Article pubs.acs.org/JPCA
Evaluation of Acceptor Strength in Thiophene Coupled Donor− Acceptor Chromophores for Optimal Design of Organic Photovoltaic Materials Muhammet E. Köse*,† Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, United States ABSTRACT: A series of thiophene coupled acceptors were systematically investigated at the density functional theory level to reveal structure−property relationships for building blocks of materials used in organic photovoltaic applications. All of the acceptor groups studied in this work retain their aromaticity when coupled to thiophene groups as estimated from their aromatic stabilization energies. However, pure chains of acceptors may adopt quinoidal geometry along the conjugated backbone depending on the structure of interest. Spearman rank order correlation has been used to assess the relationships between the computed variables such as highest occupied molecular orbital, lowest unoccupied molecular orbital, Eg, oscillator strength, exciton binding energy, aromatic stabilization energy, etc. The relative acceptor strengths were plotted and electrostatic potential maps were generated to examine the charge distribution over the chromophores. It has been found that there is no correlation between acceptor strength and electron withdrawing ability of the acceptor. Electron rich and highly electronegative atoms within acceptor groups mainly affect the charge distribution over the acceptor geometry. Exciton binding energy increases with the increasing aromatic character of the acceptor group. The acceptor strength is inversely correlated with the oscillator strength for the lowest excited state transition.
1. INTRODUCTION Organic solar cells employ low band gap donor−acceptor (D/ A) materials for better match of the absorption within the active layer with the solar spectrum. Although the D/A copolymer band gap should be small for efficient harvesting of photons in the near-infrared region of the spectrum, it is now well-known that high carrier mobility and bulk heterojunction morphology are other important aspects for improved photovoltaic activity. D/A copolymers and small molecules have shown impressive record-high power conversion efficiencies,1−3 which is promising for commercialization of organic photovoltaics (OPV) technology.4,5 In D/A conjugated materials, an electron rich donor is coupled with an electron deficient acceptor to form either the comonomer unit for an alternating copolymer or a small πconjugated molecule.3 The nature of coupling between the donor and the acceptor units determines energetic alignment of frontier orbitals, relative size of exciton, and intrinsic electronic and optical properties. There is a vast amount of literature on various types of donor and acceptor combinations that have been exploited for organic solar cell applications. Although such studies reveal some important structure−property relationships for optimum material properties in this field, a broad understanding of electronic and photophysical properties of these materials as a function of various acceptors is missing. This is because both experimental and theoretical works are usually conducted on few materials, and general comparisons © XXXX American Chemical Society
among different materials are, therefore, lacking. Theoretical studies on conjugated copolymers and small molecules have been proven to be useful in elucidating the electronic and optical properties of materials in organic photovoltaics applications.6−12 In this work, we perform a systematic theoretical investigation on a series of acceptors coupled via bis-thiophene linkages. Thiophene groups are selected for this work since thiophene based donors represent the most widely used donors in small band gap D/A copolymers. The theoretical studies were performed on 22 different acceptors (Scheme 1),10,13−26 which are widely used in the active layers of OPV devices. A number of themes were addressed, which are widely omitted in the literature. For instance, some acceptors are thought to adopt quinoidal geometry when used in the conjugated backbone of polymers.27 We highlight the fact that such geometrical features depend on the coupled units attached to the acceptor fragment. The amount of stabilization energy through either aromatic or quinoidal geometries were evaluated and compared for each acceptor. Our systematic investigation also led to evaluation of acceptor strength for all the structures studied in this work. The calculations also explore the impact of single atom substitution among various acceptors and their Received: October 8, 2012 Revised: December 4, 2012
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dx.doi.org/10.1021/jp309950f | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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Scheme 1. Chemical Structures of Acceptors and Relevant Structures Considered in This Worka
a The acceptor abbreviations stand for BT = benzo[c][1,2,5]thiadiazole, BTF = 5,6-difluorobenzo[c][1,2,5]thiadiazole, PT = [1,2,5]thiadiazolo[3,4c]pyridine, BSe = benzo[c][1,2,5]selenadiazole, PSe = [1,2,5]selenadiazolo[3,4-c]pyridine, BX = benzo[c][1,2,5]oxadiazole, PX = [1,2,5]oxadiazolo[3,4-c]pyridine, QX = quinoxaline, QXF = 6,7-difluoroquinoxaline, PP = pyrido[3,4-b]pyridine, TZ = 1,2,4,5-tetrazine, BA = 2-methyl-2Hbenzo[d][1,2,3]triazole, BAF = 5,6-difluoro-2-methyl-2H-benzo[d][1,2,3]triazole, PHT = 2-methylisoindoline-1,3-dione, B2T = benzo[1,2-c:4,5c′]bis[1,2,5]thiadiazole, TH = thiophene, TP = thieno[3,4-b]pyrazine, TB = benzo[c]thiophene, TT = thieno[3,4-b]thiophene, TTO = methyl thieno[3,4-b]thiophene-2-carboxylate, TTF = methyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate, TPD = 5-methyl-4H-thieno[3,4-c]pyrrole4,6(5H)-dione, and DPP = 2,5-dimethylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione.
Table 1. Computed Aromatic Stabilization Energy (ASE), HOMO Level, LUMO Level, HOMO−LUMO Gap, S0 → S1 Transition Energy (Eg), Exciton Binding Energy (Eb), Oscillator Strength of S0 → S1 Transition ( f), Total Coulombic Charge on Acceptor Moiety (Charge on A), and Inter-Ring C−C Single Bond Length between the Acceptor Group and Thiophene in TH-A-TH Structure acceptor
ASE (kcal·mol−1)
HOMO (eV)
LUMO (eV)
H−L gap (eV)
Eg (eV)
Eb (eV)
f (au)
charge on A (e)
C−C bond length (Å)
BT BTF PT BSe PSe BX PX QX QXF PP TZ BA BAF PHT B2T TH TP TB TT TTO TTF TPD DPP
−8.17 −9.96 −9.45 −8.16 −9.36 −8.66 −10.35 −4.67 −5.96 −4.94 −6.46 −8.69 −10.34 −8.68 −10.54 −11.18 −14.08 −13.11 −12.95 −12.55 −13.65 −14.01 −19.57
−5.36 −5.54 −5.53 −5.29 −5.46 −5.52 −5.71 −5.24 −5.45 −5.39 −6.13 −5.13 −5.33 −5.79 −5.09 −5.14 −5.00 −4.75 −4.90 −5.12 −5.17 −5.59 −4.97
−2.61 −2.73 −2.92 −2.68 −2.99 −2.70 −3.01 −2.33 −2.47 −2.60 −2.66 −1.92 −2.06 −2.21 −3.52 −1.69 −2.62 −2.16 −1.92 −2.42 −2.46 −2.19 −2.51
2.74 2.80 2.61 2.61 2.47 2.83 2.71 2.91 2.98 2.80 3.47 3.21 3.27 3.58 1.56 3.45 2.38 2.59 2.98 2.70 2.71 3.40 2.45
2.39 2.44 2.26 2.23 2.10 2.56 2.44 2.53 2.59 2.43 2.92 3.02 3.08 3.02 1.41 3.27 2.08 2.49 2.86 2.50 2.52 3.15 2.41
0.35 0.37 0.35 0.38 0.37 0.27 0.27 0.38 0.39 0.37 0.55 0.19 0.19 0.56 0.15 0.18 0.30 0.10 0.12 0.20 0.20 0.26 0.04
0.278 0.266 0.266 0.226 0.220 0.429 0.398 0.282 0.279 0.274 0.007 0.668 0.683 0.033 0.201 0.814 0.242 0.494 0.632 0.360 0.378 0.568 0.476
−0.014 −0.244 −0.057 a a −0.144 −0.118 −0.174 −0.182 −0.095 +0.420 −0.206 −0.372 +0.030 −0.084 −0.058 −0.170 −0.416 −0.176 −0.180 −0.143 −0.324 −0.216
1.4585 1.4571 1.4562 1.4593 1.4572 1.4541 1.4524 1.4651 1.4634 1.4626 1.4496 1.4573 1.4557 1.4706 1.4478 1.4466 1.4389 1.4466 1.4436 1.4425 1.4474 1.4437 1.4408
a
HF calculation for BSe and PSe incorporated chromophores failed due to the presence of Se atoms.
B
dx.doi.org/10.1021/jp309950f | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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Figure 1. Variation of frontier orbital energies in TH-A-TH structures calculated with B3LYP/6-31G(d) method.
aromatic character. In general, thiophene-based acceptors display larger aromatic character than benzene based acceptors. This is rather counterintuitive since benzene by itself is a highly aromatic molecule. The ASE results for H-A-A-H chromophores, however, show an entirely different picture. For instance, TB behaves as a highly aromatic group (−13.11 kcal·mol−1) when coupled with thiophenes on each side. A homopolymer of TB though would adopt quinoidal geometry with a stabilization energy of 4.44 kcal·mol−1. A similar picture is valid as well for TP (3.79 kcal·mol−1). Nonetheless, BT (−6.69 kcal·mol−1), QX (−10.66 kcal·mol−1), and TTO (−0.93 kcal·mol−1) would still prefer aromatic backbone in pure acceptor chains. Thus, it is rather important to consider the nature of adjacent groups attached to the acceptor unit in determining structural features of conjugated backbone. ASE calculations are also important to differentiate the mechanism of band gap reduction between coupled comonomers in low band gap material synthesis. 3.2. Acceptor Strength. We correlate the LUMO energy of TH-A-TH groups with the acceptor strength of A when coupled to thiophene units. Figure 1 shows the variation of LUMO levels of TH-A-TH in descending order from left to right. Highly fused heterocycle B2T is the strongest acceptor among all the structures studied in Scheme 1. TT by itself is a weak acceptor, yet attached ester groups (TTO) and fluorination (TTF) further increases acceptor strength. Fluorinated monomers possess low lying LUMO levels compared to nonfluorinated analogues (compare BA vs BAF, QX vs QXF, and BT vs BTF). It is also important to note that pyridine derivatives (PP, PT, PSe, and PX) are stronger acceptors than their benzene counterparts (QX, BT, BSe, and BX). This could be due to an electron withdrawing effect of nitrogen atoms in pyridine moiety.11 In BT, BX, and BSe acceptors, the only difference lies in the type of heteroatom used in the fused ring. BSe has more stable LUMO than BT, suggesting an improved resonance effect of Se in the conjugated system since both atoms have similar electronegativity (S = 2.58; Se = 2.55). BX, however, has the most stable LUMO compared to those of BSe and BT, possibly due to strong inductive electron-withdrawing effect of oxygen atom. The LUMO of TP is lower than QX, suggesting better coupling of the thiophene based acceptor with adjacent thiophene units. Nonetheless, a reverse trend is evident when one compares the frontier orbital data for TH-TPD-TH and TH-PHT-TH chromophores. Increasing acceptor strength causes localization of LUMO on the accepting moiety in TH-A-TH chromophore despite its small size. Figure 2 shows frontier orbitals of selected chromophores. It is apparent that, as the acceptor strength increases, the LUMO localizes on the acceptor group with
effects on the electronic properties. Since the calculations have been performed on a large set of acceptors, the statistical analysis of generated data allowed important correlations to be found among different variables. There is a notion in the literature relating the electron withdrawing ability of the acceptor with the acceptor strength of the monomer construct.28−30 The current study, however, presents contrasting evidence by showing the absence of correlation with the acceptor strength and the total Coulombic charge on acceptor fragment.
2. THEORETICAL METHODS Optimized geometries of all structures were obtained by using the B3LYP hybrid functional with the 6-31G(d) basis set in density functional theory (DFT). Time-dependent DFT (TDDFT) calculations were also performed with the same method and basis set. The lowest excited state transition is usually dominated by highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) transitions. Thus, it was possible to estimate exciton binding energy,9 which is the difference between HOMO−LUMO energy gap and excitation energy from TD-DFT calculations. Aromatic stabilization energy (ASE) of each acceptor was calculated by considering the total energies of the related structures as shown in Scheme 1. In order to generate an electrostatic potential map, an SCF calculation has been performed with Hartree− Fock method along with the 6-31G(d) basis set on DFT optimized geometries. The atomic charges were derived from the fits to the HF electrostatic potential by the method of Chirlian and Francl.31 All calculations were performed with Gaussian09 software packages.32 3. RESULTS AND DISCUSSION 3.1. Aromatic vs Quinoid Acceptors. The acceptors coupled with thiophene groups (TH-A-TH) display structural parameters in favor of aromatic geometry along the backbone. However, the most conclusive result can be drawn from stabilization energies when the acceptor unit is forced to adapt either aromatic or quinoid geometry as shown in Scheme 1. The aromatic stabilization energies (ASE) of the thiophene coupled chromophores were evaluated from total energies of the relevant geometries shown at the bottom of Scheme 1. The results are listed in Table 1. ASE for thiophene (−11.18 kcal·mol−1) has also been calculated as reference to the acceptor groups studied in this work. Similar calculations were also performed for chromophores incorporating pure acceptor units. It is clear that all acceptors favor aromaticity to varying degrees of magnitude when coupled with thiophene units. The largest ASE is calculated for structures such as QX and PP, whereas DPP has the lowest ASE value and hence highest C
dx.doi.org/10.1021/jp309950f | J. Phys. Chem. A XXXX, XXX, XXX−XXX
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Figure 3. Electrostatic potential map (isovalue surface, 0.05 au) of selected TH-A-TH chromophores.
regions. EPM for TB is not very different than that of TH. EPM for BT incorporated chromophore has significant color variations especially on the acceptor group. Highly electronegative nitrogen atoms withdraw significant electron density from the sulfur atom and neighboring carbon atoms. Interestingly, there is little change in charge distribution on attached thiophene groups. This is a general observation for all the acceptors studied in this work. Electron-rich and -poor atoms cause charge redistribution within the acceptor unit rather than affecting the electron density at nearby thiophene groups. The addition of fluorine atoms in TH-BTF-TH chromophore further decreases the electron density on the benzene unit compared to that on TH-BT-TH. Pyridine derivative (TH-PT-TH) causes nonsymmetric charge distribution over the chromophore. Other chromophores shown in Figure 3 have electronegative atoms N, O, and F with relatively lower electrostatic potential. N atoms abstract considerable electron density when connected directly to S atoms. Although F atoms are more electronegative than O or N atoms, the inductive effect induced by fluorine atoms on charge distribution is less pronounced compared to the resonance effect of O and N atoms.
Figure 2. HOMO/LUMO plots (isovalue surface, 0.03 au) of selected chromophores with increasing acceptor strength from top to bottom.
fewer molecular orbital coefficients on thiophene units. This is, however, not observed for HOMO plots. The localization of wave function on acceptor has implications on the magnitude of oscillator strengths for the transitions between frontier orbitals, which will be discussed below. To further understand the acceptor properties of the structures in Scheme 1, electrostatic potential maps (EPM) were plotted to reveal charge distribution in TH-A-TH chromophores (Figure 3). The blue color shows the areas of high potential, which is characterized by the absence of electrons. The red color depicts the areas of low potential and characterized by an abundance of electrons. Intermediary colors represent intermediary electrostatic potentials. EPM for TH shows relatively homogeneous color distribution with discernible positive charges on hydrogen atoms. The CC double bond regions are highly electron rich since yellow color is salient in those regions. The positions of electron rich sulfur atoms are also evident with a faint yellow color around those D
dx.doi.org/10.1021/jp309950f | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
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Table 2. Spearman Rank Order Correlation Analyses of Columnar Data Presented in Table 1; the Upper Number in Each Cell Corresponds to Correlation Coefficient, whereas the Bottom Number Shows the Magnitude of p-Value ASE
HOMO
LUMO
H−L gap
Eg
Eb
f
charge on A
C−C bond length
−0.487 0.019
−0.206 0.342 0.387 0.067
0.341 0.110 −0.459 0.027 0.563 5.3 × 10−3
0.035 0.873 −0.205 0.345 0.747
