Exciton Localization in Extended {\pi}-electron Systems: Comparison of Linear and Cyclic Structures

• Main Authors: Thiessen, Alexander, Würsch, Dominik, Jester, Stefan-S, Aggarwal, A. Vikas, Idelson, Alissa, Bange, Sebastian, Vogelsang, Jan, Höger, Sigurd, Lupton, John M
• Format: Journal Article
• Language: English
• Published: 17-06-2015
• Subjects: Physics - Chemical Physics; Physics - Materials Science
• DOI: 10.48550/arxiv.1506.05522

We employ five {\pi}-conjugated model materials of different molecular shape
--- oligomers and cyclic structures --- to investigate the extent of exciton
self-trapping and torsional motion of the molecular framework following optical
excitation. Our studies combine steady-state and transient fluorescence
spectroscopy in the ensemble with measurements of polarization anisotropy on
single molecules, supported by Monte Carlo simulations. The dimer exhibits a
significant spectral red-shift within $\sim$ 100 ps after photoexcitation which
is attributed to torsional relaxation. This relaxation mechanism is inhibited
in the structurally rigid macrocyclic analogue. However, both systems show a
high degree of exciton localization but with very different consequences: while
in the macrocycle the exciton localizes randomly on different parts of the
ring, scrambling polarization memory, in the dimer, localization leads to a
deterministic exciton position with luminescence characteristics of a dipole.
Monte Carlo simulations allow us to quantify the structural difference between
the emitting and absorbing units of the {\pi}-conjugated system in terms of
disorder parameters.