Synthetic Control of Photophysical Process and Circularly Polarized Luminescence of [5]Carbohelicene Derivatives Substituted by Maleimide Units

Abstract

A series of [5]carbohelicene derivatives substituted by electron-withdrawing maleimide and electron-donating methoxy such as maleimide-substituted [5]carbohelicene (HeliIm), and methoxy-substituted HeliIm (MeO-HeliIm) were newly designed and synthesized to examine the electrochemical properties, excited-state dynamic and circularly polarized luminescence (CPL). First, electrochemical measurements and DFT calculations of [5]carbohelicene derivatives were performed by comparing with the structural isomers: picene derivatives. Introduction of an electron-withdrawing maleimide group onto a [5]carbohelicene core contributes to the stabilized LUMO state in HeliIm as compared to that of [5]carbohelicene (Heli), whereas the energy level of HOMO state in MeO-HeliIm increases by introducing electron-donating methoxy (MeO) groups onto a HeliIm skeleton. The HOMO-LUMO gap of MeO-HeliIm is smaller than those of HeliIm and Heli, which is similar to the steady-state spectroscopic measurements. The absolute fluorescence quantum yield (Φ FL ) of HeliIm (0.37) largely increased as compared to [5]carbohelicene, Heli (0.04), whereas Φ FL of MeO-HeliIm (0.22) was slightly smaller than that of HeliIm. Theses photophysical processes including intersystem crossing are successfully explained by the kinetic discussions. Since [5]carbohelicene derivatives show the chirality, measurements of circular dichroism (CD) and circularly polarized luminescence (CPL) were successfully performed. In particular, HeliIm and MeO-HeliIm have provide excellent circularly polarized luminescence (CPL) and the values of the anisotropy factor g lum were estimated to be 2.4 × 10 −3 and 2.3 × 10 −3 , relatively. This is the first observation of CPL in [5]carbohelicene derivatives.

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 …”

“…[30] Potential applications are chiral sensing or the preparation of chiroptical systems with innovative photophysical design. [29,30,[41][42][43][44][45][46] Helicenes belongtothe most frequently reported CPL-SOM architectures, amongt hem carbo[n]helicenes, [33,[47][48][49][50][51] helicenes that incorporate heteroatoms (azahelicenes, [52][53][54][55] oxahelicenes, [34,54] thiahelicenes), [56] and metallahelicenes. [57,58] However,d espite the factt hat several BODIPY-basedc hiral architectures were found to show significant CPL effects, [39,46,[59][60][61][62][63][64] azabora [n]helicenes, that incorporate boron directly in the helical architecture, were reported only in as ingle work so far.…”

Circularly Polarised Luminescence from Helically Chiral “Confused” N,N,O,C‐Boron‐Chelated Dipyrromethenes (BODIPYs)

“…Thus there is considerable interesti nt he devel-opment of homochiral small organic fluorophores capable of CPL emission (CPL-SOMs),w hich might provide av iable alternative to chirall anthanide complexes. [8] To date several classes of CPL-SOMsh ave been investigated including those based on helicenes, [9] cyclophanes, [10] and ortho-oligo(phenylene)ethynylenes [11] as well as some interesting recent examples of thermally activated delayedC PL. [12] We and others have focusedo n the study of chiral boron-chelated dipyrromethenes (BODIPYs) due to their excellent photophysical properties (such as, high extinction coefficients (e), fluorescence quantumy ields and tuneable emission properties) and synthetic accessibility.…”

Triarylborane‐Based Helical Donor–Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

“…This is probably one of the reasons that helicenes are not considered to be suitable emissive materials for organic light‐emitting diodes (OLEDs) . As a result, considerable efforts have been committed to the design of helicene derivatives with high Φ f , by introducing various substituents and heteroatoms, or by integrating additional fluorene units in the helix core…”