Kinetic Parameters of Photo-Excited Triplet State of Pentacene Determined by Dynamic Nuclear Polarization

The lifetimes and spin-lattice relaxation time of photo-excited triplet electron of pentacene doped in p-terphenyl at room temperature have been investigated. Values of spin-lattice relaxation time previously reported in ESR studies are inconsistent with each other. In this paper, we determined these time constants based on proton signals enhanced by dynamic nuclear polarization using the electrons (Triplet-DNP). The combined analysis of dependences of proton signal intensities on the delay time of polarization transfer and laser pulse structure allows us to disentangle contributions of the lifetimes and spin-lattice relaxation time. The lifetimes of triplet sublevels with m(s) = 0 and +/- 1 were determined to be 22.3 and 88 mu s, respectively. The spin-lattice relaxation time was found to be longer than 300 mu s, hence the time evolution of the electron population in the triplet state is governed by the lifetimes. It was also found that the proton signal enhancement is limited at a high repetition rate by the partial cancellation of the electron spin polarization by the remaining population produced by the preceding laser pulses.