Ultrafast Intersystem Crossing in 1-Nitronaphthalene. An Experimental and Computational Study

Abstract

Previous studies have established that the major pathway for the first singlet excited state of 1-nitronaphthalene is intersystem crossing to the triplet manifold. In this contribution we present determinations of the decay of the S1 state of this compound in several solvents to establish the time scale of the multiplicity change as a function of the polarity and hydrogen-bonding ability of the solvent environment. The measurements were made with the femtosecond frequency up-conversion technique to follow the weak spontaneous molecular emission which precedes triplet formation. Our results show that in all environments the S1 lifetime is 100 fs or less, making 1-nitronaphthalene the organic compound with the fastest multiplicity change ever measured. We also show that the bathochromic shifts observed for the first absorption band imply changes in the relative energies of the singlet and triplet manifolds, which in turn manifest in a 2-fold increase of the fluorescence lifetime in cyclohexane compared with the polar solvents. Additionally, we performed excited-state calculations at the TD-DFT/ PBE0/6-311++G(d,p) level of theory with the PCM model for solvation. The TD-DFT theory identifies the presence of upper triplet states which can act as receiver states in this highly efficient photophysical pathway. Together, the experimental and theoretical results show that the dynamics of the S1 state in 1-nitronaphthalene represent an extreme manifestation of El-Sayed's rules due to a partial (n-pi*) character in the receiver triplets which are nearly isoenergetic with S1, determining a change in the molecular spin state within 100 fs.

Competing ultrafast intersystem crossing and internal conversion in the “channel 3” region of benzene

“…48,49 Benzene is distinct from these molecules in that it is a simple hydrocarbon with no obvious mechanism for such a large SOC. In fact, the SOC between S 1 ( 1 B 2u ) and T 2 ( 3 E 1u ) at the Franck-Condon (FC) geometry is zero by symmetry, while the S 1 ( 1 B 2u )/T 1 ( 3 B 1u ) coupling is tiny leading to the relatively long non-radiative lifetimes of the vibrational states below the onset of "channel 3".…”

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.…”

Mechanism of Ultrafast Intersystem Crossing in 2‐Nitronaphthalene

“…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.…”

“…However, for other NPAH derivatives investigated in the same study5 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 solvents6 and sub‐ps‐resolved transient absorption spectroscopy experiments7 reassured that the decay of S 1 occurs within 100 fs and it was established that relaxation within the triplet manifold (T n –T 1 ) proceeded on a time scale of 1–16 ps.…”