Optical Properties of Oligothiophene Substituted Diketopyrrolopyrrole Derivatives in the Solid Phase: Joint J- and H-Type Aggregation.

Photophysical properties of diketopyrrolopyrrole derivativessubstituted with oligothiophenes are investigated. All compounds arefound to be fluorescent both in solution and in the solid phase. Atlow temperature in the solid, fluorescence originates from excimer-likeexcited states. Comparison of absorption and fluorescence excitationspectra taken under matrix isolated conditions and on solid filmsshow the presence of both J- and H-type absorption bands in the solidphase. Quantum-chemical calculations, including exciton–phononcoupling to account for deviations from the Born–Oppenheimerapproximation, are performed to simulate the band shape of the lowestabsorption band in the molecular solid. The joint presence of J- andH-bands is explained by the presence of two molecules in the unitcell. The Davydov splitting is substantial for molecules with linearalkyl substituents on the nitrogen atom (on the order of 0.2 eV) butcan be reduced to almost zero by introducing branching at the β-carbonof the alkyl side chain. [ABSTRACT FROM AUTHOR]