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Synlett 2021; 32(19): 1879-1890
DOI: 10.1055/a-1616-5643
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Imide-Functionalized Helical PAHs: A Step towards New Chiral Functional Materials

Fridolin Saal
,
Prince Ravat  ∗

This project has received funding from the Julius-Maximilians-Universität Würzburg within the “Excellent Ideas” program.
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Dedicated to Professor Klaus Müllen

Abstract

Attachment of cyclic imide groups to polycyclic aromatic hydrocarbons (PAHs) leads to fascinating electronic and luminescence properties, with rylene diimides being a representative example. The close to unity fluorescence quantum yields and electron-acceptor properties render them suitable for application in organic electronics and photovoltaics. Recent reports show that, in line with planar PAHs, the imide functionalization has also endowed helical three-dimensional PAHs with similar beneficial photophysical properties. In this article, we have summarized the state-of-the-art research developments in the field of helicene–imide hybrid functional molecules, with a particular focus on synthesis, (chir)optical and redox properties, and applications in electronics. Additionally, we have highlighted our recent work, introducing a novel family of functional chiral molecules, namely, [n]helicene diimides, as three-dimensional relatives of rylene diimides.


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Key words

chirality - helicenes - [n]helicene diimides - rylene diimides - polycyclic aromatic hydrocarbons - functional organic materials
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Fridolin Saal(born in Schweinfurt, Germany in 1994) obtained his M. Sc. degree from the University of Würzburg in 2019 and is currently working on his PhD degree with Prince Ravat. His research interests include synthesis of [n]helicene diimides and investigation of their (chir)optical and redox properties. Prince Ravat (born in Vadodara, India in 1986) obtained his PhD degree at the Max-Planck-Institute for Polymer Research, Mainz, Germany. Since 2018 he has been a research group leader at the University of Würzburg, where he is working on his habilitation with the research focus on molecular engineering of functional chiral molecules for applications in organic electronics.

The phenomenon of chirality has long fascinated chemists, starting with Biot’s detection of optical rotation[1] and Pasteur’s discovery of the enantiomers of tartaric acid[2] in the 19th century. In the years since, the essential role of chiral compounds in biology has been studied in great detail, and chemists have devised ever more sophisticated methods for their selective synthesis. The resulting chiral molecules are used with great success in pharmaceutical and other biochemistry-related fields.[3] Application of chirality in technological applications, however, has proved to be challenging despite some early advances; for example, the first observed liquid-crystalline phase consisted of the chiral molecule cholesterol,[4] yet modern applications for liquid crystals rarely contain chiral compounds. It is only in recent years that the potential of chirality in technological applications has moved more into the spotlight,[5] since chirality endows such molecules with unique capabilities such as selective absorption and emission of circularly polarized (CP) light,[6] [7] [8] and spin-selective charge transport.[9]

One of the best-studied classes of chiral molecules is the helicenes,[10] [11] [12] [13] [14] which consist of several ortho-fused benzene rings that form a helical geometry due to their inherent steric repulsion. This geometry allows for pronounced chirality effects, such as high values of optical rotation (OR), circular dichroism (CD), and CP luminescence (CPL).[15] This has led to increasing focus on helical chiral organic molecules for applications such as chiroptical switches,[16] chirality-dependent organization, organic light-emitting diodes (OLEDs) emitting circularly polarized light (spin-LEDs),[17] CP-responsive organic field-effect transistors[18] [19] [20] (OFETs), and in stereoselective synthesis,[21] which have been detailed in multiple reviews. Still, their chiroptical properties leave much to be desired. In particular, their fluorescence quantum yield[22] is typically low due to competitive intersystem crossing pathways[23] and poor charge carrier mobility caused by a lack of effective molecular packing. Furthermore, unsubstituted helicenes often absorb light only in the ultraviolet region of the electromagnetic spectrum, necessitating suitable functional groups to induce a bathochromic shift.[24] Along with various functional groups, incorporation of either five- or six-membered cyclic imide moieties to carbo- and hetero[n]helicenes has been proved to result in favorable properties such as n-type redox properties and a bathochromic shift in absorbance and fluorescence with high fluorescence quantum yields.[25] These effects are due to the strong electron-withdrawing nature and π-conjugation of the imide groups resulting in a considerably lowered LUMO energy.[26]

Not unlike chiral compounds, dye molecules have been intensely studied since the 19th century, and they can be seen as the beginning of modern industrial chemistry. In particular, dyes containing an aromatic imide moiety have been in the spotlight of researchers since the beginning of the 20th century.[27] One of the most promising classes of functional molecules is the rylene dyes, which consist of a variable number of connected naphthalene units capped on each end by an imide group.[28] [29] [30] [31] [32] These molecules are notable for their outstanding capabilities concerning light absorbance and fluorescence as well as their captivating supramolecular organization[28,29] and charge-carrier mobilities.[33] These properties render them suitable for a multitude of applications, ranging from simple colorants as dyes and pigments to electronic devices such as OFETs,[33] OLEDs,[34] [35] and photovoltaics.[36] While planar PAHs capped with five- or six-membered imide moieties have been researched for over a century and are well established in science and technology, the development of three-dimensional PAHs bearing imide moieties is still at an early stage. Inspired by the rylene diimides, recently, an increasing number of imide-containing helicene derivatives have been synthesized to develop functional chiral molecules possessing excellent chiroptical and electronic properties.

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Figure 1 Maleimide-functionalized helicenes

In this article, we provide an overview of the emerging class of helicene–imide hybrid functional molecules, concerning their synthesis, properties, and applications with a special focus on chiroptical properties and applications in organic electronics. While several imide-containing helicenes have been synthesized in the last few years, the nature and extent of measurements performed in their respective publications vary widely. While for some compounds only the synthesis has been published, others have been subjected to extensive chiroptical measurements or used as substrates for device production. We have tried to include the most relevant conclusions for all included compounds while keeping in mind that a direct comparison is often difficult due to a lack of data for many molecules.

The published helical PAHs decorated with imide moieties broadly fall into six categories that can be divided into: (1) Helicenes containing maleimide groups—carbo[n]helicenes and their derivatives substituted with five-membered imide rings at various positions. (2) Rylene diimides with helical π-systems—a straightforward approach in which the aromatic system of a rylene dye is extended to create a helical geometry. (3) Imide-functionalized hetero[n]helicenes structurally similar to (2), but containing heteroatoms in the helicene moiety. (4) Perylene diimides (PDIs) bridged by helicenes, where multiple PDIs are connected by helicene bridges in their bay positions. (5) Propeller-shaped PDI-helicene-hybrids, where multiple PDIs are arranged around a central ring in a manner reminiscent of petals of a flower. (6) [n]Helicene diimides, where a helicene is capped with six-membered imide groups at both ends.

Helicenes Containing Maleimide Groups (Figure [1])

The first strides towards imide-functionalized helicenes were made in 1997 when compounds A1 and A2 were prepared by constructing a tetrahydro[5]helicene backbone from α-tetralone and maleimide by oxidative coupling and a subsequent Diels–Alder reaction, which was selectively functionalized and oxidized to the [5]helicene with elemental bromine.[37] [38] Both compounds were used in polymerization experiments and the supramolecular interactions of the resulting macromolecules were studied. Although a CD spectrum of the polymer of A1 with a chiral residue on the imide group was recorded, the diastereomers of the compound were not separated and the main focus of the study was not on optical or electronic effects.

After a pause of several years, similar compounds were investigated again in 2014 by Durola and co-workers, who prepared A3 and A4 from 2,7-di-tert-butylpyrene and chrysene, respectively, by Friedel–Crafts acylation followed by reduction, Perkin condensation, and Mallory photo-cyclodehydrogenation (Scheme [1]).[39] The compounds showed great stability as well as good absorbance and fluorescence properties; however, no chiroptical measurements were reported (Table [1]). In cyclic voltammetry (CV) measurements, reversible reductions were observed at –1.67 V and –1.62 V relative to Fc/Fc+, respectively, for A3 and A4, while only A3 showed reversible oxidation at 0.85 V.

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Scheme 1 Representative example for the synthesis of maleimide-functionalized helicenes via the glyoxylic Perkin reaction.[39]

In 2016, Hasobe and co-workers[25] synthesized related molecules (A5 and A6) from known dimethoxycarbonyl helicenes[40] [41] by simply hydrolyzing the ester followed by dehydration and imidization with the corresponding amino compound. Remarkably, this synthetic procedure does not necessitate a photocyclodehydrogenation step, which is often the limiting factor for batch sizes in the synthesis of helicenes.[40] The enantiomers of A5 and A6 were resolved by HPLC employing chiral stationary phase (CSP) columns (hereon HPLC-CSP) and subjected to extensive spectroscopic and chiroptical studies. The compounds showed high fluo­rescence quantum yields of 0.37 and 0.22, respectively, and fluorescence lifetimes of 9.8 and 8.0 nanoseconds. The absorption dissymmetry factors (g abs) were typical for helicenes, with g abs values of 4.8 × 10–3 and 5.7 × 10–3 for A5 and A6, respectively. The paper is notable for the performance of the first measurement of circularly polarized luminescence (CPL) on a carbo[5]helicene derivative.[25] The emission dissymmetry factor (g lum) values were determined to be 2.4 × 10–3 and 2.3 × 10–3, respectively (Table [1]). Several helicenes with two maleimide units (A7–A11) are also known from the Zhao, Haino and Durola groups.[42] [43] These were all prepared by the same synthetic route involving Perkin reactions to assemble diaryl maleic anhydride precursors that were reacted to imides with the corresponding amines and cyclized by either oxidative photocyclization or intramolecular Heck reaction. Compounds A7–A9, consisting of a single [5]helicene core (A7, A8) or connected double [4]helicenes (A9) showed fluorescence quantum yields of 0.10 to 0.15 (Table [1]). For all molecules (A7–A9), two reversible reduction waves could be observed in CV measurements. Their LUMO energies were in the range of –3.54 to –3.41 eV. Attempts were made to investigate the semiconducting properties of these molecules by assembling them into an OFET device; however, these attempts were unsuccessful due to the low melting point of the compounds. The chiroptical properties were not investigated.

Compounds A7a–A7c were separately studied by Haino and co-workers with regards to the influence of the differing imide substituents on crystal packing and optical properties.[43] While absorbance and fluorescence were largely unaffected by the different substituents, the influence on crystal structure was considerable. The authors were able to demonstrate spontaneous chiral resolution by crystallization for compound A7a, a rare phenomenon in helicenes. The enantiomers of A7a showed moderate circular dichroism, while the values for the other compounds and the fluorescence quantum yields were not determined.

Maleimide-functionalized helicenes are not limited to single helicenes. Durola and co-workers reported a family of double helicene maleimides consisting of A10 and A11.[44] Those molecules are notable for their synthesis from chrysene via the key steps of Perkin reaction and oxidative photocyclodehydrogenation. Both compounds showed two reversible reductions in CV measurements, with potentials of –1.57 V and –1.67 V for A10 and –1.37 V and –1.64 V for A11, indicating a greater interaction between the imide redox centers in the latter molecule, which can be explained by the shorter through-bond distance between two functional groups. Due to the lack of sterically stabilizing substituents, the compounds possess a low isomerization barrier, which leads to a dynamic shift between the diastereomers that is dependent on the solvent. However, this dynamic nature of the chiral centers renders the compounds less attractive for applications relying on their chiroptical properties.

Table 1 Compilation of Optical Properties of Selected Compoundsa

Compound

ε max (M–1 cm–1) at λ (nm)

|Δε| (M–1cm–1) at λ(nm)

g abs at λ (nm)

g lum at λ (nm)

φ FL

τ FL (ns)

[5]helicene[24] [45]

38 200/≈310

161/≈310

4.2 × 10–3

–

0.04

25.5

[6]helicene[24] [45]

28 300/≈320

259/≈325

9.2 × 10–3

–

0.04

14.5

[7]helicene[24] [45] [46]

11 700/≈355

242/≈350

2.0 × 10–2

6.0 × 10–3

0.02

13.8

A3

≈7000/500

≈28 000/380

–

–

–

–

–

A4

≈8000/450

≈28 000/300

–

–

–

–

–

A5

≈7500/450

≈28 000/340

≈15/450

≈55/315

4.8 × 10–3/456

2.4 × 10–3/490

0.37

9.8

A6

≈5000/415

≈32 500/325

≈10/460

≈50/360

5.7 × 10–3/475

2.3 × 10–3/525

0.22

8.0

B1

70 000/557

≈20/427

–

–

0.48

–

B2

51 000/313

90/350

2.5 × 10–3

–

–

–

B3

56 000/367

148/250

3.3 × 10–3

–

–

–

B4

56 000/386

88/280

36/380

2.2 × 10–3

–

–

–

B6

25 200/563

achiral

achiral

achiral

0.34

8.1

B7

25 000/557

460/382

7.6 × 10–3/403

5.0 × 10–4

0.31

7.8

B8

26 300/566

388/405

1.2 × 10–2/411

2.0 × 10–3

0.33

7.8

B9

23 400/556

achiral

achiral

achiral

0.29

7.4

C1

17 872/418

–

–

–

0.56

–

C2

27 296/454

–

–

–

0.14

–

C3a

67 402/700

≈30/495

≈55/310

–

–

0.025

–

C3b

80 258/702

≈

–

–

0.031

–

C4a

≈38 000/590

–

–

–

0.01

–

C4b

≈40 000/600

–

–

–

0.01

–

C5

≈90 000/310

–

1.4 × 10–2

–

–

–

D1

70 000/520

41/355

56/401

≈170/525

1.1 × 10–3/355

1.5 × 10–3/401

–

0.27

–

D3

50 000/490

134/344

214/396

6.8 × 10–3/396

–

0.41

–

D4

100 000/520

620/377

820/407

7.9 × 10–3/377

8.9 × 10–3/407

–

0.44

–

D5

≈187 000/390

350/419

≈3.0 × 10–3/430

–

–

–

D6

≈200 000/380

1760/420

≈1.4 × 10–2/430

–

–

–

E2

260 000/475

≈300/560

–

–

0.074

4.26

E3

230 000/430

≈200/560

–

–

0.12

6.36

E4

173 000/376

150 000/483 to 535

200/480

2.0 × 10–3/480

–

0.24

F1

20 570/418

34 090/288

104/325

6.69 × 10–3/327

–

0.22

3.21

F2

23 140/394

39 830/280

146/319

178/278

1.77 × 10–2/319

–

0.02

2.39

F3

25263/338

45 713/274

106/406

1.09 × 10–2/407

–

0.12

2.30

F4

≈38 000/450

–

–

–

0.47

7.64

a Values given with the approximately equal symbol (≈) have been estimated from charts or figures manually for sources in which no explicit data was given. [5]–[7]helicene have been included for comparison. A dash indicates that no experimental value could be obtained from the literature.

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Figure 2 Rylene diimides with a helical π-system

Rylene Diimides with Helical π-System (Figure [2])

Chirality in rylene imides has long been investigated from various perspectives. A popular approach is to introduce sterically demanding substituents in the bay positions of perylene diimides, leading to the formation of atropisomers. Examples of this approach have been demonstrated by the Müllen and Würthner groups.[47] [48] [49] The first strides towards a true helicene-PDI hybrid molecule were made by Müllen and co-workers in 2011 when they extended the aromatic core of terrylene diimide to include two [5]helicenes in the bay positions.[50] This was accomplished by bromination of terrylene diimide in the bay positions followed by fourfold palladium-catalyzed benzannulation with o-trimethylsilyl-phenyltriflate. The target compound (B1) was obtained as a mixture of (P,P), (M,M), and meso-isomers and all isomers proved to be configurationally stable despite the use of unsubstituted [5]helicene units, which normally rapidly racemize at room temperature.[10] Spectroscopic investigations showed that the favorable optical properties of the parent terrylene diimide were mostly retained, including an excellent fluorescence quantum yield of 0.48 and a high molar extinction coefficient of 70 000 M–1cm–1. The compound showed circular dichroism that was in line with theoretical calculations but of a relatively small magnitude as a large part of the chromophore ­retained a planar geometry.

A different approach to obtain rylene imides with a helical π-system consists of stitching helicenes and functionalized PDIs together in the bay position by Suzuki reaction and subsequent oxidative photocyclodehydrogenation. This approach was first shown by Hirsch and co-workers in 2020 to prepare compounds B2–B4.[51] Due to the utilization of [4] and [5]helicenes, the resulting molecules were configurationally unstable at room temperature. Nevertheless, the [5]helicene side of the molecule proved to be stable enough to perform circular dichroism measurements (Table [1]). All three compound B2–B4 showed g abs of 2.2 × 10–3 to 3.3 × 10–3, which are typical values for [5]helicene containing molecules.

Similar compounds B5–B9 were reported by Wang and co-workers,[52] differing mainly from B2–B4 in the number of annulated aromatic rings in the bay positions, using [8]helicenes in the place of the [4]- and [5]helicenes. The synthetic protocol used was identical to that of Hirsch and co-workers. Due to the higher inversion barrier of the [8]helicene substructure, B5–B9 are configurationally stable even at elevated temperatures of 200 °C. This enabled the isolation of all possible diastereomers of the double helicenes. Separation of the diastereomers was accomplished by standard column chromatography, and the enantiomers were resolved by HPLC-CSP. The absorption spectra showed characteristics of both helicenes and PDIs, although the PDI part of the spectrum was markedly redshifted by ca. 40 nm and the molar extinction coefficient notably decreased by a factor of four as compared to the parent PDI chromophore. All the double helicenes showed fluorescence quantum yields of ca. 0.30 along with fluorescence lifetimes of ca. 7–8 ns (Table [1]). Remarkably, the fluorescence spectra looked similar to PDI with a bathochromic shift only, indicating that the fluorescence mostly stems from the perylene diimide section of the molecule. Chiroptical investigations revealed Δε values of up to 460 M–1cm–1 and g abs values of up to 1.2 × 10–2. The g lum values were also measured, and reached values of up to 2.0 × 10–3 for B8. All double helicenes showed two reversible reductions and one oxidation peak in CV measurements, similar to the electrochemical behavior of the parent PDIs.

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Scheme 2 Examples of the synthesis of helical perylene diimide derivatives via Suzuki cross-coupling followed by photocyclodehydrogenation[51]

Imide-Functionalized Hetero[n]helicenes (Figure3)

There have been a few instances of imide-functionalized hetero[n]helicenes published in recent years. The first instance arose when the thiophene-annulated rylene imides C1 and C2 were prepared by Wang and co-workers by bromination in the bay regions followed by Stille coupling with stannylated thiophenes and Scholl reaction with ferric chloride.[53] Even though the products were not dubbed as such, they both contain multiple hetero[5]helicene structures. C1 and C2 showed fluorescence quantum yields of 0.56 and 0.16, respectively. The configurational stability of the compounds and their chiroptical properties were not investigated. In 2018, Jiang and co-workers published similar molecules C3a–b, which can be described as perylene diimides functionalized with four thiophene units in the bay regions, leading to a double hetero[5]helicene structure.[54] The compounds showed significant bathochromic shifts compared to the parent PDI chromophore, but only a very weak fluorescence with a fluorescence quantum yield of around 0.03. The enantiomers of C3a were resolved on HPLC-CSP and showed Δε values of ca. 30 M–1cm–1. The molecules showed more promise as materials for transistors as they possess clearly defined and reversible reduction and oxidation signals in the CV, and single-crystal organic field-effect transistors (SC-OFETs) made from C3a showed excellent electron transport abilities and on-off ratios. A range of very similar compounds was concurrently prepared by Zhang and co-workers, albeit without detailed studies on chiroptical or electronic properties.[55] Related compounds incorporating pyrrole rings are also known. The first such molecules (C4a–b) were prepared by Wang and co-workers in 2014 by reacting a selectively brominated and chlorinated PDI first with 3,5-dimethylaniline as a nucleophile and subsequent ring-forming palladium-catalyzed cross coupling.[56] These compounds can be described as imide-functionalized double hetero[7]helicenes. Notwithstanding their good absorption characteristics, the fluorescence quantum yields did not exceed 0.01. The compounds exhibited promising electrochemical behavior, including two reversible reductions and oxidations each. Despite being likely stable with regards to enantiomerization due to their [7]helicene structure, a separation of the enantiomers was not attempted.

A similar molecule based on perylene diimide functionalized with indoles in the bay regions (C5) was reported by Lin and co-workers in 2021 starting from a more readily available substrate; namely, a fourfold iodine-substituted PDI.[57] The thermal stability of the stereoisomers of such compounds was proven by separation of the two enantiomers on HPLC-CSP. This compound showed a remarkably large g absvalue of up to 1.4 × 10–2. An organic field-effect transistor (OFET) was fabricated from the material and it was shown that the compound possessed ambipolar charge-carrier mobility, rendering it suitable for both n- and p-type semiconducting applications. The OFETs produced from these molecules also proved to be effective at selectively detecting circularly polarized light.

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Figure 3 Hetero[n]helicenes based on π-extended rylene imides
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Figure 4 The family of helicene-bridged PDIs published by Nuckolls and co-workers.

PDIs Bridged by Helicenes (Figure [4])

Going a step further from simply substituting PDIs with carbo[n]helicenes in the bay positions, Nuckolls and co-workers published several papers on their helicene-PDI-hybrids (D1–D6) consisting of multiple PDI units bridged by helicenes of various lengths in the past few years.[58] [59] [60] [61] [62] Their compounds are some of the most promising chiral dye molecules to date. The synthetic routes of choice (Scheme [3]) were characterized by Suzuki–Miyaura cross couplings and oxidative photocyclodehydrogenation. While the simple molecules D1–D3 could be prepared in two steps in a straightforward manner with the only difference being the chosen doubly borylated linker was that the more complex compounds D4–D6 required multiple successive steps of cross-coupling and cyclization. D1 and D2 were initially investigated with respect to their electrochemical properties.[61] The smaller intramolecular distance between the imide groups in D1 led to strong through-space interaction between the imides acting as redox centers when subjected to electrochemical reduction in CV measurements. This led to four distinct reversible reductions in D1, while in D2 only two reductions could be observed since the interaction between the imide groups was too small to remove the degeneracy of the reduction potentials. Upon preparation of the larger compounds D3 and D4, which differ from D1 in bridging the PDI units with [7]helicene instead of [6]helicene and addition of a third PDI unit to the molecule, respectively, it became clear that their chiroptical properties were even more fascinating. Both the addition of one more aromatic ring in the helicene and addition of a third helicene-bridged PDI led to over proportional increases in Δε (820 M–1cm–1) and g abs (8.9 × 10–3) at 407 nm. These values could be improved by compounds D5 and D6 in which additional PDI units were annulated to the terminal PDIs, creating configurationally labile [4]helicenes in the process. These compounds were therefore dubbed ‘waggling helicene nanoribbons’ by the authors.[58] In particular, D6 delivered an outstanding Δε value of 1760 M–1cm–1 at 420 nm. Fascinatingly, the trend of an over proportional increase in Δε values from the PDI dimer to the trimer, as known from D1 and D4, remained intact even on a much higher level and the addition of a single helicene and PDI unit to D5, leading to D6, resulted in an almost tenfold increase in chiroptical response. This trend was recently attributed to a strong increase of magnetic transition dipole moment arising from an exciton-like coupling between the adjacent [6]helicene bridges in D4.[63] In combination, these properties make helicene-bridged PDIs some of the most promising compounds for chiroptical applications. Very recently, a structural analogue of D6 incorporating four [6]helicene substructures showing Δε value as high as 1920 M–1cm–1 was reported.[62]

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Scheme 3 Examples of synthetic pathways for helicene-bridged multiple PDIs published by Nuckolls and co-workers

Propeller-Shaped PDI-Helicene Hybrids (Figure [5])

Another recent development in the field of PDI-helicene hybrid molecules are propeller- and flower-shaped compounds, where several PDI units are symmetrically arranged around an aromatic core via bay region connections. The manner of connection between the parts of the molecules leads to formation of several five- (E1) and six-membered (E2) helicene-like structures. Compounds E1b and E1e were prepared by bromination of PDI in the bay region followed by Suzuki coupling and oxidative photocyclization.[64] The focus in that work lay on the suitability of the prepared molecule as a functional material for organic electronics, specifically organic photovoltaics. In such applications, the non-planarity of the molecules is a desirable trait due to inhibition of strong aggregation effects, which lead to excimer formation.[65] [66] Compound E1b showed great potential as an electron-accepting material and achieved a power conversion efficiency (PCE) of 8.28% when used as a substrate for a solution-processed bulk heterojunction organic solar cell, along with an open-circuit voltage of 1.0 V and a fill factor of 63.7%. At that time these values were among the highest reported for PDI-based organic solar cells and promising enough to warrant a multitude of further studies by Wang and co-workers. Derivatives of E1b substituted with swallow-tail alkyl chains of varying lengths (E1a–d) were investigated for their suitability in organic solar cells (OSCs).[67] Those compounds showed only small differences in their properties; however, there was a small trend towards higher PCEs with longer alkyl chains, with up to 8.64% in the largest molecule. In another study, E1b was used as an electron acceptor in an all-small-molecule OSC together with a thiophene-based donor compound to study the effect of thermal annealing treatment on the PV performance.[68] It was shown that with the thermal annealing process, the power conversion efficiency could be increased from 0.47 to 6.16%. The outstanding capabilities of E1b as an electron acceptor in OSCs were confirmed further in another study where the device based on the compound reached a PCE of 7.5% along with good absorption and external quantum efficiency over nearly the entire visible spectrum and a high open-circuit voltage of 1.14 V.[69] Investigation of the chiroptical properties of E1 was rendered difficult by problems with the separation of the enantiomers via HPLC-CSP. This problem was solved with the compound E1f and its P,P,M-diastereomer by integrating a pyrrole ring into one of the three PDI units to break the C 3 symmetry and introducing another alkyl chain to increase solubility.[70] These molecules were found to be configurationally stable and showed good absorption and emission dissymmetry factors (Table [1]). OSCs manufactured from the molecules showed PCEs of up to 8.11%.

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Figure 5 Propeller-shaped PDI-helicene hybrids and representative synthetic scheme

In the same way that three PDI units were arranged around a central benzene ring in E1, compounds E2 and E3 contain five PDIs around a corannulene core. These compounds were prepared in the same manner as E1 and were dubbed ‘Corannurylene Pentapetalae’ by the authors, on account of the flowery appearance of their structures. In contrast to E1, the larger number of PDI units arranged around the corannulene allows for multiple diastereomers including the D 5-symmetric propeller-shaped E2 as well as the C 2-symmetric E3. Two further diastereomers were predicted to be possible without prohibitive steric hindrance; however, only E2 and E3 were formed during the photocyclization step, in around equal quantities. E2 and E3 showed fluorescence quantum yields of 0.07 and 0.11, respectively. Both compounds could be resolved into their respective enantiomers via HPLC-CSP, and CD spectra were recorded for both. The CD values reached 300 and 200 M–1cm–1. Electrochemical measurements showed a good electron-accepting ability and promising properties for use of the compounds in organic electronics. Both were therefore trialed as electron acceptors in organic solar cells in a subsequent study.[71] The power conversion efficiencies reached 11.2% for E2 and 10.3% for E3, respectively, making them the most efficient PDI-based organic solar cells to date. The higher value for the D 5-symmetric E2 was attributed to a more favorable morphology caused by its increased symmetry compared to the C 2-symmetric E3. The progress in OSCs based on E1–3 as well as some other related compounds has been detailed in a recent account by Wang and co-workers.[72]

The similar compound E4—a ‘dual core’ flower-shaped molecule consisting of five PDI units arranged around two central benzene rings with one of the PDIs acting as a bridge—was prepared in 2019.[73] The synthesis was based on the same reactions as for E1, namely Suzuki cross-coupling of the bridge to the terminal PDIs with subsequent photocyclodehydrogenation. Out of the 28 possible stereoisomers, only one pair of enantiomers was formed. This mixture was separated into its constituent enantiomers by HPLC-CSP and chiroptical studies were performed. The compound shows strong absorbance over the entire visible spectrum and in the near UV as well as a high configurational stability. A fluorescence quantum yield of 0.24, Δε values of 200 M–1cm–1 and g abs of 2 × 10–3 could be achieved. These properties together with the high thermal stability make E4 a promising candidate for various applications and it provides a starting point for the synthesis of larger chiral nanographenes by extending the structure with more PDI units.

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Figure 6The helicene diimides prepared by Ravat and co-workers and Hu and co-workers and an example of a synthetic scheme

[n]Helicene Diimides (Figure [6])

Possibly the simplest helical analogue of a rylene imide was published in 2020 by our group.[26] We classified these compounds as [n]helicene diimides—the helical analogues of rylene dyes. They consist of simple carbo[n]helicenes capped with six-membered imide groups at both ends of the helix as well as methoxy groups in the fjord region. The synthesis was accomplished in a straightforward manner by reacting p-methoxyhomophthalimide with the corresponding aromatic dialdehyde in a Knoevenagel reaction and subjecting the resulting stilbene-related compound to oxidative photocyclization. It was shown that the terminal imide groups lead to intriguing electronic properties, while the methoxy groups provide enhanced configurational stability[74] as well as the favorable electronic effect of a push-pull system. In particular, the fluorescence quantum yields were considerably increased compared to the unsubstituted carbo[n]helicenes, reaching values up to 0.22 for F1 and 0.12 for F3. Notably, the fluorescence quantum yields for F2 actually decreased when compared with the corresponding carbo[n]helicene, which was attributed to a forbidden HOMO → LUMO transition for symmetry reasons. Though not being in the same league as the near-unity fluorescence quantum yields of rylene dyes, the results confirm once again the potential of the imide group as a fluorophore. All three compounds showed circular dichroism of a similar magnitude (Table [1]). The g abs values exceeded 10–2 for F2 and F3, which is above average for purely organic molecules. Of particular interest are the reduced species of the compounds, which can be probed though cyclic voltammetry and spectroelectrochemistry. As in the molecules synthesized by Nuckolls and co-workers, it was established that the imide groups communicate through space as well as though the π-system due to the small spatial distance between the redox centers. With increasing length of the helicene backbone from F1 to F3, the through-bond interactions decrease and the through-space interactions increase as the terminal imide units come closer together. The electronic properties of F1 in particular can be compared and contrasted with those of the related all-carbon molecule cethrene, where the presence of two phenalenyl fragments leads to a biradicaloid ground state.[75] [76] In the case of the more stable 13,14-dimethylcethrene, the biradicaloid ground state can be exploited to construct a chiroptical photoswitch by irradiating it with light of specific wavelengths in order to initiate ring closure and opening, respectively.[77]

A similar double helicene diimide F4 was recently published by Hu and co-workers in 2021.[78] The synthesis was accomplished by a Suzuki–Miyaura cross-coupling followed by cyclization by oxidative cyclodehydrogenation using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an oxidant. While the resulting compound did not possess the required configurational stability to perform separation of the enantiomers, it nevertheless showed interesting properties such as a very dynamic aggregation behavior and a remarkable case of mechanofluorochromism whereby the solid-state emission was hypsochromically shifted by finely grinding the crystalline substance. However, the presence of [4]helicene substructure did not allow the resolution of enantiomers.

In summary, numerous approaches towards imide-functionalized helical aromatic dyes have shown a great potential for optoelectronic applications. There have been increasing research efforts in this direction in recent years, and the number of published compounds, as well as their capabilities, are growing. Despite these successes, predicting the behavior of novel compounds remains a challenge. Further research on these fascinating compounds will lead to a better understanding of the structural and electronic factors influencing the properties and could pave the way for their application in exciting new chirality-related technologies. Moreover, most of the examples discussed in this article were synthesized as racemic mixtures, which were later resolved into enantiomers using HPLC-CSP. Enantiopure compounds obtained via this approach are limited by quantities for two main reasons: (1) high cost of preparative CSP HPLC columns, and (2) often poor separation between two isomers, especially for larger molecules. Therefore, stereoselective synthesis would further advance research on these compounds.[79] [80] We believe that the state-of-the-research presented in this article will accelerate the design process of novel helicene-imide hybrid functional molecules, which not only exhibit desired effective chiroptical response and electronic properties but will also be configurationally stable for practical applications.


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Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We thank Professor C. Lambert and Professor F. Würthner (University of Würzburg) for collaboration on [n]helicene diimide project and for their kind support.


Corresponding Author

Prince Ravat
Institut für Organische Chemie, Universität Würzburg
97074 Würzburg
Germany   

Publication History

Received: 26 July 2021

Accepted after revision: 27 August 2021

Accepted Manuscript online:
27 August 2021

Article published online:
28 September 2021

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Fridolin Saal(born in Schweinfurt, Germany in 1994) obtained his M. Sc. degree from the University of Würzburg in 2019 and is currently working on his PhD degree with Prince Ravat. His research interests include synthesis of [n]helicene diimides and investigation of their (chir)optical and redox properties. Prince Ravat (born in Vadodara, India in 1986) obtained his PhD degree at the Max-Planck-Institute for Polymer Research, Mainz, Germany. Since 2018 he has been a research group leader at the University of Würzburg, where he is working on his habilitation with the research focus on molecular engineering of functional chiral molecules for applications in organic electronics.
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Figure 1 Maleimide-functionalized helicenes
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Scheme 1 Representative example for the synthesis of maleimide-functionalized helicenes via the glyoxylic Perkin reaction.[39]
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Figure 2 Rylene diimides with a helical π-system
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Scheme 2 Examples of the synthesis of helical perylene diimide derivatives via Suzuki cross-coupling followed by photocyclodehydrogenation[51]
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Figure 3 Hetero[n]helicenes based on π-extended rylene imides
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Figure 4 The family of helicene-bridged PDIs published by Nuckolls and co-workers.
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Scheme 3 Examples of synthetic pathways for helicene-bridged multiple PDIs published by Nuckolls and co-workers
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Figure 5 Propeller-shaped PDI-helicene hybrids and representative synthetic scheme
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Figure 6The helicene diimides prepared by Ravat and co-workers and Hu and co-workers and an example of a synthetic scheme