Nonlinear optical responses and relaxation processes of photo-carriers in open-shell π-electronic systems

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We have investigated the nonlinear optical properties of two open-shell π-electronic systems: doped π-conjugated polymers and biradical molecular compounds. The chemical doping to the conjugated polymers give s rise to the formation of polarons. We performed pump-probe spectroscopy for the doped states of conjugated polymers and revealed that the doping drastically shortens the relaxation time. The pump-probe spectra clarified the interaction of the chemically induced polaron states and the photo-induced species. In the biradical molecular compounds, we revealed the enhanced third-order optical nonlinearity using the third-harmonic generation spectroscopy . Moreover, the experimental results by electromodulation spectroscopysuggest the enhancement mechanism of optical nonlinearity resulting from the degeneracy of the excited states.