Structure and Exciton Dynamics in J-Aggregates Studied by Polarization-Dependent Near-Field Scanning Optical Microscopy

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Abstract

Thin films of pseudoisocyanine (PIC) J-aggregates have been investigated by near-field scanning optical microscopy (NSOM), revealing the local molecular orientation, the molecular order, and the distance scale for exciton migration in these nanostructured films. Polarization-dependent images of J-aggregates grown in poly(vinyl sulfate) films were recorded by detecting the fluorescence from the excitonic state of the aggregates and are presented for three different excitation wavelengths corresponding to excitation to the lower (570 nm) and upper (514.5 and 488 nm) regions of the excitonic band. The images demonstrate that the fluorescence is highly polarized along the long axis of the aggregates. Aggregate absorption (i.e., fluorescence excitation) in the near field is highly polarized along the long axis of the aggregates at 570 nm, independent of polarization at 514.5 nm, and polarized perpendicular to the long axis at 488 nm. The polarization-dependent results are employed to determine the local excitonic transition dipole orientations and to determine the extent of orientational order within the aggregates. The local orientation of the PIC monomers in the aggregates is subsequently determined from the dipole orientations, and a herringbone-like structural model for the organization of the monomeric transition dipoles is proposed, based on these results. Finally, the fluorescence from the aggregates is observed to be highly polarized through curved and intersecting aggregate structures; the lack of fluorescence depolarization in these regions confirms our previous conclusion that exciton migration is limited to ≤50 nm.

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