2-Isopropylthioxanthone ITX

Technical Specifications

Product Description & Scientific Applications

ITX (2-isopropyl-9H-thioxanthen-9-one) is a Norrish Type II photoinitiator built on the thioxanthone chromophore. It forms initiating radicals bimolecularly, through its photoexcited triplet, and so it operates alongside a co-initiator.

Light absorption and excited states. ITX absorbs across the ultraviolet and into the visible, with maxima near 258 nm and 386 nm in dichloromethane and a tail to roughly 420 nm. Thioxanthone photoinitiators absorb between 380 and 420 nm, so ITX matches light-emitting-diode sources and sunlight. Absorbed light populates the excited singlet, and intersystem crossing carries the molecule to the triplet. The triplet is the reactive state and is detected by laser flash photolysis on the microsecond timescale.

Radical generation. ITX abstracts hydrogen from its triplet state rather than fragmenting, so it needs a hydrogen-donor co-initiator, and tertiary amines are the most efficient partner. The triplet and the amine form an exciplex, an electron transfers from the amine to ITX, and a proton then transfers from the carbon adjacent to nitrogen. This yields a thioxanthone ketyl radical and an α-aminoalkyl radical. The conjugated ketyl radical does not initiate polymerization and terminates chains instead; the α-aminoalkyl radical is the initiating species. The triplet abstracts hydrogen directly from alcohols and ethers as well.

Cationic sensitisation and energy transfer. The thioxanthone triplet also behaves as a photosensitiser. Transferring an electron to an onium salt, ITX sensitises iodonium and sulfonium photoinitiators for cationic polymerization, generating a thioxanthone radical cation and an initiating onium-derived radical. The combination of ITX with phosphonium salts ranks among the most efficient systems for ring-opening cationic polymerization. By triplet energy transfer, thioxanthones also sensitise other photoinitiators and cycloaddition reactions.

Applications. ITX is a commercial Type II photoinitiator for free-radical ultraviolet curing of coatings and printing inks. It also acts as the photoinitiator in negative-tone radical photoresists for multiphoton polymerization and three-dimensional microfabrication.

Limitations. The amine co-initiator can introduce odour, yellowing, and migration of photolysis byproducts from the cured film. ITX is a small molecule, so it and its photolytic products migrate out of cured films. Migration of ITX from ultraviolet-printed cartons into food is documented: a 2005 notification recorded 250 µg/L in packaged infant formula and led to the recall of over 30 million litres across Europe. This marks ITX as a recognised constraint for food-contact coatings. Under EU Regulation 10/2011 its photolytic products count as non-intentionally-added substances, held to a default migration limit of 10 µg/kg, and industry has moved toward higher-molecular-weight and polymeric photoinitiators.

Shipping Destinations

• EU & UK only: Priority delivery, 2–5 business days. Classified as dangerous goods — shipping restrictions apply outside the EU/UK & EFTA countries.