Excited-State Dynamics in Nitro-Naphthalene Derivatives: Intersystem Crossing to the Triplet Manifold in Hundreds of Femtoseconds

Abstract

Femtosecond transient absorption experiments and density functional calculations are presented for 2-methyl-1-nitronaphthalene, 2-nitronaphthalene, and 1-nitronaphthalene in cyclohexane and acetonitrile solutions. Excitation of 2-methyl-1-nitronaphthalene at 340 nm populates the Franck-Condon singlet state, which bifurcates into two barrierless decay channels with sub-200-fs lifetimes. The primary decay channel connects the Franck-Condon singlet excited state with a receiver triplet state, whereas the second, minor channel involves conformational relaxation to populate an intramolecular charge-transfer state, as previously reported for 1-nitronaphthalene (J. Chem. Phys. 2009, 113, 224518). Conversely, the experimental and computational data for 2-nitronaphthalene shows that almost the entire Franck-Condon singlet excited-state population intersystem crosses to the triplet state in less than 200 fs due to a sizable energy barrier of ca. 5 kcal/mol that must be surmounted to access the intramolecular charge-transfer state. Our results lend support to the idea that the probability of population transfer to the triplet manifold in these nitronaphthalene derivatives is controlled not only by the small energy gap between the Franck-Condon singlet excited state and the receiver triplet state but also by the region of configuration space sampled in the singlet excited-state potential energy surface at the time of excitation. It is proposed that the ultrafast intersystem crossing dynamics in these nitronaphthalene molecules most likely occurs between nonequilibrated excited states in the strongly nonadiabatic regime.

Mechanism of Ultrafast Intersystem Crossing in 2‐Nitronaphthalene

“…After photoexcitation to a S CT (ππ*) state, most of the excited‐state population is transferred to a S LE ( n π*) state through IC with a sub‐100 fs time constant τ S . This timescale is also found experimentally,3, 4 but it was attributed to ISC. In contrast, our simulations reveal that ISC in 2NN takes place on a longer timescale ( τ ISC =0.7 ps) and proceeds through two distinct reaction pathways S LE ( n π*)→T LE (ππ*) (major pathway) and S CT (ππ*)→T LE (π’π*) (minor pathway).…”

“…In contrast, ISC was expected to be considerably slower when the SOCs are very small, as is the case in organic molecules composed solely of light atoms of the first period. 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.…”

“…This is the first excited‐state dynamics study on a NN derivative using ab initio nonadiabatic molecular dynamics including singlet and triplet states on the same footing. The system 2NN has been chosen as a prototype, as this molecule seems to be the most efficient in undergoing ISC without the presence of dissociative photodegration competing pathways, as in 1NN or 2M1NN 3, 4, 29. Our results demonstrate that ISC happens with a characteristic time of approximaely 0.7 ps supporting a mechanism, that, while not on a (sub‐) 100 fs time scale, is indeed ultrafast for an organic chromophore.…”

Conformational Control in the Population of the Triplet State and Photoreactivity of Nitronaphthalene Derivatives

“…62−65 Excited-state calculations suggest that conformational relaxation of the S 1 state populates an intramolecular charge-transfer state S diss (CT). 31,34 The conformational relaxation in the S 1 state is due primarily to the rotation of the nitro group toward a close-to-perpendicular torsion angle. This S diss (CT) state is thought to eventually lead to the dissociation of the NNDs to form the ArO and NO radicals.…”

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…Examples of molecules where ISC is important range from aldehydes [37] and small aromatic compounds like benzene, [38] naphthalene, anthracene, and their carbonylic derivatives [39][40][41][42][43][44][45][46][47][48][49][50][51] to nitrocompounds. [43,[52][53][54][55][56][57][58][59][60][61] Furthermore, ISC has been reported for thio-substituted, [62][63][64][65][66][67][68] aza-substituted, [69] bromo-substituted, [70] and canonical nucleobases. [71][72][73] From a theoretical point of view, ISC can be explained by the interaction of states of different multiplicity by spin-orbit coupling (SOC), which is a relativistic effect.…”