Local Phonon Environment as a Design Element for Long-lived Excitonic Coherence: Dithia-anthracenophane Revisited

Excitonic energy transfer in light harvesting complexes, the primary process of photosynthesis, operates with near-unity efficiency. Experimental and theoretical studies suggest that quantum mechanical wave-like motion of excitons in the pigment-protein complex may be responsible for this quantum efficiency. Observed coherent exciton dynamics can be modelled completely only if we consider the interaction of the exciton with its complex environment. While it is known that the relative orientation of the chromophore units and reorganisation energy are important design elements, the role of a structured phonon environment is often not considered. The purpose of this study is to investigate the role of a structured immediate phonon environment in determining the exciton dynamics and the possibility of using it as an optimal design element. Through the case study of dithia-anthracenophane, a bichromophore using the Hierarchical Equations Of Motion formalism, we show that the experimentally observed coherent exciton dynamics can be reproduced only by considering the actual structure of the phonon environment. While the slow dephasing of quantum coherence in dithia-anthracenophane can be attributed to strong vibronic coupling to high-frequency modes, vibronic quenching is the source of long oscillation periods in population transfer. This study sheds light on the crucial role of the structure of the immediate phonon environment in determining the exciton dynamics. We conclude by proposing some design principles for sustaining long-lived coherence in molecular systems.

Vibrationally Resolved Absorption Spectra and Ultrafast Exciton Dynamics in the α-Phase and β-Phase Zinc Phthalocyanine Aggregates

The vibrationally resolved absorption spectra and ultrafast exciton dynamics in the α-phase and β-phase zinc phthalocyanine (ZnPc) aggregates are theoretically investigated by using a non-Markovian stochastic Schrödinger equation combined with...

Layered Structures of Assembled Imine-Linked Macrocycles and Two-Dimensional Covalent Organic Frameworks Give Rise to Prolonged Exciton Lifetimes

Ordered organic materials and assemblies have great potential to be tailored to have desirable properties for optoelectronic applications, such as long exciton lifetime and high directional exciton mobility. Framework materials, such as twodimensional covalent organic frameworks (2D COFs), as well as their truncated macrocyclic analogues, are versatile platforms to organize functional aromatic systems into designed assemblies and robust materials. Here we investigate the exciton dynamics in a 2D COF, its corresponding hexagonal macrocycle, and extended nanotubes comprised of stacked macrocycles. The excitonic behavior of these three systems provide an understanding of excitonic processes that occur in the plane of the covalently bonded 2D macromolecules and between layers of the nanotubes and 2D COF. The nanotube and analogous 2D COF exhibit longer excited-state lifetimes (~100 ps) compared to the individual, solvated macrocycles (<0.5 ps). These differences are attributed to the internal conversion facilitated by the internal motions of the imine linkages which are significantly reduced in the assembled macrocycles in the nanotube and 2D COF sheets in the layered structures. The exciton diffusion processes in the assembled nanotubes and 2D COF systems were characterized by the autocorrelations of the transition dipole moment of the excitons, giving the depolarization time constants for both systems to be ~1 ps. This work also reveals the anisotropic exciton dynamics related to the in-plane and inter-plane structural factors in these systems. These studies provide guidance for the design of future COF materials, where the longer excited state lifetimes imparted by assembly are beneficial for optoelectronic applications.