Solvent and alkyl substitution effects on charge-transfer mediated triplet state generation in BODIPY dyads: a combined computational and experimental study.

Donor-acceptor dyads based on BODIPYs have been recently employed to enhance the formation of triplet excited states with the process of spin-orbit charge transfer intersystem crossing (SOCT-ISC) which does not require introduction of transition metals or other heavy atoms into the molecule. In this work we compare two donor-acceptor dyads based on meso-naphthalenyl BODIPY by combining experimental and computational investigations. The photophysical and electrochemical characterization reveals a significant effect of alkylation of the BODIPY core, disfavoring the SOCT-ISC mechanism for the ethylated BODIPY dyad. This is complemented with a computational investigation carried out to rationalize the influence of ethyl substituents and solvent effects on the electronic structure and efficiency of triplet state population via charge recombination (CR) from the photoinduced electron transfer (PeT) generated charge-transfer (CT) state. Time dependent-density functional theory (TD-DFT) calculations including solvent effects and spin-orbit coupling (SOC) calculations uncover the combined role played by solvent and alkyl substitution on the lateral positions of BODIPY.

Stable Meisenheimer Complexes as Powerful Photoreductants Readily Obtained from Aza-Heteroaromatic Compounds.

Excited states of radical anions derived from the photoreduction of stable organic molecules are suggested to serve as potent reductants. However, excited states of these species are too short-lived to allow bimolecular quenching processes. Recently, the singlet excited state of Meisenheimer complexes, which possess a long-lived excited state, was identified as the competent species for the reduction of challenging organic substrates (-2.63 V vs. SCE, saturated calomel electrode). To produce reasonably stable and simply accessible different Meisenheimer complexes, the addition of nBuLi to readily available aromatic heterocycles was investigated, and the photoreactivity of the generated species was studied. In this paper, we present the straightforward preparation of a family of powerful photoreductants (*Eox<-3 V vs. SCE in their excited states, determined by DFT and time-dependent TD-DFT calculations; (DFT, density functional theory) that can induce dehalogenation of electron-rich aryl chlorides and to form C-C bond through radical cyclization. Photophysical analyses and computational studies in combination with experimental mechanistic investigations demonstrate the ability of the adduct to act as a strong electron donor under visible light irradiation.

Photoredox-catalyzed intramolecular nucleophilic amidation of alkenes with ß-lactams.

The direct nucleophilic addition of amides to unfunctionalized alkenes via photoredox catalysis represents a facile approach towards functionalized alkylamides. Unfortunately, the scarce nucleophilicity of amides and competitive side reactions limit the utility of this approach. Herein, we report an intramolecular photoredox cyclization of alkenes with ß-lactams in the presence of an acridinium photocatalyst. The approach uses an intramolecular nucleophilic addition of the ß-lactam nitrogen atom to the radical cation photogenerated in the linked alkene moiety, followed by hydrogen transfer from the hydrogen atom transfer (HAT) catalyst. This process was used to successfully prepare 2-alkylated clavam derivatives.

Light-Induced Access to Carbazole-1,3-dicarbonitrile: A Thermally Activated Delayed Fluorescent (TADF) Photocatalyst for Cobalt-Mediated Allylations.

The stability of a photocatalyst under irradiation is important in photoredox applications. In this work, we investigated the stability of a thermally activated delayed fluorescence (TADF) photocatalyst {3DPAFIPN [2,4,6-tris(diphenylamino)-5-fluoroisophthalonitrile]}, recently employed in photoredox-mediated processes, discovering that in the absence of quenchers the chromophore is unstable and is efficiently converted by irradiation with visible light into another species based on the carbazole-1,3-dicarbonitrile moiety. The new species obtained is itself a TADF emitter and finds useful applications in photoredox transformations. At the excited state, it is a strong reductant and was efficiently applied to cobalt-mediated allylation of aldehydes, whereas other TADFs (4CzIPN and 3DPAFIPN) failed to promote efficient photocatalytic cycles.

Circularly Polarized Luminescence from New Heteroleptic Eu(III) and Tb(III) Complexes.

The complexes [Eu(bpcd)(tta)], [Eu(bpcd)(Coum)], and [Tb(bpcd)(Coum)] [tta = 2-thenoyltrifluoroacetyl-acetonate, Coum = 3-acetyl-4-hydroxy-coumarin, and bpcd = N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N'-diacetate] have been synthesized and characterized from photophysical and thermodynamic points of view. The optical and chiroptical properties of these complexes, such as the total luminescence, decay curves of the Ln(III) luminescence, electronic circular dichroism, and circularly polarized luminescence, have been investigated. Interestingly, the number of coordinated solvent (methanol) molecules is sensitive to the nature of the metal ion. This number, estimated by spectroscopy, is >1 for Eu(III)-based complexes and <1 for Tb(III)-based complexes. A possible explanation for this behavior is provided via the study of the minimum energy structure obtained by density functional theory (DFT) calculations on the model complexes of the diamagnetic Y(III) and La(III) counterparts [Y(bpcd)(tta)], [Y(bpcd)(Coum)], and [La(bpcd)(Coum)]. By time-dependent DFT calculations, estimation of donor-acceptor (D-A) distances and of the energy position of the S1 and T1 ligand excited states involved in the antenna effect was possible. These data are useful for rationalizing the different sensitization efficiencies (ηsens) of the antennae toward Eu(III) and Tb(III). The tta ligand is an optimal antenna for sensitizing Eu(III) luminescence, while the Coum ligand sensitizes better Tb(III) luminescence {ϕovl = 55%; ηsens ≥ 55% for the [Tb(bpcd)(Coum)] complex}. Finally, for the [Eu(bpcd)(tta)] complex, a sizable value of glum (0.26) and a good quantum yield (26%) were measured.

Effect of the iodine atom position on the phosphorescence of BODIPY derivatives: a combined computational and experimental study.

A new BODIPY derivative (o-I-BDP) containing an iodine atom in the ortho position of the meso-linked phenyl group was prepared. Photophysical and electrochemical properties of the molecule were compared to previously reported iodo BODIPY derivatives, as well as to the non-iodinated analog. While in the case of derivatives featuring iodine substituents in the BODIPY core, efficient population of the triplet state is accompanied by a substantial positive shift of the reduction potential compared to pristine BODIPY, o-I-BDP displays phosphorescence and simultaneously maintains the electrochemical properties of unsubstituted BODIPYs. A theoretical investigation was settled to analyze results and rationalize the influence of iodine position on electronic and photophysical properties, with the purpose of preparing a fully organic phosphorescent BODIPY derivative. TD-DFT and spin-orbit coupling calculations shed light on the subtle effects played by the introduction of iodine atom in different positions of BODIPY.

Nickel-Mediated Enantioselective Photoredox Allylation of Aldehydes with Visible Light.

Here we report a practical, highly enantioselective photoredox allylation of aldehydes mediated by chiral nickel complexes with commercially available allyl acetate as the allylating agent. The methodology allows the clean stereoselective allylation of aldehydes in good to excellent yields and up to 93 % e.e. using a catalytic amount of NiCl2 (glyme) in the presence of the chiral aminoindanol-derived bis(oxazoline) as the chiral ligand. The photoredox system is constituted by the organic dye 3DPAFIPN and a Hantzsch's ester as the sacrificial reductant. The reaction proceeds under visible-light irradiation (blue LEDs, 456 nm) at 8-12 °C. Compared to other published procedures, no metal reductants (such as Zn or Mn), additives (e.g. CuI) or air-sensitive Ni(COD)2 are necessary for this reaction. Accurate DFT calculations and photophysical experiments have clarified the mechanistic picture of this stereoselective allylation reaction.

Photoredox Propargylation of Aldehydes Catalytic in Titanium.

A practical and effective photoredox propargylation of aldehydes promoted by 10 mol % of [Cp2TiCl2] is presented. No stoichiometric metals or scavengers are used for the process. A catalytic amount of the cheap and simply prepared organic dye 3DPAFIPN is used as the reductant for titanium. The reaction displayed a broad scope, and no traces of allenyl isomers were detected for simple propargyl bromide, whereas mixtures of propargyl and allenyl isomers were observed for substituted propargyl bromides.

Metallaphotoredox catalysis with organic dyes.

The use of organic dyes to promote organic reactions by photoredox catalysis is continuously expanding and was recently reviewed by Nicewicz. The synthesis of new dyes, their application in flow photoredox reactions, and their use in stereoselective and multicomponent transformations have considerably expanded the repertoire of application of organic dyes in photoredox mediated reactions. The low costs of these dyes, their tailored synthesis and availability in combination with the development of new concepts and careful catalytic cycle design (made possible by the application of fine theoretical investigations and deep understanding) are guiding the widespread application of organic dyes in the metallaphotoredox catalysis area. Developments and recent applications of different metal catalyzed processes mediated by organic dyes are covered by this review.

A Journey from Thermally Tunable Synthesis to Spectroscopy of Phenylmethanimine in Gas Phase and Solution.

Phenylmethanimine is an aromatic imine with a twofold relevance in chemistry: organic synthesis and astrochemistry. To tackle both aspects, a multidisciplinary strategy has been exploited and a new, easily accessible synthetic approach to generate stable imine-intermediates in the gas phase and in solution has been introduced. The combination of this formation pathway, based on the thermal decomposition of hydrobenzamide, with a state-of-the-art computational characterization of phenylmethanimine laid the foundation for its first laboratory observation by means of rotational electric resonance spectroscopy. Both E and Z isomers have been accurately characterized, thus providing a reliable basis to guide future astronomical observations. A further characterization has been carried out by nuclear magnetic resonance spectroscopy, showing the feasibility of this synthetic approach in solution. The temperature dependence as well as possible mechanisms of the thermolysis process have been examined.

Al(Salen) Metal Complexes in Stereoselective Catalysis.

Salen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N'-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published.

Phenoxyaluminum(salophen) Scaffolds: Synthesis, Electrochemical Properties, and Self-Assembly at Surfaces of Multifunctional Systems.

Salophens and Salens are Schiff bases generated through the condensation of two equivalents of salicylaldehyde with either 1,2-phenylenediamines or aliphatic diamines, respectively. Both ligands have been extensively exploited as key building blocks in coordination chemistry and catalysis. In particular, their metal complexes have been widely used for various catalytical transformations with high yield and selectivity. Through the modification of the phenol unit it is possible to tune the steric hindrance and electronic properties of Salophen and Salen. The introduction of long aliphatic chains in salicylaldehydes can be used to promote their self-assembly into ordered supramolecular structures on solid surfaces. Herein, we report a novel method towards the facile synthesis of robust and air-stable [Al(Salophen)] derivatives capable of undergoing spontaneous self-assembly at the graphite/solution interface forming highly-ordered nanopatterns. The new synthetic approach relies on the use of [MeAlIII (Salophen)] as a building unit to introduce, via a simple acid/base reaction with functionalized acidic phenol derivatives, selected frameworks integrating multiple functions for efficient surface decoration. STM imaging at the solid/liquid interface made it possible to monitor the formation of ordered supramolecular structures. In addition, the redox properties of the Salophen derivatives functionalized with ferrocene units in solution and on surface were unraveled by cyclic voltammetry. The use of a five-coordinate aluminum alkyl Salophen precursor enables the tailoring of new Salophen molecules capable of undergoing controlled self-assembly on HOPG, and thereby it can be exploited to introduce multiple functionalities with subnanometer precision at surfaces, ultimately forming ordered functional patterns.

Self-Assembled Two-Dimensional Supramolecular Networks Characterized by Scanning Tunneling Microscopy and Spectroscopy in Air and under Vacuum.

We combine ambient (air) and ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) investigations together with density functional theory (DFT) calculations to gain a subnanometer insight into the structure and dynamic of two-dimensional (2D) surface-supported molecular networks. The planar tetraferrocene-porphyrin molecules employed in this study undergo spontaneous self-assembly via the formation of hydrogen bonded networks at the gold substrate-solution interface. To mimic liquid phase ambient deposition conditions, film formation was accomplished in UHV by electro-spraying a solution of the molecule in chloroform onto an Au(111) substrate, thereby providing access to the full spectroscopic capabilities of STM that can be hardly attained under ambient conditions. We show that molecular assembly on Au (111) is identical in films prepared under the two different conditions, and in good agreement with the theoretical predictions. However, we observe the contrast found for a given STM bias condition to be different in ambient and UHV conditions despite the similarity of the structures, and we propose possible origins of the different imaging contrast. This approach could be valuable for the thorough characterization of surface systems that involve large molecules and are prepared mainly in ambient conditions.

Mechanistic insights into two-photon-driven photocatalysis in organic synthesis.

A mechanism based on the sequential absorption of two photons by the components of a redox couple has been recently proposed for catalysis of the energetically demanding reduction of aryl halides. Here, we analyze the suggested photochemical mechanism of this reaction, which employs perylenediimide (PDI) as a photocatalyst, on the basis of spectroscopic, electrochemical and electron paramagnetic resonance data. Our results indicate that the photoexcited PDI radical anion (*PDI˙-) cannot play the role of a photosensitizer in the aforementioned process. Instead, the reduction of 4'-bromoacetophenone likely involves *PDI˙- decomposition products. The extremely short lifetime of the photoexcited transient species, as *PDI˙-, is a major general limitation for photocatalytic schemes based on sequential two-photon excitation. In order to better understand the potential of such schemes, we discuss them in the context of the Z-scheme in natural photosynthesis.

Theory Meets Experiment for Noncovalent Complexes: The Puzzling Case of Pnicogen Interactions.

A gas-phase nitrogen-nitrogen noncovalent interaction has been unveiled in the nitroethane-trimethylamine complex in an environment free from solvent and matrix effects using rotational spectroscopy in supersonic expansion. Different quantum chemical models (NOCV/CD and NBO) agree in indicating that this interaction largely prevails over the C-H⋅⋅⋅O and C-H⋅⋅⋅N hydrogen bonds. Furthermore, a SAPT analysis shows that electrostatic and dispersion interactions play a comparable role in stabilizing the complex. The conformational landscape exploration and stationary points characterization have been performed using state-of-the-art quantum-chemical computations providing significant insights on structure determination.

Stereoselective Reactions with Chiral Schiff Base Metal Complexes.

Chiral Schiff bases are privileged chiral ligands, capable of transmitting chiral information in many stereoselective processes. Their modular nature, the possibility to modify their electronic character, and their ability to coordinate many metals, gives these chiral ligands broad utility and these have been investigated by many research groups around the world. For more than 20 years our research group has devoted attention to the application of chiral Schiff bases in the development of new stereoselective processes and in this brief survey we present our principal results and achievements in this field.

Photoredox Catalysis: The Need to Elucidate the Photochemical Mechanism.

The photocatalytic mechanism reported in a recent Communication to produce the radical anion of pyrenes postulates a highly endergonic electron transfer process. An analysis of the thermodynamics is reported together with the proposal of an alternative thermodynamically feasible mechanism.

Me2Zn-Mediated Catalytic Enantio- and Diastereoselective Addition of TosMIC to Ketones.

The first catalytic asymmetric addition of TosMIC to unactivated ketones is presented. A combination of Me2Zn and aminoalcohol catalyst promoted the aldol addition/cyclization reaction to render oxazolines possessing a fully substituted stereocenter with excellent yields (up to 92%), high enantioselectivities (up to 96%), and complete diastereoselectivity. The chiral oxazolines were then used to give, after a straightforward acid hydrolysis, enantioenriched building blocks bearing tertiary alcohol motifs such as hydroxylaldehydes, hydroxylacids, and hydroxylesters without racemization.

A Versatile Organocatalytic Approach for the Synthesis of Enantioenriched gem-Difluorinated Compounds.

The combination of a practical and highly enantioselective organocatalytic reaction, which allows the stereoselective introduction of a benzodithiol group, with a fluorination step, gives a new and effective strategy for the stereoselective synthesis of difluorinated building blocks. The benzodithiol group is a versatile and chameleonic group that can be further functionalized before fluorination, giving customized and tailored useful synthetic strategies. As an example of the application of this facile strategy, the effective enantioselective synthesis of difluoroarundic acid is described.

Synthesis of bench-stable diarylmethylium tetrafluoroborates.

A representative number of bench-stable nonsymmetric diarylcarbenium tetrafluoroborates have been isolated via the direct coupling of aryl (or heteroaryl) aldehydes and N-heteroarenes and fully characterized. They have proven to be highly stable in the presence of both EDG and EWG substituents. An (E)-iminium vinylogous substructure has been shown as the common cation scaffold by X-ray analysis and by NOE determination.