| A family of substituted 9-(2,2,2-triphenylethylidine)fluorene molecules were studied as surface immobilized light driven molecular actuators. 2-Nitro-, 2-amino-, 2-cyano-, 2-iodo-, and 2-t-butyl-9-(2,2,2-triphenylethylidene)fluorene in addition to the unsubstituted molecule, were immobilized in a poly(methyl methacrylate) (PMMA) matrix adsorbed on a fused silica substrate at room temperature. UV irradiation causes reversible spatial reorientation of the angular distribution of the dibenzofulvene rotor moieties resulting from photoisomerization of the exocyclic double bond. This reorientation was detected by multi-channel polarization-resolved normal incidence cavity ring-down spectroscopy (NICRDS) which probed the reorientation dynamics when the actuator molecule was driven by linearly polarized light. Polarized "drive" irradiation creates a "hole" in the angular distribution of the molecular transition dipoles. Changing the polarization of the drive beam refills the "hole", creating a new hole parallel to the polarization direction of the drive beam. The photo-reorientation kinetics measurements of the hole burning and refilling were fitted to a stochastic model to obtain a photo-reorientation quantum yield, phireorient. Ideally, phi reorient is equal to the photoisomerization quantum yield, but the values obtained were smaller than the photoisomerization quantum yields measured in organic solution. This may be due to the PMMA different environment, dipolar intermolecular forces, or greater steric hindrance. In the case of an unsubstituted actuator, the technique proves to be a novel method for indirectly measuring the photoisomerization quantum yield of two indistinguishable isomers, by measuring phireorient. |
