Strong solvatochromic fluorescence from the intramolecular charge-transfer state created by excited-state intramolecular proton transfer.

In this work, we report a peculiar positive solvatochromism in the keto emission of the acceptor-substituted 2-(2'-hydroxyphenyl)benzoxazoles (HBO), which originates from the excited-state intramolecular proton transfer (ESIPT) followed by the intramolecular charge transfer (ICT) and subsequent solvent relaxation. This transient evolution of enhanced ICT characteristic triggered by ESIPT, which is first observed in this work, is responsible for the novel concept of a fast hyperpolarizability modulator as well as the unique solvatochromic behavior.