2-(2-Hydroxyphenyl)Benzothiazole

As a member of the hydroxyphenyl-substituted benzothiazole class, 2-(2-hydroxyphenyl)benzothiazole (CAS 3411-95-8) is supplied as a crystalline powder with a melting range of 129–131 °C and a purity typically exceeding 98.5% by HPLC on a C18 column (UV detection at 330 nm). The photophysics are dominated by excited-state intramolecular proton transfer (ESIPT), which yields an exceptionally large Stokes shift—the absorption maximum in toluene is 345 nm (ε ≅ 1.5 × 10⁴ M⁻¹ cm⁻¹) and the keto-tautomer emission peaks near 500 nm. This shift, exceeding 150 nm, contrasts sharply with conventional benzotriazole and benzophenone UV absorbers that exhibit Stokes shifts below 30 nm, a difference that enables optical applications where signal separation or reabsorption suppression is critical. Can ESIPT-Mediated Stokes Shift Surpass Conventional UV Absorbers in Polyolefin Films? When compounded into linear low-density polyethylene (LLDPE) for greenhouse cover films, the additive is introduced as a 10% active masterbatch via a co-rotating twin-screw extruder (L/D 40, screw diameter 25 mm) operating at a melt temperature of 190 °C. Because the compound can sublime at localized hot spots, die-lip plate-out becomes a production bottleneck: die temperatures must be held below 200 °C with an accuracy of ±2 °C to prevent volatilized material from condensing on the calendering rolls. Comparative accelerated weathering per ISO 4892-2 (xenon arc, black-panel temperature 65 °C, 0.35 W/m² at 340 nm) shows that films containing 0.15 wt% of 2-(2-hydroxyphenyl)benzothiazole retain 85% of their initial tensile strength at break (tested per ASTM D638-14) after 2000 h, whereas an equivalent loading of 2-(2′-hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chlorobenzotriazole (Tinuvin 326) retains 78%. In this system, the ESIPT mechanism dissipates absorbed UV-B energy as harmless enol–keto tautomer cycling, but the effectiveness drops rapidly above 0.25 wt% due to aggregation-caused quenching in the amorphous polyethylene matrix, a limitation not observed with the benzotriazole. The additive is incompatible with amine-based processing aids; residual amines promote deprotonation of the phenolic hydroxyl, forming a yellow-coloured metal-chelate complex with iron traces from barrel wear, which can elevate the Yellowness Index (per ASTM E313) by 2.5 units after a single residence-time cycle. Extruder screws must therefore be nitrided or coated with a corrosion-resistant alloy. Aqueous dispersions of 2-(2-hydroxyphenyl)benzothiazole are post-added to pad–dry–cure formulations for polyester (PET) woven goods to impart a neutral-white optical brightening effect without the characteristic blue cast of stilbene-based brighteners. In a typical continuous pad application, a 5 g/L dispersion (particle size D50 0.8 μm, stabilised with a naphthalene-sulfonate condensate dispersant) is padded at a wet pick-up of 70%, dried at 120 °C, and thermofixed at 190 °C for 45 s. Lightfastness of the brightened fabric is assessed following ISO 105-B02:2014: after Grade 4 exposure, the chroma shift ΔC*ab measured on a spectrophotometer with D65 illuminant (10° observer) remains below 1.2, comparable to a benzoxazole-type polyester brightener but with a 15 °C higher sublimation fastness threshold, which reduces pad-roll contamination during heat-setting. A processing conflict arises when the dyebath also contains a fluorocarbon water-repellent finish applied from an alkaline bath (pH >9); under these conditions, the brightener’s hydroxy group ionises, red-shifting the emission by 18 nm and reducing the fluorescence intensity by 22%, as followed by inline fluorimetry at the frame exit.

Comparative Optical and Thermal Parameters of Selected ESIPT and Benzotriazole Additives

Parameter	2-(2-Hydroxyphenyl)benzothiazole	2-(2-Hydroxyphenyl)benzoxazole	Tinuvin 326
λmax absorption (toluene)	345 nm	330 nm	312 nm, 352 nm
λmax emission (toluene)	500 nm	460 nm	385 nm
Stokes shift	155 nm	130 nm	33 nm
Molar extinction coefficient at λmax	1.48 × 10⁴ M⁻¹ cm⁻¹	1.72 × 10⁴ M⁻¹ cm⁻¹	1.59 × 10⁴ M⁻¹ cm⁻¹
Solid-state quantum yield (integrating sphere)	0.32 ± 0.03	0.45 ± 0.05	N/A
Melting point	129–131 °C	126–128 °C	140–143 °C
TGA 5% mass loss (N₂, 10 °C/min)	225 °C	180 °C	260 °C

When Metal Ion Quenching Dictates Sensor Sensitivity Detection of transition metal ions in aqueous media exploits the rapid fluorescence turn-off that occurs upon coordination to the deprotonated phenolic oxygen and the benzothiazole nitrogen. In a homogeneous ethanol/water (1:1 v/v) buffered at pH 7.4 with HEPES, a 1.0 × 10⁻⁵ M solution of the probe exhibits a linear Stern-Volmer response towards Cu²⁺ in the range 0.1–8.0 μM with a quenching constant K_SV of 1.2 × 10⁵ M⁻¹, as measured on a scanning spectrofluorometer with excitation at 340 nm and emission monitored at 505 nm. The limit of detection, calculated as 3σ/slope, is 28 nM, which is superior to the benzoxazole analogue (LOD 54 nM) under identical conditions because the benzothiazole sulfur enhances the chelation stability. Interference from competing ions is controlled; 100-fold excesses of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Zn²⁺ cause less than 5% signal deviation, though Fe³⁺ at concentrations above 2.0 μM induces a paramagnetic quenching that deviates from the Stern-Volmer linearity and requires correction via inner-filter effect calculation per IUPAC Technical Report 2018. Immobilising the probe in a poly(vinyl chloride) matrix plasticised with o-nitrophenyl octyl ether results in a response time of 12 s (t₉₀) and a reversible relative signal recovery of 91% after EDTA washing, making it suitable for flow-injection analysis instrumentation equipped with a 470 nm LED and a photodiode detector. OLED Emitter Layer Integration and Sublimation Purity Fabrication of vacuum-deposited organic light-emitting diodes utilising 2-(2-hydroxyphenyl)benzothiazole as an emitting layer guest in a host of 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) requires triple-zone sublimation purification at 10⁻⁶ mbar and a boat temperature of 165–170 °C. The material’s sublimation enthalpy of 98 ± 4 kJ/mol necessitates deposition rates below 0.3 Å/s to avoid particle ejection from the source. Devices assembled in the architecture ITO/HAT-CN (10 nm)/NPB (40 nm)/TCTA (10 nm)/CBP: 6 wt% emitter (30 nm)/TPBi (50 nm)/LiF (1 nm)/Al (100 nm) yield an electroluminescence peak at 505 nm with a narrow full-width at half-maximum of 65 nm and a maximum external quantum efficiency (EQE) of 4.2%, as measured with a calibrated integrating sphere and a source meter (Keithley 2400). The ESIPT emitter’s large Stokes shift minimises self-absorption in the CBP host, an advantage over conventional fluorescent dopants, yet the efficiency roll-off at luminance exceeding 1000 cd/m² is more severe—dropping to 3.1%—due to the long excited-state lifetime of the keto tautomer (1.2 ns in degassed toluene), which promotes triplet–triplet annihilation. Sublimed material must be handled under a dry nitrogen atmosphere with moisture and oxygen levels below 0.1 ppm; exposure to ambient air for 30 min leads to a 15% decrease in photoluminescence quantum yield when re-measured, attributed to water-assisted tautomer quenching at the grain surface.

Typical Quality Control Specifications for Technical-Grade 2-(2-Hydroxyphenyl)benzothiazole

Test Parameter	Method	Specification
Assay (HPLC, 330 nm)	EP 2.2.29 / in-house	≥ 98.5%
Melting range	Ph.Eur. 2.2.14 / capillary	129–131 °C
Loss on drying (105 °C, 2 h)	ISO 787-2	≤ 0.5%
Residue on ignition	ISO 787-3	≤ 0.1%
Iron content (ICP-OES)	ASTM E1479-16	≤ 15 mg/kg
Particle size D50 (laser diffraction)	ISO 13320:2020	0.6–1.2 μm
Heavy metals (as Pb)	Ph.Eur. 2.4.8	≤ 10 mg/kg