Two-photon absorption induced photoluminescence in para-sexiphenyl nano-needles

We studied the photoluminescence (PL) properties of self-organized para-sexiphenyl (PSP) nano-needles with linear and nonlinear optical excitation. Excited by femtosecond pulses, two-photon absorption (TPA) induced PL emission peaks corresponding to the vibronic transitions in PSP molecules were observed. Polarization-dependent photoluminescence measurements reveal that the TPA induced PL in the highly-ordered structures, similar with the linear excitation case, is strongly dependent on the excitation polarization. The mechanism that the nonlinear transition dipole moment is oriented parallel to the long axis of the molecular for the π–π∗ absorption in PSP is suggested to explain these strong polarization effects.