Excitation-Dependent Dipoles Enable Bright-Dark State Transitions in Molecular Dimers

Organic molecules frequently possess inherent electrical asymmetry, known as permanent dipole moments, which influence how they interact with light and each other, yet are often overlooked in theoretical models. Matthew Freed from the University of Surrey and Dominic Rouse from the Universities of Glasgow and Manchester, along with colleagues, now demonstrate the significant role these permanent dipoles play in the behaviour of molecular dimers. Their research reveals that when molecules possess excitation-dependent permanent dipoles, it enables transitions between normally inaccessible ‘dark states’ within the dimer, effectively creating new pathways for energy transfer. This indirect coupling, arising from the interplay between permanent and fluctuating dipoles, generates remarkably stable dark states, potentially offering a route to improve the efficiency of light-harvesting technologies such as photovoltaic devices.

Dimer Permanent Dipoles Correct Optical Master Equation

Researchers have refined the standard model used to describe how light interacts with molecular pairs, known as dimers, by incorporating the effects of permanent electrical dipoles within the molecules. This refined model provides a more realistic representation of the dimer’s behaviour and allows for a more accurate prediction of its optical response. The approach involves mathematical transformations, beginning with a polaron transformation and followed by a Schrieffer-Wolff transformation, to derive expressions for corrected transition rates. Interestingly, the team found that even when accounting for permanent dipoles, perfectly dark states remain unaffected, but the permanent dipoles can influence the rates at which transitions occur between different energy levels. This work is important for understanding the optical properties of complex molecular systems and for developing more accurate models for energy transfer and light harvesting, potentially leading to improvements in technologies such as solar cells and organic electronics.

Dipole Interactions Govern Molecular Energy Transfer

Researchers have investigated how permanent dipole moments influence energy transfer within molecular systems, revealing a crucial role often overlooked in previous models. By constructing a detailed Hamiltonian, the team accounted for both the individual properties of each molecule and their spatial arrangement, allowing for a more accurate representation of the dimer’s behaviour. A key finding is that permanent dipoles enable transitions between bright and dark states, which is significant for applications such as light-harvesting. Furthermore, the study revealed that specific configurations, where molecules couple indirectly through their ground state, lead to particularly robust dark states. This enhanced robustness is a significant finding, potentially leading to improved designs for light-harvesting devices and other applications reliant on efficient energy transfer.

Dipole Coupling Creates Accessible Dark States

Researchers have discovered a surprising connection between permanent electrical dipoles within molecules and the creation of “dark states” in molecular dimers, with implications for improving the efficiency of light-harvesting technologies. These dark states are interesting because they represent excited states that do not readily emit light or participate in energy transfer, potentially allowing for more controlled energy flow within a material. The research demonstrates that when molecules possess permanent dipoles that change with excitation, it enables transitions between normally inaccessible dark and bright states. This occurs because permanent dipoles can indirectly couple the excited states of individual molecules, creating interference effects that lead to the formation of highly stable, localised dark states. Importantly, dark states can be engineered to be entirely localised on a single molecule within the dimer, while still allowing for energy transfer between the two excited states.

Permanent Dipoles Stabilise Molecular Dark States

This research demonstrates that permanent dipoles within molecular dimers significantly influence the behaviour of dark states, which are crucial for efficient light-harvesting and energy transfer. The team showed that when monomers possess excitation-dependent permanent dipoles, transitions between bright and dark states become possible, even in systems where dark states typically form. This occurs because permanent dipoles enable static coupling between monomers via the ground state, leading to interference effects and potentially creating more localised dark states. Furthermore, the study reveals that combining direct and indirect coupling, facilitated by these permanent dipoles, enhances the robustness of dark states against energetic disturbances. This increased robustness reduces the rate of energy recombination in noisy environments, ultimately improving the efficiency of light-harvesting and energy transport systems. Future research should investigate larger systems and incorporate a more rigorous modelling of environmental effects.