Oligophenylenevinylene phane dimers: Probing the effect of contact site onthe 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 c ompounds give insight into how bringing two chromophores into close proximi ty affects the photophysics of the pair. Thus, reaction of 4,7,12, 15-tetra bromo[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) mo dels a contact between two distyrylbenzene molecules across the central rin g. Reaction of 4,7,12,15-Br4Pc with 4-(4-tert-butylstyryl)styrene (TBSS) un der similar conditions gives 4,7,12,15-tetra(4-(4'-tert-butylstyryl)styrl)[ 2.2]-paracyclophane (5R(D)). In 5R(D) two oligophenylenevinylene units cont aining five phenyl rings are connected via their central ring. Similar reac tion 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. Com parison against the properties of 3R and 5R shows that the absorption and e mission data of 3R(D) and 5R(D) an consistent with considerable delocalizat ion between the two subunits across the paracyclophane bridge. The observed trends in the optical properties of these compounds are analyzed using col lective electronic oscillators (CEO) representing the changes induced in th e electronic density matrix upon optical excitation. Comparison of the CEO of the paracyclophane dimers with the corresponding monomers using two-dime nsional plots provides an efficient method for tracing the origin of the va rious optical and electronic transitions by identifying the underlying chan ges in charge densities and bond orders. The electronic description of 3R(D ) and 5R(D). in which the interchromophore contact is across the central ri ng, is considerably different from the description of paracyclophane dimers of similar chromophores that are connected via the terminal ring. Essentia lly no delocalization is observed for the "termini" dimers.