Spatially resolved chiroptical study of thin films of benzo[1,2‐b:4,5‐b’]dithiophene‐based oligothiophenes by synchrotron radiation electronic circular dichroism imaging (SR‐ECDi) technique

Abstract

The study of the chiroptical properties of thin-film samples of two chiral benzo [1,2b:4,5-b']dithiophene-based oligothiophenes with optoelectronic properties was carried out at Diamond B23 beamline using the synchrotron radiation electronic circular dichroism imaging (SR-ECDi) facility. The experimental results support the hypothesis that the non-reciprocal circularly polarized absorption observed in these materials is due to the combined linear dichroism and linear birefringence contributions. The mapping at 50-µm spatial resolution revealed that several species coexist in the mesoscopic chiral domains likely due to different aggregated phases, which were identified in the films by SR-ECDi but could not be detected with benchtop ECD instruments.

Reciprocal and Non‐reciprocal Chiroptical Features in Thin Films of Organic Dyes

“…More recently, the same SR‐ECD i technique has been successfully applied in the spatially resolved chiroptical study of thin film samples of chiral benzo[1,2‐ b :4,5‐ b’ ]dithiophene‐based oligothiophenes, including drop‐casted samples of 8 [33g] . Interestingly, for both front and back faces, a 31×31 grid (0.1 mm step size) was mapped at fixed wavelength of 422 nm with a beam diameter of 0.05 mm, generating 2D maps of absorption (Figure 14a–b) and ECD (Figure 14c–d); moreover, 2D colour maps of both CD iso (Figure 14e) and LDLB (Figure 14f) of the same grid array area were also calculated.…”

Extracting Pure Circular Dichroism from Hierarchically Structured CdS Magic Cluster Films

“…The main shortcoming with all CD measurements is that materials with macroscopic anisotropies can have undesired contributions to the signal from the linear dichroism (LD) and linear birefringence (LB) and the residual static birefringence (imperfection) of photoelastic modulators that are used to modulate between left and right circular polarization states. 10,11 These collective contributions have been termed "apparent CD" or "pseudo CD". 2,10 The intensity of the "chiroptic-CD" signal (also known as "true CD") that is generated from the coupling between the transition electronic dipole and transition magnetic dipole can be orders of magnitude smaller than linear anisotropy-induced CD signals (which are generated solely from electronic dipole-allowed transitions).…”

“…Circular dichroism (CD) spectroscopy is widely used for the detection and quantification of chiroptic activity by measuring the differential absorption of left- and right-handed circularly polarized light. The main shortcoming with all CD measurements is that materials with macroscopic anisotropies can have undesired contributions to the signal from the linear dichroism (LD) and linear birefringence (LB) and the residual static birefringence (imperfection) of photoelastic modulators that are used to modulate between left and right circular polarization states. , These collective contributions have been termed “apparent CD” or “pseudo CD”. , The intensity of the “chiroptic-CD” signal (also known as “true CD”) that is generated from the coupling between the transition electronic dipole and transition magnetic dipole can be orders of magnitude smaller than linear anisotropy-induced CD signals (which are generated solely from electronic dipole-allowed transitions). , It is important to note that the necessary condition for CD in an isotropic solution is chirality, but this is not the case for oriented materials: the chiroptic-CD signal (true CD) can be nonzero in an achiral system that is highly oriented . In these cases, the coupling between the electronic transition dipoles and magnetic transition dipoles or electric quadrupoles still produces the CD signal, but the chromophore is achiral.…”

Circularly Polarized Microscopy of Thin Films of Chiral Organic Dyes

“…This takes advantage of a high density photon flux joint with the optical elements and the electronics typical of conventional spectropolarimeters, such as photoelastic modulator (PEM) and lock-in detection. This concept found application in the spatially resolved ECD characterization of chiral π-conjugated materials in thin film − with a maximum spatial resolution of about 50 μm, the so-called ECD imaging or ECD i . In these conditions, one can very accurately record localized ECD spectra over the whole UV–vis spectral range and with full sensitivity, allowing the accurate detection and quantification of even small signals.…”