Thomas C. Pijper

Reversible Photochemical Control of Singlet Oxygen Generation Using Diarylethene Photochromic Switches

Reversible noninvasive control over the generation of singlet oxygen is demonstrated in a bicomponent system comprising a diarylethene photochromic switch and a porphyrin photosensitizer by selective irradiation at distinct wavelengths. The efficient generation of singlet oxygen by the photosensitizer is observed when the diarylethene unit is in the colorless open form. Singlet oxygen generation is not observed when the diarylethene is converted to the closed form. Irradiation of the closed form with visible light (>470 nm) leads to full recovery of the singlet oxygen generating ability of the porphyrin sensitizer.

Reversible photochemical control of cholesteric liquid crystals with a diamine- based diarylethene chiroptical switch

Upon addition of a chiral dopant to a nematic liquid crystal, amplification of molecular chirality can occur and consequently a cholesteric liquid crystal is formed. A major challenge in materials science consists in designing efficient chiral dopants that allow for control over chiral amplification by use of an external trigger, for example by irradiation with light, and thereby achieving the control of the dynamic and responsive structure of cholesteric liquid crystals. Here, a chiral photochromic switch bearing two chiral imine units connected viaphenyl spacers was synthesized and characterized in solution, where it can be photo-chemically converted from a colourless ring-opened form 1o to a coloured ring-closed form 1c, reversibly. We show that a small amount of 1o used as a dopant induces the formation of a stable cholesteric liquid crystal. The retention of the photochromic properties of 1, when used as a chiral dopant, allows for reversible photocontrol over the period of the cholesteric helix, and shows the highest values of helical twisting power achieved so far with diarylethene-based photoswitchable dopants.

Kinetic analysis of the thermal isomerisation pathways in an asymmetric double azobenzene switch

Here we report a photochemical and kinetic study of the thermal relaxation reaction of a double azobenzene system, in which two azobenzene photochromic units are connected via a phenyl ring. Upon UV irradiation, three thermally unstable isomers are formed. Kinetic studies using arrayed (1)H-NMR spectroscopy revealed four distinct barriers for the thermal reversion to the stable isomer. The double isomerised Z,Z-2 can revert thermally to the E,E-2 isomer via either of two isomerisation pathways. The thermal Z to E isomerisations are not significantly affected by the state of the neighbouring azo-switching unit in the meta position. These findings are supported by quantum chemical calculations on the thermal Z to E isomerisation.

Reversible light induced conductance switching of asymmetric diarylethenes on gold: surface and electronic studies

We report on the light-induced switching of conductance of a new generation of diarylethene switches embedded in an insulating matrix of dodecanethiol on Au(111), by using scanning tunneling microscopy (STM). The diarylethene switches we synthesize and study are modified diarylethenes where the thiophene unit at one side of the molecular backbone introduces an intrinsic asymmetry into the switch, which is expected to influence its photo-conductance properties. We show that reversible conversion between two distinguishable conductance states can be controlled via photoisomerisation of the switches by using alternative irradiation with UV (λ = 313 nm) or visible (λ > 420 nm) light. We addressed this phenomenon by using STM in ambient conditions, based on switching of the apparent height of the molecules which convert from 4-6 Å in their closed form to 0-1 Å in their open form. Furthermore, the levels of the frontier molecular orbital levels (HOMO and LUMO) were evaluated for these asymmetric switches by using Scanning Tunneling Spectroscopy at 77 K, which allowed us to determine a HOMO-LUMO energy gap of 2.24 eV.

An Enantioselective Synthetic Route toward Second-Generation Light-Driven Rotary Molecular Motors

Controlling the unidirectional rotary process of second-generation molecular motors demands access to these motors in their enantiomerically pure form. In this paper, we describe an enantioselective route to three new second-generation light-driven molecular motors. Their synthesis starts with the preparation of an optically active alpha-methoxy-substituted upper-half ketone involving an enzymatic resolution. The subsequent conversion of this ketone to the corresponding hydrazone by treatment with hydrazine led to full racemization. However, conversion to a TBDMS-protected hydrazone by treatment with bis-TBDMS hydrazine, prepared according to a new procedure, proceeds with nearly full retention of the stereochemical integrity. Oxidation of the TBDMS-protected hydrazone and subsequent coupling to a lower-half thioketone followed by recrystallization provided the molecular motors with >99% ee. As these are the first molecular motors that have a methoxy substituent at the stereogenic center, the photochemical and thermal isomerization steps involved in the rotary cycle of one of these new molecules were studied in detail with various spectroscopic techniques.

Electronic properties of individual diarylethene molecules studied using scanning tunneling spectroscopy

The investigation of the electronic conduction through monosulfurdiarylethene (1S-DE) immobilized in an insulating dodecanethiol matrix on a goldsurface was addressed. Scanning tunnelingspectroscopy allows to probe spatially the frontier molecular orbitals of 1S-DE at 77 K. We locally extracted the electronic highest occupied molecular orbital-lowest unoccupied molecular orbital gap of 1S-DE with the value of 1.56 eV. This attempt reveals the importance of scanning tunnelingspectroscopy as a tool to measure the charge transport properties of diarylethene towards the realization of a switching-based molecular device.

Spectroscopic and Theoretical Identification of Two Thermal Isomerization Pathways for Bistable Chiral Overcrowded Alkenes

Chiroptical molecular switches play an important role in responsive materials and dynamic molecular systems. Here we present the synthesis of four chiral overcrowded alkenes and the experimental and computational study of their photochemical and thermal behavior. By irradiation with UV light, metastable diastereoisomers with opposite helicity were generated through high yielding E-Z isomerizations. Kinetic studies on metastable 1-4 using CD spectroscopy and HPLC analysis revealed two pathways at higher temperatures for the thermal isomerization, namely a thermal E-Z isomerization (TEZI) and a thermal helix inversion (THI). These processes were also studied computationally whereby a new strategy was developed for calculating the TEZI barrier for second-generation overcrowded alkenes. To demonstrate that these overcrowded alkenes can be employed as bistable switches, photochromic cycling was performed, which showed that the alkenes display good selectivity and fatigue resistance over multiple irradiation cycles. In particular, switch 3 displayed the best performance in forward and backward photoswitching, while 1 excelled in thermal stability of the photogenerated metastable form. Overall, the alkenes studied showed a remarkable and unprecedented combination of switching properties including dynamic helicity, reversibility, selectivity, fatigue resistance, and thermal stability.