Tetra(peri‐naphthylene)anthracene: A Near‐IR Fluorophore with Four‐Stage Amphoteric Redox Properties

Abstract

A novel, benign synthetic strategy towards soluble tetra(peri‐naphthylene)anthracene (TPNA) decorated with triisopropylsilylethynyl substituents has been established. The compound is perfectly stable under ambient conditions in air and features intense and strongly bathochromically shifted UV/vis absorption and emission bands reaching to near‐IR region beyond 900 nm. Cyclic voltammetry measurements revealed four facilitated reversible redox events comprising two oxidations and two reductions. These remarkable experimental findings were corroborated by theoretical studies to identify the TPNA platform a particularly useful candidate for the development of functional near‐IR fluorophores upon appropriate functionalization.

Towards the red‐light area. A reliable synthesis of a new tetra(peri‐naphthylene)anthracene derivative decorated with triisopropylsilylethynyl moieties has been established. The soluble compound features strongly bathochromically shifted absorption and emission bands reaching into the near‐IR region and is prone to four facilitated redox steps. The experimental results, corroborated by theoretical studies, identified the π‐conjugated polycyclic scaffold as a particularly promising candidate for the development of redox‐active near‐IR fluorophores.

Introduction

Within the ongoing quest for novel technologically relevant electron acceptors several approaches have been established. ^[1] Among these approaches the decoration of the inherently electron‐rich polycyclic aromatic hydrocarbons (PAHs) with electron‐withdrawing moieties, such as cyano, ^[2] fluoro, ^[3] perfluoroalkyl, ^[4] and imide moieties[ ³ , ⁵ ] is certainly the most straightforward one. Nevertheless, next to the desired electron affinity increase these functionalities may lead to stability issues due to their inherent reactivity, which is detrimental for the use of such compounds as electron‐transport materials in organic electronics.[ ^1b , ⁶ ] A particularly appealing alternative to achieve carbonaceous electron acceptors is the incorporation of suitable non‐benzenoid moieties into the sp²‐carbon frameworks of PAHs as demonstrated by cyclopentannulated systems. ^[7] In these compounds the π‐conjugated cyclopentadiene moieties are prone to facilitated electron uptake driven by the formation of the aromatic cyclopentadienyl anion‐like moieties. ^[8] Furthermore, the five‐membered rings embedded within the hexagonal framework often lead to curved structures as illustrated by fullerene C₆₀ as the ultimate case. ^[9] In this context, bowl‐shaped corannulene (1; Figure 1) can be considered as the simplest molecular fragment of C₆₀. ^[10] Related curved indacenopicene (2) and diindenochrysene (3) with two five‐membered rings were realized and demonstrated to act as promising electron acceptors capable of forming crystalline anions upon reduction with alkali metals. ^[11] Furthermore, cyclopentannulation of acenes resulted in polycyclic scaffolds with increased electron affinity and stability.[ ⁸ , ¹² ] For example, cyclopenta[hi]aceanthrylene (4) with its low‐lying lowest unoccupied molecular orbital (LUMO) was successfully implemented upon appropriate functionalization as an electron acceptor component in organic semiconductors.[ ^8a , ^8b , ¹³ ] Decacyclene (5) with its three cyclopentadiene moieties and 36 sp² carbons represents an electron deficient fragment of C₆₀ prone to four reversible reduction steps. ^[14] Upon decoration of 5 with three imide moieties a new type of non‐fullerene acceptors was obtained. ^[15]

Figure 1.

Examples of electron‐deficient PAHs comprising cyclopentadiene moieties.

In contrast to this, the related 1.2,3.4,5.6,7.8‐tetra(peri‐naphthylene)anthracene (TPNA) with four cyclopentadiene rings fused to a common anthracene core remains largely unexplored, although it has been realized already back in the 1950s. ^[16] The original synthesis relies on a dimerization of acenaphthylene upon treatment with picric acid, followed by a twofold Diels–Alder cycloaddition with p‐benzoquinone and reduction of the intermediate diketone under harsh conditions (for a scheme representing the original synthesis, see the Supporting Information). The target compound was obtained after sublimation as olive green needles in 11 % yield. Interestingly, the solution of TPNA in 1,2,4‐trichlorobenzene was reported to show red fluorescence. However, at that time no further investigation of the emission properties was carried out. TPNA and its heteroatom‐containing analogues were predicted by theoretical studies as promising air‐stable n‐type semiconductors owing to their small internal reorganization energy. ^[17] Nevertheless, despite these interesting findings, to the best of our knowledge, no experimental follow‐up work has been reported since then.

Results and Discussion

In this work, we have developed a novel synthetic strategy towards soluble TIPS‐TPNA equipped with triisopropylsilyl (TIPS)‐ethynyl substituents. Most importantly, our synthesis does not require the use of hazardous picric acid and the final aromatization proceeds under mild conditions. Comprehensive studies identified the strongly bathochromically shifted UV/vis absorption and emission in the near‐IR region as well as four‐stage amphoteric redox behavior of the compound.

Starting from commercially available 1‐acenaphthenone the dimeric species 6 was obtained exclusively as an E isomer from a McMurry coupling in an improved yield of 65 % yield (Scheme 1).

Scheme 1.

The newly established synthesis of the soluble TIPS‐ethynyl‐substituted TIPS‐TPNA.

Subsequent treatment of 6 with an excess of p‐benzoquinone in a Diels–Alder cycloaddition reaction provided π‐expanded anthraquinone 7 in 70 % yield. In accordance with the seminal synthesis of TPNA, ^[16] the excess of p‐benzoquinone is required for the in‐situ oxidation of 6 towards the corresponding 1,3‐diene which is necessary for the Diels–Alder cycloaddition (see the Supporting Information). At the same time, p‐benzoquinone participates in the dehydrogenation of the initially formed Diels–Alder adduct towards 7. Subsequent twofold ethynylation of 7 using a large excess (50 equiv.) of lithium TIPS‐acetylide furnished diol 8 in 63 % yield solely as an anti isomer (for its X‐ray crystal structure, see the Supporting Information). Reductive aromatization with SnCl₂⋅2 H₂O and HCl in Et₂O gave the title TIPS‐TPNA in a good yield of 64 %. The compound forms a black crystalline solid which is stable at ambient conditions and soluble in common organic solvents.

The constitution of TIPS‐TPNA was unambiguously determined by X‐ray crystallographic analysis of the single crystals grown by slow evaporation of a CH₂Cl₂ solution at room temperature (Figure 2). The TIPS‐ethynyl moieties are arranged in a syn fashion pointing in the same direction. As a consequence of the steric demand imparted by the lateral substituents the anthracene core is bent with the carbon atoms C11 and C36 of the central benzene ring being located about 0.20 Å above the mean plane defined by the adjacent rings. The observed bending is within the range of other 9,10‐disubstituted anthracenes comprising sterically demanding moieties. ^[18] The planes passing through C11 and C36 cross the mean plane of the anthracene moiety at an angle of 16.4° and 17.0°, respectively (Figure 2b and c). The peri‐annulated naphthylene units are tilted away from the anthracene core with the dihedral angles ranging from 17.8° to 24.1°. The outer naphthylene flanks form a helicene‐like structure due to the steric repulsion of the hydrogen atoms at C21 and C26 as well as at C46 and C51.

Figure 2.

X‐ray crystal structure of TIPS‐TPNA (50 % probability level, hydrogens omitted for clarity). a) Top view with the relevant dihedral angles between the anthracene core and naphthylene units. Color code: gray=carbon, yellow=silicon. b) Bending of the central benzene ring of the anthracene moiety as expressed by the angles between the respective planes; blue plane (C12, C35, C37, C60), red planes (C11, C12, C60 and C35, C36, C37) c) Side view to visualize the bending of the central anthracene moiety and the helicene‐like arrangement of the adjacent naphthylene moieties (TIPS‐ethynyl substituents omitted for clarity). d) Slipped face‐to‐face dimer formed upon C(sp²)−H⋅⋅⋅π interactions (TIPS‐ethynyl substituents omitted for clarity). e) Lamellar arrangement of the individual dimers.

The crystal packing of TIPS‐TPNA is dominated by C(sp²)−H⋅⋅⋅π interactions with a distance of 2.76 Å between the naphthylene wings of two adjacent molecules (Figure 2d). ^[19] The resulting dimers are arranged in a slipped face‐to‐face manner. The dimers form a lamellar packing motif upon C(sp³)−H⋅⋅⋅π interactions between the polycyclic backbones and the isopropyl moieties. The individual dimers are separated by the bulky TIPS‐ethynyl moieties.

As assessed by density‐functional theory (DFT) calculations on the COSMO‐B3LYP‐D3 level (for details, see the Supporting Information), the syn configuration is also the most stable one in solution. Moreover, two additional conformers denoted anti and twisted were identified (Figure S18 in the Supporting Information), which are only 5 and 8 kcal⋅mol⁻¹ higher in energy, respectively. The syn conformer is in equilibrium with these two forms at room temperature. To obtain information about the local aromaticity and antiaromaticity within the polycyclic framework of TIPS‐TPNA, nucleus independent chemical shifts (NICS(0)) values at the centers of the annulated rings were calculated. ^[20] Both the anthracene core and the naphthylene subunits have clear aromatic character indicated by the negative NICS values. The induced ring currents in the naphthylene moieties are somewhat larger than in the anthracene core featuring overall less negative values. The five‐membered rings have positive NICS values, suggesting their antiaromaticity (Figure S20). At the same time, the positive values may also result from the diamagnetic ring currents of the adjacent six‐membered rings.

TIPS‐TPNA displays several absorption maxima in the UV/vis spectrum recorded in CH₂Cl₂ at room temperature (Figure 3a; for spectroscopic data of the precursors 7 (λ _max=450 nm in 1,2,4‐trichlorobenzene) and 8 (λ _max=408 nm in CH₂Cl₂), see the Supporting Information). The lowest energy maximum λ _max of TIPS‐TPNA is bathochromically shifted compared to unsubstituted TPNA (λ _max=660 nm) ^[16] and occurs at 702 nm with a tail reaching to 743 nm. This value translates into an optical band gap ($E\genfrac{}{}{0pt}{}{\mathrm{opt}}{\mathrm{g}}$ ) of 1.71 eV, which is in very good agreement with the calculated vertical excitation energy of the S₀→S₁ transition as obtained on the time‐dependent double‐hybrid density‐functional approximation (TD‐DHDFA) level (Table 1). To evaluate the solvent effects the excitation energies were calculated both with and without the implicit solvent model mimicking CH₂Cl₂ (linear‐response conductor‐like polarizable continuum model (LR‐CPCM); for details, see the Supporting Information). The results suggest only a negligible shift of <0.05 eV for most transitions (Tables S5 and S6), most likely due to rather low polarity of CH₂Cl₂. Hence, for the S₁ state the computed excitation energy is 1.73 eV (gas phase) and 1.69 eV (CH₂Cl₂). The S₀→S₁ excitation can be identified as the HOMO→LUMO transition. Although the molecule is not perfectly planar, the orbitals resemble somewhat distorted π‐like orbital shapes and can be described as the HOMO and LUMO of anthracene coupled to the naphthalene and acetylenic moieties (Figure S19). Moreover, the low‐energy absorption band experimentally exhibits a vibrational fine structure with ν≈1300 cm⁻¹ that is also present in the fluorescence spectrum (ν≈1200 cm⁻¹; see below). This observation indicates that C−C vibrations of the C(sp²) scaffold, most likely coupled to C−H deformation vibrations occurring in the same range, are excited along with the electronic transition which is in line with the nature of the excitation. The second excited state S₂ is computed at 2.24 eV (gas phase; 2.22 eV (CH₂Cl₂)) (experimental band maximum yields 2.23 eV) and also connected to a π‐π like transition (HOMO‐1→LUMO). Higher excitations become increasingly complex regarding the orbital contributions.

Figure 3.

a) UV/vis absorption and emission spectra (λ _ex=630 nm) of TIPS‐TPNA recorded in CH₂Cl₂ at room temperature. The intensity of the emission spectrum has been scaled to match the lowest absorption band. The calculated vertical gas‐phase transition energies are indicated as vertical sticks normalized to the respective oscillator strength. b) Emission spectra of TIPS‐TPNA (λ _ex=630 nm) measured in CH₂Cl₂ (λ _em=718 nm), in polystyrene matrix (λ _em=740 nm), and in spin‐coated film (λ _em=835 nm).

Table 1.

Experimental and theoretical optoelectronic data of TIPS‐TPNA.

λ max [nm][a]	ϵ [m −1 cm−1]	λ em [nm][a]	E ox [b] [V]	E red [b] [V]	IP [eV] exp.[c]// calcd.[d]	EA [eV] exp.[c]/calcd.[d]	E gap,adbatic [eV] exp.[e]/calcd.[f]	E gap,vertical [eV] exp.[g]/calcd.[h]
702	22 000	718, 786	+0.45/+0.95	−1.31/−1.66	5.25/5.06	3.49/3.43	1.76/1.63	1.71/1.69 (CH2Cl2), 1.73 (gas phase)

[a] Recorded in CH₂Cl₂ at room temperature. [b] Redox potentials from CV vs. Fc/Fc⁺ (0.1 m n‐Bu₄NPF₆ in CH₂Cl₂, scan rate 100 mV s⁻¹). [c] Ionization potential (IP_exp,sol) and electron affinity (EA_exp,sol) in solution calculated from CV measurements (IP_exp,_sol=−4.80 eV‐E _ox,1; EA_exp,sol=−4.80 eV‐E _red,1); oxidation potential of soluted Fc against vacuum (4.80 eV). ^[25] [d] Obtained from DFT calculations performed at the COSMO‐B3LYP‐D3 level of theory (ϵ=9.0). [e] Calculated from IP_exp,sol and EA_exp,sol (E _gap=EA‐IP). [f] Calculated from IP_cal and EA_cal (E _gap=EA‐IP). [g] Optical gap calculated from λ _end. [h] Obtained from TD‐DHDFA calculation on the SCS‐ωPBEPP86 level of theory with and without implicit solvent model LR‐CPCM (ϵ=9.0, see the Supporting Information).

In line with red fluorescence qualitatively reported for parent TPNA in 1,2,4‐trichlorobenzene, ^[16] the emission spectrum of TIPS‐TPNA recorded in CH₂Cl₂ at room temperature represents a mirrored image of the absorption spectrum with two distinct emission maxima at 718 nm (1.73 eV) and 786 nm (1.58 eV) and a remarkably small Stokes shift of 317 cm⁻¹. Moreover, a tail reaching to the near‐IR region at 930 nm is experimentally observed (Figure 3a). The fluorescence of TIPS‐TPNA is characterized by a photoluminescence quantum yield (PLQY) of 0.19 and a lifetime of 8.9 ns. The vertical emission (fluorescence) energy of 1.47 eV was calculated in the gas phase, which corresponds to a theoretical Stokes shift of 2070 cm⁻¹ due to the structural relaxation in the excited state. Note that vibronic effects were not considered in the calculation, and thus, the computed Stokes shift should be compared to the difference between the barycenters of the absorption and emission bands which is ≈2000 cm⁻¹. The inclusion of solvent effects within the LR‐CPCM model (see the Supporting Information) reduces the fluorescence energy to 1.35 eV. It was found in other studies that the LR‐CPM model tends to underestimate emission energies, ^[21] which is most likely also the case here.

Stimulated by the appealing spectroscopic features of TIPS‐TPNA in solution, its photoluminescence in the solid state was investigated. In spin‐coated thin films the compound shows further bathochromically shifted emission maxima at 835 nm and 918 nm with a tail reaching to 1143 nm (Figure 3b). The photoluminescence lifetime of the film was determined to be 1.5 ns. However, a strong quenching of photoluminescence was observed and the PLQY could not be reliably determined (below 0.01, for details, see the Supporting Information). Embedded in a polystyrene matrix, the emission maxima of TIPS‐TPNA are less bathochromically shifted, occurring at 739 and 809 nm with a tail reaching to 1117 nm. A PLQY of 0.06 was measured with an integrating sphere. These characteristics are quite remarkable given the apolar, transition metal‐free nature of TIPS‐TPNA and qualify the compound as an appealing candidate for the development of novel near‐IR fluorophores for diverse applications. ^[22]

The good solubility of TIPS‐TPNA enabled for the first time the investigation of the redox properties of the TPNA scaffold by cyclic voltammetry (CV; Figure 4 and Table 1). The compound shows two reversible oxidations at +0.45 V and +0.95 V (in CH₂Cl₂ vs. ferrocene/ferrocenium (Fc/Fc⁺) redox couple). More interestingly, two reversible reductions are observed at −1.31 and −1.66 V. The redox potentials were verified by square wave and differential pulse voltammetry (SWV and DPV). In other words, TIPS‐TPNA behaves as a fairly strong electron acceptor comparable to, for instance, 9,10‐anthraquinone which is reduced under comparable conditions at −1.40 V. ^[23] For comparison, the first electron uptake of fullerene C₆₀ is reported to occur at −0.98 V (in MeCN/toluene mixture). ^[24] The redox properties are in line with the calculated adiabatic ionization potential (IP) and electron affinity (EA) of TIPS‐TPNA amount to 5.06 and 3.43 eV, respectively.

Figure 4.

Cyclic, differential pulse and square wave voltammograms (CV, DPV and SWV) of TIPS‐TPNA in CH₂Cl₂ with 0.1 m nBu₄NPF₆ as supporting electrolyte (scan rate 100 mV s⁻¹).

Conclusion

In summary, we have established a practicable, nonhazardous synthesis of a new soluble tetra(peri‐naphthylene)anthracene derivative decorated with TIPS‐ethynyl moieties. Most importantly, the compound features intense, strongly bathochromically shifted absorption bands in the far‐red region and emission bands reaching the near‐IR region both in solution and the solid state. The photoluminescence quantum yield in solution amounts to a remarkable 0.19. The compound is prone to four facilitated reversible redox events: two oxidations and two reductions. These experimental findings – corroborated by theoretical studies – identify this TPNA derivative as a new type of near‐IR fluorophore with fully reversible four‐stage amphoteric redox properties. We believe that our work will provide access to new families of cyclopentannulated polycyclic aromatic hydrocarbons with properties relevant for optoelectronic applications.

Experimental Section

Experimental details, characterization data and the structures of 6 and 8 can be found in the Supporting Information.

Deposition Numbers 2199257 (6), 2199258 (8), and 2199259 (TIPS‐TPNA) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

Conflict of interest

The authors declare no conflict of interests.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supporting Information

Frisch S., Neiß C., Lindenthal S., Zorn N. F., Rominger F., Görling A., Zaumseil J., Kivala M., Chem. Eur. J. 2023, 29, e202203101.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.