Microreview: Pyrrol‐3‐yl squaraines (including indol‐3‐yl squaraines)

Pyrrol‐3‐yl squaraine dyes are prepared by the condensation of 3,4‐dihydroxycyclobut‐3‐ene‐1,2‐dione with two‐molar equivalence of a 2,5‐disubstituted pyrrole possessing a free β‐position, or substituted indoles with a similarly free β‐position. From the first reported syntheses of two indol‐3‐yl squaraines in 1966, numerous pyrrol‐3‐yl squaraines (including indol‐3‐yl squaraines) have been reported in both the scientific and patent literatures. This microreview highlights the synthesis, history, spectroscopy, and applications of pyrrol‐3‐yl squaraines from their first preparation to the present date.