Rotation, Rotation, Rotation: a study of molecular rotation for the purpose of selectivity, control, and application

Abstract

This thesis describes three projects related to the general study of control of molecular rotation. The first project discussed probes the preferred sense of rotation of NO fragments generated from the UV photodissociation of nitrosobenzene. Photodissociation experiments involving nitrosoalkanes show the rotational orientation of the generated fragments has a significant preference for v ⊥ j velocity-angular momentum vector correlation. Experiments by Keßler et al. proposed the possibility that the aryl compound nitrosobenzene may show a preferrence for the opposite v # j vector correlation. Experimental testing of this proposal here shows a measurable preference for the NO fragment to emerge from the parent molecule with a v # j recoil trajectory after the UV photodissociation of nitrosobenzene. Determination of the bipolar moment, β0 0 (22), which quantifies the degree of v − j correlation, gave an average value of 0.22, with an average standard deviation of 0.11. This demonstrates an uncommon v − j fragment trajectory for UV photodissociation experiments. The second project investigated the possiblity of directly controlling the sense of rotation of molecules by imparting rotational orientation via absorption of circularly polarized light. Simulations which calculated the degree of rotational orientation and alignment of model, symmetric, prolate rigid rotors demonstrated theoretical feasibility. As part of the same simulation, the amount of internal vibrational energy stored over the course of many photon absorptions predicted significant photodecomposition in a simple two-electronic state model having one vibrational mode. The results highlighted a concern for photostability in practical experiments. When the laser dye molecule Rhodamine 575 was held in an ion trap and continuosly irradiated with circularly polarized 514 and 488 nm laser light, it demonstrated a propensity for photodecomposition. Detection of the polarization of the emitted fluorescence also ii showed a slight rotational alignment of the molecules, though the data were inconclusive. The concept of directly controlling the rotational orientation of an ensemble of molecules by irradiation with circularly polarized light shows promise. The final project quantified the efficiency of internal rotation about a double bond during UV photoisomerization of a model molecular motor prototype. The family of dibenzofulvene derivatives investigated showed significant photoisomerization quantum yields, as high as 0.5 in the case of the iodo substituted dibenzofulvene rotor. While theoretical calculations of fulvene predict little to no photoisomerization, experimental evidence of the photoisomerization of these dibenzofulvene rotors suggests addition of aromatic benzene rings alters the potential energy surfaces of the ground and excited electronic states involved in photoisomerization to the extent that surface crossing from the excited state to the ground state occurs at nuclear coordinates much more favorable to isomerization in dibenzofulvene. The photoisomerization quantum yields showed a dependence on the substituent group, with t-butyl < nitro < cyano < iodo. The cyano and iodo substituted compounds showed evidence of a small degree of photodecomposition, which did not signifcantly affect the photoisomerization quantum yields. The substituent groups also affected the absorption spectrum of the molecule, with the nitro substituted compound absorbing into the near visible region at ∼400 nm. These three projects, while not directly related to each other, add to the overall understanding of molecular rotation and give insight into the issues involved in selectively generating a specific sense of rotation in molecules, whether by harnessing the preferred sense of rotation in UV photodissociation fragments, directly imparting specific angular momentum via absorption of circularly polarized light, or custom designing a molecular rotor for efficient photoisomerization