Tris(trifluoromethyl)germyl biphenyl conjugated molecular system with ferrocenyl substituent: Confirmation of photoinduced intramolecular charge transfer to the germanium center

Abstract

Unsymmetrical 4,4′-biphenyl derivative with tris(trifluoromethyl)germylethynyl ‒CC‒Ge(CF3)3 and ferrocenylethynyl Fc‒CC‒ substitutes have been prepared (21), its properties have been studied and compared with those full non-fluorinated analogue of trimethylgermylethynyl ‒CC‒Ge(CH3)3 compound (20). UV-visible, absorption, steady state, and time-resolved fluorescence spectra in solution for these germanium and a number of similar compounds (15-17, 22) have been investigated. The process of photoinduced electron transfer from the π-conjugated system to the germanium center is confirmed for 21, as well as for the first time discovered earlier [19] in the 4-biphenyl ‒CC‒Ge(CF3)3 molecule (9). It was found that the lifetime of the radical-ion pair 21 in the excited state (kCR2 = 1.2 × 105 s-1) is approximately an order of magnitude longer than for 9, which can be explained by the stabilizing effect of the electron donor Fc‒CC‒ group. Density-functional theory (DFT) computations was used to calculate the optimized geometry of 21 in excited state, and two main bands in the emission spectrum at laser excitation (340 and 386 nm) an excellent fit to the experimental data. An additional data on the nature of the (CF3)3Ge group, including geometry, electronegativity, and contributions 4d AOs of germanium into lowest unoccupied molecular orbital (LUMO) in a wide range of fluorinated and non-fluorinated compounds were also calculated. The molecular structure of compound 21 was determined by X-ray structural analysis. In the crystal lattice of 21, numerous intermolecular F‧‧‧F interactions shortened in comparison with the van der Waals radii were detected, which leads to the formation of quasi-two-dimensional fluorinated layers in crystals.