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

TL;DR

This study compares exciton localization and torsional relaxation in linear and cyclic { extpi}-conjugated systems using spectroscopy and simulations, revealing different localization behaviors and relaxation dynamics in dimers and macrocycles.

Contribution

It provides new insights into how molecular shape influences exciton localization and relaxation mechanisms in { extpi}-conjugated systems.

Findings

Dimer exhibits spectral red-shift due to torsional relaxation within 100 ps.

Macrocycle inhibits torsional relaxation, leading to different exciton localization behavior.

Monte Carlo simulations quantify structural disorder affecting exciton localization.

Abstract

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.