Second Harmonic Generation and Two-Photon Fluorescence as Nonlinear Optical Properties of Dipolar Mononuclear Sesquifulvalene Complexes

Abstract

In order to investigate nonlinear optical properties, in particular second harmonic generation (SHG) of organometallic complexes, numerous dipolar monocationic sesquifulvalene complexes of the general form [Mc–Z–C7H6]+ have been synthesised (Mc = metallocenyl), wherein the metallocenyl moiety Mc acts as an electron donor, and the tropylium cation C7H6+ as an electron acceptor. The mutual electronic influence of the donor and the acceptor groups is warranted by a linking spacer Z, which is a single bond [Mc = CpFeC5H4 (4a); Mc = CpRuC5H4 (4b)], an unsaturated bridge containing olefins [Z = (E-CHCH)n: n = 1, Mc = CpFeC5H4 (25a), Mc = CpRuC5H4 (25b); n = 2, Mc = CpFeC5H4 (26); n = 3, Mc = CpFeC5H4 (27)], or thiophene units [Z = 2,5-C4H2S, Mc = CpFeC5H4 (28); Z = 5,5′-(2,2′-C4H2S)2, Mc = CpFeC5H4 (29); Z = 2-(5-E-CHCH)C4H2S, Mc = CpFeC5H4 (30)]. For the salts of 4a·BF4, 4b·PF6 and 25a·PF6 X-ray structure determinations have been performed [4a·BF4: orthorhombic, Pnma, = 21.75(2), b = 9.900(2), c = 6.881(3) A, V = 1482.0(15) A3, Z = 4; 4b·PF6: monoclinic, P2(1)/c, a = 8.104(3), b = 18.206(14), c = 11.228(4) A, β = 107.59(3), V = 1579.1(15) A3, Z = 4; 25a·PF6: triclinic, P1bar, a = 10.067(6), b = 10.496(6), c = 11.418(6) A, α = 94.07(4), β = 110.96(4), γ = 102.88(5)°, V = 1083.1(11) A3, Z = 2]; the solid state structures indicate an almost coplanar arrangement of the organic π-system. Cyclic voltammetry studies reveal an irreversible one-electron reduction and an electrochemically reversible one-electron oxidation step for the ferrocenyl derivatives, whereas the ruthenocenyl derivatives demonstrate an irreversible two-electron oxidation. The redox potentials clearly indicate that the oxidation occurs at the metallocene unit, and the reduction is localised on the tropylium entity. In the electronic absorption spectra two intense bands are observed in the region 400 < λ < 900 nm which undergo strong negative solvatochromic shifts. The origin of the high-energy absorption band is assumed to be an interligand charge-transfer (LL–CT) transition, and the low-energy absorption band is assigned to a donor-acceptor charge-transfer (DA–CT) transition. Whereas the LL–CT is continuously shifted to lower energy with increasing spacer length, the energy of the DA–CT approaches a limiting value. A comparable phenomenon is also observed for the difference between the oxidation and reduction potentials obtained from electrochemical studies. Investigations concerning the nonlinear optical properties of the dipolar cationic sesquifulvalene complexes by means of hyper-Rayleigh scattering (HRS) indicate that the ferrocenyl derivatives fluoresce due to two-photon absorption whereas the ruthenocenyl congeners exhibit second harmonic generation with considerably larger first hyperpolarizability which is partly resonance-enhancement based.