Optical Properties of Oligothiophene Substituted Diketopyrrolopyrrole Derivatives in the Solid Phase: Joint J- and H‑Type Aggregation
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Abstract
Photophysical properties of diketopyrrolopyrrole derivatives
substituted with oligothiophenes are investigated. All compounds are
found to be fluorescent both in solution and in the solid phase. At
low temperature in the solid, fluorescence originates from excimer-like
excited states. Comparison of absorption and fluorescence excitation
spectra taken under matrix isolated conditions and on solid films
show the presence of both J- and H-type absorption bands in the solid
phase. Quantum-chemical calculations, including exciton–phonon
coupling to account for deviations from the Born–Oppenheimer
approximation, are performed to simulate the band shape of the lowest
absorption band in the molecular solid. The joint presence of J- and
H-bands is explained by the presence of two molecules in the unit
cell. The Davydov splitting is substantial for molecules with linear
alkyl substituents on the nitrogen atom (on the order of 0.2 eV) but
can be reduced to almost zero by introducing branching at the β-carbon
of the alkyl side chain