Molecular versus Excitonic Disorder in Individual Artificial Light-Harvesting Systems

Abstract

Natural light-harvesting antennae employ a dense array of chromophores to optimize energy transport via the formation of delocalized excited states (excitons), which are critically sensitive to spatio-energetic variations of the molecular structure. Identifying the origin and impact of such variations is highly desirable for understanding and predicting functional properties yet hard to achieve due to averaging of many overlapping responses from individual systems. Here, we overcome this problem by measuring the heterogeneity of synthetic analogues of natural antennae–self-assembled molecular nanotubes–by two complementary approaches: single-nanotube photoluminescence spectroscopy and ultrafast 2D correlation. We demonstrate remarkable homogeneity of the nanotube ensemble and reveal that ultrafast (∼50 fs) modulation of the exciton frequencies governs spectral broadening. Using multiscale exciton modeling, we show that the dominance of homogeneous broadening at the exciton level results from exchange narrowing of strong static disorder found for individual molecules within the nanotube. The detailed characterization of static and dynamic disorder at the exciton as well as the molecular level presented here opens new avenues in analyzing and predicting dynamic exciton properties, such as excitation energy transport.

Nanotube Template-Directed Formation of Strongly Coupled Dye Aggregates with Tunable Exciton Fluorescence Controlled by Switching between J- and H-Type Electronic Coupling

“…The coherence (delocalization) length is often used to describe the average number of coupled monomers in each aggregate over which excitons can be coherently delocalized . However, so far, our understanding of exciton dynamics in coupled J- and H-aggregates of dyes is very limited. , In recent years, there has been increasing interest in the development of strongly coupled dye aggregates for understanding their exciton properties and exploring their potential use in organic solar cells, functional mimics for natural light-harvesting systems, and other photo-functional materials . Recently, researchers have found that strongly coupled dye J-aggregates can potentially be used for long-range excitation energy transfer. − …”

Multiscale modeling of molecular structure and optical properties of complex supramolecular aggregates

“…38 The exciton delocalization size in these tubular aggregates generally is known to be very large; 66 the disorder distributions found in our study lead to delocalization over 450 molecules. 67 Exciton states are obtained by numerical diagonalization of the Hamiltonian. Homogeneous spectra are obtained from the ideal-cylindrically symmetric-structures assuming the same excitation energies for all molecules.…”

Vibronic coherences in light harvesting nanotubes: unravelling the role of dark states

“…[19][20][21] The possibility of controlling the morphology of the nanotube and therefore the exciton migration is a challenging goal, still limited by the difficulty to model the structure and the dynamics of such a complex system. [22][23][24][25][26][27][28][29][30][31] Despite the amount of research work carried out in the last two decades on these LHNs, 19,20,[32][33][34][35][36][37][38][39][40][41] there are still fundamental open questions related to exciton properties, mostly due to the intrinsic instability of these materials, 11,42 which prevents a thorough spectroscopic characterization.…”