Two-dimensional electronic photon echoes of a double band J-aggregate: Quantum oscillatory motion versus exciton relaxation

We investigate the first steps of exciton dynamics in the course of nanometric band-to-band energy-transfer in a bi-tubular J-aggregate by employing two-dimensional (2D) coherent electronic spectroscopy. Coupled exciton states are mapped as typical off-diagonal signals in 2D frequency-frequency correlation spectra, with sub-20 femtosecond (fs) time resolution. The experiments enable to look beneath conventional incoherent population transfer by directly probing the early 100fs quantum-kinetic regime. Our data, acquired at room temperature, indicate low-amplitude coherent motion, followed by quantum-dissipative population transfer. Exciton inter-band coherence with low intensity is observed in the high-energy cross-peak (ω3>|ω1|) as an electronic wave-packet following quantum-beat oscillation up to 100fs, whereas ongoing exciton relaxation is imaged in the low-energy cross-peak (ω3<|ω1|) as typical streaked signals along the coherence frequency ω1. Solvent induced band symmetry breaking and tube expansion create spatial disorder and densities-of-states with dipoles of minor collectivity that affect both the morphology and electronic band features by broadening the peaks and streaks and promoting the irreversibility of tube-to-tube transfer on longer time-scales.