Ultrafast singlet–triplet intersystem crossing gated by nonrelativistic nuclear dynamics

Transitions between electronic states of different spin multiplicities, known as intersystem crossing (ISC), are of fundamental importance in optoelectronics and materials chemistry. ISC has traditionally been viewed as directly driven by spin–orbit coupling (SOC), a relativistic interaction that enables spin-forbidden transitions. Recent studies suggest that vibronic interactions can modulate spin-orbit coupling and induce enhanced mixing between electronic spin states, enabling efficient ISC at specific nuclear configurations. Despite the recognized importance of vibration–spin-orbit coupling, direct manipulation of ISC dynamics through controlled nuclear motions remains challenging. Here, using ultrafast pulse-sequence spectroscopy, we accelerate ISC by nearly three orders of magnitude, reducing its timescale from 1.3 ns to 1.4 ± 0.2 ps. This dramatic enhancement is achieved by selectively tuning vibrational motions in the singlet excited state of the chalcone (E)-3-(4-(dimethylamino)phenyl)-1-(2-methoxyphenyl)prop-2-en-1-one (E-DMPO). Photoexcitation activates benzene-skeleton vibrations that drive the singlet excited state toward geometries near the singlet–triplet crossing region, resulting in rapid and efficient singlet-to-triplet population funneling. These results demonstrate that SOC, although an essential prerequisite for ISC, is not the sole determinant of ISC rates. Instead, nuclear dynamics—an intrinsic nonrelativistic factor—can critically regulate the accessibility of singlet–triplet crossing regions and thereby dominate the ISC kinetics. In other words, nuclear motion can effectively gate ISC. This insight suggests promising molecular- and vibrational-design strategies, beyond the conventional approach of enhancing SOC through heavy-atom incorporation.