On the molecular conformation of bisaromatic systems the case of 2-phenyl-2H-benzotriazoles

2-Phenyl-2H-benzotriazole exhibits a planar molecular conformation both in its ground electronic state (S₀) and its first excited singlet (S₁) and triplet state (T₁). However, introducing one or two methyl groups in the ortho positions of the phenyl ring causes the aromatic systems in the compound to lose their coplanarity in both S₀ and T₁ electronic states. On the other hand, 2-(2-methylphenyl)-2H-benzotriazole regains such coplanarity in its first excited singlet state S₁, giving rise to population inversion that could be used to generate stimulated radiation around 350 nm.

As shown in this work, the effectiveness of the ISC process in these compounds is markedly dependent on the twisting angle, θ, of the structure; accordingly, ISC occurs to a negligible extent in a planar compound such as 2-phenyl-2H-benzotriazole, where θ = 0°. This evidence supports the assumption that planar molecular forms of the TIN-P photoprotectors are more photostable than non-planar ones due to the non effective generation via ISC of their triplet states.