Photoinduced Reactivity Research Articles

Light-Induced Reactivity Switch at O2-Activating Bioinspired Copper(I) Complexes.

Using light to unveil unexplored reactivities of earth-abundant metal-oxygen intermediates is a formidable challenge, given the already remarkable oxidation ability of these species in the ground state. However, the light-induced reactivity of Cu-O2 intermediates still remains unexplored, due to the photoejection of O2 under irradiation. Herein, we describe a photoinduced reactivity switch of bioinspired O2-activating CuI complexes, based on the archetypal tris(2-pyridyl-methyl)amine (TPA) ligand. This report represents a key precedent for light-induced reactivity switch in Cu-O2 chemistry, obtained by positioning C-H substrates in close proximity of the active site. Open and caged CuI complexes displaying an internal aryl ether substrate were evaluated. Under light, a Cu-O2 mediated reaction takes place that induces a selective conversion of the internal aryl ether unit to a phenolate-CH2- moiety with excellent yields. This light-induced transformation displays high selectivity and allows easy postfunctionalization of TPA-based ligands for straightforward preparation of challenging heteroleptic structures. In the absence of light, O2 activation results in the standard oxidative cleavage of the covalently attached substrate. A reaction mechanism that supports a monomeric cupric-superoxide-dependent reactivity promoted by light is proposed on the basis of reactivity studies combined with (TD-) DFT calculations.

Role of Surface Defects on Photoinduced Reactivity in SiO2 Nanoparticles

Silica nanostructured materials find application in different fields, since they are cheap, versatile, and easy to functionalize as materials. However, silica reactivity has not been deeply investigated yet, mainly due to a poor understanding of how it is affected by superficial defects. In the present study, the electronic and optical properties of nanoparticles have been investigated using bare silica nanoparticles (SNP) and amino- or phosphonate-functionalized silica nanoparticles (SNP–APTES and SNP-phosphonate), prepared by a sol-gel procedure, and their morphology has been investigated using transmission electron microscopy (TEM) analysis. The prepared silica nanomaterials were characterized by means of reflectance and emission spectroscopies to determine the types of defects that can be found on silica nanoparticles’ surface. In order to understand the effect of surface defects on the reactivity of silica, the nanoparticles were employed for the photocatalytic degradation of Rhodamine 6G (R6G), upon selective irradiation at 320 nm, where only silica colloids absorb. The photoreaction was carried out in ethanol and in water and was monitored following the fluorescence signal of the dye. The evaluation of the fluorescent intensities allowed for the determination of the degradation efficiencies.

Ru Monoimines with Extended Excited-State Lifetimes and Geometrical Modulation of Photoinduced Mixed-Valence Interactions.

The photophysical properties of monodentate-imine ruthenium complexes do not usually fulfil the requirements for supramolecular solar energy conversion schemes. Their short excited-state lifetimes, like the 5.2 ps metal-to-ligand charge transfer (MLCT) lifetime of [Ru(py)4Cl(L)]+ with L = pz (pyrazine), preclude bimolecular or long-range photoinduced energy or electron transfer reactions. Here, we explore two strategies to extend the excited-state lifetime, based on the chemical modification of the distal N atom of pyrazine. On one hand, we used L = pzH+, where protonation stabilized MLCT states, rendering thermal population of MC states less favorable. On the other hand, we prepared a symmetric bimetallic arrangement in which L = {(μ-pz)Ru(py)4Cl} to enable hole delocalization via photoinduced mixed-valence interactions. A lifetime extension of 2 orders of magnitude is accomplished, with charge transfer excited states living 580 ps and 1.6 ns, respectively, reaching compatibility with bimolecular or long-range photoinduced reactivity. These results are similar to those obtained with Ru pentaammine analogues, suggesting that the strategy employed is of general applicability. In this context, the photoinduced mixed-valence properties of the charge transfer excited states are analyzed and compared with those of different analogues of the Creutz-Taube ion, demonstrating a geometrical modulation of the photoinduced mixed-valence properties.

Ultrafast photo-induced processes in complex environments: The role of accuracy in excited-state energy potentials and initial conditions

Light induces non-equilibrium time evolving molecular phenomena. The computational modeling of photo-induced processes in large systems, embedded in complex environments (i.e., solutions, proteins, materials), demands for a quantum and statistical mechanic treatment to achieve the required accuracy in the description of both the excited-state energy potentials and the choice of the initial conditions for dynamical simulations. On the other hand, the theoretical investigation on the atomistic scale of times and sizes of the ultrafast photo-induced reactivity and non-equilibrium relaxation dynamics right upon excitation requests tailored computational protocols. These methods often exploit hierarchic computation schemes, where a large part of the degrees of freedom are required to be treated explicitly to achieve the right accuracy. Additionally, part of the explicit system needs to be treated at ab initio level, where density functional theory, using hybrid functionals, represents a good compromise between accuracy and computational cost, when proton transfers, non-covalent interactions, and hydrogen bond dynamics play important roles. Thus, the modeling strategies presented in this review stress the importance of hierarchical quantum/molecular mechanics with effective non-periodic boundary conditions and efficient phase-sampling schemes to achieve chemical accuracy in ultrafast time-resolved spectroscopy and photo-induced phenomena. These approaches can allow explicit and accurate treatment of molecule/environment interactions, including also the electrostatic and dispersion forces of the bulk. At the same time, the specificities of the different case studies of photo-induced phenomena in solutions and biological environments are highlighted and discussed, with special attention to the computational and modeling challenges.

Action Plots in Action: In-Depth Insights into Photochemical Reactivity.

Predicting wavelength-dependent photochemical reactivity is challenging. Herein, we revive the well-established tool of measuring action spectra and adapt the technique to map wavelength-resolved covalent bond formation and cleavage in what we term "photochemical action plots". Underpinned by tunable lasers, which allow excitation of molecules with near-perfect wavelength precision, the photoinduced reactivity of several reaction classes have been mapped in detail. These include photoinduced cycloadditions and bond formation based on photochemically generated o-quinodimethanes and 1,3-dipoles such as nitrile imines as well as radical photoinitiator cleavage. Organized by reaction class, these data demonstrate that UV/vis spectra fail to act as a predictor for photochemical reactivity at a given wavelength in most of the examined reactions, with the photochemical reactivity being strongly red shifted in comparison to the absorption spectrum. We provide an encompassing perspective of the power of photochemical action plots for bond-forming reactions and their emerging applications in the design of wavelength-selective photoresists and photoresponsive soft-matter materials.

Radiationless Decay Processes of an Unnatural DNA Base: Pyrrole 2-Carbaldehyde.

Pyrrole-2-carbaldehyde (Pa) forms one of the unnatural nucleic acid bases, and as a base pair with 7-(2-thienyl)imidazo[4,5-b]pyridine (Ds), it has been known to be stable in DNA. The Ds-Pa pair is stabilized in DNA via van der Waals' interaction and shape fitting. There are some studies on the origin of its stability and reactivity in the ground state. However, for a successful unnatural base pair, it needs to be stable not only in the ground state but also upon irradiation with UV-visible light. To understand the photoinduced reactivity, we investigate the excited-state properties of the Pa base and understand the energetically feasible photoprocesses that can occur upon excitation in the UV region. Two distinct pathways are obtained. One of the pathways involves an out-of-plane mode and has some similarities with the deactivation channels in the natural pyrimidine bases. On the other hand, the second pathway involves an excited-state proton transfer.

Water-Mediated Excited State Proton Transfer of Pyranine-Acetate in Aqueous Solution: Vibrational Fingerprints from Ab Initio Molecular Dynamics.

In this work, we simulate the excited state proton transfer (ESPT) reaction involving the pyranine photoacid and an acetate molecule as proton acceptor, connected by a bridge water molecule. We employ ab initio molecular dynamics combined with an hybrid quantum/molecular mechanics (QM/MM) framework. Furthermore, a time-resolved vibrational analysis based on the wavelet-transform allows one to identify two low frequency vibrational modes that are fingerprints of the ESPT event: a ring wagging and ring breathing. Their composition suggests their key role in optimizing the structure of the proton donor–acceptor couple and promoting the ESPT event. We find that the choice of the QM/MM partition dramatically affects the photoinduced reactivity of the system. The QM subspace was gradually extended including the water molecules directly interacting with the pyranine–water–acetate system. Indeed, the ESPT reaction takes place when the hydrogen bond network around the reactive system is taken into account at full QM level.

Wave-Function Symmetry Control of Electron-Transfer Pathways within a Charge-Transfer Chromophore.

Despite a diverse manifold of excited states available, it is generally accepted that the photoinduced reactivity of charge-transfer chromophores involves only the lowest-energy excited state. Shining a visible-light laser pulse on an aqueous solution of the chromophore-quencher [Ru(tpy)(bpy)(μNC)OsIII(CN)5]- assembly (tpy = 2,2';6,2''-terpyridine and bpy = 2,2'-bipyridine), we prepared a mixture of two charge-transfer excited states with different wave-function symmetry. We were able to follow, in real time, how these states undergo separate electron-transfer reaction pathways. As a consequence, their lifetimes differ in 3 orders of magnitude. Implicit are energy barriers high enough to prevent internal conversion within early excited-state populations, shaping isolated electron-transfer channels in the excited-state potential energy surface. This is relevant not only for supramolecular donor/acceptor chemistry with restricted donor/acceptor relative orientations. These energy barriers provide a means to avoid chemical potential dissipation upon light absorption in any molecular energy conversion scheme, and our observations invite to explore wave-function symmetry-based strategies to engineer these barriers.

Photoinduced Reactivity in a Dispiro-1,2,4-trioxolane: Adamantane Ring Expansion and First Direct Observation of the Long-Lived Triplet Diradical Intermediates.

Dispiro-1,2,4-trioxolane, 1, an ozonide with efficient and broad antiparasitic activity, was synthesized and investigated using matrix isolation FTIR and EPR spectroscopies together with both B3LYP/6-311++G(3df,3dp) and M06-2X/6-311++G(3df,3dp) theoretical methods. Irradiations (λ ≥ 290 nm) of the matrix isolated 1 (Ar or N2) afforded exclusively 4-oxahomoadamantan-5-one, 4, and 1,4-cyclohexanedione, 5. These results suggested that the reaction proceeded via a dioxygen-centered diradical intermediate, formed upon homolytic cleavage of the labile peroxide bond, which regioselectively isomerized to form the more stable (secondary carbon-centered)/oxygen-centered diradical. In situ EPR measurements during the photolysis of 1 deposited in a MeTHF-matrix led to the detection of signals corresponding to two triplet species, one of which was short-lived while the other proved to be persistent at 10 K. These observations strongly support the proposed mechanism for the photogeneration of 4 and 5, which involves intramolecular rearrangement of the intermediate diradical species 2 to afford the triplet diradical 3.

Zn2+ leakage and photo-induced reactive oxidative species do not explain the full toxicity of ZnO core Quantum Dots

Metal oxide nanoparticles (NPs), and among them metal oxides Quantum Dots (QDs), exhibit a multifactorial toxicity combining metal leaching, oxidative stress and possibly direct deleterious interactions, the relative contribution of each varying according to the NP composition and surface chemistry. Their wide use in public and industrial domains requires a good understanding and even a good control of their toxicity. To address this question, we engineered ZnO QDs with different surface chemistries, expecting that they would exhibit different photo-induced reactivities and possibly different levels of interaction with biological materials. No photo-induced toxicity could be detected on whole bacterial cell toxicity assays, indicating that ROS-dependent damages, albeit real, are hidden behind a stronger source of toxicity, which was comforted by the fact that the different ZnO QDs displayed the same level of cell toxicity. However, using in vitro DNA damage assays based on quantitative PCR, significant photo-induced reactivity could be measured precisely, showing that different NPs exhibiting similar inhibitory effects on whole bacteria could differ dramatically in terms of ROS-generated damages on biomolecules. We propose that direct interactions between NPs and bacterial cell surfaces prime over any kind of intracellular damages to explain the ZnO QDs toxicity on whole bacterial cells.

Photoinduced reactivity and cytotoxicity of a series of zinc(II)–flavonolate derivative complexes

A series of Zn(II)-based complexes as photoinduced carbon-monoxide-releasing molecules (photoCORMs), viz. (L)Zn(R)]ClO4 with L = tris(pyridine-2-ylmethyl)amine (TPA), N,N-((5-estermethyl-2-pyridyl)methyl)-bis(2-pyridylmethyl) amine (5-COOCH3TPA), N,N-(bis(5-estermethyl-2-pyridyl)methyl)-(2-pyridylmethyl) amine [(5-COOCH3)2TPA], or N,N-((6-estermethyl-2-pyridyl)methyl)-bis(2-pyridylmethyl) amine (6-COOCH3TPA) and R = 4-dimethylaminoflavonol [4-N(Me)2FLH], 4-diethylaminoflavonol [4-N(Et)2FLH], or 4-methoxyflavonol (4-MeOFLH), have been prepared and characterized. When dissolved in CH3CN or 1:1 H2O:dimethylsulfoxide (DMSO) in presence of O2, visible light (λmax ranging from 417 to 462 nm) induced dioxygenase-type reactions to release CO quantitatively from these complexes. Moreover, the intracellular uptake and CO releasing reactivity were confirmed by the change of their fluorescent properties observed under fluorescence microscopy. The cytotoxicity of the complexes demonstrates that they exhibit good biocompatibility in the concentration range from 1 to 50 μM. The solid complexes are stable in air for more than 60 days, and their aqueous solutions are also stable for 96 h under anaerobic or dark condition. The remarkable stability and biocompatibility of the complexes are indispensable properties for use in pharmaceutical chemistry.

High-Pressure Synthesis of Cyclic Phosphazenes by Near-UV Photoinduced Reactivity of NH3 and Elemental Phosphorus

A comparison between the high-pressure (0.8 GPa) photoinduced reactivity of black and red phosphorus at ambient temperature in the presence of ammonia has been conducted in diamond anvil cells (DAC...

Synthesis and Photochemical Studies on Gallium and Indium Complexes of DTPA-PEG3-ArN3 for Radiolabeling Antibodies

Photochemistry is a rich source of inspiration for developing alternative methods to functionalize proteins with drug molecules, fluorophores, and radioactive probes. Here, we report the synthesis and photochemical reactivity of a modified diethylenediamine pentaacetic acid chelate that was derivatized with a light-responsive aryl azide group (DTPA-PEG3-ArN3, compound 1). The corresponding nonradioactive and radioactive nat/68Ga3+ and nat/111In3+ complexes of DTPA-PEG3-ArN3 were synthesized and their physical and photochemical properties were studied to evaluate the potential of employing this ligand system in the photochemical synthesis of radiolabeled antibodies. Photodegradation kinetics revealed that irradiation with ultraviolet light (365 nm) induced rapid photoactivation of compound 1 and the metal complexes nat/68Ga-1- and nat/111In-1-. Light-induced reactions were complete in <100 s, with measured first-order rate constants of 0.078 ± 0.045 s-1, 0.093 ± 0.009 s-1, and 0.117 ± 0.054 s-1 (n = 2, per species) for compound 1, natGa-1-, and natIn-1-, respectively. Photochemically induced bioconjugation reactions between DTPA-PEG3-ArN3 and the monoclonal antibody trastuzumab, as well as pre- and postconjugation 68Ga- and 111In-radiolabeling experiments, were performed using either a one-pot or two-step strategy. Both approaches yielded radiolabeled trastuzumab ([68Ga]GaDTPA-azepin-trastuzumab) with average radiochemical conversions of 3.9 ± 1.0% (n = 4, one-pot), and 10.0 ± 1.0% (n = 3, two-step). One-pot radiolabeling reactions with [111In]InCl3 produced the corresponding[111In]InDTPA-azepin-trastuzumab radiotracer in a similar radiochemical conversion of 5.4 ± 0.8% (n = 3). Radiochemical conversions for the desired bimolecular coupling between the chelate and the protein were comparatively low. This observation is likely caused by the high photoinduced reactivity of the compounds and subsequent competition with background reactions. Nevertheless, access to DTPA-PEG3-ArN3 increases the scope of photoradiochemical methods to include metal ions like In3+ that form complexes with higher coordination numbers.

Quantification of the Photocatalytic Self-Cleaning Ability of Non-Transparent Materials.

The photo-induced reactivity of compounds at the surface of photocatalytic materials is used to maintain the cleanliness of the surface of glass, concretes and paints. A standard method to quantify the photocatalytic self-cleaning (SC) properties of non-transparent materials was recently published. It is based on the covering of the sample surface with a defined amount of dye and on the evaluation of the reflectance spectra of the coloured surface under irradiation. The calibration of the spectral changes allowed the quantification of the surface residual dye and the evaluation of the self-cleaning kinetics. The method was tested on seven white and coloured photocatalytic materials using methylene blue (MB), rhodamine B (RhB) and metanil yellow (MY). The main by-products of the MB photocatalytic degradation at the solid/solid interface were identified, showing that MB degradation in solution follows a path quite different from that at the solid/solid interface. Also MY showed a different order of photoreactivity. Furthermore, experiments at the solid/solid interface are more trustworthy than tests in solution for evaluating the self-cleaning ability. The differences of the photocatalytic phenomena at the solid/solid interface in comparison with the most studied photoactivated processes at the solid/liquid interface are outlined. Furthermore, photocatalytic materials showed selectivity toward some specific dyes. This encourages the use of more than one dye for the evaluation of the self-cleaning ability of a photocatalytic material.

Photo-fragmentation of alkyl phosphates in the gas-phase

Alkyl phosphates are experiencing an ever increasing use due to the current arising of new applications. This implies their increasing presence in the environment so their stability and reactivity under high-energy photons, which are still unknown, need to be clarified.In this study, a mass spectrometric investigation of the ionization and fragmentation processes of four representative alkyl phosphates (dibutyl, tributyl, bis-2-ethylhexyl, and tris-2-ethylhexyl phosphate) induced by vacuum ultraviolet (VUV) radiation has been carried out. The experimental data show that fragmentation occurs through a stepwise cleavage of the bonds between the phosphate group and the alkyl chains leading to the formation of esters with a lower number of alkyl chains and the protonated orthophosphoric acid as ending product. Interestingly, the presence of charged species with mass-over-charge (m/z) values larger than the parent ion is also observed in dibutyl and bis-2-ethylhexyl phosphate, suggesting the existence of their dimers in the gas phase. This has been rationalized, with the aid of ab-initio DFT calculations, in terms of the capability of these molecules to form H-bonds. The photo-induced reactivity in the gas phase has been clarified, and its similarity with the natural bacterial degradation has been pointed out.

Recent Advances in Germanium-Based Photoinitiator Chemistry.

Acylgermanes provide an outstanding photoinduced reactivity at very useful absorption wavelengths. This encouraged multidisciplinary research groups to utilize them as highly effective and non‐toxic photoinitiators particularly for medical applications. In this Minireview, we present the most recent breakthroughs to synthesize acylgermanes. We also outline mechanistic aspects of photoinduced reactions of several acylgermane derivatives based on fundamental spectroscopic insights. These studies may aid future developments for tailor‐made photoinitiators.

Thermal-, photo- and electron-induced reactivity of hydrogen species on rutile TiO2(110) surface: Role of oxygen vacancy

Interaction of hydrogen with TiO2 plays a vital role in TiO2-based photocatalysis and thermal catalysis. In this work, we compared thermal-, photo-, and electron-induced reactivity of various types of hydrogen species on a rutile TiO2(110) surface formed by atomic H exposure at 320 and 115K by means of thermal desorption spectroscopy, X-ray photoelectron spectroscopy and low energy electron diffraction. Atomic H interaction with rutile TiO2(110) at 115K forms surface TiH hydride, surface hydroxyl group, and chemisorbed water. Upon heating, surface TiH hydride reacts to produce H2 while surface hydroxyl groups react to form both water and H2. Atomic H interaction with rutile TiO2(110) at 320K strongly reduces TiO2 due to the continuous formation and desorption of water and forms surface hydroxyl groups and likely subsurface/bulk hydrogen species. Upon heating, hydrogen forms as the only gas-phase product and its desorption activation energy decreases with the subsurface/bulk reduction extent of rutile TiO2(110). Surface TiH hydride exhibits photo-induced reactivity while both surface TiH hydride and surface hydroxyl group exhibit electro-induced reactivity. These results have important implications for understanding the hydrogen-involved thermal and photo reactions on TiO2-based catalysts.

Synthesis, Spectroscopic Behavior, and Photoinduced Reactivity of Tetraacylgermanes

Acylgermanes have been subject of great interest recently because of their low toxicity and the applicability as sources for germanium-centered radicals for visible-light induced free radical polymerization processes. We report on a novel and versatile method for the synthesis of tetraacylgermanes allowing the preparation of various tetra-substituted acylgermanes 1a–m. The formation of these derivatives was confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. UV–vis absorption spectra of the prepared compounds reveal absorption in the visible region. This transition was assigned by TD-DFT calculations. It enabled a general screening of the influence of different substitution patterns on the absorption properties. The radical formation upon irradiation was confirmed by TR-EPR spectroscopy.

High-Pressure Chemistry of Graphene Oxide in the Presence of Ar, N2, and NH3

The high pressure structural and reactive beahvior of graphene oxide (GO) in the presence of Ar, N2, and NH3 was studied in diamond anvil cells (DAC) by X-ray diffraction (XRD) and vibrational spectroscopy (FTIR and Raman), with the purpose of investigating the use of pressure for N-doping and functionalization of GO in high-density conditions. The pressure evolution of the interlayer d-spacing of GO during room temperature compression and decompression indicates the pressure-induced insertion of the selected systems between the GO layers and the stability of the GO layered structure at high pressure. Thermal and photoinduced reactivity was studied in GO with N2 and in GO with NH3 in different pressure conditions. The comparison of the infrared spectra of the recovered samples at ambient conditions with respect to the starting GO provides evidence for the occurrence of chemical reactivity of N2 and NH3 with GO, leading to N incorporation and GO functionalization, as also confirmed by the Raman spectra. Th...

Bis(mesitoyl)phosphinic acid: photo-triggered release of metaphosphorous acid in solution.

Bis(mesitoyl)phosphinic acid and its sodium salt display a unique photo-induced reactivity: both derivatives stepwise release two mesitoyl radicals and, remarkably, metaphosphorous acid (previously postulated as transient species in the gas phase), providing a new phosphorus-based reagent.