Stilbenoid dimers: Dissection of a paracyclophane chromophore

A series of paracyclophane derivatives that hold chromophores of varying conjugation lengths has been synthesized using palladium-mediated coupling reactions. These molecules mimic solid-state interactions in main-chain polychromophores and conjugated emissive polymers such as poly(p-phenylenevinylene) (PPV). Their optical properties give insight into the energetics of photoexcitations localized in a discrete chromophore relative to a state containing the through-space delocalized paracyclophane core. Thus, 4-vinyl[2.2]-paracyclophane (5) is obtained by reaction of 4-bromo[2.2]paracyclophane (3) and ethylene using Pd(OAc)(2) and P(o-tol)(3). Similar reactions starting with pseudo-o- or pseudo-p-dibromo[2.2]paracyclophane (4a and 4b, respectively) give the pseudo-o- and pseudo-p-divinyl products (6a and 6b, respectively). Using styrene instead of ethylene provides the styryl-substituted products. Thus, 4-styryl[2.2]paracyclophane (7) is obtained from 3 while pseudo-p- and pseudo-o-distyryl[2.2]paracyclophane (la and Ib) are obtained from 4a and 4b; respectively. Compounds la and Ib can be viewed as stilbene dimers that have a pair of cofacial arene units at a fixed distance. Pseudo-p-bis(4-vinyl-styryl) [2.2]paracyclophane (9) was prepared by reaction of CH2-PPh3 with pseudo-p-bis(4-carboxaldehyde-styryl)[2.2]paracyclophane. Reacting 4-(4-tert-butylstyryl)styrene with 3, 4a, or 4b under Heck-type conditions gives 4-[4-(4-tert-butylstyryl)styryl] [2.2]paracyclophane (10) and pseudo-p- and pseudo-o-bis[4-(4-tert-butylstyryl)styryl][2.2]paracyclophane (2a and 2b), respectively. The observed trends in absorption, fluorescence and radiative lifetime of these compounds are reported and analyzed using collective electronic oscillators (CEO) representing the changes induced in the reduced single-electronic density matrix upon optical excitation. Comparison of the CEO of the aggregates with the corresponding monomers using two-dimensional plots provides an efficient method for tracing the origin of the various optical transitions by identifying the underlying changes in charge densities and bond orders. For 5, 6a,b, 7, and la,b the emission is red-shifted from the "monomeric" compound and featureless, reminiscent of excimer qualities. The emissions of 9, 10, and 2a,b are similar to the "monomer" and display vibronic structure. Thus, for the smaller chromophores, emission occurs from a state containing the through-space delocalized paracyclophane core. In the situation where extended chromophores, with more stable excited states, are held together with the paracyclophane core, the photophysics of the individual chromophores dominates. The present analysis is relevant to the design and synthesis of organic molecules with desired optical properties.