Temperature-Dependent Circularly Polarized Luminescence Measurement Using KBr Pellet Method

Abstract

Circularly polarized luminescence (CPL) spectroscopy measures the difference in luminescence intensity between left-and right-circularly polarized light, and is often used to analyze the structure of chiral molecules in their excited state. Recently, it has found an increasing range of applications in the analysis of molecules that emit circularly polarized light and can be employed in 3D displays. Thus, the number of articles focusing on CPL spectroscopy has increased dramatically. However, since the luminescence dissymmetry factor (g lum ) for organic compounds is generally <|0.01|, CPL spectrometers must offer high sensitivity and produce spectra that are artifact-free for chiral molecules. Until now, the principal targets of CPL measurements have been solution samples. However, for practical device applications, it is also necessary to be able to measure the CPL spectra of solid-state samples. In addition, since electronic devices often operate at high temperatures, it is important to evaluate the thermal dependence of the CPL characteristics. Moreover, in the measurement of solid-state samples, the degree of anisotropy of the samples must be evaluated, because a large degree of anisotropy can cause artifacts. Therefore, we describe methods to evaluate the degree of anisotropy of solid-state samples and their high-temperature applications.

Circularly polarized luminescence of agglomerate emitters

“…[49] (iv) In the process of measuring CPL spectra of agglomerate state materials, to eliminate the influence of macroscopic anisotropy caused by sample preparation to some extent, samples dispersed in KBr pellet, Nujol, and isotropic film are available since the finely dispersed sample may be considered of isotropic nature. [51][52][53] Furthermore, the method that the ground agglomerate samples are sandwiched between two quartz slides is also reported. [54] Moreover, although the above-mentioned improvement of the instrument and the sample preparation methods can effectively reduce the effect of artifacts, CPL signals are also susceptible to the experimental deviations.…”

“…[50] (ii) And equipped with a double-prism monochromator, it can also reduce the influence of stray light, and so on. [51] (iii) Using a specially finetuned CPL spectrophotometer with correctly adjusted optical axes of the analyzer and a collimator lens with low intrinsic birefringence. [49] (iv) In the process of measuring CPL spectra of agglomerate state materials, to eliminate the influence of macroscopic anisotropy caused by sample preparation to some extent, samples dispersed in KBr pellet, Nujol, and isotropic film are available since the finely dispersed sample may be considered of isotropic nature.…”

“…[47] However, for practical device applications, how to measure CPL of materials in the condensed phase, especially the agglomerate state, may be more of a challenge due to the influence of artifacts caused by macroscopic anisotropies-the linear birefringence, linear dichroism, and LP luminescence. [48][49][50] And since the g lum values for chiral materials are generally small and less than ±0.01 except for partial LC systems and rare earth complexes, [51] the high sensitivity and accuracy of CPL spectrometers are the prerequisites for artifact-free spectra. Hereat, some improvements of solid-state CPL measurements have been made to remove artifacts resulted from macroscopic anisotropies.…”

“…[49] For type (b), to obtain the true CPL spectra, one should better assure that signals associated with anisotropy are excluded as much as possible by rotating the sample around the excitation light propagation axis and analyzing the collected CPL spectra from different angles. [51] In all, multiple measurements of CPL spectra are necessary for an authentic signal. In addition, these techniques for CPL tests are also applicable for CD measurements of agglomerate samples.…”

Mechanochromic Luminescence and Solid‐State Circularly Polarized Luminescence of a Chiral Diamine‐Linked Bispyrene

“…The FL and CPL spectra of the luminophores were measured using the JASCO CPL‐300 spectrofluoropolarimeter [29] at room temperature at a scattering angle of 0° upon excitation with an unpolarized, monochromatic incident light with a 10‐nm bandwidth. The excitation wavelength for all samples was 365 nm.…”