Radical-triplet Pair Mechanism Research Articles

Sample volume effects in optical overhauser dynamic nuclear polarization

The optical dynamic nuclear polarization (DNP) method has been proposed as an alternative to microwave pumping as a hyperpolarization method for solution-state NMR studies. Using continuous laser illumination to photogenerate triplet states in the presence of a persistent radical produces chemically-induced dynamic electron polarization (CIDEP) via the radical-triplet pair mechanism (RTPM), with cross-relaxation transferring this to nuclear hyperpolarization via an Overhauser mechanism. Numerical simulations have previously indicated that reducing the sample volume while maintaining a constant optical density can significantly increase the NMR signal enhancement, due to the larger steady-state concentration of triplets obtained. Here we provide the first experimental confirmation of these effects, producing a nearly five-fold increase in the optical DNP enhancement factor just by reducing the sample volume with optimal dye and radical concentrations adjusted for each optical path length. The results are supported with an in depth analysis of volume effects in the numerical model, with which they are in good qualitative agreement. These important observations will impact on the future development of the technique, with particular significance for attempts to apply DNP methods to increase sensitivity for volume-limited biological samples.

Optically-generated Overhauser dynamic nuclear polarization: A numerical analysis.

Recently, an alternative approach to dynamic nuclear polarization (DNP) in the liquid state was introduced using optical illumination instead of microwave pumping. By exciting a suitable dye to the triplet state which undergoes a diffusive encounter with a persistent radical forming a quartet-doublet pair in the encounter complex, dynamic electron polarization (DEP) is generated via the radical-triplet pair mechanism. Subsequent cross-relaxation generates nuclear polarization without the need for microwave saturation of the electronic transitions. Here, we present a theoretical justification for the initial experimental results by means of numerical simulations. These allow investigation of the effects of various experimental parameters, such as radical and dye concentrations, sample geometry, and laser power, on the DNP enhancement factors, providing targets for experimental optimization. It is predicted that reducing the sample volume will result in larger enhancements by permitting a higher concentration of triplets in a sample of increased optical density. We also explore the effects of the pulsed laser rather than continuous-wave illumination, rationalizing the failure to observe the optical DNP effect under illumination conditions common to DEP experiments. Examining the influence of the illumination duty cycle, the conditions necessary to permit the use of pulsed illumination without compromising signal enhancement are determined, which may reduce undesirable laser heating effects. This first simulation of the optical DNP method therefore underpins the further development of the technology.

A phenomenological scheme for reversed quartet mechanism of electron spin polarization in covalently linked systems of chromophore and free radical: Determination of magnitude of polarization and application to pyrene-TEMPO linked molecules.

Generation of electron spin polarization (ESP) during the bimolecular quenching of an excited chromophore by a free radical is generally explained by the radical-triplet pair mechanism, which is capable of giving the magnitudes of ESP arising from the quenching of the singlet or the triplet excited chromophore. When the chromophore and the free radical are covalently linked, although there are several mechanisms to explain the observed spin-polarized electron paramagnetic resonance signals under a variety of experimental conditions and in different chromophore-radical systems, there are no schemes that allow quantitative determination of the magnitude of ESP. In this work, we present a phenomenological scheme with this objective. In this scheme, we have incorporated several concepts of the reversed quartet mechanism of Rozenshtein et al. [J. Phys. Chem. A 109, 11144 (2005)] to our phenomenological sequential quenching scheme [V. Rane and R. Das, J. Phys. Chem. A 119, 5515 (2015)] of ESP in covalently linked chromophore-radical systems. This phenomenological reversed quartet scheme is able to explain the observed inversion of ESP with time and can also give a quantitative measure of the absorptive and emissive ESP in such systems. We have applied this scheme to the photophysical quenching of a series of newly synthesized pyrene-TEMPO molecules, where a spacer group of different lengths covalently links the pyrene chromophore and the TEMPO free radical. Given the simplicity of our scheme, reasonable estimates of the magnitudes of the ESP have been obtained in all cases.

Toward Achieving the Theoretical Limit of Electron Spin Polarization in Covalently Linked Radical-Chromophore Dyads.

Electron spin systems with non-Boltzmann spin distributions are commonly observed in photophysical and photochemical processes involving free radicals. Understanding the origin of electron spin polarization (ESP) reveals detailed insight into the spin-dependent interactions occurring in these processes. The ability to transfer ESP to the nuclear spins of the solvent to produce large nuclear polarization is itself an active area of research in the field called dynamic nuclear polarization (DNP). Harnessing ESP in such fields demands a large magnitude of ESP and importantly on free radicals that are stable. In this work, we explored various factors that could play a prominent role in generation of ESP on the stable nitroxyl radical. By exploiting the dependence of ESP on the zero-field splitting parameter DT of the chromophore triplets and a careful choice of the chromophores for efficient quenching of their excited states by a free radical, we present here our strategies for designing covalently linked chromophore-free radical dyads, which could generate high magnitudes of ESP. Syntheses and EPR studies on a series of such dyads to demonstrate successful realization of our strategy are reported here. Our work reveals that, along with the DT value and the triplet quenching efficiency, the relative orientational dynamics of the radical and chromophore moiety and an efficient intersystem crossing of the chromophore primarily govern the ESP of the chromophore-radical dyads. One such molecule, an anthraquinone moiety linked to a TEMPO free radical that optimally satisfies the above criteria, produces a very large value of ESP-about 300 times its Boltzmann value-reaching almost half of the theoretical maximum of the radical-triplet pair mechanism of ESP. Coupled with its excellent photostability in benzene and the use of ambient conditions, this molecule should prove to be highly desirable in photoinduced DNP studies and other applications such as highly sensitive magnetometers, which require generation of large nuclear spin polarization in the solvent by transferring the electron spin polarization.

Radical-Enhanced Intersystem Crossing in New Bodipy Derivatives and Application for Efficient Triplet-Triplet Annihilation Upconversion.

A long-lived triplet excited state of the well-known fluorophore boron dipyrromethene (Bodipy) was observed for the first time via efficient radical-enhanced intersystem crossing (EISC). The triplet state has been obtained in two dyads in which the Bodipy unit is linked to a nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), with two different length spacers. The photophysical properties were studied with steady-state and time-resolved transient optical spectroscopies and electron spin resonance (cw-ESR and TR-ESR). The fluorescence of Bodipy units is significantly quenched in the dyads, and the spin-polarized TEMPO signals were observed with TR-ESR, generated by a radical triplet pair mechanism. Efficient EISC (ΦT = 80%) was observed for the dyad with a shorter linker, and the triplet state lifetime of the Bodipy chromophore is exceptionally long (62 μs). The EISC takes 250 ps. Poor ISC was observed for the dyad with a longer linker. The efficient ISC and long-lived triplet excited state in this flexible system are in stark contrast to the previously studied rigid EISC systems. The EISC effect was employed for the first time to perform triplet-triplet annihilation (TTA) upconversion (quantum yield ΦUC = 6.7%).

Photoswitching of triplet-triplet annihilation upconversion with photo-generated radical from hexaphenylbiimidazole

Photoirradiation generated radical from hexaphenyl-biimidazole (HPBI) was used for reversible switching of triplet-triplet annihilation (TTA) upconversion, based on quenching of the photosensitizer triplet state by radical-triplet pair mechanism. Upon 365nm irradiation, the TTA upconversion in a system composed by a boron-dipyrromethene (BODIPY) derivative and perylene, was completely switched off due to quenching of triplet state of photosensitizer by photogenerated radical from HPBI. The upconversion was recovered after leaving the samples in darkness, due to regeneration of HPBI Dimer. The photophysical process involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV–vis absorption spectroscopy, nanosecond transient absorption spectroscopy and EPR spectroscopy.

Distance Dependence of Electron Spin Polarization during Photophysical Quenching of Excited Naphthalene by TEMPO Radical.

Quenching of excited states by a free radical is generally studied in systems where these two are separate entities freely moving in a liquid solution. Random diffusive encounters bring them together to cause the quenching and leave the spins of the radical polarized. In the dynamics of the radical-triplet pair mechanism of the generation of electron spin polarization (ESP), the distance-dependent exchange interaction plays a crucial role. To investigate how the distance between the excited molecule and the radical influences the ESP, we have covalently linked a naphthalene moiety to a TEMPO free radical through a spacer group of three different lengths. We compared the ESP process of these linked compounds with that of the usual "unlinked system" of naphthalene and TEMPO through time-resolved EPR experiments at low temperature in n-hexane solution. The time evolution of both the linked and the "unlinked system" was treated on a similar footing. The time-dependent EPR signal was analyzed by combining photophysical kinetics and time-dependent Bloch equations incorporating spin dynamics. Sequential quenching of the singlet state and the triplet state of naphthalene was seen in all the systems, as revealed through the spin-polarized TREPR spectra of opposite phase. The magnitudes of the ESP in the linked molecules were higher than those of the "unlinked system," showing that when the two moieties are held together greater mixing of quartet-doublet states takes place. The magnitudes of ESP steadily decrease with increasing the length of the spacer group. The polarization magnitudes due to triplet quenching and singlet quenching are very similar, differing by a factor of only ∼2. These characteristics show that for all the linked molecules the quenching takes place in the "weak exchange" regime and at almost the same distance of separation between the two moieties. Our results also showed that observation of small absorptive TREPR signals does not necessarily imply that its magnitude of polarization is small.

Radical-triplet pair mechanism of electron spin polarization. Detailed theoretical treatment.

Specific features of net chemically induced dynamic electron spin polarization (CIDEP) P(n), generated in liquid-phase triplet-radical (TR) quenching, are analyzed in detail within the general model, which allows for fairly simple analysis of CIDEP both numerically and analytically. This model enables one to accurately treat nonadiabatic transitions between the terms of TR-pair spin Hamiltonian, resulting in CIDEP generation. The proposed theory predicts fairly simple analytical dependence of P(n) on parameters of the model. In particular, it is shown that within the wide region of parameters the P(n) dependence on the coefficient of relative TR diffusion D(r) is described by simple linear relation P(n)(-1)(D(r)) ≈ Q0 + q̅(n)D(r) (Q0 and q̅(n) are independent of D(r)). It is also demonstrated that obtained numerical and analytical results are very helpful for the analysis of experimental data, which is demonstrated by analyzing the experimental D(r)-dependence of P(n).

A solution-processable polymer-grafted graphene oxide derivative for nonvolatile rewritable memory

A new conjugated polymer, poly{[9,9-di(triphenylamine) fluorene]-[9,9-dihexylfluorene]-[4,4′-(9H-fluorene-9,9-diyl) dibenzenamine]}(PTHF), was synthesized by the Suzuki coupling reaction and used to react with the graphene oxide (GO) containing surface-bonded acyl chloride moieties to give a solution-processable GO–PTHF. Covalent attachment of PTHF onto GO resulted in the partial fluorescence quenching of the parent polymer, suggesting energy and/or electron transfer between the excited singlet states of the PTHF moiety and the GO moiety. The absolute photoluminescence (PL) quantum yields (ΦPL) of the samples were changed from 0.59 for PTHF to 0.31 for GO–PTHF. A light-induced electron paramagnetic resonance (LEPR) technique was employed to study the photoinduced spins in the conjugated GO–PTHF. Upon irradiation with a 432 nm laser, both the polaron signals of the polymer chain at g = 2.1132, 2.0665 and 2.0197, and the signals of radical anions on GO at g// = 1.9726 and g⊥ = 1.9747 were kept unchanged, while the signal at g = 2.0002 almost completely disappeared due to a typical photoinduced selective quenching of the triplet state or a photoinduced spin polarization transfer process acting with a radical–triplet pair mechanism. A memory device based on GO–PTHF, in which the GO–PTHF film was sandwiched between the indium–tin oxide and Al electrodes, can be switched to the ON state under a negative electrical sweep, and can also be reset to the initial OFF state by a reverse (positive) electrical sweep.

Quenching dynamics study on photoinduced excited triplet duroquinone by TEMPO in 1,2-propandiol

Chemically induced dynamic electron polarization (CIDEP) spectrum and transient absorptive spectrum are recorded in photolysis of duroquinone (DQ) in 1,2-propanodiol (PG). Durosemiquinone neutral radical DQH• and PG ketyl radical \({\text{CH}}_{3}{{{\dot{\text{C}}}\text{OHCH}}}_{2}{\text{OH}}\) are produced through hydrogen transfer reaction from PG to 3DQ*. When stable radical TEMPO is added to DQ/PG solution, photolysis results in CIDEP on TEMPO, which can be interpreted as a quartet precursor radical-triplet pair mechanism (QP-RTPM). There is competition between PG and TEMPO to quench 3DQ*. The CIDEP intensity of DQH• decreases with the increase of TEMPO concentration. The quenching dynamics in photolysis of DQ/TEMPO/PG system is analyzed in detail. Based on the dynamics analysis and the measurement of the lifetime of 3DQ* by its transient absorbance decay, the quenching rate constant of 3DQ* by TEMPO in PG is obtained as 1.34 × 107 L mol−1 s−1. This quenching rate constant is closely diffusion-controlled.

CIDEP Created by the Quenching of Photo-Excited Tryptophan at Protein Surface: A Challenge to CIDEP Probing of Protein Structural Changes

Quenching of the triplet excited state of molecular tryptophan by nitroxide radical in 1,4-dioxane and water solutions was investigated by means of time-resolved electron paramagnetic resonance (EPR) and Fourier-transform (FT)-EPR. The chemically induced dynamic electron polarization (CIDEP) signals with net emissive phase were recorded at these quenching events and were analyzed through radical-triplet pair mechanism. The CIDEP time profiles were well reproduced by Bloch and kinetic equations, assuming radical-triplet pair mechanism with the appropriate quenching rate constants. From a comparison of the simulation and the experiment, CIDEP enhancement factor in 1,4-dioxane was determined to be −30 × P eq, where P eq is the spin polarization of nitroxide at thermal equilibrium. Net emissive CIDEP was also observed by FT-EPR measurements on the nitroxide quenching of the triplet excited state of tryptophan residue in α-lactalbumin. Magnitude of CIDEP created in α-lactalbumin/nitroxide system depends on the pH condition of α-lactalbumin solution, which is related to protein folding dynamics. We argue the CIDEP mechanism at the α-lactalbumin surface and propose a possibility of a novel CIDEP method to probe a protein surface and structural changes.

Charge-Transfer Controlled Exchange Interaction in Radical-Triplet Encounter Pairs as Studied by FT-EPR Spectroscopy

The exchange interaction, J, producing quartet and doublet energy separation in radical-triplet excited molecule encounter pairs, was investigated in solution by measuring chemically induced dynamic electron polarization (CIDEP) created through the radical-triplet pair mechanism. A time-resolved FT-EPR method was utilized to measure CIDEP of galvinoxyl radical by recording FID signals and an absolute magnitude of CIDEP, P(n), was determined for each radical-triplet system by detailed analysis of the time evolution curves of CIDEP. A transient FT-EPR signal phase remarkably depends on the triplet molecule. The signal phase is related to the sign of J value, which is responsible for the radical-triplet pair interaction. Most of galvinoxyl-triplet systems showed normal negative sign. An unusual positive sign was found in some systems characterized by a small energy gap, DeltaG, between the radical-triplet pair and intermolecular charge transfer (CT) states. A theoretical calculation of J value for radical-triplet encounter pairs was carried out by considering exchange integral and intermolecular CT interaction. According to the calculated J value and the diffusion theory for CIDEP magnitude, experimental Pn values were theoretically reproduced as a function of DeltaG. The present results confirm our previously reported CT model explaining the complicated nature of the sign of J value in the galvinoxyl-triplet encounter pairs. According to the proposed model for CT effect on J value and CIDEP results, nature of J value in radical-triplet pairs is discussed.

TR-EPR of single and double spin labelled C60 derivatives: observation of quartet and quintet excited states in solution

Time-resolved EPR spectra recorded after pulsed laser photoexcitation of two fullerene nitroxide radicals 1a and 1b and two fullerene nitroxide biradicals 2a and 2b in solution have been investigated at room temperature. The TR-EPR spectra of all the radicals are interpreted as arising from spin-polarized ground and excited states, the latter being obtained by the spin coupling of the nitroxide unpaired spins with the fullerene excited triplet state. The TR-EPR spectra of the monoradicals are consistent with strong (1a) or weak (1b) spin coupling, giving rise to pure quartet/doublet excited states (1a) or mixed states (1b). In the latter, the spectral simulation allowed the determination of the magnitude of the exchange coupling constant J TR between the triplet and the radical. The spin polarization of the excited and ground states is attributed to CIDEP effect induced by the radical triplet pair mechanism (RTPM). The sign of J TR is inferred from the sign of the spin polarization. In the case of biradicals 2a and 2b, the TR-EPR spectra are assigned, on the basis of hyperfine pattern and g-values, to quintet states obtained by spin coupling of the fullerene triplet photoexcited state with the two radicals. The spin polarization of the ground state is attributed to the triplet–triplet annihilation process and the sign of the polarization allows the assignment of the radical–radical exchange constant J RR.

FT-EPR study on the sign of exchange interaction in triplet naphthalene-galvinoxyl encounter pair

Chemically induced dynamic electron polarization (CIDEP) created in the quenching of triplet naphthalene by galvinoxyl were investigated by time-resolved Fourier-transform electron paramagnetic resonance (FT-EPR) measurements with monitoring a free induction decay signal of a pulsed microwave irradiation. Transient FT-EPR spectra of galvinoxyl with CIDEP were observed in various nonpolar solvents with different viscosity. A transient FT-EPR signal phase shows remarkable dependence on the viscosity: FT-EPR signal phases were absorption and emission in the solvents with low and high viscosity, respectively. Time evolutions of the FT-EPR signal of galvinoxyl were well simulated by a model of the radical-triplet pair mechanism (RTPM) for CIDEP. A sign of theJ value in the triplet naphthalene-galvinoxyl system in various solvents were discussed on the basis of the sign rule in the RTPM and the transient FT-EPR signal phase. One of possible explanation for the solvent viscosity dependence of the transient FT-EPR signal phase was pressented on the basis of hypothetical model of theJ value.

Unusually Large Dynamic Electron Polarization in an O2(1Δg)−2,2,6,6-Tetramethylpiperidine-1-oxyl Radical System

Net absorptive (Abs) chemically induced dynamic electron polarization (CIDEP) is observed in the system of the lowest excited singlet molecular oxygen O2(1Δg) and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical in benzene by means of the time-resolved ESR (TR-ESR) spectroscopy. As a result of (1) the analysis of CIDEP time profiles by the modified Bloch and kinetic equations and (2) the quenching of CIDEP by triethylamine, it is confirmed that the Abs CIDEP is created through the interaction of O2(1Δg) with TEMPO. The creation of Abs CIDEP is reasonably interpreted in terms of the radical-triplet pair mechanism (RTPM) of the O2(3Σg-)−TEMPO pair with doublet precursor. The absolute magnitude of net Abs CIDEP created on TEMPO is experimentally estimated to be 340 ± 40 in the unit of Boltzmann polarization. To our knowledge, this is one of the largest magnitudes of CIDEP measured so far by the TR-ESR method. Based upon the RTPM theory, the origin of this unusually large CIDEP is attributed to the large ...

Effect of halogen atom on electron spin polarization studied by CIDEP, using halogen substituted aromatic acyl compounds

The reaction and spin dynamics of the photocleavage reaction of 2-chloro-2′-acetylnaphthalene were studied by time-resolved FT-EPR and transient absorption (TA) spectroscopy. The photocleavage reaction from both singlet and triplet states was observed by TA and EPR experiments, although the radical cleavage reaction in the excited triplet state is energetically unfavourable. This feature has been explained by the ionic cleavage reaction due to the electro-negativity of the chlorine atoms. The time-resolved FT-EPR spectra were similar to those observed in the bromine substituted compound, 2-BAN, reported in a previous paper. The origin of the electron spin polarization was assigned to the radical triplet pair mechanism (RTPM) and free radical pair mechanism (F-pair RPM) from analysis of the time profiles of the spin polarization.

Dynamic electron polarization created by the radical-triplet pair mechanism: Application to the studies on excited state deactivation processes by free radicals

The radical-triplet pair mechanism for chemically induced dynamic electron polarization (CIDEP) created in the quenching of excited state molecules by free radicals is explained on the basis of recent time-resolved electron spin resonance spectroscopic results and theoretical studies. The CIDEP of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) and galvinoxyl radicals exhibit various CIDEP patterns of net and mutliplet types and CIDEP phases of absorption and emission. The CIDEP patterns are described by the quartet-doublet state mixings within the radical-triplet encounter pairs. The mixings by the spin-dipolar and the hyperfine interactions are responsible for the net and the multiplet patterns, respectively. The factors controlling the CIDEP phases are the spin multiplicity of the excited state quenched by radicals and the sign of the intermolecular exchange interaction of the radical-triplet encounter pairs. In particular, the intermolecular charge transfer effect on the exchange interaction is discussed much in detail from the viewpoints of CIDEP magnitudes and phases. A CIDEP creation in the O2(1Δg)-TEMPO system is also introduced and is described by the radical-triplet pair mechanisms. Applications of this CIDEP used as a probe of O2(1Δg) in condensed phase are mentioned.

Control of ferromagnetic and antiferromagnetic couplings in a galvinoxyl–triplet chrysene encounter complex through solvent polarity

Energy difference, J, between the quartet and doublet spin states of a galvinoxyl radical–triplet chrysene pair as an encounter complex in solution was investigated by measuring chemically induced dynamic electron polarization (CIDEP) of galvinoxyl with a time-resolved ESR (TR-ESR) method. All the TR-ESR spectra of galvinoxyl show net type CIDEP signals due to doublet–triplet magnetic interactions. The CIDEP phase switches from absorption in non-polar solvent to emission in polar solvent. The signals were analyzed on the basis of the radical–triplet pair mechanism (RTPM) for CIDEP creation, the results of which indicate that galvinoxyl–triplet chrysene pairs show ferromagnetic and antiferromagnetic couplings in non-polar and polar solvents, respectively. This solvent polarity-controlled switching of magnetic coupling in the galvinoxyl–triplet chrysene pair was interpreted by introducing the effect of intermolecular charge transfer interaction on the J value.

Energy Separation between Quartet and Doublet Spin States of Radical−Triplet Encounter Pairs; Unusual Ferromagnetic Interaction in a 1,1-Diphenyl-2-picrylhydrazyl and Triplet Coronene Pair

Time-resolved ESR (TR-ESR) measurements were performed for chemically induced dynamic electron polarization (CIDEP) created in various systems of the 1-diphenyl-2-picrylhydrazyl (DPPH) radical and triplet organic molecules. TR-ESR spectra of DPPH show a net emission CIDEP signal in most of the DPPH−triplet systems. The only exception was observed for a DPPH−coronene system in which the TR-ESR spectrum of DPPH presents a net absorption CIDEP signal. The origin of these net emission and absorption CIDEP signals is interpreted in terms of the radical−triplet pair mechanism (RTPM) for a triplet quenching process. To confirm that the CIDEP is created by RTPM, we simulated the time profiles of the TR-ESR signal with modified Bloch and kinetic equations. Stern−Volmer type analyses of the CIDEP intensities also support the assignment for the proposed CIDEP mechanism. According to the signal sign rule in RTPM, we examined the energy difference, J, of the quartet and doublet spin states of the radical−triplet encounter pair (RTP). J is defined by the equation J = E(2RTP) − E(4RTP) where E(2RTP) and E(4RTP) represent the energies of doublet and quartet RTP states, respectively. According to the CIDEP signal, most of the RTPs in DPPH−triplet molecule systems show a normal negative J-value, while a DPPH−triplet coronene pair shows an unusual positive J-value. The mechanism for this unusual antiferromagnetic coupling in the DPPH−triplet coronene pair is explained by introducing an intermolecular charge-transfer interaction.

Photocleavage reaction of bromine substituted aromatic acyl compounds studied by CIDEP and transient absorption spectroscopy

The photocleavage of the CBr bond in bromoacetylnaphthalene is investigated by transient absorption and time resolved EPR spectroscopy. In the transient absorption of 2-bromo-2′-acetylnaphthalene, the absorption band observed at λmax ~440 nm is assigned to the triplet state of the parent molecule. After decay of the triplet absorption, a long lived absorption band is observed at λmax ~380 nm, which is assigned to naphthoylmethyl radical. The yield of this radical is not dependent on the concentration of oxygen even though the absorption band of the triplet state was quenched by addition of oxygen. Thus we conclude that the spin multiplicity of the precursor molecule is singlet. The CW time resolved EPR spectrum shows a typical E*/A CIDEP pattern of three hyperfine lines of the naphthoylmethyl radical. This result suggests some contribution from triplet precursor molecules. However, a careful analysis of the time profile of the CIDEP intensity observed by FT-EPR revealed that the polarization is generated from the radical pair mechanism (RPM) from the encountered pair of two free naphthoylmethyl radicals and the radical-triplet pair mechanism. RPM polarization by the geminate radical pair, formed by the Br atom and the naphthoylmethyl radical, is not observed. This fact indicates that large spin-orbit coupling (Δg and/or fast spin relaxation by g anisotropy) spoils the RPM polarization. The finding is in contrast to the recent observation of RPM polarization in the Cl cleavage reaction of 1-(chloromethyl)naphthalene.