Figure 3.

Transient absorption of AXT aggregates. (a) Selected timeslices from sub-ps TA of II excited at 2.6 eV. The prompt decay of the broad NIR PIA is the only significant spectral change in the entire measurement range and reflects internal conversion from the initial 1B_u state. (b) Timeslices from ns TA show continued uniform decay of the state formed within 3 ps. (c) Integrated decay kinetics from sub-ps TA reveal instrument-limited decay of the GSB and NIR PIA (solid lines) to form the final state. (d) Equivalent sub-ps measurement of IV excited at 2.3 eV, showing similar rapid decay of the initial band in the NIR and formation of the final state within 3 ps, which (e) uniformly decays on the μs time scale. (f) Integrated decay kinetics from sub-ps TA of IV show an instrument-limited internal conversion from the initial state (thin solid line) similar to II. The slower rise at 1.94 eV reflects a gradual shift of the crossing point between GSB and PIA, related to the generation of vibrationally excited ground states following nonradiative decay of triplet pairs.²¹