Multiple mode exciton-vibrational coupling in H-aggregates: Synergistic enhancement of the quantum yield

Abstract

The impact of exciton-vibrational coupling involving fast and slow vibrational modes on absorption and emission in molecular H-aggregates is investigated using a multimode Holstein Hamiltonian. For H-aggregates composed of rigid molecules the radiative decay rate is rigorously zero due to the asymmetry of the lowest energy exciton. Increasing the Huang-Rhys factors of the coupled modes results in an increase of the radiative efficiency through increased sideband emission. Coupling to a spectrally unresolved slow mode leads to an apparent increase in the 0-0 intensity of the vibronic progression of the fast mode, thereby mimicking a distribution of molecular transition frequencies (diagonal disorder). In the intermediate coupling regime, the radiative efficiency of the aggregate exceeds the sum of the fast-mode-only and slow-mode-only radiative efficiencies. The mechanism underlying the synergistic enhancement of the radiative yield is similar to that which causes spectral splitting in the absorption spectrum of quaterthiophene crystals. [L. Silvestri et al., J. Chem. Phys. 130, 234701 (2009)]. The results here qualitatively account for the robust emission efficiency of hexaphenyl microcrystallites where the slow torsional motion involving the interphenyl dihedral angle in conjunction with the fast ring breathing mode conspire to enhance the quantum yield.

Improving the Quantum Yields of Perylene Diimide Aggregates by Increasing Molecular Hydrophobicity in Polar Media

“…As was mentioned previously, the results of the quantum yields in these PDI species initially seem to contrast the conventional wisdom of aggregation in organic conjugated molecules. The emission in H‐type aggregates is symmetry forbidden under perfect, vibrationless systems while perfect J‐type aggregates show an enhancement of quantum yields . In order to adequately describe these systems, we will discuss the underlying mechanisms for each PDI separately.…”

“…For some systems, H‐type aggregate emission is enhanced . An increase in the Huang‐Rhys factors can also improve H‐type aggregate emission . Small changes in the intermolecular interactions will affect the symmetry and, therefore, the quantum yield of these systems .…”

“…[73] An increase in the Huang-Rhys factors can also improve H-typea ggregate emission. [63] Small changes in the intermoleculari nteractions will affect the symmetry and, therefore, the quantum yield of these systems. [58,60] The increase in solvent polarity will increasem olecular confinement, which minimizes relaxation pathways in the aggregates.…”

Excitons in Carbonic Nanostructures

“…Moreover, some transformations of spectra may occur due to the forbidden character of 0-0 transition, resulting in further apparent bathochromic shifts [226,228]. The presence of strong electron-vibrational coupling may also increase the fluorescence quantum yield of H-aggregates [234].…”

“…However, as it is discussed in Section 4.2, their bright fluorescence emission is observed in many cases, and the mechanisms that remove these restrictions are many. They include non-ideal relative location and orientation of coupled chromophores, intramolecular vibrations and their coupling, and the coupling with lattice vibrations (phonons) [227,234].…”