Stefano Carli

Triplet Pathways in Diarylethene Photochromism: Photophysical and Computational Study of Dyads Containing Ruthenium(II) Polypyridine and 1,2-Bis(2-methylbenzothiophene-3-yl)maleimide Units

A 1,2-bis(2-methylbenzothiophene-3-yl)maleimide model ( DAE) and two dyads in which this photochromic unit is coupled, via a direct nitrogen-carbon bond ( Ru-DAE) or through an intervening methylene group ( Ru-CH 2-DAE ), to a ruthenium polypyridine chromophore have been synthesized. The photochemistry and photophysics of these systems have been thoroughly characterized in acetonitrile by a combination of stationary and time-resolved (nano- and femtosecond) spectroscopic methods. The diarylethene model DAE undergoes photocyclization by excitation at 448 nm, with 35% photoconversion at stationary state. The quantum yield increases from 0.22 to 0.33 upon deaeration. Photochemical cycloreversion (quantum yield, 0.51) can be carried out to completion upon excitation at lambda > 500 nm. Photocyclization takes place both from the excited singlet state (S 1), as an ultrafast (ca. 0.5 ps) process, and from the triplet state (T 1) in the microsecond time scale. In Ru-DAE and Ru-CH 2-DAE dyads, efficient photocyclization following light absorption by the ruthenium chromophore occurs with oxygen-sensitive quantum yield (0.44 and 0.22, in deaerated and aerated solution, respectively). The photoconversion efficiency is almost unitary (90%), much higher than for the photochromic DAE alone. Efficient quenching of both Ru-based MLCT phosphorescence and DAE fluorescence is observed. A complete kinetic characterization has been obtained by ps-ns time-resolved spectroscopy. Besides prompt photocyclization (0.5 ps), fast singlet energy transfer takes place from the excited diarylethene to the Ru(II) chromophore (30 ps in Ru-DAE, 150 ps in Ru-CH 2-DAE ). In the Ru(II) chromophore, prompt intersystem crossing to the MLCT triplet state is followed by triplet energy transfer to the diarylethene (1.5 ns in Ru-DAE, 40 ns in Ru-CH 2-DAE ). The triplet state of the diarylethene moiety undergoes cyclization in a microsecond time scale. The experimental results are complemented with a combined ab initio and DFT computational study whereby the potential energy surfaces (PES) for ground state (S 0) and lowest triplet state (T 1) of the diarylethene are investigated along the reaction coordinate for photocyclization/cycloreversion. At the DFT level of theory, the transition-state structures on S 0 and T 1 are similar and lean, along the reaction coordinate, toward the closed-ring form. At the transition-state geometry, the S 0 and T 1 PES are almost degenerate. Whereas on S 0 a large barrier (ca. 45 kcal mol (-1)) separates the open- and closed-ring minima, on T 1 the barriers to isomerization are modest, cyclization barrier (ca. 8 kcal mol (-1)) being smaller than cycloreversion barrier (ca. 14 kcal mol (-1)). These features account for the efficient sensitized photocyclization and inefficient sensitized cycloreversion observed with Ru-DAE. Triplet cyclization is viewed as a nonadiabatic process originating on T 1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S 0 at closed-ring geometry. A computational study of the prototypical model 1,2-bis(3-thienyl)ethene is used to benchmark DFT results against ab initio CASSCF//CASPT2 results and to demonstrate the generality of the main topological features of the S 0 and T 1 PES obtained for DAE. Altogether, the results provide strong experimental evidence and theoretical rationale for the triplet pathway in the photocyclization of photochromic diarylethenes.

Highly Stable Glassy Carbon Interfaces for Long-Term Neural Stimulation and Low-Noise Recording of Brain Activity

We report on the superior electrochemical properties, in-vivo performance and long term stability under electrical stimulation of a new electrode material fabricated from lithographically patterned glassy carbon. For a direct comparison with conventional metal electrodes, similar ultra-flexible, micro-electrocorticography (μ-ECoG) arrays with platinum (Pt) or glassy carbon (GC) electrodes were manufactured. The GC microelectrodes have more than 70% wider electrochemical window and 70% higher CTC (charge transfer capacity) than Pt microelectrodes of similar geometry. Moreover, we demonstrate that the GC microelectrodes can withstand at least 5 million pulses at 0.45 mC/cm2 charge density with less than 7.5% impedance change, while the Pt microelectrodes delaminated after 1 million pulses. Additionally, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) was selectively electrodeposited on both sets of devices to specifically reduce their impedances for smaller diameters (<60 μm). We observed that PEDOT-PSS adhered significantly better to GC than Pt, and allowed drastic reduction of electrode size while maintaining same amount of delivered current. The electrode arrays biocompatibility was demonstrated through in-vitro cell viability experiments, while acute in vivo characterization was performed in rats and showed that GC microelectrode arrays recorded somatosensory evoked potentials (SEP) with an almost twice SNR (signal-to-noise ratio) when compared to the Pt ones.

Tetracoordinated Bis-phenanthroline Copper-Complex Couple as Efficient Redox Mediators for Dye Solar Cells

A tetracoordinated redox couple, made by [Cu(2-mesityl-4,7-dimethyl-1,10-phenanthroline)2][PF6], 1, and its Cu(II) form [Cu(2-mesityl-4,7-dimethyl-1,10-phenanthroline)2][PF6]2, 2, has been synthesized, and its electrochemical and photochemical features have been investigated and compared with those of a previously published Cu(2+)/Cu(+) redox shuttle, namely, [Cu(2,9-dimethyl-1,10-phenanthroline)2][PF6], 3, and its pentacoordinated oxidized form [Cu(2,9-dimethyl-1,10-phenanthroline)2Cl][PF6], 4. The detrimental effect of the fifth Cl(-) ancillary ligand on the charge transfer kinetics of the redox shuttles has been exhaustively demonstrated. Appropriately balanced Cu-based electrolytes have been then formulated and tested in dye solar cells in combination with a π-extended benzothiadiazole dye. The bis-phenanthroline Cu-complexes, 1 and 2, have been found to provide an overall 4.4% solar energy conversion efficiency, which is more than twice that of the literature benchmark couple, 3 and 4, employing a Cl-coordinated oxidized species and even comparable with the performances of a I(-)/I3(-) electrolyte of analogous concentration. A fast counter-electrode reaction, due to the excellent electrochemical reversibility of 2, and a high electron collection efficiency, allowed through the efficient dye regeneration kinetics exerted by 1, represents two major characteristics of these copper-based electron mediators and may constitute a pivotal step toward the development of a next generation of copper-based efficient iodine-free redox shuttles.

Hematite photoanodes modified with an Fe(III) water oxidation catalyst.

Hematite photoelectrodes prepared via a hydrothermal route are functionalized with a water oxidation catalyst consisting of amorphous Fe(III) oxide, obtained by successive ionic layer adsorption and reaction. The performances of the catalyst-modified photoanodes are considerably higher than those of the parent electrodes, resulting in a nearly doubled photoanodic current in all the basic aqueous electrolytes explored in this study. The combination of electrochemical impedance spectroscopy and laser flash photolysis indicates that the presence of the catalyst results in enhanced hole trapping in surface reactive states exposed to the electrolyte, allowing for a more successful competition between charge transfer and recombination.

A New 1,3,4‐Oxadiazole‐Based Hole‐Transport Material for Efficient CH3NH3PbBr3 Perovskite Solar Cells

A new hole-transport material (HTM) based on the 1,3,4-oxadiazole moiety (H1) was prepared through a single-step synthetic pathway starting from commercially available products. Thanks to a deep HOMO level, H1 was used as HTM in CH3 NH3 PbBr3 perovskite solar cells yielding an efficiency of 5.8%. The reference HTM (Spiro-OMeTAD), under the same testing conditions, furnished a lower efficiency of 5.1%. Steady-state and time-resolved photoluminescence of the thin films showed good charge-extraction dynamics for H1 devices. In addition, H1 shows a large thermal stability and completely amorphous behavior (as evaluated by thermal gravimetric analysis and differential scanning calorimetry).

Some aspects of the charge transfer dynamics in nanostructured WO3 films

The photoanodic response of two different types of nanocrystalline WO3 electrodes prepared by following either the sol gel approach or the accelerated anodization route was explored in sulfate containing electrolytes with the aim of exploring the mechanism of charge separation at WO3/electrolyte interfaces. Combined evidence by electrochemical impedance spectroscopy and transient absorption spectroscopy indicates that hole transfer occurs through the valence band and that, under applied bias, the voltage drop involves predominantly the space charge layer of the semiconductor, controlling the photocurrent via potential-induced variations of hole density at the surface of WO3. OH radicals were found among the primary water oxidation intermediates, and are partly responsible for mediated back recombination. The generation of hydroxyl radicals suggests, however, that WO3 based materials can find promising applications in environmental photoremediation under visible light, promoting ˙OH mediated oxidation of impervious contaminants. In principle, the removal of ˙OH by organic scavengers will also optimize the photocurrent generation in photoelectrochemical cells where the generation of hydrogen can be coupled to environmental decontamination.

2,5-Dithienylpyrrole (DTP) as a donor component in DTP–π–A organic sensitizers: photophysical and photovoltaic properties

Organic dyes have been prepared to evaluate the ability of 2,5-dithienylpyrrole (DTP) to act as a donor substituent in D–π–A sensitizers for DSSCs. Using a styryl π-bridge the dyes were found to be excellent sunlight harvesters when adsorbed on TiO2 photoanodes with absorbances >3 in the 300–550 nm region. Calculations as well as transient absorption spectroscopy in both solution and on a TiO2 surface revealed that they were favourable for efficient injection and regeneration.

Multilayer poly(3,4-ethylenedioxythiophene)-dexamethasone and poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate-carbon nanotubes coatings on glassy carbon microelectrode arrays for controlled drug release

The authors present an electrochemically controlled, drug releasing neural interface composed of a glassy carbon (GC) microelectrode array combined with a multilayer poly(3,4-ethylenedioxythiophene) (PEDOT) coating. The system integrates the high stability of the GC electrode substrate, ideal for electrical stimulation and electrochemical detection of neurotransmitters, with the on-demand drug-releasing capabilities of PEDOT-dexamethasone compound, through a mechanically stable interlayer of PEDOT-polystyrene sulfonate (PSS)-carbon nanotubes (CNT). The authors demonstrate that such interlayer improves both the mechanical and electrochemical properties of the neural interface, when compared with a single PEDOT-dexamethasone coating. Moreover, the multilayer coating is able to withstand 10 × 106 biphasic pulses and delamination test with negligible change to the impedance spectra. Cross-section scanning electron microscopy images support that the PEDOT-PSS-CNT interlayer significantly improves the adhesion between the GC substrate and PEDOT-dexamethasone coating, showing no discontinuities between the three well-interconnected layers. Furthermore, the multilayer coating has superior electrochemical properties, in terms of impedance and charge transfer capabilities as compared to a single layer of either PEDOT coating or the GC substrate alone. The authors verified the drug releasing capabilities of the PEDOT-dexamethasone layer when integrated into the multilayer interface through repeated stimulation protocols in vitro, and found a pharmacologically relevant release of dexamethasone.

Incorporation of Silicon Carbide and Diamond-Like Carbon as Adhesion Promoters Improves In Vitro and In Vivo Stability of Thin-Film Glassy Carbon Electrocorticography Arrays

Thin‐film neural devices are an appealing alternative to traditional implants, although their chronic stability remains matter of investigation. In this study, a chronically stable class of thin‐film devices for electrocorticography is manufactured incorporating silicon carbide and diamond‐like carbon as adhesion promoters between glassy carbon (GC) electrodes and polyimide and between GC and platinum traces. The devices are aged in three solutions—phosphate‐buffered saline (PBS), 30 × 10−3 and 150 × 10−3m H2O2/PBS—and stressed using cyclic voltammetry (2500 cycles) and 20 million biphasic pulses. Electrochemical impedance spectroscopy (EIS) and image analysis are performed to detect eventual changes of the electrodes morphology. Results demonstrate that the devices are able to undergo chemically induced oxidative stress and electrical stimulation without failing but actually improving their electrical performance until a steady state is reached. Additionally, cell viability tests are carried out to verify the noncytotoxicity of the materials, before chronically implanting them into rat models. The behavior of the GC electrodes in vivo is monitored through EIS and sensorimotor evoked potential recordings which confirm that, with GC being activated, impedance lowers and quality of recorded signal improves. Histological analysis of the brain tissue is performed and shows no sign of severe immune reaction to the implant.

Solar Energy Conversion in Photoelectrochemical Systems

The organization of photoresponsive molecular systems and nano-materials on semiconductor surface holds great potential in the building of solar energy conversion devices where efficient energy conversion results from the optimized cooperation of several subsystems (semiconductor, dye sensitizers, redox mediator, hole transport medium), whose properties can be finely tuned through rational synthetic design. This chapter will review the fundamentals of semiconductor sensitization, a process relying on the quenching by charge transfer of molecular excited states coupled to semiconductor surfaces, and will move on by describing the structural and electronic properties of some of the most successful dye designs, used in conjunction with new electron transfer mediators in liquid electrolytes. From liquid electrolytes, a step forward is made by developing solid state hole conductors, which found their best employment in hybrid junctions with organo-halide lead perovskites, representing, at present, the most promising materials for solar-to-electric power conversion in mesoscopic solar cells. Finally, one of the most challenging tasks which can find solution by exploiting molecular level sensitized materials is discussed in detail through meaningful case studies: the production of solar fuels by photoelectrochemical water splitting.