Influence of Complex Exciton-Phonon Coupling on Optical Absorption and Energy Transfer of Quantum Aggregates

We present a theory that efficiently describes the quantum dynamics of an electronic excitation that is coupled to a continuous, highly structured phonon environment. Based on a stochastic approach to non-Markovian open quantum systems, we develop a dynamical framework that allows us to handle realistic systems where a fully quantum treatment is desired yet the usual approximation schemes fail. The capability of the method is demonstrated by calculating spectra and energy transfer dynamics of mesoscopic molecular aggregates, elucidating the transition from fully coherent to incoherent transfer.

Figure 1

The spectral density used for the calculation of spectra in Fig. 2 and energy transfer in Fig. 3. The unit of energy is the width $\Delta$ of the resulting monomer absorption spectrum [Fig. 2a].

Figure 2

(a) Absorption spectrum of the monomer [its width $\Delta$ (standard deviation) is used as the unit of energy]. (b)–(d) $J$ band spectra of ring-shaped aggregates with $C=-2.6$ $\Delta$. The values of $N$ are indicated in the figures.

Figure 3

Transfer of the electronic excitation energy on a ring-shaped 15-mer for $C=-2.6$ $\Delta$. Initially only monomer 8 is excited. (a) Without coupling to a phonon bath. (b) With coupling to a phonon bath with spectral density of Fig. 1. (c)–(e) Three of the 1000 single realizations over which the transfer in (b) is averaged.