Plasmon-Enhanced Exciton Delocalization in Squaraine-Type Molecular Aggregates

Abstract

Enlarging exciton coherence lengths in molecular aggregates is critical for enhancing the collective optical and transport properties of molecular thin film nanostructures or devices. We demonstrate that the exciton coherence length of squaraine aggregates can be increased from 10 to 24 molecular units at room temperature when preparing the aggregated thin film on a metallic rather than a dielectric substrate. Twodimensional electronic spectroscopy measurements reveal a much lower degree of inhomogeneous line broadening for aggregates on a gold film, pointing to a reduced disorder. The result is corroborated by simulations based on a Frenkel exciton model including exciton−plasmon coupling effects. The simulation shows that localized, energetically nearly resonant excitons on spatially well separated segments can be radiatively coupled via delocalized surface plasmon polariton modes at a planar molecule−gold interface. Such plasmon-enhanced delocalization of the exciton wave function is of high importance for improving the coherent transport properties of molecular aggregates on the nanoscale. Additionally, it may help tailor the collective optical response of organic materials for quantum optical applications.

Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes

“…Importantly, our work provides a firm basis for analyzing, understanding, and optimizing the aggregation properties of this class of molecules, specifically their ability to form chiral aggregate structures with large circular dichroism. The combination of large optical dipole moment, simple electronic structure, and reduced vibronic couplings of squaraine molecules results in exceptionally narrow exciton resonances, as recently demonstrated . This makes squaraine aggregates interesting candidates for applications in strong and ultrastrong coupling and, potentially, for realizing polariton condensates at room temperature, possibly with interesting chiral and nonlinear optical properties.…”

“…In the model, we directly take the frequencies and displacements of the two dominant modes at 155 and 573 cm –1 from the quantum-chemical simulations. We numerically integrate the Liouville–von Neumann equation in the Lindblad form to obtain the time-evolution of the density matrix of our system, nonperturbatively interacting with a pump and a delayed probe field. ,,− Relaxation and dephasing processes are described using Lindblad formalism. , Differential transmission spectra are obtained from the microscopic polarization, taken as the expectation value of the dipole operator for every waiting time (see the SI for details). The simulated pump–probe map, depicted in Figure b, shows the same features as the experimental map in Figure a.…”

“…The combination of large optical dipole moment, simple electronic structure, and reduced vibronic couplings of squaraine molecules results in exceptionally narrow exciton resonances, as recently demonstrated. 25 This makes squaraine aggregates interesting candidates for applications in strong and ultrastrong coupling and, potentially, for realizing polariton condensates at room temperature, possibly with interesting chiral and nonlinear optical properties. Specifically, we believe that the favorable optoelectronic properties of squaraines will be of crucial importance for analyzing the coherent energy exchange processes that are connected with strong light matter couplings in unprecedented detail.…”

“…Recently, exceptionally strong circular dichroism has been observed in thin films of squaraines . Their narrow absorption linewidths and large oscillator strengths make squaraine molecules also fascinating building blocks for studying (coherent) energy transport in dimer and trimer complexes − and for probing strong and ultrastrong coupling phenomena in aggregates coupled to plasmonic or photonic resonators. ,− …”

“…Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy (2DES) and quantum chemical simulations to investigate the vibronic wavepacket dynamics upon resonant photoexcitation in the prototypical quadrupolar squaraine molecule ProSQ-C16 , in the time domain. Our results largely confirm the energetic ordering of one- and two-photon allowed transitions separated by a ca.…”