Electronic Spectroscopy of 2‑Phenyl-1,3,2-benzodioxaborole and Its Derivatives: Important Building Blocks of Covalent Organic Frameworks

Aryl boronate esters, such as 2-phenyl-1,3,2-benzodioxaborole (1), are important components in the formation of a variety of covalent organic frameworks. The addition of substituents on the aromatic rings of aryl boronate esters has the potential to modify the structure, reactivity, and electronic properties of the resulting materials, and so, it is useful to understand at a more fundamental level the properties of these important compounds. Experimental measurements and computational investigations are presented herein that provide insight regarding the structural and electronic properties of parent aryl boronate ester 1 as well as three substituted derivatives: 2-(o-tolyl)-1,3,2-benzodioxaborole (2), 2-(2,6-dimethylphenyl)-1,3,2-benzodioxaborole (3), and 2-(4-(tert-butyl)­phenyl)-1,3,2-benzodioxaborole (4). Electronic spectroscopy combined with excited-state calculations reveal two closely spaced electronic states, S1 and S2, which appear to have excitation primarily localized on the aromatic system of the phenyl substituent or the catecholborane moiety, respectively. Interestingly, the ortho-dimethyl derivative (3) shows a significantly red-shifted electronic origin with an extensive vibronic progression of a low-frequency torsional motion about the C–B bond. Franck–Condon calculations on the ab initio determined ground- and excited-state potentials very accurately reproduce this spectrum, confirming the nonplanar ground state of this compound.