DOI: 10.1002/chem.201304857
Communication
& Photophysics
Carbonate Linkage Bearing Naphthalenediimides: Self-Assembly and Photophysical Properties Chidambar Kulkarni[a, b] and Subi J. George*[a] Dedicated to Professor C. N. R. Rao on the occasion of his 80th birthday
Abstract: Self-assembly of carbonate linkage bearing naphthalene diimides (NDI) showed unusually red-shifted excimer emission at approximately 560 nm. On the other hand, the ether linkers showed usual excimers at around 520 nm, highlighting the role of the carbonate group in tuning the molecular organization and the resultant photophysical properties of NDI.
Naphthalene diimides (NDIs) are one of the most studied class of n-type organic semiconductors for their application in organic field-effect transistors and solar cells.[1] The morphology of the active material plays a crucial role in determining their device characteristics.[2] Thus, the self-assembly properties of NDIs has been extensively investigated to control their supramolecular organization.[3] On the other hand, the optical properties of core unsubstituted NDIs are seldom explored for functional applications, as they show weak fluorescence with low quantum yields.[3a] One of the strategies utilized to overcome this limitation is the use of core-substitution of NDIs with electron-donating groups, like alkoxy and amines, to bathchromically shift the absorption spectra and increase the fluorescence quantum yield.[4] Tang and co-workers have introduced aggregation-induced enhanced emission (AIEE) strategy to increase the fluorescence quantum yield of chromophoric assemblies.[5] However, the AIEE phenomenon operates on the prerequisite that typically the molecule is nonplanar and has the freedom to undergo free-rotation about the bond connecting the chromophore and backbone moieties, like ethylene or silole; upon aggregation the rotation is restricted, minimizing the nonradiative processes, thus resulting in enhanced fluorescence quantum yield.[5a] Although this is an efficient strategy to enhance the
fluorescence quantum yield, the structural constraint on the molecule prohibits its application to conventional p-conjugated molecules, like NDIs. In this respect, our group has reported pre-associated (static) excimer emission[6] as an alternative way to enhance the fluorescence quantum yield of core-unsubstituted NDI assemblies.[7] This strategy relies on the formation of ground-state assemblies, which upon excitation lead to excimers. Excimer emission from NDIs obtained by the self-assembly approach is generally green (~ 500 nm).[7b] Few recent studies have reported the red-shifted excimer emission from coreunsubstituted NDI.[8] However, a clear understanding of such optical properties at a molecular level is lacking. Here, we report the self-assembly and unusually red-shifted excimer emission resulting from the NDI chromophores bearing a carbonate linkage. We utilize the self-assembly-based static excimer (pre-associated) approach along with the role of linker group (carbonate) to tune the optical properties of NDI, thus emphasizing the role of linkers in influencing the optical properties of NDI. Photophysical comparison of carbonate and ether linkage bearing NDIs reiterated the importance of linker groups in tuning the excimer emission. X-ray diffraction and computational studies have provided insight into the molecular organization of NDIs in the assemblies. Further, temperature-dependent chiroptical studies have been undertaken. The molecules under study (1–3) possess cholesteryl or dodecyl moieties on both imide positions of NDI chromophore through a carbonate or ether linkage (Scheme 1). Cholesteryl is
[a] C. Kulkarni, Dr. S. J. George Supramolecular Chemistry Laboratory, New Chemistry Unit Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) Jakkur P. O., Bangalore 560064 (India) E-mail: [email protected] [b] C. Kulkarni Chemistry and Physics of Materials Unit (CPMU) Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) Jakkur P. O., Bangalore 560064 (India) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304857. Chem. Eur. J. 2014, 20, 4537 – 4541
Scheme 1. a) Structure of molecules under study. b)–d) Different emissive states of 1. Photographs show the emission in different states along with the schematic of corresponding photophysical states.
4537
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication a well-known self-assembling motif, since it has the dual role of acting as a hydrophobic segment to induce aggregation and also renders a chiral bias to the molecular organization because of the presence of stereocenters.[9] Although self-assembly of cholesteryl appended chromophores has been extensively studied,[10] this is the first report of their use for the organization of NDIs. Remarkably, 1 showed different optical behavior in various solvents leading to excimer (yellow) in self-assembled and solid state (Scheme 1). To understand the origin of optical properties, emission spectra and life-time studies of 1 were compared with that of 2 and 3. Morphological investigation of 1 shows the presence of fluorescent nanoparticles in both solution and on substrates. Molecule 1 is molecularly dissolved in 1,2-dichloroethane (DCE, c = 5 10 5 m), as can be seen from the characteristic p–p* absorption features of NDI between 300–400 nm with a lmax at 379 nm (e = 2.65 104 L mol 1 cm 1; Figure 1 a).[11] The corresponding emission spectra show a maximum at 407 nm, and are a mirror image of its absorption spectra (Figure 1 b). With an increase in the percentage of acetonitrile (ACN) in DCE/ACN solvent mixture, the optical density (OD) decreases, the ratio of the vibronic features I(0–0)/I(0–1) decreases from 1.27 in pure DCE to 0.94 in 40:60 DCE/ACN mixture and the absorption maximum is blue-shifted (to 359 nm) at higher percentages of ACN (60 %). Furthermore, a broad absorption band centered at 400 nm is observed along with scattering at higher percentage of ACN in DCE (Figure 1 a). All these features indicate the formation of H-type excitonically coupled aggregates.[12] Emission spectra of 1 in DCE/ACN mixtures show an initial decrease in monomer emission intensity at 408 nm (up to 35 % of ACN; Supporting Information, Figure S1) followed by the emergence of a new structure-less band centered at 560 nm; this new band grows in intensity with increase in the amount of ACN (Figure 1 b). This yellow emissive species could be attributed to the excimer of 1 in 50:50 (v/v) of DCE/ACN. The observed emission maximum is unusually red-shifted when compared to the typical green emissive NDI excimers reported in the literature.[7b] Emission of 1 in the solid state is also centered at 560 nm as seen in DCE/ACN mixture, suggesting a similar
molecular organization in both self-assembled and solid states (inset, Figure 1 c). To understand the nature of emission arising at 560 nm, time-correlated single photon counting (TCSPC) experiments were conducted with a nanosecond excitation on a 1:1 DCE/ACN mixture containing 1 and in the film state (obtained by drop casting a solution of 1 in chloroform on a glass slide). The resultant decay profiles showed a biexponential decay with the major contribution having a life-time of 20.11 ns (92 %, DCE/ACN) and 19.02 ns (84 %, film state; Figure 1 c).[13] Significantly a long life-time suggests that the excimer formation from pure collision in the excited state is not the sole reason for the observed yellow emission. In addition, the emission from the excimer is not observed until a certain degree of aggregation is reached in the ground state, which indicates that preorganization in the ground state is indeed necessary for the formation of excimer (Figure 1 b). The excitation spectra of both the film and aggregates collected selectively at the excimer emission (550 nm) showed blue-shifted absorption maximum compared to the corresponding monomer absorption and also showed a red-shifted band at 403 nm (Supporting Information, Figures S2 and S3). This reiterates the pre-associated or static origin of the excimer emission.[6, 3d] It has also been shown that excitation of chromophores with Hdimer arrangement (in the ground-state) leads to a red-shifted emission arising from the lower energy level exciton and is attributed to excimer emission.[14] Since 1 forms H-type aggregates in the ground-state, a similar mechanism would be operative in the present instance as well. The yellow excimer formation is also observed in a variety of solvents (Supporting Information, Figure S4). To understand whether the cholesteryl group is responsible for the observed red-shifted excimer of the NDI assemblies, molecule 2 bearing a dodecyl group in place of cholesteryl was synthesized and its optical properties were studied in its aggregated state. The absorption and emission features of 2 in chloroform (c = 5 10 5 m) resemble the characteristic features of NDI monomers (Supporting Information, Figure S5). Molecule 2 aggregates in a mixture of polar DMSO and chloroform (c = 1 10 4 m) with the absorption band extending up to 425 nm.[15] Emission spectra show two prominent bands cen-
Figure 1. Self-assembly of 1. a) Absorption, and b) emission (lex = 350 nm) of 1 in ACN/DCE solvent mixture (path length, l = 10 mm). Notations 0–0 and 0–1 indicate the vibrionic transitions of NDI. Emission spectra were normalized at 407 nm. c) Life-time decay profiles of 1 in DCE/ACN solvent mixture and film state (lexc = 380 nm; lmonitored = 550 nm). Inset: the solid-state emission spectrum of 1. All solution-state studies were carried out at a concentration of 5 10 5 m. Arrows indicate the spectral changes with increasing percentage of ACN in DCE. Chem. Eur. J. 2014, 20, 4537 – 4541
www.chemeurj.org
4538
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication
Figure 2. a) Absorption and emission (lexc = 350 nm) spectra of 2 in 90 % DMSO and 10 % chloroform. b) TCSPC decay profiles of 2 monitored at two different wavelengths with a 380 nm excitation. The average life-time values are given in the graph. All spectra were recorded in a 1 cm cuvette with a concentration of 1 10 4 m.
tered at 460 and 550 nm (Figure 2 a). TCSPC studies of 2 (c = 1 10 4 m, 90 % DMSO, 10 % CHCl3) with a nanosecond excitation monitoring of the emission at 460 and 560 nm showed triexponential decay profile at both wavelengths. Life-time values monitored at 460 nm are: 1.08 ns (55.23 %), 3.27 ns (40.65 %) and 10.54 ns (4.12 %). The first two components of the decay at 460 nm is typical of NDI aggregates[7a] and amounts to about 95 % of the decay, the larger life-time of the third component (~ 5 %) could be arising from the overlap of the 560 nm band. At 560 nm, the life-time values are 3.88 ns (26 %), 9.04 ns (45.2 %) and 24.04 ns (28.8 %), which are significantly larger than the ones at 460 nm. Also, the 3.88 ns component at 560 nm can have a contribution from the emission at 460 nm. Based on the red-shifted excitation spectra compared to monomer absorption (Supporting Information, Figure S6) and the life-time studies, the emissions at 460 and 560 nm are ascribed to the J-aggregate and pre-associated excimer emission of 2, respectively. The emission from the excimeric state of 2 is even maintained in the solid state and different solvents (Supporting Information, Figures S7 and S8). Remarkably, both 1 and 2 show similar emission maxima (~550 nm) and the average life-time (~ 18 ns) values for the excimer state. Thus, the peripheral self-assembly group does not affect the photophysical properties drastically.[16] Further, the optical properties of 3 (containing an ether linker) were studied to investigate the role of linkers to influence the photophysical properties. Molecule 3 is molecularly dissolved in DCE (Supporting Information, Figure S9a) and with Chem. Eur. J. 2014, 20, 4537 – 4541
www.chemeurj.org
increasing amounts of ACN in DCE a new red-shifted band at 390 nm appears with concomitant decrease in the absorbance of the 0–0 (380 nm) and 0–1 (360 nm) vibronic bands, characteristic of NDI intermolecular interactions. The emission spectra show a broad, structure-less band centered at 520 nm beyond 50 % of ACN in DCE (Supporting Information, Figure S8b). TCSPC experiments with a nanosecond excitation showed a biexponential (t1 = 1.72 ns, 15.15 % and t2 = 10.85 ns, 84.85 %, c2 = 1.01) decay at 520 nm with long-lived species (Supporting Information, Figure S10). The position, shape and the life-time of the species emitting at 520 nm is typical of NDI pre-associated excimers reported in the literature (for a comparison see Supporting Information, Figure S11).[7b] This clearly indicates that the carbonate linkage plays an important role in controlling the organization leading to the formation of red-shifted excimer. Morphological studies were performed to further understand the molecular packing in the self-assembled aggregates of 1. AFM of a film obtained by drop casting a 60:40 (v/v) DCE/ACN solution of 1 on glass substrate shows the formation of spherical nanoparticles with a mean diameter of 300 nm (Figure 3 a). Dynamic light-scattering experiment of the same solution shows an apparent hydrodynamic radius of 200– 400 nm (Supporting Information, Figure S12), suggesting that nanoparticles are indeed self-assembled in solution. Bischolesteryl appended chromophores are known to form nanoparticles.[17] Confocal fluorescence microscopy images of the self-assembled solution of 1, sealed between glass-slides, showed yellow fluorescent nanoparticles and the emission maximum (560 nm) also matches well with that obtained from the solution-state spectroscopic studies (Supporting Information, Figure S13). Molecule 2 also formed nanoparticles as revealed by transmission electron microscopy (Supporting Information, Figure S14). Thus, pre-associated excimer emission is an alternative to AIEE mechanism for obtaining fluorescent organic nanoparticles formed by the self-assembly of p-conjugated molecules.[18] Powder XRD of 1 showed a sharp reflection at low-angle region and other higher angle reflections as well. The d-spacing corresponding to the low-angle reflection (47 ) matches with the molecular dimension (49 ) including the cholesteryl moiety (Supporting Information, Figure S15), and the reflection at around 278 (2q) shows a d-spacing of 3.28 (Figure 2 b), which could be identified as the p-stacking distance. Such a small p-stacking distance indicates the presence of strong interchromophoric interaction. Powder XRD of 2 also showed similar small angle reflection and a stacking distance of 3.6 . Sharp reflections were observed for 2 when compared 1, indicating the higher crystallinity of the former because of the ordered packing of linear alkyl chains. Computational studies on the dimer of the model compounds (obtained by replacing the cholesteryl or dodecyl moiety with a methyl group and keeping the carbonate group to reduce the computational cost) shows a stacking distance of 3.35 , in agreement with experiments (Supporting Information, Figure S16). Further, the optimized geometry of dimer and trimer show the presence of weak C H···O hydrogen
4539
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication suggests strong p–p interactions between the NDI chromophores in the self-assembled state.[19] Temperature dependent CD and UV/Vis absorption spectra of 1 (c = 5 10 5 m) in a 1:1 (v/v) solution of ACN/DCE shows that 1 exists as individual molecules beyond 40 8C and in an aggregated state below this temperature (Supporting Information, Figure S17). Further, we attempted to study the mechanism of self-assembly using temperaturedependent CD studies. However, the obtained cooling curve (Supporting Information, Figure S18) showed a reasonable fit to both isodesmic and cooperative models[20] of supramolecular polymerization. Thus, further studies will be carried out to understand the exact mechanism of polymerization. In conclusion, we have shown Figure 3. a) AFM height image of 1 obtained in the film state. b) Powder XRD pattern of 1 and 2. Inset: the zoomthe self-assembly of two novel in of the wide-angle region corresponding to the stacking distances. c) Geometry-optimized structure of the NDI derivatives bearing a carbontrimer of model compound. The solid lines indicate the C H···O and p-stacking interactions along with the correate linkage resulting in unusually sponding distances (see Supporting Information, Figure 16b for the color version of this figure). red-shifted excimer emission in both solid and self-assembled states. The ether linkage containing NDI derivative shows typibonds involving the oxygen of carbonate or imide group and cal NDI excimer emission. Detailed spectroscopic analysis has C H of ethylene spacer (Figure 3 c). These multiple interactions revealed the pre-associated (static) nature of excimer emission. result in strong interchromophoric interactions and this could These findings highlight the role of linker functional groups to possibly be the reason for the observed red-shifted excimer fine-tune the properties of self-assembled systems. Although emission. many reports in the literature have reported uncharacteristic The effect of chiral cholesteryl moiety on the organization of optical properties of chromophores, seldom has the linker or the assemblies was studied using circular dichroism (CD). CD spacer been considered to play an important role. Thus, the spectra of the self-assembly at higher percentages of ACN present work might stimulate researchers to examine the role (40 %) in DCE showed a positive bisignated Cotton effect in of spacers from an altogether different point of view. the p–p* region, suggesting the chiral bias in the supramolecular organization of NDI molecules, which is induced by the peripheral cholesteryl groups (Figure 4). The bisignated nature Acknowledgements of the CD curve, characteristic of excitonic interactions, further We thank Prof. C. N. R. Rao for his support throughout this work, JNCASR and DST, Government of India for financial support. The authors thank Mr. Amrit (DLS), Mr. Swamynathan (XRD), Dr. Basavaraj (AFM) and Peter Koreevar, Ralf Bovee (MALDI-TOF). S.J.G gratefully acknowledges Sheikh Saqr Career Award Fellowship. Keywords: excimer · naphthalene diimide · photophysics · self-assembly · supramolecular polymers [1] a) J. G. laquindanum, H. E. Katz, A. Dodabalapur, A. J. Lovinger, J. Am. Chem. Soc. 1996, 118, 11331 – 11332; b) H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li, Y.-Y. Lin, A. Dodabalapur, Nature 2000, 404, 478481; c) A. Facchetti, Mater. Today 2007, 10, 28 – 37; d) Q. Meng, W.
Figure 4. Chiroptical studies of 1. CD spectra of 1 in ACN/DCE solvent mixture at 20 8C (path length, l = 10 mm, c = 5 10 5 m). Chem. Eur. J. 2014, 20, 4537 – 4541
www.chemeurj.org
4540
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication
[2]
[3]
[4]
[5]
[6]
[7] [8]
[9]
[10]
Hu, Phys. Chem. Chem. Phys. 2012, 14, 14152 – 14164; e) T. Earmme, Y.-J. Hwang, N. M. Murari, S. Subramaniyan, S. A. Jenekhe, J. Am. Chem. Soc. 2013, 135, 14960 – 14963. a) P. M. Beaujuge, J. M. J. Frchet, J. Am. Chem. Soc. 2011, 133, 20009 – 20029; b) Z. B. Henson, K. Mllen, G. C. Bazan, Nat. Chem. 2012, 4, 699 – 704; c) J. Mei, Y. Diao, A. L. Appleton, L. Fang, Z. Bao, J. Am. Chem. Soc. 2013, 135, 6724 – 6746; d) F. Wrthner, K. Meerholz, Chem. Eur. J. 2010, 16, 9366 – 9373; e) M. Wang, F. Wudl, J. Mater. Chem. 2012, 22, 24397 – 24314; f) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, A. P. H. J. Schenning, Chem. Rev. 2005, 105, 1491 – 1546; g) S. S. Babu, V. K. Praveen, A. Ajayaghosh, Chem. Rev. 2014; DOI: 10.1021/cr400195e. a) S. V. Bhosale, C. H. Jani, S. J. Langford, Chem. Soc. Rev. 2008, 37, 331 – 342; b) M. R. Molla, S. Ghosh, Chem. Mater. 2011, 23, 95 – 105; c) N. Ponnuswamy, G. D. Pantos¸, M. M. J. Smulders, J. K. M. Sanders, J. Am. Chem. Soc. 2012, 134, 566 – 573; d) H. Shao, T. Nguyen, N. C. Romano, D. A. Modarelli, J. R. Parquette, J. Am. Chem. Soc. 2009, 131, 16374 – 16376; e) F. Aparicio, L. Snchez, Chem. Eur. J. 2013, 19, 10482 – 10486; f) T. W. Anderson, J. K. M. Sanders, G. D. Pantos¸, Org. Biomol. Chem. 2010, 8, 4274 – 4280; g) T. D. M. Bell, S. V. Bhosale, C. M. Forsyth, D. Hayne, K. P. Ghiggino, J. A. Hutchison, C. H. Jani, S. J. Langford, M. A.-P. Lee, C. P. Woodward, Chem. Commun. 2010, 46, 4881 – 4883; h) G. D. Pantos¸, P. Pengo, J. K. M. Sanders, Angew. Chem. 2007, 119, 198 – 201; Angew. Chem. Int. Ed. 2007, 46, 194 – 197. a) S. V. Bhosale, S. V. Bhosale, S. K. Bhargava, Org. Biomol. Chem. 2012, 10, 6455 – 6468; b) F. Wrthner, S. Ahmed, C. Thalacker, T. Debaerdemaeker, Chem. Eur. J. 2002, 8, 4742 – 4750. a) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Commun. 2009, 4332 – 4353; b) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40, 5361 – 5388; c) Z. Zhao, J. W. Y. Lam, B. Z. Tang, J. Mater. Chem. 2012, 22, 23726 – 23740. F. M. Winnik, Chem. Rev. 1993, 93, 587 – 614; this review details the photophysics and experimental assignment of pre-associated excimer for pyrene. These experimental characteristics of pre-associated excimer have been used to assign the nature of excimer in the present work. a) M. Kumar, S. J. George, Chem. Eur. J. 2011, 17, 11102 – 11106; b) M. Kumar, S. J. George, Nanoscale 2011, 3, 2130 – 2133. a) M. R. Molla, S. Ghosh, Chem. Eur. J. 2012, 18, 1290 – 1294; b) S. Basak, J. Nanda, A. Banerjee, Chem. Commun. 2013, 49, 6891 – 6893; c) J. B. Bodapati, H. Icil, Photochem. Photobiol. Sci. 2011, 10, 1283 – 1293. a) K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata, T. Komori, F. Ohseto, K. Ueda, S. Shinkai, J. Am. Chem. Soc. 1994, 116, 6664 – 6676; b) H. Svobodov, V. Noponen, E. Kolehmainen, E. Sievnen, RSC Adv. 2012, 2, 4985 – 5007. a) R. Wang, C. Geiger, L. Chen, B. Swanson, D. G. Whitten, J. Am. Chem. Soc. 2000, 122, 2399 – 2400; b) K. Sugiyasu, N. Fujita, S. Shinkai, Angew.
Chem. Eur. J. 2014, 20, 4537 – 4541
www.chemeurj.org
[11] [12] [13]
[14] [15]
[16]
[17] [18]
[19] [20]
Chem. 2004, 116, 1249 – 1253; Angew. Chem. Int. Ed. 2004, 43, 1229 – 1233; c) S.-I. Kawano, N. Fujita, S. Shinkai, Chem. Eur. J. 2005, 11, 4735 – 4742; d) A. Ajayaghosh, C. Vijayakumar, R. Varghese, S. J. George, Angew. Chem. 2006, 118, 470 – 474; Angew. Chem. Int. Ed. 2006, 45, 456 – 460. J. Gawron´ski, M. Brzostowska, K. Kacprzak, H. Kołbon, P. Skowronek, Chirality 2000, 12, 263 – 268. H. Shao, J. R. Parquette, Chem. Commun. 2010, 46, 4285 – 4287. The life-time components are as follows; for 1:1 ACN/DCE (v/v) A1 = 7.86 %, t1 = 5.21 ns A2 = 92.14 %, t2 = 20.11 ns, and for film state A1 = 16.06 %, t1 = 6.09 ns A2 = 83.94 %, t2 = 19.02 ns. Here A denotes the prefactor for each decay and t the corresponding life-time. N. S. S. Kumar, S. Varghese, C. H. Suresh, N. P. Rath, S. Das, J. Phys. Chem. C 2009, 113, 11927 – 11935. Molecule 2 aggregates in a range of polar and apolar solvents (Supporting Information, Figure S8) leading to same photophysical states. It has lower aggregation tendency in acetonitrile, and thus the more polar DMSO was used for the studies. We anticipate that any combination of good solvent and a bad solvent should lead to the same photophysical state. Although 1 and 2 show H- and J-type aggregation, respectively, in the ground state, on excitation there might be some structural rearrangement leading to a supramolecular organization, which is different from the ground state, thus resulting in similar red-shifted excimer emission. S. Abraham, R. K. Vijayaraghavan, S. Das, Langmuir 2009, 25, 8507 – 8513. a) A. Kaeser, A. P. H. J. Schenning, Adv. Mater. 2010, 22, 2985 – 2997; b) D. Gonzlez-Rodr guez, P. G. A. Janssen, R. Mart n-Rapffln, I. De Cat, S. De Feyter, A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem. Soc. 2010, 132, 4710 – 4719. M. Kumar, N. Jonnalagadda, S. J. George, Chem. Commun. 2012, 48, 10948 – 10950. a) P. Jonkheijm, P. van der Schoot, A. P. H. J. Schenning, E. W. Meijer, Science 2006, 313, 80 – 83; b) M. M. J. Smulders, A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem. Soc. 2008, 130, 606 – 611; c) Z. Chen, A. Lohr, C. R. Saha-Mçller, F. Wrthner, Chem. Soc. Rev. 2009, 38, 564 – 584; d) C. Kulkarni, S. Balasubramanian, S. J. George, ChemPhysChem 2013, 14, 661 – 673.
Received: December 12, 2013 Revised: February 2, 2014 Published online on March 13, 2014
4541
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication
& Photophysics
Carbonate Linkage Bearing Naphthalenediimides: Self-Assembly and Photophysical Properties Chidambar Kulkarni[a, b] and Subi J. George*[a] Dedicated to Professor C. N. R. Rao on the occasion of his 80th birthday
Abstract: Self-assembly of carbonate linkage bearing naphthalene diimides (NDI) showed unusually red-shifted excimer emission at approximately 560 nm. On the other hand, the ether linkers showed usual excimers at around 520 nm, highlighting the role of the carbonate group in tuning the molecular organization and the resultant photophysical properties of NDI.
Naphthalene diimides (NDIs) are one of the most studied class of n-type organic semiconductors for their application in organic field-effect transistors and solar cells.[1] The morphology of the active material plays a crucial role in determining their device characteristics.[2] Thus, the self-assembly properties of NDIs has been extensively investigated to control their supramolecular organization.[3] On the other hand, the optical properties of core unsubstituted NDIs are seldom explored for functional applications, as they show weak fluorescence with low quantum yields.[3a] One of the strategies utilized to overcome this limitation is the use of core-substitution of NDIs with electron-donating groups, like alkoxy and amines, to bathchromically shift the absorption spectra and increase the fluorescence quantum yield.[4] Tang and co-workers have introduced aggregation-induced enhanced emission (AIEE) strategy to increase the fluorescence quantum yield of chromophoric assemblies.[5] However, the AIEE phenomenon operates on the prerequisite that typically the molecule is nonplanar and has the freedom to undergo free-rotation about the bond connecting the chromophore and backbone moieties, like ethylene or silole; upon aggregation the rotation is restricted, minimizing the nonradiative processes, thus resulting in enhanced fluorescence quantum yield.[5a] Although this is an efficient strategy to enhance the
fluorescence quantum yield, the structural constraint on the molecule prohibits its application to conventional p-conjugated molecules, like NDIs. In this respect, our group has reported pre-associated (static) excimer emission[6] as an alternative way to enhance the fluorescence quantum yield of core-unsubstituted NDI assemblies.[7] This strategy relies on the formation of ground-state assemblies, which upon excitation lead to excimers. Excimer emission from NDIs obtained by the self-assembly approach is generally green (~ 500 nm).[7b] Few recent studies have reported the red-shifted excimer emission from coreunsubstituted NDI.[8] However, a clear understanding of such optical properties at a molecular level is lacking. Here, we report the self-assembly and unusually red-shifted excimer emission resulting from the NDI chromophores bearing a carbonate linkage. We utilize the self-assembly-based static excimer (pre-associated) approach along with the role of linker group (carbonate) to tune the optical properties of NDI, thus emphasizing the role of linkers in influencing the optical properties of NDI. Photophysical comparison of carbonate and ether linkage bearing NDIs reiterated the importance of linker groups in tuning the excimer emission. X-ray diffraction and computational studies have provided insight into the molecular organization of NDIs in the assemblies. Further, temperature-dependent chiroptical studies have been undertaken. The molecules under study (1–3) possess cholesteryl or dodecyl moieties on both imide positions of NDI chromophore through a carbonate or ether linkage (Scheme 1). Cholesteryl is
[a] C. Kulkarni, Dr. S. J. George Supramolecular Chemistry Laboratory, New Chemistry Unit Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) Jakkur P. O., Bangalore 560064 (India) E-mail: [email protected] [b] C. Kulkarni Chemistry and Physics of Materials Unit (CPMU) Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) Jakkur P. O., Bangalore 560064 (India) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304857. Chem. Eur. J. 2014, 20, 4537 – 4541
Scheme 1. a) Structure of molecules under study. b)–d) Different emissive states of 1. Photographs show the emission in different states along with the schematic of corresponding photophysical states.
4537
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication a well-known self-assembling motif, since it has the dual role of acting as a hydrophobic segment to induce aggregation and also renders a chiral bias to the molecular organization because of the presence of stereocenters.[9] Although self-assembly of cholesteryl appended chromophores has been extensively studied,[10] this is the first report of their use for the organization of NDIs. Remarkably, 1 showed different optical behavior in various solvents leading to excimer (yellow) in self-assembled and solid state (Scheme 1). To understand the origin of optical properties, emission spectra and life-time studies of 1 were compared with that of 2 and 3. Morphological investigation of 1 shows the presence of fluorescent nanoparticles in both solution and on substrates. Molecule 1 is molecularly dissolved in 1,2-dichloroethane (DCE, c = 5 10 5 m), as can be seen from the characteristic p–p* absorption features of NDI between 300–400 nm with a lmax at 379 nm (e = 2.65 104 L mol 1 cm 1; Figure 1 a).[11] The corresponding emission spectra show a maximum at 407 nm, and are a mirror image of its absorption spectra (Figure 1 b). With an increase in the percentage of acetonitrile (ACN) in DCE/ACN solvent mixture, the optical density (OD) decreases, the ratio of the vibronic features I(0–0)/I(0–1) decreases from 1.27 in pure DCE to 0.94 in 40:60 DCE/ACN mixture and the absorption maximum is blue-shifted (to 359 nm) at higher percentages of ACN (60 %). Furthermore, a broad absorption band centered at 400 nm is observed along with scattering at higher percentage of ACN in DCE (Figure 1 a). All these features indicate the formation of H-type excitonically coupled aggregates.[12] Emission spectra of 1 in DCE/ACN mixtures show an initial decrease in monomer emission intensity at 408 nm (up to 35 % of ACN; Supporting Information, Figure S1) followed by the emergence of a new structure-less band centered at 560 nm; this new band grows in intensity with increase in the amount of ACN (Figure 1 b). This yellow emissive species could be attributed to the excimer of 1 in 50:50 (v/v) of DCE/ACN. The observed emission maximum is unusually red-shifted when compared to the typical green emissive NDI excimers reported in the literature.[7b] Emission of 1 in the solid state is also centered at 560 nm as seen in DCE/ACN mixture, suggesting a similar
molecular organization in both self-assembled and solid states (inset, Figure 1 c). To understand the nature of emission arising at 560 nm, time-correlated single photon counting (TCSPC) experiments were conducted with a nanosecond excitation on a 1:1 DCE/ACN mixture containing 1 and in the film state (obtained by drop casting a solution of 1 in chloroform on a glass slide). The resultant decay profiles showed a biexponential decay with the major contribution having a life-time of 20.11 ns (92 %, DCE/ACN) and 19.02 ns (84 %, film state; Figure 1 c).[13] Significantly a long life-time suggests that the excimer formation from pure collision in the excited state is not the sole reason for the observed yellow emission. In addition, the emission from the excimer is not observed until a certain degree of aggregation is reached in the ground state, which indicates that preorganization in the ground state is indeed necessary for the formation of excimer (Figure 1 b). The excitation spectra of both the film and aggregates collected selectively at the excimer emission (550 nm) showed blue-shifted absorption maximum compared to the corresponding monomer absorption and also showed a red-shifted band at 403 nm (Supporting Information, Figures S2 and S3). This reiterates the pre-associated or static origin of the excimer emission.[6, 3d] It has also been shown that excitation of chromophores with Hdimer arrangement (in the ground-state) leads to a red-shifted emission arising from the lower energy level exciton and is attributed to excimer emission.[14] Since 1 forms H-type aggregates in the ground-state, a similar mechanism would be operative in the present instance as well. The yellow excimer formation is also observed in a variety of solvents (Supporting Information, Figure S4). To understand whether the cholesteryl group is responsible for the observed red-shifted excimer of the NDI assemblies, molecule 2 bearing a dodecyl group in place of cholesteryl was synthesized and its optical properties were studied in its aggregated state. The absorption and emission features of 2 in chloroform (c = 5 10 5 m) resemble the characteristic features of NDI monomers (Supporting Information, Figure S5). Molecule 2 aggregates in a mixture of polar DMSO and chloroform (c = 1 10 4 m) with the absorption band extending up to 425 nm.[15] Emission spectra show two prominent bands cen-
Figure 1. Self-assembly of 1. a) Absorption, and b) emission (lex = 350 nm) of 1 in ACN/DCE solvent mixture (path length, l = 10 mm). Notations 0–0 and 0–1 indicate the vibrionic transitions of NDI. Emission spectra were normalized at 407 nm. c) Life-time decay profiles of 1 in DCE/ACN solvent mixture and film state (lexc = 380 nm; lmonitored = 550 nm). Inset: the solid-state emission spectrum of 1. All solution-state studies were carried out at a concentration of 5 10 5 m. Arrows indicate the spectral changes with increasing percentage of ACN in DCE. Chem. Eur. J. 2014, 20, 4537 – 4541
www.chemeurj.org
4538
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication
Figure 2. a) Absorption and emission (lexc = 350 nm) spectra of 2 in 90 % DMSO and 10 % chloroform. b) TCSPC decay profiles of 2 monitored at two different wavelengths with a 380 nm excitation. The average life-time values are given in the graph. All spectra were recorded in a 1 cm cuvette with a concentration of 1 10 4 m.
tered at 460 and 550 nm (Figure 2 a). TCSPC studies of 2 (c = 1 10 4 m, 90 % DMSO, 10 % CHCl3) with a nanosecond excitation monitoring of the emission at 460 and 560 nm showed triexponential decay profile at both wavelengths. Life-time values monitored at 460 nm are: 1.08 ns (55.23 %), 3.27 ns (40.65 %) and 10.54 ns (4.12 %). The first two components of the decay at 460 nm is typical of NDI aggregates[7a] and amounts to about 95 % of the decay, the larger life-time of the third component (~ 5 %) could be arising from the overlap of the 560 nm band. At 560 nm, the life-time values are 3.88 ns (26 %), 9.04 ns (45.2 %) and 24.04 ns (28.8 %), which are significantly larger than the ones at 460 nm. Also, the 3.88 ns component at 560 nm can have a contribution from the emission at 460 nm. Based on the red-shifted excitation spectra compared to monomer absorption (Supporting Information, Figure S6) and the life-time studies, the emissions at 460 and 560 nm are ascribed to the J-aggregate and pre-associated excimer emission of 2, respectively. The emission from the excimeric state of 2 is even maintained in the solid state and different solvents (Supporting Information, Figures S7 and S8). Remarkably, both 1 and 2 show similar emission maxima (~550 nm) and the average life-time (~ 18 ns) values for the excimer state. Thus, the peripheral self-assembly group does not affect the photophysical properties drastically.[16] Further, the optical properties of 3 (containing an ether linker) were studied to investigate the role of linkers to influence the photophysical properties. Molecule 3 is molecularly dissolved in DCE (Supporting Information, Figure S9a) and with Chem. Eur. J. 2014, 20, 4537 – 4541
www.chemeurj.org
increasing amounts of ACN in DCE a new red-shifted band at 390 nm appears with concomitant decrease in the absorbance of the 0–0 (380 nm) and 0–1 (360 nm) vibronic bands, characteristic of NDI intermolecular interactions. The emission spectra show a broad, structure-less band centered at 520 nm beyond 50 % of ACN in DCE (Supporting Information, Figure S8b). TCSPC experiments with a nanosecond excitation showed a biexponential (t1 = 1.72 ns, 15.15 % and t2 = 10.85 ns, 84.85 %, c2 = 1.01) decay at 520 nm with long-lived species (Supporting Information, Figure S10). The position, shape and the life-time of the species emitting at 520 nm is typical of NDI pre-associated excimers reported in the literature (for a comparison see Supporting Information, Figure S11).[7b] This clearly indicates that the carbonate linkage plays an important role in controlling the organization leading to the formation of red-shifted excimer. Morphological studies were performed to further understand the molecular packing in the self-assembled aggregates of 1. AFM of a film obtained by drop casting a 60:40 (v/v) DCE/ACN solution of 1 on glass substrate shows the formation of spherical nanoparticles with a mean diameter of 300 nm (Figure 3 a). Dynamic light-scattering experiment of the same solution shows an apparent hydrodynamic radius of 200– 400 nm (Supporting Information, Figure S12), suggesting that nanoparticles are indeed self-assembled in solution. Bischolesteryl appended chromophores are known to form nanoparticles.[17] Confocal fluorescence microscopy images of the self-assembled solution of 1, sealed between glass-slides, showed yellow fluorescent nanoparticles and the emission maximum (560 nm) also matches well with that obtained from the solution-state spectroscopic studies (Supporting Information, Figure S13). Molecule 2 also formed nanoparticles as revealed by transmission electron microscopy (Supporting Information, Figure S14). Thus, pre-associated excimer emission is an alternative to AIEE mechanism for obtaining fluorescent organic nanoparticles formed by the self-assembly of p-conjugated molecules.[18] Powder XRD of 1 showed a sharp reflection at low-angle region and other higher angle reflections as well. The d-spacing corresponding to the low-angle reflection (47 ) matches with the molecular dimension (49 ) including the cholesteryl moiety (Supporting Information, Figure S15), and the reflection at around 278 (2q) shows a d-spacing of 3.28 (Figure 2 b), which could be identified as the p-stacking distance. Such a small p-stacking distance indicates the presence of strong interchromophoric interaction. Powder XRD of 2 also showed similar small angle reflection and a stacking distance of 3.6 . Sharp reflections were observed for 2 when compared 1, indicating the higher crystallinity of the former because of the ordered packing of linear alkyl chains. Computational studies on the dimer of the model compounds (obtained by replacing the cholesteryl or dodecyl moiety with a methyl group and keeping the carbonate group to reduce the computational cost) shows a stacking distance of 3.35 , in agreement with experiments (Supporting Information, Figure S16). Further, the optimized geometry of dimer and trimer show the presence of weak C H···O hydrogen
4539
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication suggests strong p–p interactions between the NDI chromophores in the self-assembled state.[19] Temperature dependent CD and UV/Vis absorption spectra of 1 (c = 5 10 5 m) in a 1:1 (v/v) solution of ACN/DCE shows that 1 exists as individual molecules beyond 40 8C and in an aggregated state below this temperature (Supporting Information, Figure S17). Further, we attempted to study the mechanism of self-assembly using temperaturedependent CD studies. However, the obtained cooling curve (Supporting Information, Figure S18) showed a reasonable fit to both isodesmic and cooperative models[20] of supramolecular polymerization. Thus, further studies will be carried out to understand the exact mechanism of polymerization. In conclusion, we have shown Figure 3. a) AFM height image of 1 obtained in the film state. b) Powder XRD pattern of 1 and 2. Inset: the zoomthe self-assembly of two novel in of the wide-angle region corresponding to the stacking distances. c) Geometry-optimized structure of the NDI derivatives bearing a carbontrimer of model compound. The solid lines indicate the C H···O and p-stacking interactions along with the correate linkage resulting in unusually sponding distances (see Supporting Information, Figure 16b for the color version of this figure). red-shifted excimer emission in both solid and self-assembled states. The ether linkage containing NDI derivative shows typibonds involving the oxygen of carbonate or imide group and cal NDI excimer emission. Detailed spectroscopic analysis has C H of ethylene spacer (Figure 3 c). These multiple interactions revealed the pre-associated (static) nature of excimer emission. result in strong interchromophoric interactions and this could These findings highlight the role of linker functional groups to possibly be the reason for the observed red-shifted excimer fine-tune the properties of self-assembled systems. Although emission. many reports in the literature have reported uncharacteristic The effect of chiral cholesteryl moiety on the organization of optical properties of chromophores, seldom has the linker or the assemblies was studied using circular dichroism (CD). CD spacer been considered to play an important role. Thus, the spectra of the self-assembly at higher percentages of ACN present work might stimulate researchers to examine the role (40 %) in DCE showed a positive bisignated Cotton effect in of spacers from an altogether different point of view. the p–p* region, suggesting the chiral bias in the supramolecular organization of NDI molecules, which is induced by the peripheral cholesteryl groups (Figure 4). The bisignated nature Acknowledgements of the CD curve, characteristic of excitonic interactions, further We thank Prof. C. N. R. Rao for his support throughout this work, JNCASR and DST, Government of India for financial support. The authors thank Mr. Amrit (DLS), Mr. Swamynathan (XRD), Dr. Basavaraj (AFM) and Peter Koreevar, Ralf Bovee (MALDI-TOF). S.J.G gratefully acknowledges Sheikh Saqr Career Award Fellowship. Keywords: excimer · naphthalene diimide · photophysics · self-assembly · supramolecular polymers [1] a) J. G. laquindanum, H. E. Katz, A. Dodabalapur, A. J. Lovinger, J. Am. Chem. Soc. 1996, 118, 11331 – 11332; b) H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Siegrist, W. Li, Y.-Y. Lin, A. Dodabalapur, Nature 2000, 404, 478481; c) A. Facchetti, Mater. Today 2007, 10, 28 – 37; d) Q. Meng, W.
Figure 4. Chiroptical studies of 1. CD spectra of 1 in ACN/DCE solvent mixture at 20 8C (path length, l = 10 mm, c = 5 10 5 m). Chem. Eur. J. 2014, 20, 4537 – 4541
www.chemeurj.org
4540
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication
[2]
[3]
[4]
[5]
[6]
[7] [8]
[9]
[10]
Hu, Phys. Chem. Chem. Phys. 2012, 14, 14152 – 14164; e) T. Earmme, Y.-J. Hwang, N. M. Murari, S. Subramaniyan, S. A. Jenekhe, J. Am. Chem. Soc. 2013, 135, 14960 – 14963. a) P. M. Beaujuge, J. M. J. Frchet, J. Am. Chem. Soc. 2011, 133, 20009 – 20029; b) Z. B. Henson, K. Mllen, G. C. Bazan, Nat. Chem. 2012, 4, 699 – 704; c) J. Mei, Y. Diao, A. L. Appleton, L. Fang, Z. Bao, J. Am. Chem. Soc. 2013, 135, 6724 – 6746; d) F. Wrthner, K. Meerholz, Chem. Eur. J. 2010, 16, 9366 – 9373; e) M. Wang, F. Wudl, J. Mater. Chem. 2012, 22, 24397 – 24314; f) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, A. P. H. J. Schenning, Chem. Rev. 2005, 105, 1491 – 1546; g) S. S. Babu, V. K. Praveen, A. Ajayaghosh, Chem. Rev. 2014; DOI: 10.1021/cr400195e. a) S. V. Bhosale, C. H. Jani, S. J. Langford, Chem. Soc. Rev. 2008, 37, 331 – 342; b) M. R. Molla, S. Ghosh, Chem. Mater. 2011, 23, 95 – 105; c) N. Ponnuswamy, G. D. Pantos¸, M. M. J. Smulders, J. K. M. Sanders, J. Am. Chem. Soc. 2012, 134, 566 – 573; d) H. Shao, T. Nguyen, N. C. Romano, D. A. Modarelli, J. R. Parquette, J. Am. Chem. Soc. 2009, 131, 16374 – 16376; e) F. Aparicio, L. Snchez, Chem. Eur. J. 2013, 19, 10482 – 10486; f) T. W. Anderson, J. K. M. Sanders, G. D. Pantos¸, Org. Biomol. Chem. 2010, 8, 4274 – 4280; g) T. D. M. Bell, S. V. Bhosale, C. M. Forsyth, D. Hayne, K. P. Ghiggino, J. A. Hutchison, C. H. Jani, S. J. Langford, M. A.-P. Lee, C. P. Woodward, Chem. Commun. 2010, 46, 4881 – 4883; h) G. D. Pantos¸, P. Pengo, J. K. M. Sanders, Angew. Chem. 2007, 119, 198 – 201; Angew. Chem. Int. Ed. 2007, 46, 194 – 197. a) S. V. Bhosale, S. V. Bhosale, S. K. Bhargava, Org. Biomol. Chem. 2012, 10, 6455 – 6468; b) F. Wrthner, S. Ahmed, C. Thalacker, T. Debaerdemaeker, Chem. Eur. J. 2002, 8, 4742 – 4750. a) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Commun. 2009, 4332 – 4353; b) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40, 5361 – 5388; c) Z. Zhao, J. W. Y. Lam, B. Z. Tang, J. Mater. Chem. 2012, 22, 23726 – 23740. F. M. Winnik, Chem. Rev. 1993, 93, 587 – 614; this review details the photophysics and experimental assignment of pre-associated excimer for pyrene. These experimental characteristics of pre-associated excimer have been used to assign the nature of excimer in the present work. a) M. Kumar, S. J. George, Chem. Eur. J. 2011, 17, 11102 – 11106; b) M. Kumar, S. J. George, Nanoscale 2011, 3, 2130 – 2133. a) M. R. Molla, S. Ghosh, Chem. Eur. J. 2012, 18, 1290 – 1294; b) S. Basak, J. Nanda, A. Banerjee, Chem. Commun. 2013, 49, 6891 – 6893; c) J. B. Bodapati, H. Icil, Photochem. Photobiol. Sci. 2011, 10, 1283 – 1293. a) K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata, T. Komori, F. Ohseto, K. Ueda, S. Shinkai, J. Am. Chem. Soc. 1994, 116, 6664 – 6676; b) H. Svobodov, V. Noponen, E. Kolehmainen, E. Sievnen, RSC Adv. 2012, 2, 4985 – 5007. a) R. Wang, C. Geiger, L. Chen, B. Swanson, D. G. Whitten, J. Am. Chem. Soc. 2000, 122, 2399 – 2400; b) K. Sugiyasu, N. Fujita, S. Shinkai, Angew.
Chem. Eur. J. 2014, 20, 4537 – 4541
www.chemeurj.org
[11] [12] [13]
[14] [15]
[16]
[17] [18]
[19] [20]
Chem. 2004, 116, 1249 – 1253; Angew. Chem. Int. Ed. 2004, 43, 1229 – 1233; c) S.-I. Kawano, N. Fujita, S. Shinkai, Chem. Eur. J. 2005, 11, 4735 – 4742; d) A. Ajayaghosh, C. Vijayakumar, R. Varghese, S. J. George, Angew. Chem. 2006, 118, 470 – 474; Angew. Chem. Int. Ed. 2006, 45, 456 – 460. J. Gawron´ski, M. Brzostowska, K. Kacprzak, H. Kołbon, P. Skowronek, Chirality 2000, 12, 263 – 268. H. Shao, J. R. Parquette, Chem. Commun. 2010, 46, 4285 – 4287. The life-time components are as follows; for 1:1 ACN/DCE (v/v) A1 = 7.86 %, t1 = 5.21 ns A2 = 92.14 %, t2 = 20.11 ns, and for film state A1 = 16.06 %, t1 = 6.09 ns A2 = 83.94 %, t2 = 19.02 ns. Here A denotes the prefactor for each decay and t the corresponding life-time. N. S. S. Kumar, S. Varghese, C. H. Suresh, N. P. Rath, S. Das, J. Phys. Chem. C 2009, 113, 11927 – 11935. Molecule 2 aggregates in a range of polar and apolar solvents (Supporting Information, Figure S8) leading to same photophysical states. It has lower aggregation tendency in acetonitrile, and thus the more polar DMSO was used for the studies. We anticipate that any combination of good solvent and a bad solvent should lead to the same photophysical state. Although 1 and 2 show H- and J-type aggregation, respectively, in the ground state, on excitation there might be some structural rearrangement leading to a supramolecular organization, which is different from the ground state, thus resulting in similar red-shifted excimer emission. S. Abraham, R. K. Vijayaraghavan, S. Das, Langmuir 2009, 25, 8507 – 8513. a) A. Kaeser, A. P. H. J. Schenning, Adv. Mater. 2010, 22, 2985 – 2997; b) D. Gonzlez-Rodr guez, P. G. A. Janssen, R. Mart n-Rapffln, I. De Cat, S. De Feyter, A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem. Soc. 2010, 132, 4710 – 4719. M. Kumar, N. Jonnalagadda, S. J. George, Chem. Commun. 2012, 48, 10948 – 10950. a) P. Jonkheijm, P. van der Schoot, A. P. H. J. Schenning, E. W. Meijer, Science 2006, 313, 80 – 83; b) M. M. J. Smulders, A. P. H. J. Schenning, E. W. Meijer, J. Am. Chem. Soc. 2008, 130, 606 – 611; c) Z. Chen, A. Lohr, C. R. Saha-Mçller, F. Wrthner, Chem. Soc. Rev. 2009, 38, 564 – 584; d) C. Kulkarni, S. Balasubramanian, S. J. George, ChemPhysChem 2013, 14, 661 – 673.
Received: December 12, 2013 Revised: February 2, 2014 Published online on March 13, 2014
4541
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
