RESUMO
We present the experimental and theoretical study of the two-photon excited polarized fluorescence of p-terphenyl dissolved in cyclohexane/paraffin. The fluorescence was produced within a two-color two-photon (2C2P) excitation scheme utilizing simultaneous absorption of two femtosecond laser pulses at 400 nm and at 800 nm with the total excitation energy of 4.649 eV. The fluorescence was detected by a time correlated single photon counting (TCSPC) system with two detectors. Using different combinations of the absorbed photon polarizations we extracted seven time-dependent molecular parameters from experiment that contain all information on the dynamics of the three-photon process under study. The analysis of the obtained molecular parameter values was based on the ab initio calculations of the vertical excitation energies and transition matrix elements in p-terphenyl and allowed for determination of the whole structure of the two-photon absorption tensor, fluorescence lifetime, and the rotational correlation time. The obtained results imply that the fluorescence in the conditions of our experiment was governed mostly by the d(z) component of the fluorescence transition dipole moment that is parallel to the molecular long axis Z. The tensor was found to be symmetric. The two-photon excitation in p-terphenyl occurs simultaneously via two channels, one of them resulting in the population of the totally symmetric excited state and the other in the population of the nontotally symmetric excited state. Moreover, the energetically allowed pure electron transitions are dipole forbidden and become allowed by vibronic coupling.
RESUMO
We present the first realization of a Two-Color Two-Photon Laser-Scanning Microscope (2c2pLSM) and UV fluorescence images of cells acquired with this technique. Fluorescence is induced by two-color two-photon absorption using the fundamental and the second harmonic of a Ti:Sa femtosecond laser. Simultaneous absorption of an 800 nm photon and a 400 nm photon energetically corresponds to one-photon absorption at 266 nm. This technique for Laser-Scanning Microscopy extends the excitation wavelength range of a Ti:Sa powered fluorescence microscope to the UV. In addition to the known advantages of multi-photon microscopy like intrinsic 3D resolution, reduced photo damage and high penetration depth 2c2pLSM offers the possibility of using standard high numeric aperture objectives for UV fluorescence imaging. The effective excitation wavelength of 266 nm corresponds especially well to the excitation spectrum of tryptophan. Hence, it is an ideal tool for label free fluorescence studies and imaging of intrinsic protein fluorescence which originates mainly from tryptophan. Thus a very sensitive natural lifetime probe can be used for monitoring protein reactions or changes in conformation. First measurements of living MIN-6 cells reveal differences between the UV fluorescence lifetimes of the nucleus and cytoplasm. The significance of this method was further demonstrated by monitoring the binding of biotin to avidin.
