solar spectrum.
The first time organic semiconductor was used as a photocatalyst for solar fuels production can be dated back to the pioneering work about poly(p-phenylene) by Yanagida et al. in 1985. [5]Due to the lack of photochemical stability of linear organic polymers, this line of work was not recovered until melon g-C 3 N 4 was reported for hydrogen evolution by means of photocatalytic water splitting. [6]From that point This work examines the role of polymer nanostructuration of phenylene vinylene (PV) conjugated porous polymers (PV-CPPs) as highly active photocatalysts for both hydrogen production and CO 2 photoreduction reactions.It is found that nanostructured PV-CPP hybrids with TiO 2 show a high increase in H 2 production being the most active example, the n-IEP-20@T-10 sample with an evolution rate of 3.24 mmol g À1 h À1 (ξ = 1.20%), that is, 8 times higher than that of its nonnanostructured and 65-fold higher than TiO 2 .In contrast, CO 2 photoreduction in both nanostructured polymers shows a significant improvement in CH 4 production compared with bare TiO 2 , and a clear change of selectivity toward C 2þ products.In particular, C 2þ /C 1 ratios are obtained with n-IEP-20-based hybrids increased by one order of magnitude that obtained for TiO 2 .The beneficial effect of this synthetic strategy is associated with an increase of the dispersion on nanostructured CPPs over TiO 2 leading to an improvement on the surface interaction between them that favors longer-lived photogenerated carriers in spatially separated redox active sites, which favor the production and selectivity to highly electron demanding products.The use of these nanostructuration strategies opens new opportunities for the production of more processable polymers for different energy technologies.
forward, there has been a meaningful interest in exploring g-C 3 N 4 as well as different conjugated polymer networks for artificial photosynthesis, particularly for photocatalytic H 2 evolution. [7,8][11][12] However, the use of organic polymers, including CPPs, for CO 2 photoreduction has been more elusive.In fact, there are few examples where CPPs appear as unique photocatalyst being more likely to prepare hybrid materials with inorganic semiconductors as counterpart. [4]longside, poly(p-phenylenevinylene) (PPV) has attracted a lot of attention due to its many important applications in organic light-emitting diodes, solar cells, and photocatalysis.In 1900, PPV was the first conjugated organic polymer employed to fabricate a light-emitting diode. [13]Later on, PPV derivative poly[2methoxy-5-(2-ethylhexyloxy)p-phenylene vinylene] (MEH-PPV) was used as a donor in the first donor-acceptor polymeric solar cell using functionalized C 60 fullerene derivatives as acceptors. [14]Furthermore, the first reported photo-induced electron transfer from a conjugated polymer to TiO 2 was studied by Hal et al. using poly(p-phenylenevinylene) and poly(thiophenes). [15]s mentioned before, a groundbreaking strategy to improve the performance of inorganic and organic semiconductors that have been used so far is the combination of both components to develop hybrid organic-inorganic materials. [16]Heterostructures of conjugated organic polymers and inorganic semiconductor TiO 2 usually enhance the intrinsic properties of the initial components due to a synergetic interaction. [17,18]Such cooperation between the original constituents may lead to an improvement of electrical, thermal, chemical, and/or mechanical properties. [19]n this line, nanostructured TiO 2 and other inorganic semiconductors have proved to be an efficient way to improve such a synergistic effect.Although nanostructured carbon-based hybrid materials, such as graphene and its derivatives [20] and carbon nanotubes [21] have been reported as photocatalytic systems in hydrogen production, nanostructuration of CPPs has received less attention.An interesting example of nanostructured CPPs by miniemulsion polymerization was described by Cooper's group. [22]These materials show hydrogen evolution rates (HERs) that are two and three times higher than the corresponding bulk polymers.However, as far as we know, there are no examples in the literature reporting hybrid photocatalysts composed of nanostructured CPP and inorganic semiconductors.
Here, we describe a series of PV-CPPs and hybrid thereof and the beneficiary effects of a nanostructuration strategy in the photocatalytic activity for solar fuels production.The higher activity of nanostructured composites is a result of a better interplay between the components due to an increase in the external surface area of the polymeric materials, which are in intimate contact with the hybrid heterostructure.
Moreover, nanostructuration of polymers IEP-20 and IEP-21 (IEP stands for IMDEA Energy Polymer) exhibited in both cases an enhanced dispersibility in aqueous media, resulting in a more homogeneous final hybrid material with a greater number of heterojunctions at the CPPs/TiO 2 interface and therefore a higher photocatalytic activity.A direct Z-scheme charge transfer mechanism has been determined by means of time-resolved fluorescence and transient absorption spectroscopy affording a clear view of the photocatalytic reduction pathways.
Stilbene-based CPPs, IEP-20 and IEP-21, were prepared by Sonogashira cross-coupling reaction (Figure 1a,b) following two different synthetic methods so as to yield bulk (Scheme S3 and S4, Supporting Information) and nanostructured CPPs (Scheme S5 and S6, Supporting Information).This strategy is represented in Figure 1 up-left (conventional bulk polymerization, b-IEP-x) and Figure 1 up-right (miniemulsion followed by solvothermal treatment polymerization conducted according to previously described procedures, [23,24] which afforded nanostructured n-IEP-x).In both methods, the synthetic conditions were optimized to achieve the desired products in high enough yields and optimal porosities.For characterization purposes, aliquots of n-IEP-x emulsions were filtered, washed thoroughly, and collected as a solid.Unless otherwise indicated, n-IEP-x refers to nanostructured polymer in dispersion.
The chemical composition and structure of IEP-20 and IEP-21 were determined by Fourier transform infrared (FTIR), 13 C-nuclear magnetic resonance (NMR), inductively coupled plasma optical emission spectroscopy (ICP-OES), and elemental analysis.The synthetic methodology did not affect the chemical structure.Thus, FTIR spectra show a characteristic ─C≡C─ weak stretching band close to 2200 cm À1 for both IEP-20 and IEP-21, as well as a shift and an increase in the C═C stretching band ≈1700 cm À1 indicative of a successful polymerization between PV-based monomers and linker 1,4-diethynylbenzene (Figure S5, Supporting Information).Additionally, there is a decrease in the intensity of the C-Br vibrational band at 1010 cm À1 as expected from polymerization.This signal corresponds to a bromine support on benzene in para substitution.Furthermore, solid-state 13 C-NMR spectra also confirmed the formation of sp-hybridized carbon atoms showing in all four polymers a couple of well-defined peaks at 90 ppm and 74-82 ppm corresponding to ─C≡C─.The rest of aromatic carbon atoms were assembled at the interval between 115 and 145 ppm, where the range between 115 and 140 ppm corresponded to C═C double bonds and the range between 125 and 150 ppm to aromatic rings (Figure S6 and S7, Supporting Information).
Elemental analysis was not in agreement with theoretical values.However, deviations between theoretical and estimated values are expected, since the porous network of the described polymers leads to bad combustion. [25,26]Ambient nitrogen was found as a trapped adsorbate in the samples (≈<1.0%)(Table S1, Supporting Information).
For each polymer, the work-up of the products ended with an aqueous sodium cyanide treatment to remove possible residual palladium nanoparticles, leftovers from the Pd(PPh 3 ) 4 catalyst that could interfere with the results of photocatalytic activity of the polymers. [27]Attending to ICP-OES analysis results, this method is successful in both cases but was more effective for bulk (≈<0.06 wt% of Pd) than for miniemulsion process (≈<0.3 wt% of Pd) (Table S2, Supporting Information).
The main difference between the materials obtained by both synthetic methods is the control of the particle size.While bulk materials showed irregular particles of sizes higher than 10 μm micrometers, nanostructured polymers exhibited future size smaller than ≈300 nm as evidenced by field-emission scanning electron microscopy (FESEM) images depicted in Figures 1c and  S8-S11, Supporting Information.
As expected, n-IEP-20 and n-IEP-21 present great dispersion and stability in an aqueous solution (see pictures in Figure 1d).Therefore, particle size distributions of different concentrated water emulsions were studied by means of dynamic light scattering.Average hydrodynamic particle size of both n-IEP-20 and n-IEP-21 was found to be 100 nm.Upon concentration of the sample, aggregation of polymer particles occurred giving rise to bigger size particles although fewer in number and in a reversible way (Figure S12 and S13, Supporting Information).This high quality of the colloidal dispersion allows the measure of their molar absorptivity (ε) (≈0.031Lg À1 cm À1 ) (Figure S14, Supporting Information), which was slightly higher for n-IEP-21.
The structural and textural properties were studied by means of PXRD and N 2 adsorption isotherms, respectively.As expected, powder X-difracction (PXRD) pattern defined IEP-20 and IEP-21 polymers as amorphous materials (Figure S15, Supporting Information).The porous nature of PV-CPPs, obtained from the two synthetic approaches, was studied by N 2 adsorption measurements at 77 K (Figure S16, Supporting Information).Surface area was calculated by BrunauerÀEmmettÀTeller (BET) theory along with other textural parameters derived from isothermal curves which are quantified in Table S4, Supporting Information.In the case of IEP-20, the bulk polymer does not show a porous structure, while miniemulsion polymerization shows a surface area of 326.76 m 2 g À1 .In contrast, n-IEP-21 showed higher BET surface area than b-IEP-21 (461.24m 2 g À1 vs 333.30m 2 g À1 , respectively).However, the real impact of the nanostructuration is related to the increase of the external surface.So, n-IEP-20 showed a 75% of external surface area over the total S BET surface area, while in the case of n-IEP-21, this percentage rose from 50% to 70%.
The thermal stabilities of PPV-based IEPs were analyzed by thermogravimetric analysis under both argon atmospheres (Figure S17, Supporting Information) and air (Figure S18, Supporting Information).The thermograms under inert atmosphere showed high thermostability and incomplete degradation of bulk and nano-IEP-20 and IEP-21, as it was described before for previous conjugated porous IEPs polymers synthesized in our research group. [17,28]At the same time, thermodegradation under air atmosphere of all PV-based IEPs produced only 6-14% of solid residue with decomposition occurring in at least two differentiated steps.The first and second degradation step for IEP-20 occurred at similar temperature for bulk and nanostructured polymers (≈338, 365 °C and 480, 477 °C, respectively).The same occurred for IEP-21 bulk and nanopolymers (≈331, 289 °C and 442, 471 °C, respectively).
Handling CPPs nanostructures dispersions promotes good quality thin film preparation for (photo)electrochemical characterization.In this sense, electrodes have been prepared over indium tin oxide (ITO) support by drop-casting technique which enables thin films with quality and stability suitable for use in an electrochemical cell (Figure 2a and S19, Supporting Information).In such manner, the conduction band (CB) and valence band (VB) levels of conjugated porous polymers n-IEP-20 and n-IEP-21 (Figure 2b) were determined by means of cyclic voltammetry measurements in a three-electrode cell configuration and respectively estimated to be at À6.07 and À3.46 eV for n-IEP-20 and À6.04 and À3.37 eV for IEP-21 (Figure S20, Supporting Information).Accordingly, the electronic bandgap of n-IEP-20 and n-IEP-21 can be approximated to 2.61 AE 0.02 and 2.67 AE 0.02 eV, respectively.Additionally, their optical bandgaps energy was established by Tauc plots obtained by the use of Kubelka-Munk function from diffuse reflectance data, showing in both cases an allowed direct transition.(Figure S21-S24, Supporting Information).The optical bandgap energy of IEP-20 and IEP-21 was determined at 2.3 and 2.4 eV, smaller than the electronic one as expected. [29]Time-dependent density functional theory (TD-DFT) calculations were used to determine the electronic vertical transitions (Table S5 and S6 The positive photopotential values of the measured OCP (Figure 2c,d) revealed that both n-IEP-20 and n-IEP-21 are p-type semiconductors, which agreed with the determined electronic structure.At the same time, n-IEP-20 and n-IEP-21 displayed both negative and positive photocurrents at different bias potentials under chopped illumination (Figure 2e,f ).This, together with the estimated energy of the Fermi levels, was not heavily n type doped, showing an interesting versatility to participate in both reduction and oxidation reactions.
As it can be observed in Figure 2b, E red (H 2 O/H 2 ) and E ox (O 2 /H 2 O) were located between the energy range of E CB and E VB of both n-IEP-20 and n-IEP-21, allowing both water-splitting reactions to take place from a thermodynamic point of view.However, few materials can catalyze both half reactions due to the well-known overpotentials and the problematic kinetics demands of oxygen evolution half reaction. [16]For this reason, methanol was used instead of water as proton donor and sacrificial agent (see vide infra).

Preparation of Hybrid Materials
The main objective of the present work was to study the impact of the CPP nanostructuration in the photocatalytic activity of hybrid based on them and inorganic semiconductors.The inorganic semiconductor of our choice was TiO 2 because is the most studied and used as benchmark, [3,30] being its hybrids the most abundant. [4]All hybrids were made up with calcined TiO 2 (Anatase) to remove any adsorbed contaminant. [31]Hybrid materials were prepared with 10 wt% of polymer loading following our previous experience in this case of hybrid, and named as IEP-x@T-10 (where x = 20 or 21).For the hydrogen production experiments, these hybrid materials were assembled in two different ways: one, grinding both components in solid state and; two, only in the case of nanostructured CPPs, adding the dispersion of nano-CPP to a likewise TiO 2 dispersion (see experimental procedure at Figure S28, Supporting Information).To differentiate if hybrids based on nanostructured polymer come from both different method the words "dispersion" and "solid" will accompany each tag.For the CO 2 reduction experiment, the powder composed of the hybrid from bulk polymer was dispersed in water and then deposited on a glass microfiber filter.The nanohybrid was prepared by mixing both dispersed components and then deposited on glass microfiber filter (see experimental details in Supporting Information).
As the TiO 2 nanocrystal size is lower than the polymer size in all their forms, the TiO 2 nanocrystals were always found to be capping the polymer.Figure 3a depicts an aggregate of a nanostructured hybrid where this morphology can be observed.Note that the aggregation is confirmed by means of backscattered electrons detection (Figure 3b).The energy-dispersive X-ray spectroscopy (EDX) mapping (Figure 3c) shows the coincidence of Ti/O and C but also some places where the polymer is not covered at all.

Photocatalytic H 2 Production
Initially, PV bare polymers were tested as photocatalysts (entries 2-5, Table 1), showing in all cases a limited hydrogen production but with higher HER and higher photonic efficiency ξ (%) [32] than TiO 2 (entry 1, Table 1).Figure 4a,b depicts the cumulative hydrogen production versus time of irradiation of polymeric photocatalysts together with TiO 2 .As it can be observed, n-IEP-21 exhibited the best performance of all polymeric materials (≈0.09 mmol g À1 h À1 ) with a HER 1.8-fold greater than TiO 2 .
Regarding to hybrid photocatalysts, a remarkable synergetic effect between inorganic and organic semiconductors was observed for all studied hybrids (Figure 4c,d).The hydrogen production rate of bare TiO 2 was boosted as far as 64-fold in the case of n-IEP-20@T-10 (dispersion) (≈3.34 mmolh À1 g À1 , ƺ = 1.20%).This is a remarkable result considering the absence of any additional noble metal cocatalyst, being as far as we know one of the highest performances reported for a nonmetal hybrid (see Table S7, Supporting Information, with bibliography data).
Furthermore, the recyclability of IEP-x@T-10 was analyzed by carrying out three photocatalytic cycles with the best catalyst n-IEP-21@T-10 (dispersion) (Figure S30, Supporting Information).This was accomplished by performing one catalytic cycle, filtering the catalyst, and redispersing it in a fresh methanol/water solution.No activity decrease was observed after the first or the second cycle when calculating the cumulative hydrogen production or the HER.On the contrary, second and third cycles exhibit higher activity toward hydrogen evolution.This could be due to the presence of cetyltrimethylammonium bromide (CTAB), used as surfactant, traces in the first cycle.The FTIR before reaction showed a small peak (≈1487 cm À1 ) associated with CTAB, which disappeared after recycling (Figure S31 and S32, Supporting Information).In spite that CTAB surfactant presence was low, we could think of CTAB acts as a sacrificial agent responsible for the higher activity nanodispersed samples.Thus, n-EP-20@T-10 (dispersion) photocatalytic control experiment that did not contain methanol was performed (see Figure S33, Supporting Information).Once the possible activity from CTAB was discarded, the increase in hydrogen production after recycling could be explained by considering that the hydrogen evolution is expressed in terms of catalyst mass and the loss of a small amount of CTAB increased the activity per gram.Also, the structural stability and catalytic behavior of IEP-X@T-10 were confirmed by FTIR spectra of before and after 6 h of irradiation (Figure S31 and S32, Supporting Information).
In spite of the low amount of Pd in the polymers (vide infra), the role of any possible residual Pd nanoparticles in photocatalytic activity toward hydrogen production was ruled out by performing a control experiment where n-IEP-20@T-10 (dispersion) and n-IEP-21@T-10 (dispersion) before Pd cleaning treatment were used as hybrid photocatalysts to show that the presence of Pd nanoparticles inside the polymer did not boost the photochemical reaction.Nondialyzed n-IEP-20@T-10 afforded lower hydrogen production than its equivalent, while nondialyzed n-IEP-21@T-10 showed little difference in analogous IEP-21 hybrid material (Figure S34 and S35, Supporting Information).These results were in harmony with the ones reported by Luping Yu's group, since both our Pd wt% residual content and our initial feeding ratio ([Pd]/[M]) of Pd(PPh 3 ) 4 catalyst (Pd) and monomer (M) were lower than the minimum threshold from which they observed significant effect on the rate of hydrogen production. [32]

Photocatalytic CO 2 Reduction
Nanostructuration methodology of CPPs was also examined in CO 2 photoreduction.For this purpose, the most active hybrids b/n-IEP-20@TiO 2 were evaluated in a gas-phase photoreactor under UV illumination and using water as electron donor.
As reported before for other CPP@TiO 2 hybrids by our group, [33] CH 4 , CO, and H 2 were found to be the major products (Figure 5).Besides, in this work methanol, ethylene, and ethane  were also detected in minor amounts (Figure 5A).Both b-IEP-20@T-10 and n-IEP-20@T-10 showed a significant improvement in CO 2 photoreduction performance compared to bare TiO 2 .In particular, CH 4 evolution increased by more than 4 times for nanostructured n-IEP-20@T-10 hybrid reaching 6.8 μmol g cat À1 h À1 (Figure 5).At the same time, hybrid materials b/n-IEP-20@T-10 showed a decrease in CO production in favor of more reduced carbon species, such as methanol and C 2þ products.We noted that these compounds were found in negligible amounts for bare TiO 2 , highlighting the superior performance of the hybrids for CO 2 photoreduction.Interestingly, both hybrid materials boosted C 2þ /C 1 ratios increasing by one order of magnitude that was obtained from TiO 2 (≈0.12 for n-IEP-20@T-10 vs 0.01 for bare TiO 2 ).In terms  of light utilization, nanostructured n-IEP-20@T-10 hybrid almost doubled the photonic efficiency toward CH 4 (0.07%) with respect to TiO 2 (0.04%).Therefore, it can be inferred that nanostructuration strongly affects the activity and selectivity toward more complex multicarbon products.These findings clearly reinforce the idea that nanostructuration produces a better synergy between the CPP and TiO 2 .Indeed, the higher external surface area of nanostructured polymers increased the number of active sites in the heterojunction, which might benefit the reduction pathways to multicarbon products.

Charge Transfer Dynamics Characterization
Understanding the synergic effect in hybrids composed of TiO 2 and conjugated porous polymer promoting the dynamic electron transfer process is key to gain insight into photocatalytic reaction rates.For this purpose, spectroscopic studies from the ns to μs timescale, using steady-state photoluminescence (PL), timeresolved PL, and transient absorption spectroscopy (TAS) were performed.
First, the emission of bare TiO 2 , n-IEP-20, n-IEP-21, and the corresponding n-IEP-20@T-10 and n-IEP-21@T-10 hybrids was measured in solid state (Figure 6a).Under steady-state conditions (λ exc = 290 nm), pristine n-IEP-20 or n-IEP-21 polymers showed a main emission band with a maximum at 570 nm; whereas TiO 2 exhibited a maximum at ≈450 nm which was quenched in the presence of n-IEP-20 or n-IEP-21 polymers in the hybrids (Figure 6a).This quenching could be induced by a charge transfer from titania to the CPP in the hybrids.Since both polymers and titania absorb UV light, an inner filter effect contribution would appear in the fluorescence of hybrids, so, it is not recommended to follow only the hybrid's steady-state fluorescence.To avoid this effect, it is preferable to follow the evolution of fluorescence lifetimes (τ F ). Thus, τ F of both n-IEP-20@T-10 and n-IEP-21@T-10 hybrids (10% of polymer loading) were measured at λ exc = 372 nm with a band pass filter centered at 450 nm where only TiO 2 emits, showing a slight quenching in τ F for hybrids regarding τ F bare TiO 2 (Figure 6b, left side and Figure S36, Supporting Information).Interestingly, when τ F of both n-IEP-20@T-10 and n-IEP-21@T-10 hybrids were measured at the same excitation wavelength but using a band pass filter centered at 550 nm (where mainly n-IEP-20 and n-IEP-21 show emission), an increase in τ F for hybrids samples than for naked CPP was observed (Figure 6b, right side, Figure S37 and Table S9, Supporting Information).Attending to literature, the small quenching of the τ F (TiO 2 ), accompanied by the corresponding increase in τ F (CPPs) by the presence of the organic counterpart was assigned to an interfacial charge transfer from the CB of TiO 2 to the VB of polymer accordingly to a direct Z-scheme charge transfer mechanism (Figure 6f ).Thus, it was observed at composites based on SrO and graphene carbon nitride (g-C 3 N 4 ), [34] phenanthraquinone-based CPPs and TiO 2 (CCP-FPA@T), [35] or truxene-based CPPs and TiO 2 (TxPP1@T). [18]owever, additional experiments in a timescale greater than a few nanoseconds are still needed.So, a complete study by TAS was carried out.First, the transient of CPP polymers, n-IEP-20 and n-IEP-21 as well as TiO 2 were analyzed (see details in Figure S38 and S39, Supporting Information).As generally happens with CPPs, bare n-IEP-20 and n-IEP-21 polymers showed poor transient absorption (Figure S37a, Supporting Information), accompanied by the complete disappearance of the kinetic curve after 100 ns after laser pulse (Figure S37b, Supporting Information).As observed in Figure S37b, Supporting Information, n-IEP-20 and n-IEP-21 showed lower transient lifetime than bare titania.
Next, we investigated the transient properties of both hybrids.To compare TAS experiments with the photocatalytic performance, all measurements were performed in the presence of MeOH 10% v/v aqueous solution as media.Photolysis at 355 nm laser of deaerated n-IEP-20@T-10 or n-IEP-21@T-10 resulted in a continuous absorption covering the complete spectral window (Figure 6c), exhibiting higher charge separation efficiency than TiO 2 , when considering the ratio between the respective ΔODs.Thus, while the TA spectrum of titania practically disappeared 500 ns after the pulse, the hybrids showed a huge absorption even upon 6 μs after pulse, it being more significant for n-IEP-20@T-10.Then, the temporal profile (λ obs = 460 nm, where TiO 2 showed its maximum transient absorption) for both polymers and TiO 2 was analyzed (Figure 6d and S40, Supporting Information).First, titania fits well to two consecutive first-order kinetics (τ = ≈55 and ≈790 ns, which corresponded to the recombination of the 95% and 5% of photogenerated electrons according to the pre-exponential values Table S10, Supporting Information).For the nanostructured hybrids, an initial fast decrease without differences (up to 100 ns) compared to bare TiO 2 was observed.Then, a huge growth in the signal of several μs was produced exhibiting higher charge separation efficiency than TiO 2 considering the respective ΔODs, mainly, from 200 to 300 ns after pulse (Figure 6d and S40, Supporting Information).The kinetic curve for hybrids experienced lived photoelectrons up to 15 μs before decreasing until complete disappearance of the signal while titania lived at around 1 μs, illustrating the high H 2 production performance provided by the hybrids.This behavior can be attributed to three major populations for the charge-separated transient state: 1) charge carriers of TiO 2 closely localized where the photochemical event occurred after bandgap excitation (shortest lifetimes up to first 100 ns scale) where probably some trapped e À recombine without reaction due to the fast e À -h þ recombination of titania, 2) migration and delocalization of the remaining of photogenerated electrons (middle lifetime fraction up to 1 μs scale), and 3) longer lifetime experienced reaching a plateau as a result of an efficient electron-hole pair transfer from TiO 2 to n-IEP-20 or n-IEP-21.Indeed, the presence of μs long-lived excited states upon direct Z-scheme electron transfer process in BiVO 4 semiconductors has been reported in the literature [36,37] and also it has been observed by us in direct Z-scheme charge transfer from TiO 2 to phenanthraquinone-based CPP [35] and truxene-based CPP. [18]According to fitted kinetic curves (Table S10, Supporting Information), the fast first component (τ = ≈55 ns for both hybrids) contributed ≈30%, followed by a growth in the absorption and lifetime from 300 to 1000 ns after pulse with τ = ≈300 and 200 ns for n-IEP-20@T-10 and n-IEP-21@T-10, respectively (≈16%).The third component contributed ≈50-55%, and corresponded with a slower decay of 5 and 4 μs up to 30 μs kinetic for n-IEP-20@T-10 and n-IEP-21@T-10, respectively.This marked enhancement of the signal for the hybrids clearly suggested not only the direct involvement of the polymer as an electron acceptor considering a slight transient absorption of the polymer partner in the UV region (Figure S21, Supporting Information) but also a reduced electron-hole recombination rate for TiO 2 that could improve the photocatalytic properties of the hybrids since the charge separation is remarkably higher.Note that when a type II heterojunction charge transfer mechanism takes place completely different transient absorption behavior. [17,33]n contrast, a clear influence of the enhancement of transient electronic states in the behavior of the reaction pathways by the presence of n-IEP-20/21 in the hybrids is illustrated in Figure 6e.
The kinetic profiles of hybrids exhibit several contributions (initial fast decrease, followed by a charge transfer as result of a growth in the signal and ending with a slower recombination), and therefore, obtaining a satisfactory average lifetime is difficult.For this reason, we calculated and compared the area under the kinetic curve with the average lifetime obtained from the equation described in Table S10, Supporting Information.When the transient analysis (area and average τ) was plotted against cumulative hydrogen production, a direct relationship was observed.The hybrids n-IEP-20@T-10 and n-IEP-21@T-10 exhibited much more photocatalytic yield compared to bare TiO 2 toward H 2 generation.A further piece of evidence of this assertion was obtained upon platinum photodeposition on the hybrid surface.As the platinum photodeposition obeys a photoreduction pathway, the resulting Pt nanoparticles must be deposited on the surface of the material responsible for the reduction process.Using FESEM experiments, we demonstrated the presence of tiny Pt nanoparticles (c.a. 2 nm) preferably on the polymer surface (Figure 7).A region where both CPP and TiO 2 coexist is detected (Figure 7b) by means of EDX mapping (Figure 7c).Analyzing deep high-resolution FESEM images, we can conclude that the region where carbon atoms are present matches with the platinum particles (Figure 7d).Thus, the polymer surface is confirmed as the place for reduction pathway.

Conclusion
The synthesis of nanostructured conjugated porous polymers has been demonstrated a powerful tool to design photoactive hybrid materials for artificial photosynthesis processes.In this work, two PV-CPPs, IEP-20 and IEP-21, have been synthesized by both bulk and miniemulsion techniques and thoroughly characterized.Thus, the photoelectronic characterization of these materials has been possible due to the successful nanostructuration.Furthermore, hybrid materials composed of CPPs and TiO 2 as the inorganic counterpart have been prepared and tested in both hydrogen production and CO 2 photoreduction.In both cases, the hybrid from nanostructured polymers boosts the solar fuels production and, in the case of CO 2 photoreduction, the selectivity changes to more demanding electron products such as methane.Besides the nanostructuration, these PV-CPPs benefit from an efficient direct Z-scheme charge transfer which has been elucidated from steady and time-resolved fluorescence and transient absorption spectroscopy.This has been confirmed by the preferential photodeposition of platinum on the polymer surface which has been analyzed by means of high-resolution images taken by FESEM. the FESEM experiments.

Figure 1 .
Figure 1.General scheme for the synthesis of IEP-20 and IEP-21 polymers.Upside: cartoons explaining differences between synthesis of bulk-CPPs and synthesis of NanoCPPs.Downside: a,b) scheme of synthesis of b/n-IEP-20 and b/n-IEP-21, respectively.c) FESEM images comparing feature size of both IEP-20 and IEP-21 using both synthesis methods.d) Appearance of b-IEP-20 and b-IEP21 aqueous dispersions compared with n-IEP-20 and n-IEP21 colloidal dispersion, several hours after preparation.
, Supporting Information) and the molecular orbitals implied on them (Figure S26 and S27, Supporting Information).The flat band potential of polymers n-IEP-20 and n-IEP-21, which was used to set the Fermi level energy (Figure 2b), was determined from light-saturated open-circuit potential (OCP) measurements at different light irradiances, giving values of À5.25 and À5.07 eV, respectively (Figure S25, Supporting Information).

Figure 2 .
Figure 2. (Photo)electrochemical measurements of polymers n-IEP-20 and n-IEP-21.a) Example of polymer n-IEP-20 thin film by drop-casting onto ITO-coated glass working electrode (WE) and its FESEM cross-section image.b) Electronic band structure of both n-IEP-20 and n-IEP-21.c,d) Photovoltage measures in OCP condition.e,f ) Linear sweep voltammetries under one sun chopped illumination showing the photocurrent density at different bias potentials.

Table 1 .
HER, accumulated hydrogen production, and photonic efficiency (ƺ) under UV-vis light of the different photocatalytic materials based on IEP-20 and IEP-21 polymers and hybrids thereof with TiO 2 .Entry Photocatalyst HER [mmol h À1 g À1 ]

Figure 4 .
Figure 4. a,b) Accumulated hydrogen production per gram of bulk and nanostructured polymers IEP-20 and IEP-21 versus time of reaction.c,d) Accumulated hydrogen production per gram of hybrid materials based on TiO 2 with a 10 wt% of CPP versus time of reaction.