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

A series of α,ω-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.