Organometallic Complexes for Non-Linear Optics. 59. Syntheses and Optical Properties of Some Octupolar (N-Heterocyclic Carbene)gold Complexes

The syntheses of octupolar alkynes 1,3,5-{4-(4-HC≡CC6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (4) and 1,3,5-{4-(4-HC≡CC6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (6), diphenylamino-substituted 1,3,5-(4-Ph2NC6H4-1-C≡C)3C6H3 (7), 1,3,5-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C)3C6H3 (8), 1,3,5-{4-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (9), and 1,3,5-{4-(4-Ph2NC6H4-1-C≡C-4-C6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (10), and (N-heterocyclic carbene)gold-appended 1,3,5-{[(NHC-iPr)Au]C≡C}3C6H3 (11), 1,3,5-{[(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C}3C6H3 (12), 1,3,5-{4-([(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (13), and 1,3,5-{4-([(NHC-iPr)Au]C≡C-4-C6H4-1-C≡C-4-C6H4-1-C≡C)-3,5-Et2C6H2-1-C≡C}3C6H3 (14) [NHC-iPr≤C-cyclo-CN(2,6-C6H3iPr2)CH≤CHN(2,6-C6H3iPr2)] are reported. The low-energy bands in the linear optical absorption spectra of all three sets of compounds are red-shifted and increase in intensity upon π-delocalizable 'arm' lengthening. The diphenylamino- and (NHC-iPr)gold-terminated compounds do not exhibit measurable second-harmonic generation as assessed by hyper-Rayleigh scattering at 1064nm using nanosecond pulses. Computational studies have been employed to rationalize the optical properties of the new compounds. Calculations on 7-10 reveal that the lowest-energy transitions with large oscillator strengths are predominantly [Ph2NC6H4] (π) → [arms+core] (π∗) in character, whereas calculations on 11-14 suggest that the low-energy transitions relate to the transfer of electron density from the Au-alkynyl core group to the terminal NHC groups.