Anomalous Triplet Mechanism Spin Polarization Induced by the Addition of Hydrochloric Acid in the Photochemical System of Xanthone in Alcohol

Koga, Takehiko; Ohara, Kouzou; Kuwata, Keiji; Murai, Hisao

doi:10.1021/jp971183n

Cited by 11 publications

(26 citation statements)

References 39 publications

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“…Apparently, in these systems there is a CIDEP mechanism that makes a dominating net absorptive signal contribution and a small signal component stemming from the RPM. The results are in agreement with CW TREPR results reported previously …”

Section: Resultssupporting

confidence: 93%

“…The polarization pattern shown in Figure a differs from the E/A* pattern previously found in a CW TREPR study of free radicals formed by the photolysis with 337 nm laser excitation of Xn in alcohols . Moreover, as shown in Figure a, with 2-methyl-1-propanol as solvent, the FT-EPR spectrum shows an almost symmetrical E/A pattern, whereas in previous TREPR work an E/A* type spectrum was observed.…”

Section: Resultscontrasting

confidence: 69%

“…If a small fraction of triplet Xn has a reaction rate approaching the triplet spin-relaxation rate, this can account for the small net E component observed. Alternatively, the radical triplet pair mechanism (RTPM) can be the source of E polarization. , Transient absorption measurements show that 3 Xn* is relatively long-lived, creating conditions where the RTPM mechanism can give rise to strong emissive polarization. − In fact, our CW TREPR experiments show that the relative intensity of the net emissive component is stronger at higher concentrations of Xn. This is consistent with what is expected for the RTPM …”

Section: Resultsmentioning

confidence: 86%

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An FT-EPR Investigation of the Anomalous CIDEP Observed in Photoinduced Reactions of Xanthone with Alcohols in the Presence of Hydrochloric Acid

Ohara¹,

Hirota²,

Martino³

et al. 1998

J. Phys. Chem. A

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CIDEP spectra from free radicals produced by the photolysis of xanthone (Xn) in 2-propanol were investigated with FT-EPR. The spectra were assigned to the 2-hydroxypropan-2-yl (2HP) and xanthone ketyl (XnH) radicals. In pure 2-propanol and 2-propanol containing 10% H2O, the spectra display low field emission/high field absorption with net emission (E*/A) type polarization. The observed CIDEP pattern is mainly due to the S−T0 radical pair mechanism (RPM) with minor contributions from the triplet mechanism (TM) and radical triplet pair mechanism (RTPM). Upon addition of HCl, the polarization changes to net absorption. The rise time of the absorptive signals is determined by the response time of the spectrometer (∼3 × 10-8 s). Transient optical absorption measurements show that the triplet state of xanthone (3Xn*) is quenched by HCl, and the change in spin polarization produced by HCl addition is attributed to this quenching process. The dependence of the CIDEP pattern and triplet xanthone lifetime on HCl concentration shows that both involve a diffusion-controlled process. The main 3Xn* quenching process was found to be nonreactive, but it is proposed that its spin selectivity produces spin polarization in the products of the hydrogen abstraction reaction that runs in parallel with this quenching process.

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Section: Resultssupporting

confidence: 93%

Section: Resultscontrasting

confidence: 69%

Section: Resultsmentioning

confidence: 86%

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An FT-EPR Investigation of the Anomalous CIDEP Observed in Photoinduced Reactions of Xanthone with Alcohols in the Presence of Hydrochloric Acid

Ohara¹,

Hirota²,

Martino³

et al. 1998

J. Phys. Chem. A

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“…It seems that the signals from (CH3)2C'OH and PA'H rise immediately after laser excitation. However, the signal of PA'H grows even after the signal of (CH3)2C'OH starts to decay, whilst the decay of the alcohol radical is much faster than that observed in the system of TCPA presented in this paper and in different system [18] where no PA is present. The time profile suggests that the initial reaction is the hydrogen abstraction reaction from 2-propanol.…”

Section: Phthalic Anhydride In 2-propanolcontrasting

confidence: 53%

Time-resolved EPR study of the photoreduction of phthalic anhydride and chlorinated phthalic anhydrides in 2-propanol

et al. 2003

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The photoreduction of phthalic anhydride (PA), 3,6-dichlorophthalic anhydride (3,6-DCPA), 4,5-dichlorophthalic anhydride (4,5-DCPA) and tetrachlorophthalic anhydride (TCPA) in 2-propanol has been studied with time-resolved electron paramagnetic resonance. The chemically induced dynamic electron polarization spectra show that the reaction takes place through the excited triplet states. From PA, cyclohexadienyl-type hydrogen adduct and ketyl radicals were observed, whereas 3,6-DCPA produced the 3,6-DCPA anion and hydrogen adduct radicals. With 4,5-DCPA only the anion radical appeared, whilst the TCPA system showed no apparent anion and adduct radical formation. These data show that the hydrogen adduct formation occurs at the 4 position in the benzene ring, but that 4,5-DCPA and TCPA do not undergo this reaction. The anion radicals of PAs are formed in subsequent deprotonation reactions of the ketyl radicals. We propose that the hydrogen adduct radical formation in PA and 3,6-DCPA takes place through direct hydrogen abstraction by the excited triplet molecules, in competition with similar abstraction by the carbonyl group to form the ketyls.

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“…It is relevant to note that recent time-resolved EPR studies of the reaction of triplet state xanthone with alcohols revealed a small dependence of spin polarization on excitation wavelength [25,26]. In this case, it is known that intersystem crossing from the excited singlet state to the triplet state of xanthone is fast and may compete with vibrational deactivation and/or internal conversion processes within the singlet excited state manifold.…”

Section: Spectroscopic Data and Theoretical Calculations Indicate Thamentioning

confidence: 99%

FT-EPR study of alkyl radicals formed in the photochemical reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) complexes

et al. 1998

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A Fourier transform EPR (FT-EPR) study was made of the photochemistry of [Re(R)(CO) 3 (a-diimine)] and [Ru(E)(R)(CO) 2 (a-diimine)] complexes, where R = alkyl or benzyl, E = I or SnPh 3 , and a-diimine = 4,4'-dimethyl-2,2'-bipyridine (DMB) or N,N'-diisopropyl-1,4-diazabutadiene (iPr-DAB). Photoexcitation of these complexes leads to homolysis of the metal-alkyl (benzyl) bonds as evident from the detection of the spectra of the alkyl (benzyl) radicals. FT-EPR spectra display strong spin polarization effects attributed to Triplet Mechanism (TM) and Radical Pair Mechanism (RPM) Chemically Induced Dynamic Electron Polarization (CIDEP). CIDEP patterns point to bond dissociation via a triplet state precursor. For a number of complexes, spin polarization was found to exhibit unusually large solvent effects, whereas for one complex the CIDEP pattern proved to be sensitive to the wavelength of laser light used to initiate bond dissociation. These effects reflect the strong dependence of CIDEP on the character of the excited states involved in the photochemical reactions and contribute to the understanding of the reaction mechanism.

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Anomalous Triplet Mechanism Spin Polarization Induced by the Addition of Hydrochloric Acid in the Photochemical System of Xanthone in Alcohol

Cited by 11 publications

References 39 publications

An FT-EPR Investigation of the Anomalous CIDEP Observed in Photoinduced Reactions of Xanthone with Alcohols in the Presence of Hydrochloric Acid

An FT-EPR Investigation of the Anomalous CIDEP Observed in Photoinduced Reactions of Xanthone with Alcohols in the Presence of Hydrochloric Acid

Time-resolved EPR study of the photoreduction of phthalic anhydride and chlorinated phthalic anhydrides in 2-propanol

FT-EPR study of alkyl radicals formed in the photochemical reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) complexes

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