Photogeneration of Chlorine Radical from a Self-Assembled Fluorous 4CzIPN•Chloride Complex: Application in C-H Bond Functionalization

The chlorine radical is a strong HAT (Hydrogen Atom Transfer) agent that is very useful for the functionalization of C(sp3)-H bonds. Albeit highly attractive, its generation from the poorly oxidizable chloride ion mediated by an excited photoredox catalyst is a difficult task. We now report that 8Rf8-4CzIPN, an electron-deficient fluorous derivative of the benchmark 4CzIPN photoredox catalyst belonging to the donor-acceptor carbazole-cyanoarene family, is not only a better photooxidant than 4CzIPN, but also becomes an excellent host for the chloride ion. Combining these two properties ultimately makes the self-assembled 8Rf8-4CzIPN•Cl- dual catalyst highly reactive in redox-neutral Giese-type C(sp3)-H bond alkylation reactions promoted by the chlorine radical. Additionally, because of its fluorous character, the efficient separation/recovery of 8Rf8-4CzIPN could be envisioned.

Cyclometalated luminescent platinum(II) complexes of dissymmetrical 2,2':4',2''-terpyridine and its self-assembled dimer presenting Pt-Ag dative bonds

A series of cyclometalated (N^C^N) Pt(II)-platinum complexes featuring a terpyridine ligand with a non-coordinating nitrogen atom and a Pt-C bond was synthesized. In the presence of Ag+, the bis(isonitrile)Pt(II) complex formed a remarkable self-assembled helicoidal dimer stabilized by coordination of Ag(I) and metallophilic Pt-Ag interactions. Its assembly was observed in the solid state and maintained in solution. All complexes show strong luminescence and multiple emitting states, which could be rationalized based on solid state X-ray structures and coordinating environment.

Divalent ansa-Octaphenyllanthanocenes: Synthesis, Structures, and EuII Luminescence

Reductive dimerization of fulvenes using low-valent metal precursors is a straightforward one-step approach to access ethylene-bridged metallocenes. This process has so far mainly been employed with fulvenes carrying one or two substituents in the exocyclic position. In this work, a new synthesis of the unsubstituted exocyclic 1,2,3,4-tetraphenylfulvene (1), its full structural characterization by NMR spectroscopy and single-crystal X-ray diffraction, as well as some photophysical properties and its first use in reductive dimerization are described. This fulvene reacted with different lanthanoid metals in thf to provide the divalent ansa-octaphenylmetallocenes [Ln(C5Ph4CH2)2(thf)n] (Ln = Sm, n = 2 (2); Ln = Eu, n = 2 (3); and Ln = Yb, n = 1 (4)). These complexes were characterized by X-ray diffraction, laser desorption/ionization time of flight mass spectrometry, and, in the case of Sm and Yb, multinuclear NMR spectroscopy, showing the influence of the ansa-bridge on solution and solid-state structures compared to previously reported unbridged metallocenes. Furthermore, the luminescence properties of the Eu ansa complex 3 were studied in solution and the solid state, revealing significant differences with the known octa- and deca-phenyleuropocenes, [Eu(C5Ph4H)2(dme)] and [Eu(C5Ph5)2].

TICT compounds by design: comparison of two naphthalimide-π-dimethylaniline conjugates of different lengths and ground state geometries

Two new twisted intramolecular charge transfer (TICT) donor-π-acceptor compounds were designed by combining a well-known electron acceptor naphthalimide unit with a classic electron donor dimethylaniline through two types of different rigid linkers. The combined steady-state and time-resolved spectroscopy of molecules in solvents of different polarities in comparison to solid-state solvation experiments of doped polymer matrixes of different polarities allowed distinguishing between solvation and conformation determined processes. The photophysical measurements revealed that non-polar solutions possess high fluorescence quantum yields of up to 70% which is a property of pre-twisted/planar molecules in the excited charge transfer (CT) states. The increase of polarity allows tuning the Stokes shift through all the visible wavelength range up to 8601 cm^-1 which is accompanied by a three orders of magnitude drop of fluorescence quantum yields. This is a result of the emerged TICT states as dimethylaniline twists to a perpendicular position against the naphthalimide core. The TICT reaction of molecules enables an additional non-radiative excitation decay channel, which is not present if the twisting is forbidden in a rigid polymer matrix. Transient absorption spectroscopy was employed to visualize the excited state dynamics and to obtain the excited state reaction constants, revealing that TICT may occur from both the Franck-Condon region and the solvated pre-twisted/planar CT states. Both molecules undergo the same photophysical processes, however, a longer linker and thus a higher excited state dipole moment determines the faster excited state reactions.

A π-Extended Phenanthrene-Fused Aza[7]helicenium as Novel Chiroptically-Active Architecture in Organic and Aqueous Media

The synthesis and characterization of an original π-extended cationic azahelicene is reported. The phenanthrene-fused aza[7]helicene derivative encompasses a total of ten aromatic fused rings leading to a dissymmetric yet helically...

Damming an electronic energy reservoir: ion-regulated electronic energy shuttling in a [2]rotaxane

We demonstrate the first example of bidirectional reversible electronic energy transfer (REET) between the mechanically bonded components of a rotaxane. Our prototypical system was designed such that photoexcitation of a chromophore in the axle results in temporary storage of electronic energy in a quasi-isoenergetic “reservoir” chromophore in the macrocycle. Over time, the emissive state of the axle is repopulated from this reservoir, resulting in long-lived, delayed luminescence. Importantly, we show that cation binding in the cavity formed by the mechanical bond perturbs the axle chromophore energy levels, modulating the REET process, and ultimately providing a luminescence read-out of cation binding. Modulation of REET processes represents an unexplored mechanism in luminescent molecular sensor development.

Delayed emission due to reversible electronic energy transfer (REET) between chromophores in the axle and macrocycle components of a rotaxane is demonstrated. The REET process can be modulated by metal ion binding in the cavity of the rotaxane.

Alkylation of the α-amino C-H bonds of anilines photocatalyzed by a DMEDA-Cu-benzophenone complex: reaction scope and mechanistic studies

The Cu(ii) complex 1 incorporating a BP chromophore is a highly active and chemoselective photocatalyst for the alkylation of α-amino C-H bonds of anilines. The reaction was shown to proceed with a broad substrate scope in the absence of additives. Extensive mechanistic studies were performed, in particular using transient absorption spectroscopy, and spectroscopic signatures of key intermediates were identified in the conditions of catalysis. Finally, the ability of 1 to act as a multitask catalyst was showcased by conducting multi-component CuAAC and olefin hydroalkylation reactions in one-pot.

Proof of principle of a purine D-A-D' ligand based ratiometric chemical sensor harnessing complexation induced intermolecular PET

A comprehensive photophysical study of a series of purines, doubly decorated at C2 and C6 positions with identical fragments ranging from electron acceptor to donor groups of different strengths, is presented. The asymmetry of substitutions creates a unique molecular D-A-D' structure possessing two independent electronic charge transfer (CT) systems attributed to each fragment and exhibiting dual-band fluorescence. Moreover, the inherent property of coordination of metal ions by purines was enriched due to a presence of nearby triazoles used as spacers for donor or acceptor fragments. New molecules present a bidentate coordination mode, which makes the assembly of several ligands with one metal cation possible. This property was exploited to create a new concept of a ratiometric chemical fluorescence sensor involving the photoinduced electron transfer between branches of different ligands as a mechanism of fluorescence modulation.

Electron injection effect in In2O3 and SnO2 nanocrystals modified by ruthenium heteroleptic complexes

In this work, the optical characteristics and conductivity under photoactivation with visible light of hybrids based on nanocrystalline SnO2 or In2O3 semiconductor matrixes and heteroleptic Ru(ii) complexes were studied. The heteroleptic Ru(ii) complexes were prepared based on 1H-imidazo[4,5-f][1,10]phenanthroline and 2,2'-bipyridine ligands. Nanocrystalline semiconductor oxides were obtained by chemical precipitation with subsequent thermal annealing and characterized by XRD, SEM and single-point BET methods. The heteroleptic Ru(ii) complexes as well as hybrid materials were characterized by time-resolved luminescence and X-ray photoelectron spectroscopy. The results showed that the surface modification of SnO2 nanoparticles with heteroleptic ruthenium complexes led to an increase in conductivity upon irradiation with light appropriate for absorption by organometallic complexes. In the case of In2O3, the deposition of Ru(ii) complexes resulted in a decrease in conductivity, apparently due to the special structure of the surface layer of the semiconductor.

Molecular engineering of logic gate types by module rearrangement in 'Pourbaix Sensors': the effect of excited-state electric fields

Two types of fluorescent logic gates are accessed from two different arrangements of the same modular components, one as an AND logic gate (1) and the other as a PASS 0 logic gate (2). The logic gates were designed with an 'electron-donor-spacer1-fluorophore-spacer2-receptor' format and demonstrated in 1 : 1 (v/v) methanol/water. The molecules consist of ferrocene as the electron donor, 4-aminonaphthalimide as the fluorophore and a tertiary alkylamine as the receptor. In the presence of high H+ and Fe3+ levels, regioisomers 1a and 1b switch 'on' as AND logic gates with fluorescence enhancement ratios of 16-fold and 10-fold, respectively, while regioisomers 2a and 2b are functionally dormant, exhibiting no fluorescence switching. The PASS 0 logic of 2a and 2b results from the transfer of an electron from the excited state fluorophore to the ferrocenium unit under oxidising conditions as predicted by DFT calculations. Time-resolved fluorescence spectroscopy provided lifetimes of 8.3 ns and 8.1 ns for 1a and 1b, respectively. The transient signal recovery rate of 1b is ∼10 ps while that of 2b is considerably longer on the nanosecond timescale. The divergent logic attributes of 1 and 2 highlight the importance of field effects and opens up a new approach for regulating logic-based molecules.

Designed Long-Lived Emission from CdSe Quantum Dots through Reversible Electronic Energy Transfer with a Surface-Bound Chromophore

The size-tunable emission of luminescent quantum dots (QDs) makes them highly interesting for applications that range from bioimaging to optoelectronics. For the same applications, engineering their luminescence lifetime, in particular, making it longer, would be as important; however, no rational approach to reach this goal is available to date. We describe a strategy to prolong the emission lifetime of QDs through electronic energy shuttling to the triplet excited state of a surface-bound molecular chromophore. To implement this idea, we made CdSe QDs of different sizes and carried out self-assembly with a pyrene derivative. We observed that the conjugates exhibit delayed luminescence, with emission decays that are prolonged by more than 3 orders of magnitude (lifetimes up to 330 μs) compared to the parent CdSe QDs. The mechanism invokes unprecedented reversible quantum dot to organic chromophore electronic energy transfer.

A novel bacteriochlorin-styrylnaphthalimide conjugate for simultaneous photodynamic therapy and fluorescence imaging

Propargyl-15²,17³-dimethoxy-13¹-amide of bacteriochlorin e (BChl) and a 4-(4-N,N-dimethylaminostyryl)-N-alkyl-1,8-naphthalimide bearing azide group in the N-alkyl fragment were conjugated by the copper(i)-catalyzed 1,3-dipolar cycloaddition to produce a novel dyad compound BChl-NI for anticancer photodynamic therapy (PDT) combining the modalities of a photosensitizer (PS) and a fluorescence imaging agent. A precise photophysical investigation of the conjugate in solution using steady-state and time-resolved optical spectroscopy revealed that the presence of the naphthalimide (NI) fragment does not decrease the photosensitizing ability of the bacteriochlorin (BChl) core as compared with BChl; however, the fluorescence of naphthalimide is completely quenched due to resonance energy transfer (RET) to BChl. It has been shown that the BChl-NI conjugate penetrates into human lung adenocarcinoma A549 cells, and accumulates in the cytoplasm where it has a mixed granular-diffuse distribution. Both NI and BChl fluorescence in vitro provides registration of bright images showing perfectly intracellular distribution of BChl-NI. The ability of NI to emit light upon excitation in imaging experiments has been found to be due to hampering of RET as a result of photodestruction of the energy acceptor BChl unit. Phototoxicity studies have shown that the BChl-NI conjugate is not toxic for A549 cells at tested concentrations (<8 μM) without light-induced activation. At the same time, the concentration-dependent killing of cells is observed upon the excitation of the bacteriochlorin moiety with red light that occurs due to reactive oxygen species formation. The presented data demonstrate that the BChl-NI conjugate is a promissing dual function agent for cancer diagnostics and therapy.

Rationalisation of a mechanism for sensing single point variants in target DNA using anthracene-tagged base discriminating probes

The fluorescence sensing mechanism for identifying single base changes in target DNA strands has been established through detailed biophysical measurements.

Light-induced piston nanoengines: ultrafast shuttling of a styryl dye inside cucurbit[7]uril

A proof of principle for an ultrafast molecular shuttle based on the light-operated movement of a styryl dye inside cucurbit[7]uril was described.

Controlling photophysics of styrylnaphthalimides through TICT, fluorescence and E,Z-photoisomerization interplay

The steady-state and time-resolved spectral properties of styrylnaphthalimides are studied in various solvents.