Organic Free Radicals as Circularly Polarized Luminescence Emitters

Abstract

Chiroptical properties of two chiral atropisomers of propeller‐like trityl‐based radical derivatives have been analyzed. A new absolute configuration (AC) assignment has been made, according to the combination of experimental and theoretical data. In this sense, their ACs have been determined through the comparison of the Cotton effects recorded by electronic circular dichroism (ECD) with the theoretical ECD of the open shell structures obtained by TD‐DFT calculations. Finally, their circularly polarized luminescence (CPL) responses have been addressed. Remarkably, this is the first description of organic free radicals as intrinsic CPL emitters. Opposite signed CPL has been detected for each pair of conformers, with acceptable luminescent dissymmetry factors (|glum|≈0.5–0.8×10−3) considering their pure organic nature. In fact, highly efficient chiral emissions have been demonstrated, according to the comparison of |glum| with their respective absorption anisotropy factors (|gabs|). This pioneering study lays the foundations for the optimization of new magnetically active organic chiral emitters.

An Enantiopure Propeller‐Like Trityl‐Brominated Radical: Bringing Together a High Racemization Barrier and an Efficient Circularly Polarized Luminescent Magnetic Emitter

“…As depicted in Figure (top), the UV/Vis electronic absorption spectrum of the TTBrM radical shows the corresponding features of the so‐called C and D bands of trityl derivatives, at 400 ( ϵ =26 797 cm −1 m −1 ) and 565 nm ( ϵ =1070 cm −1 m −1 ), respectively, ascribed to the delocalization of the unpaired electron through the aromatic rings (see more details in Section S3 from the Supporting Information file). Compared to the values for the C/D bands reported for the TTM radical at the same experimental conditions (C band: 380 nm and D band: 460/500/545), a redshift resulted for the bromine‐substituted derivative.…”

“…The | g lum ( λ )| for the TTBrM radical was estimated to be (×10 −3 ):≈0.7 (see Figures S32–S33 of he Supporting Information), lower than its previously calculated | g abs (D ECD band)| (×10 −3 ): ≈1.52 value. However, compared with the recently reported | g lum (D ECD band)|=0.5×10 −3 factor for the TTM radical, it is safe to say that a more efficient chiroptical response has been achieved with this new organic radical. Thus, the second main target of this research was also achieved.…”

“…Encouraged by the high photostability of the TTBrM radical with a half‐life ( t 1/2 ) of 4.58×10 4 s (Figure S16 of the Supporting Information) together with the revealing CPL response of the TTM radical, CPL was also evaluated for the two enantiomers of the TTBrM radical. As previously illustrated with the enantiomers of the TTM radical, the following issues were envisaged: (i) good CPL signals are only reached when both large | g lum ( λ )| and LQY values, are achieved; (ii) drastic enhancement of the LQY occurs when these molecules are confined in a rigid host; however, (iii) solid media as frozen glasses are considered as birefringent elements since they can produce optical artefacts .…”

“…Following this trend, some of us addressed the first report of organic free radicals as circularly polarized luminescence (CPL) emitters in a previous work . For such a purpose, enantiomers of tris(2,4,6‐trichlorophenyl)methyl and perchlorotriphenylmethyl radicals ( TTM and PTM radicals, respectively in Figure ) were separated and studied.…”

“…[16] Following this trend, some of us addressed the first report of organic free radicals as circularly polarized luminescence (CPL) emittersi nap revious work. [17] For such ap urpose, enantiomers of tris(2,4,6-trichlorophenyl)methyl [18] and perchlorotriphenylmethyl [19] radicals (TTM and PTM radicals, respectively in Figure1)were separateda nd studied.T heir intrinsic chirality as molecular propellers [20] and, most of all, the influence on their emissive properties were reported. In that context, opposite-signedC PL responsesw ere detected for each pair of enantiomers.T heir luminescence dissymmetry factors( j g lum (l) j)w ere also determined, considering the left-and right-handedc ircularly polarizedl ight intensities( i.e., I L (l)a nd I R (l), respectively) emitted by both isomers at ag iven wavelength (l), according to [I L (l)ÀI R (l)]/[I L (l) + I R (l)].T hose j g lum (l) j values were also compared with their respective absorption anisotropy factors or j g abs (l) j , [21] at the longest wavelength, measuredby electronicc irculard ichroism (ECD).…”