Circularly polarized (CP) light can result from CP-luminescence (CPL) or from differential absorption of left- vs. right CP light. In both cases, the relevant quantity is called dissymmetry factor g: gEM = 2(IL-IR)/ (IL+IR) for emission, and gAbs = 2(AL-AR)/ (AL+AR) for absorption, being IL and IR the intensities and similarly AL, AR the absorbances of left- and right- CP-light. While certain metal complexes are known for their strong CPL features, organic molecules usually display only modest gEM. When they are ordered in chiral supramolecular structures, some organic fluorophores can display large gEM. Measuring significant data on aggregated species and on films can be made difficult by the residual linear birefringence (LB) of the CP-detection optics, which must always be kept in mind. We shall present some systems with high gEM and discuss how can one recognize and avoid artifacts on one’s own CPL instrument. The other option for CP-light is circular dichroism, possibly with high gAbs. Chiral architectures of π-conjugated systems can be the key. In this context, a very peculiar effect needs separate and proper treatment. A way to obtain CP-light is to apply a wavelength retarder to linearly polarized light, i.e. to couple linear dichroism (LD) to linear birefringence (LB) with main axes at 45° from one another. We found some chiral molecules, which spontaneously form films with LDLB effect and which strongly polarize the transmitted light. This is not a true CD, but it is anyway a means to achieve CP-light. LDLB bears the intriguing property that the same film provides opposite chiroptical signals, when put with one or the other face in front of the light source (Figure). This leads to recognizing the orientation of a transparent film. To visualize the phenomenon, think of the sign “pub” on a transparent sign: from the opposite side it will appear “duq” and by this information, two observers can decide if they are on the same or on different sides of the same shop.

Polarizing light with chiral supramolecular structures

255th National Meeting (2018)

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