Multi-redox indenofluorene chromophores incorporating dithiafulvene donor and ene/enediyne acceptor units

Large donor–acceptor scaffolds derived from polycyclic aromatic hydrocarbons (PAHs) with tunable HOMO and LUMO energies are important for several applications, such as organic photovoltaics. Here, we present a large selection of PAHs based on central indenofluorene (IF) or fluorene cores and containing various dithiafulvene (DTF) donor units that gain aromaticity upon oxidation and a variety of acceptor units, such as vinylic diesters, enediynes, and cross-conjugated radiaannulenes (RAs) that gain aromaticity upon reduction. In some cases, the DTF units are expanded by pyrrolo annelation. The optical and redox properties of these compounds, in some cases carbon-rich, were studied by UV–vis absorption spectroscopy and cyclic voltammetry. Synthetically, the work explores IF diones or fluorenone as central building blocks by subjecting the carbonyl groups to a variety of reactions; that are, phosphite- or Lawesson’s reagent-mediated olefination reactions (to introduce DTF motifs), Ramirez/Corey–Fuchs dibromo-olefinations followed by Sonogashira couplings (to introduce enediynes motifs), and Knoevenagel condensations (to introduce the vinylic diester motif). By a subsequent Glaser–Hay coupling reaction, a RA acceptor unit was introduced to provide a DTF-IF-RA donor–acceptor scaffold with a low-energy charge-transfer absorption and multi-redox behavior.


Introduction
Tetrathiafulvalene (TTF, Figure 1) is a redox-active molecule that has been widely explored in materials chemistry and supramolecular chemistry [1][2][3][4][5][6][7][8].TTF reversibly undergoes two sequential one-electron oxidations, generating first a radical cation (TTF +• ) and subsequently a dication (TTF 2+ ) containing two 6π-aromatic 1,3-dithiolium rings.The redox properties and geometries of the redox states have been finely tuned by extending the conjugated system with various cores, such as polycyclic aromatic hydrocarbons (PAHs), resulting in so-called extended TTFs [9][10][11][12].One example of this is the introduction of an indeno [1,2-b]fluorene (IF) core [13], providing indenofluorene-extended TTFs (IF-TTFs) of the general structure shown in Figure 1.The π-system can be further expanded as well at the dithiole rings.For example, we have recently developed a synthetic protocol for fusing a pyrrole unit to one of the dithiole rings of an IF-TTF, allowing for dimerization of extended TTFs via the nitrogen atom by different linkers [14].Donor-acceptor chromophores can be obtained by replacing one of the dithiafulvene (DTF) rings of the IF-TTF by an electron acceptor.Cyclic and acyclic acetylenic scaffolds comprised of enediyne units are known to behave as good electron acceptors [15,16], and we became interested in combining the IF-DTF scaffold with such motifs to generate novel multi-redox systems.For example, the radiaannulene moiety RA shown in Figure 1 (or its truncated counterpart with one of the exocyclic enediyne units removed) [17,18] is a particularly good electron acceptor as it gains 14π-aromaticity upon reduction.In this work, we also want to further explore pyrrolo-annelated IF-DTFs with different substituents on the nitrogen atom, and the functionalization at the other end of the IF core with electron-accepting moieties.An overview of general motifs targeted in this work is shown in Figure 1.

Synthesis
The synthetic building blocks 1-8 used in this work are shown in Figure 2. The dione 1 and the ketones 4 and 6 were synthesized according to literature procedures [14,19,20], as were the 1,3-dithiole-2-thiones 2 and 3 [21].Fluorenone 5 is commercially available.The new building blocks 7 and 8 were prepared according to related literature procedures [21], as described in Supporting Information File 1.  tone 4 with Lawesson's reagent (using a recently established protocol [20]) yielded the large dimer 13 as a mixture of E and Z isomers (ca.4:1).Further functionalization of the IF-DTF ketone 11 was obtained by Ramirez/Corey-Fuchs dibromo-olefination and Knoevenagel condensation to yield vinylic dibromide 14 and diester 15, respectively, as illustrated in Scheme 2. We noted that the dibromo-olefination reaction was first discov-ered by Ramirez and co-workers [22] and used in the first step of the Corey-Fuchs reaction that ultimately provides an alkyne [23].To elucidate the properties of the donor part itself of the pyrrolo-annelated IF-DTF systems, we prepared compounds 16 and 17 containing a smaller fluorene PAH.These compounds were prepared by a Lawesson's reagent-promoted coupling between fluorenone 5 and the Ts-protected 1,3-dithiole-2-thione building blocks 2 and 3, respectively, shown in Scheme 3 (albeit in modest yields).Fluorene-based DTF compounds have previously been explored in various elaborate systems [24][25][26][27].Next, we wanted to explore IF-DTFs as motifs for acetylenic scaffolding (Scheme 4).Starting from IF-DTF building block 6, dibromo-olefinated compound 18 was obtained by a Ramirez/ Corey-Fuchs reaction.Two-fold Sonogashira couplings with trimethylsilylacetylene, ethynylbenzene, or 4-ethynylbenzonitrile yielded compounds 19-21, while two-fold Sonogashira coupling with ((2-ethynylphenyl)ethynyl)triisopropylsilane resulted in compound 22. Desilylation of the alkynes of compound 22 with tetrabutylammonium fluoride (TBAF) and subsequent intramolecular Glaser-Hay coupling of the terminal alkynes afforded the macrocyclic DTF-IF-RA scaffold 23.Molecular sieves (4 Å) were added to the reaction mixture as this has previously been shown to significantly promote the Glaser-Hay coupling [28].Compounds 20 and 21 were unfortunately very sensitive compounds that were found to easily degrade, which made their characterization somewhat difficult (vide infra).
We also targeted other enediyne acetylenic scaffolds with IF as central core as shown in Scheme 5. Starting from IF dione 1, compounds 24 and 25 were synthesized via Ramirez/ Corey-Fuchs dibromo-olefinations. Four-fold Sonogashira couplings of compound 25 with triisopropylsilylacetylene and ((2-ethynylphenyl)ethynyl)triisopropylsilane yielded compounds 26 and 27, respectively.A two-fold, intramolecular Glaser-Hay coupling of compound 27 (after desilylation) was attempted under the conditions that were successful in the synthesis of compound 23 (Scheme 4).A compound that may tentatively be assigned to 28 was observed by MALDI-MS analysis of the reaction mixture, but less than needed for an NMR sample was isolated.Furthermore, the isolated compound proved quite insoluble in all investigated deuterated solvents, and therefore it was not possible to determine the purity of the product by this method.
In an initial attempt to investigate other synthetic pathways to extended IF compounds, the reduced IF 29 was synthesized from IF dione 1 by a Wolff-Kishner reduction of the two ketones as shown in Scheme 6. Compound 29 could potentially after deprotonation be reacted with electrophiles as previously established [29] for the parent structure [30] without tert-butyl substituents.

UV-vis absorption spectroscopy
UV-vis absorption spectra of the known compound 4 [14] and new compounds 9-12 and 15 are depicted in Figure 3, and the data are presented in Table 1.A redshift of the longest-wavelength absorption maximum is observed for all new compounds compared to that of 4. For compounds 11 and 12, this indicates that the inductive electron-withdrawing or -donating influences of the substituent group (Ts group in 4 and Hex group in 11) on the nitrogen atom in the pyrrole ring have an effect on the absorption in the visible spectrum of pyrrolo-annelated IF-DTF ketones.Interestingly, the absorption of the dihydropyrrole  Table 1: UV-vis absorption data of compounds in PhMe or CH 2 Cl 2 at 25 °C (absorption maxima λ max and molar absorptivities ε).UV-vis absorption spectra of the known compound 30 [20] and new compounds 13, 16, and 17 are shown in Figure 4, and the data are presented in Table 1.Compared to compound 30, the longest-wavelength absorption maximum of compound 16 is slightly blueshifted while the absorption maximum of compound 17 is significantly blueshifted.This indicates that annelation of the dihydropyrrole ring to the DTF moiety does not change the absorption maximum significantly compared to the two SHex substituents, while annelation of a pyrrole ring results in an absorption maximum at significantly shorter wavelength.These compounds have blueshifted longest-wavelength absorp-tions relative to the donor-acceptor scaffolds incorporating a pyrrolo-annelated DTF unit.Of these compounds, the large dimer 13 stands out with a significantly redshifted and intense longest-wavelength absorption maximum (λ max at 574 nm) expanding to ca. 680 nm.(and hence a lower-energy LUMO) compared to the acyclic acetylenic scaffold of compound 22 (in line with first reduction potentials, vide infra).For compound 27, a shorter longestwavelength absorption maximum at 461 nm is observed; this is a symmetric compound for which no donor-acceptor "push-pull" system is present (albeit a broad tail to the absorption is observed), in contrast to 22 and 23.The absorption maxima of compound 26 are significantly blueshifted, presumably due to the smaller conjugated system.The same trend with a shorter longest-wavelength absorption maximum that was observed for compound 27 was also observed for this compound.electrolyte) are shown in Figure 6, and potentials against ferrocene (Fc/Fc + ) (obtained from differential pulse voltammetry, see Supporting Information File 1) are summarized in Table 2.
Compounds 11 and 15 showed two irreversible first oxidations at +0.34 V and +0.38 V vs Fc/Fc + , showing that replacing the ketone with the stronger electron withdrawing vinylic diester renders the first oxidation more difficult (by 40 mV).An anodic shift of 40 mV was also observed for the second oxidation.
Oppositely, compound 15 underwent a significantly easier first reduction than 11 (−1.00V vs −1.35 V), and it also underwent a second reduction.The pyrrolo-annelated dimer 13 showed a reversible oxidation at +0.42 V followed by an irreversible oxidation at +1.01 V, and two reversible reductions at −1.48 V and −1.81 V. Here, the acceptor properties are not promoted by incorporating an acceptor unit as in 15, but instead by the bifluorenylidene motif [32] obtained by dimerizing two pyrroloannelated IF-DTF units.Notably, the dimer 13 underwent a first oxidation more readily (by as much as 0.14 V) than the corresponding fluorene-DTF donor 17 (both containing the same N-tosylated pyrrolo-DTF unit).The low electrochemical HOMO-LUMO gap of 13 is paralleled by a low-energy longest-wavelength absorption maximum (vide supra, Figure 7).A quasi-reversible first oxidation was observed at +0.47 V for the fluorene compound 16 and an irreversible oxidation at +0.99 V. Compound 17 experienced a quasi-reversible first oxidation at +0.56 V and an irreversible oxidation at +1.07 V. Thus, the dihydropyrrolo-annelated DTF compound is more easily oxidized than the pyrrolo-annelated DTF compound.These fluorene compounds did not experience a reduction within the potential window.Of the acetylenic scaffolds studied, DTF-IF-RA 23 containing an RA moiety is the strongest acceptor, which we ascribe to gain of 14π z -aromaticity of the cyclic moiety of the reduced species (in line with previously studied RA scaffolds [17,18,34]).Indeed, it is reduced more easily by as much as 0.3 V than its corresponding acyclic counterpart, compound 22, although it contains a π-system of the same size, and it is even reduced more easily by 0.13 V than the acetylenic scaffold 27 containing acetylenic acceptor motifs at both ends of the IF core and hence no DTF donor unit.Compound 23 also undergoes a reversible, second reduction to form the dianion.This compound should gain aromaticity upon either reduction or oxidation as illustrated in Figure 9.

X-ray crystallographic analysis
Crystals suitable for single-crystal X-ray diffraction studies were obtained for compounds 25, 26, and 29.Their structures are shown in Figure 10, top, and their respective crystal packings below.All three compounds pack in a herringbone manner in the crystal structure, with the major difference that com-pound 29 is perpendicular with respect to the herringbone pattern and the related structures (see Figure 10, bottom).Compound 25 packs with an intramolecular distance of 3.41 Å between the planes of the π-systems.Neither compound 26 nor 29 shows π-π interactions in the crystal packing.The large bulkiness of the TIPS groups along with the tert-butyl groups in compound 26 prevent these interactions, while for compound 29, the lack of π-π interactions can be ascribed to the methylene bridges as the hydrogens along with the tert-butyl groups prevent good overlap of the π-systems.

Conclusion
In summary, various redox-active chromophores based on the indenofluorene scaffold were synthesized, incorporating different dithiafulvenes and acetylenic scaffolds, such as acetylenic radiaannulenes.The compounds have strong absorptions in the visible region and undergo reversible (or quasi-reversible) oxidations and reductions.We have also presented two new fluorene-extended dithiafulvenes, which also absorb strongly in the visible region and undergo one reversible oxidation, while no reductions were observed for these compounds.Systematic studies show that by small structural modifications, the optical and electrochemical HOMO-LUMO gaps can be finely tunedwith first oxidations and reductions that can be adjusted by several hundreds of millivolts for donor-acceptor IF scaffolds.Introduction of both the dithiafulvene and radiaannulene units along the indenofluorene scaffold provided a donor-acceptor compound covering a particularly broad absorption profile and with a redshifted longest-wavelength absorption maximum relative to most of the compounds (529 nm in dichloromethane), which can be related to the fact that it is both a good donor and a good acceptor as shown electrochemically.This compound stands out as gaining aromaticity in one of its appendages along the IF core upon either reduction (generation of 14π z -aromatic ring) or oxidation (generation of 1,3-dithiolium ring).
Synthetically, the work relies on using indenofluorene diones as key building blocks for performing olefination reactions, such as phosphite-or Lawesson's reagent-mediated couplings, Ramirez/Corey-Fuchs dibromo-olefinations, and Knoevenagel condensations.In particular, the acetylenic scaffolds presented in this work may be useful precursors for even more elaborate, conjugated and carbon-rich structures in future work.

UV-vis absorption spectroscopy
UV-vis absorption spectra were recorded on a Varian Cary 50 UV-vis spectrophotometer scanning between 800 and 200 nm.
All spectra were recorded with baseline correction in CH 2 Cl 2 or toluene (HPLC grades) at 25 °C in a quartz cuvette with a 10 mm path length.

Electrochemistry
Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) were obtained using an Autolab PGSTAT12 instrument and Nova 1.11 software with a scan rate of 0.1 V/s for the CVs.A silver wire immersed in a 0.1 M Bu 4 NPF 6 solution in CH 2 Cl 2 separated from the analyte solution by a frit was used as the reference electrode, a Pt wire was used as the counter electrode, and a platinum disk (diameter = 1.6 mm) or a glassy carbon disk (3 mm) was used as the working electrode.
The reference electrode was separated from the solution containing the substrate by a ceramic frit.Measured potentials were referenced to ferrocene/ferrocenium (Fc/Fc + ) redox couple, measured before and after the experiment.A 0.1 M solution of NBu 4 PF 6 was used as electrolyte.All solutions were purged with Ar prior to measurements.

Crystallography
All single crystal X-ray diffraction data for compounds 25, 26, and 29 were collected on a Bruker D8 VENTURE diffractometer equipped with a Mo Kα X-ray (λ = 0.71073 Å).The data collections were done at 100 K.All data were integrated with SAINT and a multi-scan absorption correction using SADABS was applied [35,36].The structure was solved by direct methods using SHELXT and refined by full-matrix leastsquares methods against F2 by SHELXL-2019/2 [37,38].The data for the compounds have been deposited with the Cambridge Crystallographic Data Centre [39].The CIF files (Supporting Information Files 2-4) and reports were generated using FinalCIF [40].

Compound 9
A solution of 1 (139 mg, 352 μmol) and 2 (176 mg, 534 μmol) in anhydrous toluene (5 mL) and P(OEt) 3 (10 mL) was heated to reflux for 5 h, resulting in a color change from orange to dark red.The reaction mixture was then allowed to cool to rt before it was concentrated under reduced pressure.The resulting dark red solid was purified by flash column chromatography (SiO

Compound 23
In a manner similar to [41], TBAF (1 M in THF, 0.2 mL, 0.2 mmol) was added to a solution of 22 (93 mg, 0.073 mmol) in THF (10 mL), and the reaction mixture was stirred at rt for 45 min before it was filtered through a plug of SiO

Compound 29
To a 250 mL round-bottomed flask equipped with a reflux condenser and containing a magnetic stir bar, diethylene glycol (125 mL) and KOH (2.67 g, 47.7 mmol) were added.The solution was degassed with Ar for 30 min after which 5 (461 mg, 1.17 mmol) was added.Then, N 2 H 4 •H 2 O (2.4 mL, 50.0 mmol) was added slowly, resulting in a color change to black within 30 min.The reaction was carried out under inert N 2 atmosphere.The reaction mixture was then heated to 185-190 °C for 48 h after which it was cooled to 100 °C, poured onto ice (400 mL), and acidified with aq HCl (20 mL, 6 M), resulting in an

Figure 1 :
Figure 1: Overview of structural motifs relevant for the work described herein.

Figure 9 :
Figure 9: Radical anion (left), dianion (middle), and radical cation (right) of compound 23; the radical anion has a 14π z -aromatic ring (highlighted in blue; only counting 2π-electrons of each triple bond, here defined as those in π z orbitals), the dianion has an additional 6π-aromatic cyclopentadienyl anion (highlighted in green), while the cation has a 6π-aromatic 1,3-dithiolium ring (highlighted in red).

Figure 10 :
Figure 10: ORTEP plots (50% probability) and crystal packing of compounds a) 25, b) 26, and c) 29.The respective crystal packing of each compound is shown below, in which the hydrogen atoms are omitted for clarity.Atoms are colored grey (carbon), white (hydrogen), brown (bromine), paleyellow (silicon).

Figure 11 :
Figure 11: Labels of bonds within five-membered ring.

Table 2 :
Electrochemical data from differential pulse voltammetry of compounds in CH 2 Cl 2 (with 0.1 M Bu 4 NPF 6 ) if not otherwise stated; potentials in volts vs Fc/Fc + .
a In MeCN.b E/Z ratio of 4:1.7:Comparison of properties of compounds 13 and 17.

Table 3
lists the lengths of the bonds (b-f) within the five-membered rings of the cores as well as the exocyclic C=C double bond (a) that is present in compounds 25 and 26 (for bond labels, see Figure11).A small difference in the exocyclic C=C bond length is observed between 25 and 26, with the bond in 26 being slightly longer.Bonds b and f are affected by the moiety X, with the less π-delocalized structure 29 having the longest bonds of 1.51 Å, while only minor differences are observed for bonds c, d, and e.

Table 3 :
Bond lengths (Å) within five-membered rings and of exocyclic C=C double bond (for bond assignments, see Figure11).