Regular Arrays of Rod-Shaped Molecular Photoswitches: Synthesis, Preparation, Characterization, and Selective Photoswitching within Mono- and Bilayer Systems

We assembled photoresponsive mono- and bilayer systems with well-defined properties from rod-shaped molecules equipped with different photoswitches. Using properly chosen chromophores (diarylethene-based switch and unidirectional light-driven molecular motor), we then selectively targeted layers made of the same types of photoswitches using appropriate monochromatic light. UV-vis analysis confirmed smooth and unrestricted photoisomerization. To achieve this, we synthesized a new class of triptycene-based molecular pedestals adept at forming sturdy Langmuir-Blodgett films on a water-air interface. The films were smoothly transferred to gold and quartz surfaces. Repeated deposition afforded bilayer systems: one layer containing diarylethene-based photoswitches and the other a unidirectional light-driven molecular motor. Structural analysis of both mono- and bilayer systems revealed the molecules to be tilted with carboxylic functions pointing to the surface. At least two different polymorphs differing in monolayer thickness and tilt angle (~40° and ~60°) were identified on the gold surface.

Phenyl-Substituted Cibalackrot Derivatives: Synthesis, Structure, and Solution Photophysics

Three symmetrically and three unsymmetrically substituted cibalackrot (7,14-diphenyldiindolo[3,2,1-de:3',2',1'-ij][1,5]naphthyridine-6,13-dione, 1) dyes carrying two derivatized phenyl rings have been synthesized as candidates for molecular electronics and especially for singlet fission, a process of interest for solar energy conversion. Solution measurements provided singlet and triplet excitation energies and fluorescence yields and lifetimes; conformational properties were analyzed computationally. The molecular properties are close to ideal for singlet fission. However, crystal structures, obtained by single-crystal X-ray diffraction (XRD), are rather similar to those of the polymorphs of solid 1, in which the formation of a charge-separated state followed by intersystem crossing, complemented with excimer formation, outcompetes singlet fission. Results of calculations by the approximate SIMPLE method suggest which ones among the solid derivatives are the best candidates for singlet fission, but it appears difficult to change the crystal packing in a desirable direction. We also describe the preparation of three specifically deuteriated versions of 1, expected to help sort out the mechanism of fast intersystem crossing in its charge-separated state.

Low-Temperature PM IRRAS of a Monolayer on Au: Spectra of C18D37SH

We describe an experimental setup suitable for the measurement of photomodulated IR reflection/absorption spectra of monolayers on a flat gold substrate at temperatures from room to 14 K. Results obtained for a monolayer of C₁₈D₃₇SH on gold are shown and compared with IR spectra of C₁₈D₃₇SH in a KBr pellet at these temperatures. Band widths are reduced by only ∼10%. Upon cooling below 100-150 K, the frequencies and intensities of v_as(CD₃) and v_as(CD₂) bands in the C-D stretching region in both types of spectra undergo reversible changes attributed to disappearance of gauche defects by analogy to the behavior of previously studied chains.

Time-Resolved Fluorescence Anisotropy of Bicyclo[1.1.1]pentane/Tolane-Based Molecular Rods Included in Tris(o-phenylenedioxy)cyclotriphosphazene (TPP)

We examine the fluorescence anisotropy of rod-shaped guests held inside the channels of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) host nanocrystals, characterized by powder X-ray diffraction and solid state NMR spectroscopy. We address two issues: (i) are light polarization measurements on an aqueous colloidal solution of TPP nanocrystals meaningful, or is depolarization by scattering excessive? (ii) Can measurements of the rotational mobility of the included guests be performed at low enough loading levels to suppress depolarization by intercrystallite energy transfer? We find that meaningful measurements are possible and demonstrate that the long axis of molecular rods included in TPP channels performs negligible vibrational motion.

Microcontroller based resonance tracking unit for time resolved continuous wave cavity-ringdown spectroscopy measurements

We present in this work a new tracking servoloop electronics for continuous wave cavity-ringdown absorption spectroscopy (cw-CRDS) and its application to time resolved cw-CRDS measurements by coupling the system with a pulsed laser photolysis set-up. The tracking unit significantly increases the repetition rate of the CRDS events and thus improves effective time resolution (and/or the signal-to-noise ratio) in kinetics studies with cw-CRDS in given data acquisition time. The tracking servoloop uses novel strategy to track the cavity resonances that result in a fast relocking (few ms) after the loss of tracking due to an external disturbance. The microcontroller based design is highly flexible and thus advanced tracking strategies are easy to implement by the firmware modification without the need to modify the hardware. We believe that the performance of many existing cw-CRDS experiments, not only time-resolved, can be improved with such tracking unit without any additional modification to the experiment.