Abstract
Unsymmetrically disubstituted antiaromatic indenofluorene (IF), in comparison to aromatic
pentacene counterpart with unsymmetrical disubstitution, was rare in the literature
until our recent report on indeno[1,2-b]fluorene and indeno[2,1-a]fluorene. Described herein is a straightforward access to unsymmetrically disubstituted
indeno[2,1-c]fluorenes bearing mesityl at one apical carbon and C6F5, 3,5-(CF3)2C6H3, and CCSii-Pr3 at the other apical carbon, including 4-methoxyphenyl/3,5-(CF3)2C6H3 push/pull substitution at the apical carbons with appreciable orbital density, and
a previously unknown symmetrically C6F5-disubstituted [2,1-c]IF. The electronic properties of the unsymmetrical derivatives lie halfway in between
the two symmetrical counterparts, while the 4-methoxyphenyl derivative showed the
smallest HOMO–LUMO energy gap and near-infrared absorption with intramolecular charge
transfer character. Single-crystal analyses showed 1D-columnar stacks for the unsymmetrical
motif with the C6F5 units co-facially π-stacked with the IF core, whereas symmetrically C6F5-disubstituted [2,1-c]IF, with a low-lying LUMO, showed intermolecular π–π stacks between the IFs that
resulted in good electron mobility (µ
e = 8.66 × 10−3 cm2 · V−1 · s−1) under space charge limited current measurements. Importantly, balanced ambipolar
charge-transport behaviour could be extracted for an IF series with symmetrical/unsymmetrical
substitutions, in comparison to its π-contracted pentalene congener.
Key words
indenofluorenes - indacene - polycyclic antiaromatics - ambipolar charge-transport
- charge-carrier mobility - optoelectronic properties
