Ultrafast Spectroscopy of Arylchlorodiazirines: Hammett Correlations of Excited State Lifetimes

Structure-function relationships in aryl diazirines reveal optimal design features to maximize C-H insertion

Diazirine reagents allow for the ready generation of carbenes upon photochemical, thermal, or electrical stimulation. Because carbenes formed in this way can undergo rapid insertion into any nearby C-H, O-H or N-H bond, molecules that encode diazirine functions have emerged as privileged tools in applications ranging from biological target identification and proteomics through to polymer crosslinking and adhesion. Here we use a combination of experimental and computational methods to complete the first comprehensive survey of diazirine structure-function relationships, with a particular focus on thermal activation methods. We reveal a striking ability to vary the activation energy and activation temperature of aryl diazirines through the rational manipulation of electronic properties. Significantly, we show that electron-rich diazirines have greatly enhanced efficacy toward C-H insertion, under both thermal and photochemical activation conditions. We expect these results to lead to significant improvements in diazirine-based chemical probes and polymer crosslinkers.

CaproGlu: Multifunctional tissue adhesive platform

Driven by the clinical need for a strong tissue adhesive with elastomeric material properties, a departure from legacy crosslinking chemistries was sought as a multipurpose platform for tissue mending. A fresh approach to bonding wet substrates has yielded a synthetic biomaterial that overcomes the drawbacks of free-radical and nature-inspired bioadhesives. A food-grade liquid polycaprolactone grafted with carbene precursors yields CaproGlu. The first-of-its-kind low-viscosity prepolymer is VOC-free and requires no photoinitiators. Grafted diazirine end-groups form carbene diradicals upon low energy UVA (365 nm) activation that immediately crosslink tissue surfaces; no pre-heating or animal-derived components are required. The hydrophobic polymeric environment enables metastable functional groups not possible in formulations requiring solvents or water. Activated diazirine within CaproGlu is uniquely capable of crosslinking all amino acids, even on wet tissue substrates. CaproGlu undergoes rapid liquid-to-biorubber transition within seconds of UVA exposure-features not found in any other bioadhesive. The exceptional shelf stability of CaproGlu allows gamma sterilization with no change in material properties. CaproGlu wet adhesiveness is challenged against current unmet clinical needs: anastomosis of spliced blood vessels, anesthetic muscle patches, and human platelet-mediating coatings. The versatility of CaproGlu enables both organic and inorganic composites for future bioadhesive platforms.

Photocatalytic Generation of Nitrenes for Rapid Diaziridination

A blue LED, an organic photocatalyst (rose bengal), and the Lewis acid like oxidant PhI(OAc)₂ were utilized to generate nitrene intermediates through reactions of 1,2-diols and aliphatic amines under mild reaction conditions. A versatile and rapid diaziridination strategy was established to construct functionalized 1,2-disubstituted diaziridines, diaziridines with chiral substituents, and 1,2,3-trisubstituted analogues with excellent reaction rates, yields, and stereoselectivities. Control and labeling experiments to elucidate the mechanism of this elegant metal-free photocatalyzed cyclization reaction were performed.

Substituent Effect in the First Excited Singlet State of Monosubstituted Benzenes

sEDA, pEDA, and cSAR descriptors of the substituent effect were determined for >30 monosubstituted benzenes in the first excited singlet S₁ state at the LC-ωB97XD/aug-cc-pVTZ level. It was found that in the S₁ state, the σ- and π-valence electrons are a bit less and a bit more affected, respectively, than in the S₀ state, but basically, the effect in both states remains the same. In the S₀ and S₁ states, the d(C-X) distances to the substituent's first atom and the ring perimeter correlate with the sEDA and pEDA in the appropriate states, respectively. The energies and the gap of the frontier orbitals in the two states are linearly correlated and for the HOMO(S₁), LUMO(S₁), and HOMO(S₁)-LUMO(S₁) gap correlate also with the pEDA(S₁) and cSAR(S₁) descriptors. In all studied correlations, three similar groups of substituents can be distinguished, for which correlations (i) are very good, (ii) deviate slightly, and (iii) deviate significantly. Comparison of the shape of the HOMO(S₀) and HOMO(S₁) orbitals shows that for case (i) HOMO orbitals exhibit almost perfect antisymmetry against the benzene plane, for case (ii) the antisymmetry of HOMO in one of the states is either perturbed or changed, and for case (iii) one HOMO state has σ-character.

Substituent control of the ultrafast twisted intramolecular charge transfer rate in dimethylaminochalcone derivatives

The rate of TICT relaxation in dimethylaminochalcone derivatives is shown to be controlled by acceptor strength. Variation in the charge pulling capacity of the acceptor modifies the torsional barrier along the TICT coordinate in the S₁ state, resulting in a tunable TICT relaxation rate.

Theoretical study of non-Hammett vs. Hammett behaviour in the thermolysis and photolysis of arylchlorodiazirines

Calculations show that the different Hammett behaviour of arylchlorodiazirines is due to different mechanisms in the ground and excited state.

Electronically Excited States of Borylenes

Borylenes, RB, are elusive reactive intermediates. Still not much is known about their excited states from spectroscopic experiments, and existing knowledge is limited to diatomic borylenes only. The electronic structure and geometry of borylenes with diverse substituents on boron (where R = H, F, Cl, CH3, CF3, tBu, NH2, Ph, and SiMe3) were studied by means of computational chemistry. For this purpose, geometries of borylenes in their lowest singlet and triplet states were optimized at the B3LYP/def2-TZVP level of theory. Additionally, the influence of substitution on the energies of frontier molecular orbitals, HOMO-LUMO energy gaps, singlet-triplet energy splittings, and excitation energies was investigated. Two lowest vertical singlet-singlet excitations were computed using EOM-CCSD and TD-DFT (using hybrid B3LYP, and long-range separated CAM-B3LYP and ωB97X functionals) in combination with the aug-cc-pVTZ basis set. The electronic transitions involve excitations from nonbonding sp boron orbital (HOMO) mainly to empty p(B) orbitals and partially to the orbitals of the substituent, and are of n → π* type. The results can facilitate prospective identification of borylenes, e.g., in UV-vis matrix isolation or time-resolved spectroscopy experiments.

A perspective on physical organic chemistry

A perspective on the development of mechanistic carbene chemistry is presented. The author will point out questions that have been answered, and a next generation of questions will be proposed.

Existence of a new emitting singlet state of proflavine: femtosecond dynamics of the excited state processes and quantum chemical studies in different solvents

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.

Evidence of hydrogen migration in an alkylphenyldiazirine excited state

Ultrafast photolysis (350 nm) of alkylphenyldiazirines promotes the diazirine to the S(1) excited state. Solvent and substituent effects on the excited state lifetimes indicate that the S(1) state is highly polarized and undergoes a [1,2]-H shift in concert with nitrogen extrusion in cyclohexane.