Balancing Charge Transfer and Frenkel Exciton Coupling Leads to Excimer Formation in Molecular Dimers: Implications for Singlet Fission

Abstract

Photoexcitation of molecular chromophore aggregates can form excimer states that play a significant role in photophysical processes such as charge and energy transfer as well as singlet fission. An excimer state is commonly defined as a superposition of Frenkel exciton and charge transfer states. In this work, we investigate the dynamics of excimer formation and decay in π-stacked 9,10-bis(phenylethynyl)anthracene (BPEA) covalent dimers appended to a xanthene spacer, where the electronic coupling between the two BPEA molecules is adjusted by changing their longitudinal molecular slip distances. Using exciton coupling calculations, we quantify the relative contributions of Frenkel excitons and charge transfer states and find that there is an upper and lower threshold of the charge transfer contribution for efficient excimer formation to occur. Knowing these thresholds can aid the design of molecular aggregates that optimize singlet fission.

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

“…Thirdly, the excited states of Bis-PBI are not pure Frenkel or CT diabats (Tables S3-S4), but rather (quasi)adiabatic states which consist of different degrees of LE and CR configurations. [10,44] These features are highly similar to those observed in an adiabatic singlet fission reaction. [58] Especially, in an adiabatic regime, the nuclear rearrangement determines the transition rate.…”

“…Chemie action method with double spin-flip [57] (RAS-2SF) method shows that the CR configuration in the first optically allowed state (3 %) becomes more significant with evolving into the relaxed excimer geometry (12 %), which is in line with previous calculation results by TD-DFT. [44] Consequentially, we point out the crucial features of the excimer formation mechanism; Firstly, in numerous co-facial PBI stacks, including Bis-PBI, similar rate constants of excimer formation (k = ( � 200 fs) À 1 ) were observed irrespective of the external environment [21] (e.g., solvent polarity) and exciton coupling strengths. [13,18,20,23,29,45] Secondly, the time constant of the excimer formation corresponds to that of ultrafast structural rearrangement along the interchromophore coordinate.…”

“…[23] Overall, these results underpin that the excimer state can be viewed as an adiabatic mixture of locally excited (LE) and CR configurations due to the energetic proximity of the two diabats or strong coupling. [10,23,44] To understand the excited-state dynamics of Bis-PBI, we first carried out fs-TA experiments with two different laser systems (a Ti:Sapphire system for dispersed detection with CMOS cameras and a Yb:KGW system for non-dispersed detection with photodiodes, see Supporting Information). With the dispersed detection, we have measured TA spectra from 350 nm to 1300 nm.…”

Efficient singlet fission in nanoparticles of amphipathic anthracene–tetracene dyad with broadband light harvesting ability

“…These two PIA bands are attributed to the absorption of the singlet (S 1 ) state of DPA based on the previous report. 50,51 With time delay, these two S 1 absorption bands showed an increase within the first ∼20 ps, as revealed by single-wavelength dynamics probed at ∼375 and ∼580 nm (Fig. S4, ESI†), which should be caused by vibrational cooling and rotation of the two phenyl rings at 9,10-position of DPA in the S 1 state.…”

Excimers in Multichromophoric Assemblies: Boon or Bane?

“…A recent theoretical work on covalent PBI dimers also suggested an upper threshold of CT contribution (~58 %), above which excimer formation in cofacial PBI aggregates could be suppressed. 46 In addition to the large exciton splitting, the exciton wavefunction also experiences an efficient self-localization that further safeguards it from potential trap sites in the vicinity. In view of these findings, it is prudent to take special note of two recent studies.…”