Photophysical Properties of Homo‐ and Hetero‐Aggregate Assemblies Made of N‐Annulated Perylene Derivatives

We report the absorption spectra and photophysical properties of homo and hetero-aggregate assemblies of a strongly emissive N-annulated perylene dye (P) and of a dyad made of P and a methyl viologen derivative (P-MV), in ethanol-water solutions. In homo-aggregate assemblies of P, the strong π−π* fluorescence of the isolated chromophore is replaced by an excimer emission at lower energy, with a lifetime of 900 ps, due to excimer formation from the initially prepared excitons. In homo-aggregate assemblies of P-MV, photoinduced charge separation, with formation of P+-MV- species, occurs in 3 ps and with a charge recombination of 20 ps, close to the time constants of the same processes exhibited by P-MV isolated dyad in acetonitrile or ethanol solution. In mixed hetero-aggregate P/P-MV systems, the light energy absorbed by the P components is suggested to delocalizes (by exciton formation) over various P subunits, and when a P-MV unit is reached, charge separation occurs; however, excimer emission is still present, particularly for P/P-MV ratio larger than 3:1, indicating that delocalized excitons within the hetero-aggregate systems extend over a limited, small number of P chromophores.