Two-photon spectroscopy of pi-conjugated polymers: The case of poly[1,6-bis(3,6-dihexadecyl-N-carbazolyl)-2,4-hexadiyne] (polyDCHD-HS)

The two-photon fluorescence excitation and optoacoustic spectra of red poly DCHD-HS in benzene solution at room temperature have been investigated in t he 1600-750 nm exciting wavelength region (effective two-photon energy simi lar to1.55-3.31 eV). The spectra obtained by the two detection techniques a re of comparable accuracy, and the observed band profiles are in good agree ment. Two electronic band systems have been observed and assigned to two-ph oton allowed A(g) --> A(g) transitions. Both A(g) states are at energies hi gher than that of the strongly one-photon allowed 1B(u) state. An estimate of the absolute two-photon absorptivity of polyDCHD-HS at 406.0 nm (similar to3.05 eV) is given by means of the optoacoustic method. Our results are d iscussed on the basis of the current theories on electronic states of pi -c onjugated polymers and compared with reported data on these systems.