Facile single-step synthesis of pentaaryl-substituted pyrano[3,2-b]pyrrol-5(1H)-ones showing aggregation-induced emission

Abstract

The continuing desire for multifunctional AIE materials has stimulated intensified research efforts to develop both innovative synthetic techniques and novel scaffolds. Herein, we report a new class of heterocyclic AIEgens, 1,2,3,6,7-pentaaryl substituted pyrano[3,2-b]pyrrol-5(1H)-ones, which contain an intriguing fused pyrrole/α-pyrone core. These structurally novel molecules were efficiently constructed via multicomponent reactions of easily accessible isatins, 2,3-diaryl cyclopropenones, and alkyl bromides in a single step under metal-free conditions. Subsequent late-stage modification of the resulting products was performed via the Suzuki–Miyaura coupling and reduction reactions. Furthermore, the HOMO/LUMO gaps were calculated and the single crystal structural analyses were conducted in order to gain deeper insights into the photophysical properties of these propeller-like AIEgens.

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Introduction

Aggregation-induced emission luminogens (AIEgens) represent a distinctive class of compounds that exhibit strong luminescence in both the aggregated and solid states.^1,2 These molecules overcome the inherent limitation of traditional aggregation-caused quenching (ACQ) materials, which often display non-luminescence or low emission efficiency in their aggregated states.³ The fascinating AIE phenomenon has garnered substantial interest from the scientific community, prompting numerous research groups to pursue the development of novel AIEgens and investigate their potential applications in analytical chemistry,⁴ materials science,⁵ and life sciences.⁶ Typically, a propeller-like configuration is considered the optimal structural design for compounds endowed with favorable AIE properties. This configuration effectively precludes the phenomenon of fluorescence quenching attributed to π–π stacking in the aggregated state.⁷ Thanks to the dedicated efforts of numerous scientists, an array of interesting AIEgens, such as tetraphenylethene (TPE),⁸ 9,10-distyrylanthracene (DSA),⁹ and cyanostilbene,¹⁰ have been successfully developed in accordance with the generally acknowledged mechanism of restricted intramolecular motion (RIM). Nevertheless, it is essential to recognize that some of these known AIEgens are more or less associated with intrinsic disadvantages. These include the complexity of preparation and purification, difficulties in structural modification, and instability under certain conditions, which restrict the potential applications of these compounds.¹¹ The continuing desire for multifunctional AIE materials has stimulated intensified research efforts to develop both innovative synthetic techniques and novel scaffolds.¹²

In comparison with pure hydrocarbon systems, heterocyclic AIEgens exhibit a considerable degree of structural diversity, rendering them comparatively simple to prepare and modify.¹³ Heteroatoms can influence the properties and aggregation structures of materials through electronic effects.¹⁴ The incorporation of heteroatoms into AIEgens significantly enriches the variety of AIE systems and expands the scope of AIE research. Pyrrole represents a fundamental five-membered nitrogen-containing heterocycle, featuring abundant electron density and pronounced aromaticity.¹⁵ Dong demonstrated that pentaphenyl pyrrole (PPP) shows an aggregation-induced emission enhancement phenomenon. This is attributed to the more twisted configuration which prevents parallel orientation of conjugated chromophores combined with the restricted intramolecular motion effect.¹⁶ By enlarging the conjugation and coplanarity of pyrrole to a pyrrolo[3,2-b]pyrrole core,¹⁷ it was also found that their polyaryl substituted compounds showed obvious AIE characteristics with versatile properties and applications in chloroform detection and acid response. Furthermore, α-pyrone is an important class of six-membered lactones, and Yasuda and Tani have disclosed that tetraaryl-substituted α-pyrone^18a and a fully ring-fused benzopyrano[3,2-b]pyran-2,6-dione (PPD)^18b exhibit AIE properties, respectively (Scheme 1a). Nevertheless, a fused pyrrole/α-pyrone core with polyaryl substituents has not yet been synthesized and investigated, and it is anticipated that this novel hybridized skeleton will be an intriguing candidate for AIEgens and might possess potential applications in diverse fields. As a continuation of our research efforts to develop new synthetic methods for constructing valuable heterocyclic skeletons,¹⁹ herein, we report a new class of propeller-like AIEgens, 1,2,3,6,7-pentaaryl substituted pyrano[3,2-b]pyrrol-5(1H)-ones, which could be readily synthesized from isatins, 2,3-diaryl cyclopropenones, and alkyl bromides through a single-step procedure under metal-free conditions with satisfactory yields (Scheme 1b). It is noteworthy that the bromo-substituted products can be readily functionalized via the Suzuki–Miyaura coupling reactions with aromatic boronic acids. The photoluminescence (PL) properties of these compounds were initially characterized, and then their AIE properties were elucidated through theoretical calculations and single-crystal analysis.

Scheme 1 Heterocyclic AIEgens with a pyrrole/α-pyrone core.

Results and discussion

The multicomponent reaction of isatins (1), 2,3-diaryl cyclopropenones (2), and alkyl bromides (3) was conducted in the presence of DBU in ⁱPrOH at 100 °C, which proved highly compatible and was demonstrated to be a versatile method for creating a diverse array of pyrano[3,2-b]pyrrol-5(1H)-ones (4–6), as illustrated in Scheme 2. Some representative primary bromoalkanes (–Et, –ⁿBu, –ⁿC₁₂H₂₅, –ⁿC₁₆H₃₃, methylcyclohexyl, and –Bn) and secondary bromoalkanes (–ⁱPr and cyclopentyl) gave the desired products in excellent yields (4a–4h, 78–92%). 4b was also obtained in good yields from 1-chlorobutane and 1-iodobutane. However, only a trace amount of the expected product 4i was observed using tert-butyl bromide as the substrate. Allyl bromide and bromo acetonitrile were also suitable for this reaction, furnishing 4j and 4k in 68% and 65% yields, respectively. Additionally, utilizing 1,2-dichloroethane as a substrate resulted in the formation of 4l in 61% yield. The substrate scope of isatins was subsequently investigated. Isatins bearing electron-donating groups (–Me, –ⁱPr, and –OMe) and electron-deficient groups (–F, –Cl, –Br, –OCF₃, –NO₂, and –CHO) all delivered the corresponding products in moderate to good yields (5a–5j, 67–84%), and several di-substituted isatins were also found to be compatible with the reaction (5k–5n, 49–80%). Several 2,3-diaryl cyclopropenones bearing different substituents (–Me, –^tBu, –F, –Cl, –Br) on the phenyl rings were tested to further extend the substrate scope. Fortunately, the reaction proceeded successfully and produced the desired products in satisfactory yields (6a–6e, 51–85%). 2,3-Di(thiophen-2-yl)cycloprop-2-en-1-one was also compatible for the conversion, furnishing 6f in 83% yield. Unsymmetrical 2-phenyl-3-(p-tolyl)cycloprop-2-en-1-one produced a complex mixture of inseparable regioisomers. All the synthesized compounds were characterized by ¹H/¹³C NMR and IR spectroscopy, and HRMS analysis.

Scheme 2 Substrate scope of pentaaryl pyrano[3,2-b]pyrrol-5(1H)-ones. Unless otherwise specified, X = Br for 3. Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), 3 (0.5 mmol), DBU (0.4 mmol) in ⁱPrOH (3 mL) at 100 °C in a sealed tube for 3 h; isolated yields. ^a1,2-Dichloroethane as a substrate.

Given that the incorporation of electron-donating amino groups into a luminogen typically results in electronic perturbation and modulation of its optoelectronic properties, further transformations of the obtained products were conducted. For example, the Suzuki–Miyaura coupling reactions²⁰ of the bromo-substituted product 5f with two different aromatic boronic acids afforded its derivatives 7 and 8 in excellent yields (Scheme 3a). Compound 9 was easily synthesized through a reduction reaction²¹ of 5h using a SnCl₂/HCl system (Scheme 3b). The synthesis of compound 10 commenced with the coupling reaction of 6e with four molecules of 4-(diphenylamino)phenyl boronic acid (Scheme 3c). It is worth noting that the reaction of isatin (1a) with 2,3-diphenyl cyclopropenone (2a) delivered 2-(5-oxo-2,3,6,7-tetraphenylpyrano[3,2-b]pyrrol-1(5H)-yl)benzoic acid (11) in 56% yield in the absence of alkyl halides, which could be converted into 1,2,3,6,7-pentaphenylpyrano[3,2-b]pyrrol-5(1H)-one (12) in 73% yield via a copper-catalyzed decarboxylation reaction (Scheme 3d). Furthermore, a scale-up experiment was conducted to provide further evidence of the synthetic viability and prospective applications of our methodology (Scheme 3e).

Scheme 3 Structural modification of 1,2,3,6,7-pentaaryl pyrano[3,2-b]pyrrol-5(1H)-ones and gram-scale experiment.

Based on the experimental results (see Scheme S1 in the ESI†) and previous reports,²² a possible mechanism for this novel multicomponent reaction to assemble pyrano[3,2-b]pyrrol-5(1H)-ones is depicted in Scheme 4. Firstly, the 1,4-addition of isatin (1a) to 2,3-diphenyl cyclopropenone (2a) in the presence of DBU affords intermediate A, which then progresses to give intermediate Bvia a ring-opening and intramolecular cyclization process.^22a–c Subsequently, B is subjected to a nucleophilic attack by H₂O, followed by an intramolecular rearrangement to produce intermediate D. Then, a 1,4-addition reaction between intermediate D and another molecule of 2,3-diphenyl cyclopropenone (2a) gives rise to intermediate E, which undergoes a ring-opening process to generate intermediate G. Further intramolecular electrocyclization of G furnishes intermediate 11.^22d Finally, the DBU-mediated nucleophilic substitution reaction of 11 with 1-bromobutane (3b) furnishes the product 4b as desired.

Scheme 4 Possible reaction mechanism.

To demonstrate the prospective applications of the synthesized products as optical materials, the photophysical properties of the selected pyrano[3,2-b]pyrrol-5(1H)-one derivatives 4b, 6c, 7, 9 and 10 were investigated and compared with those of previously documented AIEgens PPP and TPPP-C1.^16b,17b The normalized UV-vis absorption and photoluminescence (PL) emission spectra in THF solutions (10⁻⁵ M) are shown in Fig. 1, and the detailed data are presented in Table 1. Broad absorption bands of four pyrano[3,2-b]pyrrol-5(1H)-one derivatives 4b, 6c, 7 and 9 were observed between 361 and 378 nm with molar extinction coefficients of 23 440–95 407 M⁻¹ cm⁻¹. Compound 10 showed two distinct absorption bands, and the higher peak at 343 nm was identified as π–π* transition of the conjugated backbone, while the additional weaker shoulder peak at 396 nm should be attributed to intramolecular charge transfer (ICT) transition.²³ The emission spectra of these compounds all showed peaks at 448–500 nm with different Stokes shifts ranging from 4716 to 5379 cm⁻¹. Among them, compound 10 displays the longest emission wavelength with the highest fluorescence quantum yield up to 52.4%. Enlarging the conjugated system and introducing D–A structures by incorporating four triphenylamine groups have red-shifted both its maximum absorption and emission. The calculated HOMO and LUMO energy levels of these selected compounds are also summarized in Table 1. The results reveal that their HOMO–LUMO gaps range from 3.12 to 3.64 eV.

Fig. 1 The normalized absorption (a) and emission (b) spectra of representative aryl-substituted pyran[3,2-b]pyrrole derivatives in THF (10⁻⁵ M).

Table 1 Photophysical properties and HOMO–LUMO energies of the selected pentaaryl pyrano[3,2-b]pyrrol-5(1H)-ones

Compounds	λ abs ^a nm	λ em ^a nm	Stokes shift cm^−1 (nm)	Molar abs coeff. εmax M^−1·cm^−1	Φ F/THF ^b %	HOMO ^c eV	LUMO ^c eV	E g eV
a In THF solution at RT (10^−5 mol L^−1). b Quinine sulfate (Φ = 0.55) used as a standard. c DFT calculations were performed at the B3LYP/6-31G(d) level using the Gaussian 16 package.
4b	378	460	4716 (82)	34 639	0.93	−5.17	−1.56	3.61
6c	374	460	4999 (86)	23 440	3.14	−5.39	−1.75	3.64
7	361	448	5379 (87)	95 407	1.52	−5.13	−1.56	3.57
9	377	462	4880 (85)	38 426	1.38	−5.06	−1.42	3.64
10	343, 396	500	5253 (104)	83 130	52.4	−4.72	−1.60	3.12
PPP	306	386	6773 (80)	—	2.2	—	—	4.45
TPPP-C1	321, 372	505	7078 (133)	—	1.11	−5.18	−1.80	3.38

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Upon examination of the structures of these newly synthesized compounds, it becomes evident that five rotatable phenyl rings are linked to the central stator, a fused pyrrole/α-pyridone core, through C–C and C–N single bonds. As postulated by the RIM mechanism,²⁴ these molecules are expected to show an AIE effect. To substantiate this hypothesis, the photoluminescence (PL) behaviors were investigated in mixed solvents of THF/H₂O with different water fractions (f_w). For compound 4b, a weak emission peak at 460 nm was observed in a dilute THF solution (Fig. 2a). The fluorescence intensity of the THF/H₂O mixture was enhanced in conjunction with an increase in the water fraction (f_w). Additionally, a red-shifted as well as 6-fold emission enhancement was detected when f_w increased to 90%. Nevertheless, the fluorescence intensity exhibited a decline as f_w reached 99%. This phenomenon could be attributed to the tendency of luminogen molecules to rapidly agglomerate in a random manner, leading to the formation of less emissive particles.²⁵ The Tyndall effect of 4b in THF/H₂O illustrates the formation of aggregates (Fig. 2b). The quantum yield of 4b in THF is 0.93%. Other selected pyrano[3,2-b]pyrrol-5(1H)-one derivatives also behaved similarly (Fig. S3†). These results clearly demonstrate that the fluorescence emissions of the selected compounds are enhanced with the formation of aggregates and they exhibit AIE properties.

Fig. 2 (a) Fluorescence spectra of 4b in THF/H₂O mixtures with different water fractions (inset: the visible fluorescence under 365 nm UV light irradiation of f_w = 90%). (b) Plot of the wavelength and ratio of maximum fluorescence intensity of 4bvs. the water fraction (inset: the Tyndall effect of f_w = 90%). I₀ = emission intensity in pure THF solution, λ_ex = 375 nm, concentration: 10⁻⁵ mol L⁻¹.

To gain deeper insights into the relationship between the molecular structure and the photophysical properties of these selected aryl-substituted pyrano[3,2-b]pyrrol-5(1H)-one derivatives, density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d) level using the Gaussian 16 package (Fig. 3). The highest occupied molecular orbitals (HOMOs) of three AIE-active compounds 4b, 6c, and 7 were found to be localized on the central pyrano[3,2-b]pyrrol-5(1H)-one core and the adjacent phenyl groups at the 2,3,6-positions, while their lowest unoccupied molecular orbitals (LUMOs) were primarily distributed on N-substituted phenyl rings which contain an ester group. Apparently, the electron cloud distributions in HOMOs and LUMOs were spatially separated. A comparable distribution of the HOMO–LUMO and energy gap of compound 9 resulted in a similar absorption band at 377 nm. The HOMO of 10 was mainly localized at the benzene rings and the conjugated triphenylamine moiety at the 3,6-positions. This exhibited a higher LUMO energy level but a lower energy gap in comparison with other compounds.

Fig. 3 Molecular orbitals and energy levels of the selected aryl-substituted pyran[3,2-b]pyrroles 4b, 6c, 7, 9 and 10.

The accurate crystal structures provided the most direct evidence for elucidating the emission behaviors of these AIEgens. Single crystals of 4b (CCDC 2327902†) and 6c (CCDC 2298102†) were obtained by the slow solvent evaporation of a DCM–EtOAc–hexane mixture. Their structures were subsequently elucidated through X-ray single-crystal diffraction (Fig. 4). In the crystal, 4b exhibits a markedly twisted conformation, and the dihedral angles between its central pyrano[3,2-b]pyrrol-5(1H)-one core and the attached phenyl rings were 73.3°, 51.3°, 44.8°, 50.7°, and 68.6°, respectively (Fig. 4a). It showed a layer-by-layer staggered arrangement stacking pattern, and the distance between adjacent pyrano[3,2-b]pyrrol-5(1H)-one cores was 5.992 Å, which was larger than the distance of π–π interaction (3.6 Å) (Fig. 4b). The crystal structure of 6c also adopts a propeller-like conformation, displaying similarities in dihedral angles and stacking patterns (Fig. 4d and e). In addition, multiple inter- and intramolecular C–H⋯π and C–H⋯O interactions were identified with distances in the range of 2.356 Å–2.803 Å for 4b and 2.379 Å–2.985 Å for 6c in their single crystals (Fig. 4c and f). Such non-covalent interactions constrained the rotational motion of phenyl rings in the aggregated state, which blocked the non-radiative channel and boosted the radiative pathway, thereby enabling them to exhibit significant AIE characteristics.

Fig. 4 The crystal structures, packing patterns, and inter- and intramolecular interactions of 4b (a, b, and c) and 6c (d, e, and f).

Conclusions

In summary, a novel class of heterocyclic AIEgens, 1,2,3,6,7-pentaaryl substituted pyrano[3,2-b]pyrrol-5(1H)-ones, has been developed. These new compounds could be readily synthesized from isatins, 2,3-diaryl cyclopropenones and alkyl bromides through a single step process under metal-free conditions, with favorable yields. Subsequent functionalization of the resulting products was conducted via the Suzuki–Miyaura coupling and reduction reactions, allowing for the construction of their derivatives 7–10. The photophysical properties of representative pyran[3,2-b]pyrrole derivatives 4b, 6c, 7, 9 and 10 were investigated using UV-vis absorption spectra, PL emission spectra, DFT calculations, and analyses of single crystal structures. A spatial separation of the electron cloud distributions in HOMOs and LUMOs was observed for the selected compounds, with energy gaps ranging from 3.12 to 3.64 eV. In the crystal structures, 4b and 6c adopted a propeller-like conformation that lacks intermolecular π–π interactions but was capable of forming multiple inter- and intramolecular C–H⋯π and C–H⋯O interactions. This behavior was consistent with the typical AIE characteristics associated with the RIM mechanism. These results not only facilitate the molecular design of substituted pyrano[3,2-b]pyrrol-5(1H)-ones but also enrich the family of heterocyclic AIEgens. Efforts toward developing new multifunctional AIEgens are currently under investigation in our laboratory.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the Natural Science Foundation of Hubei Province (No. 2023AFB477) and the Science and Technology Research Project of Education Department of Hubei Province (No. B2022164) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2298102 and 2327902. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qo01600a

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