Resolution by chiral hplc of the stable free radical perchlorotriphenylmethyl: Thermodynamic and chiroptical properties

Abstract

The perchlorotriphenylmethyl radical (PTM) shows enantiomerism due to restricted internal rotations. Several chiral stationary phases have been tested in different conditions for the resolution of PTM by HPLC. Only poly(triphenylmethy1 methacrylate) provided sufficient resolution for canying out a micropreparative scale separation. Racemization of (+)-and (-)-PTM was monitored polarimetrically as a function of time. The process was found to follow strict first-order kinetics. The value of AGX303.8 = 23.6 Kcal * mol-' agrees with that determined previously by HPLC with the stopped flux technique (Crespo, Doctoral Thesis, CETS Institut Quimic de Sarrii, 1991). Following the helicity rules in the circular dichroism spectrum, (+)-PTM can be assumed to have (P) configuration and (-)-PTM (M) configuration. o 1995 Wiey-Liss, h c .

Organic Free Radicals as Circularly Polarized Luminescence Emitters

“…Supporting the preliminary results obtained from (CSP)HPLC,aslightly superior racemization barrier was found for 2,(DG°3 03 K (1) = 21.12 AE 1.77 kcal mol À1 and (DG°3 03 K (2) = 22.36 AE 1.92 kcal mol À1 ). These values are in good agreement with the ones obtained for 2 by means of polarimetric [44] or HPLC. [42] However,t he slight differences found in the actual numbers were attributed to the chosen methodologies.Accordingly to these data, at wo-ring flip might be the enantiomerization mechanism in both cases, [48,49] despite the higher steric hindrance in 2.This means that, considering the plane formed by the three aromatic carbons linked to the central radical one,t wo phenyl rings turn in the same direction, perpendicularly to this plane,while adisrotatory fashion is associated to the third one.…”

“…ECD spectra showed agood alignment to the corresponding UV/Vis electronic absorption ranges.C onsidering the propeller-shape configuration as the origin of the chiral activity,n og reat differences with respect to the UV/Vis absorption spectra were expected, due to the presence of additional elements of chirality.I nt his sense,E CD results showed perfect mirror-image Cotton effects with two main bands (B/C) detected at 335/380 nm for 1 and 339/392 nm for 2.T he absorption anisotropy factor, j g abs j= De/e, [50] of both radicals was also evaluated and the following values ( 10 À3 ) were calculated:2 .99 (at 335 nm)/1.84 (at 339 nm) (B band), 1.38 (at 380 nm)/1.44 (at 392 nm) (C band) and % 0.55 (at 545 nm)/0.95 (at 552 nm) (D band) for 1/2,r espectively. ECD response of 2 was previously reported by Irurre et al [44] and, surprisingly,Bband (instead of Cband) was the main feature of their ECD spectrum. In that occasion, the AC of both enantiomers was assigned attending to the sign of the Cotton effect of the given band, this is:( P)-(+ +)-2 and (M)-(À)-2,following the helicity rules in ECD.…”

Organic Free Radicals as Circularly Polarized Luminescence Emitters

“…Supporting the preliminary results obtained from (CSP)HPLC, a slightly superior racemization barrier was found for 2, (ΔG ‡ 303K (1) = 21.12 ± 1.77 Kcal mol -1 and (ΔG ‡ 303K (2) = 22.36 ± 1.92 Kcal mol -1 ). These values are in good agreement with the ones obtained for 2 by means of polarimetric [44] or HPLC. [42] However, the slight differences found in the actual numbers were attributed to the chosen methodologies.…”

“…The absorption anisotropy factor, |gabs| = ∆ε/ε, [50] of both radicals was also evaluated and the following values (x 10 -3 ) were calculated: 2.99 (at 335 nm)/ 1.84 (at 339 nm) (B band), 1.38 (at 380 nm)/ 1.44 (at 392 nm) (C band) and ≈ 0.55 (at 545 nm)/ 0.95 (at 552 nm) (D band) for 1/2, respectively. ECD response of 2 was previously reported by Irurre et al [44] and, surprisingly, B band (instead of C band) was the main feature of their ECD spectrum. In that occasion, the AC of both enantiomers was assigned attending to the sign of the Cotton effect of the given band, this is: (P)-(+)-2 and (M)-(-)-2, following the helicity rules in ECD.…”

“…[41] Remarkably, nothing has been said yet about the influence of the intrinsic chirality of these materials on their emissive properties, although some hints about the stability of their isolated enantiomers can be found in the literature. [42]- [44] In fact, to the best of our knowledge, the combination of chirality, luminescence and magnetism in a single organic molecule has never been addressed, despite all the advantage that the openshell character may provide. Bearing all these ideas in mind, herein we present, for the first time, a study about the intrinsic CPL response in organic free radicals, considering trityl-based radical chlorinated-derivatives for such a purpose.…”