Two-color two-photon excited fluorescence of 2-methyl-5-tert-butyl-p-quaterphenyl (DMQ): ab initio calculations and experimental determination of the molecular parameters.
The paper presents experimental and theoretical studies of two-photon excitation dynamics in 2-methyl-5-tert-butyl-p-quaterphenyl (DMQ) dissolved in cyclohexane/paraffin. Experimentally, a two-color two-photon (2C2P) excitation by two femtosecond laser pulses at 800 and 400 nm has been used in combination with the time-resolved detection of polarized molecular fluorescence. The fluorescence decay was found to be two-exponential, resulting in the molecular excited state lifetime of 753 ± 10 ps and the rotational correlation time of 724 ± 45 ps. Control over the excited and fluorescent photons polarization has been used for determination from experiment of seven independent molecular parameters. The experimental data were analyzed on the basis of the recent theoretical approach [Shternin, P. S., Gericke, K.-H., and Vasyutinskii, O. S. Mol. Phys. 2010, 108, 813-825] supported by ab initio computations of the DMQ electronic structure and transition dipole moments. The results obtained imply that the two-photon absorption tensor S is mostly diagonal and that the Szz tensor component onto the molecular long axis gives the major contribution of 93%. However, it was also found that a number of different symmetry two-photon transitions related to the dipole moment components dxdz and dydz are excited in the conditions of our measurements.
Time-resolved anisotropic two-photon spectroscopy
Abstract Two-photon excitation (TPE) in a randomly oriented liquid sample generates an anisotropic distribution of excitations that can be probed by a secondary spectroscopic transition, e.g. fluorescence or transient absorption. The orientation dependence of the secondary transition dipole (and therefore of the fluorescence or transient-absorption signal) can be exploited to generate information on molecular symmetry and orientation that is not available in two-photon absorption alone. A theoretical formalism is developed here for the orientationally averaged two-photon absorptivity taking into account the orientation of the secondary transition dipole. Spherical tensor formalism is employed to distinguish isotropic and anisotropic components of the two-photon absorptivity, and several two-photon anisotropy parameters are defined. The theory describes time-resolved detection of TPE by transient-absorption or fluorescence decay measurements. Time-resolved measurements with time-resolution that is fast relative to the rotational correlation times are shown to provide new information, including the two-photon anisotropies. Applications to TPE fluorescence anisotropy measurements and TPE induced anisotropic transient absorption are described, and illustrated by experimental measurements on bacteriorhodopsin.
Optical rotatory dispersion and absolute configuration—III
Abstract Optical rotatory dispersion measurements on pyrrolidine, piperidine, and tetrahydro-isoquinoline alkaloids indicate that the absolute configuration of the α-asymmetric center may be assigned directly from the dispersion curve, provided either that the nitrogen atom is not protonated, or that the compound contains a chromophore absorbing above 200 mμ.