Collect. Czech. Chem. Commun. 2009, 74, 131-146
Published online 2009-01-29 18:06:27

Experimental and computed dipole moments in donor–bridge–acceptor systems with p-phenylene and p-carboranediyl bridges

Ladislav Droža, Mark A. Foxb, Drahomír Hnykc,*, Paul J. Lowb, J. A. Hugh MacBrideb and Václav Všetečkaa

a Department of Organic Chemistry, Faculty of Science, Charles University, Hlavova 2030, CZ-128 40 Prague 2, Czech Republic
b Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE, UK
c Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, v.v.i., No. 1001, CZ-250 68 Husinec-Řež, Czech Republic


Dipole moments were measured for a series of substituted benzenes, biphenyls, terphenyls, C-monoaryl- and C,C′-diaryl-p-carboranes. For the donor–bridge–acceptor systems, Me2N–X–NO2, where X is 1,4-phenylene, biphenyl-4,4′-diyl, terphenyl and 1,4-C6H4-p-CB10H10C-1,4-C6H4, the measured interaction dipole moments are 1.36, 0.74, 0.51 and 0.00 D, respectively. The magnitude of the dipole moment reflects the ability of the bridge to transmit electronic effects between donor and acceptor groups. Thus, whilst the 1,4-phenylene bridges allow moderate electronic interactions between the remote groups, the p-carboranediyl unit is less efficient as a conduit for electronic effects. Averaged dipole moments computed at the DFT (B3LYP/6-31G*) level of theory from two distinct molecular conformers are in good agreement with the experimental values. Examination of the calculated electronic structures provides insight into the nature of the interactions between the donor and acceptor moieties through these 2D and 3D aromatic bridges. The most significant cooperative effect of the bridge on the dipole moment occurs in systems where there is some overlap between the HOMO and LUMO orbitals. This orbital overlap criterion may help to define the difference between “push-pull” systems in which electronic effects are mediated by the bridging moiety, and simpler systems in which the bridge acts as an electronically innocent spacer unit and through-space charge transfer/separation is dominant.

Keywords: Donor–bridge–acceptor systems; Terphenyls; p-Carboranylenes; Dipole moments; HOMO-LUMO; Electronic transmission; Carboranes; Substituent effects; DFT calculations.

References: 39 live references.