Single-photon oxidation of C₆₀ by self-sensitized singlet oxygen

Abstract

C₆₀ is regarded as the most efficient singlet oxygen (¹O₂) photosensitizer. Yet, its oxidation by self-sensitized ¹O₂ remains unclear. The literature hints both oxygen and C₆₀ must be at excited states to react, implying a two-photon process: first, oxygen is photosensitized (¹C₆₀•¹O₂); second, C₆₀ is photoexcited (¹${\mathrm{C}}_{60}^{\ast}$•¹O₂). However, this scheme is not plausible in a solvent, which would quench ¹O₂ rapidly before the second photon is absorbed. Here, we uncover a single-photon oxidation mechanism via self-sensitized ¹O₂ in solvents above an excitation energy of 3.7 eV. Using excitation spectroscopies and kinetics analysis, we deduce photoexcitation of a higher energy transient, ³${\mathrm{C}}_{60}^{{\ast}{\ast}}$•³O₂, converting to ¹${\mathrm{C}}_{60}^{\ast}$•¹O₂. Such triplet-triplet annihilation, yielding two simultaneously-excited singlets, is unique. Additionally, rate constants derived from this study allow us to predict a C₆₀ half-life of about a minute in the atmosphere, possibly explaining the scarceness of C₆₀ in the environment.

Introduction

Buckminsterfullerene (C₆₀), the most abundant fullerene as well as the most symmetric molecule in nature, has been studied extensively since its discovery in 1985¹. Among the outstanding properties of C₆₀ are ability to accept up to six electrons, high intersystem crossing (ISC) quantum yield, and long-lived triplet states², which have stimulated a thriving research effort for applications in photovoltaics³, photocatalysis⁴, and molecular probes⁵. Additionally, the latter two attributes make C₆₀ an efficient singlet oxygen (¹O₂) sensitizer, particularly promising for photodynamic therapy and environmental remediation^6,7,8.

However, C₆₀ is subject to photodegradation in these applications at ambient temperature (while thermal oxidation of C₆₀ occurs at 370 K and above)^9,10. The initial work on photooxidation (PO) of C₆₀ credited it to ozonation^11,12,13, which however is limited to excitation wavelengths shorter than 240 nm for photogeneration of O₃¹⁴. Later, however, another reactive oxygen species became the suspect, ¹O₂^15,16,17. To this end, the most seminal findings have been: (i) unless photoexcited, C₆₀ does not react with externally-produced ¹O₂¹⁶; (ii) PO can occur under UV excitation (308 nm) in O₂ ambient (albeit formation of ¹O₂ was not corroborated)¹⁷; and (iii) C₆₀O is the major photoproduct^16,17,18. Thus, the PO reaction is anticipated as ${\mathrm{C}}_{60}^{\ast} + {\,}^{1}{\mathrm{O}}_{2} \to {\mathrm{C}}_{60}{\mathrm{O}} + \frac{1}{2}{\mathrm{O}}_{2}$, where both C₆₀ and O₂ must be photoexcited. However, the mechanistic details of the photophysics and photochemistry remain unelucidated. A historical review of the C₆₀ PO literature is provided in Supplementary Note 1.

C₆₀ has long been known to be an efficient ¹O₂ sensitizer. Yet, no evidence has been shown that ¹O₂ reacts with its original C₆₀ sensitizer, which we refer to as “oxidation with self-sensitized ¹O₂”. Here, we present experimental evidence for this phenomenon. Although the lifetime of ¹O₂, τ, in air is exceptionally long (i.e., 45 min)¹⁹, it shortens to microseconds to nanoseconds in solvents. Inspired by this broad range of τ, we investigated PO of C₆₀ in hexane (C₆H₁₄), chloroform (CHCl₃), and carbon tetrachloride (CCl₄), where τ is 30, 207 and 87,000 μs, respectively²⁰. Our kinetics study reveals C₆₀ concentration decays exponentially under UV excitation and the decay rate increases with τ. We also show the decay dominantly occurs as a single-photon process above the photon energy (hυ) threshold of 3.7 eV, being the onset of 1¹A_g → 2¹H_u transition in C₆₀.

Results and discussion

Absorption spectroscopy

Figure 1a, b shows time series optical absorption spectra for our slowest and fastest PO kinetics, which occur in C₆H₁₄ and CCl₄, respectively. The major C₆₀ absorption peaks at 256 and 328 nm are seen to decrease systematically, while the baseline rises indicative of a photoproduct, which is also evident from yellowing of the solution (Fig. 1b, inset). The spectrum of the excitation source (Fig. 1c) consists of a major narrow band peaking at 310 nm with no emissions below 250 nm. Hence, the possibility of O₃ generation is ruled out¹⁴. In Fig. 1b, the noise below 255 nm is due to the high absorption of CCl₄ attenuating the optical beam. However, it does not deteriorate the accuracy of the absorption peak of C₆₀ at 260 nm in CCl₄ (see Supplementary Discussion).

Fig. 1: Time series absorbance spectra of C₆₀ under UV excitation of 3.74 mW/cm².

a In C₆H₁₄; b in CCl₄. The inset of (b) shows photos of C₆₀ solutions, unexposed and after 8 min of UV exposure. c The spectrum of the excitation source, UVP XX-15 UV lamp.

Phosphorescence spectroscopy

In Fig. 2a, the phosphorescence peak at 1273 nm substantiates photosensitization of ¹O₂ by C₆₀ in the three solvents in our study. Figure 2b plots the measured phosphorescence peak intensity versus quenching rate constant (k_q = 1/τ). Here, values of τ for the three solvents are borrowed from ref. ²⁰ as listed above. The experimental data points match with the theoretical trend (Supplementary Eq. (48)). Hence, we confidently adopt the τ values from the literature.

Fig. 2: Phosphorescence of ¹O₂ sensitized by C₆₀.

a Spectra in different solvents (under 375 nm radiation of 9.5 mW/cm² intensity). Here, [C₆₀] in C₆H₁₄ is 3.92 times higher than the usual concentration. b Match of theoretical intensity (I) with experiment (in (a)) validating the k_q values adopted from the literature. k_s2 (0.4 s⁻¹) is the sensitization rate by C₆₀. The 2 in the subscript indicates $h\upsilon$ < 3.7 eV (Supplementary Results).

Vibrational spectroscopy

PO of C₆₀ is also characterized by the Fourier-transform infrared (FTIR) spectra in Fig. 3a, where C–O, C═O, and O–H stretching vibrations are indicative of C₆₀ oxidation¹³. We anticipate the O–H groups result from the Norrish type II reaction²¹. The evolution of C–H vibrational peaks suggest fragmentation of the C₆₀ cage subsequent to PO. The peak frequencies and important assignments are shown in Fig. 3b^22,23. Detailed peak assignments are given in Supplementary Table 1.

Fig. 3: Time series FTIR spectra.

a C₆₀ in CHCl₃ under the same UV exposure conditions as in Fig. 1. b Assignment of FTIR peaks after 6 h of UV exposure (ρ: rocking; δ: bending; υ: stretching).

Mechanism of C₆₀ photooxidation in solvents

At first, we are inclined to explain the oxidation of C₆₀ by its reaction with free ¹O₂. In this model, ¹O₂ is released to the solvent after photosensitization by a C₆₀. Subsequently, it collides and reacts with a C₆₀ unless quenched by the solvent. This straightforward model (detailed in Supplementary Information; Kinetics Model) is consistent with our observation that PO rate increases with τ. Additionally, its rate is quadratic in [C₆₀] as well as radiation intensity. On the contrary, the kinetics of C₆₀, as monitored from optical absorption (Fig. 4a) suggests exponential decay, i.e., $\frac{\mathrm{d}}{{\mathrm{d}t}}\left[ {\mathrm{C}}_{60} \right] = - k_{\mathrm{pd}}\left[ {\mathrm{C}}_{60} \right]$, where k_pd is the C₆₀ photodecay rate. Additionally, Fig. 4b establishes a linear dependence of k_pd on excitation intensity, and hence a single-photon process. Thus, we rule out “oxidation with free ¹O₂” as the dominant PO mechanism. Instead, consistent with the observed exponential decay, we propose “oxidation with self-sensitized ¹O₂”, where a C₆₀ molecule photosensitizes a ¹O₂ and reacts with that same ¹O₂ in a collision complex:

$$ {\mathrm{C}}_{60} + {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{collision}}}{\mathrm{C}}_{60} \bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{hv}}{\mathrm{C}}_{60}^ \ast \bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{sensitization}}}\\ \hskip 50pt {\mathrm{C}}_{60}\bullet {\,}^{1}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{\mathrm{oxidation(?)}}{\mathrm{C}}_{60}{\mathrm{O}}$$

(1)

Fig. 4: C₆₀ oxidation kinetics.

a C₆₀ time decay in different solvents. A is optical absorbance at 256 nm (peak), A_o being the original value (prior to exposure). Although the photoproduct baseline is not subtracted, the slopes represent exponential decay rates (k_pd) with minimal error as corroborated in Supplementary Discussion. The UV exposures are same as in Fig. 1. b k_pd in CCl₄ as a function of irradiation intensity.

Yet, Scheme (1) has a flaw with the oxidation step, where C₆₀ and ¹O₂ will not react, because C₆₀ must be excited to ${\mathrm{C}}_{60}^{\ast}$. To meet this condition, a two-photon process could be proposed, where the first photon excites C₆₀ to sensitize ¹O₂ and the second one excites C₆₀ to a high energy singlet state, which thereafter reacts with ¹O₂:

$$ {\mathrm{C}}_{60} + {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{collision}}}{\mathrm{C}}_{60} \bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{h\upsilon }}{\mathrm{C}}_{60}^ \ast \bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{sensitization}}}\\ \hskip 25pt {\mathrm{C}}_{60}\bullet {\,}^1{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{h\upsilon }}{\mathrm{C}}_{60}^ \ast \bullet {\,}^1{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{oxidation}}}{\mathrm{C}}_{60}\mathrm{O}$$

(2)

However, a two-photon process is already excluded by our results (i.e., Fig. 4b). Additionally, Scheme (2) can be ruled out by fundamental considerations. Even the longest τ (0.087 s in CCl₄) is significantly shorter than the period between two subsequent excitations of C₆₀, being 3.7 s (see Supplementary Information; Kinetics Model). Therefore, before the second photon absorption occurs in Scheme (2), C₆₀•¹O₂ will relax to C₆₀•³O₂ with a high probability. In other words, the excited O₂ and excited singlet C₆₀ will hardly coincide in time.

Accordingly, we are urged to consider a scheme, which allows simultaneous excitation of O₂ and C₆₀, after which they coexist and react. Scheme (1) considers the most basic photosensitization event, where C₆₀ returns to its ground singlet state after imparting its energy to O₂. On the other hand, it is possible that C₆₀ returns to an excited singlet state, ${\mathrm{C}}_{60}^{\ast}$, (if it is photoexcited to a sufficiently high energy singlet state, ${\mathrm{C}}_{60}^{{\ast}{\ast}}$). Accordingly, Scheme (1) may be modified to:

$$ {\mathrm{C}}_{60} + {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{collision}}}{\mathrm{C}}_{60} \bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{h\upsilon }}{\mathrm{C}}_{60}^{ \ast \ast } \bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{sensitization}}}\\ \hskip 50pt {\mathrm{C}}_{60}^{\ast}\bullet {\,}^{1}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{oxidation}}}{\mathrm{C}}_{60}{\mathrm{O}}$$

(3)

The lowest energy ¹${\mathrm{C}}_{60}^{\ast}$ is 2.33 eV above the ground state. Additionally, ¹O₂ sensitization requires 0.98 eV while 0.37 eV is lost to exchange interaction during singlet-to-triplet conversion²⁴. Therefore, a minimum excitation energy (hυ) of 3.68 eV is needed for Scheme (3) (Supplementary Fig. 4) to succeed. Consistently, our investigation using 455 and 395 nm LED excitations (2.73 and 3.14 eV) with similar photon-count exposures as in Fig. 1 yielded no detectable PO, although we confirmed ¹O₂ sensitization for these excitations from the 1273 nm phosphorescence (Supplementary Fig. 1). In the below two paragraphs, we experimentally corroborate 1¹A_g → 2¹H_u is the major driver of C₆₀ PO in the solvents. Interestingly, 1¹A_g → 2¹H_u starts at 3.72 eV²⁵, being very close to the threshold energy for PO (Scheme (3)).

In Fig. 5a, excitation spectrum for ¹O₂ phosphorescence (sensitization) closely follows C₆₀ absorption spectrum from 700 nm down to 370 nm. This trend is consistent with constant and near-unity ¹O₂ photosensitization quantum yield by C₆₀, Φ_s, as established in the literature²⁴. Figure 5b shows the deconvolution of the excitation spectrum to Gaussians below 400 nm. Each band marks an optical transition. Although these transitions may also be resolved from optical absorption, their deconvolution is more facile from our excitation spectrum.

Fig. 5: Excitation spectra.

a Overlay of the absorption spectrum of C₆₀ (black) and excitation spectrum for photosensitization of ¹O₂ by C₆₀, monitored from ¹O₂ phosphorescence counts at 1270 nm (red). b Deconvolution of the phosphorescence excitation spectrum. c Overlay of normalized k_pd at different excitation wavelengths (black) and the deconvoluted 2¹H_u band (blue). Confidence interval error bars are shown (red) after 3 independent measurements.

Below 370 nm, however, Φ_s diverges from the absorption spectrum and drops significantly. On the other hand, the excitation spectrum for oxidation (i.e., k_pd) in Fig. 5c (“Methods”) exhibits a reverse trend. The PO rate, k_pd, is essentially zero for the spectral range, where Φ_s is at its maximum value of unity, but it is activated at the threshold of about 335 nm (3.70 eV), at which Φ_s is reduced to 0.37. Hence, the excitation trends in Fig. 5a, c, being spectrally different, underscore the fact that sensitization of ¹O₂ is not sufficient for the oxidation of C₆₀. Specifically, as seen in Fig. 5c, the k_pd spectrum matches the 2¹H_u band. These findings validate Scheme (3) as well as 1¹A_g → 2¹H_u being the major driver of PO. Here, the normalized k_pd values were derived from the ¹O₂ phosphorescence intensity (i.e., counts proportional to [C₆₀]) kinetics (Supplementary Fig. 3).

While 1¹A_g → 2¹H_u is the major driver of C₆₀ PO (Scheme (3)), 1¹G_u, 1¹T_1u, and 2¹G_u states can also be excited to their vibronic levels higher than 3.7 eV (from 1¹A_g), as inferred from their deconvoluted bands in Fig. 5b. However, vibrational relaxation (VR) is the fastest process, quickly quenching 1¹G_u, 1¹T_1u, and 2¹G_u to their ground vibrational levels at 3.12, 3.40, and 3.43 eV, respectively (Fig. 6). Hence, ISC from these singlet states at above 3.7 eV is expected to be outcompeted by VR. Alternatively, PO (Scheme (3)) is possible from 1¹G_u, 1¹T_1u, and 2¹G_u vibronic states, if they transition to 2¹H_u by internal conversion (IC) before VR to below 3.7 eV. Because both IC and ISC are slower than VR by an order of magnitude or more, PO from 1¹G_u, 1¹T_1u, and 2¹G_u vibronic states will be minor, but may not be negligible. In conclusion, the major PO is expected to be through direct excitation of 2¹H_u. Accordingly, k_pd spectrum (data points) in Fig. 5c follows the 2¹H_u band. However, some deviation is seen, being highest for the 3.88 eV (320 nm) data point and over the 2¹H_u Gaussian, suggestive of additional excitations contributing, possibly through 1¹G_u, 1¹T_1u, and 2¹G_u as discussed above.

Fig. 6: Jablonski diagram illustrating photooxidation of C₆₀.

To a first approximation, we adopt the energy structure of isolated C₆₀ for the C₆₀ of C₆₀•O₂. The gray curve represents the absorbance spectrum of C₆₀.

We illustrate Scheme (3) with the gray arrows in the Jablonski diagram of Fig. 6. First, C₆₀ is photoexcited through 1¹A_g(¹C₆₀) → 2¹H_u(¹${\mathrm{C}}_{60}^{{\ast}{\ast}}$). Then, ¹${\mathrm{C}}_{60}^{{\ast}{\ast}}$ transitions to ³${\mathrm{C}}_{60}^{{\ast}{\ast}}$ (2¹H_u → 2³H_u) via ISC. Subsequently, triplet–triplet annihilation (TTA)²⁶ occurs with sensitization of ¹O₂:$2^{3}{\mathrm{H}}_{\mathrm{u}}\bullet{\,}^{3}{\mathrm{O}}_{2}\mathop{\rightarrow}1^{1}{\mathrm{T}}_{\mathrm{1g}}\bullet{\,}^{1}{\mathrm{O}}_{2}$. TTA also leaves C₆₀ at an excited singlet state (1¹T_1g), which can readily react with ¹O₂:$1^{1}{\mathrm{T}}_{\mathrm{1g}}\bullet{\,}^{1}{\mathrm{O}}_{2}\mathop{\rightarrow}{\mathrm{C}}_{60}{\mathrm{O}}$. Hence, both 1¹T_1g and ¹O₂, two energetic species, are created at the same time and same place (in collision complex) and have a higher chance to react. Another useful interpretation is that a single photon’s energy ($h\upsilon$) is partially utilized in sensitizing ¹O₂ while the excess energy leaves C₆₀ at an excited state, which can react with ¹O₂. Scheme (3) may be expressed in more detail as:

$$ {\,}^{1}{\mathrm{C}}_{60} + {\,}^{3}{\mathrm{O}}_{2} \to {\,}^{1}{\mathrm{C}}_{60} \bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{UV}}}{\,}^{1}{\mathrm{C}}_{60}^{ \ast \ast } \bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{ISC}}}\\ \hskip 25pt {\,}^{3}{\mathrm{C}}_{60}^{ \ast \ast }\bullet {\,}^{3}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{\mathrm{TTA}}}{\,}^{1}{\mathrm{C}}_{60}^{\ast} \bullet {\,}^{1}{\mathrm{O}}_{2} \to {\mathrm{C}}_{60}{\mathrm{O}}$$

(4)

We provide a mathematical analysis of PO kinetics for Scheme (3), which considers all the steps as well as reverse/competing processes, such as ¹O₂ quenching, relaxation of ${\mathrm{C}}_{60}^{\ast}$, and complex dissociations (see Supplementary Information; Kinetics Model). Despite the full complexity of this model, it predicts simply an exponential decay for [C₆₀], being consistent with the measured kinetics. The model allows us to write k_pd as a function of ¹O₂ quenching rate, k_q = 1/τ. Fitting of this function to experimental data (Fig. 7) reveals the rate constants, k_ox and k_r, associated with ${\mathrm{C}}_{60}^{\ast} \bullet {\,}^{1}{\mathrm{O}}_{2}\mathop { \to }\limits^{k_{\mathrm{ox}}} {\mathrm{C}}_{60}{\mathrm{O}}$ and ${\mathrm{C}}_{60}^{\ast} \bullet {\,}^{1}{\mathrm{O}}_{2}\mathop{\longrightarrow}\limits^{{k_r}}{\mathrm{C}}_{60} \bullet {\,}^{1}{\mathrm{O}}_{2}$, respectively, which has an interesting implication, as discussed below.

Fig. 7: Fitting of theoretical k_pd expression to experimental data.

The fitted values of k_ox and k_r are given in the inset.

Mechanism of C₆₀ photooxidation in the atmosphere

PO of C₆₀ is expected to be significantly accelerated in the atmosphere thanks to dramatically prolonged τ in the air (i.e., tens of minutes). Indeed, PO can dominantly occur as a two-photon process (Scheme (2), illustrated in Supplementary Fig. 5) driven by visible and UVA photons, being abundant in solar radiation. Unlike in solvents, it is challenging to monitor PO of C₆₀ in air, since C₆₀ being at detectable concentrations in air, would quickly undergo aggregation as well as adsorption to enclosure walls. However, k_pd can be predicted from the rate constants, k_ox and k_r, which are already captured in the present work from C₆₀ dispersions in solvents. As such, we compute k_pd = 0.011 s⁻¹ for AM 1.5 solar radiation, suggesting a half-life of 63 s (see Supplementary Information; Kinetics Model). This rapid PO of C₆₀ in the atmosphere potentially explains its scarceness in the environment²⁷.

Two excited singlets by triplet–triplet annihilation

By its description in the literature, TTA involves Dexter energy transfer from a triplet to another, after which the acceptor transitions to a higher energy state (singlet), while the donor returns to its ground singlet state. On the other hand, Scheme (3) engages a unique TTA, which produces two excited singlets simultaneously and thereby enables an efficient photochemistry. This scheme is stimulating for the conception of novel efficient photochemical processes, implemented with C₆₀ or other photosensitizers.

Methods

C₆₀ solution preparation

A stock solution of C₆₀ was first prepared by dissolving 1 mg of C₆₀ (Thermo Fisher Scientific, >99.9%) in 10 mL of solvent via ultra-sonication for 30 min. Next, the solution was kept undisturbed in the dark for 30 min to let insoluble aggregates (e.g., C₆₀O) settle down. Subsequently, the supernatant was transferred by a pipette to a spectrophotometer cell (optical path length of 10 mm) filled with the same solvent until the absorbance of C₆₀ at 256 nm reaches 1.42 (as monitored by a spectrophotometer), corresponding to C₆₀ concentration of 5.67 μM. Finally, the prepared solution was stored in a sealed glass vial and kept in the dark before use. High purity of C₆₀ in the prepared C₆₀ solution is confirmed by mass spectrometry (Supplementary Fig. 2).

Excitation spectroscopy for photooxidation

Photooxidation (photodecay) rate of C₆₀, k_pd, was measured as a function of excitation wavelength using the Fluorolog-3 spectrofluorometer. In a typical acquisition, 100 μL of C₆₀ in CCl₄ solution was placed in a standard microfluorescence cuvette (Science Outlet, 10 mm optical length, 0.35 mL capacity) and excited at selected wavelengths (from 250 to 420 nm, at 10 nm intervals) using a bandpass of 5 nm. The emission (¹O₂ phosphorescence) was parked at 1270 nm with a bandpass of 30 nm. At each excitation wavelength, the acquisition was performed 3 times and an unexposed sample was employed at each acquisition. The time-series phosphorescence intensity was collected in-situ at every 2 s with detector integration time of 2 s. Hence, the monochromatic optical beam of the spectrometer served as a dual probe simultaneously for measurement and exposure. For each excitation wavelength, k_pd was derived from the exponential decay rate of the phosphorescence intensity, quantifying the decay rate of C₆₀ concentration (see Supplementary Fig. 3).