Oligophenylenevinylene phane dimers: Probing the effect of contact site on the optical properties of bichromophoric pairs

Paracyclophane derivatives have been prepared that may be considered models of bichromophoric contacts in the solid. The optical properties of these compounds give insight into how bringing two chromophores into close proximity affects the photophysics of the pair. Thus, reaction of 4,7,12, 15-tetrabromo[2.2]-paracyclophane (4,7,12,15-Br4Pc) with excess 4-tert-butylstyrene using Pd(OAc)(2) under phase transfer conditions affords 4,7,12,15-tetra(4-tert-butylstyryl)[2.2]paracyclophane (3R(D)). The connectivity of 3R(D) models a contact between two distyrylbenzene molecules across the central ring. Reaction of 4,7,12,15-Br4Pc with 4-(4-tert-butylstyryl)styrene (TBSS) under similar conditions gives 4,7,12,15-tetra(4-(4'-tert-butylstyryl)styrl)[2.2]-paracyclophane (5R(D)). In 5R(D) two oligophenylenevinylene units containing five phenyl rings are connected via their central ring. Similar reaction protocols gave 2,5-dimethyl-1,4-di(4-tert-butylstyryl)benzene (3R) and 2.5-dimethyl-1,4-bis[4-(4'-tert-butylstyryl)styryl]benzene (5R). Molecules 3R and 5R serve to give the optical properties of the monomeric units. Comparison against the properties of 3R and 5R shows that the absorption and emission data of 3R(D) and 5R(D) an consistent with considerable delocalization between the two subunits across the paracyclophane bridge. The observed trends in the optical properties of these compounds are analyzed using collective electronic oscillators (CEO) representing the changes induced in the electronic density matrix upon optical excitation. Comparison of the CEO of the paracyclophane dimers with the corresponding monomers using two-dimensional plots provides an efficient method for tracing the origin of the various optical and electronic transitions by identifying the underlying changes in charge densities and bond orders. The electronic description of 3R(D) and 5R(D). in which the interchromophore contact is across the central ring, is considerably different from the description of paracyclophane dimers of similar chromophores that are connected via the terminal ring. Essentially no delocalization is observed for the "termini" dimers.