Two-photon absorption in three-dimensional chromophores based on [2.2]-paracyclophane

A series of alpha,omega-bis donor substituted oligophenylenevinylene dimers held together by the [2.2]-paracyclophane core were synthesized to probe how the number of repeat units and through-space delocalization influence two-photon absorption cross sections. Specifically, the paracyclophane molecules are tetra(4,7,12,15)-(4'-dihexylaminostyryl)[2.2]paracyclophane (3R(D)), tetra(4,7,12,15)-(4"-(4'-dihexylaminostyryl)-styryl)[2.2]paracyclophane (5R(D)), and tetra(4,7,12,15)-(4"'-(4"-(4'-dihexylaminostyryl)styryl)styryl)[2.2]-paracyclophane (7R(D)). The compounds bis(1,4)-(4'-dihexylaminostyryl)benzene (3R) and bis(1,4)-(4"-(4'dihexylaminostyryl)styryl) benzene (5R) were also synthesized to reveal the properties of the "monomeric" counterparts. The two-photon absorption cross sections were determined by the two-photon induced fluorescence method using both femtosecond and nanosecond pulsed lasers as excitation sources. While there is a red shift in the linear absorption spectra when going from the "monomer" chromophore to the paracyclophane "dimer"(i.e., 3R --> 3R(D), 5R --> 5R(D)), there is no shift in the two-photon absorption maxima. A theoretical treatment of these trends and the dependence of transition dipole moments on molecular structure rely on calculations that interfaced time-dependent density functional theory (TDDFT) techniques with the collective electronic oscillator (CEO) program. These theoretical and experimental results indicate that intermolecular interactions can strongly affect B-u states but weakly perturb A(g) states, due to the small dipole-dipole coupling between A(g) states on the chromophores in the dimer.