Effect of Disorder on Ultrafast Exciton Dynamics Probed by Single Molecule Spectroscopy

Abstract

We present a single-molecule study unraveling the effect of static disorder on the vibrational-assisted ultrafast exciton dynamics in multichromophoric systems. For every single complex, we probe the initial exciton relaxation process by an ultrafast pump-probe approach and the coupling to vibrational modes by emission spectra, while fluorescence lifetime analysis measures the amount of static disorder. Exploiting the wide range of disorder found from complex to complex, we demonstrate that static disorder accelerates the dephasing and energy relaxation rate of the exciton. DOI: 10.1103/PhysRevLett.97.216403 PACS numbers: 71.35.Aa, 32.50.+d, 78.47.+p For many decades assemblies of coupled emitters have been studied intensively due to their intriguing properties of spectral narrowing [1] and enhancement of spontaneous emission rate (superradiance) [2]. This behavior stems from coherent excited state energy (exciton) delocalization over the aggregate [3,4]. The exploitation of exciton delocalization for a controlled transfer of energy in photonics and electronics devices as well as in novel schemes for quantum computation has sparked new interest in a wide variety of multichromophoric assemblies, such as photonic polymers [5], light harvesting complexes [6], and quantum dot arrays [7]. In practice, the performance of these systems depends on the extent of coherent exciton delocalization, a property varying in time and space due to static and dynamic disorder [8,9]. Understanding how these two parameters constrain the extent of collective excited states in multichromophoric assemblies is therefore a crucial step in the design of functional excitonically based molecular devices.Static disorder in dye assemblies mainly arises from differences in the energy of the interacting chromophores. The initial exciton created upon absorption is then confined to a region smaller than the actual size of the complex [10]. Coupling of this state to molecular vibrations and bath phonon modes (dynamic disorder) leads to subsequent ultrafast dephasing and energy relaxation through the exciton band. As a consequence, the coherence size of the exciton is further reduced [11]. Therefore, the properties of the final thermalized exciton result from the action of both static and dynamic disorder. The combined effects of these parameters on exciton dynamics have been studied so far theoretically [12] and by means of bulk spectroscopy [13]. In bulk experiments, however, the interplay of static and dynamic disorder cannot be disentangled since the average over a wide range of energy differences from molecule to molecule is measured.In this Letter we investigate the effect of static disorder on exciton dynamics in a multichromophoric system using a unique combination of several room-temperature singlemolecule spectroscopy (SMS) techniques. The coherent delocalization length of the final thermalized exciton is probed using steady-state and fast SMS methods, thus unraveling the extent of static disorder in individual aggregates. The interpla...

Localization and coherent dynamics of excitons in the two-dimensional optical spectrum of molecular J-aggregates

“…In order to study the energy dependence of the static properties of exciton states, such as their localization length, or of the dynamic properties, such as energy relaxation and the role of coherent superpositions, one can use single molecule spectroscopy, [15][16][17] or frequency resolved nonlinear optical techniques.…”

Ultrafast single-molecule photonics: Excited state dynamics in coherently coupled complexes

“…2(a)) reveal the occurrence of multilevel intensities, due to sequential photodamaging of the constituent chromophores in the complex [3,4] (Fig. 2(b)).…”

“…Despite the disorder, the single-molecule approach allows to reveal a clear correlation between the fs redistribution time and the extent of exciton delocalization [3].…”

“…We have focused particularly on coherent strongly coupled assemblies, where we find longer femtosecond time response and discrete jumps in decay times, reflecting sudden change in coupling of the quantum coupled system. In this short contribution we report on ultrafast single-molecule detection of the trimer complex of the tetraphenoxyperylene diimide chromophore (TPD) [3].…”

“…We have applied SM2P to study both intra-and intermolecular fs dynamics of single quantum systems, revealing the importance of conformational heterogeneity and the influence of coherent coupling [1][2][3].…”