Long‐Lived Excimer‐Like Emission From a π‐Stacked Trichromophore Comprising Perylene Monoimide and Perylene Diimide Units

A trichromophore has been synthesized comprising a perylene diimide sandwiched between two perylene monoimide diester (PMIDE) units and held in place by xanthene spacers. The DFT computed structure indicates π-stacking of the three polycycles, with a mean interplanar separation of 4.2Ă, but with the central unit displaced sideways. Phenyl groups inserted between xanthene and perylene help to rigidify the structure. Electronic interactions between perylene polycycles splits the absorption spectrum, which is dominated by transitions to the upper-lying H-state. Excimer-like emission is observed in solution at room temperature, for which the lifetime is ≈22 ns in toluene and ≈8 ns in CH₂Cl₂. Transient absorption spectroscopy indicates the instantaneous formation of a Franck-Condon state, which undergoes rapid vibrational cooling to populate the low-lying J-state. This species transforms over 40-60 ps into an excimer-like state wherein the exciton is delocalized over all three perylenes. A slower structural re-arrangement, complete within 1 ns, minimizes electronic interactions with one of the PMIDE units and stabilizes the resultant bipartite excimer-like state.