Isomeric Column‐Forming Esters and Imides with Varying Curvatures of the Aromatic Plane

Abstract

Dibenzo[a,j]coronene-tetracarboxylic alkyl esters and imides with either a centrosymmetric bis-peri substitution pattern or a polar bis-ortho substitution pattern form hexagonal columnar mesophases, which in the case of the imides persist at room temperature. The bis-peri isomers are obtained via a two-fold oxidative photocyclization; the bis-ortho isomers are accessed via a glyoxylic Perkin reaction of triphenylene and naphthalene building blocks. Steric congestion between the substituents and the adjacent benzo protrusion in the bis-ortho esters and imides leads to bending of the aromatic plane, which thus avoids twisting. These isomers surprisingly show a more pronounced liquid crystalline behaviour than their non-bent bis-peri homologs, accommodating non-planarity with columnar order by slipped stacking. Whereas both types of ester and the bis-peri imide show an optical behaviour typical for perylene chromophores, the strongly bent bis-ortho imide distinguishes itself notably from them by its absorption spectrum. The electron acceptor strength of the isomeric diimides is found to differ, the hexagonal (peri) diimide having a 0.20 eV lower LUMO energy than the pentagonal (ortho) isomer.

Curved Polar Dibenzocoronene Esters and Imides versus Their Planar Centrosymmetric Homologs: Photophysical and Optoelectronic Analysis

“…As shown inFig. 3, these NTOs are similar to the ones from perylene confirming the assignment of the lowest energy absorption as mainly "perylene-like" 34. The more pronounced differences in NTOs between the investigated compounds and perylene are found for 2 and 4.…”

“…to 350 nm is considered to be "coronene-like". 34 As can be seen in Table 1, the transition to S 2 exhibits only significant oscillator strengths in case of the peri -substituted compounds 1 and 3, whereas it is virtually dark for the ortho-substituted molecules 2 and 4. In contrast to this, the oscillator strengths for the states responsible for the "coronene-like" absorption are more similar for the esters 1 and 2 on the one hand and the imides 3 and 4 on the other.…”

“…[43][44][45][46] The ground (S 0 ) and first two excited states (S 1 and S 2 ) of compound 1 were considered for the vibronic calculations taking into account Franck-Condon and Herzberg-Teller terms. 47 Comparing the calculated absorption spectra with the previously published measurements, 34 it can be seen that the experimentally found trends can be reproduced: (i) the lowest energy absorption bands of the imides are red shifted relative to the esters; (ii) For the imides, a marked red shift is found when going from compound 1 to 2; (iii) For the esters, a shift in the opposite direction, i. e. a blue shift, is obtained going from 3 to 4. However, this shift is not as pronounced as for the imides.…”

“…1). 34 The experimentally obtained absorption spectra were compared to quantum chemical calculations based on density functional theory (DFT). The electronic transitions that are of special interest in spectroscopy were further analyzed.…”

“…Spectroscopic data related to the absorption and photoluminescence of thin films. The sublimation temperatures T subl at pressures p < 10 −6 mbar and intra-columnar stacking distances d are also listed 34. λ S1 /nm λ PL,max /nm ∆λ/nm T subl /°C d/nm…”