Spiro-Fused Perylene Diimide Arrays - Journal of the American

Oct 23, 2017 - The PTB7-Th:PDI blend films showed large domain size and no continuous interpenetrating network, neither of which are good for the char...
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Article Cite This: J. Am. Chem. Soc. 2017, 139, 15914-15920

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Spiro-Fused Perylene Diimide Arrays Guangpeng Gao,† Ningning Liang,†,‡ Hua Geng,† Wei Jiang,*,† Huiting Fu,†,‡ Jiajing Feng,†,‡ Jianhui Hou,† Xinliang Feng,§ and Zhaohui Wang*,† †

CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ‡ University of Chinese Academy of Sciences, Beijing 100049, PR China § Center for Advancing Electronics Dresden & Department of Chemistry and Food Chemistry, Technische Universität Dresden, Dresden 01062, Germany S Supporting Information *

ABSTRACT: The straightforward palladium-catalyzed synthesis protocol toward spiro-fused perylene diimides is developed. The reaction involves two palladium-catalyzed C−H activations and 4-fold C−C bond formation sequence from readily available precursors. This facile and step-economic approach also provides another convenient access to ethylene-bridged dimer (NDP) and further π-extended spiro system (SNTP). In addition, the molecular structure of spirodiperylenetetraimide (SDP) is illustrated to show a three-dimensional (3D) cruciform configuration, and its absorbance is distinctly red-shifted due to the significant spiroconjugation effect. With combined properties of broadened and intensive absorption, aligned LUMO levels, and unique molecular geometry, the spiro-fused PDI system represents a new kind of high-performance semiconducting framework as the electron acceptor in high-efficiency organic solar cells.



ality for various optoelectronic devices. 10 The readily derivatized perylene skeleton combining easily modifiable imide chains results in a variety of robust dye materials with intense absorbance and fluorescence, well-tunable energy level, and molecular configuration.11 The elaborate development of a homologous PDI series directly connected by different linkages was achieved by transition-metal-catalyzed or -mediated coupling (and/or C−H transformation) which has enabled distinguishing π-electron delocalization.12 Up to now, there are a pool of successful applications based on PDI scaffolding to provide compelling targets serving as n-type semiconductors in OFETs and OSCs.13 In particular, PDI-based molecules as fullerene-free electron acceptors have been actively investigated and some examples exhibited outstanding photovoltaic performances by incorporating quasi-3D oligomeric PDI architectures.14 More recently, integrating SBF subunit either on the bay-region or imide-position of PDI via a single bond yielding 3D geometry has been demonstrated to facilitate nanoscale phase domain formation for efficient charge separation and transport in OSC devices.15 We have been focusing on the precise synthesis of π-fused PDI oligomers, often called as multichromophoric PDI arrays, as promising n-type semiconductors in optoelectronic applications.16 Thus, by integrating PDI with a spirocyclic

INTRODUCTION Spiro compounds are typically defined as cruciform-shaped molecules with two rings embraced by a shared atom (e.g., carbon or heteroatom).1 The two molecular halves generally adopt almost orthogonal configuration to give rise to a rigid three-dimensional (3D) structure with a wealth of peculiar properties.2 Due to their good processability and excellent optical and thermal stabilities, the spirocyclic systems have found wide applications in a plenty of optoelectronics, such as phosphorescent organic light-emitting diodes (OLEDs),3 organic solar cells (OSCs),4 and organic field-effect transistors (OFETs).5 Notably, the outstanding representative of 9,9′spirobifluorene (SBF) has served as an ideal building block for promising extended π-systems connected by a sp3-hybridized spiro carbon (Scheme 1).6 However, the preparation of SBF core depends heavily on highly reactive organometallic reagents,7 and other synthetic routes involve in oxidation dehydrogenation of 9,9′-diarylfluorenes8 and ring closure from a ketone derivative by a strong Lewis acid9 and so on (Scheme 1). These protocols were limited by their intractable conditions and multistep procedures of the starting materials. Thus, it remains challenging to develop efficient preparative approaches, such as metal-catalyzed coupling toward π-extended systems, in particular for electron-deficient moieties fused in such spiro way. Perylene diimides (PDIs) are attracting ever-increasing academic attention with aiming at seeking desirable function© 2017 American Chemical Society

Received: August 28, 2017 Published: October 23, 2017 15914

DOI: 10.1021/jacs.7b09140 J. Am. Chem. Soc. 2017, 139, 15914−15920

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Journal of the American Chemical Society Scheme 1. Strategy for the Preparation of Spirobifluorene πExtended Structures

Scheme 2. Straightforward Synthesis Protocol of SpiroFused and Ethylene-Bridged PDI Dimers and Spiro-Fused Tetramer

skeleton, we intend to introduce spiro-fused rylenes which can yield unique physical properties due to the resulting archetypical 3D structure. In this work, we demonstrated the synthesis and characterization of novel spiro-fused PDI dimer, namely, spirodiperylenetetraimide (SDP), via palladium-catalyzed coupling reaction of 4-fold C−C bond formation directly from monobrominated PDI precursor with dibromomethane (CH2Br2). This facile and straightforward protocol also provides step-economic process for naphthodiperylenetetraimide (NDP) with ethylene as the bridge when reacting with tetrabromoethane (Br2HCCHBr2) instead of CH2Br2. Remarkably, the PDI tetramer (SNTP) alternating the spiro bond and ethylene bridge was synthesized to extend the spiro π-system. The molecular structure of SDP is fully illustrated through single-crystal X-ray diffraction analysis with a 3D cruciform configuration. Employing these spiro-fused rylenes as nonfullerene acceptors in polymer-based OSCs resulted in a high power conversion efficiency (PCE) of 7.17% with a high fill factor (FF) of 0.61, open-circuit voltage (VOC) of 0.77 V, and short-circuit current density (JSC) of 15.21 mA/cm2 for the spiro-fused tetramer.

reaction strategy encouraged us to further explore its reaction scope; it is worth mentioning that reaction of BrPDI with Br2HCCHBr2 as carbon source indeed generated ethylenebridged PDI dimer (NDP) in moderate yield of about 30% at elevating reaction temperature of 110 °C. This result suggests a more facile procedure toward ethylene-bridged perylene dimer than multistep synthesis reported previously by us and Nuckolls group independently.18 Further extension of the spiro-fused system is expected to be carried out by utilizing monobrominated dimer as the precursor coupling with CH2Br2 to construct the corresponding tetramer. The product of SNTP alternating the spiro bond and ethylene bridge from BrNDP (2) was achieved with reasonable yield (15%) under the optimized conditions (Scheme 2). These results indicated that this one-step approach to fusing two rylene moiety spirocyclically together is really efficient. Considering strong strain resulting from the short length of C−C bonds of cyclopentadiene in SDP molecule, high energy was thus needed to establish two strained five-membered carbon rings. All newly synthesized compounds were unambiguously characterized by HR-MALDI-TOF mass spectrometry and NMR spectroscopy. These compounds are soluble in common organic solvents, such as dichloromethane, chloroform, chlorobenzene, and tetrahydrofuran (THF), at room temperature. The chemical shift of central sp3-hybridized spiro carbon moved to 75.63 ppm in SDP molecule due to the shielding effect by PDI. Thermogravimetric analysis (TGA)



RESULTS AND DISCUSSION Inspired by palladium-catalyzed reaction which can readily yield fluorenes by dual C−C bond formation from 2-iodobiphenyls and alkyl halides,17 we examined 1-bromoperylenediimide (1, BrPDI) reacting with CH2Br2 catalyzed by Pd(OAc)2. A novel spiro-fused PDI dimer (SDP) with two PDI units coupled with a spirocyclic bond was surprisingly produced in fairly high yield of 81% under optimized reaction conditions. Scheme 2 shows the one-step route toward SDP in the presence of 10 mol % Pd(OAc)2, using 8 equiv of KHCO3 and 9 equiv of KOAc as the base, 2 equiv of i-PrOH as the reductant, and 1,4-dioxane as the solvent under 10 h stirring at 75 °C. This highly efficient 15915

DOI: 10.1021/jacs.7b09140 J. Am. Chem. Soc. 2017, 139, 15914−15920

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Journal of the American Chemical Society showed that all of them are thermally stable with 5% weight loss temperature (Tdeg) over 370 °C (Figure S1 and Table S3). To shed light on the reaction pathway of this high efficient cyclization, we proposed the possible mechanism involving palladacycle B that acts as the key intermediate complex according to the previous literature.17,19 As shown in Scheme 3, Scheme 3. Proposed Mechanism for the 4-fold C−C Bond Formation of SDP by Pd-Catalyzed Coupling Reaction from BrPDI (1) with CH2Br2

Figure 1. Single crystal structures of SDP with 50% thermal ellipsoids probability. The dark gray-, blue-, red-, and purple-colored atoms represent C, N, O, and spiro carbon respectively. Hydrogen atoms and long alkyl chain groups are omitted for clarify. ORTEP structure form top (a) and side views (b) and packing arrangement (c).

was found to be ca. 89.04° between two fused rings.20 With the strong strain induced by the five-membered carbon-ring in one bay-region, two blades of PDI in SDP molecule are tilted to each other; the small torsional angle of the naphthalenes in one PDI subunit is about 4.82 and 6.45°, respectively. All of the details show that the geminate five-membered carbon-ring would lead to a strained configuration. In addition, multiple interactions are observed within columns: very close contacts of O to π (3.11−3.28 Å) and π to π (3.21−3.35 Å) separately exist in two subunits of one molecule with its adjacent molecules in one column; together with close C−H···O contacts (2.58 Å) shown in Figure 1c and spiro architecture featuring solubilizing alkyl chains, the rational and ordered 3D network with balanced charge transporting properties and crystallinity can be expected. UV/vis absorption spectra of the spiro-fused PDI dimer and tetramer (SDP and SNTP) in chloroform solution reveal intense absorption bands in the range from 450 to 600 nm as shown in Figure 2. The absorptive shape of SDP assigned as perylene core with molar extinction coefficient of about 93 000 M−1 cm−1 with the lowest-energy transition at 550 nm. Contrastively, the longest absorbance maximum of ethylenebridged PDI dimer (NDP, λmax = 549 nm)18b is approximate to the value of SDP. The sharp band around 387 nm can be associated with CC bond conjugated with PDI subunits. The absorption of SNTP integrated characteristics of both SDP and NDP with enhanced light-absorbing capability, and the maximum absorption coefficient is 143 400 M−1 cm−1 at 555 nm. Likewise, the absorption wave around 399 nm is remarkably increased to 285 700 M−1 cm−1 accounted for double CC bonded PDI chromophores. Compared to the single chromophore PDI, the absorption bands of SDP, NDP, and SNTP were more or less red-shifted due to their different fusing pattern and increased conjugation length. Importantly, the absorption maximum of SDP is red-shifted about 23 nm relative to the parent PDI, as implied by the significant

the process starts with oxidative addition of BrPDI (1) to Pd(0), then palladacycle B is generated by cleaving another C− H bond at bay-position by C−H activation. The oxidative addition of CH2Br2 with bimolecular B affords Pd(IV)coordinated PDI dimer C that then undergoes reductive elimination to form Pd(II) complex D; afterward, D is cyclized to produce a Pd-involving ring. Product E then undergoes further reductive elimination to yield the spiro-carbon fused product (SDP). The reaction of BrPDI and Br2HCCHBr2 follows a similar pathway to afford intermediate I featuring two hydrogen atoms in the middle ring which subsequently undergoes oxidative dehydrogenation in situ to finally produce NDP (Scheme S1). This strategy represents the first and straightforward synthesis of spiro-fused and ethylene-bridged PDI dimers and thus provides a facile access to cyclizing and fusing two five- or six-membered rings through two palladiumcatalyzed C−H activations and 4-fold C−C bond formation from monobrominated precursors and alkyl halides. To further illustrate the molecular structure of spiro-fused dimer, we have grown crystal of SDP for single-crystal X-ray diffraction analysis by slow solvent vapor diffusion. As revealed by Figure 1, two PDI subunits are definitely connected by sp3hybridized carbon to form a cruciform-shaped structure with a torsion angle of 87.54°, in close agreement with the computed value of 89.96° (Figure S8). A torsion angle in SBF molecule 15916

DOI: 10.1021/jacs.7b09140 J. Am. Chem. Soc. 2017, 139, 15914−15920

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the short wavelength absorptions are originated from the contribution of NDP segment (Figure 2). The optical energy gaps estimated from their absorption edges in CHCl3 solution are 2.17 eV for SDP, 2.21 eV for NDP, and 2.18 eV for SNTP, respectively. Compared with the broadened peaks of PDI in film, all the three molecules showed just a few nanometer red-shifts from solutions to films (Figure S3), suggesting a relatively weaker aggregation tendency resulting from their distorted structures. This characteristic endowed the active layer with a favorable nanoscale D/A mixing morphology with increased interfacial area between donor and acceptor, which is beneficial for the efficient charge separation from the photogenerated excitons. Furthermore, the spirocyclic compounds are highly fluorescent with absolute quantum yield reaching 83.72% for SDP (Tables 1 and S3), again, emphasizing the crucial role of the rigid spiro structure. The electrochemical properties of these dyes were studied by cyclic voltammetry (CV) in CH2Cl2. Compared with the parent PDI, these dyes can accept four to eight electrons as revealed by differential pulse voltammetry curves (Figure S4). Their half-wave reduction potentials versus Fc/Fc+ were summarized in Table S4. Their first half-wave potentials were gradually less negative, which implies their enhanced electron-accepting ability from monomer, dimer, to tetramer. The LUMO energy levels were estimated to be −3.86, −3.91, −3.90, and −3.95 eV for PDI, SDP, NDP, and SNTP, respectively. These values reveal a comparable electron affinity to classical fullerene electron accepting materials. In order to demonstrate potential applications of spiro-fused PDI arrays in OSCs, we prepared the photovoltaic devices with the inverted device structure of ITO/ZnO/BHJ/MoOx/Al and used a commercial polymer material (PTB7-Th)22 as the electronic donor due to its complementary absorption and wellmatched energy levels with the PDI acceptors. The optimal donor/acceptor weight ratios (D/A) were kept at 1:1, and the photovoltaic parameters were shown in Table S7. Chlorobenzene (CB) and 1,8-diiodooctane (DIO) were employed as the solvent and additive, respectively. From Table 2, for SDP with spiro-bond constituent, a PCE of 5.18% was obtained, with an increased short-circuit current (JSC) of 13.21 mA/cm2; thus, the PCE value of spiro-bonded dimer was more than two times that of parent PDI. While the parameters of the devices based on PTB7-Th:NDP were consistent with the reported data possessing a PCE of 6.43%,23 as the length of conjugation increased, SNTP based solar cells provided the following characteristics: JSC = 15.21 mA/cm2, FF = 0.61, and PCE = 7.17%, though lower than that of ethylene-bridged PDI tetramer,24 which were slightly superior to those of non spiro NDP based OSCs. The slightly lowered VOC of 0.77 V lies in the lower LUMO energy level resulting from extended πsystem. These parameters demonstrated that the spiroconjugation in PDI dyes is beneficial to enhance the device conversion efficiency. The current density−voltage (J−V) characteristics and the external quantum efficiency (EQE) plots of these four dyes of the optimized solar cell devices were shown in Figure 4. The cells showed broad photo response from 300 to 800 nm. In particular, the EQE of PTB7-Th:SNTP approached to 75% in the range of 550−750 nm, which can be attributed to the strong absorption and efficient exiciton separation by the 3D molecular PDI geometry. In addition, the calculated JSC integrated from the EQE plots for PDI, SDP, NDP, and SNTP based devices were 9.53, 12.67, 13.50, and 14.58 mA/ cm2, respectively, consistent with the measured ones.

Figure 2. UV/vis absorption spectra of monomer PDI (black), dimers SDP (red) and NDP (blue), and tetramer SNTP (green) in CHCl3 (bottom), and a comparison with the corresponding TDDFT/631g(d)-computed spectra not including vibronic structures (top).

spiroconjugation effect21 between two fused PDI halves through a spiro bond (Table 1). This experimental conclusion Table 1. Summary of Optical and Electronic Properties of PDI, SDP, NDP, and SNTP in Solution compd

λmax [nm]a

εmax [M−1 cm−1]a

Eopt g [eV]b

Φfl [%]c

E1r [V]d

ELUMO [eV]e

PDI SDP NDP SNTP

527 550 549 555

87615 93000 82933 143400

2.30 2.17 2.21 2.18

89.36 83.72 38.36 58.14

−1.10 −1.01 −1.02 −1.00

−3.86 −3.91 −3.90 −3.95

Measured in dilute CHCl3 solution (1.0 × 10−5 M). bCalculated by the onset of absorption in CHCl3 solution according to Eopt g (eV) = (1240/λonset). cMeasured in dilute CHCl3 solution (1.0 × 10−6 M) and determined by absolute quantum yield method. dHalf-wave reductive potentials (in V vs Fc/Fc+) measured in CH2Cl2 at a scan rate of 0.1 V/s using ferrocene as an internal potential mark. eLUMO (eV) estimated by the onset of the reduction peaks and calculated according to ELUMO = −(4.8 + Ereonset) eV.

a

is further supported by a comparison of the computed absorption spectra of four PDI dyes (Figure 2). In terms of the orbital nature of the time-dependent density functional theory (TDDFT) linear response method computed electronic transitions in Figure 3, it is clear that the lowest energy peak of SDP comes from two degenerate excited state from HOMO to LUMO and HOMO to LUMO+1 transition. Due to electronic coupling between PDI segments, there are obvious energy splitting for frontier orbitals (HOMO, HOMO−1, LUMO, and LUMO+1), the HOMO−LUMO gap is reduced. Therefore, the absorption edge is red-shifted, and the absorption peak turns broader due to intramolecular coupling. The frontier orbitals of SDP molecule are strongly distributed on the entire system, though the two PDI parts are connected via a sp3carbon, shedding more light on the spiroconjugation effect resulting from these spirocyclic fused aromatics. Contrastively, the absorption edge of NDP is also minorly red-shifted, the absorption peaks are composed of HOMO to LUMO, HOMO−1 to LUMO, and HOMO−2 to LUMO transitions. Due to strong exciton coupling for NDP, a new absorption emerges around 400 nm. The entire absorption peak of SNTP can be regard as superposition of absorption from two dimers (SDP, NDP) approximately. The absorption edge is mainly originated from the contribution from SDP segment. However, 15917

DOI: 10.1021/jacs.7b09140 J. Am. Chem. Soc. 2017, 139, 15914−15920

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Figure 3. Energies and shapes of b3lyp/6-31g(d) frontier orbitals of model PDI, SDP, NDP, and SNTP.

Table 2. Photovoltaic Parameters of the Optimized PTB7-Th:PDI Acceptor Solar Cells under AM 1.5G Illumination of 100 mW/cm2 acceptor PDI SDP NDP SNTP a

VOC [V] 0.82 0.83 0.80 0.77

± ± ± ±

0.00 0.00 0.00 0.00

JSC [mA/cm2] 8.22 13.28 14.02 15.22

± ± ± ±

0.15 0.27 0.31 0.22

FF

(9.53)a (12.67)a (13.50)a (14.58)a

0.36 0.47 0.56 0.60

± ± ± ±

PCE [%] 0.00 0.01 0.01 0.01

2.46 5.12 6.25 7.07

± ± ± ±

0.05 0.07 0.11 0.09

(2.50)b (5.18)b (6.43)b (7.17)b

The JSC integrated from the EQE plots. bThe PCE maxed values.

transport, and eventually resulting in a higher JSC and FF in solar cell devices.

We measured the mobility in the optimal bulk heterojunction (BHJ) by the space-charge-limited current (SCLC) method with the device structure of ITO/MoOx/BHJ/MoOx/Al and ITO/Al/BHJ/Al for hole and electron mobilities, respectively. As shown in Figure S6 and Table S6, the measured electron mobilities are higher than the values of hole mobilities based on all blend films, but the balance of the BHJ films between hole and electron mobility was getting better when comparing with the PDI based devices. To gain insight into the morphology of the BHJ films, atomic force microscopy (AFM) was employed. There were significant differences between the blend morphology of PTB7-Th:PDI and of PTB7-Th:SDP, PTB7-Th:NDP, and PTB7-Th:SNTP shown in Figure S7. The PTB7-Th:PDI blend films showed large domain size and no continuous interpenetrating network, neither of which are good for the charge generation; thus, the PTB7-Th:PDI based solar cells showed lower JSC and FF. Obviously, the other three blends had fiber features and continuous interpenetrating network. Especially, the PTB7Th:SNTP active layer possessed more uniformed, flat surfaces with the root-mean-square (RMS) of 1.02 nm and fiber features, which was beneficial for the charge separation and



CONCLUSION

We have developed a facile and step-economic approach toward spiro-fused perylene diimides by palladium-catalyzed coupling reaction of 4-fold C−C bond formation sequence from monobrominated PDI precursors with CH2Br2. The photophysical behavior and electronic structures suggest discriminative features as the different carbon-bridge. The spiro-fused dimer showed distinct red-shifted absorbance duo to the significant spiroconjugation effect. Illustrated by singlecrystal X-ray diffraction analysis, the molecular structure of SDP possesses a 3D cruciform geometry with multiple short contacts existing between the adjacent molecules that will be beneficial to preserving rational and ordered 3D network. As a consequence, this spiroconjugation strategy is demonstrated to enhance the photovoltaic performances of reaching 7.17% for the spiro-fused tetramer in nonfullerene OSCs. Additionally, the spiro-fused PDI system still remains a continuously promising candidate for exploring their fascinating chiral and chiroptical features. 15918

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supported by the 973 Program (2014CB643502), the National Natural Science Foundation of China (NSFC) (No. 51673202 and 91427303), the Chinese Academy of Sciences (XDB12010400), and the Youth Innovation Promotion Association of Chinese Academy of Sciences.



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Figure 4. (a) J−V characteristics of the optimized PTB7-Th:PDI acceptor based solar cells under AM 1.5G illumination of 100 mW/ cm2. (b) Corresponding EQE spectra.



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b09140. Experimental details, synthesis and characterizations, TGA curves, UV absorption, CV curves, theoretical calculations, device details and characterizations for all new compounds (PDF) Crystallographic data for SDP (CIF)



REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] ORCID

Wei Jiang: 0000-0002-0153-7796 Jianhui Hou: 0000-0002-2105-6922 Zhaohui Wang: 0000-0001-5786-5660 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We thank Prof. Congyang Wang at the Institute of Chemistry, Chinese Academy of Sciences for his fruitful discussion and suggestions. We gratefully acknowledge Prof. Zhixiang Wei and Dr. Jianqi Zhang for the assistance with 2D GIWAXS measurements. We thank Dr. Dong Meng and Andong Zhang for their helpful data collection of X-ray single crystal diffraction and atomic force microscope images. This work was 15919

DOI: 10.1021/jacs.7b09140 J. Am. Chem. Soc. 2017, 139, 15914−15920

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DOI: 10.1021/jacs.7b09140 J. Am. Chem. Soc. 2017, 139, 15914−15920