Synthesis, characterization and optical properties of π-conjugated systems incorporating closo-dodecaborate clusters: new potential candidates for two-photon absorption processes

Abstract

Non-centrosymmetric π-conjugated systems incorporating closo-dodecaborate clusters, [NC–C₆H₄–C(H)N(H)–B₁₂H₁₁]⁻ (2), [NC–C₆H₄–C(H)C(H)–C₆H₄–C(H)N(H)–B₁₂H₁₁]⁻ (3), and [NC–C₆H₄–C(H)C(H)–C₆H₄–C(H)C(H)–C₆H₄–C(H)N(H)–B₁₂H₁₁]⁻ (4) have been synthesized by reaction of the monoamino derivative of B₁₂, [B₁₂H₁₁NH₃]⁻ (1), with various arylaldehydes, R–C₆H₄–CHO. These Schiff base-like compounds were fully characterized by multinuclear NMR spectroscopy and mass spectrometry. In order to evaluate these boron rich π-systems as potential materials for two-photon absorption (TPA) processes, UV linear absorption curves were recorded for 3 and 4, and comparatively studied with those of the boron-free π-systems NC–C₆H₄–C(H)N–CH₃ (5) and NC–C₆H₄–C(H)C(H)–C₆H₄–C(H)N–CH₃ (6). The donor effect of the boron cluster was evidenced by a shift to the lower energy of the absorption band in the spectra of systems incorporating B₁₂. The two photon absorption (TPA) spectrum of compound 3, obtained by the up-conversion method, shows a resonance at 720 nm with a cross-section σ_TPA of 35 × 10⁻⁵⁰ cm⁴ s photon⁻¹ molecule⁻¹. This value suggests the potential of B₁₂ clusters to be used as new donor groups for the synthesis of non-linear materials.