Investigation of dibenzoboroles having β-electrons: Toward a new type of two-dimensional nLO molecular switch?

In the present investigation, a new sequence has been presented for the reversible switching and modulation of nonlinear optical properties (NLO). The static first hyperpolarizabilities (β) of four dibenzoborole derivatives have been computed by DFT. The β values of these dibenzoborole derivatives have shown a significant increase with the attachment of F^- and/or one electron reduction. For example, 5-fluoro-5-(2, 4, 6-triisopropylphenyl)-2, 8-dimethoxy-3, 7-bithienyl-5H-dibenzo[d, b] borole ion (3·F^-) and 5-fluoro-5-phenyl-3, 7-bis-dinitrothienyl-5H-dibenzo[d, b]bor- ole ion (4·F^-) both showed β values as large as 64 × 10-30and 272 × 10-30esu, that is, about 12 times and 4 times larger than their counterparts 5-(2, 4, 6-triisopropylphenyl)-2, 8-dimethoxy-3, 7-bithienyl-5H-dibenzo[d, b]borole (3) and 5-phenyl-3, 7-bis-dinitrothienyl-5H- dibenzo[d, b]-borole (4) (without F^-), respectively. Similarly, systems 3^Red and 4^Red(one electron reduced) also showed 47 times and 15 times larger β values than their neutral forms, respectively. Interestingly, this NLO switching is two-dimensional in characteristic in which large off-diagonal hyperpolarizability tensors can be related to the charge-transfer transitions that are polarized perpendicular to the molecular dipolar axis. Density of states (DOS) and frontier molecular orbital (FMO) analysis show that the binding of F- at a boron atom and/or one electron reduction process turn off the pπ→*conjugation of vacant p-orbital of boron atom in LUMOs, resulting in a higher extent of perpendicular charge transfer (CT) and lager β values. The present investigation reveals a new idea and different means for multifunctional use of the present dibenzoborole class, especially (already synthesized) 3·F as two-dimensional NLO molecular switch with Cartesian nonlinear anisotropy as large as t]) 10.48.