The excitation intensity dependence of singlet fission dynamics of a rubrene microcrystal studied by femtosecond transient microspectroscopy

Abstract

We have investigated the excitation intensity dependence of the singlet fission in a crystalline rubrene by means of femtosecond transient absorption microspectroscopy. When a rubrene microcrystal was excited to higher energy levels than that of the lowest singlet excited (S₁) state with a 397 nm femtosecond laser pulse, a triplet excited state was formed through two pathways of the singlet fission, i.e. the direct fission from higher vibrational levels of the S₁ state with a time constant of 2.2 ps and the thermally activated fission from the S₁ state in a few tens of ps. The time constant of the thermally activated fission changed from 35 to 17 ps for increasing of the laser fluence from 0.65 to 18 mJ cm⁻² per pulse, although that of the direct fission was constant with the excitation laser intensity. On the other hand, the yield of the triplet formation was independent of the intensity. We also examined the temperature dependence of the singlet fission and demonstrated the activation energy of the thermally activated fission to be 0.21 eV. Based on the experimental results, we considered the excitation intensity dependence of the singlet fission of the rubrene crystal in terms of the effect of transient local heating on a ps time scale after femtosecond laser excitation owing to the nonradiative vibrational relaxation from the higher vibrational level to the lower one in the S₁ state.