On the Design of DonorâAcceptor Conjugated Polymers for

Article Cite This: J. Phys. Chem. C 2018, 122, 26876−26888

pubs.acs.org/JPCC

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

J. Phys. Chem. C 2018.122:26876-26888. Downloaded from pubs.acs.org by UNIV OF RHODE ISLAND on 11/30/18. For personal use only.

S Supporting Information *

ABSTRACT: A set of fluorene-based polymers with a donor−acceptor architecture has been investigated as a potential candidate 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 and stand out as potential candidates. In particular, PFO-DSeBTrT (poly (9,9′-dioctylfluorene)- 2,7-diyl-alt-(4,7-bis(thien-2yl)-2-dodecyl-benzo-(1,2c:4,5c′)- 1,2,3-triazole-2,1,3-selenodiazole)) has an absorption maximum at around 950 nm for the highest occupied molecular orbital−lowest unoccupied molecular orbital transition, covering a wider range of solar emission spectrum, and a reduction catalytic power of 0.78 eV. It also displays a calculated hydrogen-binding free energy of ΔGH = 0.02 eV, which is lower in absolute value than that of Pt (ΔGH ≈ −0.10 eV). Furthermore, the results and trends analysis provide guidance for the rational design of novel photo-electrocatalysts. ization.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, such as fluorene,22,27,29 carbazoles,26,30,31 and cyclopentadithiophene,24,32−34 to cite a few, but only a couple of acceptors (e.g., benzothiadiazole (BT), benzotriazole, and sometimes its derivatives) have been reported.35−37 Fluorene moieties are commonly used as a donor unit in D−A polymers because of 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 because in both cases, photons must be harvested to either be converted directly into electricity or to drive uphill chemical reactions.

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 photonto-chemical energy conversion. The former is achieved in the solar cell devices, whereas the latter requires the use of photoelectrocatalysts. In both strategies, the organic materials have attracted a great deal of attention because of their easy processability and functionalization. For instance, since the seminal work of Tang,1 such materials have been widely used as a photoactive layer in thin-film organic photovoltaic (OPV) devices2−4 and as photosensitizers in dye-sensitized solar cells.5−7 Additionally, these materials have been reported for applications in solar fuel production, working as photocatalysts for the hydrogen evolution reaction (HER).8−11 With the advent of the so-called donor−acceptor (D−A) polymers, the efficiency of organic solar cells has raised up to 10% in the last few years.12−18 This milestone was achieved because of 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 energy achieved by combining electron-rich donor with electron-withdrawing acceptor moieties.19−21 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 π-electron delocal© 2018 American Chemical Society

Received: September 26, 2018 Revised: October 26, 2018 Published: October 29, 2018 26876

DOI: 10.1021/acs.jpcc.8b09408 J. Phys. Chem. C 2018, 122, 26876−26888

Article

The Journal of Physical Chemistry C

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 DFT framework. Furthermore, to understand the reaction mechanism and identify the catalytic sites, the hydrogen-binding free energy (ΔGH) 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 ΔGH close to the optimal value (ΔGH = 0). By employing time-dependent DFT (TDDFT), we also show how different substitutions at the acceptor unit impact the energy levels, optical properties, and electronic transitions. An analysis of the total and partial density of states (DOS) 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 the HER.

A number of conjugated polymers have indeed been proposed as photocatalysts for the 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 a 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 polymer dots (Pdots) has been proposed as a solution for such a problem, which is achieved by further processing the photocatalyst with polystyrene grafting with carboxyl groupfunctionalized 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)-cobithiophene)], PFBT [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-2-yl)benzo-2,1,3thiadiazole)].8 First, it has been shown that the Pdots architecture displays about 5 orders of magnitude higher catalytic performance than the pristine polymers without the organic solvent. Second, it was found that the presence of the BT unit is relevant 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 6 times faster than the polymer containing only BT units, suggesting that the presence of a thiophene ring between fluorene and BT plays an important role in 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 also displayed higher stability leading to significantly greater H2 production. These results shed light on the possible routes to achieve stable and efficient organic photocatalysts. The assessment of the photocatalytic performance of the polymeric compounds (P) is 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 CT reaction.46 Therefore, the calculation of redox potentials based on the Gibbs free energy of reaction should be employed. For instance, considering the watersplitting reaction, the photocatalyst must display a potential for the P/P+ redox pair that is 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 under standard conditions).8,47 A thorough and systematic investigation of the thermodynamics is still lacking to unveil the underlying photocatalytic 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 an active layer in OPV cells but up to the present time not explored as

2. METHODOLOGY 2.1. Computational Details. Our study has been conducted within the DFT and TDDFT48 frameworks as implemented in the software Gaussian 09.49 The geometry optimizations and frequency calculations have been carried out at the M06/6-31G(d) level of theory, 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 after a benchmark study considering the B3LYP, CAM-B3LYP, and w-B97x-D functionals for geometry optimizations and electronic transition calculations. B3LYP tends to erroneously favor planar conformations due to electron self-interact 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 π-conjugated molecules requires the use of a functional that includes dispersive forces. As 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 file. The total and partial DOS as well as the molecular orbital composition analysis (using the fragment orbital framework) have been obtained with the AOMIX code.53,54 The theoretical ultraviolet−visible (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 considered the first twenty excited singlet states to obtain the transition energies and the oscillator strength that are further compared with the experimental spectra. The standard redox potential is conveniently obtained from the Gibbs free energy of the reduction reaction as E0 = −

ΔG(solv)(Red) nF

(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 followswhere 26877


Article


where E P0− /P 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.8,42,56 For the proton reduction potential, E H0 + /H2 , under standard conditions, we have used the value of 4.44 V.47 Thus, eq 4 is equivalent to 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 [self-consistent field (SCF)] as

ΔG(solv)(Red) = ΔG(g)(Red) + ΔG(solv)(Red) − ΔG(solv)(ox)

(2)

and with ΔG(g)(Red) = ΔU(g)(Red) + PV − TΔS(g)(Red), as defined. Thus, ΔU(g)(Red) and ΔS(g)(Red) correspond to the internal energy and entropic contributions, respectively. The solvation free energies of the oxidized and reduced species, ΔG(solv)(Ox) and ΔG(solv)(Red), were calculated using the polarizable continuum model55 to represent the H2O solvent with the dielectric constant 78.35.49 More specifically, the free energy of each molecular system in the gas phase is calculated as G = Eelect + EZPE + PV + U298 − TS298

E LUMO = E HOMO(SCF) + Eg (TDDFT)

(3)

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 enables 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.

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 HER, which is the main focus of this work, is then defined as CPr = |E P0− /P − E H0 + /H2|

(5)

(4)

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. 26878


Article


Figure 2. (a) Diagram representing the photocatalytic HER. (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. PFO-DSBTrT* and PFO-DSeBTrT* refer to the N triazole subunit acting as a secondary catalytic site.

For completeness and also for 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 establishes the polymer electrochemical window (or electrochemical gap). All calculated redox potentials (using different thermodynamics potentials: ΔE, ΔH, and ΔG) as well as the obtained HOMO and LUMO energies (with different methods) are shown in Tables S2−S4 in the Supporting Information 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 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 result diversity. It is important to stress that such correlation analyses aim to consolidate the adopted methodology. In the following, we will discuss only the outcomes in an aqueous environment, which is the desirable solvent for photocatalytic HER in solar fuel production. Furthermore, to further understand the reaction mechanism and evaluate the nitrogen catalytic site at different acceptor units, the hydrogen-binding free energy (ΔGH) is computed through the equation 1 i y ΔG H = Gpolymer/H − jjjGpolymer + G H2 zzz 2 k {

where Gpolymer/H, Gpolymer, and GH2 are the Gibbs free energies of the hydrogenated polymer, pristine polymer, and hydrogen molecule, respectively. 2.2. Oligomer Modeling. We are interested in fluorenebased compounds that have an electron acceptor unit such as BT and benzoselenodiazole (BS), among others, placed between thiophene (T) units. In such systems, the electronrich region defined by the fluorene unit gives a donating character to this moiety, whereas thiophene has been described as a π-bridge and the BT unit (and its derivatives) as the electron-deficient entity. To model these polymers, we first built up oligomer structures by varying the number of monomers and assessed the energetics of different conformations. Then, the frontier orbital energies were calculated as a function of the oligomer (fully optimized structures) length. This careful convergence analysis was performed for PFODTBT (see Table S1), which works as a reference compound in this study. On the basis of these results, the model containing two repeating units, with a symmetric ending containing the fluorene group, has been chosen to model all other polymeric compounds. Additionally, the long alkyl side chains were replaced by methyl groups to minimize the computational costs and convergence problems. A similar strategy has already been reported by Jespersen and co-workers for PFBT and PFO-DTBT.23 It should be pointed out that the structures have always been fully optimized without any symmetry constraint. All investigated models are illustrated in Figure 1. For pedagogical purposes, we have separated these materials into four groups, according to the kind of BT modification.

(6) 26879


Article


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 a 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 literature.56−59 This might be related to the fact that in Pati et al.,8 the CV measurements were done with nanostructured materials in water dispersion. As F8T2 is known to exhibit some peculiar properties such as self-organization in solution60 and liquid crystalline phase,61,62 it is possible that, when processed as nanoparticles, this material exhibits some unusual aggregation that strongly impacts its energy levels. As PFO-DTBT 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 studied the influence of electron-donating or -withdrawing substitution in the 5,6 positions of the BT ring. For this end, we selected the methoxyl (MeO) group as electron donating, fluorine (F) with an almost neutral character, and 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 CPr. To verify this trend, we show in Figure 2c the plot of the P−/P and P/P+ redox potentials as a function of the Hammett parameter63 (σp), 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 optical gap (see below). Considering that thermodynamics is still satisfied, this substitution is a promising approach to improve the catalytic activity in the HER. When the sulfur atom at the 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 CPr, 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 substitutions result in a small change in the P/P+ redox potential (0.70 eV for both PFO-DBTr and PFO-DTBS against 0.71 eV 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 significantly reduced to around 1.10 V and still maintained the P−/P redox potential at the appropriate level to drive the HER. This is also reflected in the optical gap that displays a near-infrared absorption edge, as shown below. These materials were first synthesized by Tam et al. (2012),64 exhibiting a configuration similar to that of bis-benzothiadiazole.65,66 Their characteristic high EA has been attributed to the presence of hypervalent sulfur65,66 in the sulfur diimide group (−NSN−) that is connected by double bonds on the thiadiazole ring. However, through ab initio calculations, Strassner and Fabian67 have demonstrated that there is no hypervalence because the d-orbitals are not involved in these bonds, hindering the sp3d hybridization in an expanded octet

Group 1 is composed of 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 PFO-DXBT [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, or MeO] contain the functionalization groups fluorine (PFODffBT), cyano (PFO-DCNBT), or methoxy (PFO-DMeOBT) on the 5,6 positions. Here, these polymers are classified as group 2. In the third group, we have considered the replacement of the sulfur atom on the thiadiazole ring by nitrogen in PFODBTr [poly(9,9′-dioctylfluorene)-2,7-diyl-(4,7-bis(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 of BTrT-derived polymers containing a hybrid benzotriazole-thiadiazole unit with an electron-deficient character, PFO-DX″BTrT, with PFODSBTrT [poly(9,9′-dioctylfluorene)-2,7-diyl-alt-(4,7-bis(thien-2-yl)-2-dodecyl-benzo-(1,2c:4,5c′)-1,2,3-triazole-2,1,3thiadiazole)] or PFO-DSeBTrT [poly (9,9′-dioctylfluorene)2,7-diyl-alt-(4,7-bis(thien-2-yl)-2-dodecyl-benzo-(1,2c:4,5c′)1,2,3-triazole-2,1,3-selenodiazole)].

3. RESULTS AND DISCUSSION 3.1. Thermodynamics Analysis. The overall photoninduced HER, catalyzed by the polymeric compounds, is depicted in the scheme of Figure 2a. 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 HER, 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 CT reaction, the potential of the P−/P redox pair must be more negative than the proton reduction potential (H+/H2 redox pair), which we have taken as the reference value under standard conditions. Figure 2b shows the potentials of the P−/P and P/P+ redox pairs for all of 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 the reference level) and using the water solvent. We start investigating the influence of the BT unit as the electron acceptor (group 1: F8T2, PFBT, and PFODTBT). 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 CPr values of 1.75, 1.41, and 1.16 V for F8T2, PFBT, and PFODTBT, respectively. This effect correlates well with the LUMO orbital energy variations (see Table S3). There is also a shift in 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 Eec = E P0+ /P − E P/P−. As it will be shown below, it has also the lowest optical gap among these three compounds. 26880


Article


Figure 3. (a) Optimized structure and spatial distribution of HOMO−LUMO for F8T2, PFBT, and PFO-DTBT (isovalue = 0.02) with the following atom color coding: carbon (gray), 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 toward 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, and PFO-DBT polymers.

S5. As one can see, ΔGH for PFBT and PFO-DTBT 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 benzotriazole as the acceptor moiety (PFO-DBTr), with ΔGH = 1.31 eV. As a matter of comparison, F8T2 has a reported ΔGH = 2.25 eV, with no catalytic activity and a similar absorption spectrum.8 Except by fluorination, the effects of functionalization and replacement of sulfur by the selenium atom on the BS ring (PFO-DTBS) leads to a decreased ΔGH. An interesting result is the trend followed by the free binding energy with respect to the substitution of S atom by Se and the insertion of the fused-ring unit, although nitrogen remains as the catalytic site. As shown in Figure 2d, by replacing S by Se in the BT unit, ΔGH decreases from 0.67 to 0.35 eV. Almost the same reduction (from 0.67 to 0.35 eV) in ΔGH is observed when the whole BT unit is substituted by 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 BTrT (PFO-DSeBTrT), ΔGH reaches the impressive value of 0.02 eV (when the N atom at N−Se−N is considered as the catalytic site), a value that is lower in absolute terms than that of Pt (ΔGH = −0.10 eV)71 and especially close to the ideal value. At the secondary catalytic site present at the N triazole unit, these values increase slightly

atom. Thus, the bond lengths in N−S−N have been determined theoretically as intermediate between double and single bonds, with four electrons shared by three centers.67,68 Our optimized structures and the 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.69,70 Actually, the ideal thermodynamic condition is given by the hydrogen-free binding energy of ΔGH = 0. Thus, a positive value for ΔGH indicates that an energy input is necessary to form the intermediate state, a quantity that is often referred as the overpotential. Our previous results have unveiled a qualitative correlation between the lowest overpotentials, calculated for N-catalytic sites in comparison to Ssites, and an increased photocatalytic activity.8 This is due to the fact that such an overpotential may compensate for the CPr (defined in eq 4) and hinder the catalytic activity. Here, we have employed the same approach to assess the effect of all investigated photocatalysts on the thermodynamics of the HER. The hydrogen-free binding energies were calculated for N-catalytic sites at different acceptors, as shown in Figure 2d, with absolute values detailed in Table 26881


Article


that these torsions are not enough to break the conjugation length, even the higher torsion angle observed for PFBT. By comparing the LUMO orbitals, one can see that the insertion of BT unit brings an interesting feature. Whereas F8T2-LUMO is well-delocalized, for PFBT and PFO-DTBT, 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. The influence of each moiety over the polymeric electronic structure was analyzed by means of the calculated DOS, as displayed in Figure 3b−d. Although these calculations were held at the ground state, a shift of the unoccupied orbital energies was made for appropriate matching with those values estimated from the TDDFT gap. First, the DOS of F8T2 shows (Figure 3b) that both fluorene and thiophene contribute 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 3a. When the bithiophene is replaced by a BT 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 3c). The localized states are mainly (up to 80% for the LUMO and 90% for LUMO + 1, for instance) formed by the unoccupied orbital from the BT moieties, whereas the HOMO has a mixed characteristic with the major contribution of the F units. For 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 3a, for both PFBT and PFO-DTBT, the LUMO is 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 BT,72 whereas other studies describe thiophene as a π-bridge (or π-spacer), considering in this case another unit (like fluorene) as the electron donor, forming a D−π−A structure.73,74 Our results show (Figure 3d) 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 the orbital from the BT units (∼67%). This is an indication that both thiophene and fluorene act as the electron donors and BT acts as the electron acceptor. D−A polymers usually have two well-defined absorption bands in the UV−vis spectrum.22−28 This kind of double-band spectra is explained by assuming that one of the low wavelength band is related to π−π* transitions, whereas the higher wavelength band is associated with the CT transitions.23 Figure 3e,f shows the comparison of the experimental UV−vis spectra for these materials and the calculated absorption spectra. F8T2 does not have the double-band-shaped

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 ΔGH is a joint effect of the presence of a heavy atom and the number of π-electrons. Tracing the relation between the electronic properties and the thermodynamic quantity expressed by the hydrogenbinding free energy, we found that ΔGH is linearly correlated with 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 toward lower energies to compose the new HOMO with the 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 are 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 starting with group 1, that is, F8T2, PF-BT, and PFO-DTBT. The optimized structures are shown in Figure 3a with the obtained torsion angles summarized in Table1. F8T2 and PFO-DTBT Table 1. Torsional Angles between F−T (θ1), T−T(θ2), F− BT (θ3), and BT−T (θ4) Groups for F8T2, PFBT, and PFODTBT, as Obtained through DFT Calculations polymer

θ1 (deg)

θ2 (deg)

F8T2 PFBT PFO-DTBT

26.8

20.2

θ3 (deg)

θ4 (deg)

39.0 27.7

11.6

structures have similar torsion angles between the fluorene (F) and thiophene (T) units. For F8T2, the torsion between T units is small compared to the F−T angle. When the T units are replaced by BT (PFBT), the torsion angle increases. However, when 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 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 because a more planar structure improves the πelectron delocalization. As depicted in Figure 3a, F8T2, PFBT, and PFO-DTBT have a delocalized HOMO orbital, indicating

Table 2. Optical Properties of the Main Singlet Excited States Obtained in the Range of 300−1000 nm with the TDDFT Methodologya group 1 F8T2 PFBT PFO-DTBT

wavelength (nm) S1: 455.80 S4: 353.60 S1: 481.98 S7: 330.09 S1: 600.58 S7: 404.76

f

character π→ π→ CT π→ CT π→

π* π*

3.262 0.331 1.382 1.593 2.220 1.610

π* π*

major contributions H H H H H H

→ L (86%) − 1 → L + 1 (71%) → L (87%) → L + 2 (85%) → L (76%) → L + 2 (80%)

nCT 0 3 4

a

In the last column, nCT is the number of CT transitions with f (oscillator strength) > 0.02. 26882


Article


Figure 4. Total and partial DOS for (a) PFO-DffBT, (b) PFO-DCNBT, (c) PFO-DMeOBT, (d) PFO-DBTr, (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, respectively.

DTBT is compared, and PFBT is also associated with the higher number of photogenerated electrons when the absorption spectrum is red-shifted, having a better match with the solar emission spectrum. In a previous work, Jespersen et al.23 employed the semiempirical 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 CT features of the first absorption peak. Here, we expand their contribution by evaluating the electronic structure and the contribution from each moiety to the total DOS of these molecules as well as by using TDDFT to describe the electronic transition, instead of ZINDO. As mentioned before, these three polymers were probed as photocatalytic materials for HER.8 Among them, PFO-DTBT exhibits the highest hydrogen (H2) generation rate because of the following factors: (i) its favorable CPr, (ii) the broad absorption spectrum leading to an improved photon harvesting capability, (iii) the presence of 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, here after, we investigate the electronic structure and absorption profiles of polymer groups 3−4 to complete the analysis of their potential application for HER. In Figure 4, the total and partial DOS 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 4a,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 PFO-DCNBT, the FOS are dominated by fluorene. In this group, the low-energy

spectrum, which is the signature of D−A polymers. Instead, it has only one band with λmax at 454 nm and an additional shoulder at 482 nm. Looking for the orbital associated with 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 π−π* transition. The spatial distribution of the main orbitals involved can be seen in Figure S4. Replacing T by the BT unit, a set of electronic transitions with a CT feature emerges. For PFBT, the first band centered at 464 nm (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 π−π* character as both the initial and final orbitals are delocalized. In this sense, PFBT comprises a D−A structure where the fluorene moieties act as the electron donor and the BT unit acts as the 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 F and BT produces an impressive red shift in the absorption spectrum of the resulting polymer (142 nm for the π−π* band and 92 nm for the CT band), leading to a broad absorption profile. This is expected because the insertion of T units increases 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 3a), preserving the CT transitions as observed in PFBT. As shown by Pati et al.,8 the faster hydrogen evolution rate from PFO26883


Article


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).

virtual states show a localization at the ground state that is directly related to the high acceptor strength of the CN substituent. Additionally, BT is also predominant at the states up to −2.5 eV for this polymer. In group 3, the thiophene valence levels predominate on the FOS from HOMO to up to −5.5 eV, when fluorene starts to have a higher contribution (see Figure 4d,e). In particular for the HOMO orbital, the BT and F units exert the same contribution. Because the triazole group promotes an increase in energy of the first LUMO + n orbitals, 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 of the hybrid benzo(triazole-thiadiazole) ring states up to −2.6 eV. It is also possible to see a higher localization of the FOS centered at around −5.0 eV, with contributions from T > BT > F. From the spatial distribution of the orbitals shown in Figure 5, it is possible to see the delocalization of HOMO for all 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 Figures 4 and 5). On the other hand, the presence of the triazole unit fused with the BT 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 because structures with small torsion angles still preserve the localized nature of the LUMO even with higher superposition of π-orbitals (for details on the torsion angles, see 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

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

that is essential for design purposes. In Figure 6, the dashed line corresponds to the PFO-DTBT spectrum, which is used as the reference. The main electronic transitions and the related information are described in Table 3, with the spatial distribution of the involved orbitals shown in Figures S5−S7. The UV−vis absorption spectra obtained through TDDFT have been compared to the experimental reported absorbances (see Figures S8,S9), showing very good correlation with mean absolute error = 35.48 nm for the estimation of the first absorption peak. One can see that the absorption spectrum from group 2 has the same “camel-back” structure as PFO-DTBT, with a decreased oscillator strength for the first absorption peak in the order PFO-DBTr > PFO-DffBT > PFO-DMeOBT > PFODCNBT. In the case of PFO-DffBT, the main absorption peaks are blue-shifted to 586.60 and 400.50 nm when compared to the analogue PFO-DTBT, in good concordance with previously reported results.75 The first absorption energy is mainly associated with the HOMO−LUMO transition, 26884


Article

The Journal of Physical Chemistry C Table 3. Description of the Contributions to the Main Transitions Calculated for the Polymers from Groups 2−4a group 1 PFO-DffBT PFO-DCNBT PFO-DMeOBT PFO-DBTr PFO-DTBS PFO-DSBTrT PFO-DSeBTrT

wavelength (nm) S1: S7: S1: S9: S1: S7: S1: S9: S1: S7: S1: S9: S1: S9:

586.60 400.50 651.11 394.76 582.74 404.43 523.40 348.15 636.20 414.15 852.00 442.36 950.50 459.25

character CT π→ CT π→ CT π→ π→ π→ CT π→ CT π→ CT π→

π* π* π* π* π* π* π* π*

f 2.192 1.913 1.723 1.470 2.175 1.832 3.580 0.454 1.739 2.018 1.858 1.676 1.540 1.922

major contributions H H H H H H H H H H H H H H

→ → → → → → → → → → → → → →

L L L L L L L L L L L L L L

(76%) + 2 (79%) (76%) + 2 (74%) (75%) + 2 (81%) (80%) + 4 (59%) (73%) + 2 (79%) (83%) + 2 (75%) (82%) + 2 (75%)

nCT 5 5 4 0 4 7 8

a

nCT is the number of charge transfer transitions with f > 0.02. Level of theory: M06/6-311G**.

As afore mentioned, there are three factors influencing the D−A polymer performance as photocatalysts: the CPr, the broad absorption spectrum, and the presence of the N sites at the acceptor moiety leading to a low overpotential for the overall HER. The CT character of the electronic transitions might also have some influence on the photocatalytic process because the electron localization in the excited state could lower the charge recombination rate improving the HER. However, this feature remains still elusive. Using such criteria, PFO-DSeBTrT stands out as a promising candidate for application in photocatalyzed 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.

which happens at even shorter wavelength (S1 = 582.74 nm) for PFO-DMeOBT, but it is red-shifted to 651.11 nm for PFODCNBT. For these three polymers, the HOMO level is welldistributed over the backbone, with a major contribution from the thiophene 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 27.3%, with 46.1% from thiophene for methoxyl-functionalized polymer PFO-DMeOBT. For these systems, the LUMO is 69−80% composed of 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 polymers 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 CT.76 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 PFODSed, S0 → S1 is a CT transition found at 636.20, 35.72 nm higher than PFO-DTBT, whereas S0 → S7 (414.15 nm) has a 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 of around 42% thiophene, 16−18% fluorene, and 32% of BTrT. For LUMO, the participation of BTrT orbitals reaches 75% for PFO-DSeBTrT, showing high localization. The latter totalizes eight CT transitions, as obtained by TDDFT.

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 HER. First, the thermodynamics of the CT reaction was 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 were assessed by employing Born−Haber thermodynamics cycles. The study was focused on the effect of the acceptor units. All investigated compounds were 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 BT 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 CPr, a relevant feature for photocatalysis application. Furthermore, a descriptor analysis based on the hydrogenfree binding energy was used to evaluate the overpotential introduced by the photocatalysts in the HER. It was found that PFO-DSeBTrT displays a ΔGH = 0.02 eV, which is lower than the value reported for Pt (ΔGH = −0.10 eV). Additionally, we found a linear correlation between ΔGH 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 ΔGH is resultant from a more effective distancing between the 26885


Article


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 for the critical reading of this manuscript. G.D. cordially thanks the Brazilian foundation CAPES (Coordenaçaõ de Aperfeiçoamento de Pessoal de Ensino Superior) for the financial support in her PhD studies, and C.F.N.M. thanks Carl Tryggers Stiftelse for the financial support.

unfavorable antibonding molecular orbitals and 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 the donor/ acceptor. For instance, we have found that the thiophene moiety, although described as a π-bridge, contributes significantly to the DOS of the HOMO − n orbitals, suggesting that this entity has a donating character. The replacement of the S atom at the BT units by an additional N (resulting in the benzotriazole) suppresses the D−A 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 BT ring. Such a shift is actually much more pronounced in the compounds containing the hybrid BT-triazole units, which cover a wider range of the solar spectrum. The combination of this feature with the favorable thermodynamics described above makes PFO-DSeBTrT a promising photocatalyst for HER. Finally, we followed a protocol, based on first-principles calculations, which can be a useful tool to select the most promising candidate material and/or to guide the synthesis of new materials for the HER. 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.

■

■

(1) Tang, C. W. Two-layer organic photovoltaic cell. Appl. Phys. Lett. 1986, 48, 183−185. (2) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. 2.5% Efficient Organic Plastic Solar Cells. Appl. Phys. Lett. 2001, 78, 841−843. (3) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; et al. A Strong Regioregularity Effect in Self-Organizing Conjugated Polymer Films and High-Efficiency Polythiophene:fullerene Solar Cells. Nat. Mater. 2006, 5, 197−203. (4) Zhang, J.; Zhu, L.; Wei, Z. Toward Over 15% Power Conversion Efficiency for Organic Solar Cells: Current Status and Perspectives. Small Methods 2017, 1, 1700258. (5) Hara, K.; Sayama, K.; Arakawa, H.; Ohga, Y.; Shinpo, A.; Suga, S. A Coumarin-Derivative Dye Sensitized Nanocrystalline TiO2 Solar Cell Having a High Solar-Energy Conversion Efficiency up to 5.6%. Chem. Commun. 2001, 569−570. (6) Ito, S.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Péchy, P.; Takata, M.; Miura, H.; et al. High-Efficiency Organic-Dye- Sensitized Solar Cells Controlled by Nanocrystalline-TiO2 Electrode Thickness. Adv. Mater. 2006, 18, 1202−1205. (7) Klfout, H.; Stewart, A.; Elkhalifa, M.; He, H. BODIPYs for DyeSensitized Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 39873− 39889. (8) Pati, P. B.; Damas, G.; Tian, L.; Fernandes, D. L. A.; Zhang, L.; Pehlivan, I. B.; Edvinsson, T.; Araujo, C. M.; Tian, H. An Experimental and Theoretical Study of an Efficient Polymer NanoPhotocatalyst for Hydrogen Evolution. Energy Environ. Sci. 2017, 10, 1372−1376. (9) Xu, Y.; Mao, N.; Zhang, C.; Wang, X.; Zeng, J.; Chen, Y.; Wang, F.; Jiang, J.-X. Rational design of donor-π-acceptor conjugated microporous polymers for photocatalytic hydrogen production. Appl. Catal., B 2018, 228, 1−9. (10) Woods, D. J.; Sprick, R. S.; Smith, C. L.; Cowan, A. J.; Cooper, A. I. A Solution-Processable Polymer Photocatalyst for Hydrogen Evolution from Water. Adv. Energy Mater. 2017, 7, 1700479. (11) Zhang, G.; Lan, Z.-A.; Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. Engl. 2016, 55, 15712−15727. (12) Liu, J.; Chen, S.; Qian, D.; Gautam, B.; Yang, G.; Zhao, J.; Bergqvist, J.; Zhang, F.; Ma, W.; Ade, H.; et al. Fast Charge Separation in a Non-Fullerene Organic Solar Cell with a Small Driving Force. Nat. Energy 2016, 1, 16089. (13) Bin, H.; Gao, L.; Zhang, Z.-G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, S.; Xue, L.; Yang, C.; Xiao, M.; et al. 11.4% Efficiency NonFullerene Polymer Solar Cells with Trialkylsilyl Substituted 2DConjugated Polymer as Donor. Nat. Commun. 2016, 7, 13651. (14) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganäs, O.; Gao, F.; Hou, J. Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734−4739. (15) Baran, D.; Kirchartz, T.; Wheeler, S.; Dimitrov, S.; Abdelsamie, M.; Gorman, J.; Ashraf, R. S.; Holliday, S.; Wadsworth, A.; Gasparini, N.; et al. Reduced Voltage Losses Yield 10% Efficient Fullerene Free Organic Solar Cells with >1 V Open Circuit Voltages. Energy Environ. Sci. 2016, 9, 3783−3793.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b09408. Benchmark for determination of frontier orbital energies for fluorene-based polymers that have been previously reported under 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 the 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 DOS on the acceptor unit upon the hydrogenation process; spatial distribution of the highest contributing orbitals involved in the two main absorption peaks; and torsional angles between different molecular groups in the polymer chain (PDF)

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.D.). *E-mail: [email protected] (C.F.N.M.). *E-mail: [email protected] (C.M.A.). Notes

The authors declare no competing financial interest.

■

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 26886


Article


ledione: toward subtractive color organic photovoltaics. Polym. Res. J. 2016, 49, 113−122. (35) Zhou, H.; Yang, L.; Price, S. C.; Knight, K. J.; You, W. Enhanced Photovoltaic Performance of Low-Bandgap Polymers with Deep LUMO Levels. Angew. Chem., Int. Ed. 2010, 49, 7992−7995. (36) Wang, L.; Cai, D.; Zheng, Q.; Tang, C.; Chen, S.-C.; Yin, Z. Low Band Gap Polymers Incorporating a Dicarboxylic Imide-Derived Acceptor Moiety for Efficient Polymer Solar Cells. ACS Macro Lett. 2013, 2, 605−608. (37) Kini, G. P.; Oh, S.; Abbas, Z.; Rasool, S.; Jahandar, M.; Song, C. E.; Lee, S. K.; Shin, W. S.; So, W.-W.; Lee, J.-C. Effects on Photovoltaic Performance of Dialkyloxy-Benzothiadiazole Copolymers by Varying the Thienoacene Donor. ACS Appl. Mater. Interfaces 2017, 9, 12617−12628. (38) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45, 723−733. (39) Steckler, T. T.; Henriksson, P.; Mollinger, S.; Lundin, A.; Salleo, A.; Andersson, M. R. Very Low Band Gap Thiadiazoloquinoxaline Donor-Acceptor Polymers as Multi-Tool Conjugated Polymers. J. Am. Chem. Soc. 2014, 136, 1190−1193. (40) Song, X.; Zhang, Y.; Li, Y.; Li, F.; Bao, X.; Ding, D.; Sun, M.; Yang, R. Fluorene Side-Chained Benzodithiophene Polymers for Low Energy Loss Solar Cells. Macromolecules 2017, 50, 6880−6887. (41) Schwab, M. G.; Hamburger, M.; Feng, X.; Shu, J.; Spiess, H. W.; Wang, X.; Antonietti, M.; Müllen, K. Photocatalytic Hydrogen Evolution through Fully Conjugated Poly(azomethine) Networks. Chem. Commun. 2010, 46, 8932. (42) Zhang, G.; Lan, Z.-A.; Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 55, 15712−15727. (43) Yanagida, S.; Kabumoto, A.; Mizumoto, K.; Pac, C.; Yoshino, K. Poly(p-phenylene)-catalysed photoreduction of water to hydrogen. J. Chem. Soc., Chem. Commun. 1985, 474−475. (44) Yang, C.; Ma, B. C.; Zhang, L.; Lin, S.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I.; Wang, X. Molecular Engineering of Conjugated Polybenzothiadiazoles for Enhanced Hydrogen Production by Photosynthesis. Angew. Chem., Int. Ed. 2016, 55, 9202−9206. (45) Wang, L.; Fernández-Terán, R.; Zhang, L.; Fernandes, D. L. A.; Tian, L.; Chen, H.; Tian, H. Organic Polymer Dots as Photocatalysts for Visible Light-Driven Hydrogen Generation. Angew. Chem., Int. Ed. 2016, 55, 12306−12310. (46) Bredas, J.-L. Mind the Gap! Mater. Horiz. 2014, 1, 17−19. (47) Pellegrin, Y.; Odobel, F. Sacrificial Electron Donor Reagents for Solar Fuel Production. C. R. Chim. 2017, 20, 283−295. (48) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. An Efficient Implementation of Time-Dependent Density-Functional Theory for the Calculation of Excitation Energies of Large Molecules. J. Chem. Phys. 1998, 109, 8218−8224. (49) Frisch, M. J.; et al. Gaussian 09, Revision D.01; Gaussian Inc., 2009. (50) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215−241. (51) Cohen, A. J.; Mori-Sanchez, P.; Yang, W. Insights into Current Limitations of Density Functional Theory. Science 2008, 321, 792− 794. (52) Cohen, A. J.; Mori-Sánchez, P.; Yang, W. Challenges for Density Functional Theory. Chem. Rev. 2012, 112, 289−320. (53) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis. http://www.sg-chem.net/, version 6.60, 2012. (54) Gorelsky, S. I.; Lever, A. B. P. Electronic Structure and Spectra of Ruthenium Diimine Complexes by Density Functional Theory and INDO/S. Comparison of the Two Methods. J. Organomet. Chem. 2001, 635, 187−196. (55) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999−3094.

(16) Li, X.; Liu, X.; Zhang, W.; Wang, H. Q.; Fang, J.; e. FullereneFree Organic Solar Cells with Efficiency over 12% Based on EDTAZnO Hybrid Cathode Interlayer. Chm. Mater. 2017, 29, 4176−4180. (17) Liu, X.; Li, X.; Li, Y.; Song, C.; Zhu, L.; Zhang, W.; Wang, H.Q.; Fang, J. High-Performance Polymer Solar Cells with PCE of 10.42% via Al-Doped ZnO Cathode Interlayer. Adv. Mater. 2016, 28, 7405−7412. (18) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148−7151. (19) Zhou, H.; Yang, L.; Stoneking, S.; You, W. A Weak Donor− Strong Acceptor Strategy to Design Ideal Polymers for Organic Solar Cells. ACS Appl. Mater. Interfaces 2010, 2, 1377−1383. (20) Zhou, H.; Yang, L.; You, W. Rational Design of High Performance Conjugated Polymers for Organic Solar Cells. Macromolecules 2012, 45, 607−632. (21) Holliday, S.; Li, Y.; Luscombe, C. K. Recent Advances in High Performance Donor-Acceptor Polymers for Organic Photovoltaics. Prog. Polym. Sci. 2017, 70, 34−51. (22) Hou, Q.; Xu, Y.; Yang, W.; Yuan, M.; Peng, J.; Cao, Y. Novel Red-Emitting Fluorene-Based Copolymers. J. Mater. Chem. 2002, 12, 2887−2892. (23) Jespersen, K. G.; Beenken, W. J. D.; Zaushitsyn, Y.; Yartsev, A.; Andersson, M.; Pullerits, T.; Sundström, V. The Electronic States of Polyfluorene Copolymers with Alternating Donor-Acceptor Units. J. Chem. Phys. 2004, 121, 12613−12617. (24) Zhang, S.; Guo, Y.; Fan, H.; Liu, Y.; Chen, H.-Y.; Yang, G.; Zhan, X.; Liu, Y.; Li, Y.; Yang, Y. Low bandgap π-conjugated copolymers based on fused thiophenes and benzothiadiazole: Synthesis and structure-property relationship study. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 5498−5508. (25) Gao, J.; Grey, J. K. Spectroscopic Studies of Energy Transfer in Fluorene Co-Polymer Blend Nanoparticles. Chem. Phys. Lett. 2012, 522, 86−91. (26) Gobalasingham, N. S.; Ekiz, S.; Pankow, R. M.; Livi, F.; Bundgaard, E.; Thompson, B. C. Carbazole-Based Copolymers via Direct Arylation Polymerization (DArP) for Suzuki-Convergent Polymer Solar Cell Performance. Polym. Chem. 2017, 8, 4393−4402. (27) Sharma, B.; Alam, F.; Dutta, V.; Jacob, J. Synthesis and Photovoltaic Studies on Novel Fluorene Based Cross-Conjugated Donor-Acceptor Type Polymers. Org. Electron. 2017, 40, 42−50. (28) Keshtov, M. L.; Khokhlov, A. R.; Kuklin, S. A.; Osipov, S. A.; Radychev, N. A.; Godovskiy, D. Y.; Konstantinov, I. O.; Sharma, G. D. Synthesis and Photovoltaic Properties Low Bandgap D-A Copolymers Based on Fluorinated Thiadiazoloquinoxaline. Org. Electron. 2017, 43, 268−276. (29) Guo, T.; Yu, L.; Zhao, B.; Ying, L.; Wu, H.; Yang, W.; Cao, Y. Blue Light-Emitting Hyperbranched Polymers Using Fluorene-CoDibenzothiophene-S,S-Dioxide as Branches. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1043−1051. (30) Li, J.; Grimsdale, A. C. Carbazole-Based Polymers for Organic Photovoltaic Devices. Chem. Soc. Rev. 2010, 39, 2399−2410. (31) Kim, J.; Kwon, Y. S.; Shin, W. S.; Moon, S.-J.; Park, T. Carbazole-Based Copolymers: Effects of Conjugation Breaks and Steric Hindrance. Macromolecules 2011, 44, 1909−1919. (32) Li, K.-C.; Huang, J.-H.; Hsu, Y.-C.; Huang, P.-J.; Chu, C.-W.; Lin, J.-T.; Wei, K.-H.; Lin, H.-C. Tunable Novel Cyclopentadithiophene-Based Copolymers Containing Various Numbers of Bithiazole and Thienyl Units for Organic Photovoltaic Cell Applications. Macromolecules 2009, 42, 3681−3693. (33) Moulé, A. J.; Tsami, A.; Bünnagel, T. W.; Forster, M.; Kronenberg, N. M.; Scharber, M.; Koppe, M.; Morana, M.; Brabec, C. J.; Meerholz, K.; et al. Two Novel Cyclopentadithiophene-Based Alternating Copolymers as Potential Donor Components for HighEfficiency Bulk-Heterojunction-Type Solar Cells. Chem. Mater. 2008, 20, 4045−4050. (34) Chang, S.-W.; Muto, T.; Kondo, T.; Liao, M.-J.; Horie, M. Double acceptor donor-acceptor alternating conjugated polymers containing cyclopentadithiophene, benzothiadiazole and thienopyrro26887


Article

The Journal of Physical Chemistry C (56) Levermore, P. A.; Jin, R.; Wang, X.; de Mello, J. C.; Bradley, D. D. C. Organic Light-Emitting Diodes Based on Poly(9,9-Dioctylfluorene-Co-Bithiophene) (F8T2). Adv. Funct. Mater. 2009, 19, 950− 957. (57) Yamanari, T.; Taima, T.; Sakai, J.; Saito, K. Origin of the OpenCircuit Voltage of Organic Thin-Film Solar Cells Based on Conjugated Polymers. Sol. Energy Mater. Sol. Cells 2009, 93, 759−761. (58) Kajii, H.; Koiwai, K.; Hirose, Y.; Ohmori, Y. Top-Gate-Type Ambipolar Organic Field-Effect Transistors with Indium-Tin Oxide Drain/source Electrodes Using Polyfluorene Derivatives. Org. Electron. 2010, 11, 509−513. (59) Sato, Y.; Kajii, H.; Ohmori, Y. Improved Performance of Polymer Photodetectors Using Indium-Tin-Oxide Modified by Phosphonic Acid-Based Self-Assembled Monolayer Treatment. Org. Electron. 2014, 15, 1753−1758. (60) Rodrigues, R. F.; Charas, A.; Morgado, J.; Maçanita, A. SelfOrganization and Excited-State Dynamics of a Fluorene−Bithiophene Copolymer (F8T2) in Solution. Macromolecules 2010, 43, 765−771. (61) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Mobility Enhancement in Conjugated Polymer Field-Effect Transistors through Chain Alignment in a Liquid-Crystalline Phase. Appl. Phys. Lett. 2000, 77, 406−408. (62) Kinder, L.; Kanicki, J.; Swensen, J.; Petroff, P. Structural Ordering in F8T2 Polyfluorene Thin-Film Transistors. Proc. SPIE 2003, 5217, 35−42. (63) Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chem. Rev. 1991, 91, 165−195. (64) Tam, T. L. D.; Ye, W.; Tan, H. H. R.; Zhou, F.; Su, H.; Mhaisalkar, S. G.; Grimsdale, A. C. Synthesis and Characterization of a Series of Annelated Benzotriazole Based Polymers with Variable Bandgap. J. Org. Chem. 2012, 77, 10035−10041. (65) Li, H.; Tam, T. L.; Lam, Y. M.; Mhaisalkar, S. G.; Grimsdale, A. C. Synthesis of Low Band Gap [1,2,5]-Thiadiazolo[3,4-g]quinoxaline and Pyrazino[2,3-g]quinoxaline Derivatives by Selective Reduction of Benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole. Org. Lett. 2011, 13, 46−49. (66) Ono, K.; Tanaka, S.; Yamashita, Y. Benzobis(thiadiazole)s Containing Hypervalent Sulfur Atoms: Novel Heterocycles with High Electron Affinity and Short Intermolecular Contacts between Heteroatoms. Angew. Chem., Int. Ed. Engl. 1994, 33, 1977−1979. (67) Strassner, T.; Fabian, J. DFT- and Post-HF-Study on Structure and Electronic Excitation of Acyclic and Cyclic Sulfur Diimides. J. Phys. Org. Chem. 1997, 10, 33−41. (68) Patel, G. D.; Feng, F.; Ohnishi, Y.-y.; Abboud, K. A.; Hirata, S.; Schanze, K. S.; Reynolds, J. R. It Takes More than an Imine: The Role of the Central Atom on the Electron-Accepting Ability of Benzotriazole and Benzothiadiazole Oligomers. J. Am. Chem. Soc. 2012, 134, 2599−2612. (69) Voiry, D.; Yamaguchi, H.; Li, J.; Silva, R.; Alves, D. C. B.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; et al. Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution. Nat. Mater. 2013, 12, 850−855. (70) Li, H.; Tsai, C.; Koh, A. L.; Cai, L.; Contryman, A. W.; Fragapane, A. H.; Zhao, J.; Han, H. S.; Manoharan, H. C.; AbildPedersen, F.; et al. Activating and optimizing MoS2 basal planes for hydrogen evolution through the formation of strained sulphur vacancies. Nat. Mater. 2016, 15, 48−53. (71) Greeley, J.; Jaramillo, T. F.; Bonde, J.; Chorkendorff, I.; Nørskov, J. K. Computational High-Throughput Screening of Electrocatalytic Materials for Hydrogen Evolution. Nat. Mater. 2006, 5, 909−913. (72) Inganäs, O.; Zhang, F.; Andersson, M. R. Alternating Polyfluorenes Collect Solar Light in Polymer Photovoltaics. Acc. Chem. Res. 2009, 42, 1731−1739. (73) Zhang, M.; Guo, X.; Li, Y. Photovoltaic Performance Improvement of D-A Copolymers Containing Bithiazole Acceptor Unit by Using Bithiophene Bridges. Macromolecules 2011, 44, 8798− 8804.

(74) Li, Z.; Huo, L.; Guo, X.; Yong, W.; Zhang, S.; Fan, H. Synthesis and photovoltaic properties of D-π-A copolymers based on thieno[3,2-b]thiophene bridge unit. Polymer 2013, 54, 6150−6157. (75) Cartwright, L.; Iraqi, A.; Zhang, Y.; Wang, T.; Lidzey, D. G. Impact of Fluorine Substitution upon the Photovoltaic Properties of Benzothiadiazole-Fluorene Alternate Copolymers. RSC Adv. 2015, 5, 46386−46394. (76) Kaya, E.; Apaydın, D. H.; Yıldız, D. E.; Toppare, L.; Cirpan, A. Solution Processable Benzotriazole and Fluorene Containing Copolymers for Photovoltaic Applications. Sol. Energy Mater. Sol. Cells 2012, 99, 321−326.

26888


On the Design of DonorâAcceptor Conjugated Polymers for

Recommend Documents

On the Design of DonorâAcceptor Conjugated Polymers for