Ultrafast exciton spectral diffusion in ultrathin organic films

　Exciton energy in molecular aggregates is subjected to dynamical fluctuations due to coupling with nuclear motions. This exciton-vibration coupling strongly affects various non-adiabatic processes in the excited state including exciton energy transport and charge transfer. Therefore, it is vital to elucidate microscopic mechanism of the coupling and nature of the bath modes.

　Spectral diffusion dynamics extracted from the two-dimensional electronic spectroscopy (2DES) provides direct access to the frequency-fluctuation correlation function of the excitonic transitions that contains information of the exciton-vibration coupling.

   We studied an ultrafast spectral diffusion of the lowest exciton in a tetracene ultrathin film by 2DES [1]. From the analysis of the nodal line slope, the frequency-fluctuation correlation function (FFCF) of the exciton band was extracted. The FFCF contains two components with decay times of 400 fs and 80 fs; while the former can be understood by a linear exciton-phonon coupling model, the latter shows an order of magnitude increase in its amplitude from 96 K to 186 K that cannot be explained by the same model. A novel scheme of the energy-gap fluctuations was examined in which an intramolecular high frequency mode causes the spectral diffusion that is enhanced through an anharmonic coupling to low frequency phonon modes.

   The same approach was applied to spectral diffusion of molecular excitons in thin films of 3,4,9,10-perylenetetracarboxylic-diimide [2]. Also in this case, a significant acceleration of the decay of the center-line slope with increasing the temperature was observed which cannot be explained by a linear system-bath coupling model with a harmonic bath, suggesting the ubiquity of the phenomena.

   These finding provides a valuable input for future theoretical predictions on ultrafast nonadiabatic dynamics of molecular excitons.