Abstract
Nitronaphthalene derivatives efficiently populate their electronically excited triplet states upon photoexcitation through ultrafast intersystem crossing (ISC). Despite having been studied extensively by time‐resolved spectroscopy, the reasons behind their ultrafast ISC remain unknown. Herein, we present the first ab initio nonadiabatic molecular dynamics study of a nitronaphthalene derivative, 2‐nitronaphthalene, including singlet and triplet states. We find that there are two distinct ISC reaction pathways involving different electronic states at distinct nuclear configurations. The high ISC efficiency is explained by the very small electronic and nuclear alterations that the chromophore needs to undergo during the singlet–triplet transition in the dominating ISC pathway after initial dynamics in the singlet manifold. The insights gained in this work are expected to shed new light on the photochemistry of other nitro polycyclic aromatic hydrocarbons that exhibit ultrafast intersystem crossing.
Highlights
The rate of intersystem crossing (ISC), that is, the non-radiative transfer between electronic states of different spin multiplicity, is directly related to the size of the spin-orbit coupling (SOC) between the states involved in the transition
We found that the initial population of the S2 state is transferred mainly to the S1 state, from which part of the trajectories undergo ISC to triplet states Tn (n = 2–6), before they relax by internal conversion (IC) within the triplet manifold to the T1 state
From the S1, the system undergoes ISC to the triplet manifold with a time constant of tISC = 0.7 ps before it relaxes to the T1 within approximately 150 fs. Comparing these time constants to experimental results, we find that our tS and tISC can be attributed to the time constants obtained for the two initial processes in the transient absorption spectroscopy experiments, that is, t1 = 0.11Æ0.05 and t2 = 2.1Æ0.1 ps for 2NN in cyclohexane.[3]
Summary
The rate of intersystem crossing (ISC), that is, the non-radiative transfer between electronic states of different spin multiplicity, is directly related to the size of the spin-orbit coupling (SOC) between the states involved in the transition. For certain classes of organic molecules, such as nitro polycyclic aromatic hydrocarbons (NPAHs)[3,4,5,6,7,8,9,10,11,12,13,14] and the closely related nitrobenzene derivatives,[15,16,17,18,19,20,21,22,23,24,25] ISC has been measured to occur in an ultrafast sub-picosecond timescale, challenging this paradigm. For other NPAH derivatives investigated in the same study[5] the fast sub-100 fs decay time was attributed to a conformational relaxation in the initially populated excited state involving the re-orientation of the nitro group. Later studies on 1NN using different solvents[6] and sub-ps-resolved transient absorption spectroscopy experiments[7] reassured that the decay of S1 occurs within 100 fs and it was established that relaxation within the triplet manifold (Tn–T1) proceeded on a time scale of 1–16 ps
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
