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.

Singlet fission in covalently bound acene dimers in solution is driven by the interplay of excitonic and singlet correlated triplet 1 (TT) states with intermediate charge-transfer states, a process which depends sensitively on the solvent environment. We use high-level electronic structure methods to explore this singlet fission process in a linked tetracene dimer, with emphasis on the symmetrybreaking mechanism for the charge-transfer (CT) states induced by low-frequency antisymmetric vibrations and polar/polarizable solvents. A combination of the second-order algebraic diagrammatic construction (ADC(2)) and density functional theory/multireference configuration interaction (DFT/MRCI) methods are employed, along with a state-specific conductor-like screening model (COSMO) solvation model in the former case. This work quantifies, for the first time, an earlier mechanistic proposal [Alvertis et al., J. Am. Chem. Soc. 2019, 141, 17558] according to which solvent-induced symmetry breaking leads to a high-energy CT state which interacts with the correlated triplet state, resulting in singlet fission. An approximate assessment of the nonadiabatic interactions between the different electronic states underscores that the CT states are essential in facilitating the transition from the bright excitonic state to the 1 (TT) state leading to singlet fission. We show that several types of symmetry-breaking inter-and intrafragment vibrations play a crucial role in a concerted mechanism with the solvent environment and with the symmetric interfragment torsion, which tunes the admixture of excitonic and CT states. This offers a new perspective on how solvent-induced symmetry-breaking CT can be understood and how it cooperates with intramolecular mechanisms in singlet fission.

The ignition sensitivity of ammonium perchlorate (APC) and ammonium periodate (API) was analyzed in terms of crystalline structure, thermal and mechanical properties, and electronic structure using density functional theory (DFT) calculations. API is more rigid, with a higher bulk modulus (K) of 25.87 GPa compared with 21.42 GPa for APC. On the other hand, the shear moduli (G) are similar, 9.75 GPa for API and 9.42 GPa for APC. With higher bulk moduli and similar shear moduli, API will experience more shear than compression in situations such as friction. Also, API presents slightly more lateral deformation than APC, with Poisson’s ratio (ν) of 0.333, compared with 0.308 for APC, and contributes to a less consistent deformation in terms of the crystal lattice. A less stable lattice structure will contribute to greater ignition sensitivity of API compared with APC. The electronic density of states (DOS) analysis showed that API also has a more ignition sensitive profile with a band gap of a semiconductor type, Δg = 2.92 eV, while APC is a typical insulator with a band gap of Δg = 6.21 eV. The analysis of the electronic structure coupled with overall higher anisotropy (shown by calculated elastic constants) could induce ignition of API in a solid phase, whereas the greater stability of APC results in a multiphase ignition mechanism. Results shown here demonstrate important properties that influence the safe handling and use of energetic materials. The observed similarities in structural, mechanical, and thermodynamic properties of API and APC and the considerably large differences in electronic properties indicate that the latter is the key to the higher ignition sensitivity of API.

The design of materials with enhanced luminescence properties is a fast-developing field due to the potential applicability of these materials as light-emitting diodes or for bioimaging. A transparent way to...