Density Functional Theory Study Predicts Low Reorganization

Letter pubs.acs.org/JPCL

Density Functional Theory Study Predicts Low Reorganization Energies for Azadipyrromethene-Based Metal Complexes Wasana Senevirathna, Cassie M. Daddario, and Geneviève Sauvé* Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States S Supporting Information *

ABSTRACT: Small internal reorganization energy is desirable for high-performance optoelectronic materials, as it facilitates both charge separation and charge transport. However, only a handful of n-type electron accepting materials are known to have small reorganization energies. Here, DFT calculations were performed to predict the reorganization energy of azadipyrromethene-based dyes and their complexes. All compounds studied were most stable in their anionic state and had high electron affinity, indicating their potential as ntype material. The homoleptic zinc(II) complexes had significantly lower reorganization energies than either the free ligands or the BF2+ chelates. The low reorganization energies of the zinc(II) complexes are explained by the large and rigid π conjugated system that extends across the two azadipyrromethene ligands via interligand π−π interactions. This work suggests that Zn(II) complexation is a novel strategy for obtaining materials that combine low internal reorganization energy with high electron affinity for the development of novel n-type optoelectronic materials. SECTION: Molecular Structure, Quantum Chemistry, and General Theory

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artificial photosynthetic systems,18−20 and are critical for the development of high efficiency OPV.21 Internal reorganization energy has been shown to decrease as the size of the conjugated portion of the molecule increases.10,12,22,23 The criteria for small internal reorganization energy include extended πconjugation, rigidity, and nonbonding character in frontier orbitals.11,12 While most organic semiconductors have internal reorganization energies greater than 0.1 eV, several p-type organic semiconductors have been reported with internal reorganization energies (λ+) of less than 0.1 eV, including phthalocyanines (0.036 to 0.098 eV),24,25 pentacene (0.093−0.098 eV),17,26 and cicum(oligo)acenes (0.057 to 0.127 eV).27 We are aware of only a few n-type acceptors with internal reorganization energies (λ−) for electron transfer of less than 0.1 eV: fullerene C60 (0.060 eV),22 octacyanopentacene (0.095 eV),28,29 and polymers of phenylethynyl-1,3,4-thiadiazole (0.07 eV).30 The high mobility n-type polymer NDI2OD-T2 has an estimated λ− of 0.27 to 0.3 eV.31 The smaller number of low λ− materials known may be due to less research with n-type organic semiconductors. It is also possible that greater structural changes can occur when a charge is added to mostly antibonding in character LUMOs (λ−) than when a charge is removed from mostly bonding in character HOMOs (λ+), because structural adjustments with charge transfer can correlate with the bonding character of the frontier molecular orbitals.11 Interestingly, Chao and co-workers demonstrated

nterest in developing n-type electron accepting organic semiconductors continues to grow as these materials are critical for several opto-electronic applications, including organic photovoltaics (OPV), organic field-effect transistors (OFET), and organic light-emitting diodes (OLED).1 The most studied n-type materials include small molecules such as rylene diimides, fullerenes, acenes, siloles, and fluorinated phthalocyanines and subphthalocyanines, diketopyrrolopyrroles, vinazene-based molecules, and 9,9′-bifluorenylidene,1−5 as well as conjugated polymers that include electron-withdrawing groups such as imides, fluorines, cyanos, or benzothiadiazoles.6−8 All are highly conjugated systems with high electron affinity. At the heart of n-type material’s function is intermolecular charge transfer: either to separate charges by accepting an electron or to transport electrons, such as through a hopping mechanism: M− + M → M + M−. According to Marcus theory, one important factor that affects the rate of charge transfer is the reorganization energy.9 Reorganization energy is the energy required for structural changes associated with the charge transfer of the molecules of interest (internal, inner-sphere, vibrational, or structural reorganization) and surrounding molecules (external or outer-sphere reorganization). In the solid state, the relevant state for devices, the external reorganization energy can be ignored because the environment is relatively stiff, and the internal reorganization energy of isolated molecules can be used as an upper bound for the charge transport energetics.10 Low internal reorganization energies of isolated molecules have been associated with higher solid-state charge carrier mobility (when combined with large electronic coupling),11−17 used for better charge separation in © 2014 American Chemical Society

Received: December 19, 2013 Accepted: February 20, 2014 Published: February 25, 2014 935

dx.doi.org/10.1021/jz402735c | J. Phys. Chem. Lett. 2014, 5, 935−941

The Journal of Physical Chemistry Letters

Letter

properties were demonstrated by fluorescence quenching experiments. DFT calculations indicated that these complexes have a similar distorted tetrahedral geometry than M(ADP)2, but with additional conjugated ‘arms’ extending in three dimensions. In addition, the frontier molecular orbitals are delocalized over the entire 3D complex. These features are expected to be highly beneficial not only for electron transfer processes, but also for charge transport in 3D.46 Here, we report the calculated internal reorganization energies of a series of azadipyrromethene-based ligands and their complexes with BF2+ and metal(II) (Co, Ni, and Zn), both for hole and electron transfer. To further evaluate these molecules, we computed and compared the adiabatic electron affinities and the ionization potential energies. The structural variations examined allowed us to better understand the factors influencing the internal reorganization energy for these types of compounds. We found that complexation with Zn(II) or Co(II) produced the largest decrease in internal reorganization energy, pointing to metal complexation as a strategy to obtain low internal reorganization energies for the development of ntype materials. The chemical structures of the azadipyrromethene ligands studied are presented in Chart 1. We investigated the effect of extending the conjugation through the pyrrolic positions (2−8) and through the distal and proximal phenyl groups (9−11). For the series with substitution at the pyrrolic positions, we looked at the effect of various electron donating and electron withdrawing groups on the phenylethynyl group (2−5), as well as varying the nature of the conjugated pyrrolic substituent with ethynylthiophene (6), phenylenevinylene (7), and thiophene (8). To investigate the effect of modifying the phenyl groups, we installed ethynylthiophene groups at the distal phenyls (9), the proximal phenyls (10), and both distal and proximal phenyls (11). Calculations were performed for pristine free ligands (L), as well as ligands coordinated with BF2+ and Zn(II). Coordination with the Zn(II) gives M(L)2 complexes with distorted tetrahedral geometry. We chose Zn(II) as the metal center of choice for comparing all the ligands because the closed-shell configuration of Zn(II) allowed for faster and simpler optimizations. To look at the effect of using other metal(II) centers, we calculated the Co(II) and Ni(II) complexes of 1 and 2. The total internal reorganization energy can be described as the vertical ionization of a neutral molecule followed by geometric relaxation, and then vertical neutralization of a charged molecules followed by geometric relaxation. Accordingly, the internal reorganization energies can be calculated using the following equations (Nelsen’s four-point method):29,47,48

that by increasing the nonbonding character of the frontier molecular orbitals, one can obtain extremely low internal reorganization energies. They used the phenalenyl radical as an example, with calculated λ+ and λ− of 0.019 and 0.035 eV, respectively. The low λ− for octacyanopentacene is ascribed to the nonbonding character contribution of the cyano groups to the LUMO.28 On the other hand, the low λ− of fullerene is explained by its rigidity and large π system that can delocalize electrons over a three-dimensional (3D) framework.20,32 In our search to find alternative n-type materials, particularly as non-fullerene electron acceptors for organic photovoltaic applications, we looked into azadipyrromethene-based dyes and their complexes, due to their high electron affinity and their intense and tunable absorption in the red to near-infrared region of the solar spectra.33−37 Figure 1a shows the chemical

Figure 1. (a) Structures of 1,3,7,9-tetraphenylazadipyrromethene and its complexes; (b) chemical structure of homoleptic metal(II) complexes of di(phenylethynyl) azadipyrromethene, and (c) calculated optimized geometry showing its three-dimensionality and one of the π-stacking distances between the two ligands.46

structure of 1,3,7,9-tetraphenylazadipyrromethene H(ADP). Free H(ADP) has a strong absorption at 600 nm, and its optical properties can be tuned by using a variety of substitutions on the phenyl groups and at the pyrrolic positions, and by chelation with boron or transition metals. Although they have mostly been considered for photodynamic therapy and biological imaging,38−41 azadipyrromethene derivatives were also tested as donors in organic solar cells.42,43 The electron accepting properties of BF2(ADP) was demonstrated in triads, where the azadipyrromethene dye was covalently bonded to electron donors such as zinc porphyrin and zinc phthalocyanine.44,45 We have recently shown that H(ADP) and their complexes have strong accepting properties by fluorescence quenching experiments using poly(3-hexylthiophene) as the donor.46 To enhance H(ADP)’s electron-accepting properties and red-shift the absorption spectra, we recently introduced homoleptic metal(II) complexes of di(phenylethynyl) azadipyrromethene, depicted in Figure 1b.46 These complexes have broad and intense absorption from 600 to 800 nm, and display two reversible reductions and two reversible oxidations, as determined by cyclic voltammetry. Their strong accepting

λ+ = λ1 + λ 2 = E0(Q +) − E0(Q 0) + E+(Q 0) − E+(Q +)

λ− = λ3 + λ4 = E0(Q −) − E0(Q 0) + E−(Q 0) − E−(Q −)

where E is energy, Q is geometry, and the subscripts 0, +, and − denote neutral, cationic, and anionic states, respectively. These variables are clearly depicted in Figure 2. The various energies were calculated by density-functional theory (DFT) using the PBEPBE functional and 6-31G(d,p) basis sets. This level of theory predicted the crystal structure of the Zn(ADP)2 complex accurately.46 Details of the calculations are given in the Supporting Information. The Supporting Information also includes the calculated HOMO, LUMO, and HOMO−LUMO 936



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Chart 1. Chemical Structures of All Ligands Studied

Figure 2. Internal reorganization energy for (a) hole transfer, λ+ = λ1 + λ2 (b) electron transfer, λ− = λ3 + λ4. E is energy, Q is geometry, and the subscripts 0, +, and − denote neutral, cationic, and anionic states, respectively. IP is ionization potential, EA is electron affinity, and the subscript v denotes vertical.

To examine the effect of various substitutions on the total internal reorganization for electron transfer, λ− is graphed for each compound in Figure 3. Generally, λ− was the highest for the free ligand. Coordination with BF2+ lowered λ−, as expected, because BF2+ coordination restricts rotation of the azadipyrromethene core,40 thus making the molecule more rigid and planar. In general, the largest decrease in λ− was obtained upon coordination with Zn(II). The smallest λ− values were 0.068 eV for 5c and 6c, followed by 0.070 eV for 2c and 3c. For comparison, we obtained a higher λ− value (0.113 eV) for the most common electron acceptor used in OPV, phenyl-C61-butyric acid methyl ester (PCBM).28 These results can be explained by the relative size of the π-conjugated systems: the larger the π-conjugated system, the smaller the λ−. In the Zn(II) complexes, the frontier molecular orbitals are delocalized over the two ligands.46 The Zn(II) center does not appear to participate in the frontier molecular orbitals; it holds the two azadipyrromethene ligands together and increases rigidity and planarity by restricting rotation of the azadipyrromethene core. Evidence for interligand π−π interactions comes

gaps for all molecules, along with a discussion of the HOMO− LUMO gap trends for the Zn(II) complexes. Table 1 reports the computed internal reorganization energies, the ionization potential (IP), and the electron affinity (EA). IP and EA were calculated as E0(Q0) − E+(Q+) and E0(Q0) − E−(Q−), respectively.28 For all the azadipyrromethene-based molecules studied, the cationic state potential energy was higher than the neutral state potential energy, giving a positive IP, and the anionic state potential energy was always lower than the neutral state energy, giving a negative EA. In other words, the lowest potential energy was obtained for the anionic state. This is in contrast to thiophene-based p-type conjugated systems where the potential energy of the cationic state is lower than that of the neutral or anionic states.10 Azadipyrromethene-based compounds therefore appear to be most stable in the anionic form, consistent with our hypothesis that azadipyrromethene-based compounds should be excellent electron acceptors.46 On the basis of this observation, we will focus our discussion on electron transfer. 937



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Table 1. Summary of Internal Reorganization Energies, Ionization Potential, and Electron Affinities Calculated for Gas Phase Molecules energy (in gas phase), eV hole transfer 1a 1b 1c 1d 1e 2a 2b 2c 2d 2e 3a 3b 3c 4a 4b 4c 5a 5b 5c 6a 6b 6c 7a 7b 7c 8a 8b 8c 9a 9b 9c 10a 10b 10c 11a 11b 11c PCBM a

electron transfer

λ+

IP

λ−

EA

0.152 0.155 0.081 0.085 0.172 0.103 0.103 0.050 0.080 0.060 0.109 0.108 0.052 0.137 0.135 0.074 0.081 0.085 0.039 0.122 0.117 0.058 0.138 0.143 0.064 0.282 0.278 0.326 0.121 0.124 -a 0.116 0.106 0.080 0.093 0.095 0.068 -a

+6.12 +6.35 +5.78 +5.52 +5.64 +5.89 +6.02 +5.55 +5.40 +5.50 +5.76 +5.89 +5.42 +5.55 +5.66 +5.18 +6.28 +6.41 +6.11 +5.63 +5.74 +5.28 +5.71 +5.85 +5.37 +5.80 +5.93 +5.44 +5.64 +5.80 -a +5.62 +5.71 +5.33 +5.38 +5.47 +5.20 -a

0.168 0.144 0.082 0.072 0.210 0.163 0.127 0.070 0.069 0.170 0.161 0.126 0.070 0.171 0.136 0.081 0.149 0.124 0.068 0.151 0.119 0.068 0.189 0.160 0.105 0.185 0.152 0.092 0.143 0.119 0.071 0.111 0.097 0.197 0.104 0.086 0.071 0.116

−1.79 −2.09 −2.01 −2.12 −2.26 −2.13 −2.34 −2.31 −2.41 −2.53 −2.08 −2.29 −2.24 −2.00 −2.22 −2.14 −2.55 −2.77 −2.89 −2.09 −2.29 −2.23 −1.95 −2.22 −2.17 −1.84 −2.10 −1.99 −2.07 −2.32 −2.28 −2.11 −2.31 −2.38 −2.29 −2.48 −2.43 −2.23

Figure 3. Comparison of the internal reorganization energy for electron transfer, λ−, for all molecules studied (a = free ligand L; b = BF2(L); c = Zn(L)2).

therefore have λ− values on par with or lower than other n-type organic compounds. The effect of various ligand substitutions on reorganization energy followed similar trends for L, BF2(L), and Zn(L)2, with the exception of 10c, which is discussed later. To simplify the discussion, we now focus on the trends for the Zn(L)2 series. The complex with unmodified azadipyrromethene, Zn(ADP)2 (1c), had a λ− of 0.082 eV. Extending the conjugation of azadipyrromethene ligands by adding phenylethynyl groups at the pyrrolic positions resulted in a modest decrease of λ− to 0.070 eV for 2c. The addition of an electron-donating methoxy group on the phenylethynyls increased λ− from 0.070 eV for 2c to 0.081 eV for 4c. Introducing electron withdrawing nitrile group (5c) on the phenylethynyls slightly decreased the λ− to 0.068 eV. Using phenylvinylene or thiophene as pyrrolic substitutents increased λ− to 0.105 and 0.092 eV for 7c and 8c, respectively. Ethynyl groups are therefore desirable when optimizing for the lowest λ−. The complex with ethynylthiophene groups on the distal phenyl (9c) and both distal and proximal phenyls (11c) had a λ− similar to that of the complex with ethynylthiophene groups at the pyrrolic positions (6c), at 0.072 and 0.071 eV for 9c and 11c, respectively. However, installing the ethynylthiophene groups in the proximal phenyls significantly increased λ− to 0.198 eV for 10c. Interestingly, the λ+ for hole transfer was unaffected, being 0.080 eV for 10c versus 0.081 eV for 1c. It is not clear why λ− of 10c was so large compared to 11c and to other zinc(II) complexes. We note that for the free ligands, adding ethynylthiophene groups on the proximal phenyl groups resulted in the lowest reorganization energies, indicating effective extension of conjugation. Comparing the nature of the metal center, Zn(II) and Co(II) gave low λ− values ∼0.07 eV for 2c and 2d, whereas using Ni(II) gave a higher λ− of 0.170 eV for 2e. The same trend was observed for 1c−e complexes. A possible explanation for this trend is that, unlike Zn(II) and Co(II), four-coordinate Ni(II) complexes (with covalently binding ligands such as azadipyrromethenes) preferably adopt a square planar geometry,52,53 but the Ni(II) complexes of azadipyrromethenes are ‘forced’ into a distorted tetrahedral geometry due to ligand−ligand interactions. Figure 4 plots the calculated LUMO energy levels versus electron affinity for all compounds studied. The data separates three groups based on the coordination of the ligands: the free ligands (green squares), the BF2+ complexes (red circles) and

Calculation failed, even after several attempts.

from analysis of the crystal structure of Zn(ADP)2,34 and from analysis of our optimized geometries for all Zn(L)2 complexes, where each of the proximal phenyl rings lay at a distance of 3.6 Å from the pyrrole group of the other azadipyrromethene core, a distance that is optimal for π-interactions.49,50 The presence of four interligand π-interactions is also expected to increase the rigidity of the complex. By forming homoleptic Zn(II) complexes, the size of the conjugation system therefore essentially doubles compared to the free ligand, and λ− roughly decreases by one-half (except for 10c and 11c). This is consistent with the λ ∼ 1/n relation derived for linear oligomers, where n is the number of heterocyclic rings.23 It is also reasonable that our metal(II) complexes have similar or lower λ− than PCBM, because they have a larger number of atoms involved in the π-conjugated system.46,51 Homoleptic metal(II) complexes of azadipyrromethene-based compounds 938



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EA and λ− can be further optimized by extending conjugation either through substitution at the pyrrolic positions or at the pdistal phenyl positions, as well as by installing electron withdrawing groups such as cyano. We also find that using ethynyl groups are more effective at lowering the internal reorganization energy than using vinylene groups, even though the latter better enhances conjugation. This work suggests that complexation with Zn(II) (or Co(II)) is a strategy for obtaining 3D materials that combine low internal reorganization energy with high electron affinity for the development of novel n-type optoelectronic materials. We have tested some of these complexes in organic solar cells, and 3c blended with the common electron donor poly(3-hexylthiophene) gave a high power conversion efficiency of 4.1%, among the highest efficiencies for fullerene-free organic solar cells reported to date. These results will be published separately.

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Figure 4. Plot of LUMO energy levels versus electron affinity for all compounds studied (a = free ligand L; b = BF2(L); c = Zn(L)2).

ASSOCIATED CONTENT

S Supporting Information *

the Zn(II) complexes (blue triangles). Since we are comparing different types of substitutions and coordinations, there is no perfect linear relationship between LUMO and EA, but there is a trend: LUMO energy levels generally decreased with increased EA. Molecules 5a−c have the cyano electron withdrawing groups that significantly lowered the LUMO energy level and increased the EA. These calculations indicate that both LUMO and EA can be independently tuned to some extent. Figure 5 illustrates the relationship between electron affinity and λ−. Generally, the free ligands had lower electron affinity

Computational details, HOMO, LUMO and HOMO−LUMO energy gaps, comparison between B3LYP functional and λ1, λ2, λ3, and λ4, and example of bond-length-alteration calculation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Science Foundation (CHEM 1148652) for funding this project and the Case High Performance Computing Cluster for computing time. We are also grateful to Prof. Thomas Gray, Prof. Alfred B. Anderson, and Roshan Fernando for fruitful discussions.

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Figure 5. Plot of electron affinity (EA) versus internal reorganization energy for electron transfer λ− for all compounds studied (a = free ligand L; b = BF2(L); c = Zn(L)2).

and higher reorganization energy, whereas BF2+ complexes had high electron affinity and intermediate reorganization energies. With the exception of 10c, the Zn complexes had both high electron affinity and low reorganization energy, two important criteria for good n-type materials.11 The complex 5c (with 4cyanophenylethynyl pyrrolic substituents) had the best combination of high electron affinity and low reorganization energy. In conclusion, we have shown that homoleptic Zn(II) (and Co(II)) complexes of azadipyrromethenes are promising candidates as n-type materials and electron acceptors. They have high electron affinity and low reorganization energies for electron transfer, which result from a large conjugated system. 939



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Density Functional Theory Study Predicts Low Reorganization

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