Abstract

The degree of molecular order has a profound influence on fundamental optical and photophysical properties [1]-[3], chemical reactivity and orientational relaxation. The control of the alignment of a photoselected molecular population is thus an important goal in any study of molecular stereodynamics[4][5].The control of initial molecular alignment using a variable laser polarisation coupled with picosecond photon counting allows the study of the dynamics of both cylindrically symmetric (θ motion) and asymmetric (θ+ϕ motion) alignment moments in an anisotropic environment [4]. However the intrinsic symmetry of electric dipole transitions does not pennit the sole creation of either alignment moment using a single beam geometry. In particular it is impossible to create an excited array in which both degrees of alignment are zero. This can be realised via the net photoselection of three synchronised laser pulses with mutually orthogonal polarisations (Figure 1) [5]. Here the technique is applied to the investigation of the pure θ diffusion of a molecular probe (Rhodamine B) in the nematic liquid crystal 5CB. Variation in the Z-polarised excitation intensity relative to the (equal) X and Y intensities (Figure 1) allows the degree of initial excited state (cylindrically symmetric) alignment ⟨P₂(0)⟩ to be tuned continuously from 0.23 to -0.06 (Figure 2) with the preservation of cylindrical symmetry. The corresponding fluorescence anisotropies for this range of initial alignment all exhibit single exponential relaxation dynamics to a steady state (⟨P₂ (∞)⟩ ^≡R_SS= 11%) with identical relaxation times (Figure 2). Notably the preparation of an initially alignment-free excited state array (Figure 2(d)) clearly shows the imposition of net molecular order by the liquid crystal environment. This is the first direct confirmation that in a moderately aligned medium (R_ss=11%) θ and ϕ motions are independent and that the relaxation dynamics are linear.

© 1998 IEEE