Tuning Near-Infrared Absorbing Donor Materials: A Study of Electronic

Article pubs.acs.org/cm

Tuning Near-Infrared Absorbing Donor Materials: A Study of Electronic, Optical, and Charge-Transport Properties of aza-BODIPYs Karl Sebastian Schellhammer,†,‡,§ Tian-Yi Li,∥ Olaf Zeika,∥ Christian Körner,∥ Karl Leo,§,∥ Frank Ortmann,*,†,‡,§ and Gianaurelio Cuniberti†,‡,§ †

Institute for Materials Science and Max Bergmann Center of Biomaterials, Technische Universität Dresden, 01062 Dresden, Germany ‡ Dresden Center for Computational Materials Science, Technische Universität Dresden, 01062 Dresden, Germany § Center for Advancing Electronics Dresden, Technische Universität Dresden, 01062 Dresden, Germany ∥ Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden, Germany S Supporting Information *

ABSTRACT: The class of 4,4′-difluoro-4-bora-3a,4a,8-triaza-s-indacenes (aza-BODIPYs) are promising near-infrared absorber materials which are successfully used in organic solar cells to extend their absorption to the near-infrared regime. We computationally studied electronic properties, internal reorganization energies, and the optical properties of more than 100 promising candidates and derived design principles, including novel functionalization routes, to improve their performance as donor materials. We synthesized and characterized several of the promising molecules, confirming the predicted trends. The best charge transport properties and absorption characteristics are obtained for naphthalene-annulated molecular cores due to optimally delocalized frontier molecular orbitals. Further optimization can be achieved by α-functionalization with fluorinated groups, β-functionalization with accepting substituents, and modification of the borondifluoride group. For such molecules, we predict a bathochromic shift in the absorption, which should not significantly reduce the open-circuit voltage. Torsional restriction of α-substituents by carbon bridges can further improve both charge transport and absorption. The theoretically and experimentally observed independence of most of the functionalization strategies makes BODIPYs an ideal material class for tailor-made absorber materials that can cover a broad range of absorption, charge transport, and energetic regimes, calling for further exploration in organic solar cell applications, fluorescence microscopy, and photodynamic therapy.

1. INTRODUCTION In recent years, organic solar cells (OSCs) based on complementary absorber materials, e.g. tandem OSCs, have attracted growing interest from both academia and industry as they provide flexible, large-area devices with up to 13% efficiency that can be produced at room temperature and at low cost.1−4 To further improve the efficiency of such technologies, organic absorber materials covering the entire sun light spectrum are needed. Unfortunately, reliable near-infrared (NIR) absorbers for application in OSCs based on vacuum evaporation of small molecules are still rare and display several disadvantageous features, e.g. peripherally functionalized metal phthalocyanines show problematic evaporation behavior, low volatility, and low open-circuit voltages in combination with C60.5−9 In this context, 4,4′-difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs, cf. Figure 1A) and especially 4,4′-difluoro-4-bora3a,4a,8-triaza-s-indacenes (aza-BODIPYs, cf. Figure 1B) have gained increasing interest as they display very promising optical properties.10−12 Although the molecules exhibit a rather small π-conjugated system, they show a strong absorption in the NIR © 2017 American Chemical Society

Figure 1. Basic BODIPY structure with IUPAC numbering (A) and aza-BODIPY structure with typical notation (B).

regime, which can be further optimized by various functionalization strategies and convenient synthesis.13 Additionally, the robust molecular core enhances the thermal stability, leading mostly to high sublimation yields. Received: February 15, 2017 Revised: June 12, 2017 Published: June 12, 2017 5525

DOI: 10.1021/acs.chemmater.7b00653 Chem. Mater. 2017, 29, 5525−5536

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Chemistry of Materials

molecular orbital (HOMO) should be low, ensuring a high opencircuit voltage and, thus, a high PCE, the energy of the lowest unoccupied molecular orbital (LUMO) should be a few 100 meV above the LUMO level of the acceptor for efficient exciton dissociation.40,41 We predict such levels by calculating the ionization potential (IP) as well as the electron affinity (EA) from the groundstate energies of the neutral and ionic molecular states in the relaxed geometry of the neutral state (indicated as q0) as follows:

These advantages have motivated their application as donor materials in vacuum-deposited planar heterojunction (PHJ) and bulk heterojunction (BHJ) OSCs, where power conversion efficiencies (PCEs) of up to 1.6 and 4.5% have been reported, respectively.14,15 Recently, fully vacuum-deposited triple junction OSCs, i.e. three serially stacked cells using complementary absorber materials, have yielded PCEs of 10.4%.16 Solution-processed BHJ OSCs have shown PCEs up to 4.7%,17,18 and PHJ devices have reached PCEs up to 4.5%.19,20 For polymer OSCs using BODIPYs as NIR sensitizer, PCEs of up to 4.3% have been obtained.21,22 Even higher PCE values up to 6.06% have been achieved by using BODIPYs as sensitizers in nanocrystalline TiO2-based solar cells.23 Despite these successful applications, studies of BODIPYs have focused mostly on functionalization at the meso-position, e.g. the standard BODIPY core or meso-phenyl and mesoethynylphenyl BODIPYs, different from the promising azaBODIPY core.10−12,21,24,25 In addition, they have mainly focused on absorption characteristics and energy levels, while charge transport properties remain largely unknown, which is similarly essential for good device performance.10−12,25−27 In this manuscript, we computationally study the effects of various structural modification strategies on the molecular properties of aza-BODIPYs to formulate design rules for improved donor materials. This includes the evaluation of energy levels, optical properties, and reorganization energies for more than 100 compounds. In particular, the absorption wavelength and oscillator strength should be high, as we are interested in materials which absorb light in the NIR regime to cover this spectral range in tandem solar cells. Further aspects such as thermal stability, which is essential for application in vacuumprocessed devices, are included in the discussion. With this study, we also continue the extensive effort of prior theoretical studies on device-relevant material parameters.28−34 We find that functionalization of aza-BODIPYs works widely independently with few exceptions being discussed. Consequently, multiple chemical functionalization strategies can be combined, which makes them a perfect class for tailor-made NIR absorber materials for application in OSCs. This is confirmed by experimental molecular characteristics of molecules that have been synthesized for the first time to our best knowledge. On the basis of the knowledge gained, we are able to determine the capabilities (but also the limitations) of the different functionalization strategies and to formulate design rules for materials simultaneously showing improved optical properties, higher charge-carrier mobility, and suitable energy levels. The application of these rules is demonstrated for newly synthesized compounds which appear superior for application in OSCs with respect to standard aza-BODIPYs.

IP = E+(q0) − E0(q0)

(1)

EA = E0(q0) − E−(q0)

(2)

with E0 as the total energy of the neutral molecule, E+ as the total energy of the cation, and E− as the total energy of the anion.42 While the geometry optimizations are performed with the B3LYP hybrid functional and the 6-311G(d,p) basis set throughout this work,43−46 the total energies are obtained with the M06-2X functional and the ccpVTZ basis set. Also here, polarization shifts have to be applied to calculate the energy levels in solution with DCM. While the IP is reduced by 1.33 eV, as shown in Figure S1A, the EA is increased by 1.44 eV, as depicted in Figure S1B. Marcus theory for hopping transport describes how the charge carrier mobility depends on the internal reorganization energy Λ of the material.14,28,47 This quantity is calculated from the adiabatic potential energy surfaces for the neutral state q0 and the ionic states q±: (2) Λ± = Λ(1) rel + Λ rel

(3)

Λ± = [E0(q±) − E0(q0)] + [E±(q0) − E±(q±)]

(4)

(2) with Λ(1) rel as the relaxation energy for the neutral molecule and Λrel as the relaxation energy for the molecule in the ionic state.28 These computational methods have shown to reliably reproduce experimental trends for organic molecules in general and BODIPYs in particular.11,14,15,28,48−50 The simulations are performed within the Gaussian09 suite.51 Above simulations are repeated for all possible stable conformers of the molecules, and the values given correspond to the most stable conformer. For the individual conformers, variations are below a few percent. Synthesis of the new aza-BODIPY compounds is described elsewhere.52 Absorption measurements in solution are performed in DCM with a Perkin Elmer l25 UV−vis spectrometer. Cyclic voltammetry (CV) measurements are recorded with a μAutolab type III potentiostat from Metrohm in single-component cells under nitrogen atmosphere with degassed DCM of high performance liquid chromatography quality as solvent and 0.1 M tetra-n-butylammonium hexafluorophosphate (TCI Europe, recrystallized from ethanol) as conducting salt. The CV curves are measured with a scan rate of 100 mV s−1 against a silver wire covered with AgCl as counter electrode and scaled to ferrocene (Fc) as internal standard. The redox potentials (Eox and Ered) are therefore deduced from the peak maxima in the measurement characteristics. With EFC HOMO = −4.78 eV, HOMO and LUMO energies of the aza-BODIPY (in solution) can then be calculated as follows

2. METHODOLOGY Time-dependent density functional theory (TD-DFT) calculations using the M06-2X functional in combination with the cc-TZVP basis set are performed for an investigation of excited state properties such as the excitation wavelength in gas phase λgas and the oscillator strength ν.35−37 The systematic shift of absorption wavelengths due to polarization effects and possible TD-DFT inaccuracies in predicting the absolute position of absorption features38,39 are resolved by shifting theoretical gas-phase absorption energies constantly by 338 and 448 meV to estimate λsol in solution with dichloromethane (DCM) and λfilm in thin film devices, respectively (cf. Figure S1C in the Supporting Information). The performance of OSCs is also strongly related to the energy levels of the donor material. While the energy of the highest occupied

Fc Fc E HOMO = E HOMO + (Eox − Eox )

(5)

Fc Fc E LUMO = E HOMO + (Eox − Ered)

(6)

Chips with a bottom-gate and bottom-contact structure for organic field-effect transistor (OFET) mobility measurements are used for sample preparation. The chips consist of a highly conductive p-doped Si wafer with a 230 nm-thick thermally grown oxide and a 30 nm-thick Au microstructure (IPMS, Dresden, Germany) for source and drain contact (channel lengths: 5−20 mm, channel width: 10 mm). The gold contacts are cleaned by a 10 min O2-plasma treatment to shift the work function of the material from 4.6 to 5.15 eV. The hole mobility is deduced from the saturation regime of the measured OFET characteristics. For further details, see ref 53. 5526


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3. RESULTS The functionalization strategies of aza-BODIPYs can be split into two groups: primary modifications of the molecular core and secondary functionalization strategies that are further applied to the benz-aza-BODIPY core. The primary functionalization routes analyzed here include the fusion of aromatic rings at the β- and α-position of the aza-BODIPY core discussed in Section 3.1 as well as modifications at the 4-boron atom presented in Section 3.2.11,15,54−57 Secondary modifications can be used to further improve the molecular characteristics or to correct for disadvantageous behavior occurring in primary functionalization routes because strong improvements of one quantity of interest are frequently at the expense of another. We analyze α-functionalization and fluorination in Section 3.3, rigidification in Section 3.4, and β-substitution in Section 3.5 as secondary modifications to the basic benz-azaBODIPY core, as it shows a balance between advantageous and disadvantageous behavior,11,23,24,58−66 and provide a global overview in Section 3.6. We further demonstrate experimentally the combination of different functionalization routes for a set of newly designed and synthesized aza-BODIPYs in Section 3.7. 3.1. Aromatic Ring Fusion. We first focus on primary modification strategies by aromatic ring fusion because they still enable for further functionalization at various open positions. The results for several molecules obtained by β-ring fusion (compounds 2 and 4−8) and α-ring fusion (3 and 9) are given in Figure 2. In general, fusing additional rings at the basic molecular core 1 leads to a strong decrease in the IP along with a weaker increase in the EA and, thus, a decrease in the fundamental gap and a red shift in absorption. For β-ring fusion (2), the IP is significantly decreased from 8.27 to 7.09 eV, and the EA remains unaffected, whereas for α-ring fusion (3), the decrease in IP is relatively weak, and EA is strongly increased. These strong differences can be inferred from the frontier molecular orbitals (MOs) of molecule 1 depicted in Figure 3. Its HOMO has an orbital pattern which is reminiscent of the HOMO states of the acenes. Fusing a phenyl ring in β position enables strong delocalization of the HOMO for 2 on these rings and a decrease in IP as the case of acenes.28 The LUMO, deviating from the orbital pattern of acenes, appears to be less susceptible for delocalization by β-ring fusion and, thus, this strategy leaves the EA almost unaffected. For α-ring fusion, we observe different trends because both the HOMO and the LUMO of 1 show similar bonding lobes between C atoms 2 and 3 (as well as 5 and 6), leading to orbital delocalization for both. Because the shifts in levels lead to a smaller fundamental gap for β-ring fusion, the red shift for molecule 2 is stronger than that for molecule 3. However, compound 3 exhibits a strongly increased oscillator strength. Although delocalization favors lower reorganization energy and improved charge transport, the reorganization energy of 3 is much higher than that for 2. We find that the fused rings of 3 are pushed out of plane because strong interactions with the BF2-group induce a destabilization of the entire molecular structure. Consequently, the lower reorganization energy, stronger red shift, and more convenient EA with respect to standard acceptor materials suggest β-ring fusion for optimized donor materials. Still, α-ring fusion, especially compound 9, might be a promising starting point to find further BODIPYbased acceptor materials because the EA is strongly increased and the reorganization energy for electron transfer is decreased

Figure 2. Electronic properties, absorption of the first excited state, and internal reorganization energies Λ for hole transfer for azaBODIPYs which are formed by aromatic ring fusion. Values in parentheses are estimates for solution with DCM according to Figure S1 in the Supporting Information. Best optical and transport properties can be expected for molecule 5. However, its high HOMO level makes an application as donor materials uncertain.

Figure 3. Frontier MOs (LUMO above, HOMO below) of azaBODIPYs 1, 2, and 3.

with respect to β-ring fusion (cf. Figure S6).67 In addition, thieno-pyrrole fused BODIPYs and Keio-Fluors appear interesting for application as donor materials in OSCs due to their strong extinction coefficients.68−72 However, as observed for nonbenzannulated aza-BODIPYs, attachment of substituents aside aromatic ring fusion at the β-position appears to be not as powerful as on the α-position (cf. compounds 1a and 1b in Figure S2). Using the above insight, molecular properties can be further optimized by designing larger aromatic groups for fusion at the 5527


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Chemistry of Materials β-position (see Figure 2). Interestingly, compound 5 outperforms even larger molecular cores such as 6, 7, and 8 regarding optical characteristics and reorganization energy. Here, the nodal plane between the β-carbons is further continued in annulation direction, which leads to an optimal delocalization of the HOMO on the fused rings and an even stronger decrease in the IP than for compound 2 as shown in Figure S5. Despite potentially low open-circuit voltages in OSCs, compound 5 also shows a superior red shift to 807 nm in thin film and an extremely low reorganization energy of 69 meV, even outperforming pentacene.28 For all molecular cores created by β-ring fusion, further functionalization at the α-position is the usually favored strategy to improve the material properties. Due to the strong similarities in the orbital structures of compounds 2, 4, and 5, such functionalization similarly affects the molecular properties, as can be seen in Figure S8. This means β-ring fusion and functionalization at the α-position can be combined widely independently. Consequently, in the following, we discuss further functionalization strategies based on molecular core 2, corresponding to the widely investigated class of benz-azaBODIPYs, because most trends are similar for other molecular cores constructed by β-ring fusion. In addition, a discussion of nonbenzannulated aza-BODIPYs based on molecular core 1 is given in the Supporting Information. 3.2. Functionalization at the 4-Boron Atom. In a recent publication, we reported that substitution of the BF2-group can improve the sublimation behavior of aza-BODIPYs, which allows their efficient application in vacuum-processed OSCs.15 Here, the systematic study of a series of modifications at the 4boron atom is reported and reveals a promising way to tune electronic properties while keeping the absorption wavelength λ and the reorganization energy Λ relatively constant (Figure 4). We first focus on compounds 10−14. With increasing EA of the groups attached at the boron atom, the energies of the frontier MOs are lowered. Best performance is expected for functionalization with cyano groups, which shows an IP increased by 0.34 eV with respect to the standard aza-BODIPY core 2. This indicates a similarly improved open-circuit voltage while keeping other properties almost unaffected. The advantageous shifts in energy levels are moderated for compounds 10a−14a and 10b−14b but still strong enough to optimize the OSC performance. In addition, while the molecular properties are similarly shifted for phenylethynyl functionalization at the α-position (10b−14b), different trends are observed for compounds 10a−14a. Here, steric hindrances and intramolecular hydrogen bonds between the B−X group at the center and phenyl groups at the α-position affect the torsional angle of these groups with respect to the molecular core and, accordingly, the delocalization of the frontier MOs (cf. Figure S15). For molecules 13a and 14a, strong steric interactions cause the largest torsional angles, smallest absorption wavelengths, and highest reorganization energies. For molecules 2a and 10a, intramolecular hydrogen bonds lead to intermediate torsional angles and characteristics. Best performance is found for 11a, enabling the substituents to arrange mostly in parallel to the molecular core. Consequently, steric effects need to be considered to obtain independent functionalization effects. For the more distant phenylethynyl substituent (2b−14b), mostly no intramolecular interactions between the substituents and the central group are present, conserving the independence of α-functionalization and functionalization at the 4-boron atom.

Figure 4. Electronic properties, absorption of the first excited state, and internal reorganization energies Λ for benz-aza-BODIPYs differing in functionalization at the 4-boron atom.

In addition to the molecules discussed in this section, several research groups have replaced the BF2-group by an alkynyl groups.11,12,57 Data for some of these molecules is presented in 5528


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Chemistry of Materials Figure S16 in the Supporting Information. Due to its still competitive characteristics, further discussion focuses on materials based on the standard BF2-group at the 4-position. 3.3. α-Substitution. All primary compounds created by βring fusion (2 and 4−8) are not stable under ambient conditions and need to be further modified by α-substitution, which also allows for an additional optimization of the molecular properties. α-Substitution can be split into different substrategies,11,12 i.e., aryl (2a and 2c−2l), alkynyl (2b and 2m), and styryl substitution (2n and 2o), presented in Figure 5, as well as heteroatom substitution (2p−w), presented in Figure 6. Despite the strong structural differences between the different substituents, the observed trends are related to a few basic design strategies. Bathochromic shifts in absorption can be achieved by promoting the delocalization of the frontier MOs and by increasing the donating character of the substituent. Correspondingly, styryl substituents (2n) give absorption wavelengths larger than those of alkynyl substituents (2b), poly aryl substituents (2j), and diaryl substituents (2a). In addition, thienyl groups further increase the absorption wavelengths with respect to phenyl-based compounds. Simultaneously, the IP of the compounds decreases. Exceptions are compounds 2f and 2g, showing both red-shifted absorption and increased IP. In the case of compound 2f, this is achieved by further attaching accepting groups at the thienyl group. As has been discussed by us recently, 2d lacks in thermal stability, preventing application in vacuum-processed OSCs because of a strong binding in the crystal structure. The replacement of the methyl group by trimethylsilyl (2e) or trifluoromethyl (2f) could also improve the sublimation behavior, because less intermolecular interactions can be expected, especially for the latter compound. A more general approach for correcting the energy levels of aza-BODIPYs while conserving the absorption properties is the partial fluorination of the substituent at the α-position, as observed for compounds 2u and 2v (Figure 6). The energy levels are gradually shifted to desired lower values, allowing for higher open-circuit voltage of the device. Moreover, materials with incompatible energy levels with respect to those of other materials used in the stack of the OSC can be made compatible, and the difference in EA between the donor and the acceptor material can be reduced to improve the exciton dissociation. Both IP and EA are increased by about 0.1 eV per fluorine atom. In contrast, full fluorination of the phenyl ring (2w) causes steric interactions with the central BF2-group, strongly increasing the torsional angle of the substituents with respect to the molecular core and thus negatively influencing both absorption properties and reorganization energies. However, partial fluorination can be considered as a powerful functionalization strategy to correct energy levels without impacting other properties. This might be especially interesting for compounds based on molecular core 5, which exhibit very promising optical and transport properties but inconvenient energy levels (cf. Figure S8). The reorganization energy is mostly increased by αsubstitution because freely movable substituents increase the degrees of freedom of the entire molecule, leading to a coupling of new normal modes to the electronic structure. In addition, the attachment of such functional groups also affects the dynamics of the core part of the molecule, which can cause a higher reorganization energy as well.50 Said differently, the functionalization can collectively lead to a stronger deformation

Figure 5. Electronic properties, absorption of the first excited state, and internal reorganization energies for benz-aza-BODIPYs differing in α-substitution. Values in parentheses are estimations for properties in solution with DCM. 5529


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Figure 7. Electronic properties, absorption of the first excited state, and internal reorganization energies for benz-aza-BODIPYs with torsionally restricted α-substitution. Best performance can be expected for restriction by carbon bridge (17a).

Figure 6. Electronic properties, absorption of the first excited state, and internal reorganization energies for benz-aza-BODIPYs differing in α-substitution. Values in parentheses are estimations for properties in solution with DCM. Reorganization energy for 2t (cyan point) is out of plotting range (425 meV). Fluorination of phenyl groups can gradually shift the energy levels without negatively influencing either optical or transport properties.

reduces the torsional angle of the groups at the α-position with respect to the molecular core, which leads to a relatively strong delocalization of the frontier MOs accompanied by a red shift in absorption (cf. Figure S21). Such rigidification freezes also low energy modes, which can cause a decrease in reorganization energy. This works most efficiently for molecule 2x (Λ = 103 meV), where intramolecular NH···F hydrogen bonds reduce the degrees of freedom of the substituents and especially compound 17a (Λ = 101 meV), where a carbon bridge covalently connects the substituents with the annulated rings, causing a nearly planar molecular structure. In the case of the structurally similar molecule 16a, the carbon bridge is too weak to affect a significant reduction in reorganization energy (Λ = 131 meV). Despite its nearly unaffected reorganization energy, the high absorption wavelength of compound 15a should be highlighted. However, this type of rigidification also strongly reduces the oscillator strength. Consequently, 17a appears to be the most promising starting point for application in OSCs. Further improvement of the absorption characteristics can be achieved by combining the rigidification with other α-

of the neutral molecule to the charged state, increasing the reorganization energy. This is further promoted by steric hindrances, as observed for compounds 2p, 2q, 2t, and 2w. In contrast, the best reorganization energies are found for alkynyl and styryl substitution, as the frontier MOs show large coefficients only localized at the triple and double bonds, respectively (Figure S17). 3.4. Rigidification. During the last years, multiple ways of torsional restriction of α-functionalization have been proposed for BODIPYs to achieve a red shift in absorption. However, only a few have been realized experimentally.11,12,61−65,73 Here, we assess these strategies for the class of aza-BODIPYs and extend them in various ways with the aim to improve both absorption and charge transport. The characteristics of the studied molecules are presented in Figure 7. The restriction 5530


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Chemistry of Materials functionalization, e.g. thienyl (17c) and polyaryl substitution (17j), but not working in an independent fashion, as observed for other functionalization strategies. All of these molecules suffer from relatively low IP, which can be compensated by fluorination of the carbon bridge (17y). 3.5. β-Substitution. Following the concept of push and pull strategies to design improved materials for OSCs, we investigate various acceptor-type β-functionalizations in addition to donor-type α-functionalizations in this section. In Figure 8, we summarize the characteristics of studied molecules showing that β-substitution can be used to directly tune the

electronic levels. Attachment of single cyano groups (18 and 19) already increases the IP by 0.7 eV without impairing the absorption properties and the reorganization energy. In addition, α-functionalization of both compounds works similar to the standard benz-aza-BODIPY core 2, showing that these types of β-functionalization can be independently combined with substitution at the α-position. In contrast, by attaching dicyano groups, which have a much stronger accepting character, the frontier MOs are strongly pulled into the direction of the substituents, leading to a strong deformation of the orbital shapes with respect to molecule 2 (cf. Figure S23). This is accompanied by a strong increase in EA and a significant red shift in absorption, giving 765 nm for compound 21 and outstanding 1023 nm for compound 20. For the latter compound, also the reorganization energies are decreased, indicating good charge transport. According to the energy levels, also an application as acceptor material or dopant seems reasonable for 20. Due to the strongly pulled frontier MOs (cf. Figure S23), α-functionalization with standard substituents such as phenyl does not have positive impact on the material properties. However, we suppose that much stronger donating substituents might be used to decrease the EA again while maintaining the advantageous characteristics of compound 20. Consequently, a balance between the accepting character of the β-substituents and the donating character of the α-substituents appears to be advantageous for the design of optimized donor materials. In addition, we also test fluorination of the annulated rings (22) because we observed successful level correction for gradual fluorination of substituents at the α-position (2u−w) in Section 3.3. Again, IP and EA are increased simultaneously, keeping the absorption wavelength almost constant. In contrast to the trends observed for 2u and 2v, the fluorination of the annulated rings induces a destabilization of the molecular structure, forcing the annulated rings to move out of plane, which strongly increases the reorganization energy. Consequently, fluorination of annulated rings should not be used for the design of donor materials. 3.6. Capabilities and Limitations of aza-BODIPYs. The computational analysis of more than 100 molecules allows the formulation of global trends not only for the individual primary and secondary functionalization strategies but also for the class of aza-BODIPYs in general. The effects discussed in the sections above for the most fundamental parameters, i.e. the absorption wavelength, the IP, and the reorganization energy for hole transfer, are summarized schematically in Figure 9. We find that delocalization of the frontier MOs leads to a bathochromic shift in the absorption as well as a reduction in the reorganization energy. However, for aza-BODIPYs, this advantageous behavior is mostly accompanied by an undesired strong decrease in the IP. In contrast, the EA is only slightly increased; as for most common functionalization routes, the delocalization for the HOMO is more pronounced than that for the LUMO. Especially, β-ring fusion is a powerful strategy with best performance observed for molecular core 5. Due to an optimal continuation of the orbital structure of the basic azaBODIPY core, this compound even outperforms molecules with larger substituents. Similar trends are observed for αsubstitution. Here, the effectiveness depends on the torsional angle of the substituents with respect to the molecular core, which controls the delocalization of the frontier MOs. Consequently, further improvement of transport and optical

Figure 8. Electronic properties, absorption of the first excited state, and internal reorganization energies for benz-aza-BODIPYs differing in β-substitution for different α-functionalization. Absorption wavelengths of dicyano-substituted compounds (blue and bright green symbols) are out of plotting range due to strong redshifts. Λ values of 20 and 21 are 51 and 209 meV, respectively. Λ values of 20a and 21a are 85 and 273 meV, respectively. Empirical polarization corrections as used throughout this work are expected to be less accurate for these push−pull compounds and are not shown here. 5531


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fusion (cf. Section S3). We also observe that functionalization at the 4-boron atom works independently if no steric hindrances are introduced. The trends deduced for azaBODIPYs appear even valid for BODIPYs in general, as independence is conserved for different substituents at the meso-position (cf. Section S4). Combining different functionalization strategies helps to systematically develop new tailormade molecules, which perform optimally for the desired application. This is demonstrated in the following section for representative newly designed and synthesized aza-BODIPYs. 3.7. Experimental Characterization of Novel Compounds. We turn to the experimental part of our work, which aims at realizing some of the compounds that have been predicted. Thereby, we combine several of the above functionalization strategies to test the various functionalization effects. In particular, benzannulation, α-substitution, and asymmetric functionalization at the 4-boron atom are applied. We focus our efforts on benzannulated aza-BODIPYs because of the well-established synthesis of the molecular core and much better properties compared to those of azaBODIPY core 1. In particular, we take advantage of the orbital delocalization leading to a strong red shift and a reduction in the internal reorganization energy. As a next step, we select different groups for α-substitution, i.e. compounds 2e, 2h, and 2i (Figure 5). Such molecules show already significant improvement of the absorption properties compared to those of 2a in the theoretical data. We synthesized the selected compounds and performed electrochemical and optical characterization (see Methodology for details). The synthesis is described elsewhere.52 The experimental electronic and optical properties of 2e, 2h, and 2i are summarized in Table 1 and show very good

Figure 9. Schematic overview on effects of eight different functionalization strategies on the absorption wavelength (λ), the ionization potential (IP), and the reorganization energy for hole transfer (Λ) for aza-BODIPYs based on a computational analysis of more than 100 molecules. The ratings of the molecular properties vary from very negative (− −) to very positive (++) according to their impact on the solar cell performance. For λ, the + + (− −) rating describes an increase (decrease) by more than 20%. For IP, + +(− −) ratings correspond to an increase (decrease) by more than 10%. Λ can be influenced more easily in the negative direction; thus, − − (++) corresponds to an increase in Λ of by more than 50% (decrease by more than 30%). (A) Primary modification strategies compared with compound 2 as a reference. (B) Secondary modification of the benzaza-BODIPY core compared with molecule 2a.

Table 1. Experimental Characteristics of Synthesized azaBODIPYs

properties can be achieved by torsionally restricting these groups. Push−pull strategies, e.g. the attachment of acceptor groups at the annulated rings and donor groups at the α-position, give further red shift in the absorption as well as modification of the energy levels. We show that an increase in the IP is obtained by partial fluorination of substituents at the α-position, attachment of strong accepting groups at the 4-boron position, as well as attachment of cyano groups at the annulated rings, which potentially removes the disadvantages of delocalization. This is accompanied by an increase in the EA, which allows tailoring of the energy levels of the donor material with respect to the acceptor material. Such strategies can improve the exciton dissociation and allow the realization of a bathochromic shift with increased IP compared to standard BODIPY compounds. Also, steric hindrances need to be considered for the design of optimized aza-BODIPYs. As observed for functionalization at the 4-boron atom, the standard BF2 group already leads to intermediate torsional angles of substituents at the α-position and correspondingly to reduced delocalization of the frontier MOs. In contrast, the smaller boronoxide gives optimized optical and transport properties. Steric hindrances also need to be considered to ensure the independence of functionalization strategies. Although most of the analysis is performed for benz-azaBODIPYs, we expect that functionalization of aza-BODIPYs works widely independently. In the Supporting Information, this is shown for representative molecules with varying β-ring

15

2a 2d15 2e 2h 2i 23e 23h

HOMO (eV)

LUMO (eV)

λmax (nm)

ε L mol−1 cm−1

−5.22 −4.96 −5.04 −4.91 −5.07 −5.13 −5.00

−3.65 −3.70 −3.81 −3.67 −3.78 −3.84 −3.74

712 793 793 762 777 797 774

106 000 95 000 89 600 70 000 65 100 98 200 104 500

μ cm2/(V s) 6.9 2.0 1.8 9.2 6.6 4.3 1.9

× × × × × × ×

10−5 10−4 10−5 10−6 10−6 10−6 10−5

agreement to the predicted properties. Both energy levels and absorption wavelengths differ only by few percent from the theoretical values in Figure 5. Most notably, the absorption maximum, which was predicted to shift to the red by 102 nm from 2a to 2e, indeed shows a red shift by 81 nm. The other compounds give similarly strong red shifts in absorption. When comparing the absorption strength of the new molecules 2e, 2h, and 2i, we however note that they show reduced extinction coefficients. Also, higher HOMO levels for 2e, 2h, and 2i appear disadvantageous for their application in OSCs. Both aspects will be addressed below and compensated by additional functionalization, namely by modifications at the 4-boron position. As demonstrated in Section 3.2, exchange of the fluorine atoms with cyano groups yields higher values for the energy levels of the frontier orbitals while maintaining the position of the absorption maximum. We noticed earlier that this 5532


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Chemistry of Materials functionalization route, however, partially suffers from steric interactions with substituents at the α-position, which tend to blueshift the absorption. To avoid this effect, we propose an asymmetric functionalization at the 4-boron atom. In fact, the substitution of a single fluorine atom by a cyano group leaves enough space for further substituents at the α-position, and synthesis can be realized conveniently. The synthesized asymmetric compounds 23e and 23h are depicted in Figure 10. From the optical characterization, we

Figure 11. Simulated values of the reorganization energy versus OFET mobilities of different BODIPYs for hole (red circles) and electron transport (blue squares). A fit function can be determined as μ(Λ) = cm 2

μ0exp(−AΛ) with μ0 = 3.0 × 10−4 Vs and A = 0.012 meV−1. However, the coefficient of determination is relatively small with R2 = 0.22.

in transfer integrals77,78 and the morphology of the transport properties, which has been studied for amorphous organic materials or polycrystalline structures79,80 and which deserves further research efforts in the future.

4. CONCLUSIONS On the basis of a systematic investigation of the electronic properties of aza-BODIPYs, we analyze design principles for this highly interesting molecule class for application as donor materials in OSCs and to deduce new functionalization routes toward optimized compounds that combine high absorption in the IR, good hole transport, and compatible energy levels. Indeed, we find that aza-BODIPYs cover a broad range of molecular properties which are dominantly affected by orbital delocalization, push−pull effects, and intramolecular steric interactions. Almost all functionalization strategies exhibit unfavorable side effects accompanying their advantages. However, we demonstrate that these can be compensated by using the wide independence of functionalization routes, allowing an optimized combination of substituents. This makes BODIPYs an ideal class for tailor-made NIR-absorbing donor materials, as demonstrated for prototypical newly synthesized molecules. Especially, the compounds with asymmetric functionalization at the 4-boron position should be highlighted as they show red-shifted absorption in combination with increased IP compared to standard azaBODIPYs, potentially leading to OSCs with improved performance. Furthermore, we demonstrate that systematic analysis of molecular properties based on density functional theory allows the preselection of the most promising candidates to guide synthesis.

Figure 10. Theoretical values for the new BODIPY core 23 and experimental absorption spectra of several compounds in solution with DCM.

find slightly red-shifted absorption by a few nanometers. This is accompanied by down-shifted energy levels compared to their BF2 analogues. In addition, both compounds also show strongly increased molar extinction coefficients. Improvement of the device relevant optoelectronic characteristics of 23e and 23h are confirmed theoretically in Figure 10. For application as donor materials in OSCs, the hole mobility is another important quantity strongly affecting the device performance.3,28,74,75 We measured the mobility for the new materials in OFET architecture.52 The results are presented in Table 1. All new compounds show smaller values in comparison to those of standard material 2a, which can be partially explained by the larger values observed for Λ. The substitution of a fluorine atom by a cyano group in 23h leads to a slight increase in the hole mobility by a factor of 2 compared to that of 2h. For 23e, the hole mobility drops by a factor of 4. Compared to the usual scatter of OFET mobilities for similar materials, these changes are considered minor, which agrees with the weakly changed reorganization energy upon cyano functionalization. To gain further insight in the correlation between the OFET mobilities of BODIPYs and the internal reorganization energy, we finally compare mobility values available in the literature with our values calculated for Λ in Figure 11.15,76 We find an apparent trend with the mobility which decreases slightly when increasing Λ. Thereby, an exponential fit function yields a rather moderate coefficient of determination of R2 = 0.22, consistent with a strong scatter of the mobility values. This indicates the relevance of additional effects such as the variance

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00653. Details on calculations and additional results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 5533


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Chemistry of Materials ORCID

(11) Lu, H.; Mack, J.; Yang, Y.; Shen, Z. Structural Modification Strategies for the Rational Design of Red/NIR Region BODIPYs. Chem. Soc. Rev. 2014, 43, 4778−4823. (12) Bessette, A.; Hanan, G. S. Design, Synthesis and Photophysical Studies of Dipyrromethene-Based Materials: Insights into Their Applications in Organic Photovoltaic Devices. Chem. Soc. Rev. 2014, 43, 3342−3405. (13) Ge, Y.; O’Shea, D. F. Azadipyrromethenes: From Tranditional Dye Chemistry to Leading Edge Applications. Chem. Soc. Rev. 2016, 45, 3846−3864. (14) Kraner, S.; Widmer, J.; Benduhn, J.; Hieckmann, E.; JägelerHoheisel, T.; Ullbrich, S.; Schütze, D.; Radke, K. S.; Cuniberti, G.; Ortmann, F.; Lorenz-Rothe, M.; Meerheim, R.; Spoltore, D.; Vandewal, K.; Koerner, C.; Leo, K. Influence of Side Groups on the Performance of Infrared Absorbing aza-BODIPY Organic Solar Cells. Phys. Status Solidi A 2015, 212, 2747−2753. (15) Lorenz-Rothe, M.; Schellhammer, K. S.; Jägeler-Hoheisel, T.; Meerheim, R.; Kraner, S.; Hein, M. P.; Schünemann, C.; Tietze, M. L.; Hummert, M.; Ortmann, F.; Cuniberti, G.; Körner, C.; Leo, K. From Fluorine to Fluorene − A Route to Thermally Stable aza-BODIPYs for Organics Solar Cell Application. Adv. Electron. Mater. 2016, 2, 1600152. (16) Meerheim, R.; Körner, C.; Oesen, B.; Leo, K. 10.4% Efficient Triple Organic Solar Cells Containing Near Infrared Absorbers. Appl. Phys. Lett. 2016, 108, 103302. (17) Liu, W.; Tang, A.; Chen, J.; Wu, Y.; Zhan, C.; Yao, J. Photocurrent Enhancement of BODIPY-Based Solution-Processed Small-Molecule Solar Cells by Dimerization via the Meso Position. ACS Appl. Mater. Interfaces 2014, 6, 22496−22505. (18) Bura, T.; Leclerc, N.; Fall, S.; Lévêque, P.; Heiser, T.; Retailleau, P.; Rihn, S.; Mirloup, A.; Ziessel, R. High-Performance SolutionProcessed Solar Cells and Ambipolar Behavior in Organic Field-Effect Transistors with Thienyl-BODIPY Scaffoldings. J. Am. Chem. Soc. 2012, 134, 17404−17407. (19) Leblebici, S. Y.; Catane, L.; Barclay, D. E.; Olson, T.; Chen, T. L.; Ma, B. Near-Infrared Azadipyrromethenes as Electron Donor for Efficient Planar Heterojunction Organic Solar Cells. ACS Appl. Mater. Interfaces 2011, 3, 4469−4474. (20) Chen, J. J.; Conron, S. M.; Erwin, P.; Dimitriou, M.; McAlahney, K.; Thompson, M. E. High-Efficiency BODIPY-Based Organic Photovoltaics. ACS Appl. Mater. Interfaces 2015, 7, 662−669. (21) Singh, S. P.; Gayathri, T. Evolution of BODIPY Dyes as Potential Sensitizers for Dye-Sensitized Solar Cells. Eur. J. Org. Chem. 2014, 2014, 4689−4707. (22) Kubo, Y.; Watanabe, K.; Nishiyabu, R.; Hata, R.; Murakami, A.; Shoda, T.; Ota, H. Near-Infrared Absorbing Boron-dibenzopyrromethenes that Serve As Light-Harvesting Sensitizers for Polymeric Solar Cells. Org. Lett. 2011, 13, 4574−4577. (23) Kubo, Y.; Eguchi, D.; Matsumoto, A.; Nishiyabu, R.; Yakushiji, H.; Shigaki, K.; Kaneko, M. Boron-Dibenzopyrromethene-Based Organic Dyes for Application in Dye-Sensitized Solar Cells. J. Mater. Chem. A 2014, 2, 5204−5211. (24) Gräf, K.; Körzdörfer, T.; Kümmel, S.; Thelakkat, M. Synthesis of Donor-Substituted Meso-Phenyl and Meso-Ethynylphenyl BODIPYs with Broad Absorption. New J. Chem. 2013, 37, 1417−1426. (25) Boens, N.; Wang, L.; Leen, V.; Yuan, P.; Verbelen, B.; Dehaen, W.; Van der Auweraer, M.; De Borggraeve, W. D.; Van Meervelt, L.; Jacobs, J.; Beljonne, D.; Tonnelé, C.; Lazzaroni, R.; Ruedas-Rama, M. J.; Orte, A.; Crovetto, L.; Talavera, E. M.; Alvarez-Pez, J. M. 8HaloBODIPYs and Their 8-(C, N, O, S) Substituted Analogues: Solvent Dependent UV-Vis Spectroscopy, Variable Temperature NMR, Crystal Structure Determination, and Quantum Chemical Calculations. J. Phys. Chem. A 2014, 118, 1576−1594. (26) Chibani, S.; Le Guennic, B.; Charaf-Eddin, A.; Maury, O.; Andraud, C.; Jacquemin, D. On the Computation of Adiabatic Energies in Aza-Boron-Dipyrromethene Dyes. J. Chem. Theory Comput. 2012, 8, 3303−3313.

Frank Ortmann: 0000-0002-5884-5749 Gianaurelio Cuniberti: 0000-0002-6574-7848 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) through the German Excellence Initiative via the Cluster of Excellence EXC 1056 “Center for Advancing Electronics Dresden” (cfaed). F.O. would like to thank the DFG for financial support (Grant OR 349/1-1). This work was partially supported by the Heinrich Böll Stiftung e.V. and the Bundesministerium für Bildung und Forschung (Projects FKZ 03IPT602A and FKZ 03IPT602X). T.-Y.L. would like to thank the China Scholarship Council (CSC 201406190164). Computational resources were provided by the Center for Information Services and High Performance Computing (ZIH) of Dresden University of Technology.

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ABBREVIATIONS BHJ, bulk heterojunction; CV, cyclic voltammetry; DCM, dichloromethane; EA, electron affinity; HOMO, highest occupied molecular orbital; IP, ionization potential; LUMO, lowest unoccupied molecular orbital; MO, molecular orbital; NIR, near-infrared; OSC, organic solar cell; PCE, power conversion efficiency; PHJ, planar heterojunction

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