Modeling the Electronic Coupling in Squaraine Thin Films: The Unusual Case of Three Davydov Components

: A theoretical study is conducted to unveil the nature of intermolecular interactions in orthorhombic 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (SQIB) crystalline thin films. The orthorhombic polymorph of SQIB crystallizes in the Pbcn space group with four equivalent molecules per unit cell (Z = 4), leading to three orthogonally polarized Davydov components and a dark state. The remarkable agreement between experimental polarized absorbance spectra and the simulated spectra utilizing the Frenkel-Holstein Hamiltonian allows one to reliably assign the observed spectroscopic features to three Davydov components originating from intermolecular Coulomb interactions. Additionally, the energetic location of the dark state, which plays an important role in nonradiative relaxation, is predicted to lie roughly midway between the lower and upper Davydov components.