Impact of Intermolecular Charge Transfer Interaction on the Absorption Spectrum of an Aza-BODIPY Dye Aggregate

Near-infrared (NIR) absorption of organic dye aggregates is desired for phototherapy and bioimaging applications to ensure deep tissue penetration and reduced scattering effects. Recently, an azadipyrromethene boron difluoride (aza-BODIPY) dye has been reported, showing an unusual two-band absorption spectrum in the aggregate, with strong absorption in the NIR region. In this work, we have used the Essential States Model (ESM) to simulate the UV/vis absorption spectrum of the aza-BODIPY dye aggregate, shining light on the origin of the large oscillator strength of the low-energy absorption band. Our results reveal that the significant red-shift of the absorption with respect to the monomer arises from the interplay between J-type Coulomb coupling and intermolecular charge transfer (ICT) interactions. The latter results in significant charge separated character in the ground state, which cannot be described by conventional exciton theory.