Real‐time Observation of Structural Dynamics Triggering Excimer Formation in a Perylene Bisimide Folda‐dimer by Ultrafast Time‐Domain Raman Spectroscopy

Abstract

In π-conjugated organic photovoltaic materials, an excimer state has been generally regarded as a trap state which hinders efficient excitation energy transport. But despite wide investigations of the excimer for overcoming the undesirable energy loss, the understanding of the relationship between the structure of the excimer in stacked organic compounds and its properties remains elusive. Here, we present the landscape of structural dynamics from the excimer formation to its relaxation in a co-facially stacked archetypical perylene bisimide folda-dimer using ultrafast time-domain Raman spectroscopy. We directly captured vibrational snapshots illustrating the ultrafast structural evolution triggering the excimer formation along the interchromophore coordinate on the complex excited-state potential surfaces and following evolution into a relaxed excimer state. Not only does this work showcase the ultrafast structural dynamics necessary for the excimer formation and control of excimer characteristics but also provides important criteria for designing the π-conjugated organic molecules.

Spectroscopic analysis of vibrational coupling in multi-molecular excited states

“…the reduction of the distance between two neighboring ZnPc monomers is in the range of 0.08 A to 0.11 A with an contrary behaviour with temperature with respect to the vibrational energy. This is a reasonable result when compared to other excimer systems for which similar values in reduction of inter-molecular distance have been reported [23,25,40,46,50,51].…”

“…First of all, the excited state interaction leading to the geometric relaxation has not been specified in detail, which actually is not necessary for the phenomenological description of the Xdimer's photon emission. In the case of excimer formation, exciton resonance [36,37] as well as a CT state [12] have been proposed as driving forces behind geometrical relaxations but, meanwhile, the consensus prevailed that only an adiabatic mixing of Frenkel and CT excitons can fully explain the observed phenomena [11,15,25,[38][39][40][41]. For example, in crystalline pentacene, the first excited state is known to have a significant admixture of CT character [42,43] and a considerable geometric reorganization accompanies its photo-excitation [44].…”

“…In fact, Tvingstedt et al successfully used a displaced harmonic oscillator model using inter-and intra-molecular vibrational modes to describe the emission of CT states formed at donoracceptor hetero-junctions [24]. The evolution from the inital excitation towards the excimer state is often considered a single-step process [22,[45][46][47], but a two step process via an intermediate state has been proposed, too [48,49], which can be part of the adiabatic evolution towards the relaxed excimer geometry [25]. Furthermore, the geometric reorganization upon photo-excitation usually comprises more than just one motion [40,50].…”

“…For example, calculations revealed the pyrene excimer formation to be governed by a superposition of a lateral convergence as shown in figure 1 a) and a horizontal sliding motion [46]. For perlyene bisimide (PBI) dimers, a relative rotation of the stacked PBI dimer is predicted to govern the geometric relaxation upon photo-excitation [9,25]. This means, the relaxation path shown in figure 1 a) between configurations (2) and (3) should be interpreted as an energetially steered pathway along which the system relaxes towards a local minimum of its PES and, hence, q can be interpreted as a generalized coordinate.…”

“…Usually, the emission from such states is characterized by a broad and unstructured spectrum which is considerably red shifted compared to the single molecule emission [2,21,22]. A unique feature of such excited multi-molecular states is the temperature dependence of their emission spectrum showing a characteristic broadening with increasing temperature [11,[23][24][25] For donor-acceptor heterojunctions in solar cells it is common since many years to describe the absorption and emission spectra of charge-transfer (CT) states in terms of Marcus theory for which the inter-and intra-molecular geometrical relaxation is mediated by low energy vibrations. Two main concepts have emerged: In the picture of dynamic disorder the broadening of the spectra is solely the product of inter-and intra-molecular vibrations [24,[26][27][28][29].…”