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 (hv_ICT ≈ 1.7-2.5 eV) and intensity (ε ≈ 5000-50000 m^-1cm^-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.