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RESEARCH PRODUCT

Mesomorphic Donor-Acceptor-substituted 1,4-Distyrylbenzenes

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description

Conjugated oligomers with terminal donor-acceptor substitution attract increasing attention in organic chemistry because of their outstanding and not seldom unexpected properties in materials science [1 – 4]. Although a variety of oligo(1,4-phenylene-vinylene)s with terminal donor-acceptor substitution (DAOPVs 1, Scheme 1) are known [5 – 22], the number of thermotropic liquid crystals among them is very low and is confined to monomers (n = 1), that means to certain push-pull substituted (E)-stilbenes [23 – 31]. The behavior of conjugatedmolecules, which are highly polar and form mesophases by self-organization, is very interesting for various applications of liquid crystals [3]. A big disadvantage of stilbenes (n = 1), compared to higher OPVs (n ≥ 2), is due to their light sensitivity. Stilbenes show four types of photoreactions in the first excited singlet state S1: E Z isomerization, concerted [2πs + 2πs] cycloadditions, [6πs] electrocyclic ring closures, and photo-crosslinking by statistical CC bond formation [32]. The extended conjugation in OPVs (n ≥ 2) prevents in their S1 state the first three processes and permits only the crosslinking by irradiation with energy-rich UV light (λ = 254 nm) [32]. The intersystem crossing (ISC) rate is normally very low in OPVs (n ≥ 1), so that triplet photoreactions can also be neglected. Among the multiple DAOPVs (n ≥ 2) which were studied in the past, thermotropic LC phases could not be found. Therefore, our chances to realize this concept were low in the beginning. (E,E)-1,4-Distyrylbenzenes with short alkoxy chains or small or large dialkyl-