Tricarbonylrhenium(I) complexes with aggregation-induced phosphorescence emission (AIPE) properties: Application to the selective detection of heparin and its main contaminant

Phosphorescent tricarbonylrhenium(I) complexes that are more emissive in the aggregate state than in solution could be very valuable probes for biological analyses, but their development is delicate. The present work focuses on the synthesis and spectroscopic study of three new complexes that differ by the nature of their positively charged substituent. The dissolved complexes were very weakly emissive. Due to electrostatic interaction and aggregation, they showed a strong aggregation-induced phosphorescence enhancement (AIPE) effect in the presence of heparin, a polyanionic macromolecule of biological interest, and a weaker effect in the presence of chondroitin sulfate, which contains fewer negative charges. The magnitude of the AIPE effect depended on the nature and number of the cationic groups borne by the complex. However, it was weaker than expected from the behavior of the parent neutral complex studied in a conventional acetonitrile/water system, which highlights the challenge of accurately predicting solid-state emission properties for this class of molecules. This work introduces rhenium(I) complexes in the field of AIPE-active probes for the detection of polyanionic biomolecules.

Oxidation and Reduction Pathways in the Knowles Hydroamination via a Photoredox-Catalyzed Radical Reaction

Systematic reaction path exploration revealed the entire mechanism of Knowles's light-promoted catalytic intramolecular hydroamination. Bond formation/cleavage competes with single electron transfer (SET) between the catalyst and substrate. These processes are described by adiabatic processes through transition states in an electronic state and non-radiative transitions through the seam of crossings (SX) between different electronic states. This study determined the energetically favorable SET path by introducing a practical computational model representing SET as non-adiabatic transitions via SXs between substrate's potential energy surfaces for different charge states adjusted based on the catalyst's redox potential. Calculations showed that the reduction and proton shuttle process proceeded concertedly. Also, the relative importance of SET paths (giving the product and leading back to the reactant) varies depending on the catalyst's redox potential, affecting the yield.

Efficient photorelease of carbon monoxide from a luminescent tricarbonyl rhenium(I) complex incorporating pyridyl-1,2,4-triazole and phosphine ligands

Precise control over the production of carbon monoxide (CO) is essential to exploit the therapeutic potential of this molecule. The development of photoactive CO-releasing molecules (PhotoCORMs) is therefore a promising route for future clinical applications. Herein, a tricarbonyl-rhenium(i) complex (1-TPP), which incorporates a phosphine moiety as ancilliary ligand for boosting the photochemical reactivity, and a pyridyltriazole bidentate ligand with appended 2-phenylbenzoxazole moiety for the purpose of photoluminescence, was synthesized and characterized from a chemical and crystallographic point of view. Upon irradiation in the near-UV range, complex 1-TPP underwent fast photoreaction, which was monitored through changes of the UV-vis absorption and phosphorescence spectra. The photoproducts (i.e. the dicarbonyl solvento complex 2 and one CO molecule) were identified using FTIR, 1H NMR and HRMS. The results were interpreted on the basis of DFT/TD-DFT calculations. The effective photochemical release of CO associated with clear optical variations (the emitted light passed from green to orange-red) could make 1-TPP the prototype of new photochemically-active agents, potentially useful for integration in photoCORM materials.

Substitution effect on the nonradiative decay and trans → cis photoisomerization route: a guideline to develop efficient cinnamate-based sunscreens

Cinnamate derivatives are very useful as UV protectors in nature and as sunscreen reagents in daily life. They convert harmful UV energy to thermal energy through effective nonradiative decay (NRD) including trans → cis photoisomerization. However, the mechanism is not simple because different photoisomeirzation routes have been observed for different substituted cinnamates. Here, we theoretically examined the substitution effects at the phenyl ring of methylcinnamate (MC), a non-substituted cinnamate, on the electronic structure and the NRD route involving trans → cis isomerization based on time-dependent density functional theory. A systematic reaction pathway search using the single-component artificial force-induced reaction method shows that the very efficient photoisomerization route of MC can be essentially described as "1ππ* (trans) → 1nπ* → T1 (3ππ*) → S0 (trans or cis)". We found that for efficient 1ππ* (trans) → 1nπ* internal conversion (IC), MC should have the substituent at the appropriate position of the phenyl ring to stabilize the highest occupied π orbital. Substitution at the para position of MC slightly lowers the 1ππ* state energy and photoisomerization occurs via a slightly less efficient "1ππ* (trans) → 3nπ* → T1 (3ππ*) → S0 (trans or cis)" pathway. Substitution at the meta or ortho positions of MC significantly lowers the 1ππ* state energy so that the energy barrier of IC (1ππ* → 1nπ*) becomes very high. This substitution leads to a much longer 1ππ* state lifetime than that of MC and para-substituted MC, and a change in the dominant photoisomerization route to "1ππ* (trans) → C[double bond, length as m-dash]C bond twisting on 1ππ* → S0 (trans or cis)". As a whole, the "1ππ* → 1nπ*" IC observed in MC is the most important initial step for the rapid change of UV energy to thermal energy. We also found that the stabilization of the π orbital (i) minimizes the energy gap between 1ππ* and 1nπ* at the 1ππ* minimum and (ii) makes the 0-0 level of 1ππ* higher than 1nπ* as observed in MC. These MC-like relationships between the 1ππ* and 1nπ* energies should be ideal to maximize the "1ππ* → 1nπ*" IC rate constant according to Marcus theory.

In Situ Solution-State Characterization of MOF-Immobilized Transition-Metal Complexes by Infrared Spectroscopy

Transition-metal catalysts immobilized on the surface of Metal-organic frameworks (MOFs) are being utilized for an ever-increasing number of reactions ranging from couplings to olefin oligomerization. While these reactions are usually performed in solution, unlike their homogeneous counterparts, the insolubility of the MOF systems makes it difficult to obtain detailed mechanistic information by in situ spectroscopic analysis in solution. In this report, we present a synthetic method to solubilize these systems by grafting oligomers on the surface of the MOF particles, making it possible to characterize these species by transmission infrared (IR) spectroscopy. The fundamental photochemistry of these catalysts was also studied and compared to that of their homogeneous counterparts. This work establishes a proof of principle for in-solution monitoring of heterogeneous catalysts.

Exploring radiative and nonradiative decay paths in indole, isoindole, quinoline, and isoquinoline

Radiative and nonradiative decay paths from the first excited singlet electronic state (S₁) in four heteroaromatics, indole, isoindole, quinoline, and isoquinoline, were systematically explored.

Luminescent Re(I) Carbonyl Complexes as Trackable PhotoCORMs for CO delivery to Cellular Targets

A family of Re(I) carbonyl complexes of general formula [ReX(CO)3(phen)]0/1+ (where X = Cl-, CF3SO3-, MeCN, PPh3, and methylimidazole) derived from 1,10-phenanthroline (phen) exhibits variable emission characteristics depending on the presence of the sixth ancillary ligand/group (X). All complexes but with X = MeCN exhibit moderate CO release upon irradiation with low-power UV light and are indefinitely stable in anaerobic/aerobic environment in solution as well as in solid state when kept under dark condition. These CO donors liberate three, one, or no CO depending on the nature of sixth ligand upon illumination as studied with the aid of time-dependent IR spectroscopy. Results of excited-state density functional theory (DFT) and time-dependent DFT calculations provided insight into the origin of the emission characteristics of these complexes. The luminescent rheinum(I) photoCORMs uniformly displayed efficient cellular internalization by the human breast adenocarcinoma cells, MDA-MB-231, while the complex with PPh3 as ancillary ligand showed moderate nuclear localization in addition to the cytosolic distribution. These species hold significant promise as theranostic photoCORMs (photoinduced CO releasing molecules), where the entry of the pro-drug can be tracked within the cellular matrices.