Abstract:A pronounced conformational preference of the 3Ј-␣-cumyl substituent in 2-(2Ј-methoxy-3Ј-␣-cumylphenyl)benzotriazole is suggested from the results of molecular dynamics simulations. This suggestion is supported by both solution NMR spectral and solid-state x-ray crystallographic data. The orientation of the ␣-cumyl substituent may have implications on the relative performance of 3Ј-substituted 2Ј-hydroxyphenylbenzotriazole light stabilizers in polar media.
“…This shielding effect is not operative in the planar configuration of the IMHB form 15b,c in the less polar solvents, acetone and THF (Figure ). These observations are analogous to those observed in the NMR studies of 4-HAP-BZT, Tinuvin P, and other benzotriazoles in solution. ,8a 2 …”
A novel UV-light stabilizer (UVLS), 3′-(2H-benzotriazol-2′′-yl)-1,1,1-tris(4′-hydroxyphenyl)-ethane or simply tris(hydroxyphenyl)ethane benzotriazole (THPE-BZT), that can be copolymerized with engineering plastic monomers has been synthesized in three steps. The compound has two solid forms, a hydrate and an anhydrous form. Surprisingly, the hydrated form is more soluble in organic solvents such as acetone. Infrared spectra reveal that the hydrated form loses water of hydration upon heating and then forms a coplanar ring structure with a strong intramolecular hydrogen bond (IMHB) between one benzotriazole nitrogen and the adjacent phenolic OH. The NMR and UV-vis spectra in less polar organic solvents are also in accordance with the IMHB structure of THPE-BZT. In polar organic solvents such as DMSO and DMF and in MeOH/water at pH >10, the IMHB is disrupted, and the disrupted form exists in equilibrium with the planar or nondisrupted form. However, THPE-BZT copolymerized into low molecular weight polysulfones and polycarbonates retains the planar IMHB structure, as indicated by the UV absorption peak at ∼335 nm. These copolymers have greater UV stability than those without any THPE-BZT because of dissipation of photon energy by molecular vibrational relaxation in the planar form. Also, polysulfone copolymers containing THPE-BZT in the range of 0.25-1.5 mol % have greater UV light stability than polysulfone containing additive-type UVLS such as 2-(2′-hydroxy-5′-t-octylphenyl)benzotriazole (Cyasorb 5411). For polycarbonates, preliminary results show that copolymers of THPE-BZT have UV-light stability comparable to or only slightly less than that of polycarbonates blended with Cyasorb 5411 or THPE-BZT as additives. But the copolymers should escape the problems of surface "blooming" and leaching by extraction experienced by noncopolymerized UVLS additives. Thermogravimetric analysis indicates that THPE-BZT is more stable than the commercial benzotriazoles 2-(2′-hydroxy-5′-methylphenyl)benzotriazole (Tinuvin P), Cyasorb 5411, and the industrial standards2-[2′-hydroxy-3′,5′-bis (1-methyl-1-phenylethyl)phenyl]benzotriazole (Tinuvin 900)sand therefore might offer superior performance, even as an additive.
“…This shielding effect is not operative in the planar configuration of the IMHB form 15b,c in the less polar solvents, acetone and THF (Figure ). These observations are analogous to those observed in the NMR studies of 4-HAP-BZT, Tinuvin P, and other benzotriazoles in solution. ,8a 2 …”
A novel UV-light stabilizer (UVLS), 3′-(2H-benzotriazol-2′′-yl)-1,1,1-tris(4′-hydroxyphenyl)-ethane or simply tris(hydroxyphenyl)ethane benzotriazole (THPE-BZT), that can be copolymerized with engineering plastic monomers has been synthesized in three steps. The compound has two solid forms, a hydrate and an anhydrous form. Surprisingly, the hydrated form is more soluble in organic solvents such as acetone. Infrared spectra reveal that the hydrated form loses water of hydration upon heating and then forms a coplanar ring structure with a strong intramolecular hydrogen bond (IMHB) between one benzotriazole nitrogen and the adjacent phenolic OH. The NMR and UV-vis spectra in less polar organic solvents are also in accordance with the IMHB structure of THPE-BZT. In polar organic solvents such as DMSO and DMF and in MeOH/water at pH >10, the IMHB is disrupted, and the disrupted form exists in equilibrium with the planar or nondisrupted form. However, THPE-BZT copolymerized into low molecular weight polysulfones and polycarbonates retains the planar IMHB structure, as indicated by the UV absorption peak at ∼335 nm. These copolymers have greater UV stability than those without any THPE-BZT because of dissipation of photon energy by molecular vibrational relaxation in the planar form. Also, polysulfone copolymers containing THPE-BZT in the range of 0.25-1.5 mol % have greater UV light stability than polysulfone containing additive-type UVLS such as 2-(2′-hydroxy-5′-t-octylphenyl)benzotriazole (Cyasorb 5411). For polycarbonates, preliminary results show that copolymers of THPE-BZT have UV-light stability comparable to or only slightly less than that of polycarbonates blended with Cyasorb 5411 or THPE-BZT as additives. But the copolymers should escape the problems of surface "blooming" and leaching by extraction experienced by noncopolymerized UVLS additives. Thermogravimetric analysis indicates that THPE-BZT is more stable than the commercial benzotriazoles 2-(2′-hydroxy-5′-methylphenyl)benzotriazole (Tinuvin P), Cyasorb 5411, and the industrial standards2-[2′-hydroxy-3′,5′-bis (1-methyl-1-phenylethyl)phenyl]benzotriazole (Tinuvin 900)sand therefore might offer superior performance, even as an additive.
“…Under the same excitation condition, the fluorescence spectrum of 3 in DMSO differs substantially in intensity (Figure ), with almost no emission being detected. On the basis of earlier reports, this low fluorescence intensity of 3 can be explained in terms of the shielding effect of the bulky o -cumyl group. The intermolecular hydrogen bond to DMSO probably becomes weakened, favoring the conformers where the intramolecular hydrogen bond is intact, suggesting that 3 exhibits a higher photostability in DMSO solutions than 1 .…”
Section: Introductionmentioning
confidence: 51%
“…A dramatic ortho effect was observed previously in benzotriazole-based stabilizers . Substitution of the hydroxyphenyl ring at the 3‘-position with a sterically bulky group was reported to protect the intramolecular hydrogen bond from polar basic environments.…”
Section: Introductionmentioning
confidence: 70%
“…In a very interesting paper by Catalan and others,21a the role of the ESIPT mechanism in the photophysics of 1 was questioned. ESIPT, it was argued, could not be taking place, since the keto tautomer produced by such a mechanism should exhibit the characteristic Stokes-shifted fluorescence at room temperature, whereas none is observed.…”
Section: Discussionmentioning
confidence: 99%
“…It has been known for some time that the intramolecular hydrogen bond in benzotriazole-based stabilizers, such as 1 , can be disrupted in polar solvents. This has been demonstrated by various methods including FT-IR, , UV−vis, ,− fluorescence, ,,,− and H-NMR. , For example FT-IR provides compelling evidence for the equilibrium shown in Scheme in DMSO solution . It occurred to us that this disruption of the intramolecular hydrogen bond should have as a result the removal of the source of photostability of 1 and 2 .…”
2-(2′-Hydroxy-5′-methylphenyl)benzotriazole, 1, Tinuvin P, and o-hydroxybenzophenone, 2, are thought to achieve exceptional photostability through highly reversible deactivation associated with their intramolecular hydrogen bonds. Excimer laser excitation (10 mJ, 20 ns at 308 nm) of these molecules in argon-bubbled hexane solution at room temperature affords no discernible transient signals (absorption or emission) between 320 and 800 nm. In DMSO solution under the same conditions, however, strong transient absorptions in the visible (λ max 410 and 425 nm for 1 and 2, respectively) are observed. The transients responsible for the absorptions are quenched by oxygen and acid, and the spectra match well with the ground state difference spectra generated from the corresponding phenolate ions. When a bulky group is incorporated ortho to the hydroxyl function such as in 2-(2′-hydroxy-3′-cumyl-5′-(1,1,3,3-tetramethylbutyl)phenyl)benzotriazole, 3, Tinuvin 928, no appreciable transient absorption is observed even in DMSO solution. These results are consistent with the disruption of the intramolecular hydrogen bond of 1 and 2 (but not 3) as the result of intermolecular hydrogen bond formation with DMSO. The "dramatic" ortho effect implies that the bulky group adjacent to the hydroxyl function sterically shields the intramolecular hydrogen bond from disruption by polar basic environments.
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