Fig. 1

Structural and optical properties of DP-Mes. a Chemical structure of DP-Mes. Orthogonal ground-state geometry shown in Supplementary Fig. 1. b DP-Mes electronic states and photophysics. Singlet fission from S₁ to ¹TT is mediated by coupling (orange arrows) to a manifold of charge-transfer states which are not directly populated, facilitating the overall marginally exothermic (~200 cm⁻¹) process^{46, 47}. Symmetries of the relevant excited electronic states are indicated in parentheses⁴⁸. c Differential transmission map of a DP-Mes thin film following excitation with a 13 fs pulse centred at 550 nm, at room temperature. The absorption (grey) and photoluminescence (orange) spectra are shown to guide spectral assignment. The primary spectral features of S₁ (stimulated emission), ¹TT (excited-state absorption) and total excited-state population (ground-state bleach) are indicated by dashed lines. The associated transient kinetics are shown above, with the weak stimulated emission trace scaled by a factor of 9 for clarity. The superimposed oscillatory modulations correspond to vibrationally coherent wavepackets formed by impulsive excitation. The transient absorption spectrum over an extended probe wavelength range is presented in Supplementary Fig. 9