Development of a Class of Easily Scalable, Electron-Deficient, Core-Extended Benzo-Fused Azadipyrromethene Derivatives (“MB-DIPY”)

We have developed a new synthetic
strategy for the preparation
of a series of isoindolin-1-imines and isoindolin-1-ones from aromatic
ketones and phthalonitrile. Self-condensation reactions of these isoindolin-1-imines
led to the formation of a novel class of benzo-fused, highly electron-deficient
core-extended azadipyrromethene chromophores (“MB-DIPY”).
The influence of temperature, catalyst, and the template ions on the
self-condensation reaction rate, yield, and stereoselectivity was
examined in detail. New chromophores (sodium, zinc, and metal-free
compounds) were characterized by NMR, UV–vis, fluorescence,
high-resolution mass spectroscopies, and in many cases, X-ray crystallography.
Their redox properties were probed by electrochemical and spectroelectrochemical
approaches that revealed the remarkable electron-accepting nature
of the new systems. Stepwise one- and two-electron reduction of the
new MB-DIPYs and their zinc complexes was investigated by spectroscopic
and spectroelectrochemical methods. Both one- and two-electron reduced
forms of all zinc complexes studied have strong absorption in the
near-infrared region up to ∼1200 nm. Unusual spectroscopic
and electrochemical properties of these dyes were correlated with
their electronic structures and excited-state natures predicted by
density functional theory (DFT) and time-dependent DFT calculations.
Despite some structural similarities with well-known aza-BODIPYs,
the new MB-DIPYs differ remarkably from them in spectroscopic and
redox properties.