Mapping the Evolution of Spatial Exciton Coherence through Time- Resolved Fluorescence Roel Tempelaar, † Frank C. Spano, ‡ Jasper Knoester, † and Thomas L. C. Jansen* ,† † Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands ‡ Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States * S Supporting Information ABSTRACT: Quantum coherence is expected to have a positive effect on the transfer efficiency of excitation energy through photosynthetic aggregates and conjugated polymers, but its significance to the functioning of these molecular assemblies remains largely unknown. We propose a new experimental means to monitor the coherence between distant molecular sites on a time scale relevant to energy transfer. Through numerical calculations, we demonstrate that the range of such spatial coherence continually scales as the 0-0 to 0-1 vibronic peak ratio in time-resolved fluorescence spectroscopy. As such, this observable allows one to monitor the coherent evolution of an excited state, displaying the large coherence length following optical excitation, and the subsequent dephasing over time. SECTION: Spectroscopy, Photochemistry, and Excited States Q uantum coherence is a hotly debated topic in the context of electronic energy transfer through light- harvesting complexes, 1-4 conjugated polymers, 5 and organic bulk heterojunction solar cells 6-8 as it involves versatile wavelike motion of the electronic excitation as opposed to slow diffusive hopping. Beating signals observed in two- dimensional spectroscopy of photosynthetic systems have been attributed to robust coherences among quantum eigenstates, 1-3 suggesting that coherent transport of the excitation holds a firm connection with the remarkable efficiency reached in natural light harvesting. Nevertheless, perhaps more than intereigenstate coherences, the spatial coherence of a quantum excitation is of direct relevance to energy transfer. Spatial coherence determines the degree of electronic correlation between molecular sites underlying a delocalized excited state. Recently, such correlation over distances has been shown theoretically to modulate transfer efficiency, 9-11 and its control is an attractive prospect for man- made devices. In this Letter, we propose the possibility to directly monitor the evolution of spatial coherence by means of time-resolved fluorescence. The proposed method relies on the vibronic progression due to coupling of the electronic excitation to a single high-frequency vibration. This is a ubiquitous phenom- enon in organic materials, with as a notable example the 1400 cm -1 symmetric vinyl-stretching mode common to conjugated molecules. In a typical spectroscopic experiment, an incoming light field interacts coherently with macroscopic fractions of a molecular assembly, preparing excitations with considerable coherence lengths. Such a sizable coherence field will subsequently decline as a result of static and dynamic fluctuations in the environment. As we will demonstrate, the 0-0 to 0-1 vibronic peak ratio in the fluorescence spectrum continually tracks the coherence length for the case of a J- aggregate. Moreover, the peak ratio accurately indicates the early decay of spatial coherence for both J- and H-aggregates. The impact of the exciton coherence length on the optical response was already long-known in the form of enhanced radiative decay in J-aggregates, 12,13 light-harvesting com- plexes, 14,15 and polyacine thin films. 16,17 Spatial coherence was also found to affect the separation between bleach and induced absorption features in pump-probe 18 and two- dimensional spectroscopy 19 of J-aggregates. However, only quite recently was a robust method proposed to quantitatively determine the degree of spatial coherence through experiment; in ref 20, it was recognized that the 0-0 to 0-1 fluorescence peak ratio provides a direct measure of the coherence length, N coh . Nevertheless, the evinced applicability of this method was restricted to thermalized molecules under steady-state con- ditions, for which the peak ratio indicates coherence averaged over the band-bottom emitting states. As such, it does not yield insight into the excitations or the dynamics relevant to energy transfer. In this Letter, we show as a proof of principle that the peak ratio serves equally well as a coherence measure at ultrafast time scales, where it instead indicates N coh of a coherently prepared wavepacket of eigenstates. Received: March 10, 2014 Accepted: April 4, 2014 Published: April 4, 2014 Letter pubs.acs.org/JPCL © 2014 American Chemical Society 1505 dx.doi.org/10.1021/jz500488u | J. Phys. Chem. Lett. 2014, 5, 1505-1510