Chemical Physics Letters 763 (2021) 138205
Available online 29 November 2020
0009-2614/© 2020 Elsevier B.V. All rights reserved.
Editor ’ s choice
Rate constant measurements for radical addition reactions with C 60 by means of time-resolved EPR and spin-echo detected pulsed
EPR spectroscopy
Hirona Takahashi
a, Hiroki Hirano
b, Kyohei Nomura
b, Kenta Hagiwara
c, Akio Kawai
b,*aDepartment of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Kita-ku, Okayama 700-0005, Japan
bDepartment of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka-shi, Kanagawa 259-1293, Japan
cDepartment of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan
A B S T R A C T
C60 fullerene shows high reactivity with free radicals and is considered as a radical trapping agent. Addition reaction of radicals to C60 was investigated by time- resolved Electron Paramagnetic Resonance (TR-EPR) and pulsed EPR methods. The structures of adduct radicals were determined by TR-EPR, and the reaction mechanism was clarified. Addition rate constants, k’s of radicals to C60 were measured by the electron spin echo detection method utilizing pulsed EPR. The measured k values were remarkably high and the reactions are diffusion-controlled. We discussed activation energy on the basis of the charge transfer effect and enthalpy change.
1. Introduction
Fullerenes such as C60 and C70 have been known to show remarkable radical quenching efficiency. Owing to this property, there is a potential to use fullerenes as a free radical trapping agent and an inhibitor for radical polymerization [1–5]. To understand reactivity of fullerenes with radicals, their rate constant, k for radical quenching reaction were measured by the transient absorption (TA) method and the spin-trapping method in 1990s [6]. However, there are large differences in literature k values for the same reaction. For example, the reported addition rate constant of CH2Ph radical to C60 ranges from 105 to 108 M−1 s−1 [7–9].
This large discrepancy might arise from the experimental difficulty in time-resolved observation of short-lived radicals. In addition, the radical quenching mechanism of C60 has not been fully explained yet because C60 adduct radicals were observed only for the limited systems by spectroscopic methods [10].
In the present study, we applied a time-resolved (TR-) EPR method to identify radical adducts of C60. TR-EPR allows the observation of short- lived radicals with dynamic electron polarization (DEP) [11–13]. Some aromatic-ketones undergo photo-dissociation to produce organic radi- cals with strong DEP. The addition reaction of these spin-polarized radicals to C60 preserves the DEP. This process gives rise to adduct radicals with strong DEP, which are easily observed by the TR-EPR method. Structures of the adduct radicals will be clearly identified by hyperfine analysis.
Recently, we have been utilizing the pulsed EPR method measuring electron spin echo (ESE) decay time, TM*, of radicals and k values were determined by analyzing the linear relationship of TM* value to mono- mer concentration. This method originally proposed by Weber and Turro and they reported that k values of radical addition reactions measured by the method agree well with those obtained by TA method [14]. In this study, we applied this ESE technique to determine k values for the elemental process of radical addition reactions to C60. This is because (1) some radicals of interest such as alkane type have no optical absorption, and (2) the radical quencher, C60 has broad absorption band for UV and visible light, which makes it difficult to observe TA of the product, namely C60-adduct radicals. We measured some rate constants for phosphinoyl and alkane type radicals and the results have been rationalized by semi-empirical arguments based on the charge transfer effects on the activation energy and the enthalpy difference in the radical addition reaction.
2. Experimental
TR-EPR measurements were made using a conventional X-band EPR spectrometer (Bruker, ELEXIS 580E) under continuous microwave operation (cw-mode) combined with a wide-band preamplifier (Bruker, ER 047 PS, 1 MHz) and a boxcar integrator (Stanford, SR-250). Light for photoexcitation was supplied by a Nd: YAG laser (Continuum, Powerlite 8000, 5–18 mJ/pulse at 355 nm). The TR-EPR signals were collected at a
* Corresponding author.
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
https://doi.org/10.1016/j.cplett.2020.138205
Received 10 September 2020; Received in revised form 11 November 2020; Accepted 16 November 2020
repetition rate of 10 Hz. The microwave power was attenuated to be 2 mW and the typical Q value of the microwave cavity (Bruker, ER 4104OR) for cw-mode was about 3600.
To determine addition rate constants of radicals, a procedure using the previously reported method [14–17] was used. The TM* of the rad- icals was measured by the EPR apparatus under pulsed-microwave operation as follows. ESE of the radicals was generated by a conven- tional Hahn echo pulse sequence with a π/2 pulse and a τ-delayed π pulse, which was initiated at t ns after the laser flash (laser flash – t – π/2 – τ – π – τ – echo). A phase cycling routine was made for all ESE mea- surements. The ESE signal intensity S(τ) was monitored by the echo-top intensity for a series of τ values. The TM* value was determined by the exponential decay fitting procedure of the S(τ) profiles on the basis of the equation
S(τ) =S0exp (
− 2τ TM*
)
(1) with S0 (initial ESE amplitude) and TM* as fitting parameters. In com- mon, decay of echo signal is promoted by spin–spin relaxation if the radical is chemically stable. Therefore, echo signal of radicals in diluted solution is well characterized by a mono-exponential decay function. In the presence of pseudo-first-order chemical reactions with C60, a radical carrying an echo signal disappears through the reaction, and echo decay should be accelerated reflecting the reaction rate. In this case, the following linear relationship was found between 1/TM* and the con- centration of C60, [C60],
1 TM*= 1
TM
+k[C60] (2)
where TM is the C60-independent relaxation time of the electron spin, and k is the rate constant for the addition reaction of the radical to C60. In the light of equation (2), addition rate constants were determined by Stern-Volmer type analyses of (TM*)−1 as a function of [C60].
Chart 1 shows structures and abbreviations of aromatic-ketones used as the radical source. (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TPO) (Tokyo Kasei), 1-hydroxy-cyclohexyl-phenyl-ketone (IRG184) (Tokyo Kasei), and 2-Hydroxy-2-methylpropiophenone (DAR1173) (Sigma Aldrich) were used without further purification. C60 was ob- tained from Frontier Carbon Corporation and used as received. Toluene (Kanto Chemical or Wako Pure Chemical) was used as a solvent without further purification. Sample solutions were deoxygenated by bubbling Ar gas. All experiments were conducted at 298 K.
Thermodynamic values of compounds concerning the addition re- actions were obtained by quantum chemical calculations using density functional theory (DFT) at UB3LYP/6-31G(d) level. The Gaussian 09 or Gaussian 16 suit of programs were utilized for the calculations [18].
3. Results and discussion
Fig. 1(a) shows a TR-EPR spectrum measured 0.5 μs after a laser irradiation for TPO in toluene solution. The spectrum for TPO shows three lines, which are polarized in net absorption phase. TPO is known to undergo α-cleavage reaction by photo-excitation as shown in Scheme 1 [19–21]. The spectral carrier radicals were assigned to benzoyl at central peak and diphenylphosphin oxide (DPPO) at both ends with
doublet hyperfine splitting due to P atom [16,22]. Fig. 1 (b) displays the TR-EPR spectrum in the presence of C60. In the spectrum, additional doublet splitting appears with hyperfine coupling constant (hfcc) of 5.17 mT and g =2.0022. These peaks were assigned to an adduct radical, which was formed by the reaction of DPPO with C60 and is denoted as DPPO-C60. The unpaired electron in DPPO transfers from the P atom to C60 when the adduct is formed. This results in smaller splitting of 5.17 mT in DPPO-C60 radical as compared to 36.3 mT of DPPO. The DFT calculation of DPPO-C60 radical gave 5.25 mT for hfcc of the P atom, aP, which agrees well with the measured value. The hfcc values of the
Chart 1.Structures and abbreviations for radical source compounds.
Fig. 1.TR-EPR spectra recorded at (a) 0.5 μs and (b) 1.0 μs after the photolysis of TPO (23.5 mM) in toluene. The concentrations of C60 are (a) 0 mM and (b) 0.81 mM. TM* measurement was performed by monitoring the peak marked with an asterisk in (a).
Scheme 1.Generation of DPPO radical by the photolysis of TPO.
Table 1
The measured and calculated hyperfine coupling constants (hfcc) for the adduct radicals of C60 generated in toluene.
Adduct radicals Hfcc/mT
measured DFT calculationa (UB3LYP/6-31G(d)) DPPO-C60 aP =5.17 (0.01) aP =5.25
Hy-CyH-C60 <0.2 aOH =0.006, aCH2 =0.0008–0.034 2Hy-Pr-C60 <0.4 aOH =0.089, aCH3 =0.005–0.023 a Hfcc’s at the optimized structure were listed.
adduct radical are listed in Table 1. The hyperfine lines of DPPO and DPPO-C60 were well separated enough to observe these radicals sepa- rately by adjusting the resonance magnetic field. Since the reaction of benzoyl is much slower [13,23,24], only the addition reaction of DPPO was considered in this system.
Next we studied addition reaction of carbon-centered radicals to C60. As shown in Scheme 2, photo-induced α-cleavage of IRG184 produces hydroxycyclohexyl (Hy-CyH) and benzoyl radicals, which are seen in a TR-EPR spectrum in Fig. 2(a). The spectrum for Hy-CyH radical exhibits linewidth alternation, which arises from puckering motion of cyclo- hexane ring [25]. Fig. 2(b) shows a TR-EPR spectrum of the same system in the presence of C60. Similar to the TPO and C60 mixture system, the adduct radical of Hy-CyH to C60 was observed. This adduct shows negligibly small hyperfine splitting, which is unresolved due to broad- ening of the spectrum. This small splitting is quite reasonable since the unpaired electron locates in C60 moiety where no hydrogen exit. This is confirmed by small hfcc’s obtained by DFT calculation for optimized structure of Hy-CyH-C60. Hfcc of hydrogen atoms in the substituent shows at most 0.034 mT as listed in Table 1. The peak of Hy-CyH-C60
looks like asymmetrical shape because the peak of benzoyl polarized in net absorption phase overlapped with that of Hy-CyH-C60.
Similarly, we examined the 2Hy-Pr radical, which was produced by photolysis of DAR1173 as described in Scheme 3 [15,16,26]. TR-EPR spectrum in Fig. 3(a) shows generation of 2Hy-Pr radical under the laser excitation of DAR1173. In the presence of C60, adduct radicals were observed in Fig. 2(b) as a single peak without hyperfine structure.
Therefore, it was confirmed that 2Hy-Pr also undergoes addition reac- tion to C60.
It was found that DPPO, Hy-CyH, 2Hy-Pr radicals undergo addition reaction with C60 by TR-EPR analysis of the adduct radicals. Then, the ESE-detected pulsed EPR method was applied to measure addition rate constants of these radicals. Time profiles of S(τ) under various C60
concentration were measured. Fig. 4(a) shows the result for the Hy-CyH and C60 system presented as an example. The profiles are well charac- terized by a single exponential decaying function. As the monomer concentration becomes higher, the decay rate apparently increases. The TM* values were determined by fitting S(τ). Fig. 4(b) displays the Stern- Volmer plots of 1/TM* for Hy-CyH versus concentration of C60. The linear fitting of the plot gives the addition rate constants, k on the basis of Equation (2). Similar analysis was made for other systems of DPPO and 2Hy-Pr radicals and the Stern-Volmer plots are shown in Fig. 5. The results are summarized in Table 2. The rate constants for addition re- action to C60 were about 109 M−1 s−1 (M =mol dm3). These values are closed to the diffusion-controlled rate constant in toluene. Some of the rate constants for other olefin monomers are shown in Table 2. It is clear that rate constant for C60 are larger than the rate constants to various conventional olefin monomers such as acrylates as reported in the pre- vious studies [14,16,17].
Beside addition reactions of photo-product DPPO, Hy-CyH and 2Hy- Pr radicals, adduct radicals, DPPO-C60, Hy-CyH-C60 and 2Hy-Pr-C60
may also undergo reaction because they have reactive C60 moiety to accept the photo-product radicals. The rate constant of DPPO-C60
radical could be measured by monitoring their hyperfine lines (Fig. 1).
However, it is difficult to observe Hy-CyH-C60 and 2Hy-Pr-C60 radicals selectively, since the spectra of their adducts with another C60 may overlap with these mono-adducts. Photo-excited triplet C60 generated
under laser irradiation is another potential reactant of photo-product radicals. As their concentration is always being much lower than C60, some experimental efforts are needed.
To better understand these rate constants, the activation barrier (Ea) for the addition reaction was estimated approximately using the following two factors, reaction enthalpy change (enthalpy effect; ΔEenth) and charge transfer in the transition state (polar effect; ΔEpol.), according Scheme 2. Generation of Hy-CyH by the photolysis of IRG184.
Fig. 2.TR-EPR spectra recorded at (a) 0.5 μs and (b) 1.5 μs after the photolysis of IRG184 (160 mM) in toluene. The concentrations of C60 are (a) 0 mM and (b) 1.0 mM. TM* measurement was performed by monitoring the peak marked with an asterisk in (a).
Scheme 3.Generation of initiator radical 2Hy-Pr by the photolysis of DAR1173.
Fig. 3.TR-EPR spectra recorded at (a) 0.5 μs and (b) 1.5 μs after the photolysis of DAR1173 (52.5 mM) in toluene. The concentrations of C60 are (a) 0 mM and (b) 0.42 mM. TM* measurement was performed by monitoring the peak marked with an asterisk in (a).
to previous studies for addition rate constants of radicals to the double bonds of monomers [15–17]. Steric effects of the reactants were neglected for this rough estimation of rate constants. These two additive effects have been discussed for radical addition reactions [27,28], and Ea
was described by the equation (3).
Ea=E0a− ΔEenth− ΔEpol (3)
Ea0 is the activation energy in the absence of enthalpy and polar effects.
The ΔEenth and ΔEpol values were evaluated by the following equations (4) and (5) [28].
ΔEenth= − 0.22ΔHr (4)
ΔEpol=(χMon− χR)2
4(ηMon+ηR) (5)
The absolute electronegativity, χ and absolute hardness, η are given by χ= (IP+EA)/2 and η= (IP− EA)/2, where IP and EA stand for ionization potential and electron affinity of reactants, respectively [29,30]. The ΔHr is the reaction enthalpy change between adduct and reactants. IP, EA, and ΔHr values were obtained by DFT calculations and the results are summarized in Tables 3 and 4. The coefficient − 0.22 of ΔHr in equation (4) was derived [28] from analysis of experimental data in Ref. [27].
Assuming the Arrhenius equation for rate constants k, the following equation (6) is introduced.
lnk= (
lnA− Ea0
RT )
+
(ΔEenth+ΔEpol
)
RT (6)
A is frequency factor, R is gas constant and T is temperature. This equation (6) suggests ln k is in proportion to (ΔEenth +ΔEpol). We plotted the measured rate constants from the literatures against (ΔEenth + ΔEpol), as marked by open squares in Fig. 6. The least-square fitting for carbon-centered radicals (black) and DPPO, that is phosphorous- centered radical (red), was obtained under assumption that the slope value is (RT)−1. These plots almost follow the fitted line. Then we added the present results for C60 in Fig. 6, with closed squares and circle.
Contrary to the previous studies, the rate constants for C60 are much higher than the fitted black and red lines and are the diffusion-controlled values in toluene (~1010 M−1 s−1). One possible reason for this large rate constant is that C60 has 12 reactive 5-carbon ring, each of which undergoes radical scavenging reaction. Although details of reaction Fig. 4. (a) Time profiles of ESE intensity, S(τ), of Hy-CyH under various C60
concentrations. (b) Stern- Volmer plots of 1/TM* of Hy-CyH versus [C60].
Fig. 5.Stern- Volmer plots of 1/TM* of the radical versus [C60]. The radicals are (a) DPPO and (b) 2Hy-Pr.
Table 2
Addition rate constants of Hy-CyH, DPPO and 2Hy-Pr to C60 and to conven- tional olefine monomers in toluene at 298 K.
radical Radical quencher k1/109 M−1 s−1
Hy-CyH C60 2.6 ±0.3
tert-butylacrylate 0.026 ±0.001a
Vinyltrimethoxysilane 0.0016 ±0.0001a
DPPO C60 1.4 ±0.2
tert-butylacrylate 0.014 ±0.001a
Vinyltrimethoxysilane 0.00055 ±0.00009a
2Hy-Pr C60 7.1 ±0.4
tert-butylacrylate 1.19b
Vinyltrimethoxysilane 0.00032 ±0.00002a a Ref. [16].
b Ref. [14].
Table 3
Reaction enthalpy change ΔHr, and activation energy shifts by enthalpy (ΔEenth) and polar (ΔEpol) effects.
Radical ΔHr/kJ mol−1 ΔEenth/kJ mol−1 ΔEpol/kJ mol−1
HyCyH −80.06 17.61 19.96
DPPO −49.33 10.85 8.11
2Hy-Pr −73.77 16.23 19.02
mechanism are not fully understood yet, it is concluded that C60
certainly works as good radical scavenger through addition reaction because of its high reactivity with radicals.
In conclusion, quenching reaction of radicals by C60 was investigated by the TR-EPR and the ESE detected pulsed EPR methods. TR-EPR spectra show direct evidences that radical addition reaction to C60 oc- curs to produce adduct radicals. The rate constant, k of the radical to C60 was determined by the Stern-Volmer analysis of the decay rate of ESE intensity of radical. It was found that k values were larger than 109 M−1 s−1 in toluene solutions, which means the reactions are diffusion- controlled. Unlike olefin monomers examined in our previous studies, the k values for C60 addition reaction are much higher than those explained by enthalpy and polar effects in the semi-empirical model calculation. It was concluded that C60 is certainly a good scavenger of radicals, which gives an adduct radical with the diffusion-controlled rate constant.
CRediT authorship contribution statement
Hirona Takahashi: Methodology, Investigation, Visualization, Writing - original draft, Supervision. Hiroki Hirano: Investigation, Validation, Data curation. Kyohei Nomura: Investigation. Kenta Hagiwara: Investigation. Akio Kawai: Conceptualization, Methodol- ogy, Writing - review & editing, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
AK express his thanks to Dr. Kunio Kondo and Dr. Ryuji Monden and their affiliation, Showa Denko K.K. for their advices and financial sup- port to this work. AK also thanks Ms. Yoko Kawai for her assistance in this research. HT thanks Grant-in-Aid for Japan Society for the Promo- tion of Sience Postdoctral Fellows No. 16J01871. This study was sup- ported in part by Grants-in-Aid for Scientific Research No. 19K05376 from the Ministry of Education, Culture, Sports, Science and Technol- ogy, Japan, by Nippon Sheet Glass Foundation for Materials Science and Engineering 2019–2021, and by a grant from Research Institute for In- tegrated Science in Kanagawa University 2020.
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