Torsion-induced fluorescence quenching in excited-state intramolecular proton transfer (ESIPT) dyes

Fluorescence quenching behaviors of four known excited-state intramolecular proton transfer (ESIPT) molecules have been studied by semiempirical and ab initio calculations. The ESIPT compounds studied in this work are assorted into two sets depending on the N-containing ring structure (5- and 6-membered rings). It has been found that twisted intramolecular charge transfer (TICT) process in the excited keto state (K^*) after ESIPT, one of the possible quenching pathways of ESIPT fluorescence, is significantly influenced by the geometrical properties of intramolecular hydrogen (H) bond associated with the N-containing ring structure. The compounds with 5-membered ring have efficient ESIPT emission with large barrier to fluorescence-quenching TICT state, due to appropriate stabilization of planar K^* through intramolecular H bond. For the compounds with 6-membered ring, however, ESIPT emission is completely quenched due to significantly lowered barrier resulting from too short H-bond length. The effect of intramolecular H bond on the TICT reaction potential has been discussed in detail from the viewpoints of molecular structure and torsional motion, with the help of elaborate model compound studies.