Photoinduced formation of singlet oxygen with PCBM–Pd polymer in solution and at the surface

Abstract

The polymeric material of PCBM–Pd was prepared using both electrochemical and chemical synthesis. The electrochemical synthesis was carried out in acetonitrile/toluene (1 : 4, v/v) solution containing PCBM, palladium acetate, and tetra(n-C₄H₉)₄NClO₄ as the supporting electrolyte. The polymeric film was grown at the Au electrode surface under cyclic voltammetry conditions. Chemical synthesis was performed in a benzene solution containing PCBM and Pd₂(dba)₃·CHCl₃. This study investigates singlet oxygen's morphology, structural properties, and photochemical generation in this synthesized n-polymer. The morphology of the PCBM–Pd polymer was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and nitrogen adsorption/desorption isotherms (BETs) were used for the structural characterization of this polymeric material. The formation of ¹O₂ by the photoactive PCBM–Pd polymer was confirmed using 2,3,4,5-tetraphenylcyclopentadienone (TPCPD) as a specific singlet oxygen quencher. Under these conditions, the photogeneration of singlet oxygen differs depending on the molar ratio of PCBM to Pd. The photochemical generation of singlet oxygen for the PCBM–Pd polymer electrochemically deposited on an ITO electrode and gold foil does not depend on the thickness of the deposited layer.

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Introduction

PCBM is a fullerene C₆₀ with an attached phenyl-butyric acid ester group that improves the material's solubility in organic solvents such as chloroform, chlorobenzene, and dichlorobenzene. Such properties are responsible for the solution processability of PCBM and, thus, its usefulness compared to the parent molecule C₆₀ in solution-processed organic electronic applications. PCBM exhibits better diffusion in an organic environment than fullerene C₆₀. It has high electron mobility and affinity, which allows it to be used as an electron acceptor in major electrochemical applications. Therefore, PCBM is used in thin film transistors^1–3 and electrical memory devices,^4–6 and most often, however, it is used in polymer organic solar cells.^7–14 These organic solar cells are a new generation of photovoltaic devices. Among the organic materials used in producing polymer solar cells mentioned in the literature, poly(3-hexylthiophen-2,5-diyl) (P3HT) and PCBM are among the most valuable examples. PCBM acts as an efficient electron acceptor and transporting material in these systems, and P3HT acts as an electron donor.

Singlet oxygen, ¹O₂, is considered an important reactive oxygen species.¹⁵ Due to the unoccupied π* orbital presence, ¹O₂ shows high selectivity towards electron-rich substances and unsaturated organic molecules.¹⁶ However, generating triplet states in organic photovoltaic (OPV) cells can be problematic because it reduces the device's efficiency in several ways. First, the presence of triplets lowers the short-circuit current density and open-circuit voltage, resulting in lower overall power conversion efficiency. Triplet excited states can also react with oxygen to form singlet oxygen that easily damages the cells’ photoactive layers, leading to the degradation of the cell and a further reduction in efficiency.¹⁷ Thus, the studies on ¹O₂ generation by photoactive layers applicable in OPVs can provide valuable information on their durability and degradation mechanism. Therefore, analysis of this process allows the design of more stable OPV cells. On the other hand, the high reactivity of singlet oxygen may be helpful in synthesizing specialty chemicals, in wastewater treatment, and in photodynamic therapy (PDT), where its cell-destroying ability is targeted to fight cancer cells.^18–20 However, forming singlet oxygen requires certain conditions. Direct excitation of oxygen from the triplet ground state to the singlet excited state is spin-forbidden, meaning that using light directly is impossible. However, it can be achieved using photosensitizers. A photosensitizer first absorbs light, which puts it into a singlet excited state (S₁). It then transitions to a triplet excited state (T₁) through intersystem crossing (ISC). In the triplet excited state, the photosensitizer can transfer its energy to oxygen in the ground (triplet) state, resulting in the formation of ¹O₂ and the return of the photosensitizer to the ground state.

Several methods have been developed to detect singlet oxygen, which can be broadly divided into indirect (trapping) and direct spectroscopic techniques. Indirect methods commonly rely on chemical traps, such as 1,3-diphenylisobenzofuran (DPBF) or 2,3,4,5-tetraphenylcyclopentadienone (TPCPD), which react selectively with ¹O₂ to form stable products that can be monitored by UV-vis absorption spectroscopy.²¹ These methods are simple and widely used; however, they are prone to interference from side reactions, trap photodegradation, and cannot provide information on the singlet oxygen lifetime or its precise quantum yield.²² Direct detection techniques, in contrast, exploit the characteristic phosphorescence emission of ¹O₂ at around 1270 nm during the transition from its excited singlet state (¹Δ_g) to the ground triplet state (³Σ_g⁻).²³ Time-resolved near-infrared (NIR) phosphorescence detection is the gold standard for reliable singlet oxygen formation and decay dynamics measurement.²⁴ This method allows for precise determination of the quantum yield (Φ_Δ) and provides insights into the kinetics of ¹O₂ generation and quenching in different environments.²⁵ In this energy transfer, both singlet excited states of oxygen, ¹Σ⁺_g and ¹Δ_g, as well as the triplet ground state ³Σ⁻_g can be formed with different rate constants (k^1Σ_T, k^1Δ_T, and k^3Σ_T). The formation proceeds via two main quenching pathways: one involving non-charge-transfer (nCT) complexes, and another involving partial charge-transfer (pCT) exciplexes. The efficiency of singlet oxygen formation depends on the excess energy in the nCT channel and on the free energy of electron transfer in the pCT channel. Notably, a rapid intersystem crossing equilibrium between singlet and triplet exciplexes appears only in the nCT pathway.²⁶

Additionally, it is known that quenching of ππ excited triplet states by O₂ occurs through internal conversion within encounter complexes and exciplexes, leading to singlet oxygen with varying efficiencies. The balance between these deactivation paths depends on the sensitizer's triplet energy, oxidation potential, and solvent polarity.²⁷

The photochemical properties of fullerenes and their derivatives have received intensive attention.^28–33 The triplet state of C₆₀ sensitizes singlet oxygen formation in high yields.³⁴ It was shown that electron-deficient fullerenes are useful sensitizers in preparative photooxygenation reactions by the intermediacy of singlet oxygen.³⁵ Unfortunately, the unfunctionalized C₆₀ and C₇₀ molecules possess poor solubility in most common solvents. The functionalization of the fullerene core with solubilizing groups makes the indirect study of their properties in polar solvents possible. For example, a palladium metallosquare with encapsulated fullerenes C₆₀ and C₇₀, which exhibits high solubility in organic solvents such as CH₃CN, CH₂Cl₂, CHCl₃, etc., can be used as a photosensitizer for the generation of singlet oxygen and for facilitating carbon–oxygen bond forming reactions with a variety of organic substrates.³⁶ Okutan et al.³⁷ investigated perylenebisimide-fullerene (PBI-fullerene) dyads. These PBI-fullerene dyads were utilized to produce efficient singlet oxygen and were applied in the photooxidation of 1,5-dihydroxy naphthalene (DHN) to produce juglone.³⁷ Photoactive layers based on the P3HT matrix containing carbon nanostructures such as C₆₀, PCBM, and SWCNTs were also studied as photosensitizers for singlet oxygen production.³² The formation of single oxygen was monitored spectroscopically in situ by oxidation of DPBF in methanol and used synthetically to produce juglone. The production yield of ¹O₂ depended on the carbon nanostructure photosensitizer and was significantly lower for mixtures of P3HT with C₆₀ and single-walled carbon nanotubes than for mixtures of P3HT with PCBM.³²

Fullerene moieties can be also bounded in η²-fasion through transition metal-containing linkers to form large macromolecular structures.³⁸ These coordination fullerene polymers can be formed under chemical or electrochemical conditions.^38–40 Chemical synthesis takes place in a non-polar solution containing fullerene and a transition metal complex in a low oxidation state.^41–44 The polymer precipitates from the solution in the form of large particles composed of spherical nanoparticles with sizes of 50 to 200 nm. Electrochemical synthesis is usually performed under cyclic voltammetry conditions in a solution containing fullerene and transition metal complex. The electrochemical reduction process taking place under such conditions leads to the formation of thin and homogeneous polymer layers on conductive surfaces.⁴⁵ Coordination polymers in which fullerene moieties were connected by Pd⁰,⁴⁵ Pt⁰,⁴⁶ Ir^I (CO)₂,^45,47 and Rh^I (CF₃COO)₂^45,47 were electrochemically synthesized. Both, pristine fullerenes and fullerene derivatives including porphyrin,⁴⁸ crown ether,⁴⁹ ferrocene^50,51 and pyrrolidine and piperazine⁵² derivatives were incorporated into a three-dimensional polymeric structure.

This work reports that the coordination polymer of PCBM and palladium atoms (PCBM–Pd) is synthesized using both chemical^42,53,54 and electrochemical^45,55,56 C₆₀–Pd formation processes. Such a polymeric material was applied as a photocatalyst to generate singlet oxygen. The PCBM–Pd polymer is practically insoluble in organic and inorganic solvents. Therefore, thin PCBM–Pd layers can be used for singlet oxygen generation in different liquid media. The photoactive performance of such thin films is presented in this work.

Results and discussion

Formation of the coordination PCBM–Pd polymer

In coordination fullerene polymers, fullerene cages are bonded through transition metal-containing units to form polymeric chains. These polymers can be produced as 1-D, 2-D, and 3-D structures. Additionally, the fullerene moiety can be functionalized with different organic groups. Coordination fullerenes formed from pristine fullerenes and materials containing organic moieties attached to the fullerene cages can be formed chemically and electrochemically. Both procedures of PCBM–Pd coordination polymer formation are shown in Scheme 1. In both cases, fullerene and fullerene derivatives are used as polymerization precursors. In chemical synthesis, the low-valence transition metal complex is carried out in benzene solution as a second precursor. In the case of palladium linkers, a zero-valent palladium complex is used as a polymerization precursor. The polymer is precipitated from the grown solution as a dark deposit. In the electrochemical procedure, polymerization occurs under electroreduction at the potentials of the first fullerene reduction step. In the same potential range, palladium(II) ions are reduced to zero-valent palladium. The metal then reacts with fullerene molecules to form monomers, which undergo polymerization. The concentration of the polymerization precursor, as in chemical polymerization, affects the spatial structure of the polymer. Under such conditions, the PCBM–Pd polymeric phase is deposited as a uniform and relatively smooth layer at the electrode surface. In these polymers, fullerene molecules are covalently bonded to transition metal atoms or their complexes, forming a polymer network. These materials exhibit electrochemical activity in the negative potential range due to the rich electroreduction of fullerene molecules.⁵⁷

Scheme 1 Formation of the coordination PCMB–Pd polymer.

Structural characterization of the PCBM–Pd polymer

The morphology of the PCBM–Pd polymer formed by electrochemical and chemical synthesis for different quantitative ratios of PCBM to palladium is shown in Fig. 1. The electrochemically produced material is initially deposited at the electrode surface in the form of a uniform layer. In the next stage of electropolymerization, aggregates of spherical particles are deposited on the initially formed polymeric layers (Fig. 1a). The number of voltammetric cycles, sweep rate, and concentration of polymerization precursors in solution can control the amount of polymeric material and the structure of the polymeric layer. In chemical synthesis, the structure of the polymeric phase depends on the ratio of polymerization precursor concentrations in the growth solution. For the 1 : 1 and 1 : 2 ratio of PCBM to palladium complex concentrations, the polymeric phase is composed of irregular particles that adhere closely to each other, creating a compact material with low porosity. This effect is much more pronounced in the case of material formed in a solution with a low concentration of the palladium complex (Fig. 1b). In a solution containing a 1 : 3 ratio of PCBM to palladium complex concentration, regular spherical particles, 150–300 nm in diameter, are formed. They aggregate together to form bulky porous materials (Fig. 1d).

Fig. 1 SEM (and inset TEM) images obtained for the PCBM–Pd polymer formed on gold foil (a) under cyclic voltammetry conditions for 20 deposition cycles in the potential range from −0.2 to −1.05 V at a sweep rate of 100 mV s⁻¹ in the solution of ACN/Tol (1 : 4, v : v) containing 0.22 mM PCBM, 3.56 mM Pd(ac)₂ and 0.1 M tetra(n-C₄H₉)₄NClO₄ as the supporting electrolyte, (b)–(d) from chemical synthesis in benzene solution containing 0.48 mM PCBM and (b) 0.24 mM, (c) 0.48 mM, and (d) 0.73 mM Pd₂(dba)₃·CHCl₃ for 24 hours of polymerization time.

The results of electron microscopy studies indicating the dependence of porosity of the formed materials on the conditions of their preparation were confirmed by the results of porosimetric tests. In panel a of Fig. 2, the nitrogen adsorption–desorption curves of chemically synthesized PCBM–Pd polymers with different component mass ratios are shown. They all represent type IV isotherms with hysteresis loops, indicating the dominance of mesopores in the synthesized materials.^58,59 The BET (Brunauer–Emmett–Teller) specific surface area (panel b of Fig. 2) of the analyzed polymers increased with increasing palladium content, reaching 22, 92, and 303 m² g⁻¹ for PCBM–Pd (1 : 1), PCBM–Pd (1 : 2), and PCBM–Pd (1 : 3), respectively (Table 1).

Fig. 2 (a) Nitrogen adsorption–desorption isotherms recorded for PCBM–Pd polymers with different component mass ratios (1 : 1, 1 : 2, and 1 : 3). (b) BET curves corresponding to the isotherms. (c) Differential pore size distributions based on DFT calculations.

Table 1 Parameters determined from nitrogen adsorption–desorption measurements for PCBM–Pd polymers with different component mass ratios

Material	BET surface area (m^2 g^−1)	S micro (m^2 g^−1)	S external (m^2 g^−1)	Cumulative volume of pores^b (cm^3 g^−1)	Average pore diameter^c (nm)
a Calculated using the t-plot method. b BJH adsorption cumulative volume of pores in the diameter range of 17.000–3000.000 Å. c Average pore size diameter calculated using the BJH method.
PCBM–Pd (1 : 1)	22	4	18	0.0627	14.1
PCBM–Pd (1 : 2)	92	27	65	0.1327	9.3
PCBM–Pd (1 : 3)	303	56	247	0.3066	7.8

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The t-plot analysis also indicated that the amount of both micropores and mesopores increased with the growth of palladium content in the obtained polymers (Table 1). Barrett–Joyner–Halenda (BJH) (Table 1) and density functional theory (DFT) (Fig. 6c) calculations confirm a decrease in average pore size with an increase in total pore volume as the mass proportion of PCBM in the polymer material decreases.

IR spectroscopy is widely used to obtain structural information in composition studies and the interaction between components. Fig. 3 shows the spectra of the PCBM–Pd (1 : 3) polymer. For comparison, the spectra obtained for pure PCBM, pure C₆₀ fullerene, and C₆₀Pd polymer are also presented.

Fig. 3 IR spectra of (a) C₆₀, (b) C₆₀Pd polymer, (c) PCBM, and (d) PCBM–Pd (1 : 3) polymer.

Pure C₆₀ fullerene gives four characteristic signals at 527, 576, 1180, and 1428 cm⁻¹ (Fig. 3a). The bands at 527 and 576 cm⁻¹ correspond to the deformation vibrations of the C₆₀ molecule, while the bands at 1180 and 1428 cm⁻¹ are attributed to the vibrations of the C=C bond in fullerene. In the case of the C₆₀–Pd₃ polymer (Fig. 3b), the signals at 486, 526, and 556 cm⁻¹ are attributed to the stretching vibrations of the C–Pd bond, and the signals at 668, 697, and 760 cm⁻¹ are attributed to the deformation vibrations of the C–Pd bond. The peaks at 1183, 1337, 1373, and 1450 cm⁻¹ are characteristic of the vibrations of the C=C region in the benzene ring.³⁹ For the fullerene derivative PCBM (Fig. 3c) at 1630 and 1737 cm⁻¹, signals from stretching and deformation vibrations of the C=O and at 1148 cm⁻¹ from the C–O–C bond in the ester group are also observed.⁶⁰ On the spectrum obtained for the PCBM–Pd (1 : 3) polymer (Fig. 3d), observed bands from both PCBM and those related to C–Pd bond vibrations indicate this polymer's formation.

In addition, calculations of the oscillatory structure for C₆₀Pd and PBCM–Pd were performed using Gaussian 09 software. The calculations were performed using DFT at the cam-B3LYP/cep-121g theory level.^61–64 Vibrational structure calculations were performed for the optimized system. No negative frequencies were observed. Theoretical infrared (IR) spectra were obtained using GaussSum⁶⁵ software. These spectra were plotted with a half-width of 10 cm and a scaling factor of 0.95. Fig. S1 in the supplementary materials presents theoretical spectra for these systems. Furthermore, theoretical calculations suggest that the bands at 486 and 526 (for C₆₀Pd) and 485 and 524 (for PCBM–Pd) are related to vibrations resulting from the deformation of C–C bonds in the fullerene molecule, which are disturbed by palladium.

X-ray photoelectron spectroscopy (XPS) analysis gave more information about the chemical composition of the chemically prepared materials. In Fig. 4, the XPS spectra of pure PCBM are presented. The high-resolution XPS spectrum of the C 1s region can be deconvoluted into several peaks corresponding to carbon atoms in different functional groups present in the analyzed sample (Fig. 4a). This spectrum shows a prominent peak at 284.8 eV, which originates from the C–C/C=C bond in C₆₀ and the phenyl group. The other components located at 286.4 eV and 288.7 eV are characteristic of epoxy (C–O) and carbonyl (C=O/O–C=O) groups, respectively, and arise from the butyric acid methyl ester side chain of PCBM. The broad peak at 290.8 eV can be assigned to a shake-up feature involving the π → π* transition in the C₆₀.^66,67 The decomposition of the O 1s spectrum gives two components about the methyl ester group, as shown in Fig. 4b. The peak at 531.9 eV corresponds to carbonyl groups (C=O/O–C=O). The peak at 533.9 eV corresponds to the epoxy group (C–O).^67,68

Fig. 4 XPS high-resolution spectra of pure PCBM: (a) C 1s region and (b) O 1s region.

Fig. 5 shows the C 1s, O 1s, and Pd 3d XPS high-resolution spectra obtained for PCBM–Pd polymers. The sample C 1s spectrum and O 1s spectrum recorded for PCBM–Pd (1 : 1) are shown in Fig. 5a and b, respectively. The C 1s spectrum recorded for PCBM–Pd obtained for different molar ratios exhibits the same components as pure PCBM (Fig. 4a). In the case of O 1s, compared to PCBM, an additional signal at 531.0 eV assigned to Pd–O can be observed. The analysis of the high-resolution XPS spectrum of the Pd 3d region reveals two doublet signals (Fig. 5c–e). The first one, located at 336.6 and 341.9 eV, represents Pd 3d_5/2 and Pd 3d_3/2, respectively, and it can be assigned to palladium atoms bonded to fullerene moieties.^69,70 The second pair of peaks at 338.3 and 343.75 eV also appear in the Pd 3d spectrum, which agrees with the values reported for Pd(IV).^71–73 Studies using TEM confirmed the presence of Pd nanoparticles on the surface of the synthesized PCBM–Pd polymer, with their amount increasing with increasing concentration of the precursor during the polymerization process. Thus, there is a possibility of oxidation of metallic Pd nanoparticles to PdO₂ during the sample drying process.⁷² Hence, signals originating from Pd(IV) may appear.

Fig. 5 XPS high-resolution spectra of (a) C 1s region, (b) O 1s region, (c) Pd 3d region recorded for PCBM–Pd (1 : 1), (d) Pd 3d region for PCBM–Pd (1 : 2), and (e) Pd 3d region recorded for PCBM–Pd (1 : 3).

The TGA stability curves of PCBM and polymers of PCBM–Pd in an air atmosphere are presented in Fig. 6. PCBM shows stability up to about 400 °C. In a temperature range from 400 to 600 °C, PCMB undergoes very rapid degradation associated with the decomposition of C₆₀ molecules. When 600 °C is reached, PCBM is almost completely volatilized, leaving a total residue of ca. 0.3%. This result is consistent with the literature on TGA thermograms for PCBM in an air atmosphere.⁷⁴ PCBM–Pd polymers begin to degrade slightly earlier at about 300 °C. Still, at 700 °C, the value of total residual palladium mass is equal to 10.53, 22.68, and 30.41 wt% for PCBM–Pd (1 : 1), PCBM–Pd (1 : 2), and PCBM–Pd (1 : 3), respectively. The PCBM to Pd molar ratios determined from the TGA curves are equal to 1 : 0.95, 1 : 2.08, and 1 : 2.85, for materials formed in solutions with a PCBM to Pd ratio of 1 : 1, 1 : 2, and 1 : 3, respectively. A good agreement of the PCMB to Pd molar ratio in the grown solution and the formed material indicates 1-D, 2-D, and 3-D polymeric structures depending on the solution composition, as was observed for the process of C₆₀–Pd formation.⁴² However, in the case of PCMB, one of the sites available in the carbon lattice for binding palladium is blocked by an ester moiety. A theoretical value of the PCMB to Pd molar ratio of 1 : 2.5 should be expected in this case. Slightly higher values obtained from thermogravimetric measurements indicate the formation of palladium nanoparticles, as indicated by the results of XPS studies discussed earlier.

Fig. 6 TGA curves of (1) PCBM, (2) PCBM–Pd (1 : 1), (3) PCBM–Pd (1 : 2), and (4) PCBM–Pd (1 : 3) polymer.

Singlet oxygen photogeneration

Quantitative analysis applies a chemical trap and is a standard method to assess singlet oxygen production. Singlet oxygen is a highly reactive form that can be generated in various chemical and biological processes, including photosensitization reactions. Chemical traps are compounds that readily react with singlet oxygen to produce stable reaction products. Besides, it must be highly reactive towards singlet oxygen, so the reaction must be swift, competing with other slower routes. One common trap is TPCPD.^75–77 The trapping molecule is irreversibly oxidized, and then, typically, a decrease in its absorbance is monitored. The change in absorbance is proportional to the amount of singlet oxygen generated.

Therefore, the dominant degradation pathway of PCBM–Pd should involve singlet oxygen, which is generated from triplet excited states located at PCBM.^78,79 For PCBM, triplets can be formed via intersystem crossing from the PCBM singlet states after light excitation, before being quenched by oxygen via energy transfer, forming singlet oxygen.⁷⁹

Fig. 7 shows UV-vis absorption spectra of the PCBM–Pd (1 : 3) polymer and TPCPD in 1,2-dichlorobenzene solution. The UV-vis spectrum of pristine PCBM in 1,2-dichlorobenzene is also shown for comparison. The spectrum in the UV range below 350 nm, both for pure PCBM and polymer of PCBM–Pd, corresponds to the strong allowed transition of the fullerene core.⁸⁰ In the visible region, where the transition is forbidden mainly by the symmetry of the highly symmetric C₆₀ molecule, much more prominent peaks are observed in the range from 400 to 700 nm in pure PCBM and PCBM–Pd, suggesting that the fullerene derivative side chain only interferes with the forbidden transitions of the fullerene core in the visible region as a result of breaking the symmetry of the original C₆₀ molecule by assigning a [6,6]-addition to C₆₀.^80,81 The UV-vis spectra of the TPCPD solution show a characteristic absorption peak in the range of 420–700 nm. Absorption is assumed to decrease TPCPD at 510 nm and is directly proportional to its reaction with singlet oxygen.

Fig. 7 UV-vis absorption spectra of PCBM (black line), PCBM–Pd (1 : 3) polymer (red line), and TPCPD (blue line) in 1,2-dichlorobenzene solution. The inset shows the UV-vis absorption spectrum for PCBM (black line) and PCBM–Pd polymer (1 : 3) (red line) recorded in a 1,2-dichlorobenzene solution on a broader wavelength range.

Fig. 8 presents the UV-vis spectra of a 0.01 mM solution of TPCPD in 1,2-dichlorobenzene without photosensitizer of PCBM or PCBM–Pd recorded every 2 minutes during the irradiation of this solution for 20 minutes. As you can see, small changes in the absorption band intensity at 510 nm indicate that this molecule is stable under light illumination with a xenon lamp, can be used in 1,2-dichlorobenzene, and is selective toward ¹O₂. Therefore, the pure solution of TPCPD excludes its self-decomposition under lighting.

Fig. 8 UV-vis absorption spectra of a 0.01 mM solution of TPCPD in 1,2-dichlorobenzene upon illumination with a xenon lamp as a light source for 20 minutes.

Fullerene electron acceptors are widely used in organic electronics, and PCBM is the most commonly employed electron acceptor in organic solar cells. This is down to the low-lying excited states of the fullerene anion, which promote fast charge separation and high solubility in organic solvents, making solvent processing possible.⁸² Most studies use PCBM as an acceptor when testing new polymers, architectures, or upscaling methods. However, despite being well established, the photo-oxidation of fullerene electron acceptors is far less studied than that of polymers. The photochemical generation of singlet oxygen, ¹O₂, using PCBM typically involves sensitizing molecular oxygen (O₂) by a photosensitizer, such as PCBM, under light irradiation. PCBM absorbs light, typically in the visible or near-infrared region. The absorption of a photon promotes the sensitizer to an excited state. PCBM can undergo intersystem crossing to a triplet state in the excited state. The triplet state of PCBM can then transfer its energy to ground-state molecular oxygen, O₂, producing singlet oxygen. The energy transfer process involves the transfer of energy from the excited state of PCBM to ground-state molecular oxygen, resulting in the formation of singlet oxygen. PCBM sensitizes the molecular oxygen in its ground state (³O₂) to produce ¹O₂. The overall reaction can be represented as:

PCBM + hν → PCBM* (1)

PCBM* + O₂→ PCBM + ¹O₂ (2)

Fig. 9a–c shows the sets of UV-vis spectra of TPCPD in 1,2-dichlorobenzene recorded during irradiation of the chemically synthesized PCBM–Pd (1 : 3) polymer added in varying amounts: (a) 50 μl, (b) 100 μl, and (c) 150 μl. In all materials tested, a decrease in the absorption band located at 510 nm was observed with increased irradiation time of the PCBM–Pd polymer. These drops in the absorption band indicate the reaction of TPCPD with singlet oxygen photogenerated by the polymer of PCBM–Pd. The comparison of the decrease in the intensity of the TPCPD band at 510 nm over time (Fig. 9d) confirms a much faster oxidation of TPCPD with an increase in the polymer's concentration in solution. UV-vis absorption spectra obtained for pure PCBM as a photosensitizer dissolved in various amounts in a solution of 1,2-dichlorobenzene containing 0.01 mM TPCPD were very similar to those obtained in the case of the PCBM–Pd polymer synthesized in a PCBM to Pd₂(dba)₃·CHCl₃ molar ratio of 1 : 1 (Fig. S2 in SI). An increase in the photooxidation rate of the polymer is also observed as its concentration in the solution increases. Thus, the studies indicate that the PCBM in the polymer is the source of singlet oxygen. An increase in the ratio of Pd concentration to PCBM concentration during the chemical synthesis of the PCBM–Pd polymer leads to much slower changes in absorbance associated with the photooxidation of this polymer. This is due to the decrease in the amount of the fullerene derivative in the polymer.

Fig. 9 UV-vis absorption spectra of 0.01 mM solution of TPCPD in 1,2-dichlorobenzene recorded during illumination of (a) 0.03 mM, (b) 0.06 mM, and (c) 0.09 mM of PCBM–Pd (1 : 3). (d) Time-dependent decrease in TPCPD absorbance at 510 nm in the presence of a polymer.

Reaction rate constants for different amounts of PCBM–Pd polymers in solution as a source of active oxygen were determined by linear regression as the slope of the line and are presented in Table 2. These changes related to the different rates of decrease in the absorption bands for polymers with different PCBM to Pd₂(dba)₃·CHCl₃ concentration ratios depend on the amount of PCBM in the sample. For the PCBM–Pd system (1 : 1), where there are more PCBM in one volume, the changes in these ratios are much higher compared to PCBM–Pd (1 : 3). In addition, the rate of singlet oxygen generation may also be related to the structure of the polymer being formed. In the case of agglomerated spherical particles, which form as the ratio of palladium to PCBM increases, it is much more difficult for oxygen to penetrate more aggregated areas. The photooxidation of fullerene derivative polymers can, therefore, be altered by changing their concentration and structure by increasing their crystallinity or aggregation, and can be used to control these properties.

Table 2 TPCPD oxidation's rate constants were determined considering the decrease in absorbance at 510 nm

Photoactive polymer	k [mM min^−1]
50 μl PCBM–Pd (1 : 1)	6.92 × 10^−2
100 μl PCBM–Pd (1 : 1)	9.72 × 10^−2
150 μl PCBM–Pd (1 : 1)	15.86 × 10^−2
50 μl PCBM–Pd (1 : 2)	4.19 × 10^−2
100 μl PCBM–Pd (1 : 2)	6.50 × 10^−2
150 μl PCBM–Pd (1 : 2)	9.03 × 10^−2
50 μl PCBM–Pd (1 : 3)	2.86 × 10^−2
100 μl PCBM–Pd (1 : 3)	4.09 × 10^−2
150 μl PCBM–Pd (1 : 3)	6.60 × 10^−2

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A similar set of UV-vis spectra was obtained during irradiation of the PCBM–Pd layer of different thicknesses, electropolymerized onto the ITO electrode and thin gold foil (Fig. S3 in SI). Examples of UV-vis spectra of PCBM–Pd electrochemically synthesized on gold foil are shown in Fig. 10. The studies carried out show that the number of cycles, and thus the amount of deposited polymeric material, does not affect the rate of the photooxidation layers, because it is a typical surface process. However, much better adhesion can be seen in the case of layers deposited on gold foil.

Fig. 10 UV-vis absorption spectra of 0.01 mM solution of TPCPD in 1,2-dichlorobenzene recorded during illumination of PCBM–Pd polymer layers electrodeposited on the gold foil electrode for: (a) 5, and (b) 20 cycles in the range −0.2 : −1.05 V. Insets: Time-dependent decrease in TPCPD absorbance at 510 nm in the presence of a polymer illuminated with a xenon lamp.

Experimental

Materials

PCBM ([6,6]phenyl-C₆₁-butyric acid methyl ester), Pd(ac)₂ (palladium acetate), and Pd₂(dba)₃·CHCl₃ (tris(dibenzylideneacetone)dipalladium(0)-chloroform adduct) were used as received from Aldrich Chemical Co., Ltd. The supporting electrolyte for electrochemical measurements, tetra(n-C₄H₉)₄NClO₄, was used as received from Sigma-Aldrich Chemical Co. Toluene, benzene, 1,2-dichlorobenzene, acetonitrile, and dichloromethane, obtained from Aldrich, were used without additional purification. 2,3,4,5-Tetraphenylcyclopentadienone (TPCPD) obtained from Acros Organics was used in benzene as a specific ¹O₂ quencher in photoactive properties.

Instrumentation

Electrochemical polymerization of PCBM–Pd was performed on an AUTOLAB Model 283 Potentiostat/Galvanostat (EG&G Princeton Applied Research, Oak Ridge, TN, USA) with a three-electrode cell. The AUTOLAB system was controlled by the same manufacturer with GPES 4.9 software. The ITO electrode and thin gold foil were used as the working electrode. The reference electrode was made by immersing a silver wire in 0.01 M silver nitrate and 0.09 M tetra(n-butyl)ammonium perchlorate in acetonitrile. The reference electrode was separated from the working electrode by using a ceramic tip (Bioanalytical Systems Inc.). The counter electrode was a platinum tab with an area of approximately 0.5 cm².

The morphology of the obtained materials was secondary-electron imaged using an Inspect S50 scanning-electron microscope (FEI Company, Hillsboro, Oregon, USA). The accelerating voltage of the electron beam was 20 or 25 keV, and the average working distance was 10 mm.

TEM images were obtained using a Tecnai G² 20 X-TWIN microscope (FEI Company, Hillsboro, Oregon, USA) with a LaB6 emitter and an HAADF detector operating at 120/200 kV. Ametek's EDX instrument and software (Berwyn, Pennsylvania, USA) were used for energy-dispersive X-ray fluorescence measurements. This software facilitates standardless quantification of atomic weight and atomic percentage. The accelerating voltage for the electron beam was 20 keV, and the working distance was 10 mm.

The DRIFT spectra were recorded using a Magna IR 550 Series II spectrometer with a spectral resolution of 4 cm⁻¹.

Thermogravimetric analysis (TGA) was performed using a Mettler Toledo Star TGA/DSC system. Nitrogen was used as the purge gas (0.1 dm³ min⁻¹). Samples weighing 2 mg were placed in aluminum pans and heated from 50 to 1000 °C at a heating rate of 10 °C min⁻¹.

Nitrogen adsorption/desorption isotherms (BETs) were recorded at 77.15 K with a Micrometrics ASAP 2020 automatic sorption analyzer.

X-ray photoelectron spectroscopy (XPS) analysis was conducted using an AXIS Supra+ instrument (Kratos Analytical) equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV, operating at 10 mA, 15 kV). The system base pressure was equal to p_b = 1.7 × 10⁻⁹ Torr. The pass energy was set to 160 eV (scanning step 0.9 eV) or 20 eV (scanning step 0.05 eV) for survey spectra and high-resolution spectra, respectively. For the charging effect compensation, the Kratos charge neutralizer system was used. The binding energy scale was calibrated concerning the C–C component of C 1s spectra (284.8 eV). The acquired spectra were analysed using CASA XPS® software and embedded algorithms. The components of the high-resolution spectra were presented with Gaussian (70%) and Lorentzian (30%) lines, while the background was with Shirley's function.

Singlet oxygen (¹O₂) photogeneration by PCBM–Pd₃ was investigated using a Hewlett–Packard 8452A UV-vis spectrometer and a standard 10 mm × 4 mm quartz cuvette (Hellma Analytics). A xenon lamp was employed as an excitation light source and was arranged perpendicular to the direction of UV-vis spectra acquisition.

Synthesis of the PCBM–Pd polymer

The PCBM–Pd polymer was synthesized using electrochemical reduction and chemical precipitation. These syntheses were carried out identically to form the C₆₀Pd_n polymer.^{42,45,53–56}

Electrochemical synthesis was performed in acetonitrile/toluene (1 : 4, v/v) solution containing 0.22 mM PCBM, 3.56 mM palladium acetate, and 0.1 M tetra(n-C₄H₉)₄NClO₄ as the supporting electrolyte on the ITO electrode and gold foil electrode. The film was grown at the electrode surface under cyclic voltammetry conditions in the potential range of −0.2 to −1.05 V at a potential sweep rate of 100 mV s⁻¹ (Fig. S4 in SI). The increase in current values in subsequent cycles indicates the formation of a new solid phase on the electrode surface. Therefore, in this case, the polymer was deposited at the electrode surface during electropolymerization. The thickness of the deposited polymer layer depended on the number of cycles used in cyclic voltammetry.

Chemical synthesis was performed in benzene solution containing 0.48 mM PCBM and respectively 0.24 mM, 0.48 mM, or 0.73 mM Pd₂(dba)₃·CHCl₃ according to the procedure proposed by Nagashima,⁴² obtaining a polymer with the appropriate molar ratio of PCBM to Pd₂(dba)₃·CHCl₃ of 1 : 1, 1 : 2, 1 : 3 (PCBM–Pd (1 : 1), PCBM–Pd (1 : 2), PCBM–Pd (1 : 3)), respectively. By chemical synthesis, a black polymeric material was obtained.

Procedure for singlet oxygen photogeneration

Singlet oxygen photogeneration by PCBM and the PCBM–Pd polymer produced by chemical or electrochemical synthesis was investigated using a 0.01 mM solution of TPCPD, i.e., a specific ¹O₂ chemical quencher, in 1,2-dichlorobenzene. TPCPD was employed as an absorption probe to confirm the generation of ¹O₂ by the PCBM–Pd polymer qualitatively. For this purpose, the pure PCBM and the chemically synthesized PCBM–Pd polymer were dissolved in 1,2-dichlorobenzene to obtain a 0.3 mM solution. Different amounts of PCBM–Pd solution (0.3 mM), namely 50, 100, and 150, were added to 0.5 mL of TPCPD solution (0.01 mM) to give concentrations of 0.03, 0.06, and 0.09 mM, respectively. On the other hand, the electrochemically synthesized PCBM–Pd polymer was deposited for 5, 10, or 20 cycles directly on the ITO electrode or thin gold foil electrode, which fitted the size of the cuvette. Then the modified foil or ITO electrode in this way was placed in a cuvette containing a solution of TPCPD in 1,2-dichlorobenzene. The progress of the reactions was given as a drop in the absorbance of TPCPD at 510 nm.

Conclusions

In this paper, we show the formation of a new PCBM–Pd polymer synthesized by both chemical and electrochemical synthesis. The electrochemically formed material was deposited as a uniform layer on the surface of which there were agglomerates of spherical particles. In chemical synthesis, the formed polymer depends on the concentration of polymerization precursors in the growth solution. In the case of a 3 : 1 ratio of Pd(0) complex to PCBM, the polymer precipitates from solution in the form of spherical particles with a size of 150–300 nm in diameter. The PCBM to Pd ratio in this polymer is close to 2.5, indicating 3-dimensional structure formation. Such a material exhibits porous morphology with a pore size of 3.5 nm. The obtained IR spectra confirmed the formation of the PCBM–Pd polymer. The nitrogen adsorption–desorption isotherms displayed a typical IV-type isotherm, characteristic of mesoporous materials with a pore size distribution of 3.5 nm for this polymer.

The photocatalytic activity of PCBM–Pd toward singlet oxygen evolution was examined. Photogeneration of singlet oxygen in phenyl-C₆₁-butyric acid methyl ester (PCBM) can occur through the photosensitization process in the presence of TPCPD as a specific ¹O₂ quencher. This work also represents the first demonstration of singlet oxygen generation and the specific photochemical sensitizing ability of the PCBM–Pd polymer. This photoactive polymer can, therefore, be considered as an alternative in the heterogeneous oxidation reactions activated by visible light.

Author contributions

Conceptualization: M. W.-Z., A. B.-G.; investigation: M. W.-Z., A. B.-G., J. B, D. M. B., A. M., K. W.; methodology: M. W.-Z., A. B.-G.; visualization: M. W.-Z., A. B.-G., J. B., D. M. B. A. M.; writing – original draft: M. W.-Z; writing – review & editing: M. W.-Z., A. B.-G. and K. W.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included in the SI. Additional datasets generated and analyzed during the current study are available from the corresponding author upon reasonable request. For inquiries related to synthetic data and experiments, please contact Monika Wysocka-Żołopa (monia@uwb.edu.pl) and Agata Blacha-Grzechnik (Agata.Blacha-Grzechnik@polsl.pl). See DOI: https://doi.org/10.1039/d5nj02494c

Acknowledgements

The equipment in the Center of Synthesis and Analysis BioNanoTechno of the University of Bialystok was funded by the EU as part of the Operational Program Development of Eastern Poland, projects: POPW.01.03.00-20-034/09 and POPW.01.03.00-004/11.

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