50-fs photoinduced intramolecular charge separation in triphenylmethane lactones

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 S-1 state. In malachite green lactone the products of charge separation are observed 150 fs after excitation to the S-2 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.