Physicochemical and Electronic Properties of Cationic [6]Helicenes: from Chemical and Electrochemical Stabilities to Far‐Red (Polarized) Luminescence

Abstract

Physicochemical properties of cationic dioxa 1, azaoxa 2 and diaza 3 [6]helicenes demonstrate a much higher chemical stability of diaza adduct 3 (pKR+ 20.4, E1false/2red −0.72 V) compared to azaoxa 2 (pKR+ 15.2, E1false/2red −0.45 V) and dioxa 1 (pKR+ 8.8, E1false/2red −0.12 V) analogues. The fluorescence of these cationic chromophores was established and ranges from the orange to the far-red regions. From 1 to 3, a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31% and 9.8 ns at 658 nm) are observed. The triplet quantum yields and the circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core was achieved through selective and orthogonal post-functionalization reactions (12 examples, compounds 4-15). The electronic absorption is modulated from the orange to far-red spectral range (560-731 nm) while fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70% (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first-principles calculations.

Ion Transfer Voltammetry at Thin Films Based on Functionalized Cationic [6]Helicenes

“…The structures of the functionalized [6]helicenes are shown in Figure . The synthesized compounds were characterized on the basis of electrochemical techniques (cyclic voltammetry, CV) in bulk organic solution and in the thin polymeric membranes . All compounds were found to be highly hydrophobic and therefore insoluble in pure water but soluble in aprotic solvents such as tetrahydrofuran or acetonitrile.…”

“…These derivatives, and novel didodecyl diaza + [6]helicene (diaza + (C 12 ) 2 ), were synthesized in one step from a single common advanced synthetic intermediate following reported procedures .…”

Efficient Annihilation Electrochemiluminescence of Cationic Helicene Luminophores

“…The series of [4], [6]helicenes and triangulene 1–11 was investigated by recording cyclic voltammetry in acetonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as supporting electrolyte. The electrochemical behavior of some of the compounds was previously described in several independent contributions and they were gathered here with new voltammetric characterizations in order to draw a full structure/activity relationship. Briefly, the 1‐electron reduction of the cation forms a neutral species and the electron transfer reaction is reversible at a scan rate of 0.1 V s −1 .…”

“…Among these derivatives, functionalized cationic [4] and [6]helicenes exhibit a relatively‐large structural diversity (Figure ) and these compounds are readily prepared via versatile late stage post‐functionalization strategies . In relation to this study, these moieties are luminophores with original absorption and emission properties tunable up to the near‐infrared wavelengths . More specifically, these derivatives contain nitrogen and/or oxygen heteroatoms that bridge the aromatic all‐carbon rings within the helical core.…”

Azabora[5]helicene Charge‐Transfer Dyes Show Efficient and Spectrally Variable Circularly Polarized Luminescence

“…Potential applications are chiral sensing or the preparation of chiroptical systems with innovative photophysical design . Helicenes belong to the most frequently reported CPL‐SOM architectures, among them carbo[ n ]helicenes, helicenes that incorporate heteroatoms (azahelicenes, oxahelicenes, thiahelicenes), and metallahelicenes …”