Observation of Splitting of EPR Spectral Lines without Any

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Observation of Splitting of EPR Spectral Lines without Any Concomitant Splitting in Energy Levels Vinayak Rane and Ranjan Das* Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India ABSTRACT: Splitting of spectral lines is generally associated with corresponding splitting of appropriate energy levels. However, here we report our observation of the splittings of hyperfine lines in the time-resolved EPR spectrum of a stable free radical that participates in the quenching of an excited molecule. The observed splitting does not arise from any splitting of the energy levels of the radical, nor due to Torrey oscillations but is a culmination of a detailed interplay of photophysical and magnetic resonance dynamics of the quenching process. In particular, sequential quenching of an excited singlet and triplet states by the free radical, generation of opposite electron spin polarization, and time-dependent EPR line width evolving according to the Bloch equations contribute to the appearance of unusual lineshapes of the observed EPR spectrum. This effect is sufficiently general and should always be observable in the time-resolved EPR experiments on free radicals involved in photophysical quenching of excited molecules. We also point out that this effect cannot be seen in Fourier transform EPR spectroscopy.

I. INTRODUCTION Since the first proposal of stationary electronic states of hydrogen atom by Niels Bohr, the existence of well-defined discrete energy levels and their association with the observed spectra of various atomic and molecular systems have been well-established. This is now a corner stone of all spectroscopic studies: a one-to-one correspondence between the observed lines in a given spectrum and the existence of discrete energy levels involved with the transitions that generate the spectrum. It is therefore highly unusual to observe spectral lines to which no specific energy levels could be assigned. However, in magnetic resonance experiments, several experimental conditions are well-known where such spectral lines are seen. One is the magnetic field or frequency modulation-induced sidebands, which are observed while recording steady-state NMR spectra.1 The recorded spectrum is superimposed with the harmonics of modulating frequency, which are symmetrically placed with respect to the true magnetic resonance signal. Spinning the NMR sample along the direction of the external field to reduce its inhomogeneity also produces sidebands of the spinning frequency symmetrically placed at the Larmor frequency of transition.2 In a similar manner, spinning of NMR powder samples are carried out at the magic angle to reduce the powder-pattern spectral width arising from dipolar interactions among magnetic nuclei. Such spinning also produces sidebands.3 In steady-state EPR experiments, magnetic field modulation and phase-sensitive detection at the modulation frequency also produces sidebands, which are, however, not easily resolved.4 Nevertheless, these sidebands contribute to the observed widths of the EPR spectra. The appearance of the sidebands in all these cases is due to the generation of sum and difference frequencies of the carrier frequency and the © 2014 American Chemical Society

harmonics of the modulating frequency. There is no splitting of the energy levels, and they have no bearing on the dynamics of the observed transitions. In this work, we report our observation of splitting of hyperfine lines of the time-resolved EPR spectrum of a stable free radical, which participates in the quenching of an excited moiety attached to itself. The splittings do not arise from any time-dependent splitting of the energy levels of the free radical. At the same time, its origin is very different from the situations mentioned above where modulation of one kind or another is responsible for the appearance of splittings. Our analysis showed that the appearance of splittings in the recorded time-resolved EPR spectra is a net result of a detailed ultrafast quenching dynamics, coupled with the evolution of the magnetization according to the Bloch equations. No splitting of energy levels of the spin system, and consequently, no intrinsic splitting of the EPR spectrum occurs. Nevertheless, observation and analyses of such spectral line shape could give important insight into the dynamics of the process. Our molecular system is the stable nitroxyl free radical 2,2′,6,6′-tetramethylpiperidine N-oxyl (TEMPO), covalently linked to a naphthalene moiety [Nap-CH2O-TEMPO, Chart 1]. When the naphthalene moiety is excited by light, the TEMPO moiety sequentially quenches first the singlet naphthalene to produce triplet naphthalene and then the triplet naphthalene to produce singlet naphthalene in the ground state. In each step, the TEMPO radical experiences electron spin polarization, and its line width of the spinReceived: August 14, 2014 Revised: August 28, 2014 Published: September 11, 2014 8689

dx.doi.org/10.1021/jp508231m | J. Phys. Chem. A 2014, 118, 8689−8694

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Ground state absorption and steady state fluorescence measurements were done on a UV/vis spectrophotometer (PerkinElmer model Lambda 25) and a spectrofluorimeter (Jobin-Yvon model Fluorolog-3), respectively. Fluorescence lifetimes were recorded by a picosecond time-correlated single photon counting spectrometer. The wavelength of excitation was 295 nm. No degassing was done for these measurements, since no effect of oxygen was observed on the lifetimes of the linked molecules. This was attributed to the efficient quenching of the naphthalene moiety by the covalently linked TEMPO free radical. A nanosecond laser flash photolysis setup was used for recording transient excited state absorption spectra. All these experiments were done with n-hexane as the solvent.

Chart 1. Structure of Naphthalene-TEMPO Linked Molecule (Nap-CH2O-TEMPO)

polarized EPR spectra evolve with time. The resultant line shape of the EPR spectrum of the TEMPO moiety shows the appearance of splitting in certain intermediate times, which are dictated by the details of the dynamics of the photophysical processes.

III. RESULTS AND DISCUSSION The steady-state EPR spectrum of Nap-CH2O-TEMPO is shown in Figure 1A. The spectrum was recorded in the usual

II. EXPERIMENTAL SECTION Synthesis of Nap-CH2O-TEMPO. This molecule was synthesized by following the protocol developed by Dr. Amol Deorukhkar in our laboratory. 4-Hydroxy TEMPO (1.72 g, 10 mmol) was dissolved in 10 mL of dry THF and cooled to 0 °C. Sodium hydride (0.29 g, 12 mmol) was added gradually with constant stirring. The reaction was stirred for 30 min at low temperature, followed by addition of 2-bromomethyl naphthalene (2.2 g, 10 mmol). The reaction mixture was then warmed to room temperature and stirred until completion (1 day), as monitored by TLC. The reaction mixture was diluted with water. The resulting solution was extracted with ethyl acetate (3 × 50 mL). The combined organic layers were dried with sodium sulfate, filtered, concentrated, and purified by column chromatography using 0 to 10% ethyl acetate/hexane as the eluent to provide an orange solid powder of Nap-CH2OTEMPO. 1H NMR δ (CD3OD): 7.85 (m, 4H), 7.48 (br-s, 3H), 4.71 (s, 1H), 3.82 (br-s, 1H), 2.12 (d, 2H), 1.80 (m, 2H), 1.36 (m, 6H), 1.25 (m, 6H). Instruments. Steady-state and time-resolved EPR spectra were recorded in a laboratory-built X-band EPR spectrometer.5 For all the time-resolved EPR spectra presented here, the photolyzing source was an excimer laser (Coherent model: COMPex Pro 110) operating at a repetition rate of 60 Hz, giving about 180 mJ of energy per pulse at 248 nm. The energy reaching the sample was about 8 mJ/pulse. However, early experiments were carried out using the fourth harmonic of an Nd:YAG laser (Quantel model: YG-981C; wavelength: 266 nm; repetition rate: 30 Hz; and energy output: about 30 mJ/ pulse). The sample in n-hexane was degassed by bubbling with pure and dry IOLAR grade nitrogen and passed through an EPR flat cell with a path length of 0.5 mm. For preliminary TREPR experiments, the solution from the exit was put back into the sample reservoir, but final EPR spectra for quantitative analyses were recorded while discarding the sample coming out from the cavity after photolysis. The temperature of the sample was changed by passing the liquid through a cooling coil, which was kept immersed in a low-temperature bath of ethanol. The temperature of the bath was controlled by pouring liquid nitrogen into it. This way the temperature of the sample could be controlled to within ±2 °C. In order to obtain the TREPR spectra with a high signal-to-noise ratio, which were suitable for quantitative analyses, we lowered the sample temperature to the lowest possible value. We could reach about −50 °C in this way. During the recording of the time-resolved EPR spectra, the cavity was flushed with commercial oxygen gas.6

Figure 1. (A) Steady-state first derivative EPR spectrum of NapCH2O-TEMPO linked molecule in degassed n-hexane solution (concentration 3.0 mM) near 0 °C. (B) Steady-state absorptive EPR spectrum of Nap-CH2O-TEMPO, obtained by numerical integration of the spectrum A. (C) Time-resolved EPR spectrum of Nap-CH2OTEMPO (concentration 1.5 mM) recorded in the time window from 150 to 250 ns, after a 248 nm laser pulse. The preamplifier of the timeresolved EPR spectrometer was AC coupled. The spectrum is emissively spin-polarized and shows splitting of all three 14N hyperfine lines. Temperature: − 45 °C. Microwave power: 5 mW for both the spectra. A: absorption and E: emission.

first-derivative mode, employing a 100 kHz magnetic field modulation and a phase sensitive detection. The spectrum in Figure 1B is the absorptive mode, obtained by numerically integrating the spectrum in Figure 1A. The steady-state EPR spectra show three hyperfine lines of equal intensity, arising from the Fermi contact interaction between the unpaired electron of the free radical and the nuclear spin of 14N nucleus of the nitroxyl group. The intensity of an EPR transition is proportional to the electron spin polarization, P ≡ (nβ − nα)/ (nβ + nα), where nα and nβ are the number of α and β spins, respectively. For the steady-state EPR spectrum shown in Figure 1, P is governed by the Boltzmann distribution of electron spins in the various Zeeman levels in the magnetic field of the EPR spectrometer, and the value of the polarization at thermal equilibrium, Peq, is of the order of +0.001. On photoexcitation, naphthalene is known to go to a triplet excited state via a singlet excited state. The time constant of this intersystem crossing process is estimated to be ∼120 ns, based on the triplet quantum yield.7 When a nanosecond pulse of UV laser light excites the naphthalene moiety, the excited state is 8690

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A spin-correlated radical pair, generated by a light pulse, often produces splitting of energy levels due to their exchange or dipolar interaction because of the close proximity of the two radicals.12,13 Spin-selective population of these energy levels produces “antiphase” type of line shape for each of the EPR transitions. When a covalent linkage holds the two radicals of the pair in a frozen solution, their anisotropic interactions are not averaged out. For such a system, the time-resolved EPR spectra are known to produce several unusual shapes, in general.14−17 For such mechanisms to operate, the excited state must be present during the observation of the EPR spectra. From our inability to detect any excited species in the nanosecond laser flash photolysis setup, we attributed the lifetime of the exited states to be less than 50 ns. In addition, the experiments were carried out in liquid solutions where anisotropic interactions are expected to get averaged out appreciably. Thus, we did not consider the doublet-quartet spin-correlated pair likely to produce the observed EPR line shape as late as 200 ns after photoexcitation. The evolution of the EPR spectra of the nonequilibrium magnetization of the radical under the influence of the Zeeman magnetic field and the microwave radiation used to detect the EPR spectrum is governed by the Bloch equations

quenched by the free radical in its proximity. The quenching process is usually a physical process involving a spin-flip through an enhanced intersystem crossing (EISC) caused by the free radical. The EPR spectrum of the radical, recorded by integrating the time-dependent microwave signal for 150−250 ns after the laser pulse while scanning the Zeeman magnetic field, is shown in Figure 1C. This spectrum was recorded in a direct detection mode, without using any magnetic field modulation. Spectra, recorded is this manner, appear absorptive or emissive, depending on the sign of P. All three hyperfine lines of the spectrum in Figure 1C were seen to be emissive and split into well-resolved doublets. Before discussing the implications of the above observation, we make a few remarks on the primary photophysical step. The ground state absorption spectrum of Nap-CH2O-TEMPO was found to be a superposition of the spectrum due to the naphthalene moiety and the TEMPO moiety, in 1:1 ratio. Moreover, its absorbance at 248 nm, the wavelength of our experiment, was almost equally divided by these moieties. Hence, the primary photophysical event is the direct excitation of naphthalene and TEMPO in equal proportion, leading to an equal mixture of these two excited species. Our aim was to excite only the naphthalene moiety, and not the TEMPO, and study the photophysical quenching by the latter. To that end, initial experiments were carried out using a 266 nm laser light, where the molar absorbance of the naphthalene moiety is about 5 times higher than that of the TEMPO moiety. The timeresolved EPR spectra did not show any noticeable difference whether the exciting light was 266 or 248 nm. This showed that the direct excitation of the TEMPO moiety did not contribute to any change of the magnetization. We therefore decided to use the 248 nm excimer laser for all our experiments, as this laser was much more stable and easy to use. One obvious explanation for the origin of the doublet splitting of all the hyperfine lines could be that the photoexcitation of Nap-CH2O-TEMPO generates a new radical species similar to the initial TEMPO, but with an additional hyperfine coupling to a spin-1/2 nucleus, possibly a proton. This possibility was ruled out as no photochemical reaction was detected, and the Nap-CH2O-TEMPO was found to be photostable in the solvent n-hexane, used here. Quenching of an excited electronic state by a free radical through an EISC process is spin selective.8,9 The overall spin state of the excited chromophore (here, naphthalene) and the radical can be doublet or quartet. Intersystem crossing between them, where the zero-field splitting of the triplet moiety acts as the dominant perturbation, drives the spin polarization of the radical from the Boltzmann distribution to a non-Boltzmann distribution. The resulting polarization could have P > Peq > 0 or P < 0. Its magnitude depends on the mixing of the quartet and doublet spin states. As the quartet is usually higher than the doublet, the sign is positive when an excited singlet is quenched and negative when a triplet is quenched. This, briefly, is the radical triplet pair mechanism (RTPM) of electron spin polarization of a radical that quenches an excited molecule.8,9 Transient EPR spectra of excited doublet and quartet states of chromophore−radical linked molecules have been reported.10,11 The spectra of these two spin states differ primarily in their g values and time-dependence. The splittings seen in Figure 1C could not be attributed to two such species with different spin states, as they showed exactly the same time dependence, and with the passage of time, the magnitude of the splitting decreased, eventually leading to a single merged line.

dM x M = − x − ΔωMy dt T2 dM y dt

= +ΔωMx −

My T2

Mx(0) = Mx0 + ω1Mz

dM z M − Mz∞ = −ω1My − z dt T1

My(0) = M y0 Mz(0) = Mz0

(1)

Here ω1 = γB1; ω0 = γB0 ; Δω ≡ ω0 − ω

and M0x, M0y , M0z are the initial nonequilibrium magnetization after the pulse perturbation (quenching of the excited state by the radical, in our case).18,19 An important consequence of these coupled differential equations is that the magnetization My(t), the component that usually produces the EPR spectra, shows oscillations, called Torrey oscillations, as a function of time and magnetic field. The frequency of the oscillation depends on the intensity of the microwave magnetic field B1 and the relaxation times T1 and T2.20 Thus, if the B1 field is not insignificantly small, the oscillations along the magnetic field domain can appear as structures that can be mistaken as hyperfine splittings in the EPR spectra, as shown in the timeresolved EPR spectra of the duroquinone anion radical.21 In a linked system of carotenoid−porphyrin−diquinone tetrad, spectral distortion in the time-resolved EPR spectra at times shorter than T2 is seen. In particular, spectra at earlier times appear to be very different from those at later times, which “may lead to misinterpretation of the radical intermediates participating in the reaction route.”22 This well-known effect of high microwave magnetic field, however, was not the cause of the splitting seen in Figure 1C, as the effect was independent of the intensity of the B1 magnetic field. Another effect of the time evolution of the magnetization is the time-dependent line width: the EPR spectrum, after a perturbation of the spin system, starts with a broad line, reaching the steady-state width characterized by its T2 with the passage of time. Broad EPR lines that evolve to narrow lines 8691

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have been reported for transient radicals.23,24 Figure 2 shows this effect in simulated EPR signals calculated according to eq 1.

Then at about 300 ns, the spectrum changed to a completely emissive mode, with peculiar shapes in between. The signal after 300 ns steadily went toward the thermally equilibrated spectrum, with a characteristic time of T1. Inversion of EPR lines from totally emissive to totally absorptive with peculiar shapes in between, similar to the ones shown in Figure 3A, has been reported by Basu et al.24 In their system, the time-resolved EPR spectra are for spin-polarized radicals, which are involved in degenerate electron-exchange reactions. In contrast, in our system, no chemical reaction was involved. Therefore, only the photophysical processes should be the underlying cause for the appearance of the splittings. The time dependence of the EPR spectra in Figure 3A revealed that, at an early stage of evolution, the spectra were associated with a positive P ≫ Peq. This is the characteristic signature of change of electron spin polarization of the radical due to the quenching of the singlet excited state of naphthalene. The later spectra, similarly, were associated with a negative P (with |P| ≫ Peq) and were the result of the quenching of the triplet state of naphthalene. The intramolecular quenching processes are very fast, with a time constant of 0) and PT (< 0) are the spin polarization due to the respective quenching process. To obtain the time evolution of the magnetization of the free radical and the EPR spectra, the Mz component of the Bloch equations was modified in the following manner8,9 dM z M − Mz∞ = −ω1My − z + PSkqs[S − R ] dt T1 + PTkqt[T1 − R ]

(2)

The coupled differential equations, based on the various pathways of Scheme 1, were solved by the Laplace transform technique. After the laser pulse at t = 0, the initial concentrations of all the intermediate species in Scheme 1 was zero, except the concentration of the excited singlet naphthalene. This gave the time-dependence of [S − R] and [T1 − R]. These expressions are used in eq 2, and the modified Bloch equations were also solved by the Laplace transform technique. The initial magnetization was the thermally equilibrated magnetization of the radical. The analytical expression of the My component was obtained as a function of time and the Zeeman magnetic field B0. The time constants of the quenching processes were either determined or estimated from time-resolved optical measurements. Similarly, the relaxation times and the microwave magnetic field B1 were also measured experimentally. The only adjustable parameters were the polarization PS and PT, which were reported in units of Peq. The time-dependent magnetization thus obtained was convolved with the response time of the EPR spectrometer. The simulated EPR spectra at different times after the laser pulse were calculated. The calculated EPR spectra, shown in Figure 3B, reproduced the experimental spectra recorded at different times after the laser pulse. The values of PS and PT remained the same for all these time-resolved EPR spectra. The origin of the peculiar lineshapes of the time-resolved EPR spectra, as seen here, stems from the evolution of the magnetization due to the sequential quenching of the naphthalene excited states. The sudden change in the magnetization makes the line width very broad, which evolves under the Bloch equations to an equilibrium line width characterized by T2. The sequential quenching of the excited states gives rise to the overall EPR line shape to be a superposition of an absorptive narrower component arising from those radicals which quench the excited singlet state earlier and a broader emissive component from those which quench the excited triplet state later. In addition to that, the change in the sign of P from positive to negative, due to the sequential quenching of naphthalene, produces the observed line shape that changes from an overall absorptive to an overall emissive phase and causes the apparent splittings of various

IV. CONCLUSIONS We have shown that time-resolved EPR spectra of a radical can show resolved lines, resembling extra hyperfine lines, when the radical participates in the photophysical quenching of excited singlet and triplet states of a molecule in a sequential manner. The appearance of such splittings does not reflect any splitting of the energy levels associated with the EPR transitions but is a net result of time-dependent evolution of the EPR line shape. This evolution of the line shape is controlled by the detailed dynamics of the quenching process and the time-dependence of the magnetization according to the Bloch equations. We also noted that such time-dependent evolution of line shape and the possible appearance of splittings cannot be seen in the EPR spectra recorded by Fourier-transform EPR techniques. Failure to recognize the origin of such splittings can lead to misinterpretation of the spectra and incorrect details of the dynamics of the photophysical and photochemical pathways.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 8693

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Notes

(18) Atkins, P. W.; McLauchlan, K. A.; Percival, P. W. Electron SpinLattice Relaxation Times from the Decay of E.S.R. Emission Spectra. Mol. Phys. 1973, 25, 281−296. (19) Percival, P. W.; Hyde, J. S. Pulsed EPR Spectrometer, II. Rev. Sci. Instrum. 1975, 46, 1522−1529. (20) Hore, P. J.; McLauchlan, K. A. Chemically Induced Dynamic Electron Polarization (CIDEP) and Spin-Relaxation Measurements by Flash-Photolysis Electron Paramagnetic Resonance Methods. J. Magn. Reson. 1979, 36, 129−134. (21) Hore, P. J.; McLauchlan, K. A.; Frydkjaer, S.; Muus, L. T. Structure in Time-resolved ESR Spectra. Chem. Phys. Lett. 1981, 77, 127−130. (22) Hasharoni, K.; Levanon, H.; Bowman, M. K.; Norris, J. R.; Gust, D.; Moore, T. A.; Moore, A. L. Analysis of Time-Resolved CW-EPR Spectra of Short-Lived Radicals at Different Times after Laser Excitation. Appl. Magn. Reson. 1990, 1, 357−368. (23) Verma, N. C.; Fessenden, R. W. Time Resolved ESR Spectroscopy. II. The Behavior of H Atom Signals. J. Chem. Phys. 1973, 58, 2501−2506. (24) Basu, S.; McLauchlan, K. A.; Ritchie, A. J. D. Time-resolved Electron Spin Resonance with Electron Spin Polarization (CIDEP) as a Sensitive Probe of Degenerate Electron Exchange Reactions. Chem. Phys. Lett. 1984, 105, 447−450. (25) Bowman, M. K. Fourier Transform Electron Spin Resonance. In Modern Pulsed and Continuous-Wave Electron Spin Resonance; Kevan, L.; Bowman, M. K., Eds., John Wiley: New York, 1990; Chapter 1.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank Prof. Art van der Est for fruitful discussions. We also thank Mr Krishnendu Kundu for many useful, constructive, and stimulating suggestions and help with the simulations described here.

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REFERENCES

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