Monitoring of Orientation in Molecular Ensembles by Polarization Sensitive Nonlinear Microscopy

TL;DR

This study combines two-photon fluorescence and second harmonic generation microscopy with polarization analysis to investigate molecular orientation distributions in complex environments, providing detailed insights into molecular behavior and interactions.

Contribution

It introduces a polarization-sensitive microscopy technique to analyze molecular orientation distributions, applicable to various molecular arrangements and environments.

Findings

Polarized TPF reveals properties of incoherent emission and excitation transfer.

SHG detects slight deviations from centrosymmetry induced by external fields.

Combined signals elucidate molecular behavior in complex environments.

Abstract

We present high resolution two-photon excitation microscopy studies combining two-photon fluorescence (TPF) and second harmonic generation (SHG) in order to probe orientational distributions of molecular ensembles at room temperature. A detailed polarization analysis of TPF and SHG signals is used in order to unravel the parameters of the molecular orientational statistical distribution, using a technique which can be extended and generalized to a broad variety of molecular arrangements. A polymer film containing molecules active for TPF and/or SHG emission is studied as a model system. Polarized TPF is shown to provide information on specific properties pertaining to incoherent emission in molecular media, such as excitation transfer. SHG, being highly sensitive to a slight departure from centrosymmetry such as induced by an external electric field in the medium, complements TPF. The response of each signal to a variable excitation polarization allows investigation of molecular behavior in complex environments which affect their orientations and interactions.