Computational Modeling of Exciton Localization in Self-Assembled Perylene Helices: Effects of Thermal Motion and Aggregate Size

Abstract

Effects of aggregation on excited states properties in solution of perylene diimide self-assembled helixlike structures of different size are investigated by means of first principle Density Functional Theory (DFT), Time-Dependent DFT (TD-DFT) and classical Molecular Dynamic (MD) simulations. Excited state analysis based on the 1-particle transition density matrices is then used to study the exciton nature and its delocalization as a function of the thermal motion and aggregate size. Overall the results point to a rather small delocalization of the Frenkel excitonic state even in large aggregates also related to a concerted motion of blocks of 4 monomers along the MD trajectories. Although dynamic effects do not remarkably affect the calculated position and shape of the absorption spectrum, they cause the appearance of several low-energy states of charge transfer character, and hence of weak intensity (dark states), that might be populated along the ultrafast exciton relaxation process potentially influencing the charge separation processes in PDI-sensitized photoactive heterointerfaces.

Aggregates of polar dyes: beyond the exciton model

“…Modest disorder effects are expected in crystalline systems, but they are important for aggregates in solution. Molecular dynamics and more generally multiscale approaches are powerful tools for addressing disorder that occurs in aggregates in solution 49,50 as related to the conformational motion of the aggregate itself, as well as to the fluctuating electric field associated with polar solvation. However, treating electronic and vibrational degrees of freedom using a truly non-adiabatic approach is challenging for large disordered systems, and requires the development of new approximation techniques.…”

Insight into the self-assembly of water-soluble perylene bisimide derivatives through a combined computational and experimental approach

“…PBI functionalised with L-DOPA at the imide position, 5 which has two additional hydroxyl groups that can hydrogen bond per L-DOPA substituent, a more systematic computational approach might be beneficial. An alternative might be sampling structures from molecular dynamics (MD), a strategy recently employed by Segalina et al, 36 but this might require many more TD-DFT calculations. The barriers between the different classes of aggregates also should be low enough for switching between them to be observable during a MD run.…”

“…4,5,12 Recently Segalina and co-workers computationally studied aggregates of up to seven units for a PBI functionalised with quaternary nitrogen groups. 36 Here, we employ the computational approach sketched out above in combination with experimental spectroscopy, neutron scattering and rheology to understand the changes upon gelation of amino acid functionalised PBIs using PBI-A, 3 PBI functionalised with alanine at the imide nitrogen position (see Scheme 1), as an example. In contrast to most previous work, as discussed above, we explicitly model PBI-A aggregates and consider aggregates beyond a stacked dimer.…”

Optical properties of photodynamic therapy drugs in different environments: the paradigmatic case of temoporfin

“…3 and ESI †) due to the significant destabilization of the ground state rather than the excited state, as is common for similar p-conjugated systems. 42,44,[101][102][103] Although the influence of the rotation of the peripheral substituents on the absorption spectral maxima is expected to be marginal, it may, nonetheless, induce a broadening of the absorption and emission bands, and more notably, may have a key role in inducing thermal quenching of the luminescence. While the possibility to take into account the out-of-plane deformation effects via a reasonable sampling of the conformational space is well established and has been thoroughly demonstrated in similar systems, 36,42,66,101 the effects of the peripheral dihedral rotations require much deeper attention, due to their strong non-harmonic behaviour.…”