Helically Locked Tethered Twistacenes

Abstract

Twisting linear acenes out of planarity affects their electronic and optical properties, and induces chirality. However, it is difficult to isolate the effect of twisting from the substituent effect. Moreover, many twistacenes (twisted acenes) readily racemize in solution. Here, we introduce a series of twistacenes having an anthracene backbone diagonally tethered by an n-alkyl bridge, which induces a twist of various angles. This allows us to systematically monitor the effect of twisting on electronic and optical properties. We find that absorption is bathochromically shifted with increasing twist, while fluorescence quantum efficiency drops dramatically. The tethered twistacenes were isolated to their enantiomerically pure form, displaying strong chiroptical properties and anisotropy factor ( g-value). No racemization was observed even upon prolonged heating, rendering these tethered twistacenes suitable as enantiopure helical building units for π-conjugated backbones.

Chiroptical Properties of Twisted Acenes: Experimental and Computational Study

“…In addition, measuring the ECD at different temperatures across the 20-60 8Cr ange resulted in only slight reversible changes in the spectra, which indicates the conformational rigidity imposed by the tether (Figure S16, in the Supporting Information). [13] As the twist angle increases from Ant-C6 to Ant-C3,t he Cotton effect increases for both the 1 L a and 1 B b transitions, which also undergo ab athochromic shift, consistent with the trend that was calculated for parent anthracene. The spectral changes observedu pon twisting are more pronounced for the ECD spectra than for the UV/Vis spectra (Figure S13 a, in the Supporting Information).…”

“…To investigate experimentally the effect of twisting on the chiroptical properties of acenes,w es eparatedaseries of enantiopure tethered twistacenes with twistingd ihedrala ngles ranging from 22 to 438 ( % 7-148 per ring) by using preparative chiral HPLC. [13] To the best of our knowledge, this constitutes the first series of enantiopure twisted acenes.T his allows us to comparet he chiroptical properties calculated for parent acenes with our model system of helically locked twistacenes, Ant-Cn.…”

“…Recently,w ei ntroduced tetheredt wistacenes as an ew familyo fh elicallyl ocked conjugated backbones. [13] Unlike nontethered acenes,w hich are conformationally unstable ( Figure 1a), our diagonal tethering "locks" the back-and-forth flip, thus preventingr acemization( Figure 1b). By using different tetherl engths, we controlled the twisting angle, and were able to:( i) isolate stable M and P enantiomers; and (ii)keep the number of substituents around the acene backbonec onstant, thus allowing us to monitor the effecto ft wisting, while minimizingt he substituent effect.…”

Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps

“…Embedding helical motifs into polyaromatic structures [1] provides access to inherently curved and multilayered aromatic molecules with exceptional chiroptical properties and the potential to be used in organic electronics. [2][3][4][5][6] Recent activity in this field has produced adiverse range of new chiral aromatics,i ncluding helicenes fused to large polycyclic structures, [7][8][9][10][11] helically hindered PA Hs, [12,13] helical nanoribbons, [2][3][4] and multi-helicene architectures. [14][15][16][17][18][19][20][21][22][23][24][25][26][27] Inclusion of heteroatoms or nonbenzenoid rings in such systems can have ap rofound effect on their electronic and chiroptical characteristics.H owever,s uch modifications are synthetically challenging and thus rarely attempted.…”

Pseudopotential‐fragment spectroscopy for organic molecules and carbon allotropes

“…The phrase “twisted acene,” later “twistacene,” was introduced in 2004, then more thoroughly described in 2006 . The electronic and optical properties of their parent acenes are altered by this twisting, which introduces chirality to the molecule . A study by Bedi et al describes a system allowing the “helical‐locking” of an acene at a specific torsion angle, and it is to this publication that we turn for geometric data.…”