Crystalline Diradical Dianions and Radical Anions of Indenofluorenediones

Abstract

Fluorenone derivatives represent promising candidates for electron-transport materials in organic electronic devices. Given that anionic species serve as electron-transfer carriers in electron-transport materials, it is highly desirable to isolate and characterize the radical anions and dianions of indenofluorened derivatives (IFO). In this work, the reduction of three indenofluorenedione derivatives (IFO, 1, 2 and 3) with potassium resulted in three radical anion salts (1K[(crypt-222)], 2K[(crypt-222)] and 3K) and one dianion salt (2[K(crypt-222)]₂). Single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy reveal that 1K[(crypt-222)] and 2K[(crypt-222)] have a full delocalization of the unpaired electron which is supported by calculated spin density distributions. We demonstrate that the polarization of electron spin in 3K is induced by potassium ion coordination through single-crystal X-ray structure analysis and DFT calculations, suggesting the electrostatic effect by potassium ion has a significant influence on the spin density modulation. Superconducting quantum interference device (SQUID) measurements and DFT calculations show that 2[K(crypt-222)]₂ has an open-shell singlet base with a large singlet-triplet energy gap (ΔE_os-t = −7.40 kcal mol⁻¹) so that the excited triplet state is not accessible at room temperature.

1. Introduction

The field of organic semiconductors has matured significantly and achieved commercial viability [1,2,3,4]. While numerous hole-transporting organic materials have been developed, only a limited number of electron-deficient compounds demonstrate reliable electron-transport properties [5,6]. The key issue in the fabrication of efficient organic p-n junctions and complementary integrated circuits is the development of the high-performance n-type materials [7]. Therefore, the design of new highly efficient n-type materials is now receiving significant attention. Among organic semiconductors, polycyclic conjugated hydrocarbons (PCHs) have emerged as particularly promising materials due to their tunable electronic structures and charge transport characteristics [8]. In the PCHs, indenofluorene (IF) can offer a highly planar 6-5-6-5-6 π-fused-ring backbones and the bridge carbons for efficient synthetic modifications and functionalization [8]. The carbonyl (C=O) group, with a negative resonance effect on reducing the electron density, is one of the most important electron-withdrawing groups employed in the n-channel organic semiconductors [8,9]. The compounds are indenofluorenedione derivatives (IFO) when the bridge carbons in IF are functionalized as carbonyl groups [8].

Fluorenone is a promising building block for electron-transport materials in organic electronic devices, including organic field-effect transistors (OFETs), organic photovoltaic devices (OPVs), and organic light-emitting diodes (OLEDs) owing to its electron acceptor capability derived from the cyclopentadienone central ring and highly electron-deficient carbonyl [10,11,12,13,14,15,16,17,18]. In particular, IFO (indenofluorenedione derivatives [7,19,20,21,22,23,24,25,26,27] and truxenone derivatives [1,28,29,30,31,32,33,34]) has showed promising n-type material properties. For example, the Yamashita group [23] successfully synthesized and characterized novel indenofluorenedione derivatives used in OFET with high n-channel charge mobility. Moreover, a derivative of truxenone with electron mobility values above 1 cm² V⁻¹ S⁻¹ was discovered in 2018 by Delgado, Golemme, and Gómez-Lor [19].

Anionic species are electron-transfer carriers in electron-transport materials [35], so it is highly desirable to isolate and characterize the series of radical anions and dianions of IFO. Radicals delocalized over π-conjugated systems are gaining increasing attention for their potential applications as functional materials [36,37,38]. IFO radical anionic species have been studied since the 1950s [39,40]. However, the detailed properties of those radicals have not yet been fully examined, except through spectroscopic analysis. Herein, the geometries and electronic structures of radical anions and dianions of IFOs 1, 2, and 3 (Scheme 1) were investigated by single-crystal X-ray structure analysis, EPR measurements, and SQUID measurements, in conjunction with DFT calculations.

2. Materials and Methods

All experiments were carried out in a dry nitrogen atmosphere using standard Schlenk techniques and a glovebox. Solvents were dried by standard methods. The NMR spectra were recorded on Bruker spectrometers (AV400 and AV500) (Bruker, FÄLLANDEN, Switzerland). EPR spectra were obtained using Bruker EMX plus-6/1 X-band (Bruker, Baden-Württemberg, Germany). UV–vis spectra were recorded on a Lambda 750 spectrometer (PerkinElmer, MA, USA). Magnetic measurements were performed using a Quantum Design SQUID VSM magnetometer (Quantum Design, CA, USA). The crystal samples were mounted on a glass capillary in perfluorinated oil and measured in a cold N₂ flow. The single crystal X-ray structures were collected on Bruker D8 CMOS detectors (Bruker, Karlsruhe, Germany) at low temperature. The crystal structures were solved by direct methods and refined on F² with the SHELX-97 or Olex2 1.2 software packages. Commercially available reagents were purchased from Aldrich, Energy, or Alfa-Assar and used as received. The indeno [3,2-b]fluorene-6,12-dione (1), indeno [2,1-b]fluorene-10,12-dione (2), and truxenone (3) were synthesized following previously published methods [41,42,43]. The radical anion salts and diradical diaion salt were synthesized as described the Supplementary Materials. All the calculations were carried out using the Gaussian 16 program [44]. The electronic structures of the studied compounds were optimized by density functional theory (DFT) combined with the M052X [45,46,47,48] functional and 6-31G(d) basis set. To further uncover the absorption properties of the studied complexes, we also obtained the UV–vis spectra at the time-dependent DFT (TDDFT)/UM052X/6-311++G(d,p) level, in conjuction with THF (SMD model) [49]. Then, the approximate spin projection (ASP) method [50,51,52,53] was utilized to evaluate the energy gap between the singlet state and triplet state. Details about the computations are given in the Supplementary Materials.

3. Results and Discussion

The cyclic voltammogram of 1 and 2 show two quasi-reversible redox processes at −1.721 and −1.243 V for 1 and −1.360 and −1.918 V for 2, respectively, versus the Ag/Ag⁺ reference electrode (Figure S6). To this end, we conducted one- or two-electron reductions of compounds 1, 2, and 3 using potassium graphite (KC₈) in THF, both with and without 2,2,2-cryptand, yielding three radical anion salts, i.e., 1K[(crypt-222)], 2K[(crypt-222)], and 3K, and a dianion salt, i.e., 2K₂[(crypt-222)]₂, respectively (Scheme 1 and Supplementary Materials). The reduced products were consequently studied by single-crystal X-ray structure analysis, EPR spectroscopy, SQUID measurements, and UV/Vis absorption spectroscopy, in conjunction with DFT calculations.

Single crystals of 1K[(crypt-222)], 2K[(crypt-222)], and 3K suitable for X-ray diffraction analysis were obtained by recrystallization from the saturated THF solution at 5 °C, −20 °C and room temperature, respectively. 1K[(crypt-222)] crystallized in the orthorhombic space group Pbcn with a centrosymmetric geometry. In the structure of 1K[(crypt-222)] (Figure 1a), each potassium ion is coordinated by a single 2,2,2-cryptand and remains entirely isolated from the anionic 1^−• moiety, demonstrating that no interactions occur between the potassium ions and oxygen atoms. The 1K[(crypt-222)] shows a nearly planar IFO backbone. The shortest distance between the radical anions in 1K[(crypt-222)] is d_C-C = 3.394(4) Å (Figure S3), while there is no π–π stacking interaction between 1^−•. The O–C bond length (1.251(2) Å) in 1^−• is longer than that in neutral 1 (1.211(3) Å, Figure S1). 2K[(crypt-222)] crystallizes in the monoclinic space group P2₁/c with an approximate orthorhombic symmetry, whereas the two O–C bonds in solid states are slightly different (1.244(4) and 1.237(4) Å), which can be attributed to the different distances between the O atoms and the potassium ion (Figure 1b). The shortest distance between the radical anions in 2K[(crypt-222)] is d_O-C = 3.313(4) Å (Figure S4). Similarly to the 1^−•, there is no π–π stacking interaction between the 2^−• radical anions.

3K crystallizes in orthorhombic space group P2₁/c. The 3K is three-dimensional network of 3^−• moieties and potassium ions (Figure 2b). In the crystal structure of 3K, each potassium ion is coordinated to four 3^−• moieties and one THF molecule, and each 3^−• moiety is connected to four potassium ions (Figure 2c,d). Two neighboring 3^−• moieties are parallel with a distance of less than 3.4 Å (Figure S2), indicating the existence of π–π stacking interactions between them. Given the planar π-conjugated units and specific parallel π-stacking interactions, the solid state of 3K exhibits pancake bonds [54], which are supported by the absence of a NIR band in the UV–vis spectrum of 3K (SI, Figure S11 and Table S10). The distances between the potassium ions and oxygen atoms in neighboring 3^−• moieties are 2.689(2), 2.744(2), 2.775(3), and 2.767(3) Å, respectively. The length of the C1–O1 (1.275(4) Å) bond in 3K coordinating to two potassium ions is much longer than those of C19–O2 (1.230(4) Å) and C10–O3 (1.239(4) Å) and is very similar to those in 2[K(crypt-222)]₂ (1.272(2) and 1.271(2) Å). The three C–O bonds in 3K are significantly differently attributed to the different coordination environments (Figure 2c,d). The discussion of bond lengths suggests that the unpaired electron is mostly delocalized on one side in 3K (C₅O₁).

Theoretical calculations were performed to study the electronic structure of 3K. The crystal structure of 3^−• was taken as the starting geometry for optimization at the UM052X/6-31G(d) level. The optimized structure of 3^−• is inconsistent with the crystallographic analysis. After considering potassium ion, the calculation was initially carried out based on the crystal structure without geometry optimization for overly large-scale molecules. Significantly different from optimized structure of 3^−•, the unpaired electron in crystal structure (Figure 3b) is mainly delocalized on the C₅O₁ moiety (+0.62e) coordinating to two potassium ions, which is consistent with the above analysis on bonds. The polarization of electron spin in 3K is induced by potassium ion coordination, suggesting the electrostatic effect by potassium ion has a significant influence on the spin density modulation. A similar behavior has been reported for a PTO radical anion system [55].

To fully investigate the electronic structures of 1K[(crypt-222)], 2K[(crypt-222)], and 3K, we carried out EPR measurements. The solution EPR spectra of 1K[(crypt-222)] and 2K[(crypt-222)] at room temperature show complicated signals with the g-tensor of 2.0043 for 1K[(crypt-222)] and 2.0041 for 2K[(crypt-222)] (Figure 4a,b). The signal of 1K[(crypt-222)] can be satisfactorily simulated with five sets of proton hyperfine coupling constants (a(H¹)) for center benzene unit (7.84 G) and periphery benzene unit (7.11, 5.75, 2.27, and 0.01 G), suggesting the central symmetry of 1K[(crypt-222)] in THF and a full delocalization of the unpaired electron, which is supported by calculated spin density distribution (UM052X/6-31G (d) level, Figure 4c). The solution EPR spectrum of 2K[(crypt-222)] arises from couplings with six sets of proton hyperfine coupling constants (the a(H¹) on the center benzene unit are 12.73 and 3.36 G and the a(H1) on the surrounding benzene unit are 6.70, 4.48, 4.28, and 2.80 G), indicating the C₂ symmetry of 2K[(crypt-222)] in solution and the full delocalization of the unpaired electron, which is also supported by calculated spin density distribution (UM052X/6-31G (d) level, Figure 4d). The solution EPR spectrum of 3K shows a single peak with g = 2.0039 (Figure S7). However, the hyperfine splitting was not observed, and the EPR spectrum of 3K cannot be simulated.

Crystals of 2[K(crypt-222)]₂ suitable for X-ray crystallographic analysis were obtained from the THF solution at room temperature, whereas attempts to grow crystals of the potassium salt of 1^2−•• and 3^2−•• failed. 2[K(crypt-222)]₂ crystallizes in the triclinic space group P$\overline{1}$. Crystallographic analysis reveal that each potassium ion is coordinated by one 2,2,2-cryptand, fully separated from the 2²⁻ anion part (Figure 5). Notably, both potassium ions do not interact with the O atoms in anion part. The shortest distance between the dianions in 2K[(crypt-222)]₂ is d_C-C = 3.605(3) Å (Figure S5). Similarly to the cases of 1^−• and 2^−•, there is no π–π stacking between 2^2−••. Upon the initiation of two-electron reduction, the dianionic backbones in 2[K(crypt-222)]₂ maintain planarity. Two potassium ions in 2[K(crypt-222)]₂ are almost equivalent, with the O1–C15 and O2–C19 distances of 1.271 and 1.272 Å, respectively. Compared with anion 2^−•, the C–O bonds in 2²⁻ is elongated.

The magnetic properties of 2[K(crypt-222)]₂ were examined by EPR and SQUID measurements. The detection EPR spectrum for 2[K(crypt-222)]₂ does not show the signal typical for spin-triplet species and the solid of 2[K(crypt-222)]₂ is SQUID inactive (Figure S8), suggesting that 2[K(crypt-222)]₂ has a large singlet-triplet energy gap. To further understand the ground-state electronic structure of 2²⁻, calculations for 2²⁻ were performed. The geometry optimizations of the closed-shell singlet (CS) state, open shell singlet (OS) state, and triplet (T) state were performed at the (U)M052X/6-31G(d) level. It has an open-shell singlet ground state with energy gap ΔE_os-t of −7.40 kcal mol⁻¹ (Table S6), which is so large that its excited triplet state cannot be accessible at room temperature, which is consistent with the experimental results. The S² value of 0.7502 for the OS state (Table S6) deviates from the theoretical ideal of S² = 1.0 for a pure OS state. This intermediate S² value may arise from a mixed ground state involving contributions from both CS and OS states, rather than a pure OS state. The Mulliken spin-density distribution reveals that the unpaired electrons are delocalized throughout the whole π-system (Figure 6).

4. Conclusions

In summary, we isolated and characterized three potassium salts of IFO radical anions, namely 1^−•, 2^−•, and 3^−•, and a dianion 2^2−••. The polarization of electron spin in 3K is induced by potassium ion coordination, suggesting that the electrostatic effect by potassium ion has a great influence on the spin density modulation. Experimental and theoretical results demonstrate that the dianion 2^2−•• is an open-shell singlet in the ground state with a very large singlet–triplet energy gap so that their excited triplet states cannot be thermally accessible at room temperature. This work provides the fundamental properties of the IFO based anionic species and also lays an experimental foundation for the study of the mechanism of the electron-transport materials based on IFO.