Engineering the HOMOLUMO gap of indeno[1,2-]fluorene

A direct, efficient and versatile strategy for the modulation of optoelectronic and magnetic properties of indeno[1,2- b ]fluorene has been developed. 4-Substituted-2,6-dimethylphenyl acetylene groups placed in the apical carbon of the five-membered rings lead to redshifted absorption maxima ( max ranging from 600700 nm) and considerable narrowing of the HOMOLUMO energy gap (down to 1.5 eV). Experimental and theoretical data show an increase in the diradical character ( y ) and a decrease of the singlet-triplet energy gap. Moreover, we have investigated the single-molecule conductance of the antiaromatic indeno[1,2- b ]fluorene for the first time by including thiomethyl (-SMe) anchor groups on the phenylacetylene moiety. Conductance values one order of magnitude higher than those of a reference linear 3-ring para -phenylene ethylene have been found, despite the longer length of the S-to-S molecular junction. First principles transport calculations support this high conductance value. The insertion of phenyl acetylenes at the reactive positions of indeno[1,2- b ]fluorene leads to tune its optoelectronic and magnetic properties. Furthermore, its single-molecule conductance has been investigated for the first time.