Controlling the Spatial Orientation of Molecular Actuators: Polarized Photoisomerization of 2-Nitro-9-(2,2,2-triphenylethylidene)fluorene in a Thin Polymer Matrix

Proton-Gated Photoisomerization of Amino-Substituted Dibenzofulvene Rotors

Derivatives of 9-(2,2,2-triphenylethylidene)-fluorene (TEF) undergo E/Z photoisomerization and are candidates for light-powered molecular actuators and switches. The 2-substituted derivatives bearing an amino group (ATEF) or a dimethylamino group (DTEF) are weakly photoactive in the absence of acids, but protonation increases photoisomerization quantum yields by factors of 30-60. Such compounds may be useful for incorporation into pH-switchable photoactive devices.

Synthesis and Photoisomerization of Substituted Dibenzofulvene Molecular Rotors

The synthesis, spectral and structural characterization, and photoisomerization of a family of 2-substituted dibenzofulvene molecular actuators based on (2,2,2-triphenylethylidene)fluorene (TEF) are reported. The 2-substituted species investigated are nitro (NTEF), cyano (CTEF), and iodo (ITEF). X-ray structures of these three compounds and three intermediates were determined to assign alkene configuration and investigate the effects of the 2-substituents on steric gearing. The addition-elimination reaction of Z-9 with trityl anion to form Z-10 proceeded with complete retention of configuration. Rates of photoisomerization were measured at irradiation wavelengths between 266-355 nm in acetonitrile/dioxane solutions at room temperature. Photoisomerization quantum yields (φ) were calculated by means of a mathematical model that accounts for a certain degree of photodecomposition in the cases of CTEF and ITEF. Quantum yields vary significantly with substituent, having maximum values of φ=0.26 for NTEF, 0.39 for CTEF, and 0.50 for ITEF. NTEF is photochemically robust and has a large quantum yield for photoisomerization in the near-UV, making it a particularly promising drive rotor moiety for light-powered molecular devices.

Polarized normal-incidence cavity ring-down spectroscopy: probing spiropyran photochromism in thin PMMA films and toluene

A normal-incidence geometry, polarization-resolved cavity ring-down spectroscopy technique (polarized NICRDS) is described to probe the polarized absorbance of surface-adsorbed thin films and short-path length liquid samples. The technique is demonstrated by a kinetic study of the photochromic behavior of the spiropyran dye 6,8-dibromo-1', 3'-dihydro-1', 3', 3'-trimethylspiro[2H-1-benzopyran-2, 2'-(2H)-indole]. The technique is shown potentially to have monolayer coverage sensitivity and can measure the angular orientation distribution of analyte molecules. The photochromic kinetics of 6,8-dibromoBIPS in toluene solution were qualitatively consistent with a previous study of this molecule using conventional absorption spectroscopy. The absorption polarizations and slow ring-closing kinetics measured in a thin poly(methyl methacrylate) film are consistent with a strong interaction of the spiropyran and merocyanine forms with the polymer matrix.