Ultrafast Intersystem Crossing in Xanthone from Wavepacket Dynamics

Most aromatic ketones containing first-row elements undergo unexpectedly fast intersystem crossing in a few tens of picoseconds and a quantum yield close to unity. Among them, xanthone (9H-xanthen-9-one) possesses one of the fastest singlet-triplet rates of only similar to 1.5 ps. The exact mechanism of this unusually fast transition is still under debate. Here, we perform wavepacket dynamics of the photochemistry of xanthone in the gas phase and in polar solvents. We show that xanthone follows El-Sayed's rule for intersystem crossing. From the second singlet excited state, the mechanism is sequential: (i) an internal conversion between singlets (1)pi pi* -> (1)n pi* (85 fs), (ii) an intersystem crossing (1)n pi* -> (3)pi pi* (2.0 ps), and (iii) an internal conversion between triplets (3)pi pi* -> (3)n pi* (602 fs). Each transfer finds its origin in a barrierless access to electronic state intersections. These intersections are close to minimum energy structures, allowing for efficient transitions from the initial singlet state to the triplets.