Kinetik einer Diradikal-Ionenpaarbildung organischer Nitroaromaten mit n-Elektronendonatoren durch photochemische Elektronenübertragung und deren Temperaturabhängigkeit

Abstract

The electron spin resonance spectra (ESR) which were obtained when solutions of nitroaromatics in tetrahydrofuran (THF) were irradiated within the scope of the charge transfer band (λ=436 mμ), demonstrate the presence of nitroaromatic·– radical anions connected with the radical cation THF·+ in an ion pair. Only the nitroaromatic·— ESR spectrum is observed, but not that of THF·+, because of an electron exchange with THF molecules in large excess. During irradiation at temperatures within 263°<T<293°K there is an increase of lESR to a steady state value which increases in intensity when the temperature decreases. The rate of decay of the steady state intensity, when the light is shut off, is first order. The decay time τ is dependent upon temperature and grows from minutes to hours and days when the temperature decreases. The nitroaromatic radical ions may be “frozen” at temperatures below ca. 230°K. When irradiation is shut off at low temperatures, where τ is large, the signal intensity reversibly decreases and the ESR lines become broader and lastly undetectable when the temperature decreases, and vice versa. During irradiation at temperatures within 160°<T<248°K, l(t) ESR reaches a maximum and decreases during prolonged irradiation. On the basis of all experimental observations it may be assumed that primarily the EDA complex°C in the normal state will be excited in a singlet state 1C [e. g. TNB- ↑ ... THF+ ↓ ... THF] with an appreciable interaction between unpaired electrons (spins antiparallel). 1C turns over to an isomeric ion pair biradical 2C [e. g. TNB-↖ ... THF+ ↗... THF] of maximum Coulomb and solvation energy with no or only small spin interaction and with a remarkable longevity, as the configuration of the biradical 2C does not satisfy the symmetry requirements permitting favorable overlapping between the odd electrons in aromatic radical ions (e. g. TNB·– and THF·+). At low temperature an association of nitroaromatic with nitroaromatic·– to X [e. g. TNB ... TNB·– ... THF·+ ... THF] may be assumed, hence the ESR lines are broadened and the ESR signal is undetectable. By h v-absorption after prolonged irradiation X will be slowly transferred into diamagnetic Y not detectable by ESR. On the basis of the scheme quantitative expressions of the increase of the concentration of 2C which is proportional to the signal intensity lESR are obtained. The curves of the growth of the signal intensity as a function of time are calculated in good quantitative accordance with the experimental observation.