Non-Kasha fluorescence of pyrene emerges from a dynamic equilibrium between excited states

Abstract

Pyrene fluorescence after a high-energy electronic excitation exhibits a prominent band shoulder not present after excitation at low energies. The standard assignment of this shoulder as a non-Kasha emission from the second-excited state (S2) has been recently questioned. To elucidate this issue, we simulated the fluorescence of pyrene using two different theoretical approaches based on vertical convolution and nonadiabatic dynamics with nuclear ensembles. To conduct the necessary nonadiabatic dynamics simulations with high-lying electronic states and deal with fluorescence timescales of about 100 ns of this large molecule, we developed new computational protocols. The results from both approaches confirm that the band shoulder is, in fact, due to S2 emission. We show that the non-Kasha behavior is a dynamic-equilibrium effect not caused by a metastable S2 minimum. However, it requires considerable vibrational energy, which can only be achieved in collisionless regimes after transitions into highly excited states. This strict condition explains why the S2 emission was not observed in some experiments.

Surface hopping modeling of charge and energy transfer in active environments

“…Large systems, like those including the environment, may be especially prone to this problem, which can be addressed with different techniques. 6,[35][36][37] In general, conventional surface hopping will perform well if 1) the light pulse is shorter than the excited-state dynamics, 2) the nuclei move fast like quasi-classical particles, 3) there are no significant recoherences between nuclear wave packets, and 4) non-local effects can be neglected.…”

Surface hopping modeling of charge and energy transfer in complex environments

“…Large systems, like those including the environment, may be especially prone to this problem, which can be addressed with different techniques. 6,33,34 In general, conventional surface hopping will perform well if 1) the light pulse is shorter than the excited-state dynamics, 2) the nuclei move fast like quasi-classical particles, 3) there are no significant recoherences between nuclear wave packets, and 4) non-local effects can be neglected. However, even if these conditions are not satisfied, specific surface hopping implementations may be available to extend its validity domain, for instance, including fast nuclear degrees of freedom in the quantum partition 35 or explicitly accounting for the electromagnetic field in the Hamiltonian.…”

“…In a vacuum, it has a marked non-Kasha fluorescence due to thermal activation of the S2 state. 6 This fluorescence disappears in high gas concentrations thanks to vibrational cooling to the environment. As a final example, take acceptor donor complexes at organic heterojunctions composed of thiophene oligomers (electron donors) and fullerenes (electron acceptors).…”

Newton-X Platform: New Software Developments for Surface Hopping and Nuclear Ensembles

“…The size-extensivity problem is already noticeable for medium-sized molecules. In ref ( 49 ), for example, reducing the kinetic energy available for back hopping was imperative to adequately describe the excited-state dynamics of pyrene (26 atoms) after excitation into S 7 .…”

“…Note that the sum over the states K in eq 33 implies that non-Kasha emissions 81 are also considered. See ref ( 49 ) for an example of simulations including fluorescence from the first and second excited states of pyrene.…”