Abstract
Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral opto-electronics. Here we discuss the temperature-controlled assembly of enantiopure prolinol functionalized squaraines with opposite handedness into intrinsically circular dichroic, molecular J-aggregates in spincasted thin films. By Mueller matrix spectroscopy we accurately probe an extraordinary high excitonic circular dichroism, which is not amplified by mesoscopic ordering effects. At maximum, CD values of 1000 mdeg/nm are reached and, after accounting for reflection losses related to the thin film nature, we obtain a film thickness independent dissymmetry factor g = 0.75. The large oscillator strength of the corresponding absorption within the deep-red spectral range translates into a negative real part of the dielectric function in the spectral vicinity of the exciton resonance. Thereby, we provide a new small molecular benchmark material for the development of organic thin film based chiroptics.
Highlights
Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral optoelectronics
For organic matter thin films without mesoscopic structural ordering, the layer thickness normalized ellipticity typically is below 10 mdeg/ nm due to a weak overall absorbance, and the corresponding absorbance normalized dissymmetry factor g ranges between 10−4 to 10−2 14–17
We show that thermal annealing of such thin films leads to a giant and sharp excitonic CD within the deep-red spectral range located around 780 nm
Summary
Molecular chirality and the inherently connected differential absorption of circular polarized light (CD) combined with semiconducting properties offers great potential for chiral optoelectronics. Nature has evolved with single handedness meaning that essentially all biological processes are intrinsically chiral[1] This is well recognized for instance in pharmacological applications, but the enantiopure, i.e. homochiral, synthesis of drugs remains a challenge for chemists. Other applications such as chiral optoelectronics (chiroptics) not focused on the underlying mechanism of molecular recognition processes or structural ordering in optoelectronic thin films[2,3] are still rare. Extreme g-values exceeding 0.318 or even up to 18,19 have been reported for polyfluorene thin films These values include thickness and orientation dependent, pseudoCD effects originating from cross-terms between linear dichroism and birefringence[20] as well as selective CP reflection due to longrange cholesteric ordering (Bragg reflection)[21,22,23]. Thereby, we provide a new small molecular benchmark material for the development of organic thin film based chiroptics
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