EXPERIMENTAL AND COMPUTED DIPOLE MOMENTS IN DONOR–BRIDGE–ACCEPTOR SYSTEMS WITH p -PHENYLENE AND p -CARBORANEDIYL BRIDGES Ladislav DROŽ a1 , Mark A. FOX b1 , Drahomír HNYK c, *, Paul J. LOW b2 , J. A. Hugh MacBRIDE b3 and Václav VŠETEČKA a2 a Department of Organic Chemistry, Faculty of Science, Charles University, Hlavova 2030, CZ-128 40 Prague 2, Czech Republic; e-mail: 1 droz@prfdec.natur.cuni.cz, 2 vsetecka@prfdec.natur.cuni.cz b Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE, UK; e-mail: 1 m.a.fox@durham.ac.uk, 2 p.j.low@durham.ac.uk, 3 j.a.h.macbride@durham.ac.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; e-mail: hnyk@iic.cas.cz Received July 1, 2008 Accepted September 15, 2008 Published online January 29, 2009 Dedicated to the memory of Professor Otto Exner. 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, Me 2 N–X–NO 2 , where X is 1,4-phenylene, biphenyl-4,4 ′-diyl, terphenyl and 1,4-C 6 H 4 -p-CB 10 H 10 C-1,4-C 6 H 4 , 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 agree- ment with the experimental values. Examination of the calculated electronic structures pro- vides 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 moi- ety, and simpler systems in which the bridge acts as an electronically innocent spacer unit and through-space charge transfer/separation is dominant. Ke y w o rd s : Donor–bridge–acceptor systems; Terphenyls; p-Carboranylenes; Dipole moments; HOMO-LUMO; Electronic transmission; Carboranes; Substituent effects, DFT calculations. Collect. Czech. Chem. Commun. 2009, Vol. 74, No. 1, pp. 131–146 Dipole Moments in Donor–Bridge–Acceptor Systems 131 © 2009 Institute of Organic Chemistry and Biochemistry doi:10.1135/cccc2008154