Exciton coupling in diketopyrrolopyrrole–helicene derivatives leads to red and near-infrared circularly polarized luminescence

Abstract

Helicene-bis-diketopyrrolopyrrole derivatives display strong circular dichroism and red circularly polarized emission originating from exciton coupling chirality.

Helicenes Grafted with 1,1,4,4‐Tetracyanobutadiene Moieties: π‐Helical Push–Pull Systems with Strong Electronic Circular Dichroism and Two‐Photon Absorption

“…Indeed, whereas the precursor P ‐ 2 c displays a negative ECD band at 280 nm (Δ ϵ =−224 m −1 cm −1 ) and a positive one at 341 nm (Δ ϵ =+ 320 m −1 cm −1 ), typical of helicene derivatives and corresponding to the classical π–π* transitions (see ref. ), P ‐ 1 b and P ‐ 1 c show two negative bands at 243 nm (Δ ϵ =−188 m −1 cm −1 for 1 b , c ) and 300 nm ( 1 b : Δ ϵ =−34 m −1 cm −1 , 1 c : Δ ϵ =−97 m −1 cm −1 ), and one positive ECD band in the range 340–346 nm (Δ ϵ =+125 m −1 cm −1 for 1 b , c ). In line with the UV/Vis absorption spectra, P ‐ 1 b , c also show additional ECD bands in the range 390–550 nm, with higher intensity and a bisignate signature (Δ ϵ =−10 m −1 cm −1 at 397 nm and +55 m −1 cm −1 at 480 nm) in the bis‐substituted P ‐ 1 c system as compared to the mono‐substituted P ‐ 1 b derivative (Δ ϵ =+14 m −1 cm −1 at 480 nm) (see inset in Figure ).…”

“…UV/Vis one‐photon absorption spectra of 1 a – c were recorded from solutions in dichloromethane, and are displayed in Figure along with the absorption spectrum of bis‐trimethylsilylethynyl‐helicene precursor 2 c . The functionalization of the carbo[4] or carbo[6]helicene core by one or two TCBD unit(s) strongly modifies its absorption spectrum, with the appearance of new broad and intense visible bands between 390 and 550 nm.…”

“…For the assignment of the calculated spectra of 2 c , see ref. . See also the Supporting Information.…”

“…Of particular interest is the possibility of exciton coupling (EC) ECD for the bis‐substituted helicene‐TCBD 1 c . We discuss this system in relation to a carbo[6]helicene functionalized with two diketopyrrolopyrrole (DPP) groups, for which we recently clearly demonstrated EC ECD and corresponding CPL in the low‐energy (long‐wavelength) region . The TCBD‐N(Me)(Ts) substituent differs in crucial ways from DPP in that its frontier molecular FOs are much lower in energy overall, namely 2.3 and 0.9 eV lower than for DPPBr for the HOFO and LUFO, respectively, and there is also a 1.4 eV larger energy gap between these FOs (BHLYP calculations, see the Supporting Information).…”

Significant Enhancement of Absorption and Luminescence Dissymmetry Factors in the Far‐Red Region: A Zinc(II) Homoleptic Helicate Formed by a Pair of Achiral Dipyrromethene Ligands

“…This may have been apparent in CD, where an effective exciton coupling should be observed, but now turned out to be also applicable to the materials that emit strong CPL in the far‐red wavelength region. Recently, a related approach has been reported, in which conjugated diketopyrrolopyrrole units were covalently incorporated into a chiral helicene scaffold to provide an intense chiroptical response …”