Synthesis, structure and optical properties of tetraphenylethene derivatives with through-space conjugation between benzene and various planar chromophores

Abstract

Through-space conjugation is conducive to efficient multidimensional exciton diffusion and charge transport in optoelectronic functional materials. Herein, a series of tetraphenylethene derivatives with through-space conjugation formed between benzene and various aromatic rings are synthesized and characterized. Their crystal structures, frontier orbital distributions and aggregation-enhanced emission properties are investigated.

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π–π interactions, along with cation–π¹ and CH–π interactions,²etc., are very important noncovalent weak interactions, which play a vital role in manipulating molecular geometries, ranging over biology, chemistry, and materials.³ When it comes to π–π interactions, the two interplaying aromatic rings can pile up in different manners of parallel face-to-face, parallel offset, and edge-to-face (T-shape),⁴ resulting in different degrees of interactions. The efficient π–π interaction, generally occurring in the two closely face-to-face stacked aromatic rings, is also referred to as through-space conjugation. Intense electron clouds of the frontier molecular orbitals can delocalize onto the inter-ring region of the two through-space conjugated aromatic rings. The pioneering synthesis of [2.2]paracyclophane,⁵ where two benzene rings are stacked in an orientation of parallel face-to-face, provided a perfect model for studying the through-space conjugation. Recent reports further demonstrate that through-space conjugation can occur in many molecules that possess well-defined and nonfluxional structures with two chromophores arranged in close proximity,⁶ and allow for effective multidimensional exciton diffusion and charge transport in optoelectronic functional materials.

In addition to the myriad of [2.2]paracyclophane derivatives including small molecules, oligomers,⁷ and polymers,⁸ various other skeletons featuring through-space conjugation have also been developed, including derivatives of naphthalene,⁹ diarylbiphenylene,¹⁰ xanthene,¹¹ bicyclo[4.4.1]undecanone,¹² hexaphenylbenzene,¹³ dibenzofulvene,¹⁴ indenofluorene,¹⁵ and so forth. However, most studied through-space conjugation is based on two identical aromatic rings, and that which happens in two different kinds of aromatic rings is much rarer, leaving much space for further digging deep through-space conjugated molecules and deciphering the influence of through-space conjugation on molecular properties.

Recently, a novel folded skeleton with intramolecular through-space conjugation has been developed in our study on tetraphenylethene (TPE) derivatives, which is a stereospecific product of McMurry coupling aided by substrate modulation.¹⁶ Most TPE derivatives¹⁷ exhibit aggregation-induced emission (AIE) characteristics, but the novel folded conformation endows TPE derivatives with another brand new emission experience, and allow for novel optoelectronic functionalities. To further expand the family of through-space conjugated molecules, in this work, we wish to report a series of new TPE derivatives (namely o-TPEAr), where through-space conjugation is formed between benzene and different planar chromophores. These new TPE derivatives were readily obtained by Suzuki–Miyaura coupling of o-TPEBr with the corresponding arylboronic acids as illustrated in Scheme 1. Various aromatic substituents including benzene, naphthalene, anthracene, phenanthrene and pyrene were introduced onto the ortho-position of the phenyl ring in the TPE moiety. The detailed characterization data of NMR and mass spectra are given in the ESI.† The molecular structures of o-TPEPh, o-2-TPENp, o-TPEAn, o-TPEPa and o-TPEPy were further confirmed by X-ray diffraction crystallography analyses on their single crystals, which were grown from THF/ethanol mixtures via slow diffusion and evaporation of the solvents. The parameters for the crystal structures are described in the ESI.† As shown in Fig. 1, the incorporated planar aromatic groups are stacked with the phenyl rings in TPE in a face-to-face, slightly slipped manner. However, different aromatic substituents will give rise to varied stacking choice to the phenyl rings in TPE. For instance, the small-sized benzene and naphthalene rings tend to stack with Φ₁ as indicated in Scheme 1, while large anthracene, phenanthrene and pyrene rings prefer to stack with Φ₂. The two stacked aromatic rings are not totally parallel. The tilted angles are 9.95°, 15.72°, 12.51°, 25.76°, and 14.40° for o-TPEPh, o-2-TPENp, o-TPEAn, o-TPEPa and o-TPEPy, respectively (Table 1 and Fig. S1–S5†). And they are staggered and overlapped in varied degrees. The shortest distances between the two stacked aromatic rings are in the range of 2.97–3.44 Å, being shorter than the typical π–π stacking interaction distance of 3.5 Å, implying the existence of π–π interactions, that is through-space conjugation.

Scheme 1 Synthetic routes to o-TPEAr.

Fig. 1 Crystal structures of o-TPEAr with indicated distances (Å) between π–π stacked aromatic rings.

Table 1 Tilted angles of two cofacially stacked rings in o-TPEAr

Compound	Cofacial pairs	Tilted angles (°)
o-TPEPh	Ph-Φ1	9.95
o-2-TPENp	Np-Φ1	15.72
o-TPEAn	An-Φ2	12.51
o-TPEPa	Pa-Φ2	25.76
o-TPEPy	Py-Φ2	14.40

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Intramolecular through-space conjugation is an interesting characteristic of π-electron cloud overlapping between cofacial aromatic rings.^7a,16c Density functional theory (DFT) calculations, using the ωB97XD functional with the 6-31g* basis set, were carried out to further verify the occurrence of through-space conjugation in o-TPEAr. The amplitude plots of important frontier molecular orbitals ranging from HOMO−2 to LUMO+2 are displayed in Fig. 2. It can be seen that the transannular π-electron cloud overlapping appears in π-stacked aromatic rings. Despite the slipped and staggered stacking of the two phenyl rings in o-TPEPh (Fig. 1 and S6†), obvious cofacial π-electron cloud overlapping still occurs, as shown in its LUMO+1 orbital. o-2-TPENp and o-TPEAn, which possess large planar chromophores, also show π-electron cloud overlapping in the LUMO+1 orbital and HOMO−2 orbital, respectively. For o-TPEPa and o-TPEPy, these features are not so distinct, probably owing to the unsuitable electronic structures as well as energy levels of phenanthrene and pyrene relative to benzene.

Fig. 2 Molecular orbital amplitude plots ranging from HOMO−2 to LUMO+2 of o-TPEAr in the ground state calculated at the ωB97XD/6-31g* level.

The absorption spectra of o-TPEAr in THF solutions are shown in Fig. 3A. The spectral profiles and maximum absorption peaks varying from 287 to 391 nm (Table 2) are closely related to the type of introduced planar chromophore. The o-TPEAr are nonfluorescent or weakly fluorescent in dilute THF solutions, and their absolute fluorescence quantum yields (Φ_F) fall in the range of 0.8–4.6%. With regard to o-TPEAn and o-TPEPy that possess large fluorescent chromophores, they show much lower Φ_F values than isolated anthracene (36%) and pyrene (32%).¹⁸ These results are well consistent with many TPE derivatives, such as those with aromatic substituents at the para-positions of the phenyl ring in TPE, no matter mono-substitution¹⁹ (TPEAr) or bis-substitution.²⁰ It can be interpreted as that intramolecular rotation, which is ubiquitous in many other TPE derivatives,²¹ activates the nonradiative decay of excitons, and thus leads to fluorescence quenching in the solution state. But their Φ_F values are much higher than those of TPEAr (0.019–0.34%), due to lowered rotation by the increased molecular rigidity derived from intramolecular through-space conjugation.

Fig. 3 (A) Absorption spectra of o-TPEAr in THF solution (10 μM). Photoluminescence (PL) spectra of (B) o-TPEPh, (C) o-2-TPENp, (D) o-TPEAn, (E) o-TPEPa, and (F) o-TPEPy in THF–water mixtures with different water fractions (f_w); λ_ex = 330 nm.

Table 2 Photophysical properties of o-TPEAr

	λ abs (nm)	λ em (nm)			Φ F ^d (%)		α AIE
	Soln^a	Aggr^b	Film^c	Crystal	Soln^a	Film^c
a In THF solution (10 μM). b In THF/water mixtures (V/V 1/99). c Film drop-cast on a quartz plate. d Absolute fluorescence quantum yield measured by an integrating sphere. e AIE effect determined as ΦF (Film)/ΦF (Soln).
o-TPEPh	313	466	466	445	1.9	55	29
o-2-TPENp	287	468	467	425	2.1	56	27
o-TPEAn	391	504	501	432	4.6	61	13
o-TPEPa	303	473	472	465	0.8	54	68
o-TPEPy	350	490	484	424	3.3	68	21

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The o-TPEAr are much more fluorescent in the aggregated state. Fig. 3B–F display photoluminescence (PL) spectral changes of o-TPEAr with the addition of water to their THF solutions. Because water is a non-solvent for o-TPEAr, the addition of a large amount of water will make the molecules form aggregates, and thus alter their emission properties. Taking o-TPEAn as an example, it shows an emission spectrum in the blue region with fine structures at around 400 and 422 nm stemming from an anthracene moiety when the water fraction (f_w, vol%) is lower than 80%, but when f_w increases to 90% or more, the blue emission band disappears and a long-wavelength emission is swiftly enhanced. Similar PL spectral changes can also be observed in o-TPEPh, o-2-TPENp, o-TPEPa and o-TPEPy, except that their blue emission bands (f_w < 80%) are relatively weaker. These results clearly demonstrate that these new TPE derivatives have aggregation-enhanced emission (AEE) properties.¹⁷ The disappearance of blue emission bands and the appearance of long-wavelength emission bands should result from the restriction of intramolecular rotation, because the aromatic rings that do not form intramolecular π-stacking can rotate freely, and thus impair the electronic coupling between building blocks. When the intramolecular rotation is restricted in the aggregated state, the emission is stemmed from the radiative decay of all the conjugated molecules.

Like many other TPE derivatives, o-TPEAr are strongly fluorescent in solid films. The PL spectra of their solid films are given in Fig. S11A,† and the detailed data are listed in Table 2. The emission peaks of the solid films of o-TPEPh, o-2-TPENp, o-TPEAn, o-TPEPa, and o-TPEPy are located at 466, 467, 501, 472, and 484 nm, respectively, exhibiting variations with the types of incorporated chromophores. These PL peaks are very close to those in the aggregated state, i.e. the corresponding emission peaks are located at 466, 468, 504, 473 and 490 nm, respectively, in THF–water mixtures (f_w = 99%) (Table 2). The Φ_F values of the solid films are in the range of 54–68%, which are greatly improved with respect to those in solutions. The corresponding values of the AIE effect (α_AIE) are in the range of 13–68. The crystals of o-TPEAr show PL emissions peaking at 424–465 nm (Table 2 and Fig. S11B†), which are blue-shifted relative to those of amorphous films. Similar blue-shifted emissions from amorphous films to crystals have also been observed and studied on many AIE-active molecules, which are attributed to the lowered reorganization energy in more rigid crystals,²² with the help of multiple inter- and intramolecular CH–π interactions (Fig. S6–S10†).

In conclusion, a series of TPE derivatives substituted with various planar chromophores at the ortho-position of the phenyl ring in TPE are synthesized and characterized. The crystallography analysis reveals that the introduced chromophores adopt a close cofacial π–π stacking pattern with the phenyl rings in TPE, and theoretical calculation further demonstrates the apparent through-space conjugation characteristic of transannular π-electron cloud overlapping between π-stacked aromatic rings. But different aromatic rings as well as different geometries show significant impacts on the formation of intramolecular through-space conjugation. These TPE derivatives exhibit better PL emission properties than similar TPEAr in the literature, owing to the enhanced molecular rigidity by through-space conjugation. Their PL emissions are further improved in the aggregated state, exhibiting AEE features. These through-space conjugated TPE derivatives are good models for the mechanistic studies of some photophysical phenomena, such as AIE, and will provide deep insights into the structure–property correlation of various luminescent materials.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (51273053), the 973 Program (2015CB655004 and 2013CB834702), the Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306035), the Guangdong Innovative Research Team Program (201101C0105067115), the Natural Science Foundation of Guangdong Province (2016A030312002), the ITC-CNERC14S01 and the Fundamental Research Funds for the Central Universities (2015PT020 and 2015ZY013).

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Footnote

† Electronic supplementary information (ESI) available: Experimental, single crystal data, molecular packing patterns, photoluminescence spectra of o-TPEAr in films and crystals, and ¹H NMR spectra. CCDC 955717, 1432503–1432506. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qo00204h

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