A set of monodisperse bent donor–acceptor–donor-type conjugated borazine oligomers, BnNn+1 (n=1–4), incorporating electron-rich triarylamine donor and electron-deficient triarylborane acceptor units has been prepared through an iterative synthetic approach that takes advantage of highly selective silicon–boron and tin–boron exchange reactions. The effect of chain elongation on the electrochemical, one- and two-photon properties and excited-state photodynamics has been investigated. Strong intramolecular charge transfer (ICT) from the arylamine donors to boryl-centered acceptor sites results in emissions with high quantum yields (Φ_fl>0.5) in the range of 400–500 nm. Solvatochromic effects lead to solvent shifts as large as ∼70 nm for the shortest member (n=1) and gradually decrease with chain elongation. The oligomers exhibit strong two-photon absorption (2PA) in the visible spectral region with 2PA cross sections as large as 1410 GM (n=4), and broadband excited-state absorption (ESA) attributed to long-lived singlet–singlet and radical cation/anion absorption. The excited-state dynamics also show sensitivity to the solvent environment. Electrochemical observations and DFT calculations (B3LYP/6-31G*) reveal spatially separated HOMO and LUMO levels resulting in highly fluorescent oligomers with strong ICT character. The BnNn+1 oligomers have been used to demonstrate the detection of cyanide anions with association constants of log K>7.

Six anionic pentamethine dyes with different 2,2-difluoro-4-aryl-1,3,2(2 H)-dioxaborin-6-yl termini were synthesized and isolated as tetra-n-octylammonium salts with a variety of aryl groups appended to increase conjugation beyond the dioxaborine termini. The increased conjugation was expected to decrease the energy of the lowest-lying excited state, and increase the transition dipole moment linking this state to the ground state, which would be anticipated to result in an increase in the real part of the third-order polarizability, Re(γ). UV/Vis-NIR absorption spectroscopy indicates that the absorption maxima in DMSO vary from 691 to 761 nm, with the longest wavelength transitions observed for a derivative where the aryl group is 4-nitrophenyl. Closed-aperture Z-scan measurements at 1.3 μm in DMSO indicate that Re(γ) varies from −2.9×10⁻³³ to −5.4×10⁻³³ esu in these systems. The largest magnitude of Re(γ) was observed for a dye with E-4-styrylphenyl aryl groups. This result can be rationalized using a two-state expression which relates Re(γ) to the energy and transition dipole moment of the transition from the ground state to the lowest-lying excited state. A nonamethine analogue of this compound was also synthesized and exhibits a slightly larger Re(γ) with respect to a previously reported bis(dioxaborine)-terminated nonamethine. The extension of conjugation beyond the dioxaborine termini seems to result in an overall increase in Re(γ). However, the effects are smaller than those found by increasing conjugation in the polymethine bridge due to reduced participation of terminal groups in the HOMO.