Premium
Electron Delocalization in Cross‐Conjugated p ‐Phenylenevinylidene Oligomers
Author(s) -
Klokkenburg Mark,
Lutz Martin,
Spek Anthony L.,
van der Maas John H.,
van Walree Cornelis A.
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200204671
Subject(s) - delocalized electron , conjugated system , homo/lumo , molecular orbital , crystallography , chemistry , raman spectroscopy , electronic structure , ground state , atomic orbital , bond order , photochemistry , materials science , electron , chemical physics , computational chemistry , bond length , crystal structure , atomic physics , molecule , physics , polymer , organic chemistry , quantum mechanics , optics
The synthesis, structure, and electronic properties of a series of cross‐conjugated p ‐phenylenevinylidene oligomers with one to four double bonds are reported. The X‐ray crystal structure of the compound with two double bonds reveals a nonplanar conformation with torsion angles about the C(phenylene)–C(vinylidene) and C(phenyl)–C(vinylidene) formal single bonds of 39.5(2)° and 30.5(2)°, respectively. Admixture of quinoid character in the ground state is observed. Infrared and Raman spectroscopy do not provide a clear picture of the degree of electron delocalization in the series, since the CC stretching mode does not adequately reflect the CC bond order and has a local nature. In contrast, electronic spectra and electrochemical data, as well as AM1 and PPP/SCF calculations, reveal that the cross‐conjugated compounds basically behave as linearly π‐conjugated systems in the sense that molecular orbitals are delocalized over the entire structure and systematically change in energy. The electronic interaction between the repeating units is, however, not very strong, which has the consequence that spatial extension of the molecular orbitals does not lead to a red shift of the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) electronic transition. This is related to the feature that the modest narrowing of the HOMO–LUMO gap with the chain length is accompanied by a relatively large reduction of electron repulsion. This finding implies that care should be taken in the use of electronic spectra for the evaluation of conjugation phenomena.