Anharmonic Molecular Motion Drives Resonance Energy Transfer in peri- Arylene Dyads

Abstract

Spectral and dynamical properties of molecular donor-acceptor systems strongly depend on the steric arrangement of the constituents with exciton coupling J as a key control parameter. In the present work we study two peri-arylene based dyads with orthogonal and parallel transition dipoles for donor and acceptor moieties, respectively. We show that the anharmonic multi-well character of the orthogonal dyad's intramolecular potential explains findings from both stationary and time-resolved absorption experiments. While for a parallel dyad, standard quantum chemical estimates of J at 0 K are in good agreement with experimental observations, J becomes vanishingly small for the orthogonal dyad, in contrast to its ultrafast experimental transfer times. This discrepancy is not resolved even by accounting for harmonic fluctuations along normal coordinates. We resolve this problem by supplementing quantum chemical approaches with dynamical sampling of fluctuating geometries. In contrast to the moderate Gaussian fluctuations of J for the parallel dyad, fluctuations for the orthogonal dyad are found to follow non-Gaussian statistics leading to significantly higher effective J in good agreement with experimental observations. In effort to apply a unified framework for treating the dynamics of optical coherence and excitonic populations of both dyads, we employ a vibronic approach treating electronic and selected vibrational degrees on an equal footing. This vibronic model is used to model absorption and fluorescence spectra as well as donor-acceptor transport dynamics and covers the more traditional categories of Förster and Redfield transport as limiting cases.

Keywords: Förster transport; MD/QC; perylene dyads; ultrafast energy transfer; vibronic transport.

Figure 1

Simulated donor-acceptor systems. (A) Perylene-benzoperylene bisimide dyad in orthogonal arrangement. The distance between centers of the transition charge centroids is 16 Angstrom. (B) S-13-obisim-terrylene dyad in parallel arrangement.

Figure 2

Experimental and simulated absorption (blue lines) and fluorescence (red lines) spectra for acceptor (top), donor (middle), and dyad (bottom). The dashed black line combines absorption of donor and acceptor S_A + S_D. Left: Orthogonal dyad. Right: Parallel dyad.

Figure 3

Population evolutions for the orthogonal (A) and parallel (B) dyad. Parameters are the same as for Figure 2. Population evolution of donor (light blue), acceptor (dark blue) states as well as populations of resonant levels around the color-coded energies. The inset shows these energies with respect to the absorption (full blue area) and emission spectrum (gray dashed lines).

Figure 4

Histogram of coupling strengths for the orthogonal dyad (left). Mean and standard deviations are 〈J〉 = 0cm⁻¹, $\sqrt{\langle J^2\rangle -{\langle J\rangle }^2}=19$cm⁻¹. (Right) Parallel dyad and 〈J〉 = −247cm⁻¹, $\sqrt{\langle J^2\rangle -{\langle J\rangle }^2}=11$cm⁻¹.

Figure 5

Normal mode displacement probability distribution for selected an harmonic normal mode 3 of the orthogonal dyad. Statistics are derived from 20,000 configurations over 800 ps MD trajectories. Thick lines correspond to a displacement according to a harmonic approximation, adopted in normal mode analysis.