Theoretical Analysis of the Relaxation Dynamics in Perylene Bisimide Dimers Excited by Femtosecond Laser Pulses

Abstract

We present a model for the relaxation dynamics in perylene bisimide dimers, which is based on ab initio electronic structure and quantum dynamics calculations including effects of dissipation. The excited-state dynamics proceeds via a mixing of electronic states of local Frenkel and charge-transfer characters, which becomes effective upon a small distortion of the dimer geometry. In this way, it is possible to explain the fast depopulation of the photoexcited state, which we characterize by femtosecond transient absorption measurements. The combined theoretical and experimental analysis hints at a trapping mechanism, which involves nonadiabatic and dissipative dynamics in an excited-state vibronic manifold and provides an atomistic picture that might prove valuable for future design of photovoltaic materials.

The dimer-approach to characterize opto-electronic properties of and exciton trapping and diffusion in organic semiconductor aggregates and crystals

“…6, upper panel, which also contains calculated CT-characters (lower panel). Despite the rather small changes in the atomic positions in going from q = 0 to q = 1, which are in the order of 0.01 Å, 53 the effect on the energetic order of the states is pronounced. The photoexcited state with quantum number n = 2 is of Frenkel-type and the energetically closest state (n = 3) is of CT-type.…”

“…Taking the interaction with a surrounding into account we solved a stochastic Schrödinger equation, 97 and the thus obtained population dynamics are in very good agreement with experiment. 53 The overall picture of the dynamics which takes place in the dimer after femtosecond excitation of the higher lying Frenkel state (n = 2) is illustrated in Fig. 7, upper panel, and it reads as follows.…”

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…More recent quantum dynamic studies corroborate further the involvement of charge-transfer states that, depending on the respective geometry of the dimer stack, may cross the energy surface of the Frenkel exciton state and become the lowest-energy state at particular stacking geometries. [20] Experimentally it proved to be challenging to monitor the ultrafast relaxation from the initial exciton state to the excimer state. Five different PBI aggregates, including PBI 2 dimers and PBI 1 oligomers, were investigated by femtosecond transient absorption spectroscopy and time-resolved anisotropy measurements in MCH.…”

Exciton dynamics in perturbed vibronic molecular aggregates

“…If the localization remains stable for some time τ l , it is possible that an electronic relaxation process takes place subject to the condition that the relaxation time τ r is shorter than τ l . This, for example, was observed in femtosecond experiments on perylene bisimide aggregates where the photoexcited state decays on a time-scale of about 200 fs 17 . Because such processes trap the exciton, it is highly desirable to avoid a localization maintained on a longer time-scale.…”