Locally induced Stark shifts of collective excitonic modes in polyradical aggregates.

Active control of dark long-lived excitonic states in molecular aggregates using local electric fields is a pivotal challenge for advancing nanoscale optoelectronics and quantum device engineering. This experimental study investigates the collective excitonic states in aggregates composed of radical chromophores. With the strong optical enhancement provided by tip-enhanced photoluminescence spectroscopy, we observe fingerprints of bright and dark excitonic modes appearing due to interexciton coupling and induced changes in their spectra with the electric field locally applied within the nanocavity gap. The analysis of the Stark shifts revealed a divergent behavior of the bright states in asymmetric measurement positions of the nanocavity above the aggregates. The observed complex behavior is discussed considering the influence of the field, molecule arrangement, nanocavity coupling, and electrostatic charge inhomogeneities in the clusters. This sensitivity to the external parameters demonstrates an effective means of control over radical excitonic aggregates.