Figure 7.

Ultrafast triplet formation. (a) Sub-30 fs TA measurements of aggregate II in the visible and NIR spectral regions show a direct transition from the initial singlet (solid) to triplets, already evident by 150 fs. No other states (such as 2A_g) are observed. Equivalent measurements on aggregates (b) III and (c) IV reveal similar behavior. Arrows indicate the direction of the primary spectral changes. (d–f) Species extracted with a genetic algorithm using a two-state model agree well with the characteristics of 1B_u and triplet excitons, and no additional states are needed to describe the data. (g–i) Integrated decay kinetics for each aggregate type (symbols) can all be globally fitted with biexponential functions (lines) capturing the initial singlet fission process and subsequent relaxation of the triplet pair. The singlet fission time constant varies with aggregate structure, from 85 and 90 fs for II and III to 65 fs for IV.