Planarized and Twisted Intramolecular Charge Transfer: A Concept for Fluorophores Showing Two Independent Rotations in Excited State.

TICT (twisted intramolecular charge transfer) compounds are characterized by showing a rotation around a single bond in the excited state: starting from an almost planar geometry in the ground state, a twisted system is formed in the electronic excited state. The previously reported PLICT (planarized intramolecular charge transfer) compounds show inverse behavior: starting from a twisted geometry in the electronic ground state, a planarized system is formed in the excited state by rotation around a single bond. Here, a concept for planarized and twisted intramolecular charge transfer (PLATICT) states is presented which amalgamates both (TICT and PLICT) effects. Due to an intramolecular charge transfer, both a twisting around one single bond and a planarization around another one occurs. In sum, the PLATICT system shows two independent rotations around different axes in the excited state. By means of quantum chemical calculations (TD-cam-B3LYP and CC2) and experimental studies, it is demonstrated that N-aryl-substituted 1-aminoindoles are able to form photoinduced PLATICT states. In the fluorescence spectra of N-aryl-substituted 1-aminoindoles with a methoxycarbonyl or a cyano group as substituent in the aryl ring, very large Stokes shifts (ca. 18 000 cm-1 ; >250 nm) are observed. The two independent rotations in the excited state, the very large Stokes shifts and their easy availability starting from indoline, make them very attractive for use as optical switches and motors in various fields of chemistry.