DOI: 10.1021/cg900939c
Extended 7,7,8,8-Tetracyano-p-quinodimethane-Based Acceptors: How Molecular Shape and Packing Impact Electron Accepting Behavior Mamoun M. Bader,*,† Phuong-Truc T. Pham,‡ Basant R. Nassar,† Hui Lin,† Yu Xia,§ and C. Daniel Frisbie§
2009, Vol. 9 4599–4601
†
Department of Chemistry, Pennsylvania State University, Hazleton, Pennsylvania, ‡Department of Chemistry, Pennsylvania State University, Worthington Scranton, Pennsylvania, and §Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota Received August 8, 2009
ABSTRACT: Structural analysis of π-extended 7,7,8,8-tetracyano-p-quinodimethane (TCNQ)-based molecules reveal molecular and packing features which may help better understand the weak electron accepting features for these molecules compared with TCNQ, in solution (cyclic voltammetry measurements), while showing varying trends in thin solid films (photoinduced electron transfer measurements). On the basis of the current findings, we propose that unsymmetrical compounds in this series hold promise as good candidates for use in solar cell applications as their molecular features allow for better solid-state packing, a property likely conducive for better charge transport. Electron acceptor/donor pairs of compatible molecular shapes rather than the usually sought planar shapes may offer a route worth pursuing. 7,7,8,8-Tetracyano-p-quinodimethane (TCNQ) is an electron acceptor that has played an important role in the area of organic electronics.1 It has been shown to form charge transfer complexes with a variety of electron donors.2 Fine tuning of the electron accepting ability of TCNQ has been explored by means of introducing substituents3 or by annulations (extension of the π-system).4 One of the main applications for electron acceptors lies in the development of solar cells. On the molecular level, two key parameters are important in the design of these molecules, namely, molecular shape, as it impacts packing in the solid state, and the ability to accept and transport electrons. While molecular shape can be examined by crystal structural analysis, the ability to accept electrons can be approximately estimated by cyclic voltammetry (CV) measurements in solution.5 The prototype electron acceptor currently used in this field is Buckminsterfullerene (C60). It has several drawbacks that potentially limit its commercial use.6 Over the past few years, several TCNQ-based materials have been synthesized.7 Hanack and co-workers reported a series of extended π-system molecules,8 which were expected to enhance the stability of the product of the electron transfer to the acceptor (the radical anion) through the reduction of intramolecular electronic repulsions. On the basis of CV data however, this π-system extension resulted in TCNQ derivatives that were poorer electron acceptors as more negative reduction potentials were obtained compared with TCNQ.
Contrary to this finding, the observed trends in solution CV measurements do not hold true in solid thin films. Studies of *To whom correspondence should be addressed. E-mail:
[email protected]. r 2009 American Chemical Society
photoinduced electron transfer reaction rates from conjugated polymers such as polyphenylenevinylenes and polyalkyl-thiophenes (as donors) onto fused π-extended TCNQ molecules (as acceptors) reveal that these rates were more efficient for unsymmetrical acceptors 2 and 3 than for the symmetrical acceptor 4. Molecules 2 and 3 were found to undergo photoinduced electron transfer reactions virtually identical to C60.9 This is rather intriguing as it suggests that either shape and/or solid-state packing of these materials would play a key role in dictating the nature of the interface and degree of overlap between the donor and acceptor layers in solar cells. Several studies directed specifically toward addressing these issues have been reported.10 Very recently, Frechet and co-workers have demonstrated the use of a solution-processed electron donor layer containing the cone-shaped molecule subnaphthalocyanine,11 in building organic photovoltaic cells. This is an important and encouraging finding as it shows that charge transfer is still possible in donor and/or acceptor molecules not having the usually sought planar shape. Interestingly, higher solubility is also seen in nonplanar compared with planar molecules, a desirable feature from a processing viewpoint. In this communication, we report the solid-state packing of some π-extended TCNQ-based acceptors which generally adopt a nonplanar butterfly shape. Our goal is to gain some insights and a better understanding of the role that molecular shape and packing may play in the different behaviors observed in solution and the solid state for this class of compounds. Three molecules considered in this study are 13,13,14,14-tetracyano-5,12-tetracenequinodimethane (TCTQ, 2); 7,12-bis (dicyanomethylene)7,12-dihydrobenz[a]anthracene (BDCNBA; 3); 15,15,16,16-tetracyano-6,13-pentacenequinodimethane (6,13 -TCPQ; 4). Syntheses were carried out according to published procedures by reacting the corresponding dione with malonitrile in dry pyridine using TiCl4 as a catalyst.8 Crystals of 2 and 4 were grown by means of physical vapor transport (PVT) in a horizontal quartz tube with temperature gradient of ∼10 °C/cm,12 while those of 3 were grown from ethyl acetate. Their structural data are summarized in Table 1. We note that the three molecules under study crystallize in different space groups with different packing patterns. Loss of molecular planarity is observed in all three molecules as they all adopt butterfly-type structures in which the TCNQ ring adopts a boat conformation (Figure 1). The angles between the aromatic rings (the wings of the butterfly) are about the same for all three molecules (114o), while there is a considerable distortion in the naphthalene part of molecule 3. Torsions between the TCNQ moiety and the ring are 42.45°, 38.21°, and 41.92° for 2, 3, and 4, respectively. These values are comparable with the only two known structures in this series, namely, 11,11,12,12-tetracyano-9,10-anthraquinodimethane TCANQ13 Published on Web 10/06/2009
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Table 1. Crystallographic Information and Selected Structural Features for Compounds 2-4 compound
TCTQ 2
formula formula wt crystal system space group color a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) temp (K) Z G.O.F.
C24H10N4 354.36 orthorhombic Pnma yellow rod 9.0021(9) 16.3690(16) 11.8678(12) 90 90 90 1748.8(3) 173(2) K 4 1.011
BDCNBA. 2CH3COOEt 3 C28H18N4O2 442.46 triclinic P1 yellow plate 8.6501(11) 10.2167(13) 14.3111(18) 69.761(2)° 87.523(2)° 76.670(2)° 1153.5(3) 173(2) K 2 1.063
6,13TCPQ 4 C28H12N4 404.42 monoclinic P21/n yellow needle 7.6186(7) 21.104(2) 12.226(1) 90 92.714(2)° 90 1963.5(3) 173(2) K 4 1.011
Figure 1. Molecular structure and packing for (a) TCTQ, (b) BDCNBA, and (c) 6,13-TCPQ; TCNQ is planar.18 *Solvent molecules are not shown for clarity.
(35.4°) and a pseudo polymorph of 3 (49.5°).14 Better packing is observed in the unsymmetrical molecules 2 and 3 where “longer wings” of the butterfly form π-stacks with distances of approximately 3.6 A˚. This is clearly seen in Figure 1 where both 2 and 3 form stacks involving the longer napthalene portions or “wings” of the structures. Such preferential stacking is, however, lost in the symmetrical molecule 4. These structural features offer some insights at the molecular level into why these molecules show better electron accepting ability in the solid state, despite their common lack of planarity and “poor” (compared with TCNQ) solution CV data.9,15 Although our current work only focuses on the acceptors’ molecular shape and packing, other issues need to be considered in addressing the observed anomalies such as alignment of molecules at the interface, the nature of the donor layer, and the processing of these layers.16 To further study the role of molecular structure, we also have carried out DFT-B3LYP level calculations on these molecules (gas phase isolated molecules) using Spartan 06 software.17
Figure 2. DFT Calculated HOMO-LUMO levels for compounds 1-4.
The main features of the results of these calculations are as follows: (1) The LUMO-HOMO levels are nearly the same for all three molecules, with calculated band gaps of 3.17, 3.03, and 3.24 eV for 2, 3, and 4, respectively (compared to 2.51 eV calculated for TCNQ), Figure 2. (2) Calculated molecular geometries and shapes were in good agreement with the X-ray experimental data, lending further confidence in this level of theoretical study, whereas other theoretical models have predicted planar structures for these molecules.19 (3) The highest occupied molecular orbital (HOMO) localizes on the naphthalene moiety in 2 and 3, whereas it is uniformly distributed in 4, and the lowest unoccupied molecular orbital (LUMO) localizes on the tetracyano group without overlapping the HOMO, which illustrates that the HOMO and LUMO are completely separated. We also note that none of these molecules showed field effect in thin film or in single crystal device configurations. On the basis of this study, we conclude that for annulated or π-extended TCNQ-based molecules the less symmetrical molecules show better electron accepting ability in the solid state due to their molecular shape and packing. Molecules of this type (with lower molecular symmetry) are worth pursuing as acceptors, despite the somewhat discouraging results CV data may suggest. Furthermore, since most of the acceptor and donor layers in solar cells are amorphous or semi crystalline, perhaps designing donor molecules with bent molecular shapes for better overlap with these butterfly-shaped (or other non planar) acceptors might offer an additional advantage and approach to consider in the design of molecular organic donor/acceptor pairs for solar cells. Acknowledgment. The MRSEC Program of the National Science Foundation under Award Number DMR-0212302 primarily supported this work. Funding from the University College at Pennsylvania State University is also acknowledged. The authors also acknowledge B. E. Kucera, L. M. Hinkle, V. G. Young, Jr., and the X-ray Crystallographic Laboratory at the University of Minnesota. Supporting Information Available: Crystallographic information (CIF) and detailed X-ray data are available free of charge via the Internet at http://pubs.acs.org.
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