Two-photon absorption and excited-state energy-transfer properties of a new multibranched molecule

This paper presents a molecular design for a two-photon absorbing structure, which contains a two-dimensional molecular architecture with four arms and one bridging unit, each of these segments having an extended pi -conjugation and capable of producing two-photon absorption in different spectral ranges. Two chromophore units of a triphenylamine derivative dye, (N,N-di[4-{4-(5-[4-tert-butylphenyl]-1,3,4-oxadiazol-2-yl)styryl}phenyl]amino)benzene, were linked with a pi -conjugated di-2-thienylethenylene bridging unit to form a new multibranched compound, bis(5-[4-{N,N-di(4-[4-{5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl}styryl]phenyl)amino}styryl]thien-2-yl)ethenylene. Linear absorption spectra and luminescence spectra of the component and the multibranched chromophores revealed a hypochromic effect for the multibranched dye. A very effective energy transfer from the excited arm units to the pi -conjugated bridging unit is observed. The two-photon cross-sections were measured using the femtosecond Z-scan technique at several excitation wavelengths in the 620-800 nm range. Comparing the nonlinear absorption coefficients observed for the constituent chromophore dyes with that for the multibranched compound, we found that the arm units make the dominant contribution to the two-photon absorption when the wavelength of excitation is longer than 720 nm.