Fluorescence-detected two-photon circular dichroism of Gd3+ in trigonal Na3[Gd(C4H4O5)3] · 2NaClO4 · 6H2O Original Research Article

Fluorescence-detected two-photon circular dichroism measurements are reported for Gd3+ in single, uniaxial crystals of Na3[Gd(C4H4O5)3] · 2NaClO4 · 6H2O. These measurements were obtained from experiments in which sample fluorescence is excited by a sequence of one-color optical pulses modulated between left- and right-circular polarization states. Excitation is along the unique (optic) axis of the crystal sample, and fluorescence detection is along the direction of excitation. The fluorescence intensities produced by the left- and right-circularly polarized excitation pulses are measured separately and then added and subtracted to obtain the sum and difference quantities, I = I(L) + I(R) and ΔI = I(L) − I(R). Measurements of ΔI and I as a function of excitation wavelength yield spectra related to the circular dichroism (CD) and unpolarized absorption spectra of the sample, and ratios of ΔI to I/2 correspond exactly to absorption dissymmetry factors obtained from direct measurements of CD/absorption spectra. In the study reported here, ΔI and I excitation spectra were measured over spectral regions that correspond to two-photon excitation of 8S7/2 → 6P7/2, 6P5/2, and 6P3/2 transitions within the 4f7 electronic configuration of Gd3+ in Na3[Gd(C4H4O5)3] · 2NaClO4 · 6H2O. The two-photon circular dichroism (TPCD) spectra and absorption dissymmetry factors obtained from these measurements are the first to be reported for any optically active system. The results pose interesting new challenges for detailed mechanistic analyses of two-photon absorption processes, and they provide information about molecular transition polarizabilities that are unique to chiral systems. Furthermore, they suggest that TPCD measurements can yield structural information that both supplements and complements that obtainable from ordinary (one-photon) circular dichroism experiments.