Fig. 3.

Energy diagram of MB monomer and dimer. Plain lines represent radiative transitions. Small downward dashed lines represent nonradiative processes. The energy of the $T_1:T_2$ absorption is converted into a photoacoustic relaxation signal. The double bar indicates a discontinuity in the transition diagram: the dimer triplet undergoes electron transfer ($\sim 40\mathrm{ns}$) which leads to the formation of oppositely charged radicals that relax to the ground state via charge recombination ($\sim {10}^{-6}$ to ${10}^{-4}\mathrm{s}$). $S_0$: ground state, $S_1$: singlet excited state, $T_1$: primary triplet excited state, $T_1$: secondary triplet excited state, $S_1^{\prime }$ and $S_1^{\prime \prime }$: split exciton states, ISC: intersystem crossing, IC: internal conversion. Bottom: possible configurations of MB monomer and H-type dimer.