The time-resolved absorption-detected magnetic resonance spectrum of the polymethylene linked biradical: Effect of the exchange integral

The time-resolved absorption-detected magnetic resonance spectrum of the polymethylene linked biradical generated in the photolysis of alpha-(xanthene-2-carbonyloxy)-omega-(xanthene-2-carbonyloxy)dodecane was observed. The central part of the spectrum shows an increase in the absorbance by applying high microwave power. This is rationalized by the deceleration of the intersystem crossing due to the state locking that serves to slow down the intramolecular recombination reaction. Both wing portions show an increase in the recombination rate by microwave radiation, i.e. acceleration of the intersystem crossing. This is ascribed to transitions from the \T-+1> and \T--1> states to a singlet state having a slight \T-0> character. We propose that the broadness of the time resolved spectrum is due to the wide distribution of the exchange integral and its fluctuation (modulated exchange integral) by the motion of the polymethylene chain.