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. 2022 Dec 2;7(49):45732–45739. doi: 10.1021/acsomega.2c07168

Aggregation-Induced Emission of Contorted Polycondensed Aromatic Hydrocarbons Made by Edge Extension Using a Palladium-Catalyzed Cyclopentannulation Reaction

Noorullah Baig †,‡, Suchetha Shetty †,‡, Rajeshwari Tiwari §, Sumit Kumar Pramanik §, Bassam Alameddine †,‡,*
PMCID: PMC9753205  PMID: 36530321

Abstract

graphic file with name ao2c07168_0009.jpg

Contorted polycyclic aromatic hydrocarbons (PAHs), CPA1–2 and CPB1–2, bearing peripheral five-membered rings were synthesized employing a palladium-catalyzed cyclopentannulation reaction using specially designed diaryl acetylene synthons TPE and TPEN with commercially available dibromo- anthracene DBA and bianthracene DBBA derivatives. The resulting target compounds CPA1–2 and CPB1–2 were isolated in excellent yield and found to be highly soluble in common organic solvents, which allowed for their structural characterization and investigation of the photophysical properties, disclosing their aggregation-induced emission (AIE) properties in THF at selective concentration ranges of water fractions in the solvent mixture. Examination of the contorted PAH structures by means of density functional theory (DFT) revealed higher electronic conjugation in the more rigid and planar anthracene-containing CPA1–2 derivatives when compared to the twisted bianthracene-bearing moieties CBPA1–2 with HOMO–LUMO bandgaps (ΔE) of ∼2.32 eV for the former PAHs and ∼2.78 eV for the latter ones.

Introduction

Contorted polycyclic aromatic hydrocarbons (PAHs) are conjugated compounds whose out-of-plane geometry is mostly caused by an intramolecular steric congestion.1,2 PAHs bearing cyclopentadienide units, which can be considered as cross-sectional units of fullerenes, disclose prominent optoelectronic3−8 properties and, consequently, are employed in organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs), and organic photovoltaics (OPVs).9,10 Thus, there is a growing interest in exploring versatile and efficient synthetic strategies to make new contorted PAH structures and explore their chemical and physical properties.9 Among others, palladium- and nickel-catalyzed cross-coupling reactions, notably, Heck, Negishi, Suzuki–Miyaura, and Sonogashira,11,12 are widely utilized in the synthesis of PAHs.13−15 In addition, cycloaddition reactions have traditionally been used in the synthesis of large PAHs,16 namely, Diels–Alder reaction, which proved to be a versatile synthetic approach to make large aromatic compounds.17 There are several reports in the literature that describe the employment of alkyne derivatives as potential substrates to expand the size of PAHs via stereospecific annulation reactions of the aromatic edges,18−23 thus resulting in the formation of unique conjugated structures bearing fused benzenoid groups.24−26 The increased PAH size caused by the hitherto mentioned ring expansion generally leads to reduced band gap energies, which improves the charge transport properties.27,28 Nevertheless, if annulation affords sterically hindered fused rings, the resulting PAH will have a distorted structure instead of a planar one, giving rise to significant aggregation.29 To the best of our knowledge, few examples can be found in the literature on the synthesis of PAHs using cyclopentannulation reaction methodology.24,26,29−32

Generally, luminescent materials display higher emission intensity when they are present in solution, which diminishes substantially upon aggregation or when these materials are in the solid state. Such quenching of the emission intensity is known as “aggregation caused quenching (ACQ)” effect, which is an intractable challenge for optoelectronic devices.33 In 2001, Tang et al.(34) reported the synthesis of new organic compounds whose emission is only enhanced upon aggregation coining this photophysical phenomenon as aggregation-induced emission (AIE), thus allowing for the development of various small molecules and polymers for various applications, among others, photonics, nuclear and particle physics, quantum electronics, and medical diagnostics.35−37 While the mechanism behind the AIE phenomenon is still debated, most research focuses on the synthesis of organic molecules with rotation axes and whose restricted intramolecular rotation (RIR) caused by aggregation leads to AIE.33 Additionally, other materials have been developed revealing AIE behavior but whose mechanisms are based on the restriction of intramolecular charge transfer (ICT), twisted intramolecular charge transfer (TICT), cis–trans isomerization, planarization, J-type aggregates, etc.38−41

In this work, we reveal the synthesis of a several cyclopentannulated anthracene derivatives CPA1–2 and CPB1–2 employing a palladium-catalyzed cyclopentannulation reaction, which interestingly reveal AIE properties.

Results and Discussion

Synthesis

The diaryl acetylene synthons, TPE and TPEN, were synthesized following reported procedure,30,42,43 and their structures were confirmed by 1H- and 13C-nuclear magnetic resonance (NMR), high-resolution mass spectrometry (EI-HRMS), FTIR (synthetic procedures (i) and (ii), in addition to Figures S1, S5, and S9 in the Supporting Information).

Scheme 1 elucidates the two-fold palladium-catalyzed cyclopentannulation reaction carried out using either TPE or TPEN with each of the commercially available 9,10-dibromoanthracene, DBA and 10,10′-dibromo-9,9′-bianthracene DBBA, affording the four PAH-extended derivatives, CPA1–2 and CPB1–2. Desired compounds CPA1(30)CPA2, CPB1–2 were isolated in excellent yields (86–90%) and have excellent solubility in common organic solvents. It is noteworthy that the formation of CPA2 and CPB1–2 was confirmed by 1H- and 13C-NMR spectroscopy, EI-HRMS, FTIR, and UV–visible absorption and fluorescence spectroscopy (see Figures 1–4 and Figures S2–S4, S6–S8, S10 and S11, and S12–S16 in the Supporting Information).

Scheme 1. Synthesis of CPA1–2 and CPB1–2.

Scheme 1

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Figure 1.

Figure 1

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Comparative 1H-NMR of TPE, DBBA, and CPB1 recorded in CDCl3.

Figure 4.

Figure 4

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Normalized absorption (left) and emission (right) spectra of compound CPA1 in different solvents.

Figure 1 portrays the comparative 1H-NMR spectra of both synthons TPE and DBBA in addition to target PAH CPB1, which clearly confirms the presence of all the desired peaks for the latter and the absence of those of the starting material. Thus, the aromatic protons of CPB1 were detected in the range of 7.8–7.0 ppm (see Figure 1 and Figure S3 in the Supporting Information). The distinctive aromatic protons in DBBA detected as a doublet at 8.74 ppm (c.f. peaks labeled a in Figure 1) completely disappear in the 1H-NMR spectrum of CPB1. In addition, the 1H-NMR spectrum of the latter target compound divulges the characteristic peaks, which can be attributed to the t-butyl moieties at 1.4 and 1.3 ppm, respectively (see Figure S3, in the Supporting Information). Furthermore, the 13C-NMR spectrum displays all the peaks expected for CPB1 with the aromatic ones ranging from 149.4 to122.6 ppm as well as the two peaks detected at 34.5 and 31.6 ppm, which are assigned to the characteristic sp3 carbons of the t-butyl groups (see Figure S7, in the Supporting Information). Likewise, 1H- and 13C-NMR spectra of CPA2 and CPB2 corroborate their formation by divulging all their chemical shifts (see Figures S2, S4, S6, and S8 in the Supporting Information).

The formation of the newly synthesized compounds CPA2 and CPB1–2 was also confirmed by electron impact high-resolution mass spectrometry (EI-HRMS), which disclosed their corresponding exact mass peaks. The isotopic patterns of the desired compounds reveal their high purity, as evidenced from Figure 2, which shows the similarity between the experimental and calculated isotopic peaks of CPB1.

Figure 2.

Figure 2

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EI-HRMS spectrum of CPB1 (inset: isotopic distribution at m/z).

Figure 3 illustrates the comparative FT-IR absorption spectra of the starting materials DBBA and TPE and their corresponding target compound CPB1. It is noteworthy that neither the characteristic stretching vibrations of the C-Br groups in DBBA, which are observed at 741 cm–1,44 nor the fingerprint stretching vibrations of the alkynyl group (CΞC) in TPE, which are detected at 2208 cm–1,45 are noticed in the FT-IR absorption spectrum of the target compound CPB1, therefore proving the complete cyclopentannulation reaction. It is noticeable that the FT-IR spectrum of CPB1 discloses pronounced aliphatic C–H and aromatic C–H stretching vibrations at 2953 and 1428 cm–1, respectively. The same is also observed at 832 cm–1, which is attributed to C=C bending vibrations.46 This further confirms the formation of the desired doubly cyclopentannulated PAH compound CBP1.

Figure 3.

Figure 3

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Comparative FT-IR absorption spectra of DBBA, TPE, and target compound CPB1.

Photophysical Properties

The UV–Vis absorption spectroscopy and emission characteristics of PAHs CPA1–2 and CPB1–2 were investigated in different solvents of various polarities, namely, hexane, toluene, xylene, THF, acetonitrile, and dimethyl sulfoxide (DMSO) (see Figure 4 below and Figures S14–S16 in the Supporting Information). The UV–Vis absorption spectrum of CPA1 in all the solvents show absorption bands with a maximum peak detected at 394 nm in addition to two other absorption bands at 440 and 465 nm in addition to a shoulder peak in the visible region at 625 nm. On the other hand, the UV–Vis absorption spectrum of PAH CPA2, i.e., which contains the naphthyl pending groups, exhibits an absorption peak maximum at 494 nm in toluene, xylene, THF, and DMSO but is blue-shifted in hexane and acetonitrile by 131 and 5 nm, respectively (see Figure S14 in the Supporting Information). UV–Vis absorption spectra of compounds CPB1–2 display the same features in all the solvents, revealing a strong absorption band at ∼377 and ∼ 420nm, respectively, which could be explained by the π–π* transition47 of the bianthracene core48 (see Figures S15 and S16 in the Supporting Information).

The photoluminescence (PL) properties of PAHs CPA1–2 and CPB1–2 were also explored using the same solvents employed to study their UV–Vis absorption. Emission spectra of CPA1 disclose the same pattern in hexane, toluene, xylene, and DMSO with two peaks whose λmax values are detected at 329 and 355 nm. As noticed from Figure 4, CPA1 portrayed red-shifted peaks in THF and ACN by 109 and 32 nm, respectively, which indicates that the solvent effect was more conspicuous in emission than UV–Vis absorption. On the other hand, negligible red shifts in the range of 6–8 nm were noted in the emission spectra of CPA2 and CPB2 upon changing the solvent from a non-polar solvent to polar (see Figure S14 and S16 in the Supporting Information). Interestingly, the emission spectra of CPB1, which reveals an emission maximum at 329 nm in the polar solvent DMSO, was red shifted by 81 nm in the non-polar solvent hexane and therefore displaying the emission maximum at 410 nm (see Figure S15 in the Supporting Information).

Aggregation-Induced Emission (AIE) Properties

Given that the newly synthesized PAHs CPA1–2 and CPB1–2 have various rotation centers, which could allow them to act as typical AIE chromophores, where the rotation of the peripheral phenyl rings decays the excitation energy and reduces the luminescence intensity in solution,49 several experiments were carried out to investigate the AIE properties of CPA1–2 and CBP1–2 (see Figures S17 and S18 in the Supporting Information) Interestingly, AIE properties were observed for all the hitherto mentioned PAHs when they were tested in various THF/H2O solvent mixtures with an increasing water concentration from 0% to 100%.

Figure 5 illustrates the AIE study, which was carried out for CPB2 where the solution of the latter in pure THF exhibits a very weak emission spectrum but whose intensity increases when raising the water fraction (fw) in the medium (Figure 5A,B), which implies that the emission intensity increases with the formation of aggregates caused by the addition of water. The plot of the emission intensity of CPB2 versus water concentration clearly shows the enhanced emission (c.f. inset image taken under 365 nm excitation in Figure 5B). The fluorescence intensity of CPB2 reaches the maximum in a 30:70 concentration of the THF/H2O mixture before it starts decreasing, as fw increases, showing a very weak emission peak in pure water. The enhancement of the PL intensity in the aggregated state (i.e., fw = 70%) proves the AIE property of the CPB2 whose rotational aromatic moieties interlinked through the conjugated spacers result in an interlocked network upon aggregation.33 It is worth mentioning that the decrease in the PL intensity of CPB2 after reaching an fw of 70% can be explained by two reasons: (i) the formation of large aggregates where only the molecules, which are present at the surface, will emit light upon excitation, thus leading to a decrease in the overall fluorescence intensity50 or (ii) the solute molecules aggregate as amorphous nanoparticles, which leads to a reduction in the PL intensity unlike the formation of crystalline nanoparticles, which enhances it.51 The aqueous mixtures with higher fw but where precipitation was not observed suggest that the size of the aggregates is in the nanoscale dimension. Therefore, dynamic light-scattering (DLS) analysis of CPB2 in THF/H2O, namely, the solutions with fw of 70% and 90%, revealed average aggregates sizes of ∼90 d.nm and ∼340 d.nm, respectively (Figure 5C). The correlation of the aforementioned results with the emission intensity change observed in Figure 5B clearly indicates that the emission decrease observed at an fw of 90% is because only the molecules at the surface of the large aggregates are emitting light.52 Similarly, the other target compounds CPA1–2 and CPB1 display AIE properties but at different water fraction ranges, with average aggregate sizes for CPA1 of ∼415 d.nm and ∼820 d.nm at fw of 60% and 90%, respectively, while those for CPA2 have aggregates size of ∼365 d.nm and ∼825 d.nm at fw of 70% and 90%, respectively. CPB1 disclosed aggregate sizes of ∼165 d.nm and ∼553 d.nm at fw of 40% and 90%, respectively (see Figures S19–S21 in the Supporting Information).53

Figure 5.

Figure 5

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(A) Emission spectra of CPB2 in THF/water mixture (0–100%); (B) plot of maximum emission intensity of CPB2 versus water fraction; (C) dynamic light-scattering (DLS) spectra, expressed as diameter values in nanometers (d.nm), of CPB2 in THF/water, wt %; 70 (black dashed line) and 90 (red dashed line).

Electronic Structure

The optimized structures of CPA1–2 and CPB1–2 using the B3LYP/6-31G* basis set show similar orbital distribution of the HOMO and LUMO energy levels with electron conjugation majorly located in the cyclopentannulated anthracene and bianthracene structures but is disrupted at the peripheral aryl rings whose deviation from planarity prevents their conjugation with the central aromatic core (Figure 6). It is worth mentioning that the peripheral phenyl groups in the anthracene-containing PAH CPA1 disclose similar dihedral angles of 58.1° and 61.8°, whereas the more sterically demanding naphthyl groups in CPA2 reveal a dihedral angle of 76.2° and reduced deviation for the phenyl groups by 43.6° from the central core. On the other hand, the skewed bianthracene backbone in CPBA1–2 displays dihedral angles of 86.9° and 88.5°, respectively. The PAHs portray similar behavior to CPA1–2 where the pending phenyl groups in CPBA1 are out of plane by ∼64.3° whereas the cumbersome naphthyl groups in CPBA2 portray a dihedral angle of 78.4° while the neighboring phenyl units are twisted from the planar core by 45.3°. The calculated HOMO and LUMO values of CPA1–2 are analogous (Figure 6), revealing band gaps ΔE of ∼2.32 eV. On the other hand, CPB1–2 displays higher ΔE values of 2.78 and 2.81 eV, respectively, which could be explained by the disruption of conjugation caused by the skewed structure of the bianthracene core.

Figure 6.

Figure 6

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Optimized structures and molecular orbital amplitude plots of HOMO and LUMO energy levels of CPA1–2 and CPB1–2 calculated using the B3LYP/6-31G* basis set.

Conclusions

New large and contorted PAHs were obtained in excellent yield employing a mild palladium-catalyzed cyclopentannulation reaction condition. Investigation of the photophysical properties of target compounds CPA1–2 and CPBA1–2 revealed their aggregation-induced emission (AIE) behavior in THF/H2O solvent mixtures. Structural optimization using density function theory (DFT) calculations at the B3LYP/6-31G* basis set reveals a higher electronic conjugation in the more rigid and planar anthracene-containing CPA1–2 derivatives when compared to the twisted bianthracene-bearing moieties CBPA1–2. The reported versatile synthesis allows for designing bent PAH structures by extending their aromatic core from the edges and therefore tuning their photophysical properties for applications in various fields, namely, optoelectronics.

Experimental Section

General Information

All the reactions were carried out under an inert atmosphere using dry argon. All chemical reagents were used as purchased without further purification from Merck (Darmstadt, Germany) and HiMedia (Mumbai, India), unless otherwise specified. The ethynyl aryl synthons 1,2-bis(4-(tert-butyl)phenyl)ethyne42 (TPE), 1-((4-(tert-butyl)phenyl)ethynyl)naphthalene30,43 (TPEN), and 1,2,6,7-tetrakis(4-(tert-butyl)phenyl)cyclopenta[hi]aceanthrylene30 (CPA1) were synthesized following the literature. The solvents, namely, hexane, dichloromethane (DCM), tetrahydrofuran (THF), toluene, methanol, diisopropylamine, acetonitrile (ACN), and dimethylformamide (DMF), were dried over molecular sieves and deoxygenated by bubbling with argon gas for 30 min. Thin-layer chromatography (TLC) was performed on aluminum sheets coated with silica gel 60 F254 and revealed using a UV lamp. NMR spectra (1H: 600 MHz, 13C: 150 MHz) were recorded using a JEOL resonance ECZ600R spectrometer at 25 °C, using CDCl3 as a solvent with the chemical shifts (δ) given in ppm and referred to tetramethylsilane (TMS). Electron impact high-resolution mass spectra (EI-HRMS) were recorded on a Thermo (DFS) with a standard PFK (perfluorokerosene) as a lock mass. The analyzed data is converted to accurate mass employing X-Calibur accurate mass calculation software. UV–Vis spectra were recorded on a Shimadzu UV1800 spectrophotometer. Photoluminescence (PL) spectra were recorded on an Agilent G9800 Cary Eclipse Fluorescence spectrophotometer. FT-IR spectra were recorded on an Agilent Cary 630 FTIR instrument. Both size and size distribution of the dye molecule and their aggregates were measured at 25 °C by dynamic light-scattering (DLS) using a Brookhaven instruments Zetapals (USA) and expressed as diameter values in nanometers (d.nm).

Synthetic Procedure of PAHs

Synthesis of 2,7-Bis(4-(tert-butyl)phenyl)-1,6-di(naphthalen-1-yl)cyclopenta[hi]aceanthrylene CPA2 (Procedure A)

A Schlenk tube was charged under argon with 9,10-dibromoanthracene DBA (23 mg, 0.07 mmol, 1 equiv), TPEN (43 mg, 0.15 mmol, 2.2 equiv), Pd2(dba)3 (6.4 mg, 7.0 μmol, 0.1 eq), tris(o-tolyl)phosphine (P(o-tol)3, 2.1 mg, 7.0 μmol), KOAc (34 mg, 0.35 mmol), LiCl (6.0 mg, 0.14 mmol), and DMF/toluene (4 mL, 1:1 v/v), and the solution was refluxed overnight. The solvent was evaporated under reduced pressure, and the resulting mixture was dissolved in DCM and extracted with a saturated solution of NaHCO3 (20 mL × 2). The combined organic layer was washed with deionized water (50 mL × 3) and concentrated, and the product was purified by silica gel column chromatography using DCM/hexane (50:50 v/v) as the eluent affording a green solid (45 mg, 90%). 1H-NMR (600 MHz, CDCl3, ppm): δ 7.98–7.93 (brm, 4H,), 7.86–7.75 (m, 4H,), 7.62–7.55 (m, 6H,) 7.45–7.42 (m, 6H,), 7.31 (m, 4H,), 7.12–7.07 (m, 4H,), 1.35 (s, 18H,). 13C-NMR (150 MHz, CD2Cl2, ppm): δ 151.8, 133.6, 133.3, 131.4, 130.6, 130.3, 129.3, 129.0, 128.6, 128.4, 128.3, 128.1, 127.2, 126.7, 126.4, 126.3, 125.5, 125.3, 125.1, 121.2, 120.4, 34.6, 31.4, 31.2. EI-HRMS: m/z calculated for M•+ C58H46 742.3600 found 742.3674.

Synthesis of 1,1′,2,2′-Tetrakis(4-(tert-butyl)phenyl)-6,6′-biaceanthrylene CPB1

CPB1 was prepared following procedure A with 10,10′-dibromo-9,9′-bianthracene DBBA (51 mg, 0.1 mmol, 1 equiv), TPE (63 mg, 0.22 mmol, 2.2 equiv), Pd2(dba)3 (9.0 mg, 10 μmol, 0.1 equiv), P(o-tol)3 (3.0 mg, 10 μmol), KOAc (49 mg, 0.5 mmol), and LiCl (8.5 mg, 0.2 mmol). Brick red solid (80 mg, 87%). 1H-NMR (600 MHz, CDCl3, ppm): δ 7.88 (d, 2H, J = 8.5 Hz,), 7.85 (d, 2H, J = 6.3 Hz,), 7.54 (d, 4H, J = 8.6 Hz,), 7.50 (d, 4H, J = 8.6 Hz,), 7.37 (d, 6H, J = 8.6 Hz,), 7.34 (m, 6H,), 7.22 (d, 4H, J = 8.6 Hz,), 7.07 (t, 2H, J = 5.7 Hz,), 1.44 (s, 18H, –), 1.33 (s, 18H,), 13C-NMR (150 MHz, CDCl3, ppm): δ 149.4, 140.3, 136.0, 135.0, 133.9, 132.3, 130.4, 129.9, 129.4, 128.0, 127.9, 127.5, 126.9, 126.0, 125.3, 125.0, 124.6, 122.6, 34.5, 31.6; EI-HRMS: m/z calculated for M•+ C72H66 930.5159 found 930.5156.

Synthesis of 1,1′-Bis(4-(tert-butyl)phenyl)-2,2′-di(naphthalen-1-yl)-6,6′-biaceanthrylene CPB2

CPB2 was prepared following procedure A with 10,10′-dibromo-9,9′-bianthracene DBBA (36 mg, 0.07 mmol, 1 equiv), TPEN (44 mg, 0.15 mmol, 2.2 equiv), Pd2(dba)3 (6.4 mg, 7.0 μmol, 0.1 equiv), P(o-tol)3 (2.1 mg, 7.0 μmol), KOAc (34 mg, 0.35 mmol), and LiCl (5.9 mg, 0.14 mmol). Brick red solid (55 mg, 86%). 1H-NMR (600 MHz, CDCl3, ppm): δ 8.14–8.12 (m, 2H,), 8.01 (d, 2H, J = 8.6 Hz,), 7.96–7.93 (m, 1H,), 7.88–7.81 (m, 2H,), 7.75 (m, 1H,), 7.67 (m, 1H,), 7.62 (m, 1H,), 7.58–7.57 (m, 3H,), 7.52 (brq, 2H,), 7.46–7.36 (m, 8H,), 7.31–7.28 (m, 6H,), 7.22 (d, 2H, J = 6.9 Hz,), 7.19 (m, 2H,), 7.10 (brs, 1H, ArH), 7.01–6.92 (m, 2H,), 1.26 (s, 18H,), 13C-NMR (150 MHz, CDCl3, ppm): δ 149.6, 140.1, 136.0, 138.3, 136.1, 133.7, 132.2, 131.4, 130.6, 130.3, 129.5, 129.07, 128.4, 128.3, 128.1, 127.5, 127.1, 127.0, 126.5, 126.1, 125.9, 125.5, 125.1, 124.9, 124.2, 34.5, 31.4; EI-HRMS: m/z calculated for M•+ C72H54 918.4226 found 918.4256.

Acknowledgments

The project was partially supported by Kuwait Foundation for the Advancement of Sciences (KFAS) under project code PN17-34SC-01.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07168.

  • Experimental procedure, characterization of all compounds, and copies of NMR, EI-HRMS, FT-IR, UV–Vis absorption and Emission spectra and AIE studies (PDF)

The authors declare no competing financial interest.

Notes

The raw data required to reproduce these findings are available upon request.

Supplementary Material

ao2c07168_si_001.pdf (2.6MB, pdf)

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