Thienoacene‐Based Conjugated Porous Polymer/TiO2 Hybrids as Photocatalysts in Artificial Photosynthesis

Herein, the design and synthesis of a couple of CPPs based on thienoacene units (named as IEP‐14 and IEP‐15, stand for IMDEA Energy Polymer numbers 14 and 15) are described, which show high BET surface areas, good photo(thermal) stabilities, and appropriate electronic alignment with TiO2 to prepare hybrids (named as IEP‐x@T‐10, X = 14 and 15, being 10 wt% of polymer loading). It is shown that the simultaneous UV–vis irradiation of both materials leads to better H2 production (ca. 925 and 827 µmol g−1 h−1 by IEP‐15@T‐10 and IEP‐14@T‐10, 12 and 11‐fold higher production than bare TiO2) than the solely irradiation at visible of the CPPs (ca. 124 and 90 µmol g−1 h−1 by IEP‐15@T‐10 and IEP‐14@T‐10 when TiO2 is photocatalytically inactive). The reason is attributed to the charge‐transfer mechanisms that occur between the counterparts of the hybrid material: in the first case it consists in a Z‐scheme charge transfer mechanism, while in the second one is a sensitization charge transfer mechanism. Both mechanisms are elucidated by advanced techniques. Furthermore, in a gas phase CO2 photoreduction test, IEP‐15@T‐10 shows sixfold higher CH4 evolution than TiO2, which result in a selectivity shift from CO to CH4 (i.e., >26% greater selectivity than bare TiO2).


Introduction
Mimicking nature is often a useful starting point for chemical processes design.This is the case of Artificial Photosynthesis (AP), which comprises hydrogen production, CO 2 photoreduction, and nitrogen fixation processes. [1]Thus, AP emulates the natural photosynthesis performed by plants, by using water, CO 2 , O 2, and/or N 2 as raw materials to promote the synthesis of solar fuels and valuable chemicals instead of nutrients.To do so, a DOI: 10.1002/adsu.202300330photocatalyst must provide: 1) efficient light absorption; 2) long-lived chargeseparated states; and 3) minimal backreaction rates and product crossover. [2]In this sense, a solely photocatalyst is not able to meet all the requirements, and, for this reason, it has been necessary the use of hybrid materials.Hybrids composed by the heterojunction between an organic semiconductor (OS) and an inorganic semiconductor (IS) are named organicinorganic hybrids (OIHs). [3]These OIH are in a new aged because we are attempting to the renascence of OS, with the apparition of a novel generation consisting on hyper-crosslinked networks such as conjugated porous polymers (CPPs) and its crystalline analogues covalent organic frameworks (COFs). [4]hese polymeric networks show the advantage of having higher chemical, thermochemical, and photochemical stabilities than their linear analogues polymers, properties that are required to develop robust photocatalysts.Furthermore, these materials show micro and meso porosity, something that could be an advantage in gas-phase reactions.
In this sense, our research group has developed during the last years a research line focused in the design of new hybrid materials based on CPPs and TiO 2 .TiO 2 is the most popular photocatalyst whose photocatalytic behavior is widely known.It allows us to study the hybrid materials and determine the specific contributions of the CPPs in the photocatalytic performance and ignore unrelated side effects. [5]hus, we have described the artificial photosynthetic activity of hybrid materials comprising BODIPY and BOPHYbased CPPs, [6,7] truxene-based CPPs [8] as well as phenanthrenequinone-based CPPs. [9]n the look for moieties to prepare new CPPs, we found that thiophene and thioacenes are among the most important ones.Thiophene is an electron-donating compound because it contains sulfur atoms with two pairs of free electrons.These electrons can be transferred and delocalized around the polymeric network.It confers thiophene-based CPPs improved conductivity of electrons and holes.For this reason, CPPs based on this type of building blocks have been widely employed in fields such as batteries, [10,11] field-effect transistors, [12] solar cells, [13] or photocatalysis, [14,15] among others.Thus, several authors have employed building blocks based on thiophene moieties in the synthesis of a huge number of CPPs.For instance, Jiang's group carried out the synthesis of two CPPs by the self-polymerization of thiophene and molecules of bithiophene through FeCl 3 -catalyzed oxidative coupling. [10]Xu's group synthesized also CPPs with thiophene functionalized with carboxylic acid groups for iodine capture [16] and carbon dioxide adsorption. [17]Yanze et al. obtained other CPP through the Sonogashira coupling reaction. [18]ere, we describe two organic-inorganic heterostructures composed of two new thienoacene-based CPPs as organic counterpart and TiO 2 as inorganic semiconductor.Both hydrogen production and CO 2 photoreduction as artificial photosynthesis processes are presented with good results.In order to clarify the charge transfer mechanism responsible of their success, an intensive photophysical study by means of steady state and time-resolved fluorescence as well as transient absorption spectroscopy was carried out.We found differences in the charge transfer mechanisms depending if both organic and inorganic semiconductors are excited with UV-vis light irradiation, or if only the CPP is excited under visible light.Thus, different charge transfer processes led into distinct efficiencies towards solar fuels production.

Synthesis and Characterization of CPPs
Two novel thienoacene-based CPPs, IEP-14 and IEP-15, were synthetized through the Sonogashira cross-coupling reaction between 1,2,4,5-Tetra(2′-bromo-5′-thienyl)benzene (TA-2) and the conjugated aromatic linkers 1 and 2, respectively.The difference between both monomers is the number of connection points, two in the case of linker 1 (A2) and three in the case of linker 2 (A3).Considering that the main monomer TA-2 has four (B4), combining this one with linkers 1 and 2 leads to more or less branched polymers (A4B2 and A4B3 respectively).The polymerization reactions were driven under inert atmosphere, in presence of a palladium catalyst, Pd(PPh 3 ) 4 , and Et 3 N as organic base, as it was reported in a previous publication of our group (Scheme 1). [7]hese CPPs were fully characterized through numerous characterization techniques.The porous nature of the materials led to inaccurate elemental analysis (Table S1 at Supporting Information).The chemical structure was confirmed by means of solidstate 13 C-NMR and FTIR.Thus, Figure 1a depicts solid state 13 C-NMR of both CPPs.Signals located at ca. 123-145 ppm are assigned to the aromatic carbons in benzene and thiophene moieties.Also, a couple of signals that appears at ca. 80 and 90 ppm, they are related to the carbon-carbon triple bonds.Furthermore, FTIR spectra (Figure 1b) show bands of low intensity at 2900 cm −1 , which correspond to the stretching of the bonds ═C─H which are present in the aromatic moieties.Bands at ca. 2200 cm −1 are due to the stretching of the C≡C bonds formed during the polymerization through Sonogashira reaction.
The bands located ca. 1600-1700 cm −1 correspond to the stretching of the C═C bonds of the aromatic rings.Furthermore, other band appears at 1500 cm −1 in IEP-15 spectrum due to the stretching of the C-N bond present in the linker selected for the polymerization.Thermogravimetric analysis (TGA) was employed to measure the thermal stabilities of CPPs.In both polymeric networks the single step thermal decomposition under air conditions start at ca. 350 °C and it is almost complete, producing only ca.0.2% of solid residue (Figure S6, Supporting Information).However, same experiments carried out under argon atmosphere showed that the degradation of the polymers led to carbonaceous materials, retaining the 70% of the whole mass (exemplified with IEP-14 at Figure S7, Supporting Information).
As the polymerization process by Sonogashira cross-coupling reaction involves the use of palladium species, the work-up used after the synthesis of the polymers implies a washing step with sodium cyanine solution in order to remove the palladium nanoparticles. [19]To demonstrate the effectiveness of this washing step, the palladium content of each sample was measured by inductively coupled plasma mass spectrometry (ICP-MS) affording 0.043 wt% and 0.019 wt% for IEP-14 and IEP-15, respectively.
On the other hand, textural and structural properties of both CPPs were evaluated.Figure S8 (Supporting Information) depicts N 2 isotherm plots of both CPPs.Their surface area was analyzed by Brunauer-Emmett-Teller (BET) theory.As expected, considering their molecular structure, IEP-14 showed almost the double surface area (ca.1340 m 2 g −1 ) than IEP-15 (ca.747 m 2 g −1 ).Cumulative pore volume (ca.1.2 cc g −1 ) was studied by employing a DFT calculus model.Both CPPs are amorphous materials without any order at large scale, as can be checked by means of powder X-ray diffraction (PXRD) in Figure S9 (Supporting Information).Moreover, the morphology of the materials was studied with scanning electron microscopy (SEM).They consist in small smooth aggregated polymer particles (Figures S10 and S11, Supporting Information).
Furthermore, optoelectronic characterization of the CPPs was done in order to determine the energy diagram of both organic semiconductors (Figure 1d).For this purpose, reflectance diffuse spectroscopy measurements were driven to determine the optical bandgap energy (Figure S12, Supporting Information).The linear dependence of the square of the absorption coefficient on photon energy also reveals the existence of a direct allowed transition in the polymer (Tauc plots for direct are depicted at Figure S13, Supporting Information).On the other hand, the HOMO and LUMO positions were determined by means of cyclic voltammetry experiments (Figure S14, Supporting Information).TD-DFT calculations show similar energy differences between HOMO and LUMO orbitals for all polymers.Figure 1d and Tables S3 and S4 (Supporting Information) show the most important vertical transitions, while Figures S15 and S16 (Supporting Information) show the molecular orbitals implied in this optical measurement.Table S2 (Supporting Information) depicts all these values for comparative purposes.Note that obtained elec-trochemical bandgap usually led to higher values due to the exciton binding energy. [20]hus, both semi-reactions of oxidation (blue dash line) and reduction of water (red dash line) are located between the conduction and valence bands of both CPPs and titanium dioxide.So, from a thermodynamic point of view, these photocatalysts are suitable materials to catalyze the water photoreduction and also the water photooxidation reactions.Nevertheless, the reaction did not take place due to unfavorable kinetics of water oxidation, which avoids the oxygen evolution.For this reason, methanol is employed as a sacrificial agent in all the photocatalytic experiments (i.e., MeOH/CO 2 , green dash line).
On the other hand, our area of interest is not the polymers themselves, but their hybrid materials with titanium dioxide.For this reason, we have prepared a couple of hybrid materials using TiO 2 (calcinated PC500, anatase phase) as inorganic counterpart and 10 wt% loading of each polymer.They have been named as IEP-X@T-10 (X = 14 and 15).[8][9] The morphology of the hybrids consists in a core of polymer surrounded by TiO 2 nanocrystals (7.5 nm of particle size [21] ). Figure 2 depicts the FESEM images of one of the hybrids and the homogeneous distribution of both components in the composite is confirmed by its EDS images (Figure S18, Supporting Information).This hybrid morphology lead to negligible differences on the BET surface areas (126 and 131 m 2 g −1 , for IEP-14@T10 and IEP-15@T10, respectively) compared with the bare TiO 2 (ca.117.6 m 2 g −1 ) (Figure S19, Supporting Information) in accordance with other examples described by us. [7]On the other hand, the UVvis spectra of both hybrids show the contribution the polymer, broaden the absorption to 00-450 nm (Figure S20, Supporting Information).

Photocatalytic Hydrogen Production Reaction and Charge Transfer Mechanism Elucidation
The photocatalytic activity of the thienoacene-based CPPs and TiO 2 hybrids thereof was evaluated in hydrogen production reaction.(see experimental set-up in Figure S21, Supporting Information and emission spectra of the lamps in Figure S22, Supporting Information).The battery of experiments includes the use of two different irradiation sources: A) Xe solar simulator lamp and, B) Hg UV-vis lamp.Table 1 summarizes the most relevant results.
As both polymers show a good absorption in the visible region of the spectra (see Figure 1c), IEP-14, IEP-15 and their hybrid materials were tested in hydrogen production reaction under visible light irradiation, in which conditions TiO 2 is unable to produce hydrogen by itself.Figure 3a shows the cumulative hydrogen production over time for IEP-14@T-10 and IEP-15@T-10, and Figure 3b the photonic efficiency index (see entries 2 and 3 of Table 1).These materials produce similar hydrogen amount, 90 and 124 μmol g −1 h −1 , respectively.Note that neither pristine tita-nium dioxide nor both bare polymers show photocatalytic activity under these conditions.To explain these results, we could wonder to different options: 1) both polymers, IEP-14 and IEP-15, act as sensitizers of the TiO 2 (Figure 3c) or; 2) the small amount of UV irradiation present in the solar simulator source is enough to activate the hybrid as a whole.To discard one or another option, we have calculated the theoretical amount of hydrogen that would be produced by the small UV contribution of the solar simulator (see Section S6 of Supporting Information).Our estimations discard that the UV would be able to produce these amounts of hydrogen and confirmed the contribution of the sensitization mechanism when the reaction is driven under solar simulator irradiation.To provide spectroscopic evidence that supports the electron transfer through a photosensitized mechanism in both hybrids under solar simulator irradiation, steady-state photoluminescence (PL), time-resolved photoluminescence (TR-PL) and transient absorption spectroscopy (TAS) experiments at  exc ≈ 450 nm (where only the polymer absorbs) were carried out with the pristine materials and the hybrids.After excitation at 450 nm, IEP-14 and IEP-15 show emission in solid state with a fluorescence maximum centered at ca. 600 nm (Figure S28, Supporting Information).In the case of IEP-15, this band is certainly much more intense than in IEP-14.
The decay fitting for IEP-14 and IEP-15 ( exc = 445 nm,  obs = 600 nm) employing a three-exponential function exhibited similar average lifetime (<> = 1.38 and 1.47 ns, respectively) which correspond to two main contributions of  ≈ 0.35 and 1.6 ns with a 30 or 50% of pre-exponential value (A i ), respectively (Figure 3d and Table S8, Supporting Information, entries 1 and 2).The <> of both polymers were quenched in the hybrids (0.67 and 0.79 ns) for IEP-14@T-10 and IEP-15@T-10, respectively (Figure 3d), accompanied of a sharp decrease in the pre-exponential contribution of the  polymer forming part of the hybrid (see Table S8, Supporting Information entries 3 and 4).This result suggested a charge transfer from the polymer to titania.
Subsequently, to delve deeper more into this idea, TAS experiments at  exc = 450 nm (again where only the polymer absorbs) were carried out with IEP-15@T-10 (Figure 3e).As a control, no signal was observed measuring TiO 2 upon exposure at 450 nm of laser pulse (Figure 3e, gray).However, measuring the hybrid material, a clear appearance of a kinetic trace attributed to TiO 2 was observed (Figure 3f, black), accompanied of an increase in the absorption intensity of the transient spectrum in the hybrid, as an unequivocal proof of an electron transfer from the conduction band of IEP-15 to TiO 2 (Figure 3e, black).All these results are in accordance with a photosensitized charge transfer mechanism.Combining all these evidences, it was demonstrated that hybrids show an efficient electron-transfer mechanism via sensitization when they are irradiated under visible light (Figure 3c).
As the hydrogen production results under visible light irradiation were not outstanding, we decided to perform new experiments using UV-Vis irradiation (entries 5 and 6, Table 1).Figure 4a shows the cumulative hydrogen production of the hy-brid materials over time, and Figure 4b shows the photonic efficiency index of each sample.In spite of the photocatalytic inactivity of the bare polymers, both IEP-14@T-10 and IEP-15@T-10 hybrid materials showed excellent photocatalytic activity in hydrogen production reactions.IEP-15@T-10 was the best one of this series, with a hydrogen production of 925 μmol g −1 h −1 and 0.34% of photonic efficiency (almost 13-fold higher than bare TiO 2 ).
In order to increase the efficiency of our materials in hydrogen production reaction, IEP-15@T-10 was selected to perform an experiment employing photo-deposited platinum nanoparticles (ca. 1 wt%) as electron trapping co-catalyst (see entry 8, Table 1). [22]Figure 4c shows the cumulative hydrogen production over time, and Figure 4d the photonic efficiency index.Pt/IEP-15@T-10 shows almost 3 times higher photocatalytic activity than Pt/TiO 2 (24 275 μmol g −1 h −1 vs 8900 μmol g −1 h −1 ).Regarding literature, this result is among the higher for hybrid based on TiO 2 (Table S5, Supporting Information).To demonstrate the stability of Pt/IEP-15@T-10 sample the repeating experiments for 3 cycles of 6 h each were carried out (Figure S23, Supporting Information).The FTIR before and after the recycling experiment shows the sample unaltered (Figure S24, Supporting Information).
To elucidate the charge-transfer mechanism between TiO 2 and both IEP-14 and IEP-15 when the UV light participates in the reaction, experiments by means of PL, TR-PL and TAS have been carried out.
Firstly, the photoluminescence emission of the TiO 2 was quenched by the presence of IEP-14 and IEP-15 (Figure S29, Supporting Information).In order to discard a filter effect of the fluorescence of TiO 2 due to absorption by the polymer, TR-PL experiments ( exc = 372 nm), were carried out (Figure 5a).The quenching of the fluorescence emission of the TiO 2 was confirmed by a slight but consistent decrease in the TiO 2 average <> when it is forming part of the hybrids (Figure 5a left side and Table S8, Supporting Information, entries 8-10).Note that using a bandpass filter centered at 450 nm we detected only the emission of titania without traces of CPP (the decay fit was well adjusted as mono-exponential function).
Focusing the detection for the CPPs in the hybrids ( exc = 372 nm,  obs = 600 nm), a huge increase of the <> from ≈ 1.45 to ≈6 ns was observed for IEP-14@T-10 and IEP-15@T-10 (Figure 5a right side and Table S8, Supporting Information, entries 11-14).The three-exponential fitting revealed a small contribution (≈16%) of the (TiO 2 ) because titania emits above 600 nm (Figure S29, Supporting Information) and a third component that corresponds to the IRF instrument (≈61.2 ps).The observed behavior upon analyzing how change the lifetime in the hybrids compared to the pristine materials agrees with a Zscheme charge transfer pathway as it is described by our group in other hybrids based on truxene and phenanthrenequinone-CPPs. [8,9]In Z-scheme processes, electrons are transferred from the conduction band (CB) of the TiO 2 to the valence band (VB) of the polymer, the fluorescence lifetime of TiO 2 decreases in the hybrids whereas the increase for the organic counterpart (Figure 5b).
Furthermore, TAS experiments were driven in order to support the Z-scheme transfer pathway.TAS of TiO 2 ( exc = 355 nm, N 2 ) dispersed in 10 vol% MeOH, resulted in a continuous absorption covering in all spectral windows with a maximum at ca. 460 nm (black trace of Figure 5c and Figure S30, Supporting Information), where TiO 2 shows its maximum transient absorption. [23,8,24]The kinetic profile of TiO 2 ( obs = 460 nm) fits well to a mono-exponential kinetic profile immediately after laser pulse ( Trans = 53 ns) followed by large contribution in the microsecond scale (Figure S31, Supporting Information).
The influence of the polymers in the TiO 2 transient lifetime of the hybrid materials was evaluated.For IEP-14@T-10 or IEP-15@T-10, an enhancement of transient absorption (Figure 5c, green and violet traces) as well as an increase of the transient lifetime was observed at microsecond scale, indicating an efficient synergy between TiO 2 and IEP-14 or IEP-15 (Figure 5d).The behavior observed in these hybrids can be explained by three events after band-gap excitation: i) part of the charge is recombined during the photochemical event (shortest lifetimes up to first 50 ns scale), ii) migration and delocalization of the electrons (middle lifetime fraction up to 1-2 μs scale) and iii) growth in the absorption (Figure 5c) accompanied with a longer lifetime as a result of an efficient electron transfer followed by a subsequent slower decay.Thus, an increase of the absorption at longer timescale is attributed to a higher efficiency in the electron transfer from the CB of TiO 2 to the VB of the polymers.It leads to a huge enhance in the photocatalytic activity in hydrogen production reaction.
Finally, to confirm unequivocally the Z-scheme mechanism charge-transfer, analysis of FESEM and TEM images of Pt/IEP-15@T-10 sample were carried out (Figure 2 and Figure 6, respectively).In spite of the hybrid morphology described vide infra, some parts of the polymer are not completely covered.On one hand, Figure 2e depicts the EDS analysis of one of these regions and Figure 2b-d are successive magnifications where both polymer and TiO 2 nanocrystals can be recognized: TiO 2 appears as white particles while IEP-15 appears as a dark continuous and sticky material.Figure 2d shows a high-resolution FESEM image (see also raw information at Figure S32, Supporting Information) where tiny platinum nanoparticles can be appreciated over the polymer surface as brilliant points.On the other hand, Figure 6 shows the results taken by TEM.Herein, a region where the polymers are not covered with TiO 2 was analyzed.So, in Figure 6c it could be appreciated tiny nanoparticles deposited over a con-tinuous and amorphous material.The metallic nature of these nanoparticles can be envisioned by means of high-angle annular dark-field imaging (HAADF) mode in a STEM image (Figure 6d) where they appear as brilliant points.All this means that the reduction pathway preferentially takes play on the polymer surface in accordance with the Z-scheme proposed (Figure 5b).

CO 2 Photoreduction
The photoactivity of the best-performing hybrid, IEP-15@T-10, was further tested in the gas-phase CO 2 photoreduction with water under UV illumination (see experimental set-up in Figure S25 (Supporting Information) and emission spectrum of the lamps Figure S26, Supporting Information).Figure 7a shows the cumulative production of C1-C2 compounds, compared to that of benchmark TiO 2 .Both samples were mainly active towards CO and CH 4 formation, and they also showed some C 2 H 4 and C 2 H 6 production.Interestingly, the hybrid IEP-15@T-10 showed a 6fold higher CH 4 evolution than TiO 2 (Figure 7b), which resulted in a selectivity shift from CO to CH 4 (i.e., >26% greater selectivity).This production is 6-times higher than the sum of cumulative CH 4 productions of the counterparts (Table S6, Supporting Information), and highlights the synergy between the organic-inorganic heterostructure.Besides, the hybrid IEP-15@T-10 exhibited a 2.5-fold higher C 2 production, mainly C 2 H 6 , than bare TiO 2 (Figure S27, Supporting Information).Accordingly, the superior photoactivity of the heterojunction was also confirmed in terms of its photonic efficiency towards CH 4 , which was seven times higher than that reached by TiO 2 under UV illumination (0.04 and <0.01%, respectively).
The change of selectivity and the increase on the photoactivity are due to the more efficient Z-scheme charge transfer mechanism within the heterojunction.The combination of the polymer and TiO 2 leads to longer excited states of photogenerated charges, resulting in a change of selectivity towards the formation of more electron-demanding products.
Notably, the selectivity of IEP-15@T-10 to methane production and C2 products outperformed most of the state-of-the-art organic-inorganic hybrids for gas-phase CO 2 photoreduction, in the absence of any metal co-catalyst or dopant and under UV illumination (Table S7, Supporting Information).

Conclusions
Two novel thienoacene-based CPPs (IEP-14 and IEP-15) have been tested in hydrogen production reaction under visible and UV-vis irradiation.These CPPs did not show any photocatalytic activity themselves.On the contrary, hybrid materials with 10 wt% of polymer loading proved to be active in both hydrogen production reaction and CO 2 photoreduction.Importantly, this work demonstrated the importance of simultaneous excitation of both inorganic and organic counterparts of hybrid material.Thus, when the irradiation comes from a solar simulator, sensitization is the main charge transfer mechanism and both IEP-14@T-10 and IEP-15@T-10 showed hydrogen production, while the bare TiO 2 kept inactive.However, when both materials absorb light under UV-Vis irradiation the hydrogen production as well as the photonic efficiency increased, and IEP-14@T-10 and IEP-15@T-10 exhibited hydrogen productions 11 and 12-fold higher than bare TiO 2 , respectively.In this case, Z-scheme was proposed as the most likely charge-transfer pathway.Several experiments done by means of TAS, steady-state and time-resolved fluorescence support both theories.This opened the door to the use of our best material, IEP-15@T-10, in a gas-phase reactor to test the photoreduction reaction of CO 2 .Here, the use of a hybrid led to a change in the selectivity to more demanding electron products like CH 4 as well as other C2 products.

Experimental Section
Materials: Unless otherwise noted, all reagents, including solvents, were commercially available and were used without further purification.
Synthesis of IEP-14: Following the procedure described by Liras et al., [19] TA-2 (200 mg, 0.28 mmol), monomer 1 (70 mg, 0.55 mmol), Pd(PPh 3 ) 4 (13 mg, 0.01 mmol) and CuI (4 mg, 0.02 mmol) were placed in a Schlenk tube in DMF/Et 3 N 1:1 (8 mL).The mixture was stirred under argon atmosphere at 90 °C for 3 d (Scheme S4, Supporting Information).The polymer was filtered and washed with DCM, EtOAc, THF and ether.Then, it was added to a solution of THF/H 2 O (1:1) v/v with NaCN.The solution was stirred overnight.Finally, the polymer was filtered again and washed with H 2 O and ACN.The compound IEP-14 (170 mg, 94%) was obtained as an orange solid, insoluble in any solvent.Synthesis of IEP-15: Following the procedure described by Liras et al., [19] TA-2 (150 mg, 0.21 mmol), monomer 2 (86 mg, 0.27 mmol), Pd(PPh 3 ) 4 (9 mg, 0.01 mmol) and CuI (3 mg, 0.02 mmol) were placed in a Schlenk tube in DMF/Et 3 N 1:1 (8 mL).The mixture was stirred under argon atmosphere at 90 °C for 3 days (Scheme 1 and Scheme S5, Supporting Information).The polymer was filtered and washed with DCM, EtOAc, THF and ether.Then, it was added to a solution of THF/H 2 O (1:1) v/v with NaCN.The solution was stirred overnight.Finally, the polymer was filtered again and washed with H 2 O and ACN.The compound IEP-15 (155 mg, 91%) was obtained as an orange solid, insoluble in any solvent.Hybrid Materials Synthesis: Hybrid materials composed of CPPs and titanium dioxide (anatase, PC500) are physical mixtures of both materials.These materials are named as IEP-X@T-Y (X = 14 or 15, Y = wt% of polymer loading).The samples were prepared following the method depicted in Figure S15 (Supporting Information).First, TiO 2 was calcined at 400 °C for 4 h in order to remove all the possible organic impurities.Then, both polymer and TiO 2 were added in a round bottom flask, in an ACN/H 2 O 1:1 v/v mixture.The mixture was sonicated for 15 min and the solvents were removed using a rotary evaporator at 50 °C.The hybrid was dried and mixed with a mortar and pestle.
Hydrogen Production Set-Up: Photocatalytic experiments were conducted in a slurry photoreactor, which consisted of a three-mouth cylindrical flask made of glass with an effective volume of 130 mL.For each experiment, 0.025 g of photocatalyst was added to a H 2 O/MeOH (9:1) v/v aqueous solution (130 mL).After that, the reactor was tightly closed and maintained at a temperature of 20 °C by a cooling system.The reaction temperature was measured with a thermocouple situated on the photoreactor wall.Argon was then flown through the suspension at 60 mL min -1 and atmospheric pressure.The suspension was magnetically stirred under darkness until air was removed (verified by GC) and then irradiated by a 150 W medium-pressure Hg UV lamp or a 184 W Xe UV-VIS lamp (see lamps spectra in Figure S19, Supporting Information).H 2 evolution was monitored every 3.8 min by means of a Varian micro-GC equipped with two channels with a molecular sieve and a PPQ column, respectively (see cartoon of the set up at Figure S17, Supporting Information).For those reactions in which Pt was employed as cocatalyst, the metal was photo-deposited in situ on the hybrid material by dissolving the appropriate amount of H 2 PtCl 6 •6H 2 O to obtain a nominal loading of 1 wt% Pt.
CO 2 Photoreduction Set-Up: Gas-phase CO 2 photoreduction experiments were performed in continuous-flow mode using a home-made reaction system, which includes a photoreactor (280 mL) made of steel and equipped with a borosilicate window for irradiation.Powdered samples (60 mg) were deposited on a glass microfiber filter and fitted inside the reactor.A mixture of CO 2 (99.9999%,Praxair) and water vapor was fed into the system (CO 2 :H 2 O = 7.25 molar ratio) and the reaction conditions were set at 2 bars and 50 °C.UV illumination was performed using four 6 W lamps ( max = 365 nm) with an average intensity of 71.7 W m −2 (measured by a Blue-Wave spectrometer in the range 330-400 nm).Reaction products were detected in continuous mode with a gas chromatograph (GC, Agilent 7890 A) equipped with two separation branches and two sampling loops.First separation branch included two semi-capillary columns (Molesieve 5A and HP Plot Q), a thermal conductivity detector (TCD), a flame ionization detector (FID), and a methanizer.Second separation branch contained a capillary column (CP-Sil 5B) and a second FID.
For a typical experiment, the reactor was degassed under vacuum at 80 °C and then purged with Argon (100 mL min −1 for 1 h) to remove any residual organics weakly adsorbed on the catalyst surface.Then, the catalyst was exposed to the CO 2 :H 2 O mixture for 1 h to establish an adsorptiondesorption balance.Photocatalytic tests were performed for 15 h under UV illumination, and repeated at least twice to guarantee the reproducibility.No products were detected at 50°C under dark conditions or under UV illumination without catalyst.
Photonic Efficiency Calculations: Photonic efficiency ( ) is the ratio between the useful energy delivered or bound and the energy supplied, i.e., energy output/energy input.In the context of photocatalysis in heterogeneous media, quantum efficiency and photonic efficiency refer to the use of wavelength in a range ( 1 - 2 ) for absorbed radiation in the case in the case of quantum efficiency and for incident radiation in the case of photonic efficiency. [27]Therefore, photonic efficiency ( ) was calculated as the ratio between the produced hydrogen and the photon flux irradiated by the lamp (Equation 1).The photon flux is the number of photons of a particular wavelength, per time interval (spectral photon radiance) coming from all directions and absorbed by a system, integrated over the whole volume and averaged per volume. [27]This parameter was calculated from the lamp emission spectrum, recorded with a StellarNet UVNb-50 radiometer connected to an optical fiber.For that purpose, spectral irradiance (W m −2 nm −1 ) in the whole lamp spectrum was determined at distances from the lamp corresponding to the internal and external walls of the reactor (considering the geometry of the reactor as a cylinder), and the decrease of irradiance between both distances was fitted to the known inverse proportionality between irradiance and the square of the distance to the irradiation source.
where HER is the produced hydrogen and q the incident spectral photon flux in the selected wavelength range.*Dividend of the equation is multiplied by 2 because each H 2 molecule production requires the consumption of two photons.

Figure 1 .
Figure 1.CPPs characterization.a) 13 C NMR spectra of both IEP-14 and IEP-15; b) FTIR spectra of both polymers compared with TA-1 molecule; c) UV-Vis Absorption from Kubelka Munck function and main vertical transition of both IEP-14 and IEP-15.Inset: orbital structure of HOMO and LUMO calculated by TD-DFT; d) Conduction and valence bands of both CPPs and TiO 2 (valence band position calculated from cyclic voltammetry measurements) and red-ox pairs involved in hydrogen production and CO 2 photoreduction reactions.

Figure 2 .
Figure 2. Hybrid morphology.a) FESEM image of IEP-15@T-10/Pt hybrid and b-d) their magnifications.e) EDS images of the white dashed region of image (a) being EDS merge of all, C atoms (red), S atoms (yellow), Ti atoms (blue), O atoms (green) and Pt atoms (violet).

Figure 6 .
Figure 6.a) TEM image and b,c) its successive magnifications of Pt/IEP-15@T-10 sample.d) STEM image in HAADF mode of image d.