Undecabenzo[7]superhelicene: A Helical Nanographene Ribbon as a Circularly Polarized Luminescence Emitter

Abstract

We report the synthesis and characterization of an enantiopure superhelicene nanographene constituted by two saddleshaped and one planar hexabenzocoronene (HBC) units arranged in a helicoidal shape. This is, to the best of our knowledge, the first undecabenzo[7]carbohelicene, i.e., the first fully π-extended [7]helicene. Racemic resolution allowed its chiroptical properties analysis revealing dissymmetry factors in the range of 2×10-3 both in the absorption and in the emission measurements. Remarkably, nonlinear photophysical analysis showed two-photon absorption crosssection of 870 GM at 800 nm and a perfect overlapping between linear, non-linear and chiral emission.

Amplification of Dissymmetry Factors in π-Extended [7]- and [9]Helicenes

“…Another research direction in helicene chemistry is the lateral extension of π-conjugated systems. 9 − 20 With more extensive conjugation, π-extended helicenes can be regarded as nanosolenoids and are predicted to possess intriguing electronic, magnetic, and spin properties. 21 − 23 In addition, their fascinating chiroptical features, such as circular dichroism (CD) and circularly polarized luminescence (CPL), have been intensively studied and are valuable for circularly polarized organic light-emitting diodes and bioimaging applications.…”

“…However, the potential of such π-extended helicenes as CPL emitters has not been well explored. For example, an excellent Φ (>0.80) was achieved by a HBC-fused oxa[7]superhelicene, but its g lum was only 2 × 10 –4 ( Scheme 1 A); 10 , 29 a moderate g lum (2 × 10 –3 ) and a low Φ (0.098) were reported for another HBC-based undecabenzo[7]superhelicene ( Scheme 1 B); 9 , 23 and in a series of PDI-embedding double [8]helicenes, only moderate values of g lum (up to 2 × 10 –3 ) and Φ (up to 0.30) were observed ( Scheme 1 C). 19 After the initial submission of this manuscript, Santoro, Schuster, Nuckolls et al reported amplified CD signals by extending the helical length, but did not study the CPL performance.…”

A Family of Superhelicenes: Easily Tunable, Chiral Nanographenes by Merging Helicity with Planar π Systems

“…Among these, conformationally stable π-extended [n]-helicenes (with n ≥ 5) have lately received increased attention. [3] Such helicenes commonly consist of large planar segments, that is, the "πextension" and one or more twisted parts, that is, the "helicene".…”

“…Such a versatility is rather uncommon and renders our superhelicenes to stand out from the vast majority of published π-extended helicenes. [3] In the original synthesis of oxa- [7]-superhelicene, [13] we started from a diphenyl ether precursor. In the final step of the synthesis, it was necessary not only to close the hexaarylbenzenes, but also the furan ring to obtain the extended helicene.…”

B,N-Embedded Double Hetero[7]helicenes with Strong Chiroptical Responses in the Visible Light Region

“…In recent years, enlarging the π-conjugated plane of helicenes and fusing multiple helicenes into a PAH core have led to a variety of elegant molecules that extend chiroptical responses to the visible spectral range. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Nevertheless, the absorption dissymmetry factors (gabs) are generally in the range of 10 -4 ~ 10 -3 , 30 with only very few compounds showing gabs on the order of 10 -2 . [12][13][14][15][16][17] Therefore, it is highly demanding to develop novel helicene materials with high gabs in the visible range for future chiral optoelectronic applications.…”