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On the Design of Donor-Acceptor Conjugated Polymers for Photocatalytic Hydrogen Evolution Reaction: First-Principles Theory Based Assessment Giane Damas, Cleber Fabiano N. Marchiori, and C. Moyses Graça Araujo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09408 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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The Journal of Physical Chemistry

On the Design of Donor-Acceptor Conjugated Polymers for Photocatalytic Hydrogen Evolution Reaction: First-principles Theory Based Assessment

Giane Damas*, Cleber F. N. Marchiori* and C. Moyses Araujo*

1

Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden.

Abstract A set of fluorene-based polymers with donor-acceptor architecture have been investigated as potential candidates for photocatalytic hydrogen evolution reaction. A design protocol has been employed based on first-principles theory and focusing on the following properties: (i) broad absorption spectrum to promote a higher number of photogenerated electron-hole pairs, (ii) suitable redox potentials and (iii) appropriate reaction thermodynamics using the hydrogen binding energy as a descriptor. We have found that the polymers containing a fused-ring acceptor formed by benzo(triazole-thiadiazole) or benzo(triazole-selenodiazole) units display a suitable combination of such properties standing out as potential candidates. In particular, PFO-DSeBTrT has an absorption maximum at around 950 nm for the HOMO-LUMO transition, covering a wider range of solar emission spectrum, and a reduction catalytic power of 0.78 eV. It displays also a calculated hydrogen binding free energy of ∆GH = 0.02 eV, which is lower in absolute value than that of Pt (∆GH ~ -0.10 eV). Furthermore, the results and trends analysis provide a guidance for rational design of novel photo-electro-catalysts.

Corresponding

authors:

[email protected],

[email protected]

and

[email protected]

ACS Paragon Plus Environment

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1. Introduction Solar energy harvesting is one of the most promising strategies to meet the demand for a renewable and clean energy system. There are two primary approaches that have been pursued in this context involving either photon-to-electricity or photon-to-chemical energy conversion. The former is achieved in the solar cell devices while the latter requires the use of photo-electro-catalysts. In both strategies, the organic materials have attracted a great deal of attention due to their easy processability and functionalization. For instance, since the seminal work of Tang1 such materials have been widely used as photoactive layer in thin film photovoltaic devices (OPV)2–4 and as photosensitizers in Dye-sensitized solar cells (DSSC)5–7. Additionally, these materials have been reported for applications in solar fuel production working as photocatalysts for the hydrogen evolution reaction (HER)8,9–11. With the advent of the so-called donor-acceptor (D-A) polymers, the efficiency of organic solar cells was raised up to 10% in the last few years12–18. This milestone was achieved due to improvement in both optical and electrical properties of these new materials. One of the most important features of D-A polymers is the low band-gap (LBG) energy achieved by combining electron-rich (donor) with electron withdrawing (acceptor) moieties19–21. The molecular orbital interaction between the donor and acceptor units induces a more planar conformation, improving the conjugation length and consequently lowering the polymer band gap. As a result, the more planar conjugated backbone enhances the p-electrons delocalization.20 Moreover, these D-A polymers usually have a broad absorption spectrum22–28 with a characteristic double-band shape where the low-energy band displays a charge transfer (CT) nature. 23 Several different donor units are available in the literature, like fluorine,22,27,29 carbazoles,26,30,31 cyclopentadithiophene,

24,32–34

to cite a few but, only a couple of

acceptors (e.g. benzothiadiazole, benzotriazole and sometimes its derivatives) have been


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reported.35–37 Fluorene moieties are commonly used as donor unit in D-A polymers due to the notable properties of the polyfluorene itself, as for instance its thermal and chemical stability as well as its high absorption coefficient.20 The features of D-A polymers have an important impact on the optical characteristics of these kind of materials. By combining the proper donor and acceptor moieties, one can shift and/or widen the absorption spectrum achieving much better matching with the solar emission spectrum and also tune the position of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO),28,38–40 which in turn tune the polymer redox potentials. This is actually an important attribute common for the photovoltaic and photocatalytic processes since in both cases photons must be harvested to either be converted directly into electricity or to drive uphill chemical reactions. A number of conjugated polymers have indeed been proposed as photocatalysts for the hydrogen evolution reaction (HER),10,11,41,42 a trend initiated after the pioneer work by Yanagida et al. was published in 1985.43 In their report, poly(p-phenylene) was irradiated at λ>290 nm to give 8.30 µmol of H2, a modest production in the presence of diethylamine as sacrificial electron donor. Following this publication, g-C3N4, polythiophene-based materials and polybenzothiadiazoles are among the polymeric materials that have been synthesized for HER purpose.10,11,41,42 One general hurdle in this application is that the pristine polymeric systems display low solubility in water demanding the experiments to be performed in organic solvents or water/organic solvent mixtures.44 The stabilization of nanostructured polymerdots (Pdots) has been proposed as a solution for such problem, which is achieved by further processing the photocatalyst with polystyrene grafting with carboxyl-group-functionalized ethylene oxide (PS-PEG-COOH).8,45 Three distinct Pdots have been investigated employing the following D-A polymers: F8T2 (poly(9,9-dioctylfluorenyl-2,7-diyl)-co-


3


bithiophene)),

PFBT

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(poly(9,9’-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-2,1,3-

thiadiazole)) and PFO-DTBT (poly(2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2yl)benzo-2,1,3-thiadiazole)).8 First, it has been shown that the Pdots architectures display about five orders of magnitude higher catalytic performance than that of the pristine polymers without organic solvent. Second, it was found that the presence of BT unit is relevant in order to achieve the HER catalytic activity. This observation combined with density functional theory (DFT) calculations indicated that N heteroatoms act as the actual reactive sites.8 Another important result is that the polymer containing DTBT units showed a reaction rate that is six times faster than that of the polymer containing only BT units, suggesting that the presence of a thiophene ring between the fluorene and benzothiadiazole plays an important role on the photocatalytic process. This is mainly due to the broader optical response of PFO-DTBT (up to 700 nm) as compared to PFBT. The former displayed also higher stability leading to significantly greater H2 production. These results shed light on possible routes to achieve stable and efficient organic photocatalysts. The assessment of the photocatalytic performance of the polymeric compounds (P) are commonly carried out through the analysis of the ground-state frontier orbital energies.46 However, unlike in solid-state inorganic compounds where such an approach is more justifiable, the polymers undergo significant atomic relaxation and free energy variation upon photon-induced charge transfer reaction.46 Therefore, the calculation of redox potentials based on the Gibbs free energy of reaction should be employed. For instance, considering the water splitting reaction, the photocatalyst must display a potential for the P/P+ redox pair more positive than the water oxidation potential (H2O/O2 redox pair),41 whereas the potential of the P-/P redox pair must be more negative than the proton reduction potential (H+/H2 redox pair, which has an absolute value lying around 4.44 V at standard conditions).8,47 Thus, a thorough and systematic investigation of the


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thermodynamics is still lacking in order to unveil the underlying photo-catalytic mechanisms to rationally design suitable materials. In this work, we have studied a set of fluorene-based polymers with different acceptor moieties, most of them already synthesized envisioning the application as active layer in OPVs but, up to the present time, not explored as photocatalysts. Our main goal is to initially provide a theoretical assessment of the compounds’ redox potentials obtained from the Gibbs free energy of reaction, which is calculated within the density functional theory (DFT) framework. Furthermore, in order to understand the reaction mechanism and identify the catalytic sites, the hydrogen binding free energy (∆GH) has been calculated. For instance, we have found that the substitution of the S atom by Se in a fused-ring acceptor strongly leads to a ∆GH close to the optimal value (∆GH = 0). By employing timedependent DFT (TDDFT), we also show how different substitutions at the acceptor unit impacts on the energy levels, optical properties and electronic transitions. An analysis of the total and partial density of states has provided the contribution of each moiety to the composition of the electronic states. Our results also shed light on the CT character of the electronic transitions. In summary, a combined set of properties based on thermodynamic, electronic and optical properties is used for the first time as a suitable methodology to select promising organic photocatalysts for HER. 2. Methodology 2.1- Computational Details Our study has been conducted within the density functional theory and time-dependent density functional theory48 frameworks as implemented in the software Gaussian0949. The geometry optimizations and frequency calculations have been carried out at the theory level M06/6-31G(d), followed by single-point total energy calculations, as well as TDDFT calculations, with larger basis set - M06/6-311G(d,p). The M0650 functional was chosen


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after a benchmark study considering the B3LYP, CAM-B3LYP and w-B97x-D for geometry optimization as well as for the electronic transitions calculations. B3LYP tends to erroneously favor planar conformations due to electron selfinteract misrepresentation, as well as the neglected dispersive forces,51,52 and also underestimates the energies of electronic states with a CT character.8 Thus, a proper description of the electronic properties of p-conjugated molecules requires the use of a functional that includes dispersive forces. Since a comparative study of different functionals is beyond the scope of this work, here we present only the results obtained with M06. For some results obtained with the other aforementioned functionals see Table S1 in the Supporting Information (SI) file. The total and partial density of states (DOS), as well as the molecular orbital composition analysis (using the fragment orbital framework) have been obtained with AOMIX code.53,54 The theoretical UV-Vis absorption spectrum for the oligomers has been computed at the same level of theory by using the full linear method for TDDFT.48 At this step, we have considered the first twenty excited singlet states to obtain the transition energies and the oscillator strength that are further compared with the experimental spectra (see Figures S8 and S9 in the SI file). The standard redox potential is conveniently obtained from the Gibbs free energy of the reduction reaction such as:

𝐸" = −

∆' ()*+ ,-. /0

(1)

where F and n are the Faraday constant (23.06 kcal mol-1 V-1) and the number of electrons involved in the redox reaction, respectively. The reduction free energy in solution is evaluated using the Born-Haber thermodynamic cycle as follows:


6

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where,

∆𝐺(3456) 𝑅𝑒𝑑 = ∆𝐺(;) 𝑅𝑒𝑑 + ∆𝐺(3456) 𝑅𝑒𝑑 − ∆𝐺(3456) 𝑜𝑥

and

with

∆𝐺(;) 𝑅𝑒𝑑 = ∆𝑈(;) 𝑅𝑒𝑑 + 𝑃𝑉 − 𝑇∆𝑆 ; 𝑅𝑒𝑑 ,

as

(2)

defined.

Thus,

∆𝑈(;) 𝑅𝑒𝑑 and ∆𝑆(;) 𝑅𝑒𝑑 correspond to the internal energy and entropic contributions, respectively. The solvation free energies of the oxidized and reduced species, ∆𝐺(3456) 𝑂𝑥 and ∆𝐺(3456) 𝑅𝑒𝑑 , were calculated using the polarizable continuum model (PCM)55 to represent the H2O-solvent with dielectric constant 78.35.49 More specifically, the free energy of each molecular system in the gas phase is calculated as

𝐺 = 𝐸-5-EF + 𝐸GHI + 𝑃𝑉 + 𝑈JKL − 𝑇𝑆JKL ,

(3)

where Eelect and EZPE are the electronic total energy and the zero-point energy, respectively. The last two terms in Eq. 3 are the finite temperature contributions for the internal energy and entropy, which have been evaluated at 298 K including vibrational, rotational and translational components (statistical mechanics contributions). The catalytic power (CPr) for the hydrogen evolution reaction, which is the main focus of this work, is then defined as


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𝐶𝑃N = 𝐸H"O /H − 𝐸Q" R /QS ,

(4)

where 𝐸H"O /H is the standard potential of the polymer redox reaction with P- representing the polymer reduced state and P representing the polymer ground-state. It is common in the literature to estimate the CPr through either the enthalpy of the reduction reaction (the electron affinity - EA) or the LUMO orbital energy only (based on the Koopman’s theorem). Here, we are carefully employing the full Gibbs free energy to have a proper assessment of the redox reaction thermodynamics.8,42,57 For the proton reduction potential, 𝐸Q" R /QS , under standard conditions, we have used the value of 4.44 V.47 Thus, Eq. 4 is equivalent to simply take the module of the P-/P redox pair potential referred to the natural hydrogen electrode (NHE). The LUMO energy has also been estimated through the lowest excitation energy (approximately the HOMO-LUMO gap, Eg (TDDFT)), obtained from TDDFT, and the selfconsistent HOMO orbital energy, EHOMO (SCF), such as

𝐸TUVW = 𝐸QWVW 𝑆𝐶𝐹 + 𝐸; 𝑇𝐷𝐷𝐹𝑇 .

(5)

Here, the HOMO→LUMO is a vertical transition, as the molecular structure is not optimized at the excited state. Therefore, although providing a good agreement with the experimental values, this approach offers an approximation of the excitation process that enable us to make relative comparisons and further property predictions. These calculations have been carried out mainly to benchmark our theory level against the experimental results where the LUMO is obtained from optical measurements.


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For completeness and also benchmark purpose, the potential of the P/P+ redox pair has also been calculated, which is related to the polymer ionization potential. Here, P+ represents the polymer oxidized state. The difference between P/P+ and P-/P redox potentials stablish the polymer electrochemical window (or electrochemical gap). All the calculated redox potentials (using different thermodynamics potentials: ∆𝐸, ∆𝐻 and ∆𝐺) as well as the obtained HOMO and LUMO energies (with different methods) are shown in Tables S2-S4 in the SI file. We have carried out additional calculations to benchmark our results against some experimental values (using the respective solvents in the theoretical assessment) available in the literature (see summarized results in Table S4). The correlation curves are presented in the Figure S1. As can be seen, the combination of our polymeric models (shown below) and theory-level has been able to catch the trends of an experimental results diversity. It is important to stress that such correlations analyses aim to consolidate the adopted methodology. In the following, we will discuss only the outcomes in aqueous environment, which is the desirable solvent for photocatalytic HER in the solar fuel production. Furthermore, in order to further understand the reaction mechanism and evaluate the nitrogen catalytic site at different acceptor units, the hydrogen binding free energy (∆GH) have been computed through the equation: _

∆𝐺Q = 𝐺[45\]-N:Q − 𝐺[45\]-N + 𝐺QS , J

(6)

where Gpolymer:H, Gpolymer and GQS are the Gibbs free energies of the hydrogenated polymer, pristine polymer and hydrogen molecule, respectively.


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2.2 - Oligomers modeling We are interested in fluorene-based compounds that have an electron acceptor unity like benzothiadiazole (BT) and benzoselenodiazole (BS), among others, placed between thiophene (T) units. In such systems, the electron-rich region defined by the fluorene unit gives to this moiety a donating character, whereas thiophene has been described as a πbridge and the BT unit (and its derivatives) as the electron-deficient entity. To model these polymers, we have first built up oligomer structures with varying the number of monomers and assessed the energetics of different conformations. Then, the frontier orbital energies have been calculated as a function of the oligomer (fully optimized structures) length. This careful convergence analysis has been performed for PFO-DTBT (see Table S1), which works as a reference compound in this study. Based on these results, the model containing two repeating units, with a symmetric ending containing the fluorene group, has been chosen to model all the other polymeric compounds. Additionally, the long alkyl side chains were replaced by methyl groups in order to minimize the computational costs and convergence problems. Similar strategy has already been reported by Jespersen and coworkers for the PFBT and PFO-DTBT23. It should be point out that structures have always been fully optimized without any symmetry constraint. All the investigated models are illustrated in Figure 1.


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Figure 1 – Molecular structures of the compounds that are discussed in this study. In the fluorene unit, the original alkyl chains have been replaced by methyl groups, as indicated by R.

For pedagogical purposes, we have separated these materials into four groups, according to the kind of the BT modification. The group 1 is composed by the compounds that we have previously studied, namely F8T2, PFBT and PFO-DTBT.8 As illustrated in Figure 1, the polymers represented by the general acronym PFODXBT

(poly

(2,7(9,9’-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)-5,6-X-benzo-2,1,3-

thiadiazole, where X=F; CN and MeO) contains the functionalization groups fluorine (PFO-DffBT), cyano (PFO-DCNBT) or methoxy (PFO-DMeOBT) on the 5,6 positions. Here, these polymers are classified as group 2.


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In the third group, we have considered the replacement of the sulfur atom on thiadiazole ring by nitrogen in PFO-DBTr (poly[9,9’-dioctylfluorene)-2,7-diyl-(4,7bis(thien-2-yl) 2-dodecyl-benzo-1,2,3-triazole) or selenium, as abbreviated by PFO-DTBS (poly (2,7(9,9’-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)-benzo-2,1,3-selenodiazole). The last group is composed by BTrT-derived polymers containing a hybrid benzotriazole-thiadiazole unit with an electron-deficient character, PFO-DX’’BTrT, with PFO-DSBTrT (poly (9,9’-dioctylfluorene)-2,7-diyl-alt-(4,7-bis(thien-2-yl) 2-dodecylbenzo-(1,2c:4,5c’)-1,2,3-triazole-2,1,3-thiadiazole)) or PFO-DSeBTrT (poly (9,9’dioctylfluorene)-2,7-diyl-alt-(4,7-bis(thien-2-yl)-2-dodecyl-benzo-(1,2c:4,5c’)-1,2,3triazole-2,1,3-selenodiazole)).

3. Results and Discussions 3.1 The thermodynamics analysis The overall photon-induced hydrogen evolution reaction, catalyzed by the polymeric compounds, is depicted in the scheme of Figure 2(a). In the first step (I), the polymer absorbs the photon energy promoting an electronic transition and creating an electron-hole pair. In the second step (II), the hole is consumed through the oxidation of a sacrificial reagent. Here, we are assuming that this redox process is faster than the H2 evolution reaction, which is supported by the fact that it is usually not a proton coupled process. Thus, the polymeric compound is stabilized in the state P- that is subsequently oxidized (step III) promoting the HER and returning to the ground-state (P). Therefore, to satisfy the thermodynamics of the charge transfer reaction, the potential of the P-/P redox pair must


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be more negative than the proton reduction potential (H+/H2 redox pair), which we have been taken as the reference value under standard conditions. Figure 2(b) shows the potentials of the P-/P and P/P+ redox pairs for all the materials grouped according to the modification on the acceptor unit. As mentioned above the potentials are calculated from the full Gibbs free energy and referred to the NHE (including also the H+/H2 redox pair potential that works as reference level) and using water solvent. We start investigating the influence of the benzothiadiazole unit (BT) as the electron acceptor (group 1: F8T2, PFBT and PFO-DTBT). As can be seen, all these compounds fulfill the thermodynamics requirement to drive the photo-induced proton reduction. However, there is a shift of the P-/P redox potential toward less negative values leading to the CP values of 1.75, 1.41 and 1.16 V for F8T2, PFBT and PFO-DTBT, respectively. This effect correlates well with the LUMO orbital energy variations (see the last column in Table S3). There is also a shift on the P/P+ redox potential for less positive values when moving from PFBT to PFO-DTBT. Thus, the latter polymer has the narrowest electrochemical gap, defined as 𝐸-E = 𝐸H"R /H − 𝐸H/HO . As it will be shown below, it has also the lowest optical gap among these three compounds. It should be pointed out that the calculated redox potentials for F8T2 display higher discrepancy from the values measured by cyclic voltammetry (CV).8 This is not the case for PFBT and PFO-DTBT, which display good agreement with the values measured under the same conditions. On the other hand, our results for F8T2 fairly agree with other values found in the literature57–60. This might be related to the fact that in our previous study,8 the CV measurements were done with nanostructured materials in water dispersion. As the F8T2 is known to exhibit some peculiar properties like self-organization in solution61 and


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liquid crystalline phase62,63, it is possible that, when processed as nanoparticles this material exhibits some unusual aggregation that strongly impacts on its energy levels. Since PFO-DTBT has displayed the best catalytic performance in our previous study, it will be used here as a reference polymer for comparison with the other modified compounds. Starting with group 2, we have studied the influence of electron donating or withdrawing substitution in the 5,6 positions of the benzothiadiazole ring. For this end, we have selected the methoxyl (MeO) group as electron donating, fluorine (F) with almost neutral character, and the cyano (CN) as an electron withdrawing substituent. As one can see, both redox potentials follow the electron withdrawing strength with the CN-based material (PFO-DCNBT) having the lowest lying P-/P redox potential and consequently the lowest CP. To verify this trend, we show in Figure 2(c) the plot of the P-/P and P/P+ redox potentials as a function of the Hammett parameter64 (sp), which is related to the electron withdrawing strength of the functionalized-polymers. There is a linear relation enabling one to predict the potentials using this parameter as a descriptor. However, the different slopes of the curves lead to a reduction of the electrochemical gap with the CN substituted compound displaying the lowest value of 1.89 V, an effect that correlates also with the


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optical gap (see below). Considering that thermodynamics is still satisfied, this substitution is a promising approach to improve the catalytic activity in the HER reaction.

Figure 2 – (a) Diagram representing the photocatalytic hydrogen evolution reaction. (b) Redox potentials (P-/P and P/P+ pairs) diagram comparing different electron acceptor entities composing the polymer chain and the proton reduction energy. (c) Redox potentials as a function of the Hammett parameter obtained for PFO-DMeOBT, PFO-DffBT and PFO-DCNBT. (d) Hydrogen binding free energy calculated for polymers of groups 2 (purple), 3 (green) and 4 (blue). PFBT and PFO-DTBT are shown in orange. The PFO-DSBTrT* and PFO-DSeBTrT* refer to the N triazole subunit acting as a secondary catalytic site.

When the sulfur atom at BT unit is replaced by N or Se (group 3), there is a variation in the P-/P redox potential for Se (PFO-DTBS) while a negligible change was obtained for the N-containing polymer (PFO-DBTr), when compared to PFO-DTBT. Concerning the


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catalytic power, both materials are suitable for HER, although PFO-DBTr displays some disadvantages from the optical properties perspective, as will be discussed later. Furthermore, both N and Se substitution result in a small change in the P/P+ redox potential (0.70 eV for both PFO-DBTr and PFO-DTBS, against 0.71 for PFO-DTBT). Very interesting results came out for the polymers containing the fused-ring acceptors (group 4: PFO-DSBTrT and PFO-DSeBTrT). The calculated electrochemical gap has been significantly reduced to around 1.1 V and still maintaining the P-/P redox potential at the appropriate level to drive HER. This is also reflected on the optical gap that displays a near-infrared absorption edge, as shown below. These materials were first synthesized by Tam et al. (2012)65, exhibiting similar configuration to bisbenzothiadiazole.66,67 Their characteristic high electron affinity has been attributed to the presence of a hypervalent sulfur66,67 in the sulfur diimide group (-N=S=N-) that is connected by double bonds on the thiadiazole ring. However, through ab initio calculations, Strassner & Fabian68 have demonstrated that there is no hypervalence because the dorbitals are not involved in these bonds, hindering the sp3d hybridization in an expanded octet atom. Thus, the bond lengths in the N-S-N have been determined theoretically as intermediate between double and single bonds, with four electrons shared by three centers.68,69 Our optimized structures and calculated electronic structure corroborate with those previous results and analyses. A descriptor analysis based on the hydrogen free binding energy was used to evaluate the nitrogen and sulfur atoms as catalytic sites in F8T2, PFBT and PFO-DTBT.8 The hydrogenation of the catalytic site should take place at intermediate energies disabling too strong and too weak interactions.70,71 Actually, the ideal thermodynamic condition is given by the hydrogen free binding energy of ∆GH = 0. Thus, a positive value for ∆GH indicates that an energy input is necessary to form the intermediate state, a quantity that is


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often referred as overpotential. Our previous results have unveiled a qualitative correlation between the lowest overpotentials, calculated for N-catalytic sites in comparison to S-sites, and an increased photocatalytic activity.8 This is due to the fact that such overpotential may compensate for the catalytic power (defined in Eq. 4) and hinder the catalytic activity. Here, we have employed the same approach to assess the effect of all the investigated photocatalysts on the thermodynamics of the HER. The hydrogen free binding energies were calculated for N-catalytic sites at different acceptor, as shown in Figure 2(d), with absolute values detailed in Table S5. As one can see, the ∆GH for PFBT and PFODTBT polymers are very close to each other (within about 0.70 eV), showing no significant divergence from the previous published results. The highest overpotential is found for the benzotriazole as acceptor moiety (PFO-DBTr), with ∆GH = 1.31 eV. As a matter of comparison, F8T2 has a reported ∆GH = 2.25 eV, with no catalytic activity and a similar absorption spectrum.8 Except by fluorination, the effects of functionalization and replacement of sulphur by selenium atom on the benzoselenodiazole ring (PFO-DTBS) leads to a decreased ∆GH. An interesting result is the trend followed by the free binding energy in respect to the substitution of S atom by Se and the insertion of the fused-ring unit, although the nitrogen remains as the catalytic site. As shown in Figure 2(d), by replacing S by Se in the BT unit the ∆GH decreases from 0.67 eV to 0.35 eV. Almost the same reduction (from 0.67 eV to 0.35 eV) in ∆GH is observed when the whole BT unit is substituted by the BTrT (one should keep in mind that both BT and BTrT units have the S atom in their composition). However, when the Se atom is inserted in the BTrT (PFO-DSeBTrT) the ∆GH reaches the impressive value of 0.02 eV (when the N atom at the N-Se-N is considered as the catalytic site), a value that is lower in absolute terms than that of Pt (∆GH = -0.10 eV)

72

and

especially close to the ideal value. At the secondary catalytic site present at the N triazole


17


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unit, these values increase slightly to around 0.55-0.66 eV for the fused-ring-based systems, which lies close to the PFO-DTBT value (0.67 eV). These results indicate that the low ∆GH is a joint effect of presence of heavy atom and number of p-electrons. Tracing the relation between the electronic properties and the thermodynamic quantity expressed by the hydrogen binding free energy, we have found that ∆GH is linearly correlated to the polymer energy gap with an absolute correlation coefficient that remains unchanged after hydrogenation, as displayed in Figure S2. In the latter case, the intermediate state is formed by populating a previous unoccupied orbital, which is now moved towards lower energies to compose the new HOMO with major character given by the acceptor unit. The hybridized molecular orbitals formed upon the N-H chemical bond are distanced from the antibonding states by around 2.68-4.06 eV, where the extent of such separation gives the bond strength, a feature that is well-predicted by the molecular orbital theory for chemical bonds. This effect is observed by considering the energy gap of the acceptor unit and the hydrogen bonding and antibonding states, which is displayed in Figure S3.

3.2 Structural, electronic structure and optical properties In this section, we will present and discuss the electronic structure and optical properties stating with the group 1, i.e. F8T2, PF-BT and PFO-DTBT. The optimized structures are shown in Figure 3(a) with the obtained torsion angles summarized in Table1. F8T2 and PFO-DTBT structures have similar torsion angles between the Fluorene (F) and Thiophene (T) units. For the F8T2 the torsion between T units is small compared to F-T angle. When the T units are replaced by BT (PFBT) the torsion angle increases. However, when the BT is inserted between the thiophene rings, the F-T angles are slightly increased and the T-BT torsion angle is almost half of the T-T angle in F8T2. This makes the PFO-DTBT backbone


18

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more planar, increasing the conjugation length. It is well known that the torsion angles deeply affect the energy levels as well as the spatial distribution of the molecular orbitals since a more planar structure improves the p-electrons delocalization. As depicted in Figure 3(a), F8T2, PFBT and PFO-DTBT have a delocalized HOMO orbital, indicating that these torsions are not enough to break the conjugation length, even the higher torsion angle observed for PFBT.

Table 1- Torsional angles between F-T (θ1), T-T(θ2), F-BT (θ3) and BT-T (θ4) groups for F8T2, PFBT and PFO-DTBT, as obtained through DFT calculations.

Polymer

θ1

θ2

θ3

θ4

F8T2

26.8°

20.2°

-

-

PFBT

-

-

39.0°

-

PFO-DTBT

27.7°

-

-

11.6°

By comparing the LUMO orbitals, one can see that the insertion of BT unit brings an interesting feature. While the F8T2-LUMO is well delocalized, for PFBT and PFODTBT the LUMO is more localized at the BT units. The main effect of this behavior will be discussed later, in light of the absorption spectra of these materials.


19


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Figure 3 – (a) Optimized structure and spatial distribution of HOMO-LUMO for F8T2, PFBT, PFO-DTBT (isovalue = 0.02) with the atom color coding: carbon (grey), hydrogen (white), nitrogen (blue) and sulfur (yellow). Total and Partial DOS for (b) F8T2, (c) PFBT and (d) PFO-DTBT. The empty states were shifted towards more negative values in accordance with the TDDFT predictions for the energy gap. (e) Measured and (f) calculated (TDDFT) UV-Vis spectra of F8T2, PFBT, PFODBT polymers.

The influence of each moiety over the polymeric electronic structure has been analyzed by means of the calculated density of states, as displayed in Figures 3(b)-(d). Although these calculations were held at the ground state, a shift on the unoccupied orbitals energies was made for the appropriate matching with those values estimated from the


20

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TDDFT gap. First, the DOS of F8T2 shows (Figure 3 (b)) that both fluorene and thiophene contributes to the composition of the frontier orbitals. In particular, the LUMO has an almost equal contribution of F and T, indicating that both HOMO and LUMO should be delocalized over the chain. This result corroborates with the orbital distributions shown in Figure 3(a). When the bithiophene is replaced by a benzothiadiazole unit, a well-known strong electron acceptor,19–21 a peak indicating the formation of a set of localized states is observed around the LUMO energy (Figure 3 (c)). The localized states are mainly (up to 80% for the LUMO and 90% for LUMO+1, for instance) formed by unoccupied orbital from the BT moieties while the HOMO has a mixed characteristic with the major contribution of the F units. For the PFO-DTBT, the insertion of T between the F and BT units preserve the localized nature of the first unoccupied orbitals. As depicted in Figure 3(a), for both PFBT and PFO-DTBT, the LUMO are localized over the BT units. There are different interpretations for the role of thiophene in D-A polymers. Some works state that it acts as an electron donor unit when combined with benzothiadiazole73, whereas other studies describe the thiophene as a p-bridge (or p-spacer), considering in this case another unit (like fluorene) as the electron donor, forming a D-p-A structure74,75. Our results show (Figure 3 (d)) that T (~46%) units have a higher contribution to the HOMO of the PFO-DTBT than F (~24%), and the LUMO is mainly formed by orbital from the BT units (~67%). This is an indication that both thiophene and fluorene act as the electron donors and the BT as the electron acceptor. D-A polymers usually have two well-defined absorption band in the UV-Vis spectrum22–28. This kind of double-band spectra is explained by assuming that one of the low wavelength band is related to p-p* transitions while the higher wavelength band is associated with the CT transitions23. Figures 3 (e) and (f) show the comparison of the experimental UV-Vis spectra for these materials and the calculated absorption spectra. The


21


Page 22 of 46

F8T2 does not have the double-band shaped spectrum, which is the signature of D-A polymers. Instead it has only one band with lmax at 454 nm and an additional shoulder at 482 nm. Looking for the orbital associated to the calculated electronic transitions, summarized in Table 2, it is possible to see that, for F8T2, the two main electronic transitions (S0®S1 and S0®S2) correspond to the excitation where both the initial and final states are delocalized, characterizing a p - p* transition. The spatial distribution of the main orbitals involved can be seen in Figure S4. Replacing T by BT unity a set of electronic transitions with a CT feature emerges. For PFBT, the first band centered at 464nm (S0®S1) corresponds to an excitation from a delocalized orbital to an orbital mainly localized at the BT units, resulting in a CT transition. The same characteristic is observed for S0®S1 and S0®S4 transitions. Moreover, the transition S0®S4 preserves the p - p* character, since both the initial and final orbital are delocalized. In this sense, PFBT compose a D-A structure where the Fluorene moieties act as the electron donor and the benzothiadiazole unit acts as electron acceptor, whereas F8T2 does not result in a D-A polymer (for the number of CT transitions, see Table 2). The insertion of T units between the F and BT produces an impressive red-shift in the absorption spectrum of the resulting polymer (142 nm for the p - p* band and 92 nm for the CT band) leading to a broad absorption profile. This is expected since the insertion of T units increase the conjugation of the chain stabilizing a more planar conformation (as it is possible to see by the torsion angles showed in Table 1). Despite the more planar backbone, a more localized distribution of the LUMO is observed (see Figure 3(a)), preserving the CT transitions as observed in PFBT. As shown by our group,8 the faster hydrogen evolution rate from PFO-DTBT compared and PFBT is associated also with the


22

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higher number of photogenerated electrons when the absorption spectrum is red-shifted, having a better match with the solar emission spectrum.

Table 2 – Optical Properties of the main singlet excited states obtained at the range 300-1000 nm with TDDFT methodology. In the last column, nCT is the number of charge transfer transitions with f (oscillator strength)> 0.02. Group 1

Wavelength

Character

f

Major contributions

nCT

(nm) F8T2

PFBT

PFO-DTBT

S1:455.80

π→π*

3.262

H→L

(86%)

S4:353.60

π→π*

0.331

H-1→L+1

(71%)

S1:481.98

CT

1.382

H→L

(87%)

S7:330.09

π→π*

1.593

H→L+2

(85%)

S1:600.58

CT

2.220

H→L

(76%)

S7:404.76

π→π*

1.610

H→L+2

(80%)

0

3

4

In a previous work, Jespersen et al23 employed the semi-empirical method (ZINDO) to describe the UV/Vis Spectrum from PFBT and PFO-DTBT,23 finding a similar nature from the frontier orbitals for these polymers, as well as the charge transfer features of the first absorption peak. Here, we expand their contribution by evaluating the electronic structure, the contribution from each moiety to the total density of states (DOS) of these molecules, as well as by using TDDFT to describe the electronic transition, instead to ZINDO. As mentioned before, these three polymers were probed as photocatalytic materials for HER8. Among them, the PFO-DTBT exhibits the highest hydrogen (H2) generation rate due to the following factors: (i) its favorable catalytic power (CP), (ii) the broad absorption spectrum leading to an improved photon harvesting capability, (iii) the


23


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presence of the N sites, which act as the catalytic reactive center and (iv) the localized nature of the electrons, in the excited state, at the BT units. In light of these main features, from hereinafter we investigate the electronic structure and absorption profiles of polymer groups 3-4 in order to complete the analysis of their potential application for HER.

Figure 4– Total and Partial DOS for: a) PFO-DffBT, b) PFO-DCNBT, c) PFO-DMeOBT, d) PFODBTr, e) PFO-DTBS, f) PFO-DSBTrT and g) PFO-DSeBTrT. The red, blue and gray colors represent the contribution from fluorene, thiophene and the corresponding acceptor unit.

In Figure 4, the total and partial density of states (DOS and pDOS) are presented for each polymer. The frontier occupied states (FOS), located at around -5.6 eV are dominated by the thiophene rings for PFO-DffBT and PFO-DMeOBT (Figure 4 (a) and (b)) with a secondary contribution from fluorene. All of them have a minor contribution of the acceptor units for the FOS. On the other hand, in the same energy range for PFODCNBT, the FOS are dominated by fluorene. In this group, low-energy virtual states show a localization at the ground state that is directly related to high acceptor strength of the CN


24

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substituent. Additionally, BT is also predominant at the states up to -2.5 eV for this polymer. In the group 3, the thiophene valence levels are predominate on the FOS from HOMO to up to -5.5 eV, when fluorene starts to have a higher contribution (see Figure 4 (d) and (e)). In particular for the HOMO orbital, the BT and F units exert the same contribution. Since the triazole group promotes an increase in energy of the first LUMO+n orbitals, a higher hybridization is found with other molecular orbitals increasing its delocalization. For PFO-DTBS, there is no significant change in the DOS profile compared to PFO-DTBT. For the last group, there is a highly localized peak centered at around -3.2 eV composed mainly by the hybrid benzo(triazole-thiadiazole) ring states up to -2.6 eV. It


25


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is also possible to see a higher localization of the FOS centered at around -5.0 eV, with contributions from T>BT>F.

Figure 5 - Optimized structures (left) and frontier orbital plots for the polymers from groups 1-3. Level of theory: M06/6-311G**, isovalue: 0.02. The color coding represents selenium (orange) and oxygen (red).

From the spatial distribution of the orbitals shown in Figure 5, it is possible to see the delocalization of HOMO for all the materials analyzed whereas, for most of them, the LUMO is localized around the acceptor moieties in agreement with the DOS features shown in Figure 4. The exception is the triazole containing material (PFO-DBTr) for which the LUMO is also delocalized (see Figure 4 and Figure 5). On the other hand, the presence of the triazole unit fused with the benzothiadiazole group promotes the LUMO localization of the resulting material. In general, our results indicate that the localization/delocalization of the unoccupied orbital is not related to the conformation of the conjugated backbone,


26

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since structures with small torsion angles still preserve the localized nature of the LUMO instead of delocalizing it due to the higher superposition of the p orbitals (for details on the torsion angles see the Table S6). The localized nature of the first unoccupied orbitals have a deep impact on the absorption profile of these materials. We have evaluated the UV/Vis spectra for the polymers of groups 2-4 (see Figure 6), discussing the nature of the main electronic transitions to establish a structure-property relation that is essential for design purposes. In Figure 6, the dashed line corresponds to the PFO-DTBT spectrum, which is used as reference. The main electronic transitions and related information are described in Table 3, with spatial distribution of the involved orbitals shown in Figures S5-S7. For comparison, the correlation between theoretical and experimental absorbance values is depicted in


27


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Figure S9, showing a very good accuracy with MAE=35.48 nm for the estimation of the first absorption peak.

Figure 6 – Theoretical UV-Vis spectra of polymers from groups 2, 3 and 4. PFO-DTBT is displayed in dashed lines for comparison.

One can see that the absorption spectrum from group 2 have the same ‘camelback’ structure as PFO-DTBT, with a decreased oscillator strength for the first absorption peak in the order PFO-DBTr> PFO-DffBT> PFO-DMeOBT> PFO-DCNBT. In the case of PFO-DffBT, the main absorption peaks are blue-shifted to 586.60 nm and 400.50 nm when compared to the analogue PFO-DTBT, in a good concordance with previously reported results76. The first absorption energy is mainly associated to the HOMO-LUMO transition, which happens at even shorter wavelength (S1=582.74 nm) for PFO-DMeOBT, but it is red-shifted to 651.11 nm for PFO-DCNBT. For these three polymers, the HOMO level is well distributed over the backbone, with a major contribution from the thiophene


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rings (around 40-46%) for PFO-DffBT and PFO-DCNBT, followed by the fluorene moiety (28-39%). On the other hand, the fluorene moiety contributes in 27.3%, with 46.1% from thiophene for methoxyl-functionalized polymer PFO-DMeOBT. For these systems, the LUMO is 69-80% composed by the functionalized-BT ring and 10-17% from thiophene, which confirms its localization over the acceptor unit, indicating that S0→S1 is a CT transition. In this group, the cyano and fluorine-functionalized polymer exhibit five CT transitions between the two main adsorption peaks (nCT=5). However, the second adsorption peak comes from a π→π* transition between the HOMO®LUMO+2 level for these systems, where these virtual levels are found to be highly delocalized over the molecule. Experimentally, PFO-DBTr is found to have a single adsorption peak at 474 nm that has been attributed to an internal charge transfer. 77 Nonetheless, our results show that this HOMO-LUMO transition (S1=523.38 nm) has a π→π* character, considering the delocalized nature of the frontier orbitals. The highest contribution for the HOMO level is given by thiophene (45.9%), followed by F (21.7%) and BT (22.6%), but the delocalized LUMO level also shows a high percentage of thiophene character (31.7%), besides the dominant benzotriazole contribution (39.0%). The second transition at 348.15 nm has the same π→π* character, corresponding to a HOMO→ LUMO+4 transition. In PFO-DSed, S0→ S1 is a CT transition found at 636.20 nm, 35.72 nm higher than PFO-DTBT, whereas S0→ S7 (414.15 nm) has π→π* character. For the hybrid benzo(triazole-thiadiazole) derivatives (group 4), a much broader absorption spectrum can be observed. The first adsorption maximum (the CT transition) is strongly red-shifted to 852.00 nm (PFO-DSBTrT) and 950.50 nm (PFO-DSeBTrT). In these systems, we find a HOMO orbital composed by around 42% thiophene, 16-18% fluorene and 32% of BTrT. For LUMO, the participation of BTrT orbitals reaches 75% for


29


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PFO-DSeBTrT, showing a high localization. The latter totalizes eight CT transitions, as obtained by TDDFT.

Table 3 – Description of the contributions to the main transitions calculated for the polymers from groups 24. nCT is the number of charge transfer transitions with f> 0.02. Level of theory:M06/6-311G**. Group 1

Wavelength

Character

f

Major contributions

nCT

(nm) PFO-DffBT

PFO-DCNBT

PFO-DMeOBT

PFO-DBTr

PFO-DTBS

PFO-DSBTrT

PFO-DSeBTrT

S1:586.60

CT

2.192

H→L

(76%)

S7:400.50

π→π*

1.913

H→L+2

(79%)

S1:.651.11

CT

1.723

H→L

(76%)

S9:394.76

π→π*

1.470

H→L+2

(74%)

S1:582.74

CT

2.175

H→L

(75%)

S7:404.43

π→π*

1.832

H→L+2

(81%)

S1:523.40

π→π*

3.580

H→L

(80%)

S9:348.15

π→π*

0.454

H→L+4

(59%)

S1:636.20

CT

1.739

H→L

(73%)

S7:414.15

π→π*

2.018

H→L+2

(79%)

S1:852.00

CT

1.858

H→L

(83%)

S9:442.36

π→π*

1.676

H→L+2

(75%)

S1950.50

CT

1.540

H→L

(82%)

S9:459.25

π→π*

1.922

H→L+2

(75%)

5

5

4

0

4

7

8

As afore mentioned, three are the factors influencing the D-A polymers performance as photocatalysts: the catalytic power, the broad absorption spectrum and the presence of the N sites at the acceptor moiety leading to a low overpotential for the overall HER reaction. The charge transfer character of the electronic transitions might also have some influence on the photocatalytic process, since the electron localization in the excited


30

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state could lower the charge recombination rate improving the HER. However, this feature remains still elusive. Using such criteria, the PFO-DSeBTrT stands out as a promising candidate for application in photo-catalysed HER. Notably, the HOMO levels (and the P+/P redox pair potential) of polymers could also influence the reaction kinetics of formation of reduced polymers for the subsequent proton reduction, which has to be taken into account in a practical experiment.

4. Conclusions In this work, we have carried out a theoretical assessment, based on first-principles theory, of the key properties controlling the performance of donor-acceptor conjugated polymers as photocatalysts for hydrogen evolution reaction. First, the thermodynamics of the charge transfer reaction have been investigated by means of the calculated redox potentials obtained through the Gibbs free energy of redox reactions and using the DFT framework. The solvation effects have been assessed by employing Born-Haber thermodynamics cycles. The study has been focused on the effect of the acceptor units. All investigated compounds have been found to display favorable thermodynamics for HER, which comes out from the analysis of the P/P- redox pair potential. In particular, the analysis of the substitution in the 5,6 positions of the benzothiadiazole ring shows that the redox potentials follow the electron withdrawing strength with the CN-based material (PFO-DCNBT) having the narrowest electrochemical gap but still keeping a significant CP, a relevant feature for photocatalysis application. Furthermore, a descriptor analysis based on the hydrogen free binding energy was used to evaluate the overpotential introduced by the photocatalysts in the HER. It was found that the PFO-DSeBTrT displays a ∆GH = 0.02 eV, which is lower than the value reported for Pt (∆GH = -0.10 eV). Additionally, we found a linear correlation between ∆GH


31


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and the polymer energy gap as well as with the H energy gap, when H is bound to the N site. Thus, one might expect that a lower positive ∆GH is resultant from a more effective distancing between the unfavourable antibonding molecular orbitals to the bonding states, further leading to an enhanced N-H bond strength. The electronic structure and optical properties have also been studied using TDDFT and DFT. The results have unveiled the role of different chemical units as donor/acceptor. For instance, we have found that the thiophene moiety, although described as a p-bridge, contributes significantly to the density of states of the HOMO-n orbitals, suggesting that this entity has a donating character. The replacement of the S atom at the benzothiadiazole units by an additional N (resulting in the benzotriazole) suppress the DA characteristics, at least when this unit is combined with fluorene and thiophene moieties. Following the electrochemical gap, there is a significant red-shift of the CN substitution in the 5,6 positions of the benzothiadiazole ring. Such shift is actually much more pronounced in the compounds containing the hybrid benzothiadiazole-triazole units, which covers a wider range of the solar spectrum. The combination of this feature with the favorable thermodynamics described above makes the PFO-DSeBTrT a promising photocatalysts for HER. Finally, we followed a protocol, based on first-principles calculations, which can be a useful tool to select the most promising candidate materials and/or to guide the synthesis of new materials for hydrogen evolution reactions. It is based mainly on the following three properties: (i) broad absorption spectrum to promote a higher number of photogenerated electron-hole pairs, (ii) suitable redox potentials and (iii) appropriate reaction thermodynamics using the hydrogen binding energy as a descriptor.


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Acknowledgments This project has support from the Swedish Research Council (VR) and STandUP for Energy collaboration, with infrastructure provided by the Swedish National Infrastructure for Computing (SNIC) at the PDC Center for High Performance Computing and National Supercomputer Centre at Linköping University (NSC). We would like to thank Dr. Haining Tian for the discussions and critical reading of this manuscript. The author GBD cordially thanks the brazilian foundation CAPES (Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior) for the financial support in her Phd studies and CFNM thanks Carl Tryggers Stiftelse for the financial support.

Supporting Information The Supporting Information is available free of charge on the ACS Publication Website. Benchmark for determination of frontier orbitals energies for fluorene-based polymers that have been previously reported in different experimental conditions; correlation between the redox potentials computed in the appropriate solvent relative to the experimental HOMO/LUMO energies; comparison between the polymer-hydrogen binding energies with presence or absence of thermal corrections to the internal energy; correlation between the hydrogen binding free energy and the polymer energy gap; variation of the partial density of states on the acceptor unit upon the hydrogenation process; spatial distribution of the highest contributing orbitals involved in the two main absorption peaks; torsional angles between different molecular groups in the polymer chain.

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Figure 1 – Molecular structures of the compounds that are discussed in this study. In the fluorene unit, the original alkyl chains have been replaced by methyl groups, as indicated by R. 594x584mm (144 x 144 DPI)


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Figure 2 – (a) Diagram representing the photocatalytic hydrogen evolution reaction. (b) Redox potentials (P-/P and P/P+ pairs) diagram comparing different electron acceptor entities composing the polymer chain and the proton reduction energy. (c) Redox potentials as a function of the Hammett parameter obtained for PFO-DMeOBT, PFO-DffBT and PFO-DCNBT. (d) Hydrogen binding free energy calculated for polymers of groups 2 (purple), 3 (green) and 4 (blue). PFBT and PFO-DTBT are shown in orange. The PFO-DSBTrT* and PFO-DSeBTrT* refer to the N triazole subunit acting as a secondary catalytic site. 493x390mm (144 x 144 DPI)



Figure 3 – (a) Optimized structure and spatial distribution of HOMO-LUMO for F8T2, PFBT, PFO-DTBT (isovalue = 0.02) with the atom color coding: carbon (grey), hydrogen (white), nitrogen (blue) and sulfur (yellow). Total and Partial DOS for (b) F8T2, (c) PFBT and (d) PFO-DTBT. The empty states were shifted towards more negative values in accordance with the TDDFT predictions for the energy gap. (e) Measured and (f) calculated (TDDFT) UV-Vis spectra of F8T2, PFBT, PFO-DBT polymers. 271x310mm (150 x 150 DPI)


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Figure 5 - Optimized structures (left) and frontier orbital plots for the polymers from groups 1-3. Level of theory: M06/6-311G**, isovalue: 0.02. The color coding represents selenium (orange) and oxygen (red). 337x266mm (300 x 300 DPI)


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Figure 6 – Theoretical UV-Vis spectra of polymers from groups 2, 3 and 4. PFO-DTBT is displayed in dashed lines for comparison. 1164x899mm (72 x 72 DPI)



TOC abstract 663x593mm (72 x 72 DPI)


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On the Design of Donor-Acceptor Conjugated ... - ACS Publications

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