Synthesis, Structures and Nonlinear Optical Properties of Novel D−π−A Chromophores: Intramolecular Charge Transfer from 1,3‐Dithiole or Ferrocene Moieties to Polynitrofluorene or Dicyanomethylene Moieties through Conjugated Linkers

Electron donor−π−acceptor chromophores 5 , 9 , 11 , 18−20 , 21 , 22 , 27 , 28a , 28c , 31 , 32 , 34−36 , 38a−c , 41a , 41c , and 42 have been synthesised. The donor units are 1,3‐dithiole and ferrocene; conjugated ethylenic, phenyl, phenylenevinylene, thienyl, bithienyl, terthienyl, or thienylenevinylene linkers act as a central π‐electron relay unit, and dicyanomethylene and polynitrofluorene groups as the acceptor unit. The electronic absorption spectra display a broad low‐energy intramolecular charge transfer band in the visible region (500−700 nm) the energy (hν ICT ≈ 1.7−2.5 eV) and intensity (ϵ ≈ 5000−50000 M −1 cm −1 ) of which depend substantially on the nature of both D and A moieties and on the structure of the linker unit. Nonlinear optical properties have been evaluated using the EFISH technique: the highest μβ(0) values are observed for 38b [(900±300)×10 −48 esu] and 42 [(1800±300)×10 −48 esu] establishing that polynitrofluorene is a promising acceptor terminal moiety in this context. The molecular and electronic structures of 49 and 50 have been calculated by the RHF/6‐31G(d)//RHF/6‐31G(d) ab initio method. The HOMO is located mostly in the 1,3‐dithiolium ring, and the LUMO mostly at the dicyanomethylene fragment (and the phenyl ring of 50 ) although the electronic population at C2 of the 1,3‐dithiolium rings is also considerable. The X‐ray crystal structures of 9 , 18 and 27 are reported. In all three structures the conjugated π‐systems are effectively planar with extensive π‐electron delocalisation between the donor and acceptor moieties. The planar conformation of 18 gives rise to a close intramolecular S ··· S contact of 3.095(3) Å between the dithiole and thiophene units.