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

Abstract

We present results of single and three pulse photoselection studies of the dynamics of a fluorescent probe (Oxazine 4) in the nematic phase of the liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB). The combination of these techniques with time resolved fluorescence anisotropy measurements allows the unambiguous determination of the full angular motion of the probe together with the ground and excited state degrees of equilibrium (steady state) alignment. The restricted JHRPHWU\ LPSRVHG E\ WKH QHPDWLF KRVW LV VHHQ WR LPSDUW D PDUNHG DQLVRWURS\ LQ DQG φ GLIIXVLRQ ! ~4 ).

Time-resolved stimulated emission depletion in two-photon excited states

“…Considerable detail that could be explored in fluorescence lifetime and anisotropy studies is thus lost. The interaction of polarized light with an ordered molecular array has been shown to yield information on detailed local order and the full angular motion of a molecular probe [6][7][8][9][10]. The local environment on average experiences wholly unpolarized excitation and the possibility of distinguishing between θ and φ orientations (and motions) within the probe environment is lost ( [11] and A.J.…”

“…This loss of information is potentially serious: recent studies in our group of fluorescent probe dynamics within ordered hosts have shown that the imposition of order has a profound influence on rotational diffusion dynamics. A marked difference in θ and φ diffusion rates and their temperature dependence is observed [10,12,13]. The second limitation arises from the selection rules for spontaneous electric dipole transitions, which restrict the degree of order that can be probed in the excited state.…”

“…Measurement of R(d), R(u) and S (via F R ) as a function of the PUMP DUMP delay allows the time evolution of α ex 40 to be determined from eqn (2). This will be of particular importance in the study of probe motion in ordered environments where significant contributions to the molecular distribution function arise from higher-order moments, and the simple symmetry relationships that govern the relaxation dynamics for different moments are lifted [6,8,10]. The validity of this approach can be tested by measurement of the relaxation dynamics of α ex 40 (t) created by two-photon excitation of fluorescein in ethylene glycol.…”