Chemically Induced Dynamic Electron Polarization. IV. The Photolysis of 1,4-Naphthoquinone

Abstract

Electron spin resonance emission was studied from the photochemically produced 1,4-naphthoseniiquinone radical in liquid methanol, ethanol, isopropanol, ethylene glycol, and in acetic acid in the presence of 2,6-di-tert-butylphenol. This chemically induced dynamic electron polarization is due to the optically spin polarized triplets of the parent quinone and their subsequent hydrogen abstraction reaction with retention of the polarization in the resultant semiquinone radicals. In the acetic acid -2,6-di-tert-butylphenol system, emission was observed also from the phenoxy counter radical. The magnitude of the polarization at constant microwave power was found to be temperature dependent. The cornparison of the photochemical theory and the radical-pair theory is examined in the light of some o f the results obtained.-.On a Ctudie I'Cmission de resonance Clectronique de spin du radical de la naphtosemiquinone-l,4, preparee photochimiquement dans le methanol, I'ethanol, I'isopropanol, I'ethylene glycol et I'acide acetique en presence de diterbutyl-2,6 phenol liquides. Cette polarisation electronique dynarnique, chimiquement produite, est dfie aux etats triplets, dont les spins sont optiquement polarises de la quinone parente, qui subit par la suite une reaction d'abstraction d'un atome d'hydrogene accompagnte de la retention de la polarisation dans le radical semiquinonique resultant. Dans le systeme acide acetiquediterbutyl-2,6 phenol on observe aussi une emission dfie au radical complementaire phenoxy. On a trouvk que I'importance de la polarisation depend de la temperature lorsque la puissance de la microonde reste constante. La ihtorie photochimique et la theorie des "paiies de radicaux" sont comparees a la lumiere de quelques-uns des resultats obtenus.[ Introduction Recently we have reported the observation of the totally emissive e.s.r. of 1,Cnaphthosemiquinone radicals produced in the pulsed photolysis of 1,4-naphthoquinone in isopropanol (1). It was concluded that a spin polarization arising from intersystem crossing to the triplet quinone may result in a spin polarization for the doublet radical, when the chemical hydrogen abstraction rate is comparable to the spin-lattice relaxation rate of the triplet molecule. A formal theoretical development of this model has been given (2). The model also predicts that the counter radical from the hydrogen donor must have the same sign of electron polarization. Thus, in a recent communication (3) on the observation of a photo-radical pair of durosemiquinone radical and the 2,6-di-tert-butylphenoxy radical, the results that both radicals are totally emissive support strongly the photochemical theory. In the present paper, we describe the detailed study of the photolysis of 1,4-naphthoquinone in a variety of solvents. The chemically induced dynamic electron polarization (c.i.d.e.p.) results of this system are discussed in the light of the

Time-Resolved ESR Study on the Photochemistry of Naphthoquinones Included in Cyclodextrins

“…The spectral pattern is emissive with a slight E/A pattern (emission in low field and absorption in high field); namely, E*/A. The emission implies that reaction takes place by way of the excited triplet state of NQ in α-CD, and this is called the triplet mechanism (TM), because the TM polarization of the photoreaction of NQ has been known to be emissive. − The S−T - 1 mixing under the conditions of a large exchange interaction ( J ) and long time interaction in the intermediate radical pair may also induce emissive TM-like polarization. The contribution of this mechanism cannot be excluded.…”

Chemically induced dynamic electron and nuclear polarization in the photolysis of 1,4-benzoquinone in 2-propanol. Radical-pair theory of the photochemical model

“…In the CIDEP study of the photoreduction of 1,4-benzoquinone in 2-propanol, the semiquinone radical was readily observed but the counter 2-propanol radical escaped experimental observation for similar reasons involved in other substituted quinone studies. 13 It was, however, established that the semiquinone radical polarization is in the totally emissive mode. Figure 1 represents a typical transient polarized ESR signal at a constant field showing the initially emissive signal relaxing to the thermal equilibrated absorptive mode.…”