Room-Temperature Fluorescence Lifetime of Pseudoisocyanine (PIC) J Excitons with Various Aggregate Morphologies in Relation to Microcavity Polariton Formation

Yuki Obara,Masaru Oda,Keita Saitoh,Toshiro Tani

doi:10.3390/ijms13055851

Abstract

The results of room-temperature fluorescence lifetime measurements are reported for the excitation of J aggregates (Js) of pseudoisocyanine chloride (PIC-Cl) prepared in potassium polyvinyl sulfate (PVS) polymer thin films, their aqueous solutions, and NaCl aqueous solutions. Variations of the microscopic morphologies of the aggregates were investigated. The results show that fluorescence decay features correlated to the morphology change. The observed fluorescence lifetime and quantum efficiency of PIC J aggregates (PIC-Js) in a NaCl aqueous solution were 310 ps and 28%, respectively. The lifetime of the fibril-shaped macroaggregates prepared in PVS thin films was below the instrumental time resolution of 5 ps, and the efficiency decreased to below 3%. The results indicate that PIC-Js prepared with PVS polymers have an increased nonradiative contribution to the excitation deactivation process. In particular, macro-Js with isolated fibril-shaped structures revealed nonradiative pathway(s) that are closely associated to the specific packaging morphology of the constituent meso-Js. The possibility of a destructive effect on the formation of cavity-polaritons is also discussed.

Highlights

Dynamic processes of Frenkel excitons in self-assembled molecular J aggregates (Js) have been intensively investigated over the last few decades
We observed room-temperature fluorescence decay profiles of PIC J aggregates (PIC-Js) prepared with anionic polyelectrolyte polyvinyl sulfate (PVS) polymers with various macroaggregate morphologies, i.e., in PVS polymer thin films and in their aqueous solutions
The numbers indicate the difference in the excitation wavelengths, corresponding to those in Figure 1; (f) Decay profiles of pseudoisocyanine chloride (PIC-Cl) Js prepared in NaCl aqueous solution

Summary

Introduction

Dynamic processes of Frenkel excitons in self-assembled molecular J aggregates (Js) have been intensively investigated over the last few decades. Planar quantum microcavities (QMCs), which contain Js as active media, are gaining increased attention This regime is identified by the observation of anti-crossing behaviors between the cavity-photon and J-exciton dispersion curves near their resonance energy; the minimum energy corresponds to the vacuum Rabi splitting (2Δ). Well-defined cavity–polariton features were observed in the QMC structures containing PIC-Js as the active media, even in a fairly low-Q metal-mirror cavity at room temperature. In the case of PIC-Js prepared in anionic polyelectrolyte PVS polymer films, isolated fibril-shaped macroaggregate structures are obtained [22,23,24,25,26] These structures are presumably bundles of multiple, almost parallel, constituent mesoaggregates. Preliminary results of fluorescence decay in various types of macroaggregates in PVS-based matrices and in saline aqueous solutions are presented here

Experimental

Results and Discussion

Conclusion