Diradical approach toward organic near infrared dyes

Abstract

Organic near infrared (NIR) dyes have many technical applications in materials science and bio-technology. So far, two major strategies have been used for the molecular design: (1) extension of the π-conjugations length and (2) construction of a push-pull type structure. Although these methods gave many stable NIR absorbing and emitting dyes, synthesis of organic dyes with intensive absorption beyond 800nm or even 1000nm is still a challenging task for chemists. In this Digest, we introduce a new diradical approach for the design of organic NIR dyes, by careful tuning the diradical character of a series of quinoidal pro-aromatic compounds, including zethrenes, extended para-quinodimethanes and their analogues. Stable organic dyes with very intense absorption beyond 1000nm were achieved and the relationships between the molecular structure, diradical character and the optical property are discussed. Our studies demonstrate that π-conjugated molecules with a small to moderate diradical character could be good candidates of organic NIR dyes if properly stabilized.