Micro- and macro-scopic second-order non-linear optical properties of fulvene compounds

A number of fulvene compounds, which can polarize to give rise to intramolecular charge transfer (ICT), have been synthesized, and the second-order molecular hyperpolarizability (β)and dipole moment (µg) have been calculated by using the MOPAC program. The macroscopic non-linearity (second-harmonic generation; SHG) evaluated by the Kurtz powder method is discussed in relation to microscopic β and µg. The large β and the small µg are characteristic of these compounds and contribute to the SHG activity Among these compounds, ω-(p-methoxyphenyl)benzofulvene (MPBF) was found to exhibit large SHG activity and could be grown into large crystals. X-Ray diffraction analysis of MPBF shows non-centrosymmetrically cylindrical packing along the b-axis. The effective non-linear optical coefficient deff of MPBF, which was evaluated from the bulk crystal cut to the phase-matching direction, corresponds to the value (7.0 pm V–1) of potassium titanylphosphate (KTP), although the hardness is low (24 Vickers hardness).