Regioisomerism in Symmetric Dimethyl Dialdehydes Dictates their Photochemical Reactivity.

We herein report the first light-driven selective monoderivatization (desymmetrization) of two chemically equivalent carbonyl groups in a single chromophore. By comparing of four symmetric regioisomers, featuring two equivalent ortho-methylbenzaldehyde units, we identify dimethyltherephtalaldehydes (DMTAs) which can be activated in a dual wavelength-selective fashion. Under visible light and UV-light irradiation, DMTAs undergo two consecutive Diels-Alder reactions exhibiting near-quantitative endo-selectivity (>99%) and provide excellent yields (96-98%). The influence of the regioisomerism of the dialdehydes on their photochemical behavior is profound, evidenced by an in-depth investigation of their photochemical performance. We exemplify the capability of the photosystems via the synthesis of complex Diels-Alder adducts with various dienophiles, including alkynes.

Two-colour light activated covalent bond formation.

We introduce a photochemical bond forming system, where two colours of light are required to trigger covalent bond formation. Specifically, we exploit a visible light cis/trans isomerization of chlorinated azobenzene, which can only undergo reaction with a photochemically generated ketene in its cis state. Detailed photophysical mapping of the reaction efficiencies at a wide range of monochromatic wavelengths revealed the optimum irradiation conditions. Subsequent small molecule and polymer ligation experiments illustrated that only the application of both colours of light affords the reaction product. We further extend the functionality to a photo reversible ketene moiety and translate the concept into material science. The presented reaction system holds promise to be employed as a two-colour resist.

Orange-Light-Induced Photochemistry Gated by pH and Confined Environments.

We introduce a new photochemically active compound, i.e., pyridinepyrene (PyPy), entailing a pH-active moiety that effects a significant halochromic shift into orange-light (λ = 590 nm) activatable photoreactivity while concomitantly exerting control over its reaction pathways. With blue light (λ = 450 nm) in neutral to basic pH, a [2 + 2] photocycloaddition can be triggered to form a cyclobutene ring in a reversible fashion. If the pH is decreased to acidic conditions, resulting in a halochromic absorption shift, photocycloaddition on the small-molecule level is blocked due to repulsive interactions and exclusive trans-cis isomerization is observed. Through implementation of PyPy into the confined environment of a single-chain nanoparticle (SCNP) design, one can overcome the repulsive forces and exploit the halochromic shift for orange light (λ = 590 nm)-induced cycloaddition and formation of macromolecular three-dimensional (3D) architectures.

Sequence-independent activation of photocycloadditions using two colours of light.

We exploit two reactive chromophores to establish sequence-independent photochemical activation, employing ortho-methyl benzaldehyde (oMBA) and N,N-(dimethylamino)pyrene aryl tetrazole (APAT) with N-(2-hydroxy)ethyl maleimide (NHEM), without any additives. Critically, the order of the irradiation sequence is irrelevant, as the shorter wavelength does not activate the higher wavelength activated species. Therefore, full sequence-independent λ-orthogonality is achieved through differences in both the reaction quantum yields (Φ r,oMBA and Φ r,APAT) and wavelength-dependent reactivity profiles of the employed chromophores.

A Self-Catalyzed Visible Light Driven Thiol Ligation.

We introduce a highly efficient ligation system based on a visible light-induced rearrangement affording a thiophenol which rapidly undergoes thiol-Michael additions. Unlike conventional light-triggered thiol-ene/yne systems, which rely on the use of photocaged bases/nucleophiles, (organo)-photo catalysts, or radical photoinitiators, our system provides a light-induced reaction in the absence of any additives. The ligation is self-catalyzed via the pyridine mediated deprotonation of the photochemically generated thiophenol. Subsequently, the thiol-Michael reaction between the thiophenol anion and electron deficient alkynes/alkenes proceeds additive-free. Hereby, the underlying photoinduced rearrangement of o-thiopyrinidylbenzaldehyde (oTPyB) generating the free thiol is described for the first time. We studied the influence of various reactions conditions as well as solvents and substrates. We exemplify our findings in a polymer end group modification and obtained macromolecules with excellent end group fidelity.

Light-induced Ligation of o-Quinodimethanes with Gated Fluorescence Self-reporting.

We introduce a highly efficient photoligation system, affording a pro-fluorescent Diels-Alder product that, on demand, converts into an intensively fluorescent naphthalene via E1 elimination in the presence of catalytic amounts of acid. The Diels-Alder reaction of the photocaged diene (o-quinodimethane ether or thioether) with electron-deficient alkynes is induced by UV or visible light. In contrast to previously reported ligation techniques directly leading to fluorescent products, the fluorescence is turned on after the photoligation. Thus, the light absorption of the fluorophore does not undermine the photoligation via competitive absorption, and as a result, photobleaching or side reactions of the fluorophore are not observed. Critically, the gated generation of a fluorescent product allows for fluorometric determination of the conversion. We employ a simple synthesis strategy for heterobifunctional electron-deficient alkynes allowing for facile functionalization of payload molecules.