Picosecond polarized fluorescence studies of anisotropic fluid media. I. Theory

Abstract

New polarized time resolved fluorescence techniques are implemented to determine the full angular motion of a probe molecule in an anisotropic environment. Studies of rhodamine 6G and resorufin molecules aligned in a free ethylene glycol jet show that the presence of net molecular order is accompanied by a distinct anisotropy in alignment relaxation following photoselection. Diffusion coefficients for and motion ͑D ʈ and D Ќ ͒ in a jet fixed axis system are determined from the cylindrically symmetric and asymmetric alignment relaxation rates for the isotropic and anisotropic regions of the jet. The presence of net negative molecular alignment as the free jet is formed is seen to correspond to restricted motion (D ʈ ϽD Ќ), with a net positive steady state alignment the anisotropy in the diffusion dynamics is reversed. The differences in D ʈ and D Ќ are attributed to anisotropy in the solvent viscosity as a consequence of flow. The combination of linear and circular polarization techniques is seen to provide useful information on cylindrical asymmetry and relaxation dynamics hitherto unobserved by conventional fluorescence polarization techniques.

Single- and two-photon time-resolved polarized fluorescence studies of probe molecule dynamics in nematic liquid crystals

“…Recent work in our group has shown for an ordered medium (i.e. a non random distribution of probe environments) polarised time resolved fluorescence when coupled with a variable excitation polarisation can yield hitherto unobserved information on molecular order and dynamics [6][7][8][9] . Variation of the (linear) excitation polarisation in the laboratory frame allows the preparation of a range of non-equilibrium probe orientational distributions within the (ordered) local environment.…”

Single and three pulse photoselection of dye probes in nematic liquid crystals

“…Principally these are the non-parallelism of absorption and emission transition dipole moments 3 , depolarisation due arising from sample concentration and path length effects 19 and depolarisation by the collection optics. The A parameter is determined by the departure of the initial fluorescence anisotropy from its theoretical value in an isotropic sample 4 .…”

“…Analysis of the subsequent fluorescence anisotropy signals yields the ground and excited state degrees of molecular alignment together with the intrinsic rates for θ and φ diffusion in the laboratory axis system. This approach has been highly successful in the study of systems exhibiting moderate probe alignment where the departure from (otherwise) isotropic diffusion dynamics is well described by first order perturbation theory 3 . In single pulse photoselection the creation of the maximum probe alignment orthogonal to the laboratory Z axis (in the case of liquid crystals the nematic director) is achieved with an excitation polarisation angle of 90°4.…”

“…Picosecond fluorescence anisotropy studies of molecular probes have been shown to yield useful information on molecular order, orientational dynamics and photophysical processes in both molecular and biomolecular systems 1,11 . Recent work in our group has shown that it is possible to obtain information on the order and full angular motion of a fluorescent probe in an ordered environment using single pulse variable photoselection and picosecond polarised fluorescence [2][3][4] . Here the variation of the excitation (linear) polarisation gives rise to the creation of varying degrees of (non-equilibrium) cylindrically symmetric and asymmetric probe alignment.…”

“…,Q DQ RUGHUHG PHGLXP LW LV FUXFLDO WR DFKLHYH D IXOO UHVROXWLRQ RI WKH RULHQWDWLRQDO ERWK DQG φ) dynamics as the imposition of order gives rise to a marked departure from conventional (isotropic) molecular relaxation [1][2][3][4][5] . Molecular order has to date been probed using a range of optical processes including Raman scattering 6 , laser induced fluorescence [7][8][9] and surface second harmonic generation 10 .…”