FULL PAPER DOI: 10.1002/chem.201301431
Synthesis, Crystal Structure and Photophysical Properties of Pyrene–Helicene Hybrids Anne-Catherine Bdard,[a] Anna Vlassova,[a] Augusto C. Hernandez-Perez,[a] Andr Bessette,[a] Garry S. Hanan,[a] Matthew A. Heuft,[b] and Shawn K. Collins*[a] Abstract: Synthesis of helically chiral aromatics resulting from fusion of pyrene and [4]- or [5]helicene has been accomplished using photoredox catalysis employing a Cu-based sensitizer as the key step. Photocyclisation experiments for the synthesis of the target compounds were carried out in batch and using continuous flow strategies. The solid-state structures, UV/Vis absorption spectra and fluorescence spectra of the pyrene–helicene hybrids
were investigated and compared to that of the parent [5]helicene to discern the effects of merging a pyrene moiety within a helicene skeleton. The studies demonstrated that pyrene–helicene hybrids adopt co-planar or stacked arrangements in the solid state, in conKeywords: continuous flow · chirality · helicenes · helical structures · pyrenes
trast to the solid-state structure of the parent [5]helicene. The UV/Vis and fluorescence spectra of the pyrene–helicene hybrids exhibited strong redshifts when compared to the parent [5]helicene. DFT calculations suggest that the strategy of extending the p surface in the y axis of the helicenes increased their HOMO levels while also decreasing their LUMO levels, resulting in significantly reduced band gaps.
Introduction Rigid, shape persistent carbon rich structures continue to attract attention in materials chemistry[1, 2] as novel liquid crystals,[3] organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs).[4] Among carbon-rich materials, the helicenes are viewed as unique candidates for optical or electronic materials due to their inherent helical chirality and extended conju- Figure 1. Extension of the helicene brids. gated structure.[5] Modification of the helicene skeleton and subsequent investigation of physical properties has been limited by the synthetic methods available for their construction.[6] The most common structural variation studied among
[a] A.-C. Bdard, A. Vlassova, A. C. Hernandez-Perez, A. Bessette, Prof. Dr. G. S. Hanan, Prof. Dr. S. K. Collins Dpartement de Chimie Centre for Green Chemistry and Catalysis Universit de Montral, CP 6128 Station Downtown Montral, Qubec H3C 3J7 (Canada) E-mail: [email protected] [b] M. A. Heuft Xerox Research Centre of Canada, Xerox Corporation 2660 Speakman Dr., Mississauga, ON, L5K 2L1 (Canada) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301431.
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skeleton in a non-traditional manner leads to novel helicene–pyrene hy-
helicenes is the preparation of higher helicenes (extension about the z axis) most commonly achieved by UV-mediated photocyclisation[7] or [2+2+2]-cycloACHTUNGREtriACHTUNGREmerACHTUNGREisaACHTUNGREtion (Figure 1).[8] The incorporation of heteroatoms within the carbon skeleton is also common due to the variety of methods available for their installation.[9] However, synthesis and investigation of helicene derivatives resulting from extension of the p skeleton along the x- or y axis, resulting in compounds with a significant increase in the p surface, is rare.[10–12] Less than a handful of examples of helical aromatics with expanded p surfaces have appeared in the literature including helicene–triphenylene hybrids prepared by asymmetric Rh-catalysed [2+2+2]-cycloACHTUNGREtriACHTUNGREmerACHTUNGREisaACHTUNGREtion,[10] helicene–perylene hybrids formed by UV-mediated photocyclisation,[11]
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and most recently, contorted octabenzocircumbiphenyls synthesised by Scholl reaction.[12] The scarcity of “wider” helicenes is surprising given the potential for these novel structures to exhibit intriguing physical properties, crystallinity and enhanced solubility relative to other traditional planar aromatics.[13] Herein we describe the first synthesis of three members of a new family of helical aromatic structures 1, 2 and 3, that are hybrids of pyrene and helicene skeletons. The X-ray crystal structures, optical and physical properties are described and compared to the parent [5]helicene to discern the effects of merging a pyrene moiety within a helicene skeleton.
57 % yield by treatment with stoichiometric amounts of CuBr2 in MeOH. The bromide 6 could be subjected to carboxymethylation conditions and the resulting ester was reduced to provide the pyrenyl alcohol 7. Note that the reduction to alcohol 7 must be undertaken at room temperature or lower otherwise over-reduction to the corresponding alkane becomes problematic. The alcohol 7 could also be transformed to the corresponding bromide (73 % yield) and subsequently treated with triphenylphosphine to afford the phosphonium salt 8 (99 % yield). With the alcohol 7 in hand, the synthesis of the pyrene– helicene hybrid 1 via photoredox chemistry was investigated (Scheme 2).[17–20] The alcohol 7 was oxidised and the corresponding aldehyde underwent Wittig olefination to afford
Results and Discussion The synthesis of the desired helicene–pyrene hybrids 1 and 2 required functionalisation at the 2-position of pyrene (Scheme 1). As the last step in the synthesis was a photo-
Scheme 2. Synthesis of pyrene–[5]helicene hybrid 1.
Scheme 1. Synthesis of key pyrenyl intermediates, alcohol 7 and phosphonium salt 8.
ACHTUNGREcyclisation of a 1,2-diaryl olefin precursor prepared by Wittig reaction, the pyrenyl alcohol 7 and corresponding phosphonium salt 8 were identified as key intermediates. Functionalisation of the pyrene motif selectively at the 2-position is a challenging transformation that has garnered much attention.[14] First, tert-butylation of the 2-position was accomplished, which effectively desymmetrised pyrene and enhanced its solubility.[15] Subsequently, an Ir-catalysed borylACHTUNGREaACHTUNGREtion allowed for the formation of the pinacol borane substituted 5.[16] Considerable effort was made to reduce the Ir-catalyst loading and the borylation reaction time to maximise synthetic efficiency. Disappointingly, long reaction times and 5 mol % of the iridium catalyst were always required, but good yields of 5 (55 %) could be obtained and gram quantities of material could be produced. The pinacol borane functionality was then converted to bromide 6 in
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the pyrene 9 as a mixture of cis and trans isomers. The pyrene derivative 9 was then subjected to previously developed photoredox cyclisation conditions.[21] A Cu-based sensitizer[22] was prepared in situ; copper complex CuACHTUNGRE(MeCN)4BF4, xantphos (12) and neocuproine (11) were added sequentially to one flask in THF to form the desired sensitizer. The mixture was then added to another flask containing 9 and the oxidant system (I2/propylene oxide) and irradiated with visible light for 5 days. Disappointingly, the photochemical transformation afforded the desired pyrene– helicene hybrid 1 in only 23 % isolated yield and the remaining mass balance was starting material 9 as a mixture of cis and trans isomers. In an effort to improve the yields and reaction times, the synthesis of pyrene–helicene hybrid 1 was also investigated using a flow-chemistry approach. Using the identical reaction conditions as in the batch reaction, the reaction solution was recycled through fluorinated ethylene propylene (FEP) tubing three times to afford pyrene–helicene 1 with an isolated yield of 45 %, a substantial improvement when compared to the batch photochemical protocol.
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The pyrenyl alcohol 7 also served as a key building block for the synthesis of a second pyrene–helicene hybrid 2. The pyrene–helicene hybrid 2 is electronically modified when compared to hydrid 1, as it is substituted with three methoxy groups and is a fusion of pyrene and a [4]helicene (Scheme 3). Wittig reaction of the aldehyde 13 with the
Scheme 4. Synthesis of key 4-substituted pyrenyl intermediate 18.
Scheme 3. Synthesis of pyrene–[4]helicene hybrid 2.
ylide formed from deprotonation of the phosphonium salt 8 with nBuLi afforded the styrenyl product 14 in 68 % yield. The photocatalytic transformation of 14 to the desired pyrene–helicene hybrid 2 was again conducted using the in situ prepared Cu-based photocatalyst and irradiation for 5 days. Once again, the batch photochemical process provided only low yields of the desired pyrene–helicene hybrid 2 (12 %) and the starting material 14. The synthesis of pyrene–helicene hybrid 2 was subsequently investigated using a flow-chemistry approach. Gratifyingly, the continuous flow synthesis under the optimised reaction conditions afforded the desired pyrene–helicene hybrid 2 in 41 % isolated yield. In contrast to the helicene–pyrene hybrids 1 and 2, the synthesis of the helicene–pyrene hybrid 3 required functionalisation at the 4-position of pyrene (Scheme 4). Functionalisation of the pyrene motif selectively at the 4-position employing an Ir-catalysed borylation has been reported.[14a] Consequently, di-tBu pyrene 15 was subjected to similar borylation conditions, except [IrACHTUNGRE(COD)ACHTUNGRE(OMe)]2 (10 mol %) was used as catalyst and afforded the desired product 16 in 96 % yield after 48 h. Once functionalisation was achieved, a series of functional group interconversions including transformation to the corresponding bromide 17 (CuBr2, 99 % yield), carboxymethylation, and reduction (97 % yield over two steps) afforded the pyrenyl alcohol 18. The pyrenyl alcohol 18 also served as the key building block for the synthesis of stilbenoid precursor 19 (Scheme 5). Following oxidation of 18, Wittig reaction with the ylide formed from deprotonation of the phosphonium
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Scheme 5. Synthesis of pyrene–[5]helicene hybrid 3.
salt 8 with nBuLi afforded the styrenyl product 19 in 71 % yield. The photocatalytic transformation of 19 to the desired pyrene–helicene hybrid 3 would afford a pyrene–helicene hybrid in which the fusion of the pyrene motif to the helicene core would occur through the 4-position. Photocyclisation was again conducted using the in situ prepared Cubased photocatalyst under batch conditions. Following irradiation for 5 days, low yield of the desired pyrene–helicene hybrid 3 (27 %) was observed. Conversely, good yields of the pyrene–helicene hybrid 3 were observed when employing a continuous-flow approach under the optimised reaction conditions (46 % isolated yield). Upon synthesis of pyrene–helicene hybrid 1, an analysis of its solid-state structure was undertaken. X-ray quality crystals of pyrene–helicene hybrid 1 were obtained from slow diffusion of hexanes into a CHCl3 solution of 1 and the
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Figure 2. X-ray crystallographic analysis of the molecular packing of [5]helicene 1 a (a: side view; b: top view) and pyrene–helicene hybrid 1 (c: side view; d: top view).
subsequent X-ray crystallographic analysis was compared to that of the parent [5]helicene (Figure 2).[23] The solid-state structure of [5]helicene depicts its twisted helical shape, and a distortion angle of 51.28 was found between the planes defined by the terminal aromatic rings. Every third molecule of the unit cell of [5]helicene is arranged perpendicular to two other molecules, however, the distances between the aryl C H bonds and the centroids of the adjacent [5]helicene molecules is too long to denote any strong interaction. Some p overlap between the centroids of two of the aryl units (rings 1 and 2, Figure 2 b) is probable given the intermolecular distance (3.72 ). The solid-state structure of pyrene–helicene derivative 1 also depicts a similar twisted helical conformation, but the distortion angle of 43.38 (measured using the same benzene rings as for [5]helicene) was significantly smaller than in the parent [5]helicene (51.28). The difference of approximately 88 in helical pitch is significant, given that previous studies on [6]helicenes found that only 3–48 differences in helical pitch were possible through substitution of the terminal aromatic rings.[24] In addition to the large change in distortion angle, the packing structure of the pyrene–helicene derivative 1 is also different. The solidstate structure of pyrene–helicene 1 displayed layers of the aromatic molecules, and an intermolecular distance of 3.98 was found between the centroids of the two “central” benzene rings (ring 3, Figure 2 d). Additional C H···p interactions are observed in the solid state between the tBu group and the centroid of the terminal aromatic ring (ring 4,
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Figure 2 c). The layered structure of hybrid 1 shows increased p overlap in the solid state when compared to [5]helicene (Figure 2 b and d). The co-planar or stacked-like arrangements shown in the solid-state structure of pyrene– helicene 1 are typically important when developing organic electronic materials.[25] Next, the UV/Vis absorption spectra of [5]helicene and pyrene–helicenes 1, 2 and 3 were investigated (Figure 3). The UV/Vis spectrum of the pyrene–helicene hybrid 1 is
Figure 3. UV/Vis absorption spectra for [5]helicene (black line), pyrene– helicene hybrid 1 (dashed line), pyrene–helicene hybrid 2 (double line) and pyrene–helicene hybrid 3 (dotted line). All spectra were recorded in CH2Cl2 solution.
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red-shifted when compared to the spectrum of [5]helicene. Although there is no significant absorption for [5]helicene past approximately 350 nm, the hybrid helicene 1 exhibits an extended absorption to about 440 nm. These spectral differences are most likely a result of the extended conjugation in the p structure of the hybrid helicene 1, suggesting a higher HOMO and lower oxidation potential, which are typically desired characteristics when designing organic materials.[13 a] It should be noted that UV/Vis absorption spectra of [7]helicene, which contains as many aromatic rings as helicene hybrid 1, only tails to about 400 nm, whereas the helicene hybrid has a significant absorption band at 407 nm and tails up to 430 nm, suggesting that the increase of the p surface in the y axis as opposed to the z axis of helicenes could lead to more significant red-shifts.[26] When the UV/Vis spectra of pyrene helicenes 1 and 3 were compared, they displayed similar bathochromic shifts in their spectra when compared to [5]helicene, however, the helicene 1 in which the pyrene was fused via its 2-position (pyrene helicene hybrid 3 is fused at the 4-position of pyrene), showed much more significant absorption at longer wavelengths. Since modifying the HOMO and oxidation potentials of materials
is possible through the incorporation of electron-donating groups, we examined the UV/Vis absorption spectra of pyrene–helicene hybrid 2 bearing three methoxy groups. The pyrene–helicene hybrid 2 also displayed a similar bathochromic shift in its spectrum when compared to [5]helicene 1 a, although 2 was less red-shifted (lmax = ~ 385 nm) than hybrid 1. The excitation and fluorescence spectra for [5]helicene and the pyrene–helicene hybrids 1, 2 and 3 were also investigated (Figure 4). The fluorescence spectra follow a similar trend as was observed for the UV/Vis absorption data. Although fluorescence is observed for [5]helicene between approximately 360 and 450 nm (violet-emitting) and tails to 500 nm, the fluorescence spectrum of both pyrene–helicene hybrids 1 and 2 is red-shifted again: for the pyrene–[5]helicene hybrid 1 fluorescence is blue-emitting and was observed between wavelengths of about 350–550 nm and tails to 600 nm; for the pyrene–[4]helicene hybrid 2 fluorescence is once again blue-emitting and observed between wavelengths approximately 420–550 nm and tails to 600 nm. To gain insights into the structure–property relationships between [5]helicene 1 a and the pyrene helicene hybrids 1–3,
Figure 4. Spectral data: a) [5]helicene: excitation spectrum (dotted line, grey, 375 nm) and fluorescence spectrum (solid line, black, 344 nm); b) pyrene– helicene hybrid 1: excitation spectrum (dotted line, grey, 451 nm) and fluorescence spectrum (solid line, black, 338 nm); c) pyrene–helicene hybrid 2: excitation spectrum (dotted line, grey, 447 nm) and fluorescence spectrum (solid line, black, 330 nm); d) pyrene–helicene hybrid 3: excitation spectrum (dotted line, grey, 423 nm) and fluorescence spectrum (solid line, black, 334 nm). All spectra were recorded in CH2Cl2 solution.
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Figure 5. Calculated HOMO (left) and LUMO (right) of: a) [5]helicene 1 a, b) pyrene–helicene hybrids 1, c) 2, and d) 3 using the B3LYP/6-31GACHTUNGRE(d,p) method with IEFPCM of dichloromethane.
quantum-chemical calculations were performed. The geometries were constructed and optimised from the crystallographic structures of 1 a or 1 (for hybrid compounds 1–3) by using density functional theory (DFT) with the B3LYP function and the 6-31GACHTUNGRE(d,p) basis sets. The IEFPCM solvation continuum of dichloromethane was further applied to match the photophysical characterisations performed. HOMO and LUMO representations are shown in Figure 5. The electronic distribution of both the HOMO and the LUMO in the parent [5]helicene 1 a revealed delocalisation spread over the entire molecule (Figure 5 a). Corresponding HOMO and LUMO energy levels versus vacuum were calculated to be 5.53 and 1.34 eV, respectively (Table 1). The theoretical band gap (Egap theo) of 4.19 eV is slightly higher than that obtained from optical measurements (Egap ppt = 3.59 eV).[27] Al-
Table 1. Summary of electrochemical and optical properties.[a] Helicene 1a 1 2 3
HOMO [eV] 5.53 5.10 5.00 5.24
LUMO [eV] 1.34 1.82 1.63 1.62
Egap [eV] theoretical optical[b]
F[c]
4.19 3.27 3.37 3.62
0.04[29] 0.02 0.02 0.03
3.59 2.91 2.99 3.14
[a] HOMO and LUMO were calculated using the B3LYP/6-31GACHTUNGRE(d,p) method with IEFPCM of dichloromethane. [b] The optical band gap was estimated from the intersection between the absorption and emission bands. [c] Quantum yields were calculated relative to [5]helicene in degassed dichloromethane (F = 0.04).[29]
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though it is known that DFT tends to underestimate the intramolecular stabilisation in helical structures, this method was used considering its relatively abundant usage in recent literature for similar systems.[28] When comparing the HOMO/LUMO of [5]helicene 1 a to the pyrene hybrids, the electronic density is mainly located on the pyrene moiety in each case. This trend held upon variation of the helicene length, substitution position, and in both the HOMO and LUMO molecular orbitals. The most dramatic example of this effect is in the hybrid 3, in which the distribution of the p surface in the HOMO is significantly localised on the pyrene moiety. The calculations also suggest that extending the p surface in the y axis of the helicenes tends to increase their HOMO levels (from 5.53 eV for 1 a up to 5.00 eV for 2) while decreasing their LUMO levels (from 1.34 eV for 1 a down to 1.82 eV for 1). Such combined behaviour results in significantly reduced band gaps (from 4.19 eV in reference 1 a down to 3.27 eV for 1). Noteworthy, the trend in calculated band gap (Egap theo) variation over the series is closely similar to the one obtained from optical measurements (Egap opt). Finally, the helicene hybrid 3 appears to provide a better delocalisation in the p system that stabilises the HOMO level more than in the helicene hybrid 1 ( 5.24 vs. 5.10 eV, respectively).[30]
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Conclusion In summary, pyrene–helicene hybrids have been prepared via a visible-light photoredox cyclisation. The process employs a copper-based photoredox catalyst and the key photocyclisation step was accelerated and improved when using continuous flow techniques. The pyrene–helicene hybrids 1 and 2 are constructed through functionalisation of the 2position of pyrene, a traditionally difficult position to functionalise and the corresponding structures of which have attracted increased attention for their properties. In addition, the pyrene–helicene hybrids represent novel helicene structures with an increased p surface. The physical properties of hybrid 1 were compared to the parent [5]helicene, and Xray crystal structure analysis revealed greater p overlap for hybrid 1 and a smaller distortion angle in the “twist” of the helicene in the solid state than [5]helicene. Spectrophotometric analysis of the helicenes displayed significant redshifts for the pyrene–helicene hybrids 1, 2 and 3 compared to the parent [5]helicene in both the UV/Vis and the fluorescence spectra. Calculations suggest that the strategy of extending the p surface in the y axis of the helicenes increased their HOMO levels while also decreasing their LUMO levels, resulting in significantly reduced band gaps. Future work is focused on the synthesis of pyrene–helicene hybrids containing two pyrene fragments and configurationally stable members of the family, which can be prepared asymmetrically or resolved to study their optical properties.
Acknowledgements The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC), Universit de Montreal, Xerox Research Centre of Canada and the Centre for Green Chemistry and Catalysis (CGCC) for generous funding. The Canadian Foundation for Innovation (CFI) is thanked for funding the acquisition of the flow chemistry infrastructure. Benoit DechÞnes-Simard is thanked for help in acquiring the X-ray data. lodie Rousset and Alexandre Therrien are thanked for help in acquiring the physical data. A.-C.B. thanks the NSERC for a Vanier graduate scholarship. A.B. thanks NSERC, FQRNT and SaintJean Photochemicals Inc. for a BMP-Innovation grant as well as CSACS for financial support.
[1] a) Fullerenes (Eds.: A. Hirsch, M. Brettreich), Wiley-VCH, Weinheim, 2005; b) Carbon Nanotubes: Advanced Topics in the Synthesis Structure, Properties and Applications; Topics in Applied Physics (Eds.: A. Jorio, G. Dresselhaus, M. S. Dresselhaus), Springer, Berlin, 2008; c) A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183 – 191; d) M. D. Watson, A. Fechtenkçtter, K. Mllen, Chem. Rev. 2001, 101, 1267 – 1300. [2] a) Y. Rubin, F. Diederich, in Stimulating Concepts in Chemistry (Eds.: F. Vçgtle, J. F. Stoddart, M. Shibasaki), Wiley-VCH, Weinheim, 2000, pp. 163 – 186; b) S. Becker, K. Mllen, in Stimulating Concepts in Chemistry (Eds.: F. Vçgtle, J. F. Stoddart, M. Shibasaki), Wiley-VCH, Weinheim, 2000, pp. 317 – 337. [3] S. Kumar, Liq. Cryst. 2004, 31, 1037 – 1059. [4] For reviews, see: a) O. Ingans, F. Zhang, K. Tvingstedt, L. M. Andersson, S. Hellstrçm, M. R. Andersson, Adv. Mater. 2010, 22, E100 – E116; b) R. Abbel, A. P. H. J. Schenning, E. W. Meijer, J.
Chem. Eur. J. 2013, 19, 16295 – 16302
[5]
[6]
[7]
[8]
[9]
[10] [11] [12]
[13]
[14]
[15] [16]
[17] [18] [19] [20]
Polym. Sci. Part A 2009, 47, 4215 – 4233; c) A. C. Grimsdale, K. Mllen, Macromol. Rapid Commun. 2007, 28, 1676 – 1702. H. Hopf, in Classics in Hydrocarbon Chemistry: Syntheses, Concepts, Perspectives, Wiley-VCH, Weinheim, 2000, pp. 323 – 330; b) P. J. Smith, J. F. Liebman, H. Hopf, I. Stary´, I. G. Star, B. Halton, in Strained Hydrocarbons (Ed.: H. Dodziuk), 2009, pp. 147 – 203; c) I. G. Stara, I. Stary, Sci. Synth. 2010, 45, 885 – 953; d) Y. Shen, C.F. Chen, Chem. Rev. 2012, 112, 1463 – 1535; e) A. Rajca, M. Miyasaka, in Functional Organic Materials (Eds.: T. J. J. Mller, U. H. F. Bunz), Wiley-VCH, Weinheim, 2007, pp. 547 – 581; f) C. Nuckolls, R. F. Shao, W. G. Jang, N. A. Clark, D. M. Walba, T. J. Katz, Chem. Mater. 2002, 14, 773 – 776; g) T. J. Katz, Angew. Chem. 2000, 112, 1997 – 1999; Angew. Chem. Int. Ed. 2000, 39, 1921 – 1923. a) A. Urbano, Angew. Chem. 2003, 115, 4116 – 4119; Angew. Chem. Int. Ed. 2003, 42, 3986 – 3989; b) M. Gingras, Chem. Soc. Rev. 2013, 42, 968 – 1006; c) M. Gingras, G. Felix, R. Peresutti, Chem. Soc. Rev. 2013, 42, 1007 – 1050; d) M. Gingras, Chem. Soc. Rev. 2013, 42, 1051 – 1095. a) K. B. Joergensen, Molecules 2010, 15, 4334 – 4358; b) F. B. Mallory, C. W. Mallory, Org. React. 1984, 30, 1 – 456; c) M. Flammang-Barbieux, J. Nasielski, R. H. Martin, Tetrahedron Lett. 1967, 8, 743 – 744; d) R. H. Martin, M. Baes, Tetrahedron 1975, 31, 2135 – 2137. For examples of asymmetric helicene synthesis via transition metalcatalysed [2 + 2 + 2] cycloaddition, see: a) F. Teply´, I. G. Star, I. Stary´, A. Kollrovicˇ, D. Sˇaman, Sˇ. Vyskocˇil, P. Fiedler, J. Org. Chem. 2003, 68, 5193 – 5197; b) K. Tanaka, A. Kamisawa, T. Suda, K. Noguchi, M. Hirano, J. Am. Chem. Soc. 2007, 129, 12078 – 12079; c) K. Tanaka, N. Fukawa, T. Suda, K. Noguchi, Angew. Chem. 2009, 121, 5578 – 5581; Angew. Chem. Int. Ed. 2009, 48, 5470 – 5473; d) P. Sehnal, I. G. Star, D. Sˇaman, M. Tichy´, J. M sˇek, J. Cvacˇka, L. Rul sˇek, J. Chocholousˇov, J. Vacek, G. Goryl, M. Szymonski, I. C sarov, I. Stary´, Proc. Natl. Acad. Sci. USA 2009, 106, 13169 – 13174. a) K. Nakano, Y. Hidehira, K. Takahashi, T. Hiyama, K. Nozaki, Angew. Chem. 2005, 117, 7298 – 7300; Angew. Chem. Int. Ed. 2005, 44, 7136 – 7138; b) S. D. Dreher, D. J. Weix, T. J. Katz, J. Org. Chem. 1999, 64, 3671 – 3678; c) G. Gottarelli, G. Proni, G. P. Spada, D. Fabbri, S. Gladiali, C. Rosini, J. Org. Chem. 1996, 61, 2013 – 2019. Y. Sawada, S. Furumi, A. Takai, M. Takeuchi, K. Noguchi, K. Tanaka, J. Am. Chem. Soc. 2012, 134, 4080 – 4083. J. Kelber, M.-F. Achard, F. Durola, H. Bock, Angew. Chem. 2012, 124, 5290 – 5293; Angew. Chem. Int. Ed. 2012, 51, 5200 – 5203. S. Xiao, S. J. Kang, Y. Wu, S. Ahn, J. B. Kim, Y.-L. Loo, T. Siegrist, M. L. Steigerwald, H. Li, C. Nuckolls, Chem. Sci. 2013, 4, 2018 – 2023. a) M. Bendikov, F. Wudl, D. F. Perepichka, Chem. Rev. 2004, 104, 4891 – 4946; b) H. M. Duong, M. Bendikov, D. Steiger, Q. Zhang, G. Sonmez, J. Yamada, F. Wudl, Org. Lett. 2003, 5, 4433 – 4436; c) M. M. Payne, S. R. Parkin, J. E. Anthony, J. Am. Chem. Soc. 2005, 127, 8028 – 8029. a) Z. Liu, Y. Wang, Y. Chen, J. Liu, Q. Fang, C. Kleeberg, T. B. Marder, J. Org. Chem. 2012, 77, 7124 – 7128; b) A. S. Batsanov, J. A. K. Howard, D. Albesa-Jove, J. C. Collings, Z. Liu, I. A. I. Mkhalid, M.-H. Thibault, T. B. Marder, Cryst. Growth Des. 2012, 12, 2794 – 2802. Y. Miura, E. Yamano, A. Tanaka, J. Org. Chem. 1994, 59, 3294 – 3300. a) D. N. Coventry, A. S. Batsanov, A. E. Goeta, J. A. K. Howard, T. B. Marder, R. N. Perutz, Chem. Commun. 2005, 2172 – 2174; b) J. F. Hartwig, Acc. Chem. Res. 2012, 45, 864 – 873. D. A. Nicewicz, D. W. C. MacMillan, Science 2008, 322, 77 – 80. T. P. Yoon, M. A. Ischay, J. Du, Nat. Chem. 2010, 2, 527 – 532. J. M. R. Narayanam, C. R. J. Stephenson, Chem. Soc. Rev. 2011, 40, 102 – 113. For some other examples of catalysts used for visible-light mediated transformations, see: a) F. Su, S. C. Mathew, L. Moehlmann, M. Antonietti, X. Wang, S. Blechert, Angew. Chem. 2011, 123, 683 – 686; Angew. Chem. Int. Ed. 2011, 50, 657 – 660; b) F. Su, S. C. Mathew, G.
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www.chemeurj.org
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S. K. Collins et al.
[21] [22]
[23]
[24] [25] [26] [27]
[28]
Lipner, X. Fu, M. Antonietti, S. Blechert, X. Wang, J. Am. Chem. Soc. 2010, 132, 16299 – 16301. A. C. Hernandez-Perez, A. Vlassova, S. K. Collins, Org. Lett. 2012, 14, 2988 – 2991. a) J. D. Slinker, J. Rivnay, J. S. Moskowitz, J. B. Parker, S. Bernhard, H. D. AbruÇa, G. G. Malliaras, J. Mater. Chem. 2007, 17, 2976 – 2988; b) D. G. Cuttell, S.-M. Kuang, P. E. Fanwick, D. R. McMillin, R. A. Walton, J. Am. Chem. Soc. 2002, 124, 6 – 7. CCDC-933555 ([5]helicene) and 933556 (pyrene – helicene hybrid 1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Y. Nakai, T. Mori, Y. Inoue, J. Phys. Chem. A 2013, 117, 83 – 93. J. E. Anthony, Chem. Rev. 2006, 106, 5028 – 5048. R. H. Martin, M. J. Marchant, Tetrahedron 1974, 30, 343 – 345. To the best of our knowledge, detailed electrochemistry for [5]helicene has not been reported in order to assess the accuracy of the calculated values. DFT has been shown to underestimate intramolecular stabilisation in helical structures. See: a) L. Rulisek, O. Exner, L. Cwiklik, P.
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Jungwirth, I. Stary´, L. Pospisil, Z. Havlas, J. Phys. Chem. C 2007, 111, 14948 – 14955. For other examples of computer modelling of helical structures, see: b) T. Kaseyama, S. Furumi, X. Zhang, K. Tanaka, M. Takeuchi, Angew. Chem. 2011, 123, 3768 – 3771; Angew. Chem. Int. Ed. 2011, 50, 3684 – 3687; c) K. Nakano, H. Oyama, Y. Nishimura, S. Nakasako, K. Nozaki, Angew. Chem. 2012, 124, 719 – 723; Angew. Chem. Int. Ed. 2012, 51, 695 – 699; d) D. Mys´liwiec, M. Ste˛pien´, Angew. Chem. 2013, 125, 1757 – 1761; Angew. Chem. Int. Ed. 2013, 52, 1713 – 1717. [29] J. B. Birks, D. J. S. Birch, E. Cordemans, E. Vander Donckt, Chem. Phys. Lett. 1976, 43, 33 – 36. [30] Yamato and co-workers recently synthesised a series of pyrenecored [4]helicenes substituted at the 4- and 9-positions and observed stabilisation of the HOMO level. See: J.-Y. Hu, A. Paudel, N. Seto, X. Feng, M. Era, T. Matsumoto, J. Tanaka, M. R. J. Elsegood, C. Redshaw, T. Yamato, Org. Biomol. Chem. 2013, 11, 2186 – 2197.
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Received: April 15, 2013 Revised: August 13, 2013 Published online: October 21, 2013
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Synthesis, Crystal Structure and Photophysical Properties of Pyrene–Helicene Hybrids Anne-Catherine Bdard,[a] Anna Vlassova,[a] Augusto C. Hernandez-Perez,[a] Andr Bessette,[a] Garry S. Hanan,[a] Matthew A. Heuft,[b] and Shawn K. Collins*[a] Abstract: Synthesis of helically chiral aromatics resulting from fusion of pyrene and [4]- or [5]helicene has been accomplished using photoredox catalysis employing a Cu-based sensitizer as the key step. Photocyclisation experiments for the synthesis of the target compounds were carried out in batch and using continuous flow strategies. The solid-state structures, UV/Vis absorption spectra and fluorescence spectra of the pyrene–helicene hybrids
were investigated and compared to that of the parent [5]helicene to discern the effects of merging a pyrene moiety within a helicene skeleton. The studies demonstrated that pyrene–helicene hybrids adopt co-planar or stacked arrangements in the solid state, in conKeywords: continuous flow · chirality · helicenes · helical structures · pyrenes
trast to the solid-state structure of the parent [5]helicene. The UV/Vis and fluorescence spectra of the pyrene–helicene hybrids exhibited strong redshifts when compared to the parent [5]helicene. DFT calculations suggest that the strategy of extending the p surface in the y axis of the helicenes increased their HOMO levels while also decreasing their LUMO levels, resulting in significantly reduced band gaps.
Introduction Rigid, shape persistent carbon rich structures continue to attract attention in materials chemistry[1, 2] as novel liquid crystals,[3] organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs).[4] Among carbon-rich materials, the helicenes are viewed as unique candidates for optical or electronic materials due to their inherent helical chirality and extended conju- Figure 1. Extension of the helicene brids. gated structure.[5] Modification of the helicene skeleton and subsequent investigation of physical properties has been limited by the synthetic methods available for their construction.[6] The most common structural variation studied among
[a] A.-C. Bdard, A. Vlassova, A. C. Hernandez-Perez, A. Bessette, Prof. Dr. G. S. Hanan, Prof. Dr. S. K. Collins Dpartement de Chimie Centre for Green Chemistry and Catalysis Universit de Montral, CP 6128 Station Downtown Montral, Qubec H3C 3J7 (Canada) E-mail: [email protected] [b] M. A. Heuft Xerox Research Centre of Canada, Xerox Corporation 2660 Speakman Dr., Mississauga, ON, L5K 2L1 (Canada) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301431.
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skeleton in a non-traditional manner leads to novel helicene–pyrene hy-
helicenes is the preparation of higher helicenes (extension about the z axis) most commonly achieved by UV-mediated photocyclisation[7] or [2+2+2]-cycloACHTUNGREtriACHTUNGREmerACHTUNGREisaACHTUNGREtion (Figure 1).[8] The incorporation of heteroatoms within the carbon skeleton is also common due to the variety of methods available for their installation.[9] However, synthesis and investigation of helicene derivatives resulting from extension of the p skeleton along the x- or y axis, resulting in compounds with a significant increase in the p surface, is rare.[10–12] Less than a handful of examples of helical aromatics with expanded p surfaces have appeared in the literature including helicene–triphenylene hybrids prepared by asymmetric Rh-catalysed [2+2+2]-cycloACHTUNGREtriACHTUNGREmerACHTUNGREisaACHTUNGREtion,[10] helicene–perylene hybrids formed by UV-mediated photocyclisation,[11]
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and most recently, contorted octabenzocircumbiphenyls synthesised by Scholl reaction.[12] The scarcity of “wider” helicenes is surprising given the potential for these novel structures to exhibit intriguing physical properties, crystallinity and enhanced solubility relative to other traditional planar aromatics.[13] Herein we describe the first synthesis of three members of a new family of helical aromatic structures 1, 2 and 3, that are hybrids of pyrene and helicene skeletons. The X-ray crystal structures, optical and physical properties are described and compared to the parent [5]helicene to discern the effects of merging a pyrene moiety within a helicene skeleton.
57 % yield by treatment with stoichiometric amounts of CuBr2 in MeOH. The bromide 6 could be subjected to carboxymethylation conditions and the resulting ester was reduced to provide the pyrenyl alcohol 7. Note that the reduction to alcohol 7 must be undertaken at room temperature or lower otherwise over-reduction to the corresponding alkane becomes problematic. The alcohol 7 could also be transformed to the corresponding bromide (73 % yield) and subsequently treated with triphenylphosphine to afford the phosphonium salt 8 (99 % yield). With the alcohol 7 in hand, the synthesis of the pyrene– helicene hybrid 1 via photoredox chemistry was investigated (Scheme 2).[17–20] The alcohol 7 was oxidised and the corresponding aldehyde underwent Wittig olefination to afford
Results and Discussion The synthesis of the desired helicene–pyrene hybrids 1 and 2 required functionalisation at the 2-position of pyrene (Scheme 1). As the last step in the synthesis was a photo-
Scheme 2. Synthesis of pyrene–[5]helicene hybrid 1.
Scheme 1. Synthesis of key pyrenyl intermediates, alcohol 7 and phosphonium salt 8.
ACHTUNGREcyclisation of a 1,2-diaryl olefin precursor prepared by Wittig reaction, the pyrenyl alcohol 7 and corresponding phosphonium salt 8 were identified as key intermediates. Functionalisation of the pyrene motif selectively at the 2-position is a challenging transformation that has garnered much attention.[14] First, tert-butylation of the 2-position was accomplished, which effectively desymmetrised pyrene and enhanced its solubility.[15] Subsequently, an Ir-catalysed borylACHTUNGREaACHTUNGREtion allowed for the formation of the pinacol borane substituted 5.[16] Considerable effort was made to reduce the Ir-catalyst loading and the borylation reaction time to maximise synthetic efficiency. Disappointingly, long reaction times and 5 mol % of the iridium catalyst were always required, but good yields of 5 (55 %) could be obtained and gram quantities of material could be produced. The pinacol borane functionality was then converted to bromide 6 in
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the pyrene 9 as a mixture of cis and trans isomers. The pyrene derivative 9 was then subjected to previously developed photoredox cyclisation conditions.[21] A Cu-based sensitizer[22] was prepared in situ; copper complex CuACHTUNGRE(MeCN)4BF4, xantphos (12) and neocuproine (11) were added sequentially to one flask in THF to form the desired sensitizer. The mixture was then added to another flask containing 9 and the oxidant system (I2/propylene oxide) and irradiated with visible light for 5 days. Disappointingly, the photochemical transformation afforded the desired pyrene– helicene hybrid 1 in only 23 % isolated yield and the remaining mass balance was starting material 9 as a mixture of cis and trans isomers. In an effort to improve the yields and reaction times, the synthesis of pyrene–helicene hybrid 1 was also investigated using a flow-chemistry approach. Using the identical reaction conditions as in the batch reaction, the reaction solution was recycled through fluorinated ethylene propylene (FEP) tubing three times to afford pyrene–helicene 1 with an isolated yield of 45 %, a substantial improvement when compared to the batch photochemical protocol.
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The pyrenyl alcohol 7 also served as a key building block for the synthesis of a second pyrene–helicene hybrid 2. The pyrene–helicene hybrid 2 is electronically modified when compared to hydrid 1, as it is substituted with three methoxy groups and is a fusion of pyrene and a [4]helicene (Scheme 3). Wittig reaction of the aldehyde 13 with the
Scheme 4. Synthesis of key 4-substituted pyrenyl intermediate 18.
Scheme 3. Synthesis of pyrene–[4]helicene hybrid 2.
ylide formed from deprotonation of the phosphonium salt 8 with nBuLi afforded the styrenyl product 14 in 68 % yield. The photocatalytic transformation of 14 to the desired pyrene–helicene hybrid 2 was again conducted using the in situ prepared Cu-based photocatalyst and irradiation for 5 days. Once again, the batch photochemical process provided only low yields of the desired pyrene–helicene hybrid 2 (12 %) and the starting material 14. The synthesis of pyrene–helicene hybrid 2 was subsequently investigated using a flow-chemistry approach. Gratifyingly, the continuous flow synthesis under the optimised reaction conditions afforded the desired pyrene–helicene hybrid 2 in 41 % isolated yield. In contrast to the helicene–pyrene hybrids 1 and 2, the synthesis of the helicene–pyrene hybrid 3 required functionalisation at the 4-position of pyrene (Scheme 4). Functionalisation of the pyrene motif selectively at the 4-position employing an Ir-catalysed borylation has been reported.[14a] Consequently, di-tBu pyrene 15 was subjected to similar borylation conditions, except [IrACHTUNGRE(COD)ACHTUNGRE(OMe)]2 (10 mol %) was used as catalyst and afforded the desired product 16 in 96 % yield after 48 h. Once functionalisation was achieved, a series of functional group interconversions including transformation to the corresponding bromide 17 (CuBr2, 99 % yield), carboxymethylation, and reduction (97 % yield over two steps) afforded the pyrenyl alcohol 18. The pyrenyl alcohol 18 also served as the key building block for the synthesis of stilbenoid precursor 19 (Scheme 5). Following oxidation of 18, Wittig reaction with the ylide formed from deprotonation of the phosphonium
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Scheme 5. Synthesis of pyrene–[5]helicene hybrid 3.
salt 8 with nBuLi afforded the styrenyl product 19 in 71 % yield. The photocatalytic transformation of 19 to the desired pyrene–helicene hybrid 3 would afford a pyrene–helicene hybrid in which the fusion of the pyrene motif to the helicene core would occur through the 4-position. Photocyclisation was again conducted using the in situ prepared Cubased photocatalyst under batch conditions. Following irradiation for 5 days, low yield of the desired pyrene–helicene hybrid 3 (27 %) was observed. Conversely, good yields of the pyrene–helicene hybrid 3 were observed when employing a continuous-flow approach under the optimised reaction conditions (46 % isolated yield). Upon synthesis of pyrene–helicene hybrid 1, an analysis of its solid-state structure was undertaken. X-ray quality crystals of pyrene–helicene hybrid 1 were obtained from slow diffusion of hexanes into a CHCl3 solution of 1 and the
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Figure 2. X-ray crystallographic analysis of the molecular packing of [5]helicene 1 a (a: side view; b: top view) and pyrene–helicene hybrid 1 (c: side view; d: top view).
subsequent X-ray crystallographic analysis was compared to that of the parent [5]helicene (Figure 2).[23] The solid-state structure of [5]helicene depicts its twisted helical shape, and a distortion angle of 51.28 was found between the planes defined by the terminal aromatic rings. Every third molecule of the unit cell of [5]helicene is arranged perpendicular to two other molecules, however, the distances between the aryl C H bonds and the centroids of the adjacent [5]helicene molecules is too long to denote any strong interaction. Some p overlap between the centroids of two of the aryl units (rings 1 and 2, Figure 2 b) is probable given the intermolecular distance (3.72 ). The solid-state structure of pyrene–helicene derivative 1 also depicts a similar twisted helical conformation, but the distortion angle of 43.38 (measured using the same benzene rings as for [5]helicene) was significantly smaller than in the parent [5]helicene (51.28). The difference of approximately 88 in helical pitch is significant, given that previous studies on [6]helicenes found that only 3–48 differences in helical pitch were possible through substitution of the terminal aromatic rings.[24] In addition to the large change in distortion angle, the packing structure of the pyrene–helicene derivative 1 is also different. The solidstate structure of pyrene–helicene 1 displayed layers of the aromatic molecules, and an intermolecular distance of 3.98 was found between the centroids of the two “central” benzene rings (ring 3, Figure 2 d). Additional C H···p interactions are observed in the solid state between the tBu group and the centroid of the terminal aromatic ring (ring 4,
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Figure 2 c). The layered structure of hybrid 1 shows increased p overlap in the solid state when compared to [5]helicene (Figure 2 b and d). The co-planar or stacked-like arrangements shown in the solid-state structure of pyrene– helicene 1 are typically important when developing organic electronic materials.[25] Next, the UV/Vis absorption spectra of [5]helicene and pyrene–helicenes 1, 2 and 3 were investigated (Figure 3). The UV/Vis spectrum of the pyrene–helicene hybrid 1 is
Figure 3. UV/Vis absorption spectra for [5]helicene (black line), pyrene– helicene hybrid 1 (dashed line), pyrene–helicene hybrid 2 (double line) and pyrene–helicene hybrid 3 (dotted line). All spectra were recorded in CH2Cl2 solution.
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red-shifted when compared to the spectrum of [5]helicene. Although there is no significant absorption for [5]helicene past approximately 350 nm, the hybrid helicene 1 exhibits an extended absorption to about 440 nm. These spectral differences are most likely a result of the extended conjugation in the p structure of the hybrid helicene 1, suggesting a higher HOMO and lower oxidation potential, which are typically desired characteristics when designing organic materials.[13 a] It should be noted that UV/Vis absorption spectra of [7]helicene, which contains as many aromatic rings as helicene hybrid 1, only tails to about 400 nm, whereas the helicene hybrid has a significant absorption band at 407 nm and tails up to 430 nm, suggesting that the increase of the p surface in the y axis as opposed to the z axis of helicenes could lead to more significant red-shifts.[26] When the UV/Vis spectra of pyrene helicenes 1 and 3 were compared, they displayed similar bathochromic shifts in their spectra when compared to [5]helicene, however, the helicene 1 in which the pyrene was fused via its 2-position (pyrene helicene hybrid 3 is fused at the 4-position of pyrene), showed much more significant absorption at longer wavelengths. Since modifying the HOMO and oxidation potentials of materials
is possible through the incorporation of electron-donating groups, we examined the UV/Vis absorption spectra of pyrene–helicene hybrid 2 bearing three methoxy groups. The pyrene–helicene hybrid 2 also displayed a similar bathochromic shift in its spectrum when compared to [5]helicene 1 a, although 2 was less red-shifted (lmax = ~ 385 nm) than hybrid 1. The excitation and fluorescence spectra for [5]helicene and the pyrene–helicene hybrids 1, 2 and 3 were also investigated (Figure 4). The fluorescence spectra follow a similar trend as was observed for the UV/Vis absorption data. Although fluorescence is observed for [5]helicene between approximately 360 and 450 nm (violet-emitting) and tails to 500 nm, the fluorescence spectrum of both pyrene–helicene hybrids 1 and 2 is red-shifted again: for the pyrene–[5]helicene hybrid 1 fluorescence is blue-emitting and was observed between wavelengths of about 350–550 nm and tails to 600 nm; for the pyrene–[4]helicene hybrid 2 fluorescence is once again blue-emitting and observed between wavelengths approximately 420–550 nm and tails to 600 nm. To gain insights into the structure–property relationships between [5]helicene 1 a and the pyrene helicene hybrids 1–3,
Figure 4. Spectral data: a) [5]helicene: excitation spectrum (dotted line, grey, 375 nm) and fluorescence spectrum (solid line, black, 344 nm); b) pyrene– helicene hybrid 1: excitation spectrum (dotted line, grey, 451 nm) and fluorescence spectrum (solid line, black, 338 nm); c) pyrene–helicene hybrid 2: excitation spectrum (dotted line, grey, 447 nm) and fluorescence spectrum (solid line, black, 330 nm); d) pyrene–helicene hybrid 3: excitation spectrum (dotted line, grey, 423 nm) and fluorescence spectrum (solid line, black, 334 nm). All spectra were recorded in CH2Cl2 solution.
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Figure 5. Calculated HOMO (left) and LUMO (right) of: a) [5]helicene 1 a, b) pyrene–helicene hybrids 1, c) 2, and d) 3 using the B3LYP/6-31GACHTUNGRE(d,p) method with IEFPCM of dichloromethane.
quantum-chemical calculations were performed. The geometries were constructed and optimised from the crystallographic structures of 1 a or 1 (for hybrid compounds 1–3) by using density functional theory (DFT) with the B3LYP function and the 6-31GACHTUNGRE(d,p) basis sets. The IEFPCM solvation continuum of dichloromethane was further applied to match the photophysical characterisations performed. HOMO and LUMO representations are shown in Figure 5. The electronic distribution of both the HOMO and the LUMO in the parent [5]helicene 1 a revealed delocalisation spread over the entire molecule (Figure 5 a). Corresponding HOMO and LUMO energy levels versus vacuum were calculated to be 5.53 and 1.34 eV, respectively (Table 1). The theoretical band gap (Egap theo) of 4.19 eV is slightly higher than that obtained from optical measurements (Egap ppt = 3.59 eV).[27] Al-
Table 1. Summary of electrochemical and optical properties.[a] Helicene 1a 1 2 3
HOMO [eV] 5.53 5.10 5.00 5.24
LUMO [eV] 1.34 1.82 1.63 1.62
Egap [eV] theoretical optical[b]
F[c]
4.19 3.27 3.37 3.62
0.04[29] 0.02 0.02 0.03
3.59 2.91 2.99 3.14
[a] HOMO and LUMO were calculated using the B3LYP/6-31GACHTUNGRE(d,p) method with IEFPCM of dichloromethane. [b] The optical band gap was estimated from the intersection between the absorption and emission bands. [c] Quantum yields were calculated relative to [5]helicene in degassed dichloromethane (F = 0.04).[29]
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though it is known that DFT tends to underestimate the intramolecular stabilisation in helical structures, this method was used considering its relatively abundant usage in recent literature for similar systems.[28] When comparing the HOMO/LUMO of [5]helicene 1 a to the pyrene hybrids, the electronic density is mainly located on the pyrene moiety in each case. This trend held upon variation of the helicene length, substitution position, and in both the HOMO and LUMO molecular orbitals. The most dramatic example of this effect is in the hybrid 3, in which the distribution of the p surface in the HOMO is significantly localised on the pyrene moiety. The calculations also suggest that extending the p surface in the y axis of the helicenes tends to increase their HOMO levels (from 5.53 eV for 1 a up to 5.00 eV for 2) while decreasing their LUMO levels (from 1.34 eV for 1 a down to 1.82 eV for 1). Such combined behaviour results in significantly reduced band gaps (from 4.19 eV in reference 1 a down to 3.27 eV for 1). Noteworthy, the trend in calculated band gap (Egap theo) variation over the series is closely similar to the one obtained from optical measurements (Egap opt). Finally, the helicene hybrid 3 appears to provide a better delocalisation in the p system that stabilises the HOMO level more than in the helicene hybrid 1 ( 5.24 vs. 5.10 eV, respectively).[30]
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Conclusion In summary, pyrene–helicene hybrids have been prepared via a visible-light photoredox cyclisation. The process employs a copper-based photoredox catalyst and the key photocyclisation step was accelerated and improved when using continuous flow techniques. The pyrene–helicene hybrids 1 and 2 are constructed through functionalisation of the 2position of pyrene, a traditionally difficult position to functionalise and the corresponding structures of which have attracted increased attention for their properties. In addition, the pyrene–helicene hybrids represent novel helicene structures with an increased p surface. The physical properties of hybrid 1 were compared to the parent [5]helicene, and Xray crystal structure analysis revealed greater p overlap for hybrid 1 and a smaller distortion angle in the “twist” of the helicene in the solid state than [5]helicene. Spectrophotometric analysis of the helicenes displayed significant redshifts for the pyrene–helicene hybrids 1, 2 and 3 compared to the parent [5]helicene in both the UV/Vis and the fluorescence spectra. Calculations suggest that the strategy of extending the p surface in the y axis of the helicenes increased their HOMO levels while also decreasing their LUMO levels, resulting in significantly reduced band gaps. Future work is focused on the synthesis of pyrene–helicene hybrids containing two pyrene fragments and configurationally stable members of the family, which can be prepared asymmetrically or resolved to study their optical properties.
Acknowledgements The authors acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC), Universit de Montreal, Xerox Research Centre of Canada and the Centre for Green Chemistry and Catalysis (CGCC) for generous funding. The Canadian Foundation for Innovation (CFI) is thanked for funding the acquisition of the flow chemistry infrastructure. Benoit DechÞnes-Simard is thanked for help in acquiring the X-ray data. lodie Rousset and Alexandre Therrien are thanked for help in acquiring the physical data. A.-C.B. thanks the NSERC for a Vanier graduate scholarship. A.B. thanks NSERC, FQRNT and SaintJean Photochemicals Inc. for a BMP-Innovation grant as well as CSACS for financial support.
[1] a) Fullerenes (Eds.: A. Hirsch, M. Brettreich), Wiley-VCH, Weinheim, 2005; b) Carbon Nanotubes: Advanced Topics in the Synthesis Structure, Properties and Applications; Topics in Applied Physics (Eds.: A. Jorio, G. Dresselhaus, M. S. Dresselhaus), Springer, Berlin, 2008; c) A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183 – 191; d) M. D. Watson, A. Fechtenkçtter, K. Mllen, Chem. Rev. 2001, 101, 1267 – 1300. [2] a) Y. Rubin, F. Diederich, in Stimulating Concepts in Chemistry (Eds.: F. Vçgtle, J. F. Stoddart, M. Shibasaki), Wiley-VCH, Weinheim, 2000, pp. 163 – 186; b) S. Becker, K. Mllen, in Stimulating Concepts in Chemistry (Eds.: F. Vçgtle, J. F. Stoddart, M. Shibasaki), Wiley-VCH, Weinheim, 2000, pp. 317 – 337. [3] S. Kumar, Liq. Cryst. 2004, 31, 1037 – 1059. [4] For reviews, see: a) O. Ingans, F. Zhang, K. Tvingstedt, L. M. Andersson, S. Hellstrçm, M. R. Andersson, Adv. Mater. 2010, 22, E100 – E116; b) R. Abbel, A. P. H. J. Schenning, E. W. Meijer, J.
Chem. Eur. J. 2013, 19, 16295 – 16302
[5]
[6]
[7]
[8]
[9]
[10] [11] [12]
[13]
[14]
[15] [16]
[17] [18] [19] [20]
Polym. Sci. Part A 2009, 47, 4215 – 4233; c) A. C. Grimsdale, K. Mllen, Macromol. Rapid Commun. 2007, 28, 1676 – 1702. H. Hopf, in Classics in Hydrocarbon Chemistry: Syntheses, Concepts, Perspectives, Wiley-VCH, Weinheim, 2000, pp. 323 – 330; b) P. J. Smith, J. F. Liebman, H. Hopf, I. Stary´, I. G. Star, B. Halton, in Strained Hydrocarbons (Ed.: H. Dodziuk), 2009, pp. 147 – 203; c) I. G. Stara, I. Stary, Sci. Synth. 2010, 45, 885 – 953; d) Y. Shen, C.F. Chen, Chem. Rev. 2012, 112, 1463 – 1535; e) A. Rajca, M. Miyasaka, in Functional Organic Materials (Eds.: T. J. J. Mller, U. H. F. Bunz), Wiley-VCH, Weinheim, 2007, pp. 547 – 581; f) C. Nuckolls, R. F. Shao, W. G. Jang, N. A. Clark, D. M. Walba, T. J. Katz, Chem. Mater. 2002, 14, 773 – 776; g) T. J. Katz, Angew. Chem. 2000, 112, 1997 – 1999; Angew. Chem. Int. Ed. 2000, 39, 1921 – 1923. a) A. Urbano, Angew. Chem. 2003, 115, 4116 – 4119; Angew. Chem. Int. Ed. 2003, 42, 3986 – 3989; b) M. Gingras, Chem. Soc. Rev. 2013, 42, 968 – 1006; c) M. Gingras, G. Felix, R. Peresutti, Chem. Soc. Rev. 2013, 42, 1007 – 1050; d) M. Gingras, Chem. Soc. Rev. 2013, 42, 1051 – 1095. a) K. B. Joergensen, Molecules 2010, 15, 4334 – 4358; b) F. B. Mallory, C. W. Mallory, Org. React. 1984, 30, 1 – 456; c) M. Flammang-Barbieux, J. Nasielski, R. H. Martin, Tetrahedron Lett. 1967, 8, 743 – 744; d) R. H. Martin, M. Baes, Tetrahedron 1975, 31, 2135 – 2137. For examples of asymmetric helicene synthesis via transition metalcatalysed [2 + 2 + 2] cycloaddition, see: a) F. Teply´, I. G. Star, I. Stary´, A. Kollrovicˇ, D. Sˇaman, Sˇ. Vyskocˇil, P. Fiedler, J. Org. Chem. 2003, 68, 5193 – 5197; b) K. Tanaka, A. Kamisawa, T. Suda, K. Noguchi, M. Hirano, J. Am. Chem. Soc. 2007, 129, 12078 – 12079; c) K. Tanaka, N. Fukawa, T. Suda, K. Noguchi, Angew. Chem. 2009, 121, 5578 – 5581; Angew. Chem. Int. Ed. 2009, 48, 5470 – 5473; d) P. Sehnal, I. G. Star, D. Sˇaman, M. Tichy´, J. M sˇek, J. Cvacˇka, L. Rul sˇek, J. Chocholousˇov, J. Vacek, G. Goryl, M. Szymonski, I. C sarov, I. Stary´, Proc. Natl. Acad. Sci. USA 2009, 106, 13169 – 13174. a) K. Nakano, Y. Hidehira, K. Takahashi, T. Hiyama, K. Nozaki, Angew. Chem. 2005, 117, 7298 – 7300; Angew. Chem. Int. Ed. 2005, 44, 7136 – 7138; b) S. D. Dreher, D. J. Weix, T. J. Katz, J. Org. Chem. 1999, 64, 3671 – 3678; c) G. Gottarelli, G. Proni, G. P. Spada, D. Fabbri, S. Gladiali, C. Rosini, J. Org. Chem. 1996, 61, 2013 – 2019. Y. Sawada, S. Furumi, A. Takai, M. Takeuchi, K. Noguchi, K. Tanaka, J. Am. Chem. Soc. 2012, 134, 4080 – 4083. J. Kelber, M.-F. Achard, F. Durola, H. Bock, Angew. Chem. 2012, 124, 5290 – 5293; Angew. Chem. Int. Ed. 2012, 51, 5200 – 5203. S. Xiao, S. J. Kang, Y. Wu, S. Ahn, J. B. Kim, Y.-L. Loo, T. Siegrist, M. L. Steigerwald, H. Li, C. Nuckolls, Chem. Sci. 2013, 4, 2018 – 2023. a) M. Bendikov, F. Wudl, D. F. Perepichka, Chem. Rev. 2004, 104, 4891 – 4946; b) H. M. Duong, M. Bendikov, D. Steiger, Q. Zhang, G. Sonmez, J. Yamada, F. Wudl, Org. Lett. 2003, 5, 4433 – 4436; c) M. M. Payne, S. R. Parkin, J. E. Anthony, J. Am. Chem. Soc. 2005, 127, 8028 – 8029. a) Z. Liu, Y. Wang, Y. Chen, J. Liu, Q. Fang, C. Kleeberg, T. B. Marder, J. Org. Chem. 2012, 77, 7124 – 7128; b) A. S. Batsanov, J. A. K. Howard, D. Albesa-Jove, J. C. Collings, Z. Liu, I. A. I. Mkhalid, M.-H. Thibault, T. B. Marder, Cryst. Growth Des. 2012, 12, 2794 – 2802. Y. Miura, E. Yamano, A. Tanaka, J. Org. Chem. 1994, 59, 3294 – 3300. a) D. N. Coventry, A. S. Batsanov, A. E. Goeta, J. A. K. Howard, T. B. Marder, R. N. Perutz, Chem. Commun. 2005, 2172 – 2174; b) J. F. Hartwig, Acc. Chem. Res. 2012, 45, 864 – 873. D. A. Nicewicz, D. W. C. MacMillan, Science 2008, 322, 77 – 80. T. P. Yoon, M. A. Ischay, J. Du, Nat. Chem. 2010, 2, 527 – 532. J. M. R. Narayanam, C. R. J. Stephenson, Chem. Soc. Rev. 2011, 40, 102 – 113. For some other examples of catalysts used for visible-light mediated transformations, see: a) F. Su, S. C. Mathew, L. Moehlmann, M. Antonietti, X. Wang, S. Blechert, Angew. Chem. 2011, 123, 683 – 686; Angew. Chem. Int. Ed. 2011, 50, 657 – 660; b) F. Su, S. C. Mathew, G.
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
16301
S. K. Collins et al.
[21] [22]
[23]
[24] [25] [26] [27]
[28]
Lipner, X. Fu, M. Antonietti, S. Blechert, X. Wang, J. Am. Chem. Soc. 2010, 132, 16299 – 16301. A. C. Hernandez-Perez, A. Vlassova, S. K. Collins, Org. Lett. 2012, 14, 2988 – 2991. a) J. D. Slinker, J. Rivnay, J. S. Moskowitz, J. B. Parker, S. Bernhard, H. D. AbruÇa, G. G. Malliaras, J. Mater. Chem. 2007, 17, 2976 – 2988; b) D. G. Cuttell, S.-M. Kuang, P. E. Fanwick, D. R. McMillin, R. A. Walton, J. Am. Chem. Soc. 2002, 124, 6 – 7. CCDC-933555 ([5]helicene) and 933556 (pyrene – helicene hybrid 1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif. Y. Nakai, T. Mori, Y. Inoue, J. Phys. Chem. A 2013, 117, 83 – 93. J. E. Anthony, Chem. Rev. 2006, 106, 5028 – 5048. R. H. Martin, M. J. Marchant, Tetrahedron 1974, 30, 343 – 345. To the best of our knowledge, detailed electrochemistry for [5]helicene has not been reported in order to assess the accuracy of the calculated values. DFT has been shown to underestimate intramolecular stabilisation in helical structures. See: a) L. Rulisek, O. Exner, L. Cwiklik, P.
16302
www.chemeurj.org
Jungwirth, I. Stary´, L. Pospisil, Z. Havlas, J. Phys. Chem. C 2007, 111, 14948 – 14955. For other examples of computer modelling of helical structures, see: b) T. Kaseyama, S. Furumi, X. Zhang, K. Tanaka, M. Takeuchi, Angew. Chem. 2011, 123, 3768 – 3771; Angew. Chem. Int. Ed. 2011, 50, 3684 – 3687; c) K. Nakano, H. Oyama, Y. Nishimura, S. Nakasako, K. Nozaki, Angew. Chem. 2012, 124, 719 – 723; Angew. Chem. Int. Ed. 2012, 51, 695 – 699; d) D. Mys´liwiec, M. Ste˛pien´, Angew. Chem. 2013, 125, 1757 – 1761; Angew. Chem. Int. Ed. 2013, 52, 1713 – 1717. [29] J. B. Birks, D. J. S. Birch, E. Cordemans, E. Vander Donckt, Chem. Phys. Lett. 1976, 43, 33 – 36. [30] Yamato and co-workers recently synthesised a series of pyrenecored [4]helicenes substituted at the 4- and 9-positions and observed stabilisation of the HOMO level. See: J.-Y. Hu, A. Paudel, N. Seto, X. Feng, M. Era, T. Matsumoto, J. Tanaka, M. R. J. Elsegood, C. Redshaw, T. Yamato, Org. Biomol. Chem. 2013, 11, 2186 – 2197.
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: April 15, 2013 Revised: August 13, 2013 Published online: October 21, 2013
Chem. Eur. J. 2013, 19, 16295 – 16302
