Synthesis of a new type of dibenzopyrromethene–boron complex with near-infrared absorption property

A new type of π-extended 4-bora-3a,4a-diaza-s-indacene (BODIPY dye), bis(isoindole)-derived benzo[1,3,2]oxazaborinine 1, has been synthesized by seven-step procedure from 2-methoxybenzoic acid methyl ester. The benzannulation of the pyrrole ring and the formation of a structurally strained intramolecular B–O ring enable the dye to absorb near-infrared light at ca. 750 nm with a molecular extinction coefficient (ε) of ca 8.3 × 104 M−1 cm−1 in THF. The absorption properties are discussed on the basis of DFT calculations. Interestingly, a film of 5,5-dihexyloxy derivative 1b, which was fabricated by a spin-coating procedure on a glass plate, exhibited a dramatic bathochromic shift of absorbance as compared to the solution, with λmax of 922 nm.