50-fs Photoinduced Intramolecular Charge Separation in Triphenylmethane Lactones

Abstract

Upon excitation of their structural subunits, phenolphthalein and malachite green lactone undergo ultrafast intramolecular charge separation with formation of a long-lived charge-transfer state (intramolecular “radical ion pair”). The kinetics of the primary charge separation in these molecules in aprotic solvents has been studied using femtosecond pump−probe spectroscopy. The phenol radical cation is formed by electron transfer within 50 fs after excitation of phenolphthalein to the S1 state. In malachite green lactone the products of charge separation are observed 150 fs after excitation to the S2 state. The results demonstrate that electron transfer in these molecules occurs faster than the time scale of inertial solvation dynamics. An intramolecular vibrational mode to promote charge separation is indicated.