Photochemistry of 1,N-Diiodoalkanes

This article reviews the photochemistry of 1,n-diiodoalkanes, with special emphasis on the involved intermediates. Photolysis of 1,1-diiodomethane produces homolytic cleavage of the C–I bond to give the α-iodo radical, which in solid matrix cannot dissociate but collapses to isodiiodomethane. However, in solution subsequent electron-transfer produces the α-iodo cation, which is able to achieve cyclo-propanation of olefins. In the case of 1,2-diiodides, their photolysis gives rise to β-iodoalkyl radicals which usually lose iodine very rapidly, forming a carbon-carbon double bond. Homolytic cleavage of 1,n-diiodoalkanes (n>2) affords n-iodo radicals which are stabilized by participation of the remaining iodine atom. The cyclic hypervalent iodine radicals thus generated have low reactivity towards oxygen and characteristic UV spectra whose λmax depends on the ring size. These intermediates can be photolyzed, with iodine extrusion and formation of 1,2-diphenylcycloalkanes. On the other hand, photoinduced electron-transfer between carbanionic nucleophiles and 1,n-diiodoalkanes produces n-iodo radicals, which couple with the nucleophile leading to radical anions. Intramolecular electron-transfer from the radical anion to the σ* MO of the remaining C–I bond may compete with intermolecular electron-transfer to the substrate. This type of processes have been used for the synthesis of mono- and disubstituted adamantane derivatives. Finally, photoinduced electron-transfer between aromatic amines and 1,n-diiodoperfluoroalkanes (n>2) allows the preparation of fluorine containing heteroaromatic compounds.

Nitroxide amide-BODIPY probe behavior in fibroblasts analyzed by advanced fluorescence microscopy

A novel synthesized nitroxide amide-BODIPY prefluorescent probe was used to study cellular redox balance that modulates nitroxide/hydroxylamine ratio in cultured human fibroblasts. FLIM quantitatively differentiated between nitroxide states of the cytoplasm-localized probe imaged by TIRF, monitoring nitroxide depletion by hydrogen peroxide; eluding incorrect interpretation if only fluorescence intensity is considered.

The photochemical alkylation and reduction of heteroarenes

The functionalization of heteroarenes has been integral to the structural diversification of medicinally active molecules such as quinolines, pyridines, and phenanthridines. Electron-deficient heteroarenes are electronically compatible to react with relatively nucleophilic free radicals such as hydroxyalkyl. However, the radical functionalization of such heteroarenes has been marked by the use of transition-metal catalyzed processes that require initiators and stoichiometric oxidants. Herein, we describe the photochemical alkylation of quinolines, pyridines and phenanthridines, where through direct excitation of the protonated heterocycle, alcohols and ethers, such as methanol and THF, can serve as alkylating agents. We also report the discovery of a photochemical reduction of these heteroarenes using only iPrOH and HCl. Mechanistic studies to elucidate the underlying mechanism of these transformations, and preliminary results on catalytic methylations are also reported.

Polynuclear gold(i) complexes in photoredox catalysis: understanding their reactivity through characterization and kinetic analysis

The light mediated reduction of unactivated carbon–halogen bonds using polynuclear gold(i) complexes provides a mild and temporally controlled route to the generation of C–H, and C–C bonds.

Opportunistic use of tetrachloroaurate photolysis in the generation of reductive species for the production of gold nanostructures

The photolysis of gold salts is rarely viewed as the initiation for gold nanoparticle (AuNP) formation. Yet, photolysis of AuCl(4)(-) generates chlorine atoms whose rich hydrogen transfer chemistry can readily generate strongly reducing radicals. Interesting precursors include hydrogen peroxide, 2-propanol, 1,4-cyclohexadiene and tetrahydrofuran; all of them yield strongly reducing radicals. Further, this group of substrates has been selected because of the innocuous and volatile nature of reagents and products, thus allowing a remarkably clean synthesis of gold nanostructures. In the case of H(2)O(2) the by-products are water and oxygen. The methodology reported here opens the door to particles that can be modified in situ or post-synthesis with custom surface covering without concern for chemical debris from the nanostructure synthesis.

Cucurbituril complexes cross the cell membrane

Cucurbiturils (CBs) of the appropriate size (CB[7] and CB[8]) form strong guest-host complexes in phosphate buffer solution (PBS) with acridine orange (AO) and pyronine Y (PYY) with 1 : 1 and 2 : 1 stoichiometries for CB[7] and CB[8] complexes, respectively. Binding constants in the range 0.87-1.60 x 10(6) M(-1) and 5.2-6.3 x 10(13) M(-2) were determined by titration with fluorescence spectroscopy for 1 : 1 and 2 : 1 complexes, respectively. These binding constants in PBS and the eight-fold excess of CBs minimize the presence of free dye in solution and also stabilize the host-guest complex in the culture medium. Images showing that the CB complexes can cross the cell membrane of 3T3 cells have been acquired using fluorescence microscopy. Given the current importance of supramolecular CB complexes and the search for new drug delivery systems, the present findings open avenues for the use of CBs as nanocapsules to transport drugs into the cells.

Methylene blue encapsulation in cucurbit[7]uril: laser flash photolysis and near-IR luminescence studies of the interaction with oxygen

The effect of methylene blue (MB) encapsulation in cucurbit[7]uril (CB[7]) on triplet excited-state behavior and singlet oxygen (1O2) generation has been studied by using laser flash photolysis (LFP) and time-resolved near-IR luminescence spectroscopy. The lifetime of the triplet excited state of MB is longer in the CB[7] cavity (140 micros for MB-CB[7] vs 79.5 micros for aqueous MB). Cucurbituril also protects the dye triplets from quenching by oxygen, reducing the quenching rate constant [kq(O2)] from 2.6x10(9) M(-1) s(-1) to 0.2x10(9) M(-1) s(-1). The quantum yield of 1O2 production in the air-equilibrated D2O solutions is similar for free MB and for MB-CB[7], and the singlet oxygen lifetime is approximately 70 micros, suggesting its decay occurs in the aqueous (D2O) phase. The generation of singlet oxygen is delayed by CB[7]; this is attributed to the time required for oxygen to access the CB[7] nanocavity and react with the MB triplet. Thus, the rate-limiting step for sensitization is the entry of oxygen into the CB[7] cavity. Encapsulation inside CB[7] increases the relative efficiency of photoinduced MB2+* dication-radical generation, for which a modest yield is observed.

Photoinduced formation and characterization of electron-hole pairs in azaxanthylium-derivatized short single-walled carbon nanotubes

2-Azaxanthone, a nitrogenated derivative of the well-studied organic chromophore xanthone, has been covalently bound through 2-(ethylthio)ethylamido linkers to the carboxylic acid groups of short, soluble single-walled carbon nanotubes (CNTs) of 450 nm average length, and the resulting azaxanthylium-functionalized CNTs (AZX-CNT, 8.5 wt % AZX content) characterized by solution (1)H NMR, Raman and IR spectroscopy and thermogravimetric analysis. Comparison of the quenching of the triplet excited state of AZX (steady-state and time-resolved) and of the transient optical spectra of CNTs and AZX-CNT shows that the covalent linkage boosts the interaction between the azaxanthylium moiety and the short CNT units. The triplet excited state of the azaxanthylium derivative is quenched by CNT with and without covalent bonding, but when it is covalently bonded, the singular transient spectrum is compatible with the photogeneration of electron holes through electron transfer from CNT to excited azaxanthylium units.

How drug photodegradation studies led to the promise of new therapies and some fundamental carbanion reaction dynamics along the way

The photodegradation of nonsteroidal anti-inflammatory drugs (NSAIDs), a class of medications that includes aspirin and ibuprofen, has generated considerable interest since the 1990s, largely because of the phototoxic and photoallergic effects that frequently accompany their therapeutic use. Among NSAIDs, ketoprofen, which contains a benzophenone chromophore, has been extensively studied, reflecting both its notorious adverse effects and the fascination that photochemists have with benzophenone. The photochemistry of ketoprofen involves the intermediacy of an easily detectable carbanion with a remarkable lifetime of 200 ns in water; its life expectancy can in fact be extended to minutes under carefully controlled anhydrous conditions. Over the past decade, we have used some key properties of the ketoprofen carbanion to conduct mechanistic studies on carbanions under various conditions. In particular, its ease of photogeneration provides the temporal control required for kinetic studies, which, combined with its long lifetime and readily detectable visible absorption, have enabled extensive laser flash photolysis work. These studies have led to an intimate understanding of the reaction dynamics for carbanions in solution, including the determination of absolute rate constants for protonation, S(N)2, and elimination reactions. Together they provide excellent exemplars of reactivity patterns that today are part of all introductory curricula in organic chemistry and illustrate the fundamentals of nucleophilic substitution paradigms. More recently, we have begun to exploit the photochemistry of ketoprofenate and have developed the ketoprofenate photocage, a valuable tool for the photocontrolled cleavage of protecting groups and concomitant drug release. The photorelease has been illustrated with ibuprofen, among many other molecules. These photocages have been further improved with the use of the xanthone chromophore; the goal is the release of antiviral agents taking advantage of the improved UVA absorption of xanthone (xanthonate photocages). In this Account, we survey our work of the past few years on the photochemistry of ketoprofen and related chromophores. Beginning with studies on the phototoxicity of ketoprofen, we have made the journey to new prodrug candidates, unraveling mechanistic elements of aroyl-substituted benzyl carbanions along the way.

Direct time-resolved detection of singlet oxygen in zeolite-based photocatalysts

Singlet oxygen has been characterized spectroscopically as a product of the exposure of suspensions of zeolites containing oxidation catalysts. Spectroscopic and lifetime studies show that a part of the singlet oxygen formed reacts within the zeolite porous structure, while a significant fraction escapes and becomes available for reaction in the bulk media. The liquid phase plays a key role in determining intra- and extracavity dynamics.

Free radical sensor based on CdSe quantum dots with added 4-amino-2,2,6,6-tetramethylpiperidine oxide functionality

The association and resulting fluorescence quenching of CdSe quantum dots by 4-amino-2,2,6,6-tetramethylpiperidine oxide (4-amino-TEMPO), a persistent nitroxide, have been examined using electron paramagnetic resonance (EPR) and fluorescence spectroscopy. EPR data suggest binding constants around (8 +/- 4) x 10(6) M(-1) for green (2.4-2.5 nm) nanoparticles, and the application of Job's method indicates that the preferred mode of binding involves one or two quencher molecules per quantum dot, although more quenchers could bind at high concentrations of 4-amino-TEMPO. Fluorescence quenching by 4-amino-TEMPO is at least 3 orders of magnitude more efficient than by TEMPO itself, reflecting the strong binding confirmed by the EPR data. Stern-Volmer plots are nonlinear and in light of the EPR data probably reflect ready accessibility of the CdSe surface to one or two 4-amino-TEMPO molecules, while additional quenchers can only bind if they displace trioctylphosphine oxide ligands. Quantum dot-4-amino-TEMPO complexes can be used as free radical sensors, since the fluorescence (quenched by the nitroxide) is readily restored when radicals are trapped to form alkoxyamines.

Transient Enol Isomers of Dibenzoylmethane and Avobenzone as Efficient Hydrogen Donors toward a Nitroxide Pre-fluorescent Probe†

Dibenzoylmethane and avobenzone photochemistry involves the formation of transient enol isomers (Z and E). Conjugation of the OH group with the carbonyl group in these transient isomers reduces the OH bond energy. A fast reduction of the pre-fluorescent probe (C343T) was observed after addition to photolysed DBM samples in nonpolar solvents. This can be attributed to a hydrogen transfer reaction from the transient E accumulated. While no quenching for Z by TEMPO was detected in the laser flash photolysis timescale. Theoretical calculations of spin densities distribution of the radical formed from Z and E showed a more delocalized distribution that would indicate a low reactivity towards oxygen. Our results suggest that DBM and avobenzone can effectively behave as photoantioxidants or photoactivated antioxidants under conditions where its enol isomers can be accumulated.

Effect of Hexafluorobenzene on the Photophysics of Pyrene

Absorption and fluorescence spectroscopy studies reveal the formation of a weak complex between pyrene and C(6)F(6) even in very dilute systems. The complex affects the photophysics of pyrene and reveals a combination of static and dynamic-quenching phenomena in both polar and nonpolar solvents. The results are supported by computational studies that shed light on the structure of the complex and the interactions involved and suggest that ground and excited-state interactions are of comparable magnitude; the association is believed to be driven by quadrupolar interactions. Understanding these interactions in solution is important for applications that aim at controlling the regio- or stereoselectivity of organic reactions.

Effect of UVC-Induced Damage to DNA on the Intercalation of Thiazole Orange: A Convenient Reporter for DNA Damage†

We report a novel method of identifying damage to DNA leading to the loss of intercalation sites. Thiazole orange (TO), an intercalating cyanine dye, fluoresces strongly when intercalated in DNA, but not free in solution. Upon UVC-induced damage to DNA, the change in TO fluorescence is greater than the change in any of the other spectral or biochemical indicators (absorbance, circular dichroism and agarose gel electrophoresis), thus providing a fast screening method to identify damage to DNA. The method is geared toward high levels of damage, such as those that may result during radiation treatment of food products.

Photolysis of an alkoxyamine using intramolecular energy transfer from a quinoline antenna—towards photo-induced living radical polymerization

The photolysis of an alkoxyamine triad comprised of the 1-phenylethyl moiety, 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) and 2-methyl-3-hydroxy quinoline leads to energy transfer from the quinoline moiety (antenna) and cleavage of the C-O alkoxyamine bond, suggesting its possible applications in low temperature living free radical polymerization.

Comparative study of the quenching of core and core-shell CdSe quantum dots by binding and non-binding nitroxides

The quenching of core and core-shell CdSe quantum dots by TEMPO and 4-amino-TEMPO has been examined using steady state fluorescence spectroscopy. The efficiency of quenching is strongly dependent on the nanoparticle size, the binding properties of the nitroxide, and the presence or not of a protective shell, ZnS in our systems. The shell reduces the quenching efficiency significantly only in the case of binding nitroxides, such as 4-amino-TEMPO. Downward quenching plots revealing bimodal quenching characterize the Stern-Volmer plots obtained for 4-amino-TEMPO. The slope characteristic of the low concentrations regime depends strongly on the presence of a shell. For example, for particles with a 2.4 nm core, emitting at 525 nm the concentrations of 4-amino-TEMPO required to reduce the emission to one half are 0.65 microM and 0.08 mM for core and core-shell nanoparticles, respectively. Surprisingly, in the high concentration regime, a single Stern-Volmer slope of about 4000 M-1 seems to accommodate all systems. We speculate that this value is characteristic of the exchange of TOPO ligands by 4-amino-TEMPO.

Synthesis and characterization of a new fluorescent probe for reactive oxygen species

We report the development of a new fluorescence sensor for reactive oxygen species (ROS) based on a benzofurazan structure. The sensor, NBFhd, is initially non-fluorescent and reacts with peroxyl radicals by hydrogen transfer in an aqueous medium under neutral conditions to release the fluorescent N-methyl-4-amino-7-nitrobenzofurazan (NBF) and 1,4-benzoquinone. NBFhd shows excellent contrast and no interference in the region of cell autofluorescence and is a new tool to detect ROS in homogeneous and biological systems.