The femtosecond second hyperpoJarizability due to the nonresonant electronic polarization for two intramolecular charge-transfer molecules, 6-propionyl-2-dimethylamino naphthalene and 2-anilinonaphthalene-6-sulfonic acid which possess the great permanent dipole-moment difference between the exciU:;d state and the ground state, is obtained from the measurement of the ultrafast change in refractive index usi ng a time-resolved interferometer. It is found that the second hyperpolarizability increases parabolically from 5X 10-34 to 4.3x 1O-32 esu with increasi ng ·the dipole-moment difference from 7.7 to 40 0, and this quantitative reJationshjp is well explained by a quantum-mechanical equation describing the electronic second hyperpolarizability of o ne molecule with the permanent dipole-moment difference. e 1999 American I"stitute of Physics.[S0003-6951 (99)02025-2)Recently, highly third-order nonlinear optical materials with a femtosecond response and no absorption have ·come to be desired for the development of the ultrafast opticalcontrol devices. 1 One o f the materials satisfying these optical properties is anticipated to be the organic compound having the nonlineari ty caused by intramolecular charge transfer (lcr) in the nonresonant region.2 However, the investigation of the femtosecond nonresonant dynamics of the third-order optical no nlinearity for molecules possessing the large ICf hard ly has been carried out so far.In this letter, we report that ICT molecules, with the great difference 16.,uI=I,uslIl( ;;a 200) betwe. en the permanent electric dipole moment of the excited state j)-~e and that of the ground state J.t", have the highly nonlinear electronic polarization even in the nonresonant region, and the third-order nonlinearity responds instantaneously within the duration of the femtosecond pump pulse. Furthermore, it is found experimentally that the angular-averaged second hyperpolarizability (-y) of the molecule indicates a parabolic dependence as an increasing function of the dipole-moment difference 16.,u1 over the wide range from 16.,u1;c5 to 40 O.The large ICT compounds studied in this investigation are 6-propionyl-2-dimethylainino n·phthalene (PRODAN) and 2-anilinonaphthalene-6-sulfonic acid (2,6-ANS). They were obtained from Molecular Probes, Inc. Their ultrafast th ird-order nonlinear optical responses were examined for the sample solutions dissolved in ethanol (EtOH) at the di~ lute concentration. The absorption peak of the PRODAN solution is at 360 nm with the long-wavelength absorption edge Since PRODAN has a good electron donor of the d imethyl amino group and a good electron acceptor of the p ro~ pionyl group, attached to the 2 and 6 positions of the large lUlphthalene ring, it shows a great charge-transfer character. The dipole-moment difference [6..u1 between the first excited electronic state and the ground state is known to be 20 D on the basis of the established meas urement of the Stokes shif o f the absorption and emission spectra. 3 Similarly, 2,6-ANS 1 has a good ele<;tron ...
