Anthanthrene as a Super-Extended Tetraphenylethylene for

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Anthanthrene as a Super-Extended Tetraphenylethylene for Aggregation-Induced Emission Maude Desroches and Jean-François Morin* Département de Chimie et Centre de Recherche sur les Matériaux Avancés (CERMA) Université Laval, Pavillon A.-Vachon 1045 Ave de la Médecine, Québec G1 V 0A6, Canada S Supporting Information *

ABSTRACT: Seven π-extended derivatives of the known tetraphenylethylene (TPE) have been studied for their aggregation-induced emission properties. Modulation of the emission properties of these anthanthronebased luminogens has been accomplished through simple chemical modifications on four different positions of the anthanthrone core. The optical properties, along with X-ray diffraction and DFT-optimized structures, were studied. This work provides a new, easily accessible bridging unit for the TPE framework with a red-shifted emission.

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phenylamino)phenyl)fumaronitrile (NPAFN),11 and malononitrile-substituted pyran12 exhibit AIE properties as a result of restricted molecular motion in the solid state. Several applications can take advantage of the increased photoluminescence intensity in the solid state such as chemical sensors for the detection of trinitrotoluene,13 chirality,14 and metal ions,15 among others.16 Also, fine-tuning of the substituents on the TPE framework has been used to develop OLEDs from blue to red.6 Our group has been interested in the 4,10-dibromoanthanthrone dye for different applications.17−19 The conjugated anthanthrone core is a quinoidal polycyclic hydrocarbon made of two fused anthracenes. When the 6 and 12 positions are substituted with bis(diphenylmethylene) moieties, the resulting molecule can be seen as an extended version of the tetraphenylethene unit known for its aggregation-induced emission properties (Figure 1). Because of the high steric bulk between the anthanthrone core and its adjacent phenyl groups, this molecule can replicate the propeller shape of TPE that allows restricted intramolecular motion. While similar to the TPE in general structure, the anthanthrone core benefits from a wide range of properties upon very small changes either in the 4,10 or 6,12 conjugation channels.20,21 It was demonstrated that the steric bulk between these adjacent moieties is so high that it is the driving force for a double-to-single bond transformation, along with the creation of a stable biradical species.19 Most recently, an isoelectronic nitrogen-based dicationic analogue was shown to be open-shell, where the diradicals recombination is prevented by steric bulk.22 Several anthracene-based motifs have demonstrated some aggregation-induced emission in the green and yellow region of the visible spectrum.23,24 With its extended conjugation pathway, it was hypothesized that the anthanthrone

ggregation-induced emission has been extensively studied for the past 16 years.1,2 It has shed light on a solid-state phenomenon that consists of increased photoluminescence intensity with aggregation, while in most organic molecules, the exact opposite effect is observed. In fact, fluorescence is usually inhibited by π−π interactions and stacking, a phenomenon called aggregation-caused quenching.3−5 This quenching has hampered the uses of these molecules in light-emitting devices (OLEDs), as emission in the thin-film state is required.6 Thus, molecules exhibiting enhanced fluorescence in the solid, thinfilm, or aggregated state is certainly a desirable feature. One of the best strategies to obtain AIE properties is to design molecules that contain a fixed, rigid part (stator) bearing several moving parts (rotors) whose intramolecular motions can be inhibited upon aggregation, enabling an accessible radiative deactivation pathway.7 The most common example of such systems is tetraphenylethene (TPE) (Figure 1), an almost nonemissive chromophore in solution that can be highly emissive in the solid state due to its twisted structure that prevents π−π stacking.8,9 Other classes of molecules, such as 1,1,2,3,4,5-hexaphenylsilole (HPS),10 bis(4-(N-(1-naphthyl)-

Figure 1. Structure of TPE and its extended version with the anthanthrene dye. © XXXX American Chemical Society

Received: February 7, 2018

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DOI: 10.1021/acs.orglett.8b00452 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

Figure 2. Seven anthanthrene-based derivatives studied for AIE.

Scheme 1. Synthetic Scheme of Compound 3

Table 1. Summary of the Optical Properties of Compounds 1−7 1 2 3 4 5 6 7

λmax (nm)

ελmax (M−1 cm−1)

λonset (nm)

λexc (nm)

λem (nm)

Eg (eV)

ϕsolution (%)

ϕsolid (%)

485 488 513 576 562 513 627

43937 45717 23937 17147 20123 30794 15481

521 526 578 635 615 543 712

470 488 612 565 550 612 630

515 559 711 674 651 644 749

2.38 2.36 2.15 1.95 2.02 2.28 1.74

0.3 2.6

1.3 ± 0.4 1.3 ± 0.4

1.4 4.5

6.4 ± 1 18.6 ± 2

a Calculated at the onset of the absorption spectrum, Eg was determined at λonset, standard for solution QY fluorescein 0.1 M NaOH (91%) for 1 and 2, and cresyl violet in ethanol (57.8%) for 4 and 5.

core as the stator could produce AIE-active fluorophores in the red to near-infrared (NIR) region, which is of great interest for bioimaging.25 Herein, we report the synthesis and characterization of seven anthanthrone-based derivatives with different substituents in the 4,10 and 6,12 positions (Figure 2). The influence of the bis(diphenylmethylene) versus diphenylamines at the 6 and 12 positions, and the planarity of the polycyclic aromatic core on the photoluminescence intensity is discussed. These molecules have been studied by UV−visible absorption and emission spectroscopy and X-ray crystallography. DFT

calculations were also performed to obtain the optimized structures. Compounds 1,17 2,19 4 and 5,22 and 6 and 717 were synthesized according to a previously reported method. Compound 3 was synthesized from compound 7 by a Ramirez olefination with PPh3 and CCl4 in the microwave at 150 °C to afford compound 8 in 99% yield. Compound 8 was subjected to a 4-fold Suzuki−Miyaura coupling with 4-cyanophenylboronic acid pinacol ester at 50 °C to afford compound 3 in 54% yield (Scheme 1). B


Letter

Organic Letters The optical properties of all compounds are reported in Table 1. As expected, the nature of the functional groups positioned at the 4, 6, 10, and 12 positions has a significant influence on the optical properties of the anthanthrene and anthanthrone core. In the bis(diphenylmethylene) series (compounds 1−3), the addition of a diphenylamine unit at the 4 and 10 positions (compound 3) induces a bathochromic shift of 25 nm of the absorption spectrum relative to the TIPSacetylene derivative 2. The same transformation on anthanthrone (compound 6 to 7) produces an even larger bathochromic shift (114 nm) due to the creation of a stronger donor−acceptor complex. In the case of compounds 4 and 5, the replacement of TIPS-acetylene by diphenylamine resulted in a small hypsochromic shift, which can be attributed to the loss of conjugation and the absence of a donor−acceptor complex. These small substitution changes also impacted significantly the photoluminescence spectra of the anthanthrone and anthanthrene derivatives. In fact, emission in solution from green (515 nm, compound 1) to red (749 nm, compound 7) can be obtained. Solution quantum yields were measured for 1, 2, 4, and 5 (0.3, 2.6, 1.4, and 4.5%, respectively) and emit very weakly. The AIE properties of compounds 1−7 have been studied by addition of distilled water fractions in THF solution of the chromophore. Because of their similar structure, both compounds 1 and 2 showed analogous behavior with increased water fraction ( f w) (Figures 3 and S5, and Figure S6,

Figure 4. Front view and side view of the optimized structure of compound 3. H atoms have been removed for clarity.

interactions occur between the neighboring phenyl rings, reducing the rotational movements both in solution and in the solid state. From these observations, it is interesting to note that the presence of a propeller-shaped triphenylamine unit does not guarantee enhanced photoluminescence in the aggregated state. For the 6,12-bis(diphenylamine) series, compounds 4 and 5 (Figure S8 and Figure 5, respectively) both showed AIE upon

Figure 5. (a) Absorption and emission spectra, (b) emission spectra as a function of the water fraction of compound 5 in THF.

addition of water to reach a maximum at f w of 90%. The photoluminescence is more intense in the case of compound 5, compared to 4, with the only structural difference residing in the substituents at the 4 and 10 positions. Indeed, the presence of diphenylamine units increased the maximum photoluminescence intensity, compared to the triisopropylsilyl groups. The presence of four additional rotors in 5 could explain the enhanced AIE effects. The solid-state quantum yield of 4 and 5 were measured and 5 has the highest value of 18.6% followed by 4 with 6.4%. Compound 5 has the most dramatic increase in its quantum yield value from solution to solid state, while not being the most emissive of the series. This trend is also reproduced in the aggregated state, with 5 having the highest increase in the emission intensity from f w = 0 to 90% (Figures S8c and S9). The aggregation-induced emission behavior of compound 5 stands in sharp contrast with the previously drawn conclusions for the diphenylmethylene series in compound 3. Again, the explanation could reside in the X-ray diffraction structure of compound 4.22 The diphenylamine units at the 6 and 12 positions are arranged in a trigonal planar geometry, meaning that no significant steric bulk between the anthanthrone core and the adjacent phenyl rings is present, resulting in a perfectly planar aromatic core. This geometry, also replicated in the DFT-optimized structure of 5 (Figure S20), should provide plenty of space for the free-rotation of the diphenylamines at the 4 and 10 positions in solution, which is not the case with compound 3. Interestingly, compounds 1 and 4 show very different behaviors in the aggregated state. The maximum photo-

Figure 3. (a) Absorption and emission spectra, (b) emission spectra in function of the water fraction of compound 1 in THF.

respectively). The photoluminescence was low until f w = 30% with a dramatic increase to a maximum at 40% water in THF. The solid-state quantum yields were measured and both compounds 1 and 2 have a low value of 1.3%. From solution to solid-state, there is an increase of 1% for 1 and 1.3% for 2. This increased intensity trend is also observed in the aggregated state with 2 having a more intense increase than 1 from f w = 30 to 40% (Figures S5 and S6c, respectively). In the case of compound 3, no significant photoluminescence was observed upon addition of water (Figure S7), meaning that the presence of diphenylamine units at the 4 and 10 positions is detrimental for the restriction of intramolecular motions. From the X-ray diffraction data of compound 2,19 the structure of 3 was optimized (Figure 4 and Figure S18). It was demonstrated that because of the steric congestion between the diphenylmethylene and the anthanthrone core, compound 2 adopts a butterfly structure, which is also replicated in the optimized structure of 3. One major difference dwells in the trigonal planar geometry of the diphenylamine units at the 4 and 10 positions of 3, increasing the steric bulk on the butterfly structure. One can assume that the high steric congestion prevents the free rotation of the phenyl both in solution and in the solid state. It is also possible that intramolecular πC


Organic Letters

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luminescence intensity was reached for compound 1 at a much lower f w value than for 4 (40 and 90%, respectively), which could be attributed to the poorer solubility of compound 1. Figure 6a shows solutions of compounds 1, 2, 4, and 5 at

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00452. General information, synthetic details, NMR spectra, UV−vis spectra, DRX analysis, DFT geometry, and SEM images (PDF) Accession Codes

CCDC 1581831 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jean-François Morin: 0000-0002-9259-9051 Notes

The authors declare no competing financial interest.

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Figure 6. (a) Photoluminescence at maximum water fraction of 1, 2, 4, and 5, left to right, (b) CIE coordinates of 4 (X) and 5 (+), (c) SEM image of aggregated 2, scale 10 μm, (d) SEM image of aggregated 4, scale 100 μm.

ACKNOWLEDGMENTS M.D. thanks the NSERC for a Ph.D. scholarship. We also thank Thierry Maris at Université de Montréal for the DRX analysis, Thaksen Vasant Jadhav and Dmitrii Perepichka from McGill University for solid-state quantum yield measurements, and André Ferland at Université Laval for SEM analysis.

different f w values, corresponding to the maximum photoluminescence intensity. Compounds 4 and 5 both show emission in the red, and their CIE coordinates are (0.55, 0.28) and (0.62, 0.37), respectively (Figure 6b). These coordinates are very close to the pure red (0.67, 0.33) especially for compound 5, which could be interesting for the development of OLEDs and for bioimaging applications. SEM images of the dried samples 1, 2, 4, and 5 from THF were compared to the aggregated samples with water at their maximum fluorescence intensity (Figure 6c,d and Figures S12− S15). It is clear that all of the samples in the presence of a nonsolvent contain large domains of aggregates. The ketones series (compounds 6 and 7, Figures S10 and S11, respectively), do not show strong photoluminescence, most probably due to the absence of phenyl rotors in 6. As for 7, the photoluminescence intensity is very low but still shows AIE at a f w of 70%, which could be attributed to the diphenylamine units at the 4 and 10 positions (see the Supporting Information, X-ray diffraction details). In both compounds 6 and 7, the presence of fluorescence quenching ketones could also result in very low emissive properties, therefore not showing strong AIE behavior. In conclusion, we have studied seven new AIE luminogens bearing an extended polycyclic aromatic core, anthanthrone, to replicate the propeller shape of the AIE-active TPE. Slight changes in the substituents at the 4,10 and 6,12 positions have a dramatic effect on the optical and AIE properties. The extended conjugation of the anthanthrone stator allows emission in the NIR with compound 5 having the closest CIE coordinates to pure red.

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REFERENCES

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Anthanthrene as a Super-Extended Tetraphenylethylene for

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