Deep-red circularly polarised luminescent C70 derivatives.

Optically active fullerenes, including C60 and C70 derivatives carrying organic substituents, are used in a range of applications because of their unique spectroscopic, catalytic, and chiral recognition properties. However, their inherent photoexcited chirality is yet to be elucidated because of their very poor fluorescence quantum yield (Φf). We synthesised a new chiral C70 derivative, X70A, with 20% yield, by reacting bis-borylated xanthene with C70 in a one-step double addition reaction, followed by a successful optical resolution. The isolation of two separate X70A enantiomers was confirmed by mirror-image circular dichroism spectroscopy in the range of 300-750 nm. In toluene, the enantiomeric pair of X70A clearly revealed mirror-image circularly polarised luminescence (CPL) spectra with a high |glum| value of 7.0 ×  10-3 at 690 nm. The first fullerene-based deep-red CPL of X70A should provide a new guideline for the design of chiral nanocarbon materials.

Introduction

Achiral buckminsterfullerene (C60) and [5,6]-fullerene (C70) adopt highly symmetrical spherical and elliptical structures, respectively, allowing them to be utilised as n-type molecular semiconductors and building blocks of molecular conductors/magnets owing to the uniqueness of their energetically low-lying lowest unoccupied molecular orbitals (LUMOs)[1,2]. The LUMO characteristics of C60 and C70 in solution permit reversible acceptance and release of up to six electrons via an electrochemical redox process[3]. In addition, K3C60[4] and Rb3C60[5] have been shown to exhibit superconductivity at critical temperatures (Tc) of ~ 20 and 30 K, respectively. In addition, a recent study indicated the possibility of Tc =  ~ 150 K when femtosecond laser pulses excite the phonon modes of K3C60 at 0.3 GPa[6]. Furthermore, the charge-transfer (CT) complex of C60 with an electron-donating molecule has been reported to undergo a paramagnetic-ferromagnetic transition at Tc =  ~ 17 K[7]. Moreover, photoinduced CT processes between fullerene derivatives and π-conjugated polymers have been found to efficiently generate electron and hole carriers with enhanced mobilities, thereby improving the performances of organic photovoltaic solar cells[8,9].

Early photoluminescence (PL) studies have elucidated that fullerenes can emit fluorescence (FL), but in very low quantum yields (Φf)[10-13], and that the singlet (S1)–triplet (T1) intersystem crossing (ISC) occurs nearly quantitatively due to large spin–orbit coupling (SOC)[14]. Thus, a thermally activated delayed FL (TADF) is possible owing to the small S1–T1 energy gap (ΔES-T)[15-19]. However, the lack of a high Φf at the S1–S0 transition remains an obstacle when fullerene derivatives are applied to several photonic applications.

Molecular chirality and helicity play key roles in biomolecular and human-made materials science, facilitating the introduction of a perturbation to the photoexcited and ground states. In solution, the majority of chiral organic luminophores exhibit circularly polarised luminescence (CPL) in the UV–visible region[20]. For example, a few helicenes, as helical nanocarbon molecules, have been shown to emit CPL in the visible region up to 800 nm[21,22]. However, nanocarbon materials that exhibit a large dissymmetry factor over long wavelength regions, such as the deep-red region, have rarely been investigated. This is due to the fact that the molecular design that can simultaneously achieve chirality and an effective π-conjugation whose absorption reaches the deep-red region is still unexplored. In this context, we attempt the rational design of the π surfaces of C70[23-25] to produce chiral deep-red luminophores. As a result, a chiral C70 derivative, X70A, was synthesised by reacting bis-borylated xanthene with C70 in a one-step double addition reaction, followed by successful purification by high performance liquid chromatography (HPLC) using a chiral separation column.

Thus, we herein report the first deep-red mirror-image CPL spectra at 690 nm, originating from a pair of chiral fullerene derivatives, which are associated with the corresponding mirror-image circular dichroism (CD) spectra upon the dissolution of left-handed and right-handed X70A in toluene. We believe that the results obtained for this CPL-exhibiting X70A will provide useful guidelines for the future material design of nanocarbon light-emitting materials that emit in the deep-red to near infrared (NIR) region with high g-values[26-28].

Results

Molecular and reaction design

Due to the fact that C70 possesses five non-equivalent carbon atoms (a–e, see Fig. 1a), the a and b atoms, which have a high angular distortion because of their proximities to the two poles, inherently exhibit a high reactivity toward several nucleophiles[29]. The reactivities of the α-site (a–b double bond) and the β-site (c–c double bond) are higher than those of other sites, facilitating the rational design of double bond-selective reactions[30-33]. It should be noted here that when two C–C double bonds are inequivalent to a symmetrical plane, C60 and C70 form chiral electronic structures due to symmetry breaking. In 1998, Diederich et al. synthesised racemic mixtures of bis-adducts of C60 and C70 by the Bingel reaction[34], leading to the successful resolution of enantiomerically pure compounds.

Figure 1

Schematic representation of the short distance tether-directed remote functionalisation of C70. (a) Ball-and-stick model of C70 with carbon labels: a (red), b (orange), c (yellow), d (green), and e (purple). (b) Sequential reaction of the tethered-substrate with C70, which is bound to carbon a in the first addition reaction. Here, the second addition reaction is limited to the β site. The reaction leads to chiral racemic products given the equivalence of the β sites on the two sides adjacent to the α site. (c) Reaction of C70 with bis-borylated xanthene 1 to afford X70. cod = cyclooctadiene.

Following a nucleophilic reaction of the substrate on the most reactive carbon a of C70, the second addition reaction can be performed regioselectively on the successive β-site (c–c double bond), which produces a chiral bis-adduct (Fig. 1b). This tether-directed remote functionalisation[35-37] was first achieved for the selective preparation of a tris-adduct of C60[35]. In this study, we reacted two boronic acids, with a fixed short distance, on C70 with a rhodium catalyst using Itami’s method[38,39]. The xanthene skeleton was selected as a very short-tethered boronic acid. Bis-borylated xanthene 1 was prepared from 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene in 72% yield[40].

Isolation and characterisation of the X70 family

Bis-borylated xanthene 1 was reacted with C70 in the presence of a catalytic amount of [Rh(cod)(MeCN)2]BF4 in H2O/o-dichlorobenzene (1/4) at 60 °C for 6 h to yield the desired xanthene adducts (Fig. 1c)[39]. The obtained chromatogram (toluene/hexane = 1:1, v/v, COSMOSIL Buckyprep column, Nacalai Tesque Inc.) of the reaction mixture is shown in Supplementary Fig. S1. Mass spectrometric analysis of the products revealed that the first eluent contained bis-xanthene adducts, while the subsequent fractions contained the six mono-xanthene adducts (Supplementary Figs. S2–S7), i.e., X70A, X70B, X70C, X70D, X70E, and X70F. The obtained yield of X70A was moderately high (~ 20%).

The 1H and 13C nuclear magnetic resonance (NMR) spectra of X70A, X70B, and X70F are shown in Fig. 2 and Supplementary Figs. S8–S13. In Fig. 2, the red and blue circles indicate the proton peaks of xanthene and fullerene, respectively. Asymmetric X70A was characterised by four doublet peaks arising from the xanthene component, and two singlet peaks originating from the fullerene framework. The two singlet peaks observed for the fullerene framework indicate that the C70 carbon atoms at the 1,3-positions (carbons a and c) reacted preferentially. Formation of the highly symmetric X70B was confirmed by the observation of two doublet peaks corresponding to xanthene, and one singlet peak originating from the fullerene framework, indicating that the equivalent carbon b had reacted.

Figure 2

1H NMR spectra of a series of X70 and illustration of the replacement positions on C70. (a) X70A in CDCl3, (b) X70B in CDCl3, and (c) X70F in CDCl3. * indicates an impurity peak. Red circles are assigned to the peaks of xanthene, blue circles are assigned to the peaks of the fullerene protons, and a purple circle is assigned to the peak of the hydroxy group. Ca, Cb, and Cc represent the xanthene carbon atoms; Ha, Hb, and Hc represent the hydrogen atoms attached to carbons a, b, and c, respectively. The structures of X70A, X70B, and X70F determined by the single-crystal X-ray analysis are shown in Fig. 3.

Figure 3

Structures of X70A, X70B, and X70F. Molecular structures of (a) X70A, (b) X70B, and (c) X70F and their ORTEP diagrams for single-crystal X-ray structures of (d) X70A, (e) X70B, and (f) X70F with 25% thermal ellipsoids. Solvent molecules and disordered parts are omitted for clarity.

The structures of X70A and X70B were determined by single-crystal X-ray structure analysis (Fig. 3a,b,d,e, and Supplementary Tables S1–S2). X70A is a product of double addition reactions at the 1,3-positions of the top six-membered ring of C70, which results in the generation of a chiral structure. Alternatively, X70B was formed from the addition reaction at carbon b at the 1,4-positions of C70, as supported by the 1H NMR results. X70A and X70B were named using the official fullerene IUPAC nomenclature[25], as indicated in Fig. 4 and Supplementary Fig. S14.

Figure 4

Schlegel diagrams of X70A, X70B, and X70F with enantiomeric numbering schemes: systematic numbering recommended by IUPAC; arrows indicate the direction of the numbering commencement. The full names for these compounds are listed in the Supplementary Information.

High-resolution matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (HR–MALDI–TOF–MS) of X70E and X70F detected X70A plus 16 and 32 mass units, respectively, suggesting that one and two oxygen atoms are inserted into the mono-xanthene adducts. The 1H NMR spectrum of X70F in CDCl3 was similar to that of X70A, exhibiting four doublet peaks originating from the xanthene component, in addition to two singlet peaks at 6.44 and 4.86 ppm (Fig. 2c).

The structure of X70F was unambiguously determined by X-ray diffraction analysis, where it was found that one of the C–C bonds was cleaved by oxygen (Figs. 3c,f, 4; Supplementary Table S3). This compound possesses one hydroxyl group on the xanthene moiety, and one oxygen atom at the β-site, indicating the presence of an epoxide structure. We confirmed that the singlet peak at 6.44 ppm in its 1H NMR spectrum disappeared after the addition of methanol-d4, indicating that this peak is derived from the hydroxyl group.

Although the formation mechanism of X70F remains unclear, we observed the conversion of X70A to X70F in solution under air, and so, X70F is considered to be produced by the air oxidation of X70A following its generation. The plausible structures and names for X70C, X70D, and X70E estimated from 1H and 13C NMR analyses (Supplementary Figs. S15–S19) are shown in Supplementary Fig. S20.

Photophysical properties of the enantiomerically purified X70A and X70F

The UV–visible absorption spectra of X70A, X70F, and pristine C70 are shown in Fig. 5a and Supplementary Table S4. In the UV–visible absorption spectra of X70A and X70F, the characteristic peaks of C70 were greatly suppressed and broadened, exhibiting the typical absorption shape of monosubstituted fullerenes, as reported in the literature[30].

Figure 5

UV–visible absorption and CD spectra. (a) UV–vis absorption spectra of X70A (blue line), X70F (red line), and for comparison, C70 (black line) in toluene. (b) Blue and red lines represent the CD spectra in toluene for the first (X70A1: 9.6 × 10−6 M) and second (X70A2: 5.1 × 10−6 M) fractions, respectively. (c) Blue and red lines represent the CD spectra in toluene of the first (X70F1: 6.1 × 10−6 M) and second (X70F2: 5.1 × 10−6 M) fractions, respectively.

Subsequently, the racemates of X70A and X70F were subjected to enantiomeric resolution using chiral separation column chromatography. Although the chromatogram of X70A in hexane/i-PrOH (4:1) did not show well-separated peaks after 24 cycles of the racemate, the first and second halves of the peaks clearly provided mirror-signed CD spectral profiles, indicating that enantiomeric resolution was possible (Supplementary Fig. S21). Furthermore, four repetitions of the enantiomeric resolution process using the first half of the peak succeeded in isolating mostly enantiomerically purified X70A[41,42]. Another chiral X70A was obtained using the second half of the peak of X70A, following six repetitions of the recycling enantiomeric resolution process (Supplementary Fig. S22). Enantiomerically purified X70Fs were obtained using the same separation procedure (Supplementary Figs. S23, S24). As a result, X70A1 and X70A2, as the first and second fractions, respectively, revealed ideal mirror-image CD spectra between 300 and 750 nm (Fig. 5b). The first and second fractions of X70F were named X70F1 and X70F2, respectively (Fig. 5c).

To determine the absolute structures of chiral fullerenes X70A and X70F, we compared the experimental CD spectra with the density functional theory (DFT)-calculated spectra between 300 and 750 nm using the SpecDis software package (Supplementary Figs. S25–S27; Supplementary Tables S5–S7)[43,44]. Both CD spectra (i.e. for X70A and X70F) were simulated with high similarity factors (0.89 for X70A and 0.81 for X70F, respectively), and thus, the absolute structure of the second eluent X70A2 was determined with high accuracy as (f,sA)-7,25-xantheno-7,8,22,25-tetrahydro(C70-D5)[5,6]fullerene, and the first eluent X70A1 was determined as (f,sC)-7,25-xantheno-7,8,22,25-tetrahydro(C70-D5)[5,6]fullerene (Fig. 4). In addition, the absolute structure of the second eluent X70F2 was determined as (f,sC)-25-(5′-hydroxyxanthenyl)-7,22-epoxy-7,8,22,25-tetrahydro(C70-D5)[5,6]fullerene, while the first eluent X70F1 was determined to be (f,sA)-25-(5′-hydroxyxanthenyl)-7,22-epoxy-7,8,22,25-tetrahydro(C70-D5)[5,6]fullerene.

The FL spectra of X70A and X70F are depicted in Fig. 6a along with that of C70 excited at 500 nm in degassed toluene at 20 °C for comparison. As indicated, X70A and X70F exhibit similar broad FL bands with vibronic shoulders in the range 600–850 nm. Based on a previous report that the Φf of C70 in toluene at 20 °C was ~ 0.06%[13], the relative Φf values of X70A and X70F were determined to be 0.1 and 0.2%, respectively (Fig. 6a). These enhancements are probably due to a lowering symmetry accompanying a weak polarity led by two substituents of the fullerene π-systems; C70 adopts achiral D5, while X70A and X70F are chiral C1-symmetry. Although the FL emission at ~ 700 nm cannot be detected with the naked eye, photosensitivity experiments carried out using a crystal silicon-based digital camera with detection up to 950 nm allowed the deep-red emission to be captured (inset of Fig. 6a). The FL lifetimes of X70A and X70F in deaerated toluene (3.0 × 10−5 M) were determined to be 0.99 and 1.31 ns, respectively, from which, we can determine the radiative (kf) and non-radiative (knr) rate constants to be 1.0 × 106 s−1 and 1.0 × 109 s−1 for X70A and 1.5 × 106 s−1 and 1.5 × 109 s−1 for X70F, respectively.

Figure 6

FL and CPL/PL spectra of X70A and X70F in toluene. (a) FL spectra of X70A (blue line), X70F (red line), and C70 (black line) in toluene excited at 500 nm with the absorbance adjusted at 0.1. Inset: photographic image of the deep-red FL from X70A1 excited at 450 nm taken using a digital camera (Leica, ASA 6400, f1.8, SS1/4). (b) CPL (top) and PL (bottom) spectra of X70A in toluene excited at 410 nm. Blue, red, and grey lines represent the CPL/PL spectra of the first and second fractions of X70A and the intact C70, respectively. (c) CPL (top) and PL (bottom) spectra of X70F in toluene excited at 500 nm. Blue and red lines represent the CPL/PL spectra of the first and second fractions, respectively. Note that X70F showed no obvious CPL spectra.

X70A1 and X70A2 clearly display mirror-image CPL spectra (Fig. 6b). To the best of our knowledge, these CPL spectra are the first ones observed for fullerene-based compounds, although optically active C76[45] and C60 adducts[46] were previously found to exhibit FL. The absolute glum values of X70A1 and X70A2, |glum|, were moderately high: 7.0 × 10−3 (λex = 410 nm, λem = 690 nm). It should be noted here that this glum value is among the highest in the deep-red to NIR regions for purely organic compounds that do not contain lanthanide metals[47]. No obvious CPL signals were observed for X70F1 and X70F2 (Fig. 6c). To account for this observation, we calculated the intersection of the electric and magnetic transition dipole moments of X70A and X70F. Interestingly, although X70F possesses two orthogonal electric and magnetic dipole moments, X70A does not (Supplementary Fig. S28). The orthogonal electric and magnetic dipole moments should result in the cancellation of the |glum| values, thereby accounting for the reduced CPL observed for these compounds[48]. Since the π-conjugated systems of both X70A and X70F are identical, the reason for the difference in the angles between the two types of moments can be attributed to the subtle electronic and steric effects of the substituents. In recent years, molecular design to control the angle and strength of the two transition moments has been studied intensively[49,50].

Discussion

To date, investigations into the photochemistry of fullerenes have mainly focused on subsequent electron transfer after photoexcitation and triplet energy transfer, for example, through the generation of singlet oxygen. In this study, we successfully synthesised and characterised a family of xanthene-attached C70 derivatives, X70, via a facile one-step reaction from C70. Furthermore, enantiomeric separation from the racemates of X70A and X70F was also achieved, and these compounds were found to exhibit significantly strong emission properties than C70. The enantiomeric pair of X70A clearly revealed ideal mirror-image CPL spectra ranging from the deep-red to NIR regions with a high |glum| value of 7 × 10−3 at 690 nm as a purely organic fluorophore. The corresponding Φf values are small because the singlet excited state of C70 is converted to the triplet excited state with an efficiency close to 100%. Also, the deep-red emission Φf of the fluorophores is small, owing to the smooth non-radiative pathway; hence, it is reasonable to aim for a molecular design that gives a large dissymmetry factor (g-value) for the deep-red luminophore.

It is well known that the transition electric dipole moment () is larger than the transition magnetic dipole moment (), and the glum value is inversely proportional to the absolute value of from the following relationship:

As can be seen from this equation, the molar absorption coefficient, which is directly proportional to the absolute value of , and the g-value generally have a trade-off relationship[51]. Thus, fullerenes with smaller molar absorption coefficients should be used for the S0–S1 forbidden transitions to achieve chiral luminophores with high g-values.

Although the Φf value of X70A is small, this is the first step in developing chiral fullerene luminescence. In this study, by comparing C70 with X70A and X70F, we have found that the improvement of fluorescence quantum yield can be achieved by a lower symmetrization on the fullerene π-system associated with an introduction of polar substituent(s), and that the difference of substituted pattern on the fullerene also changes the strength and angle of electric and magnetic transition dipole moments (Supplementary Fig. S28) and thus greatly affects the dissymmetry factor. We believe that the strategy for developing a molecule that can exhibit a high g-value in the deep-red region is valuable and can be applied for molecular design in the near future[26-28].

Methods

General methods

C70 (purchased from SES Research Inc.) was purified using a Buckyprep column and degassed at 20 °C prior to carrying out any spectroscopic measurements. See the Supplementary Methods for further details.

Syntheses of X70A–X70F

A Schlenk flask was flame-dried under vacuum and filled with argon. Dry o-dichlorobenzene (140 mL) and H2O (36 mL) were added to this flask under a stream of argon. After performing three freeze–pump–thaw cycles, [Rh(cod)(MeCN)2]BF4 (45 mg, 0.18 mmol), C70 (500 mg, 0.59 mmol), and 2,7-di-tert-butyl-9,9-dimethylxanthene-4,5-diboronic acid (275 mg, 0.71 mmol) were added to the flask under a stream of argon. After stirring the mixture at 60 °C for 6 h, it was cooled to 20 °C. The organic layer was separated, passed through a pad of Celite and silica gel, and washed with toluene. The filtrate was concentrated and purified using a Buckyprep column (toluene/hexane (v/v) = 1:1 eluent) to afford X70A (140 mg, 20%), X70B (0.8 mg, 0.1%), X70C (12 mg, 1.0%), X70D (13 mg, 1.1%), X70E (0.7 mg, 0.1%), and X70F (3.1 mg, 0.3%) as brown solids. Spectral data for all compounds are provided in the Supplementary Information.

HPLC purification

Preparative HPLC system was constructed using a ϕ10 × 250 mm Buckyprep column (Nacalai Tesque Inc., Kyoto, Japan), a JASCO UV-2075 Plus detector, and a JASCO PU-2086 Plus pump. Eluent: toluene/hexane = 1/1, v/v Temperature: 20 °C, flow rate: 3.0 mL/min, injection volume: 3.0 mL, and detection: UV absorption at 326 nm. Chiral resolutions of X70A and X70F were performed at 20 °C using a ϕ10 × 250 mm Cholester column (Nacalai Tesque Inc.) fitted to a recycling preparative HPLC system, which was constructed using a JASCO UV-2075 Plus detector and a JASCO PU-2086 Plus pump. Eluent: hexane/i-PrOH = 4/1 (v/v), flow rate: 4.5 mL/min, injection volume: 3.0 mL, and detection: UV absorption at 326 nm.

CD measurements

The CD spectra were recorded using a JASCO J-820 spectropolarimeter.

CPL measurements and analysis

Artefact-free PL and CPL spectra were obtained using a JASCO CPL-200 spectrofluoropolarimeter, which allowed us to avoid second- and third-order stray light due to diffraction grating. The spectrofluoropolarimeter was designed as a prism-based spectrometer with a forward scattering angle of 0°, and it was equipped with focusing and collecting lenses. In addition, a movable cuvette holder fitted on an optical rail enabled adjustment of the best focal point to maximise the PL and CPL signals. Simultaneous CPL and PL measurements allowed the quantitative evaluation of the degree of CPL efficiency relative to the PL, known as Kuhn’s dissymmetry factor (glum), which is defined as glum = (IL − IR)/[(IL + IR)/2], where IL and IR refer to the intensities of the left- and right-handed CPL, respectively. The glum value was evaluated as glum = [ellipticity (mdeg)/32,980/ln10] / PL amplitude (Volts) at the CPL extremum.

Supplementary Information 1.