Photphysical properties of tetrahydroisoquinoline: evidence for flipped intramolecular charge transfer (FICT) emission

Photphysical properties of tetrahydroisoquinoline (THIQ) have been studied in various solvents. It has been found that in aprotic solvents a secondary amine THIQ shows a charge transfer emission band in addition to the expected locally excited xylene type emission band, though there is no corresponding charge transfer band in the absorption spectrum. It is ascribed to a charge transfer process in the excited state after flipping the N-atom across the average molecular plane. The ratio of FICT emission to locally excited xylene type emission is sensitive to the pH of the solution. This is due to the blocking of the lone pair of the nitrogen atom of THIQ by H+.