Delocalization of Coherent Triplet Excitons in Linear Rigid Rod Conjugated Oligomers

Christian Hintze, Patrick Korf, Frank Degen, Friederike Schütze, Stefan Mecking, Ulrich E. Steiner, and Malte Drescher J. Phys. Chem. Lett., 2017, 8 (3), pp 690–695 DOI: 10.1021/acs.jpclett.6b02869

Abstract: In this work, the triplet state delocalization in a series of monodisperse oligo(p-phenyleneethynylene)s (OPEs) is studied by pulsed electron paramagnetic resonance (EPR) and pulsed electron nuclear double resonance (ENDOR) determining zero-field splitting, optical spin polarization, and proton hyperfine couplings. Neither the zero-field splitting parameters nor the optical spin polarization change significantly with OPE chain length, in contrast to the hyperfine coupling constants, which showed a systematic decrease with chain length n according to a 2/(1 + n) decay law. The results provide striking evidence for the Frenkel-type nature of the triplet excitons exhibiting full coherent delocalization in the OPEs under investigation with up to five OPE repeat units and with a spin density distribution described by a nodeless particle in the box wave function. The same model is successfully applied to recently published data on π-conjugated porphyrin oligomers.